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Doping effects in CuGeO3
ELSEVIER
Physica B 225 (1996) 177 190
Doping effects in CuGeO3 M. Weiden a'*, W. Richter a, C. Geibel a, F. Steglich a, P. Lemmens b, B. Eisener b, M. Brinkmann b, G. Giintherodt b aFG Technische Physik, TH Darmstadt, Hochschulstr. 8, 64289 Darmstadt, Germany b2. Physikalisches Institut, RWTH Aachen, Templergraben 55, 52056 Aachen, Germany Received 26 January 1996
Abstract
We investigated the effect of doping on the crystallographic, dynamic and magnetic properties of (Cul _ zZnz)GeO3 as well as Cu(Gel _xmx)O3 with A = A1, Si, Ga, As, In, Sn, Sb and Ti. A large solubility was only observed for Zn, Si and Ti. For those elements, the lattice parameters show a linear dependence on doping within the whole solubility range. The spin-Peierls transition temperature Tsp decreases linearly with doping, though the slope is strongly dependent on the dopant. For Zn-doping, a N6el-like ground state which exhibits spin-flop behavior appears for higher doping after the complete disappearance of the spin-Peierls ground state. Si- and Ti-doped samples exhibit a similar phase in a coexistence with the spin-Peierls ground state. The analysis of the susceptibility in the spin-Peierls phase for low Zn content indicates a reduction of the dimerization within the Cu chains and the energy gap upon doping. Whereas for Si and Ti all phonon frequencies changed linearly with the doping level, we observed in Zn-doped samples for some phonons a clear minimum at the composition z = 0.02 where the spin-Peierls transition disappears, suggesting a strong spin-phonon coupling for these modes. For higher Zn concentrations, no lattice dimerization can be observed by Raman scattering.
I. Introduction
Quantum effects in low-dimensional spin systems are presently a subject of high interest. In this context, the compound CuGeO3 has recently [1-5] attracted much attention, since it is the only inorganic compound presenting a transition into a spin-Peierls (SP) ground state. The crystal structure of CuGeO3 is characterized by an orthorhombic unit cell [6] which is built up by quasi-one-dimensional Cu chains alon~ the caxis with a short C u - C u distance of 2.941 A. These chains are separated by G e - O chains which leads to a quite large C u - C u distance along a- and b-axis * Corresponding author.
of 4.81,~ and 4.235A, respectively. Along the C u - O - C u chains, a strong antiferromagnetic (AF) coupling with an exchange coupling constant Jc ,~ 8 8 K [1] is observed giving rise to strong one-dimensional AF fluctuations at higher temperatures I-7]. Perpendicular to the chains, the coupling is one order (along b) or even two orders (along a) of magnitude smaller I-8]. Below the transition temperature Tse ~ 14.5K, the system condensates into a nonmagnetic SP ground state, separated from the excited spin triplet by an energy gap Eg ~ 2.1 meV. This SP transition is caused by a magnetically driven lattice dimerization along the Cu chains. Till now, besides CuGeO3 only some organic materials (see e.g. [9-11] and references therein) are
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M. Weiden et al. / Physica B 225 (1996) 177-190
known to show a SP ground state. In all these compounds, studies of doping effects on the transition are quite difficult due to the low solubility of dopants. In contrast, doping can be rather easily performed in CuGeO3, both on the Cu as well as on the Ge site. This compound is therefore the first possibility to study the effects of larger amounts of impurities on the magnetic ground states and energy excitations of a spin-Peierls system. Several investigations of doped CuGeO3 have already been published [12-16]. Although there is agreement on some of the observations, there are strong discrepancies in other aspects, e.g. on the effect of Si-doping and on the nature of a second anomaly observed for Zn-doping: for example, neutron experiments supposed a N~el state [17], whereas/~+SR studies [18, 19] suggest a spin-glass state. No structural data have so far been published for doped CuGeO3. We have, therefore, performed a systematic doping study putting emphasis on structural and magnetic properties.
2. Experimental details Polycrystalline samples of (CUl - zZnz)GeOa and Cu(Get -xA~)O3 with A = AI, Si, Ga, In, Sn, and Ti (for details concerning the stoichiometry, see Table 1) were prepared by a standard solid-state reaction. Since inhomogeneity might be a strong
Table 1 An overview of all investigated samples Dopant A=A1
Concentrations (x)
0.005,0.01,0.02,0.03,0.04, 0.05, 0.1, 0.2 A = Si 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.15 A = Ga 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.15, 0.2, 0.25 A = In, Sn 0.02, 0.05, 0.1, 0.2 A = As, Sb 0.01, 0.02, 0.03 A = Ti 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.1, 0.15 Zn 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09
problem in such doping studies, special attention was given to improving the quality of the samples: stoichiometric mixtures of the oxides (Alfa specpure) were thoroughly grounded, heated to 950°C for 100h, regrinded and heated again for another 100 h. In a third step, the samples were pressed into pellets and heated again. No significant weight loss was observed during the whole preparation procedure. Reaction and annealing temperatures were optimized in order to get the lowest residual susceptibility at T ~ Tsp in pure CuGeO3. Since Sidoping seems to increase the melting point of the doped Cu(Gel -xSix)O3, we increased the sintering temperature to 1000°C for this series. The As- and Sb-doped samples were prepared in sealed quartz ampoules under an atmosphere of oxygen because of the high dissociation pressure of the corresponding oxides. All samples were analyzed by X-ray diffraction using Si as an internal standard. Magnetic measurements within the temperature range 2 K ~< T ~< 350 K and 0 ~< B ~< 5.5 T were carried out using a SQUID-magnetometer (MPMS, Quantum Design). Raman scattering experiments were performed with different excitation lines of an Ar-Laser using a DILOR-XY spectrometer and a nitrogen cooled CCD as detector. A quasi-backscattering geometry was used with the sample cooled in an optical cryostat. Special emphasis was placed on the analysis of phonon frequencies and half-widths as a function of the substitution level and the detection of additional substitution-induced phonon modes. The analysis of magnetic scattering below and above Tsp will be published elsewhere [16, 20].
Spurious phases in x >~ 0.02...0.03 None x >/0.03... 0.04 All samples All samples x/> 0.07 x/> 0.08
3. Impurity effects due to single-crystal growth A careful analysis of the published results clearly indicates systematic differences between single crystals and polycrystalline samples. These differences are more pronounced in doped CuGeO3, but are already visible in the pure compound. Presently, most investigations on single crystals are performed using large crystals grown in an image furnace. In Fig. 1, we compare the typical susceptibility of such a crystal (taken from [15], with
M. Weiden et al. / Physica B 225 (1996) 177-190
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similar results for example in Ref. [17]) with the behavior of two other samples prepared within this work in a different way: a polycrystalline sample and a single crystal grown from CuO flux at a very slow cooling rate. Image-furnace-grown crystals reveal a much higher residual susceptibility for T ~ Tsp than flux-grown crystals or polycrystalline samples. A likely cause for this enhanced susceptibility might be some disorder or oxygen defficiency caused by the reduction of Cu 2 ÷ to Cu ÷ during the crystal growth. To gain more insight into the origin of these defects, we performed thermogravity (TG) and differential thermal analysis (DTA) on pure CuGeO3. In air, TG revealed an abrupt weight loss at l l00°C. At the same temperature, DTA showed a sharp increase indicating some sort of transition (Fig. 2). The lost weight is partly recovered on slow cooling below this transition temperature. After this DTA measurement, the X-ray spectra revealed a large amount of GeO2 and Cu20 besides CuGeO3. In a flowing Ar atmosphere (99.999%), the weight loss already takes place at 1000°C and is not recovered on cooling. This clearly shows that CuGeO3 looses oxygen at 1100°C (in air) and starts to decompose. Since the crystal growth is typically performed in this temperature range, these results suggest that
Fig. 2. TG (solid line) and DTA ([]) measurements of 63 mg of pure CuGeO 3. The measurements were performed in air.
partial reduction of Cu 2+ to Cu + during the crystal growth and incomplete reoxydation of the large crystals during cooling are likely to cause the observed higher residual susceptibility. Besides disturbing the lattice, this leads to an intrinsic doping of Cu ÷ within the chains. Such a cutting of the Cu z + chains by the nonmagnetic Cu ÷ ions should have a rather similar effect as Zn-doping (see below). Attributing the increase in z(T) to oxygen defects, a defficiency in CuGeO3_~ of 6 ,,~ 0.0024 (!) would be sufficient to account for the larger z(T) of the single crystal of Ref. [15] in Fig. 1. Since doping can change the melting point and/or the stability against reduction, we preferred to investigate the intrinsic effect of dopants on polycrystalline samples since they can be synthesized at much lower temperatures where reduction can be avoided.
4. Results
4.1. Crystallographic properties, solubility limits and lattice parameters The solubilities of the different dopants were determined by extrapolating the intensity of the reflexes of the foreign phases in the X-ray pattern to zero. They were further checked indirectly by analyzing the composition dependence of characteristic features in the susceptibility measurements.
M. Weiden et aL / Physica B 225 (1996) 177-190
180
The largest solubility was observed for the substitution of Ge by the isoelectronic Si (more than 15at°/0). In contrast, our results indicate that the solubility limit for the isoelectronic Sn and the elements of the 5th period, As and Sb, is below our detection limit of 0.5 at%. This also was the case for In. The limit for the other investigated elements of the 3rd period, AI and Ga, is rather small, around 2 and 3 at°/0, respectively. In contrast, substitution of Ge by Ti is possible up to 6 at% and of Cu by Zn up to 8 at%, respectively. The solubility limits for all dopants are given in Table 1. For the elements with a larger solubility, i.e. Si, Ti and Zn, we determined the composition dependence of the lattice parameters. Si-doping has the largest effect on the lattice parameters. Up to the highest investigated doping level of x = 0.15, the aand c-axes as well as the unit-cell volume decrease linearly, whereas b increases (Fig. 3). The largest effect, - 0 . 5 % at x = 0.15, is observed along the c-axis, i.e. along the direction of the Cu-chains. The opposite trend is observed for Ti-doping: the b-axis decreases, whereas the a- and c-axes increase (Table 2). The effect of Ti on the a- and c-axes is approximately a factor of three smaller compared to the effect of Si. This could be expected as this value roughly corresponds to the ratio of the difference of the ionic radii: (Rsl,+ - Ree,+)/(Rri,+ R~e,+) ~ - 3 . In contrast, the decrease of the b-axis is much larger than expected from comparing the ionic radii. The effect of Zn-doping on the lattice is quite small as a consequence of the nearly equivalent ionic radii of Cu 2+ (Rcu2+ = 0.57A) and Zn 2+ (Rz,~+ = 0.60 A). Here, the a-axis decreases whereas the c-axis is increasing (Table 2). No significant variation for both the b-axis and the volume could be resolved within the scattering of the data.
4.2. Electronic and dynamic properties
In pure C u G e O 3 , three excitations related to the SP state appear in the Raman spectra below Tsp: a singlet-triplet excitation (30cm-1), two-magnon scattering (30-240cm-1) and an additional zonefolded phonon (370cm -1) [16,20,36]. The onset and the line shape of these excitations are a valuable tool to determine the degree of dimerization in
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M. Weiden et al. / Physica B 225 (1996) 177-190
the C u - O - C u chains and the coherence of the SP state. Significant shifts of the phonon frequencies were observed for Si-, Ti- and Zn-doping and studied in dependence of concentration and temperature. The observed shifts of the phonon frequencies are closely related to the changes in the lattice parameters discussed above. Si-substitution leads to linear shifts of all observed phonon modes with increasing substitution level, without showing large broadenings of the line widths. At x = 0.05, these shifts range from + 0.8 to - 0 . 5 % (Table 3). An additional phonon is observed at 669cm -1 for x ~> 0.01. This excitation shows a linear increase of intensity with substitution. Due to the absence of any other second phase we attribute it to an intrinsic phonon which becomes Raman-allowed due to the symmetry breaking of the dopant. Ti substitution also leads to two additional phonons at 454 and 610cm-1. For both dopants, the singlet-triplet excitation is not observable for x > 0.01, and the two-magnon scattering vanishes for x > 0.02. In the case of Zn substitution, we observe pronounced minima at z = 0.02 in the concentration dependencies of the frequencies of some phonons, whereas others reveal a linear behavior (Fig. 4). This minimum occurs at the highest Zn concentration where a SP transition can still be observed in
Table 3 Shift of the phonon frequencies as a function of Si-doping. The slope sabs is calculated using v(x)= v(0)+ Sabs* 100X, X from Cu(Gel _xSix)O3. The relative shift Srel is calculated using v(x) = v(0)(l + Sre~* 100X) and reveals strong differences, though the observed dependence is perfectly linear for all phonons up to x = 0.05
susceptibility measurements. As it is not caused by a change in the doping dependence of the lattice parameters, which is linear for the whole concentration range, it may indicate a change in the spin-phonon coupling due to the doping. No such extremum was observed for any phonon in Si- or Ti-doped samples. In Fig. 5, we compare the dependence of the frequency of the phonon at 185cm -1 on doping with Si, Ti, and Zn. The singlet-triplet excitation is not observable in any Zn-doped sample. This strong suppression is also observed for the two-magnon scattering which vanishes for z > 0.01. In the case of Ga substitution a continous broadening of the phonon line widths instead of frequency shifts or additional phonon modes was observed. For the phonon at 592cm-1, the broadening reaches 12% at x = 0.02. This broadening indicates a considerable amount of disorder induced by substitution. The leveling off of the line width marks the solubility limit reached for this substitution at x ~ 0.03, as it was already detected within the crystallographic analysis. To investigate the lattice dynamics of the SP transition, we measured the intensity of the folded phonon at 370cm -1 as a function of temperature for Cu(Gel-xGax)CuO3 with x = 0, 0.01 and 0.02, respectively. It thus is a tool to measure the development of the crystallographic dimerization. In Fig. 6, we present the intensity of this folded phonon, normalized to the intensity of the phonon at 390cm-~ which presents an only very weak temperature dependence. For the pure compound, we find a sharp increase at Tsp. The behavior can be well-fitted using I = Io(1 --
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with fl = 0.52. This is a different behavior than observed for the spin gap Eg v e r s u s T where we obtain fl = 0.10, in good agreement with neutron experiments [30] and published Raman measurements [36]. For the doped compounds, the behavior changes to a linear evolution of the phonon intensity with decreasing temperature. Furthermore, the slope of this evolution decreases with increasing doping level. To investigate the possibility of doping charge carriers by substituting Ga 3 + for Ge 4+, the Raman
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Fig. 6. Intensity of the phonon at 370cm-1 as a function of T (after subtracting the background) normalized to the temperatureindependent phonon at 390cm-L CuGeO3 (11) reveals a sharp increase (Eq. (1)), whereas 2% Ga ([3) leads to a linear dependence.
efficiency o f t h e p h o n o n at 592 c m - 1 w a s m e a s u r e d in s a m p l e s w i t h x = 0 a n d 0.02 as a f u n c t i o n o f t h e e n e r g y o f t h e i n c i d e n t e x c i t a t i o n line (Fig. 7). I n pure CuGeO3, a resonance-like enhancement of
the p h o n o n i n t e n s i t y is o b s e r v e d at a w a v e l e n g t h o f 2 = 487 n m (2.53 eV). T h i s m a x i m u m is c o n n e c ted w i t h a c h a r g e - e x c i t a t i o n g a p t h a t r e s o n a n t l y i n c r e a s e s t h e p h o n o n intensity. Its e n e r g y
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corresponds to the bright blue color of the samples. In G a substituted samples this color changes slightly to a greyish tone. The corresponding R a m a n experiments for x = 0.02 show a decrease of the R a m a n efficiency and a slight shift of the m a x i m u m to higher wavelength 2 = 4 9 7 n m (2.50eV), i.e. a reduction of the excitation gap. However, DC-conductivity measurements showed insulating behavior in all substituted samples. The charge remains localized at the defect site. This result is in perfect agreement with the analysis of Terasaki et al. [21], who proposed carrier doping to be difficult due to a narrow tpd band of the undoped CuGeO3.
4.3. Magnetic phase diagrams The magnetic phase diagrams were investigated by means of susceptibility (z(T, B)) and magnetization (M(B)) measurements. The results of the z(T)-measurements as a function of doping for Zn, Si and Ti are shown in Figs. 8(a), 9(a) and 10, respectively. The pronounced decrease in Z (T) near T ~ 14 K corresponds to the SP transition. As the transition broadens slightly with increasing doping, the use of the inflection point for the determination of the transition was not reliable. The transition temperature is therefore derived by fitting a linear function above and below the SP transition. Tsp is
183
defined as the crossing temperature of these two lines. The same procedure was performed for the determination of TN1 and TN2, see below. Tsp decreases for all kinds of dopants on the Ge site as well as on the Cu site. This decrease is rather fast for Zn (Tsp = 12.6K for z = 0.02, see Fig. ll(a)), somewhat weaker for Ti (Tsp = 13.0K at x = 0.06) and very small for Si (Tsp = 14.2 K at x = 0.1). For these three dopants, the application of a magnetic field leads to a reduction of Tsp. As already observed in pure CuGeO3 [1], and in agreement with theoretical predictions [22, 23], this reduction is proportional to B e
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where the calculated value Ath. . . . tical varies between 0.32 and 0.44 [22, 24, 25]. We observe a correlation between the A coefficient and the slope of OTsp/O(x,z). Compared to the value for the pure compound (A = 0.47), A increases to 0.61 for 2 a t % Zn ( T s p = 12.6K), 0.55 for 3 a t % Ti ( T s p = 13.6K) and even decreases to 0.36 for 2 a t % Si ( T s p = 1 4 . 5 K ) . These measured A coefficients represent an average over the three crystal axes as we never observed any preferred orientations in polycrystalline samples. Thus, we consider the anisotropy of the crystal axes as observed by Ref. [26] for the pure c o m p o u n d not to play an important role in this analysis. For Zn-doping, a SP transition was observed for 0 ~< z ~< 0.02 only, whereas a second anomaly of antiferromagnetic type at low temperature TN1 appeared for z/> 0.04. N o transition down to 2 K was found for z = 0.03. TN1 shifts slightly to lower temperatures with increasing x, but is still observable up to the solubility limit. For z = 0.06, we also performed measurements of z ( T ) in various fields and of M(B) at various temperatures. TN1 is shifting slightly to lower temperatures with increasing field and vanishes for B ~ 1 T (Fig. 8(b)). Above TN1, the magnetization is strictly linear up to the highest investigated field of B = 5.5 T. Below TN1, the magnetization is linear only up to B = 0.2 T. At higher fields, an upturn in M(B) for 0.3 T ~< B ~< 1.4 T is observed.
M. Weiden et al. / Physica B 225 (1996) 177-190
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In contrast to the results for Zn, the SP transition could be observed at least up to 10at% in Si and up to the solubility limit of 6 at% in Ti. In both cases, a second antiferromagnetic-like anomaly appears below Tsp at TN2 ~ 5 K. For Si, it is visible in the doping range 0.02 ~< x ~< 0.04, whereas for Ti doping, it is shifted to x >/0.04. Thus, the lower transition coexists with the SP transition in both cases. Coexistence here (and throughout the whole paper) means the observance of both transitions within the same sample, as we are presently not able to clarify whether the N6el state replaces the SP state or really coexists in a microscopic way. Macroscopic phase separation due to sample inhomogeneities can be ruled out as the N6el state vanishes again for higher Si-doping. We also investigated the field dependence of this
low-temperature transition in Cu(Geo.9sSio.02)O 3 (Fig. 9b). TN2 decreases very weakly for 0 T ~< B ~< 1.0T. At larger fields, the slope ~ T N 2 / ~ B increases strongly. For B > 1.5T, the transition seems to be suppressed completely. The comparison of the behavior of the low-temperature phase of Zn-doped samples on the one hand and Si/Ti-doped samples on the other hand via M(B) measurements reveals some similarities. Above TN1/TN2,the magnetization is strictly linear in the whole field range (B~< 5.5T). Below TN1/TN2, a clear non-linear behavior for all three dopants is observed. In Fig. 11, we plot the deviation of M(B) at T = 2.7K from a linear field dependence for a 2 at% Si-, 5 at% Ti- and 6 at% Zn-doped sample. All samples show a clear upturn around B ~ 1 T. Above this upturn, Si and Ti keep
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a constant value above the linear extrapolation, whereas Zn reveals a weak m a x i m u m near B ~ 1.5T and drops significantly below the linear extrapolation for higher fields. This different behavior at higher fields might be related to the presence or absence, respectively, of a SP transition at higher temperatures (and thus also at higher fields). The series C u ( G e l - x A x ) O 3 with A = A1, G a show a decrease of Tsp with increasing doping. Due to the low a m o u n t of doping allotted in both cases (see Table 1), however, it was not possible to determine whether or not this dependence of Tsp is linear as in the case o f A = Si, Ti or not (Fig. 1 l(a)). Furthermore, no anomally in z(T) at lower temperatures was observed, probably also a consequence of the low solubility limit.
5. Analysis and discussion
5.1. Magneticphase diagram The effect of Zn-doping was already investigated by several other groups [12, 13, 17, 27]. Our results for OTsp/~z as well as the observation of a second anomaly for higher doping levels at low temperatures are in good agreement with the published results. However, Hase et al. [12] and Oseroff et al. [13] observed a coexistence of the SP phase and the low-temperature phase for z = 0.03, whereas we did not observe such an anomaly for this stoichiometry (Fig. 8(a)), indicating that there is no coexistence of the two phases in the Zn-doped samples. The comparison of our results with the published ones suggests that
186
M. Weiden et aL / Physica B 225 (1996) 177-190 '
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homogeneity of the dopant distribution is crucial for doping investigations. Hase et al. [12] and Tchernyshyov et al. [27] also observed a difference between zero-field cooled (zfc) and field cooled (fc) z(T) measurements and concluded that the low-temperature phase is spin-glass like. In contrast, Oseroff et al. [13] and Lussier et al. [17] concluded from specific heat and neutron measurements that this phase corresponds to a N~el state. We performed detailed measurements of fc and zfc z(T) for various samples with z > 0.03 and found no evidence for any hysteresis within our resolution of less than 0.1%. From the absence of hysteresis, but mostly from the upturn in the magnetization, we conclude that antiferromagnetic order, with a broadened spin-flop transition for B ,~ 0.8T, rather than a spin-glass state develops at low temperatures.
4
5
6
7
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d o p i n g (%) Fig. 11. (a) Tsv as a function of dopant and the amount of doping. The solid lines are guide to the eye, they were also used to determine the slope as discussed in the test. (b) Tm (for Zn) and TN2 (for Si, Ti) as a function of doping.
A recent theory proposed by Lu et al. [22] for non-magnetic doping within the Cu chains (applicable for the Zn-doped samples) yields to a linear decrease of Tsp with small doping amounts:
Tsv = T ~ ° ( 1 - sn),
(3)
with n the doping concentration and s = 11.1 for in-chain doping. These authors also predict the vanishing of the energy gap around ni ~ 2.8% and the existence of a gapless SP phase for higher nl which could lead to a spin-glass state due to frustration. About 10at% of doping should destroy the SP ground state completely. The decrease of Tsp we observe for (Cul-zZn,)GeO3 is linear with a slope of s ,~ 6.76 (Fig. 11 (a)). Furthermore, the SP transition vanishes completely for z = 0.03. Both values agree reasonably with the
187
M. Weiden et al. / Physica B 225 (1996) 177 190
theoretical predictions, as was already observed by Hase et al. [12]. As far as the effects of doping on the Ge site are concerned, much less results have been published. Furthermore, these results are not consistent with each other: in polycrystals, Oseroffet al. [13] found an only very weak decrease of Tsp when doping with Si, whereas Poirier et al. [15] and Renard et al. [35] noticed a rather fast suppression in independent measurements on the same single crystals. From an investigation of a 0.7at% doped single crystal, where both transitions at Tsp at TN2 were observed, they proposed that TN2 corresponds to the formation of a N6el state which exhibits a spinflop transition at B ~ 1 T. Our results indicate a very small dependence of Tsp upon Si-doping. The decrease is linear up to 10at% with a slope ofs ~ 0.32 (Eq. (3)) (Fig. 1l(a)). For higher doping (x > 0.1), no SP transition was observed (Fig. 8(a)). Thus, concerning Tsp our results support those of Ref. [13] and are in strong disagreement with [15, 35]. On the other hand, our results on the magnetization of a 2 at% Si-doped sample can be well-described with the proposed spin-flop transition (Fig. 8(b)). If one takes into account that anisotropies are smeared out in polycrystalline material, our results on the phase diagram of the AF phase are in agreement with those of Ref. [15]. Since defects seem to affect Tsp much more than TN2, this discrepancy of the composition dependency can be explained by a larger amount of defects in the single crystals used in Refs. [15, 35] as was already discussed above. To our knowledge, no data have been published for Ti-doping. Our results indicate that Ti has an effect rather similar to that of Si, though in the former case the depression of Tsp is much more pronounced with a slope of s ~ 1.62. Also, the range where TN2 and Tsp coexist is shifted to higher x values. The magnetization of those Si- and Tidoped samples which exhibit both transitions are very similar: above the transition TN2, M(B) is strictly linear. Below TN2, M(B) is linear only up to B ~ 0.4 T and B ~ 0.6 T, respectively, and shows an upturn for higher fields due to a spin-flop-like transition (Fig. 12). Thus, the low-temperature phase in Ti-doped CuGeO3 is probably of the same nature as the one in Si-doped CuGeOa.
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B (T) Fig. 12. The difference between measured magnetization and a low-field linear extrapolation at T = 2.7 K. A M is calculated independently for each composition as A M = M ( B ) - M u ..... where M~i.... is always derived from an extrapolation of the low-field dependence as Mn .... = m * B. We obtained (in units of A / T m ) m = 307 for 2% Si, m = 371 for 5% Ti, and m = 441 for 6% Zn, respectively.
For both Si- and Ti-doping, the reduction of Tsp is accompanied by a reduction of the unit-cell volume. Since the pressure dependence of the SP transition is positive [5, 14, 34], the main effect of doping on the Ge site cannot be due to a volume change, but has to be attributed to the disorder in the coupling between neighboring Cu 2 + moments introduced by the doping. Comparing the magnetic phase diagrams of Si-, Ti- and Zn-doped samples, there are obvious similarities. All doping series exhibit a linear suppression of Tse. All series also reveal the existence of an antiferromagnetic ground state, showing a spin-flop transition in an external magnetic field for some doping level. At the doping level where this antiferromagnetic phase appears, its transition temperature TN is the same for all dopants: TN TN1 ~ TN2 ~ 5 K , and shifts to lower temperatures with increasing doping level. All these similarities suggest that the low-temperature phase is the same for all dopants and that it corresponds to the antiferromagnetic state of disturbed CuGeO3. A simple explanation would be that the disorder induced by doping destroys the one-dimensional character locally and leads to a classical three-dimensional magnetic ordered state. Since the SP- and the AFstate coexist for some doping levels, the magnetic
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M. Weiden et al. / Physica B 225 (1996) 177-190
degrees of freedom have to be distributed between these two-order parameters. The differences between the phase diagrams as a function of doping ( T s p , T N 1 , TN2 =f(x,z)) for Zn, Si, Ti would indicate that both order parameters behave rather independently. It seems that some amount of disorder is needed to create the AF state. This would explain why less Si than Ti or Zn is needed since Si has a much greater effect on the lattice: (Roe4+ - - Rsi4+) ~ --3 (R~e,+ - - RTi,+) ~ --3 (Rc,~+ -- Rz,2+). In contrast, the SP phase is more sensitive to the doping site. The effect of in-chain doping is much stronger which can be seen by the complete suppression of the SP phase with 3 at% Zn. This is probably the reason why no coexistence can be observed for Zn-doping: though short-range correlations still exist for z > 0.03 as can be seen in high-temperature z(T) data, the long-range SP phase is destroyed before the AF phase can form.
5.2. Energy gap and dimerization The effect of small doping levels can be analyzed in more details for (Cul-zZn,)GeO3 using the theory by Bulaevskii [28] for the behavior of g(T) in the SP regime. In this theory which is valid for a static system, z(T) is given by g2p2 a~(-Y ) e x p ( j ~ _ y ) ) z(T) - ___)__
(4)
where p is the effective moment and J is the coupling constant within the antiferromagnetic coupled chains. J can be derived from the high-temperature z(T) results, using the theory by Bonner and Fisher [29] for uniform one-dimensional magnetic chains, and was calculated from g(T)-measurements to be Jc = 88K [1], though neutron experiments [30] and measurements of the saturation field [31] suggest higher coupling constants of 120 and 183K, respectively, a(y) and A (~) are tabulated values as a function of the dimerization parameter y. In this model, ~ = 1 is the limit for a uniform antiferromagnetically coupled Heisenberg chain. The energy gap E~ can be obtained by combining this theory with the
mean-field approximation of Pytte [33] and Bray et al. [34] to be Eg=l.637fiJ,
fi_l-7
1 + 7"
(5)
For the pure compound, using this theory with and J as free parameters we obtain y = 0.614, J = 86.6 K and, consequently, Eg = 34 K (see also [1], with a higher value for ~). fi = 0.24 is slightly larger than the value obtained by theoretical analysis of the two-magnon spectra [20]. The value of J is in a remarkable agreement with the value Jc obtained from the analysis of the high-temperature susceptibility results. From neutron experiments [30], Eg = 25 K was determined which is also in reasonable agreement with the results of the present study. In the doped samples, an additional Curie-Weiss (CW) term and a constant offset, Z0, appear. The meaning of Zo will be discussed below. The CWterm can be associated with the contribution of unpaired Cu 2÷ spins (inset Fig. 13): Zn cuts the infinite chains into finite pieces, half of them having an odd number of Cu 2 + ions and thus an effective spin ½. We therefore fitted the low-temperature region of z(T) to get the CW constants of this contribution. The values obtained are in remarkable agreement (within less than 10% difference) with the ones we expect from the amount of unpaired Cu 2 + spins due to the Zn-doping. After subtracting this contribution, the susceptibility was fitted to Eq. (4) (Fig. 14) for temperatures up to about 3 ° below Tsp, with J being fixed at the value obtained for the pure sample. The only fitting parameters were 7 and an additional scaling factor c, the latter being multiplied with the theoretical function (4). This factor c < 1 takes into account that, due to the doping, only part of the sample condensates into the SP-phase below Tsv. We obtained ~ = 0.656 for z = 0.005 and 7 = 0.699 for z = 0.01. This implies a reduction of the gap Eg of 13% and ~26%, respectively. This reduction is nearly linear with doping in contrast to the prediction by Lu et al. [22], who arrived at a sublinear suppression of Eg. The extension of this analysis to higher Zn content or for other dopants was not possible, as the low-temperature tail in the z(T) results in not purely CW like for those samples.
189
M. Weiden et aL / Physica B 225 (1996) 177-190 I
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T (K) Fig. 13. The low-temperaturebehavior of z(T) fitted with the theory of Bulaevskii[17] (solidline)for (Cuo.995Zno.0os)GeO 3. The inset shows the substracted Curie-Weiss-likecontribution. A fit with similar quality was obtained for z = 0.01 and the pure compound.
The increase of 7 with increasing doping level indicates, within this theory, that the dimerization of the Cu chains decreases. An extrapolation of leads to a vanishing of the dimerization (7 = 1) around z = 0.03, the same concentration where the z ( T ) measurements reveal no SP transition. This critical concentration agrees with our Raman results. There, the folded phonon at 370cm -1 provides a possibility to measure directly the crystallographic dimerization. In the case of Si- and Ti-doping, this phonon can be seen for the same concentration range where the SP transition is observed in z ( T ) . With Zn-doping, a rather fast suppression of the phonon intensity is observed. Already with 2 a t % it vanishes completely. This corresponds to the disappearance of the spin gap at z = 0.03 as observed in z ( T ). These results indicate that both characteristics of the SP transition, the energy gap as well as the dimerization, vanish completely with z = 0.03, which is at variance to the existence ofa gapless SP state for 0.03 < z < 0.1 as proposed by Lu et al. 1-22]. The rather small decrease of E~ (26% for z = 0.01) contrasts with the very strong reduction of the SP derived anomaly observed in z ( T ) already at the lowest doping level for all dopants.
This latter reduction is related to an increase of the CW contribution and of the constant background susceptibility Zo. Whereas the CW contribution can be attributed to non-dimerized Cu 2÷ spins due to disorder, the constant contribution Zo suggests the presence of impurity levels inside the SP gap. Thus, the main effect of doping is mainly to fill up the gap, rather than to reduce the gap energy.
6. Conclusion Our results indicate that the main effect of doping on the Ge-site is to introduce disorder which suppresses Tsp linearly. For higher doping amounts, an antiferromagnetically ordered phase can be observed at lower temperatures which exhibits spin-flop behavior in an external magnetic field. This phase coexists with the SP phase. The same N6el phase also appears in the case of Zndoping, though no coexistence is observed due to the fast suppression of the SP phase owing to the very efficient in-chain doping. In contrast to theoretical predictions, our results indicate that Zn-doping leads to a complete
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M. Weiden et al. / Physica B 225 (1996) 177-190
destruction of both the energy gap and the dimeriza t i o n w i t h i n the chains.
Acknowledgements W e w i s h to t h a n k D r . C o r d i e r , w h o p e r f o r m e d the T G a n d D T A m e a s u r e m e n t s . T h i s w o r k w a s s u p p o r t e d b y S F B 252, S F B 341 a n d B M B F p r o j e c t N o . 13N6586.
References [1] M. Hase, I. Terasaki and K. Uchinokura, Phys. Rev. Lett. 70 (1993) 3651. [2] M. Hase, I. Terasaki, K. Uchinokura, M. Tokunaga, N. Miura and H. Obara, Phys. Rev. B 48 (1993) 9616. [3] K. Hirota, D.E. Cox, J.E. Lorenzo, G. Shirane, J.M. Tranquada, M. Hase, K. Uchinokura, H. Kojima, Y. Shibuya and I. Tanaka, Phys. Rev. Lett. 73 (1994) 736. [4] K. Le Dang, G. Dhalenne, J.P. Renard, A. Revcolevschi and P. Veillet, Solid State. Commun. 91 (1994) 927. [5] M. Weiden, J. K6hler, G. Sparn, M. K6ppen, M. Lang, C. Geibel and F. Steglich, Z. Phys. B 98 (1995) 167. [6] H. V611enkle, A. Wittmann and H. Nowotny, Mh. Chemie 98 (1967) 1352. [7] C.H. Chen and S-W. Cheong, Phys. Rev. B 51 (1995) 6777. [8] H. Kuroe, T. Sekine, M. Hase, Y. Sasago, K. Uchinokura, H. Kojima, I. Tanaka and Y. Shibuya, Phys. Rev. B 50 (1994) 16468. [9] S. Huizinga, J. Kommandeur, G.A. Sawatzky, B.T. Thole, K. Kopinga, W.J.M. de Jonge and J. Roos, Phys. Rev. B 19 (1979) 4723. [10] S.D. Obertelli, R.H. Friend, D.R. Talham, M. Kurmoo and P. Day, J. Phys.: Condens. Matter 1 (1989) 5671. [11] M. Kurmoo, M.A. Green, P. Day, C. Bellitto, G. Staulo, F.L. Pratt and W. Hayes, Synthetic Metals 55-57 (1993) 2380. [12] M. Hase, I. Terasaki, Y. Sasago, K. Uchinokura and H. Obara, Phys. Rev. Lett. 71 (1993) 4059. [13] S.B. Oseroff, S-W. Cheong, B. Aktas, M.F. Hundley, Z. Fisk and L.W. Rupp Jr., Phys. Rev. Lett. 74 (1995) 1450. [14] H. Winkelmann, M. Braden, A. Revcolvschi, G. Dhalenne, E. Gamper and B. Biichner, Phys. Rev. B 51 (1995) 12884.
[15] M. Poirier, R. Beaudry, M. Castonguay, M.L. Plumer, G. Quirion, F.S. Ravazi, A. Revcolevschi and G. Dhalenne, Phys. Rev. B, submitted. [16] P. Lemmens, B. Eisener, M. Brinkmann, L.V. Gasparov, G. Giintherodt, P.V. Dongen, W. Richter, M. Weiden, C. Geibel and F. Steglich, Physica B 223&224 (1996) 535. [17] J.G. Lussier, S.M. Coad, D.F. McMorrow and D. McK. Paul, J. Phys.: Condens. Matter 7 (1995) L325. [18] J.L. Garcia-Munoz, M. Suaaidi and B. Martinez, Phys. Rev. B 52 (1995) 4288. [19] A. Sohma, H. Okajima, T. Yokoo, A. Yamashita, J. Akimitsu, K. Nishiyama and K. Nagamine, J. Phys. Soc. Japan 64 (1995) 3060. [20] P. Lemmens, M. Fischer, B. Eisener, G. Gfintherodt, P.V. Dongen, M. Weiden, C. Geibel and F. Steglich, to be published. [21] I.Terasaki, R. Itti, N. Koshizuka, M. Hase, I. Tsukuda and K. Uchinokura, Phys. Rev. B 52 (1995) 295. [22] Z. Lu, Z. Su and L. Yu, Phys. Rev. Lett. 27 (1994) 1276. [23] L.N. Bulaevskii, A.I. Buzdin and D.I. Khomskii, Solid State Commun. 27 (1978) 5. [24] L.N. Bulaevskii, A.I. Buzdin and D.I. Khomskii, Solid State Commun. 27 (1978) 5. [25] M.C. Cross, Phys. Rev. B 20 (1979) 4606. [26] R.K. Kremer, Sol. State Commun. 96 (1995) 417. [27] O. Tchernyshyov, A.S. Blaer, A. Keren, K. Kojima, G.M. Luke, W.D. Wu, Y.J. Uemura, M. Hase, K. Uchinokura, Y. Ajiro, T. Asano and M. Mekata, J. Magn. Magn. Mater. 140-144 (1995) 1687. [28] L.N. Bulaevskii, Soy. Phys.-Solid State 11 (1969) 921. [29] J.C. Bonner and M.E. Fisher, Phys. Rev. A 135 (1964) A640. [30] M. Nishi, O. Fujita and J. Akimitsu, Phys. Rev. B 50 (1994) 6508. [31] H. Nojiri, Y. Shimamoto, N. Miura, M. Hase, K. Uchinokura, H. Kojima, I. Tanaka and Y. Shibuya, Phys. Rev. B 52 (1995) 12749. [32] E. Pytte, Phys. Rev. B 10 (1974) 4637. [33] J.W. Bray, H.R. Hart Jr., L.V. Interrante, I.S. Jacobs, J.S. Kasper, G.D. Watkins, S.H. Wee and J.C. Bonner, Phys. Rev. Lett. 35 (1975) 744. [34] H. Takahashi, N. M6ri, O. Fujita, J. Akimitsu and T. Matsumoto, Solid State Commun. 95 (1995) 817. [35] J.P. Renard, K. LeDang, P. Veillet, G. Dhalenne, A. Revcolevschi and L.P. Regnault, Europhys. Lett. 30 (1995) 475. [36] H. Kuroe, T. Sekine, M. Hase, Y. Sasago, K. Uchinokura, H. Kojima, I. Tanaka and Y. Shibuya, Phys. Rev. B 50 (1994) 16468.
Physica B 225 (1996) 177 190
Doping effects in CuGeO3 M. Weiden a'*, W. Richter a, C. Geibel a, F. Steglich a, P. Lemmens b, B. Eisener b, M. Brinkmann b, G. Giintherodt b aFG Technische Physik, TH Darmstadt, Hochschulstr. 8, 64289 Darmstadt, Germany b2. Physikalisches Institut, RWTH Aachen, Templergraben 55, 52056 Aachen, Germany Received 26 January 1996
Abstract
We investigated the effect of doping on the crystallographic, dynamic and magnetic properties of (Cul _ zZnz)GeO3 as well as Cu(Gel _xmx)O3 with A = A1, Si, Ga, As, In, Sn, Sb and Ti. A large solubility was only observed for Zn, Si and Ti. For those elements, the lattice parameters show a linear dependence on doping within the whole solubility range. The spin-Peierls transition temperature Tsp decreases linearly with doping, though the slope is strongly dependent on the dopant. For Zn-doping, a N6el-like ground state which exhibits spin-flop behavior appears for higher doping after the complete disappearance of the spin-Peierls ground state. Si- and Ti-doped samples exhibit a similar phase in a coexistence with the spin-Peierls ground state. The analysis of the susceptibility in the spin-Peierls phase for low Zn content indicates a reduction of the dimerization within the Cu chains and the energy gap upon doping. Whereas for Si and Ti all phonon frequencies changed linearly with the doping level, we observed in Zn-doped samples for some phonons a clear minimum at the composition z = 0.02 where the spin-Peierls transition disappears, suggesting a strong spin-phonon coupling for these modes. For higher Zn concentrations, no lattice dimerization can be observed by Raman scattering.
I. Introduction
Quantum effects in low-dimensional spin systems are presently a subject of high interest. In this context, the compound CuGeO3 has recently [1-5] attracted much attention, since it is the only inorganic compound presenting a transition into a spin-Peierls (SP) ground state. The crystal structure of CuGeO3 is characterized by an orthorhombic unit cell [6] which is built up by quasi-one-dimensional Cu chains alon~ the caxis with a short C u - C u distance of 2.941 A. These chains are separated by G e - O chains which leads to a quite large C u - C u distance along a- and b-axis * Corresponding author.
of 4.81,~ and 4.235A, respectively. Along the C u - O - C u chains, a strong antiferromagnetic (AF) coupling with an exchange coupling constant Jc ,~ 8 8 K [1] is observed giving rise to strong one-dimensional AF fluctuations at higher temperatures I-7]. Perpendicular to the chains, the coupling is one order (along b) or even two orders (along a) of magnitude smaller I-8]. Below the transition temperature Tse ~ 14.5K, the system condensates into a nonmagnetic SP ground state, separated from the excited spin triplet by an energy gap Eg ~ 2.1 meV. This SP transition is caused by a magnetically driven lattice dimerization along the Cu chains. Till now, besides CuGeO3 only some organic materials (see e.g. [9-11] and references therein) are
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M. Weiden et al. / Physica B 225 (1996) 177-190
known to show a SP ground state. In all these compounds, studies of doping effects on the transition are quite difficult due to the low solubility of dopants. In contrast, doping can be rather easily performed in CuGeO3, both on the Cu as well as on the Ge site. This compound is therefore the first possibility to study the effects of larger amounts of impurities on the magnetic ground states and energy excitations of a spin-Peierls system. Several investigations of doped CuGeO3 have already been published [12-16]. Although there is agreement on some of the observations, there are strong discrepancies in other aspects, e.g. on the effect of Si-doping and on the nature of a second anomaly observed for Zn-doping: for example, neutron experiments supposed a N~el state [17], whereas/~+SR studies [18, 19] suggest a spin-glass state. No structural data have so far been published for doped CuGeO3. We have, therefore, performed a systematic doping study putting emphasis on structural and magnetic properties.
2. Experimental details Polycrystalline samples of (CUl - zZnz)GeOa and Cu(Get -xA~)O3 with A = AI, Si, Ga, In, Sn, and Ti (for details concerning the stoichiometry, see Table 1) were prepared by a standard solid-state reaction. Since inhomogeneity might be a strong
Table 1 An overview of all investigated samples Dopant A=A1
Concentrations (x)
0.005,0.01,0.02,0.03,0.04, 0.05, 0.1, 0.2 A = Si 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.15 A = Ga 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.15, 0.2, 0.25 A = In, Sn 0.02, 0.05, 0.1, 0.2 A = As, Sb 0.01, 0.02, 0.03 A = Ti 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.1, 0.15 Zn 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09
problem in such doping studies, special attention was given to improving the quality of the samples: stoichiometric mixtures of the oxides (Alfa specpure) were thoroughly grounded, heated to 950°C for 100h, regrinded and heated again for another 100 h. In a third step, the samples were pressed into pellets and heated again. No significant weight loss was observed during the whole preparation procedure. Reaction and annealing temperatures were optimized in order to get the lowest residual susceptibility at T ~ Tsp in pure CuGeO3. Since Sidoping seems to increase the melting point of the doped Cu(Gel -xSix)O3, we increased the sintering temperature to 1000°C for this series. The As- and Sb-doped samples were prepared in sealed quartz ampoules under an atmosphere of oxygen because of the high dissociation pressure of the corresponding oxides. All samples were analyzed by X-ray diffraction using Si as an internal standard. Magnetic measurements within the temperature range 2 K ~< T ~< 350 K and 0 ~< B ~< 5.5 T were carried out using a SQUID-magnetometer (MPMS, Quantum Design). Raman scattering experiments were performed with different excitation lines of an Ar-Laser using a DILOR-XY spectrometer and a nitrogen cooled CCD as detector. A quasi-backscattering geometry was used with the sample cooled in an optical cryostat. Special emphasis was placed on the analysis of phonon frequencies and half-widths as a function of the substitution level and the detection of additional substitution-induced phonon modes. The analysis of magnetic scattering below and above Tsp will be published elsewhere [16, 20].
Spurious phases in x >~ 0.02...0.03 None x >/0.03... 0.04 All samples All samples x/> 0.07 x/> 0.08
3. Impurity effects due to single-crystal growth A careful analysis of the published results clearly indicates systematic differences between single crystals and polycrystalline samples. These differences are more pronounced in doped CuGeO3, but are already visible in the pure compound. Presently, most investigations on single crystals are performed using large crystals grown in an image furnace. In Fig. 1, we compare the typical susceptibility of such a crystal (taken from [15], with
M. Weiden et al. / Physica B 225 (1996) 177-190
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T (K) Fig. 1. Comparison of z ( T ) of different samples of CuGeO3. Note that the increase at low temperatures caused by impurities is strongly dependent on the preparation. (D) is a single crystal prepared in an image furnace (taken from Ref. [ 15]), (O) is a flux grown crystal, (/x) a polycrystalline sample, both prepared within this work.
similar results for example in Ref. [17]) with the behavior of two other samples prepared within this work in a different way: a polycrystalline sample and a single crystal grown from CuO flux at a very slow cooling rate. Image-furnace-grown crystals reveal a much higher residual susceptibility for T ~ Tsp than flux-grown crystals or polycrystalline samples. A likely cause for this enhanced susceptibility might be some disorder or oxygen defficiency caused by the reduction of Cu 2 ÷ to Cu ÷ during the crystal growth. To gain more insight into the origin of these defects, we performed thermogravity (TG) and differential thermal analysis (DTA) on pure CuGeO3. In air, TG revealed an abrupt weight loss at l l00°C. At the same temperature, DTA showed a sharp increase indicating some sort of transition (Fig. 2). The lost weight is partly recovered on slow cooling below this transition temperature. After this DTA measurement, the X-ray spectra revealed a large amount of GeO2 and Cu20 besides CuGeO3. In a flowing Ar atmosphere (99.999%), the weight loss already takes place at 1000°C and is not recovered on cooling. This clearly shows that CuGeO3 looses oxygen at 1100°C (in air) and starts to decompose. Since the crystal growth is typically performed in this temperature range, these results suggest that
Fig. 2. TG (solid line) and DTA ([]) measurements of 63 mg of pure CuGeO 3. The measurements were performed in air.
partial reduction of Cu 2+ to Cu + during the crystal growth and incomplete reoxydation of the large crystals during cooling are likely to cause the observed higher residual susceptibility. Besides disturbing the lattice, this leads to an intrinsic doping of Cu ÷ within the chains. Such a cutting of the Cu z + chains by the nonmagnetic Cu ÷ ions should have a rather similar effect as Zn-doping (see below). Attributing the increase in z(T) to oxygen defects, a defficiency in CuGeO3_~ of 6 ,,~ 0.0024 (!) would be sufficient to account for the larger z(T) of the single crystal of Ref. [15] in Fig. 1. Since doping can change the melting point and/or the stability against reduction, we preferred to investigate the intrinsic effect of dopants on polycrystalline samples since they can be synthesized at much lower temperatures where reduction can be avoided.
4. Results
4.1. Crystallographic properties, solubility limits and lattice parameters The solubilities of the different dopants were determined by extrapolating the intensity of the reflexes of the foreign phases in the X-ray pattern to zero. They were further checked indirectly by analyzing the composition dependence of characteristic features in the susceptibility measurements.
M. Weiden et aL / Physica B 225 (1996) 177-190
180
The largest solubility was observed for the substitution of Ge by the isoelectronic Si (more than 15at°/0). In contrast, our results indicate that the solubility limit for the isoelectronic Sn and the elements of the 5th period, As and Sb, is below our detection limit of 0.5 at%. This also was the case for In. The limit for the other investigated elements of the 3rd period, AI and Ga, is rather small, around 2 and 3 at°/0, respectively. In contrast, substitution of Ge by Ti is possible up to 6 at% and of Cu by Zn up to 8 at%, respectively. The solubility limits for all dopants are given in Table 1. For the elements with a larger solubility, i.e. Si, Ti and Zn, we determined the composition dependence of the lattice parameters. Si-doping has the largest effect on the lattice parameters. Up to the highest investigated doping level of x = 0.15, the aand c-axes as well as the unit-cell volume decrease linearly, whereas b increases (Fig. 3). The largest effect, - 0 . 5 % at x = 0.15, is observed along the c-axis, i.e. along the direction of the Cu-chains. The opposite trend is observed for Ti-doping: the b-axis decreases, whereas the a- and c-axes increase (Table 2). The effect of Ti on the a- and c-axes is approximately a factor of three smaller compared to the effect of Si. This could be expected as this value roughly corresponds to the ratio of the difference of the ionic radii: (Rsl,+ - Ree,+)/(Rri,+ R~e,+) ~ - 3 . In contrast, the decrease of the b-axis is much larger than expected from comparing the ionic radii. The effect of Zn-doping on the lattice is quite small as a consequence of the nearly equivalent ionic radii of Cu 2+ (Rcu2+ = 0.57A) and Zn 2+ (Rz,~+ = 0.60 A). Here, the a-axis decreases whereas the c-axis is increasing (Table 2). No significant variation for both the b-axis and the volume could be resolved within the scattering of the data.
4.2. Electronic and dynamic properties
In pure C u G e O 3 , three excitations related to the SP state appear in the Raman spectra below Tsp: a singlet-triplet excitation (30cm-1), two-magnon scattering (30-240cm-1) and an additional zonefolded phonon (370cm -1) [16,20,36]. The onset and the line shape of these excitations are a valuable tool to determine the degree of dimerization in
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M. Weiden et al. / Physica B 225 (1996) 177-190
the C u - O - C u chains and the coherence of the SP state. Significant shifts of the phonon frequencies were observed for Si-, Ti- and Zn-doping and studied in dependence of concentration and temperature. The observed shifts of the phonon frequencies are closely related to the changes in the lattice parameters discussed above. Si-substitution leads to linear shifts of all observed phonon modes with increasing substitution level, without showing large broadenings of the line widths. At x = 0.05, these shifts range from + 0.8 to - 0 . 5 % (Table 3). An additional phonon is observed at 669cm -1 for x ~> 0.01. This excitation shows a linear increase of intensity with substitution. Due to the absence of any other second phase we attribute it to an intrinsic phonon which becomes Raman-allowed due to the symmetry breaking of the dopant. Ti substitution also leads to two additional phonons at 454 and 610cm-1. For both dopants, the singlet-triplet excitation is not observable for x > 0.01, and the two-magnon scattering vanishes for x > 0.02. In the case of Zn substitution, we observe pronounced minima at z = 0.02 in the concentration dependencies of the frequencies of some phonons, whereas others reveal a linear behavior (Fig. 4). This minimum occurs at the highest Zn concentration where a SP transition can still be observed in
Table 3 Shift of the phonon frequencies as a function of Si-doping. The slope sabs is calculated using v(x)= v(0)+ Sabs* 100X, X from Cu(Gel _xSix)O3. The relative shift Srel is calculated using v(x) = v(0)(l + Sre~* 100X) and reveals strong differences, though the observed dependence is perfectly linear for all phonons up to x = 0.05
susceptibility measurements. As it is not caused by a change in the doping dependence of the lattice parameters, which is linear for the whole concentration range, it may indicate a change in the spin-phonon coupling due to the doping. No such extremum was observed for any phonon in Si- or Ti-doped samples. In Fig. 5, we compare the dependence of the frequency of the phonon at 185cm -1 on doping with Si, Ti, and Zn. The singlet-triplet excitation is not observable in any Zn-doped sample. This strong suppression is also observed for the two-magnon scattering which vanishes for z > 0.01. In the case of Ga substitution a continous broadening of the phonon line widths instead of frequency shifts or additional phonon modes was observed. For the phonon at 592cm-1, the broadening reaches 12% at x = 0.02. This broadening indicates a considerable amount of disorder induced by substitution. The leveling off of the line width marks the solubility limit reached for this substitution at x ~ 0.03, as it was already detected within the crystallographic analysis. To investigate the lattice dynamics of the SP transition, we measured the intensity of the folded phonon at 370cm -1 as a function of temperature for Cu(Gel-xGax)CuO3 with x = 0, 0.01 and 0.02, respectively. It thus is a tool to measure the development of the crystallographic dimerization. In Fig. 6, we present the intensity of this folded phonon, normalized to the intensity of the phonon at 390cm-~ which presents an only very weak temperature dependence. For the pure compound, we find a sharp increase at Tsp. The behavior can be well-fitted using I = Io(1 --
Phonon frequency (cm- 1)
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with fl = 0.52. This is a different behavior than observed for the spin gap Eg v e r s u s T where we obtain fl = 0.10, in good agreement with neutron experiments [30] and published Raman measurements [36]. For the doped compounds, the behavior changes to a linear evolution of the phonon intensity with decreasing temperature. Furthermore, the slope of this evolution decreases with increasing doping level. To investigate the possibility of doping charge carriers by substituting Ga 3 + for Ge 4+, the Raman
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Fig. 6. Intensity of the phonon at 370cm-1 as a function of T (after subtracting the background) normalized to the temperatureindependent phonon at 390cm-L CuGeO3 (11) reveals a sharp increase (Eq. (1)), whereas 2% Ga ([3) leads to a linear dependence.
efficiency o f t h e p h o n o n at 592 c m - 1 w a s m e a s u r e d in s a m p l e s w i t h x = 0 a n d 0.02 as a f u n c t i o n o f t h e e n e r g y o f t h e i n c i d e n t e x c i t a t i o n line (Fig. 7). I n pure CuGeO3, a resonance-like enhancement of
the p h o n o n i n t e n s i t y is o b s e r v e d at a w a v e l e n g t h o f 2 = 487 n m (2.53 eV). T h i s m a x i m u m is c o n n e c ted w i t h a c h a r g e - e x c i t a t i o n g a p t h a t r e s o n a n t l y i n c r e a s e s t h e p h o n o n intensity. Its e n e r g y
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corresponds to the bright blue color of the samples. In G a substituted samples this color changes slightly to a greyish tone. The corresponding R a m a n experiments for x = 0.02 show a decrease of the R a m a n efficiency and a slight shift of the m a x i m u m to higher wavelength 2 = 4 9 7 n m (2.50eV), i.e. a reduction of the excitation gap. However, DC-conductivity measurements showed insulating behavior in all substituted samples. The charge remains localized at the defect site. This result is in perfect agreement with the analysis of Terasaki et al. [21], who proposed carrier doping to be difficult due to a narrow tpd band of the undoped CuGeO3.
4.3. Magnetic phase diagrams The magnetic phase diagrams were investigated by means of susceptibility (z(T, B)) and magnetization (M(B)) measurements. The results of the z(T)-measurements as a function of doping for Zn, Si and Ti are shown in Figs. 8(a), 9(a) and 10, respectively. The pronounced decrease in Z (T) near T ~ 14 K corresponds to the SP transition. As the transition broadens slightly with increasing doping, the use of the inflection point for the determination of the transition was not reliable. The transition temperature is therefore derived by fitting a linear function above and below the SP transition. Tsp is
183
defined as the crossing temperature of these two lines. The same procedure was performed for the determination of TN1 and TN2, see below. Tsp decreases for all kinds of dopants on the Ge site as well as on the Cu site. This decrease is rather fast for Zn (Tsp = 12.6K for z = 0.02, see Fig. ll(a)), somewhat weaker for Ti (Tsp = 13.0K at x = 0.06) and very small for Si (Tsp = 14.2 K at x = 0.1). For these three dopants, the application of a magnetic field leads to a reduction of Tsp. As already observed in pure CuGeO3 [1], and in agreement with theoretical predictions [22, 23], this reduction is proportional to B e
Tsv = Tsv(o) 1 - a \kB-T-~sP(o,J ]
(2)
where the calculated value Ath. . . . tical varies between 0.32 and 0.44 [22, 24, 25]. We observe a correlation between the A coefficient and the slope of OTsp/O(x,z). Compared to the value for the pure compound (A = 0.47), A increases to 0.61 for 2 a t % Zn ( T s p = 12.6K), 0.55 for 3 a t % Ti ( T s p = 13.6K) and even decreases to 0.36 for 2 a t % Si ( T s p = 1 4 . 5 K ) . These measured A coefficients represent an average over the three crystal axes as we never observed any preferred orientations in polycrystalline samples. Thus, we consider the anisotropy of the crystal axes as observed by Ref. [26] for the pure c o m p o u n d not to play an important role in this analysis. For Zn-doping, a SP transition was observed for 0 ~< z ~< 0.02 only, whereas a second anomaly of antiferromagnetic type at low temperature TN1 appeared for z/> 0.04. N o transition down to 2 K was found for z = 0.03. TN1 shifts slightly to lower temperatures with increasing x, but is still observable up to the solubility limit. For z = 0.06, we also performed measurements of z ( T ) in various fields and of M(B) at various temperatures. TN1 is shifting slightly to lower temperatures with increasing field and vanishes for B ~ 1 T (Fig. 8(b)). Above TN1, the magnetization is strictly linear up to the highest investigated field of B = 5.5 T. Below TN1, the magnetization is linear only up to B = 0.2 T. At higher fields, an upturn in M(B) for 0.3 T ~< B ~< 1.4 T is observed.
M. Weiden et al. / Physica B 225 (1996) 177-190
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In contrast to the results for Zn, the SP transition could be observed at least up to 10at% in Si and up to the solubility limit of 6 at% in Ti. In both cases, a second antiferromagnetic-like anomaly appears below Tsp at TN2 ~ 5 K. For Si, it is visible in the doping range 0.02 ~< x ~< 0.04, whereas for Ti doping, it is shifted to x >/0.04. Thus, the lower transition coexists with the SP transition in both cases. Coexistence here (and throughout the whole paper) means the observance of both transitions within the same sample, as we are presently not able to clarify whether the N6el state replaces the SP state or really coexists in a microscopic way. Macroscopic phase separation due to sample inhomogeneities can be ruled out as the N6el state vanishes again for higher Si-doping. We also investigated the field dependence of this
low-temperature transition in Cu(Geo.9sSio.02)O 3 (Fig. 9b). TN2 decreases very weakly for 0 T ~< B ~< 1.0T. At larger fields, the slope ~ T N 2 / ~ B increases strongly. For B > 1.5T, the transition seems to be suppressed completely. The comparison of the behavior of the low-temperature phase of Zn-doped samples on the one hand and Si/Ti-doped samples on the other hand via M(B) measurements reveals some similarities. Above TN1/TN2,the magnetization is strictly linear in the whole field range (B~< 5.5T). Below TN1/TN2, a clear non-linear behavior for all three dopants is observed. In Fig. 11, we plot the deviation of M(B) at T = 2.7K from a linear field dependence for a 2 at% Si-, 5 at% Ti- and 6 at% Zn-doped sample. All samples show a clear upturn around B ~ 1 T. Above this upturn, Si and Ti keep
M. Weiden et al. / Physica B 225 (1996) 177-190
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a constant value above the linear extrapolation, whereas Zn reveals a weak m a x i m u m near B ~ 1.5T and drops significantly below the linear extrapolation for higher fields. This different behavior at higher fields might be related to the presence or absence, respectively, of a SP transition at higher temperatures (and thus also at higher fields). The series C u ( G e l - x A x ) O 3 with A = A1, G a show a decrease of Tsp with increasing doping. Due to the low a m o u n t of doping allotted in both cases (see Table 1), however, it was not possible to determine whether or not this dependence of Tsp is linear as in the case o f A = Si, Ti or not (Fig. 1 l(a)). Furthermore, no anomally in z(T) at lower temperatures was observed, probably also a consequence of the low solubility limit.
5. Analysis and discussion
5.1. Magneticphase diagram The effect of Zn-doping was already investigated by several other groups [12, 13, 17, 27]. Our results for OTsp/~z as well as the observation of a second anomaly for higher doping levels at low temperatures are in good agreement with the published results. However, Hase et al. [12] and Oseroff et al. [13] observed a coexistence of the SP phase and the low-temperature phase for z = 0.03, whereas we did not observe such an anomaly for this stoichiometry (Fig. 8(a)), indicating that there is no coexistence of the two phases in the Zn-doped samples. The comparison of our results with the published ones suggests that
186
M. Weiden et aL / Physica B 225 (1996) 177-190 '
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homogeneity of the dopant distribution is crucial for doping investigations. Hase et al. [12] and Tchernyshyov et al. [27] also observed a difference between zero-field cooled (zfc) and field cooled (fc) z(T) measurements and concluded that the low-temperature phase is spin-glass like. In contrast, Oseroff et al. [13] and Lussier et al. [17] concluded from specific heat and neutron measurements that this phase corresponds to a N~el state. We performed detailed measurements of fc and zfc z(T) for various samples with z > 0.03 and found no evidence for any hysteresis within our resolution of less than 0.1%. From the absence of hysteresis, but mostly from the upturn in the magnetization, we conclude that antiferromagnetic order, with a broadened spin-flop transition for B ,~ 0.8T, rather than a spin-glass state develops at low temperatures.
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A recent theory proposed by Lu et al. [22] for non-magnetic doping within the Cu chains (applicable for the Zn-doped samples) yields to a linear decrease of Tsp with small doping amounts:
Tsv = T ~ ° ( 1 - sn),
(3)
with n the doping concentration and s = 11.1 for in-chain doping. These authors also predict the vanishing of the energy gap around ni ~ 2.8% and the existence of a gapless SP phase for higher nl which could lead to a spin-glass state due to frustration. About 10at% of doping should destroy the SP ground state completely. The decrease of Tsp we observe for (Cul-zZn,)GeO3 is linear with a slope of s ,~ 6.76 (Fig. 11 (a)). Furthermore, the SP transition vanishes completely for z = 0.03. Both values agree reasonably with the
187
M. Weiden et al. / Physica B 225 (1996) 177 190
theoretical predictions, as was already observed by Hase et al. [12]. As far as the effects of doping on the Ge site are concerned, much less results have been published. Furthermore, these results are not consistent with each other: in polycrystals, Oseroffet al. [13] found an only very weak decrease of Tsp when doping with Si, whereas Poirier et al. [15] and Renard et al. [35] noticed a rather fast suppression in independent measurements on the same single crystals. From an investigation of a 0.7at% doped single crystal, where both transitions at Tsp at TN2 were observed, they proposed that TN2 corresponds to the formation of a N6el state which exhibits a spinflop transition at B ~ 1 T. Our results indicate a very small dependence of Tsp upon Si-doping. The decrease is linear up to 10at% with a slope ofs ~ 0.32 (Eq. (3)) (Fig. 1l(a)). For higher doping (x > 0.1), no SP transition was observed (Fig. 8(a)). Thus, concerning Tsp our results support those of Ref. [13] and are in strong disagreement with [15, 35]. On the other hand, our results on the magnetization of a 2 at% Si-doped sample can be well-described with the proposed spin-flop transition (Fig. 8(b)). If one takes into account that anisotropies are smeared out in polycrystalline material, our results on the phase diagram of the AF phase are in agreement with those of Ref. [15]. Since defects seem to affect Tsp much more than TN2, this discrepancy of the composition dependency can be explained by a larger amount of defects in the single crystals used in Refs. [15, 35] as was already discussed above. To our knowledge, no data have been published for Ti-doping. Our results indicate that Ti has an effect rather similar to that of Si, though in the former case the depression of Tsp is much more pronounced with a slope of s ~ 1.62. Also, the range where TN2 and Tsp coexist is shifted to higher x values. The magnetization of those Si- and Tidoped samples which exhibit both transitions are very similar: above the transition TN2, M(B) is strictly linear. Below TN2, M(B) is linear only up to B ~ 0.4 T and B ~ 0.6 T, respectively, and shows an upturn for higher fields due to a spin-flop-like transition (Fig. 12). Thus, the low-temperature phase in Ti-doped CuGeO3 is probably of the same nature as the one in Si-doped CuGeOa.
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B (T) Fig. 12. The difference between measured magnetization and a low-field linear extrapolation at T = 2.7 K. A M is calculated independently for each composition as A M = M ( B ) - M u ..... where M~i.... is always derived from an extrapolation of the low-field dependence as Mn .... = m * B. We obtained (in units of A / T m ) m = 307 for 2% Si, m = 371 for 5% Ti, and m = 441 for 6% Zn, respectively.
For both Si- and Ti-doping, the reduction of Tsp is accompanied by a reduction of the unit-cell volume. Since the pressure dependence of the SP transition is positive [5, 14, 34], the main effect of doping on the Ge site cannot be due to a volume change, but has to be attributed to the disorder in the coupling between neighboring Cu 2 + moments introduced by the doping. Comparing the magnetic phase diagrams of Si-, Ti- and Zn-doped samples, there are obvious similarities. All doping series exhibit a linear suppression of Tse. All series also reveal the existence of an antiferromagnetic ground state, showing a spin-flop transition in an external magnetic field for some doping level. At the doping level where this antiferromagnetic phase appears, its transition temperature TN is the same for all dopants: TN TN1 ~ TN2 ~ 5 K , and shifts to lower temperatures with increasing doping level. All these similarities suggest that the low-temperature phase is the same for all dopants and that it corresponds to the antiferromagnetic state of disturbed CuGeO3. A simple explanation would be that the disorder induced by doping destroys the one-dimensional character locally and leads to a classical three-dimensional magnetic ordered state. Since the SP- and the AFstate coexist for some doping levels, the magnetic
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degrees of freedom have to be distributed between these two-order parameters. The differences between the phase diagrams as a function of doping ( T s p , T N 1 , TN2 =f(x,z)) for Zn, Si, Ti would indicate that both order parameters behave rather independently. It seems that some amount of disorder is needed to create the AF state. This would explain why less Si than Ti or Zn is needed since Si has a much greater effect on the lattice: (Roe4+ - - Rsi4+) ~ --3 (R~e,+ - - RTi,+) ~ --3 (Rc,~+ -- Rz,2+). In contrast, the SP phase is more sensitive to the doping site. The effect of in-chain doping is much stronger which can be seen by the complete suppression of the SP phase with 3 at% Zn. This is probably the reason why no coexistence can be observed for Zn-doping: though short-range correlations still exist for z > 0.03 as can be seen in high-temperature z(T) data, the long-range SP phase is destroyed before the AF phase can form.
5.2. Energy gap and dimerization The effect of small doping levels can be analyzed in more details for (Cul-zZn,)GeO3 using the theory by Bulaevskii [28] for the behavior of g(T) in the SP regime. In this theory which is valid for a static system, z(T) is given by g2p2 a~(-Y ) e x p ( j ~ _ y ) ) z(T) - ___)__
(4)
where p is the effective moment and J is the coupling constant within the antiferromagnetic coupled chains. J can be derived from the high-temperature z(T) results, using the theory by Bonner and Fisher [29] for uniform one-dimensional magnetic chains, and was calculated from g(T)-measurements to be Jc = 88K [1], though neutron experiments [30] and measurements of the saturation field [31] suggest higher coupling constants of 120 and 183K, respectively, a(y) and A (~) are tabulated values as a function of the dimerization parameter y. In this model, ~ = 1 is the limit for a uniform antiferromagnetically coupled Heisenberg chain. The energy gap E~ can be obtained by combining this theory with the
mean-field approximation of Pytte [33] and Bray et al. [34] to be Eg=l.637fiJ,
fi_l-7
1 + 7"
(5)
For the pure compound, using this theory with and J as free parameters we obtain y = 0.614, J = 86.6 K and, consequently, Eg = 34 K (see also [1], with a higher value for ~). fi = 0.24 is slightly larger than the value obtained by theoretical analysis of the two-magnon spectra [20]. The value of J is in a remarkable agreement with the value Jc obtained from the analysis of the high-temperature susceptibility results. From neutron experiments [30], Eg = 25 K was determined which is also in reasonable agreement with the results of the present study. In the doped samples, an additional Curie-Weiss (CW) term and a constant offset, Z0, appear. The meaning of Zo will be discussed below. The CWterm can be associated with the contribution of unpaired Cu 2÷ spins (inset Fig. 13): Zn cuts the infinite chains into finite pieces, half of them having an odd number of Cu 2 + ions and thus an effective spin ½. We therefore fitted the low-temperature region of z(T) to get the CW constants of this contribution. The values obtained are in remarkable agreement (within less than 10% difference) with the ones we expect from the amount of unpaired Cu 2 + spins due to the Zn-doping. After subtracting this contribution, the susceptibility was fitted to Eq. (4) (Fig. 14) for temperatures up to about 3 ° below Tsp, with J being fixed at the value obtained for the pure sample. The only fitting parameters were 7 and an additional scaling factor c, the latter being multiplied with the theoretical function (4). This factor c < 1 takes into account that, due to the doping, only part of the sample condensates into the SP-phase below Tsv. We obtained ~ = 0.656 for z = 0.005 and 7 = 0.699 for z = 0.01. This implies a reduction of the gap Eg of 13% and ~26%, respectively. This reduction is nearly linear with doping in contrast to the prediction by Lu et al. [22], who arrived at a sublinear suppression of Eg. The extension of this analysis to higher Zn content or for other dopants was not possible, as the low-temperature tail in the z(T) results in not purely CW like for those samples.
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M. Weiden et aL / Physica B 225 (1996) 177-190 I
I
I
measur~edlt
I
I
4x10"5
I
/
1.5x105
t~
[l~',i" 3x10.5
I/I
1.0xl0 "5
5.0x10 6
0 T (K)
~J:~~-~-[]
0.0
~
I
2
i
1
,
4
I
,
6
,
8
~ 'Z-Zcw Bulaevskii-fit I
10
i
I
12
T (K) Fig. 13. The low-temperaturebehavior of z(T) fitted with the theory of Bulaevskii[17] (solidline)for (Cuo.995Zno.0os)GeO 3. The inset shows the substracted Curie-Weiss-likecontribution. A fit with similar quality was obtained for z = 0.01 and the pure compound.
The increase of 7 with increasing doping level indicates, within this theory, that the dimerization of the Cu chains decreases. An extrapolation of leads to a vanishing of the dimerization (7 = 1) around z = 0.03, the same concentration where the z ( T ) measurements reveal no SP transition. This critical concentration agrees with our Raman results. There, the folded phonon at 370cm -1 provides a possibility to measure directly the crystallographic dimerization. In the case of Si- and Ti-doping, this phonon can be seen for the same concentration range where the SP transition is observed in z ( T ) . With Zn-doping, a rather fast suppression of the phonon intensity is observed. Already with 2 a t % it vanishes completely. This corresponds to the disappearance of the spin gap at z = 0.03 as observed in z ( T ). These results indicate that both characteristics of the SP transition, the energy gap as well as the dimerization, vanish completely with z = 0.03, which is at variance to the existence ofa gapless SP state for 0.03 < z < 0.1 as proposed by Lu et al. 1-22]. The rather small decrease of E~ (26% for z = 0.01) contrasts with the very strong reduction of the SP derived anomaly observed in z ( T ) already at the lowest doping level for all dopants.
This latter reduction is related to an increase of the CW contribution and of the constant background susceptibility Zo. Whereas the CW contribution can be attributed to non-dimerized Cu 2÷ spins due to disorder, the constant contribution Zo suggests the presence of impurity levels inside the SP gap. Thus, the main effect of doping is mainly to fill up the gap, rather than to reduce the gap energy.
6. Conclusion Our results indicate that the main effect of doping on the Ge-site is to introduce disorder which suppresses Tsp linearly. For higher doping amounts, an antiferromagnetically ordered phase can be observed at lower temperatures which exhibits spin-flop behavior in an external magnetic field. This phase coexists with the SP phase. The same N6el phase also appears in the case of Zndoping, though no coexistence is observed due to the fast suppression of the SP phase owing to the very efficient in-chain doping. In contrast to theoretical predictions, our results indicate that Zn-doping leads to a complete
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M. Weiden et al. / Physica B 225 (1996) 177-190
destruction of both the energy gap and the dimeriza t i o n w i t h i n the chains.
Acknowledgements W e w i s h to t h a n k D r . C o r d i e r , w h o p e r f o r m e d the T G a n d D T A m e a s u r e m e n t s . T h i s w o r k w a s s u p p o r t e d b y S F B 252, S F B 341 a n d B M B F p r o j e c t N o . 13N6586.
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