Spectroscopic studies of the high-Tc superconducting cuprate perovskites

Physica 147B (1987) 166-174 North-Holland, Amsterdam

SPECTROSCOPIC STUDIES OF THE HIGH-T~ SUPERCONDUCTING CUPRATE PEROVSKITES I. O N Y S Z K I E W l C Z , M. K O R A L E W S K I , P. C Z A R N E C K I , R. MICNAS and S. R O B A S Z K I E W I C Z Institute of Physics, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznah, Poland Received 20 August 1987 We present XRD, ESR, IR, DR(UV, VIS) as well as resistivity and magnetic susceptibility data of single phase superconducting perovskites YBa2Cu30x and EuBazCu30x. A rich phonon structure for both oxide compounds has been observed with a peak at: 72, 108, 135, 148, 172, 570 and 620cm-1. The edge of the low energy side was observed at 55 cm-1. The estimated carrier density for YBa2Cu30x is (1-2) × 1021c m 3. A weak ESR signal has also been observed for superconducting phases which is associated with the lattice defects. We also report ESR and optical measurements of constitutent materials as well as not superconductingphases Y2Cu205 and BaCuO2.

1. Introduction Since G. Bednorz and A. Miiller's discovery of superconductivity in the mixed-phase L a - B a Cu-O compound system [1] the same p h e n o m e n o n has been also observed in other compounds of cuprate perovskites [2-4]. The crystal chemistry and physical properties of the new Y - B a - C u - O and E u - B a - C u - O high-T c superconducting systems are widely investigated by such methods as X-ray and neutron diffraction, magnetic measurements but, surprisingly "enough, much less attention has been paid to their vibrational and radiospectroscopic spectra. The spectroscopic studies of electronic and phonon structure can be crucial for the understanding of the origin of the superconductivity in these compounds, as well as for a search for new high- T c materials. In this paper we report the results of studies of pure single phase superconduting perovskites: YBa2Cu3Ox and E u B a 2 C u 3 0 x. By the powder X-ray diffraction method the structure of both compounds has been identified as orthorhombic distorted perovskite with the space group Pmmm. The superconducting transition at Tc = 90.7 K and 84.0 K for well timely stabilized Y-

B a - C u - O and E u - B a - C u - O , respectively, has been established by resistivity and ac-susceptibility measurements. The properties of the normal state were studied by ESR, the infrared (4000-40 cm-1) and the diffuse reflection (50000-4000 c m - 1) spectroscopy. We think also that at the present stage as much as possible experimental information about the normal phase should be gathered in order to get better insight into the origin of superconductivity of these materials.

2. Experimental 2.1. Preparation o f samples

The compounds examined were prepared with nominal compositions represented by M B a 2 C u 3 0 6 5 ( M = Y , Eu) through solid-state reaction of stoichiometric amounts of high-purity Y203 or Eu203, C u O 3 and B a C O 3. They were ground, mixed, pressed into pellets and sintered at about 1220 K in oxygen atmosphere and finally slowly cooled. To determine the conditions for obtaining single-phase compounds we subjected the M - B a - C u - O systems to thermogravimetric

0378-4363 / 87 / $03.50 t~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

I. Onyszkiewicz et al. / Spectroscopic studies o f high-T c perovskites TG

Mixture EuBa2cu306, 5

= ~

i

400

500

600

700

800

900

i

I

1000 1100 1200 T[K]

Fig. 1. DTA, TG, and DTG of EuBa2Cu306. 5 mixture. Experimental conditions: Heating rate 10K/min. DTA 1/5, DTG 1/15, sample mass = 460 mg, temperature range: 293 to 1273 K, reference material A1203.

analysis (TGA) and to differential thermal analysis (DTA) experiments (fig. 1). On D T A curves, besides endothermal effects corresponding to polymorphous transitions on BaCO 3 [5], we observed a pronounced endothermic peak, which appeared above 1190 K in the range 11901230 K. This effect was accompanied by a mass loss well marked on T G and D T G curves. According to the results of studies carried out on the samples obtained in temperatures above this endothermic effect, this peak corresponds not only to BaCO 3 decomposition but also to the solid-state reactions in M - B a - C u - O systems. In order to carry out some additional studies, two compounds Y2Cu20 5 and BaCuO 2 were obtained from the appropriate oxides and carbonate-BaCO 3 as a result of high temperature solid-state reactions. Y2Cu20 5 and BaCuO 2 may appear as additional phases in the process of obtaining of the high-temperature superconducting Y - B a - C u oxide, BaCuO 2 also for E u - B a Cu oxide [6, 7]

167

radiations and a conventional X-ray diffractometer (the 0 - 0 scan technique) for polycrystalline materials. ESR spectra were recorded at RT as the first derivative of the absorption signal, using the conventional X-band (9.6 GHz) reflection spectrometer with 100 KHz - field modulation. The magnetic field was controlled with a digital nuclear magnetometer. The dc resistivity as a function of temperature was measured by employing the standard fourprobe method. The current density was of about 12 i~A/cm 2 for both superconducting compounds. The ac magnetic susceptibility versus temperature was measured by the mutual inductance method, with an ac frequency of 20 Hz and a magnetic field strength equal to 3 mT. The temperature was measured by a Copper-Constantan thermocouple situated on the sample. Optical investigations included: diffuse reflection (DR) and infrared spectroscopy (IR). The spectra were examined in the frequency range 40-50000 cm-1. The IR spectra were recorded in Nujol in the frequency range 40-600 cm -1 using a Perkin Elmer (model 180) spectrometer. In the frequency range 400-3800 cm -1 the IR spectra were recorded on a Digilab spectrometer (model FTS 14) with double precision and resolution of 2 cm- 1. 400 spectra were sampled for a spectrum. The samples for these measurements were mixed thoroughly with dry KBr (the concentration is 1 wt%) and pressed into pellets. The diffuse reflectance spectra of well powdered samples diluted in MgO were recorded at RT over the range ll000-50000cm -1 on a Specord M 40 (Zeiss Jena) and over the range 4000-25000 cm -1 on a Beckman UV 5240 spectrometer fitted with a diffuse reflectance attachment. We used MgO as reflectance standards. All measurements were performed on pieces taken from the same pellet.

2.2. Experimental method 3. Results and discussion

X-ray powder diffraction (XRD) measurements were made at room temperature (RT) on the pellet samples. We used Co K~ and Cu K s

The compounds obtained were identified through XRD study. The diffraction patterns

I. Onyszkiewicz et al. / Spectroscopic studies of high-T c perovskites

168

(a)

T : 295

YBa2Cu3Ox

5'5 Co

L5

Lo

~s of?]

3'o

is

2b

T =295 EuBa2Cu3Ox

s'5

s'o ~'s

(b)

Lo

3's

3'0

2'5

2b

15

10

5

e['] Fig. 2. X-ray powder diffractograms of YBa2Cu3Ox(Cu K~) (a) and E u B a 2 C u 3 0 x (Co K~) (b) perovskite oxides at RT.

shown in figs. 2a and b indicate that single-phase samples were obtained with nominal composition of YBa2Cu30 x and E u B a 2 C u 3 0 x. The oxygen stoichiometry x = 7 - 8 was not determined for the samples studied. The compounds form an orthorhombically distorted perovskite structure with the space group Pmmm. The lattice parameters are given in table I. An insignificant increase in the lattice parameters of E u B a 2 C u 3 0 ~ in comparison to YBa2Cu20~ is related to the greater ionic radius of the Eu +3

cation replacing the y+3 cation in the crystal lattice. These results are in agreement with those from other authors [8]. The superconducting transitions in MBa2Cu30 x were established by resistivity and ac magnetic susceptibility studies. Fig. 3 shows the resistivity vs. temperature for YBa2Cu30 x and E u B a 2 C u 3 0 x. The higher T c is observed for YBa2Cu30 x (see Table I). The temperature dependences of the electric resistivity in both cases exhibit a linear temperature dependence in the normal state. The samples show zero resistance to the limit of the resolution in our mearurement at 8 9 K and 7 6 K for YBa2Cu30 x and EuBa2Cu3Ox, respectively. The critical temperatures found from the resistive midpoint are also presented in table I along with ATc(R-IO/90% ) and the residual resistivity ratio R ( 3 0 0 K ) / R(100 K). The transitions are rather sha W. Additional studies with the ac method show a decrease in resistance of the samples at least by 5 orders of magnitude. At R T the specific resistivity p of YBa2Cu3Ox and E u B a 2 C u 3 0 x has been estimated to be of the order of 1200 ixllcm and 1800 Ixf~cm, respectively. The above results are consistent with most of the experimental data reported elsewhere [9-11]. We observed the reproducibility in the resistance measurements during a few cooling and heating cycles. The variation of Xac(T) for YBa2Cu30 x is shown in fig. 4. For EuBa2Cu3Ox we were not able to measure Xac below 7 7 K in our experimental set-up. • _ y Ba2Cu30 x

~

• - Eu Bo2Cu30x

Table I Properties of the M B a 2 C u 3 0 x samples

f

~

[ ~

Sample:

YBa2Cu30 x

EuBa2Cu30 ~

Crystallographic data

a = 0.3826 n m b = 0.3890 n m c = 1.1670 n m

a = 0.3845 n m b = 0.3910 n m c = 1.1720 n m

'

lO

.E

Color

Grey-black

Grey-black

Relative hardness

Hard, dense

Hard, dense

R(300)/R(100)

2.5

1.7

T¢(R-mid)

91 K

84 K

ATc(R-IO/90% )

2.4 K

7K

IIC

80

100

120

1/,0

160

180

200

2..20

2/,0

260 280 T[K]

300

Fig. 3. T e m p e r a t u r e dependence of the resistivity of Y B a 2 C u 3 0 x and E u B a 2 C u 3 0 x. A n inset shows the enlarged version for Y B a 2 C u 3 0 x in the vicinity of To.

169

I. Onyszkiewicz et al. I Spectroscopic studies of high-T c perovskites 0.0 >.-0.05

T=295 X-band

-0.10

YBa2Cu30 x H=3rnT =20Hz

6 o _0.15

,cu2o 10rot

-020

B

-Q2! dO

85

go

g5

T[K]

'(~0

) fX..S~2cu3°~

Fig. 4. Temperature dependence of the ac-magnetic susceptibility of YBazCu3Ox.

In the normal state YBa2Cu30 x is a very weak paramagnet whereas in the superconducting state its X ~- - 0 . 2 5 (77 K) which is close to the results reported by others [10, 11]. This value means that about 1 of the sample volume is in the superconducting state at 77 K. E S R investigations were performed not only for M B a z C u 3 0 x but also for Y 2 C u 2 0 5 and B a C u O 2. The E S R method is sensitive enough to detect even trace amounts of such impurity phases in the superconductors studied, whereas those amounts cannot be detected by the X R D method. As it can be seen from the data presented in fig. 5 the E S R signal of Y 2 C u 2 0 5 is analogous to the spectra of uniaxial systems and can be described with anisotropy of the g-factor [12]. The calculated g values for Y 2 C u 2 0 5 are gll = 2 . 3 9 and g± =2.04. The value of the parameter G=(gll-2)/(g±-2)(..~lO), being much higher than 4, clearly indicates the existence of nonnegligible exchange coupling between Cu ÷2 ions in Y 2 C u 2 0 5 and therefore the measured g-values do not reflect the symmetry of the individual polyhedron of the Cu rE ion. A paramagnetic signal of that kind was not observed for the superconducting YBa2Cu30 x studied at RT. For B a C u O 2 we observed a broad ESR line with the isotropic g-factor equal to 2.06. Thus, we may conclude that a sample with T c = 9 1 K does not contain the impurity phases Y 2 C u 2 0 5 neither Y2Cu2_xBaxO 5 [7,13] nor B a C u O 2. The E S R signal of E u B a 2 C u 3 0 x (fig. 5) is of the same character as that of YBa2CuaOx, however the Eu-perovskite signal is

10mT

Fig. 5. ESR spectra (9.6 GHz) of YzCu2Os, YBa2Cu30 x and EuBa2Cu30 x at RT.

slightly stronger (both signals were recorded in strong gain). Treating E S R spectra of superconducting phases as signals with isotropic g-factor, we obtain the rattier surprising values of g = 2.36 and g = 2.34 (linewidths of both are comparable: A B p p ~ 1 2 m T ) for YBa2Cu30 x and EuBaECUaOx, respectively. For EUBaECUaOx, the Eu +3 ion cannot be a paramagnetic centre because of its electron configuration 4f 6 (the ground state is 7Fo). An admixture phase of the Y2CuzOs type must also be excluded, as E u 2 0 3 forms with C u e the compound E u e f u O 4 [14]. Therefore the observed E S R signals must be of another origin. We suppose that a complex defect strongly coupled to the lattice vibrations [15, 16] may be the source of these signals. The local mixed-valence state Cu ÷2 to Cu ÷3 may be the origin of this defect, resulting both from the oxygen nonstoichiometry and as well as from the introduction of Ba ÷2 ions. It is known that freeion Cu ÷3 has the configuration 3d 8 and its ground-state term is a 3F. At the axially symmetrical site the Cu ÷3 ion exhibits a spectrum typical of a center with S = 1. Such a spectrum is observed for Cu ÷3 ion entering the AI203 lattice as a substitutional impurity for AI ÷3 but with the small anisotropy g-values and with similar values

170

1. Onyszkiewicz et al. / Spectroscopic studies of high-T~ perovskites

for gs [17]. However in an octahedral crystal field, the value of the g-factor for the Cu ÷3 ion is expected to be in the range from 2.15 to 2.35 [18]. Taking into account the line broadening and g values, appearance of defects of the Cu +2Vo pairs in the lattice is probable too. Such a defect would produce a change in symmetry of the crystalline field acting on the nearest cupric ions. An example may be the appearance of the Cu+2-Vo defect in BaTiO 3 [16] which results in a strongly anisotropic g; the tensor g components for this case are: gll = 3.13 and g± = 2.08 thus ( g = ~a2g-L + gll = 2.43). It is also known that the isotropic g factor is highly susceptible to the changes in nearest neighbourhood of paramagnetic ion, e.g. to the change in the ligand type. In our case, the observed g-values as well as the line shape, may suggest the inequivalent positions of a paramagnetic center in the crystalline lattice. Any attempt to determine the precise symmetry of these lines would probably fail, among other reasons because of a very low concentration of the defects. Further temperature E S R studies are necessary to solve this problem. An attempt was undertaken to find the active fundamental IR modes for the high-T~ superconducting M - B a - C u - O system. In order to do that we took additionally IR spectra of Eu203, C u 2 0 , C H O , Y203, BaO (obtained from BaCo 3 decomposition), Y2Cu2Os, B a C u O 2 as well as of the mixtures containing Y203 (or E u 2 0 3 ) , BaCO 3 and CuO in the proportions suitable for obtaining the superconducting compound, however before sintering them. Some of the IR spectra are given in figs. 6, 7 and 8, Several important features of these IR spectra seem to be evident: i) The IR spectra of C u 2 0 , CuO, BaO, Y203 and Eu203 are consistent with those in the catalogue [19] and therefore we shall not analyse them in detail. ii) Having compared the IR spectra of the initial components, of the mixtures MBa2Cu306. 5 and of the obtained superconductors MBa2Cu3Ox, we checked that in the technological conditions applied the initial components had totally reacted.

T = 295K KBr disc YBa2Cu30x Superconductor

ft

~\~_, _ _ ~ ~ ~ LY/I

~cuo2

400 800 1200 1600 2000 2400 2800 3200 3600 Wave number[cm"1] Fig. 6. I R spectra of YBa2Cu30,, EUBaECU3Ox, YECUzOs, BaCuO 2 and YBazCu306. s mixture in the 3800-400cm -1 spectral region at RT.

<

<

,oo

8oo

Wavenurnber [cm1]

Fig. 7. IR spectra of YBa2Cu3Ox, EuBa2Cu30,, Y2CuzOs, CuO and Cu20 in the 800-400 cm -I spectral region at RT.

iii) In the IR spectrum of YBa2Cu30 x there are no characteristic bands at 692cm -1 for BaCuO 2 and at 482cm -1 and 528cm -1 for

I. Onyszkiewicz et al. / Spectroscopic studies of high-T~ perovskites 100

T=295t<

.

~

~/

-100 e

5c E

g

-50

0

3~0 3b0 2~0 2~0

,~0

~0

s'0 °

Wavenum~r [~m"] Fig. 8. IR spectra of YBa2Cu30 ~ and YBa2Cu306. 5 mixture in the 400-40cm -~ spectral region at RT.

Y2Cu2Os. This additionally confirms that YBa2Cu3Ox is in the single phase state. iv) In the Y B a C u 3 0 x IR spectrum we observe the appearance of a broad band with its maximum at about 110 c m -1, which is one of characteristic frequencies of the perovskite structure [19, 20, 21, 22]. This band was not observed in the IR spectrum of the mixture before sintering or in the IR spectra of the appropriate components. v) It should be noted that the spectra of YBa2Cu30 x and E u B a 2 C u 3 0 ~ are almost identical in the range 400-3800 cm -1. The only difference is that for E u B a 2 C u 3 0 x the shoulders - 4 6 0 cm -1 and 620cm -~ are more pronounced on the bands at about 400 cm -1 and 570 cm -1 respectively. The X R D data [23] as well as the results of neutrons diffraction structural studies [24] revealed several types of anion configurations around cations to appear in the new superconductors. These configurations are, so called, rhombic prisms and apicaUy elongated pyramids and they have not been found in typical perovskites. Different types of configurations and the possibility of mixed valent copper ions to occur makes it difficult to ascribe unambiguously the observed phonons to definite lattice vibrations of the oxygen deficient perovskite structure. Since the superconductors studied are rhombic-distorted perovskites of a space group P m m m (a ~-- b), their IR spectra reveal the active modes of Blu, B2u and B3u type. According to the generally

171

asumed notation [22] the types of vibrations v1, 7,2, 1,3, v4 found for perovskites of this kind may be tentatively ascribed as follows: i) vl: The bands above 450cm -1 to about 700 cm -1 may be ascribed to valence vibrations of C u - O , stretching vibration in the o x y g e n - c o p p e r polyhedrons. ii) 1,2: The band with the maximum at about 460cm -1 may be ascribed to the bending vibration of O - C u - O . iii) 1,3: The bands below 200 cm -1 may be ascribed to deformation vibrations of o x y g e n copper polyhedrons with simultaneous cation displacement and significant contribution of deformation vibrations of yttrium polyhedrons. -1 iv) v4: The bands within the range 250-400 cm may be ascribed to bending vibrations of C u - O and Y - O . The lowest energy vibration may be related to translational lattice vibrations. A broad band at about 135 cm -1 may appear due to a few kinds of vibrations. In classical perovskites (CaTiO3, BaTiO3) a phonon of a frequency of about 135cm -1 is not observed [20,21]. Vibration at this frequency was observed in the structures where a Cu ion is in the coordination 4, e.g. in Cs2CuC14 [25]. A vibration of a frequency - 1 2 0 c m -1 was also observed for Y203 which structure revealed a great number of unoccupied anion sites orderly distributed in the lattice [26]. Part of the weak bands found above 700 cm-1 in the spectra MBa2Cu30 x may be associated with harmonic vibrations. We should point out that in the entire spectral region studied we observed broad bands which could indicate an overlapping of some electronic bands and their hybridization. Besides the IR spectra we also have made D R spectra at R T of all components, mixtures MBa2fu306.5, superconducting phases MBa2Cu30 x and of the compounds B a C u O 2 and Y2Cu20 5. As an example three most important spectra are depicted in fig. 9. In all those spectra three broad bands with their maxima at about 42000cm -1, 30000cm -~ and 13500cm -1 can be distinguished. The broad intensive band with the maximum at --13500 cm -1 appears as a result of electron transitions of d - d type in the Cu ÷2 ion

172

I. Onyszkiewicz et al. / Spectroscopic studies of high-T c perovskites =

l

T= 295K

~

~

.,

~-

50

40

YBa2Cu30 x Superconductor I

30 WOVenumber['03Cm-'] 20

10

Fig. 9. Diffuse reflection spectra of YBa2Cu30~, Y2Cu205 and YBa2Cu306. s mixture in 50000-4000 cm -1 at RT.

ascribed to the transition Eg---~T2g. The other two bands arise from electronic transitions from ligand orbitals (O2p) to the d-type orbital of the cupric ion (Cu3d). For sintered materials the d - d band is changed into a continuous band with the shortwave edge similar to those in the mixture spectra (fig. 9). This may be explained by a change in the symmetry of the copper site and implied by this change different splitting of the d - d levels of Cu 2÷ in the crystalline field of the deformed polyhedra. Such a conclusion is supported by the X R D data which reveal several cation configurations in the samples studied• Additionally there is a possibility of Cu 3÷ ions to be formed which may also contribute to the contJnuous band starting at - 1 3 0 0 0 c m - k They are isoelectronic with the Ni 2÷ ion which gives rise to intensive bands associated with electronic d - d transitions at: 8500, 1400 and 23000cm -1 [27]• Besides, Cu 3÷ gives rise to an absorption edge begining before 40000 cm -1 as it has been found for that ion in A l 2 0 3 matrices [17]. The observed decreasing of absorpt]on above 40000 cm indicates that the number of Cu 3÷ ions is low. This last statement is in agreement with the suggestion that we made from our E S R spectra of superconducting M B a 2 C u 3 0 x. On the grounds of the IR investigations we supposed that the vibration at 620 cm -1 (fig. 7) may be ascribed to the valence vibration of Cu 1÷, however this supposition is not confirmed by the D R spectrum of YBa2Cu30 x, as in the presence of Cu ~+, the bands related to the d - d type •

•

-1

•

transitions should disappear• Moreover Cu ~÷ in the visible region of the spectrum should produce four bands as it is observed by D R [28] and they are not clearly seen in the YBa2Cu30 x spectrum• Continuous bands that begin at about 1300cm -1 and a broad band in the visible and U V region can be related with a great number of almost fiat bands existing in the upper part of the valence band [29]. The D R spectrum of sintered YBa2Cu30 x does not reveal any of the bands characteristic for Y2Cu20 5 (fig. 9) (especially that at 25000 cm -1) which additionally confirms, besides our other data, that the superconducting samples studied are single-phase systems. Similarly to the normal metals [30] Y B a e C u 3 0 x becomes transparent in the U V region (fig. 9). This fact allowed us to find the plasma frequency, top, for this material. Because of the dilution effect we could only estimate the value of top to be about 45000 c m - k This permitted an estimation of the charge density n (taking e~ = 5 and m* = me) to be of about (1-2) × 1021 cm -3. Such a low density of carriers seems to be one of the characteristics of the new high-T c superconductors and is i n agreement with other independent measurements [31]. From the available parameters p and n one can estimate the mean free path of the charge carriers l using the standard expression for the electron gas: l = ( 3 a r 2 ) X / 3 h / ( e 2 p n 2 / 3 ) . For p = (500-1000) t~D cm and n = 2 × 1021 cm -3 one gets l = 14.6-7•3/~. Such a short mean free path, comparable to the unit dimensions can be an indication of correlated transport•

4. Final remarks

•

The temperature dependences of resistivity and susceptibility obtained in our study for YBaECU30 x and EUBaECU30~ show the same general tendency as the dependences obtained for the samples reported as being close to stoichiometric YBa2Cu30 7 [3, 4, 10]. We should like to point out an important problem related with the investigation of high T c oxides, namely the time effect. Within a short time (a few days) after the substances had been obtained, they showed sig-

I. Onyszkiewicz et al. / Spectroscopic studies of high-T c perovskites

nificantly higher transition temperatures to the superconducting state of about several tens of degrees. The measurements presented here were performed for samples with stabilized T c (about 60 days after preparation). The results reported in this paper enables us to draw the following conclusions: i) A verified and controlled technology ensures obtaining single-phase materials. ii) Spectroscopic investigations confirm the possibility of the occurence of three types of oxygen configurations around the copper ions in the crystal lattice structure of the new superconducting cuprate perovskites. iii) The ESR method allowed to detect at RT the presence of complex lattice defects which can be related to Cu2+-V0 or/and C u 2+ ~ C u 3+.

iv) The difficulties in observation of the bands with standard spectrometers prove a great diversity of structural groups to be present in MBa2Cu30x at the microscale, which result in the bands broadening and appearance of a continuous band. v) A rich phonon structure creates favourable conditions for phonon assisted superconductivity to arise. vi) The optical edge of the low-energy side of the optical spectrum is found to be 55 cm -1 vii) Comparison of our IR spectra with corresponding spectra of various quality samples reported by Maeno et al. [32] suggests that any "good" superconducting samples should exhibit a continuous background begining at 800cm -~. However, the above statement needs further experimental evidence. viii) The charge carrier density estimated from optical investigation is close to the carrier density determined for other superconductors of L a - B a - C u - O type. One should point out that the nearly temperature-independent susceptibility with no trace of Curie-like contribution obtained for our samples above Tc is in disagreement with the earlier results of Cava et al. [3] concerning YBaCuO, who found Curie-Weiss behaviour. In our opinion these earlier results may be associated with

173

the presence of the impurity phases, eg. BaCuO 2 (see also Junod et al. [33]) or Y2Cu2_xBaxOs. Particularly interesting is the linear temperature behaviour of the resistivity down to To, which is also characteristic for L a - B a ( S r ) - C u - O materials [34]. Theoretically such a behaviour is expected to hold only for T > OD although experimentally it is sometimes observed down to temperatures of the order of OD/2 or even OD/3. The fact that in these new high-T c materials the linearity in p(T) is observed down to OD/4 in Y - B a - C u - O (O D ~ 400 K [35]) and to OD/10 in L a - B a ( S r ) - C u - O ( O D = 4 0 0 - 5 0 0 K [36]) is rather surprising and merits a serious question of its origin. Recently it has been shown [37] that such a behaviour down to T ~ OD can be explained within the standard electron-phonon dissipation mechanism provided the electronic structure is bidimensional and the number of carriers is small. Both these conditions are well satisfied for the systems under consideration. A different mechanism based on electron-electron scattering giving also rise to a linear in T law for resistivity has been proposed by Lee and Read [381.

Acknowledgements The authors would like to thank Prof. dr. hab. J. Pietrzak for his encouragement for this work. We also thank Dr. Z. Kruczyfiski for help in obtaining the ESR spectra as well as A. Por~bska M. Sci. for technical assistance in obtaining the Y2Cu205 and BaCuO 2 phases. This work was supported partially by the Polish Academy of Sciences under the project CPBP 01.04.II and CPBP 01.12.6.4.

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[4]

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I. Onyszkiewicz et al. / Spectroscopic studies o f high-T c perovskites

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