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Metal to insulator transition in Mn substituted underdoped NdBa2Cu3O7−y: Evidence of strong charge localization
Physica C 469 (2009) 1971–1976
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Metal to insulator transition in Mn substituted underdoped NdBa2Cu3O7y: Evidence of strong charge localization T.A. Jamadar, Ajay Kumar Ghosh * Condensed Matter Physics Research Centre, Department of Physics, Jadavpur University, Kolkata 700 032, India
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Article history: Received 13 July 2009 Accepted 30 July 2009 Available online 11 August 2009 PACS: 74.25.Fy 74.62.c 74.72.h Keywords: Cuprate superconductor Metal to insulator transition Transport properties
a b s t r a c t We have synthesized underdoped NdBa2Cu3O7y (NBCO) and NdBa2Cu3xMnxO7y (x = 0.1, 0.2, and 0.3) samples. The analysis of the lattice parameters has been done by using the X-ray diffraction (XRD) method. Using the Scanning Electron Microscope (SEM) the granular nature as well as the intergranular networks has been studied. The Energy Dispersive X-ray (EDX) and Rutherford Backscattering Spectroscopy (RBS) studies confirm the substitution of Mn in the Cu-sites. The transport measurements in several undoped and Mn-substituted NBCO samples have been carried out. We have observed an indication of the metal to insulator transition as a result of the strong charge localization induced by Mn substitution. The applicability of various conductivity equations has been verified for comparison. Estimations of the activation energy and localization length have been carried out and discussed. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The degradation of the superconducting transition temperature in high-Tc materials has been observed to be induced by the several factors such as substitution, irradiation and application of the magnetic field [1–5]. Despite of the lowering in the critical temperature several other physical properties are drastically modified including the properties related to the critical current density. The enhancement in the critical current density involves optimization of both the granular and intergranular modifications in the sample. The degradation of superconducting properties has been studied in several systems such as YBCO and Bi-2212 [6]. The composite phases of the superconductors have been studied for investigation of the related issues [7]. Like other cuprate superconductors the structure of NdBa2Cu3O7y (NBCO) or Nd-123 reveals that it consists of layered structure with CuO2 planes. Substitution by divalent transition metals for Cu offers an attractive method of introducing structural disorder preserving the doping level in particular. The previous studies have revealed that the nonmagnetic Zn2+ induces a drastic reduction in Tc which is almost three times larger than that caused by the magnetic Ni2+ ions [1,2]. Magnetic impurities are said to be coupled via the exchange interaction and acts as weaker scattering centers. However, the nonmagnetic * Corresponding author. Tel.: +91 33 2413 8917; fax: +91 33 2414 6584. E-mail address: [email protected] (A.K. Ghosh). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.07.008
Zn doping exhibits that the superconducting features are strongly reduced. The observation is evident from the resonance feature studied by inelastic neutron scattering [3] and it is also consistent with the density of states at the Fermi level reported by others [4]. In addition to the finite density of states at Fermi level, the life time of quasi particle involved in superconducting pairing mechanism is strongly lowered [8]. Abrikosov and Gorkov (AG) have shown that the magnetic impurities act as pair breaking centers in superconductor using the field theoretical approach and it has been supported by several experiments in low-Tc superconductors [9]. The suppression of the critical temperature has also been found in cuprate high temperature superconductors [1,2]. A small amount of Cu around Zn may exhibit small magnetic moment in CuO2 plane which is able to act as pair breaking centre [10]. Both the magnetic and nonmagnetic impurities strongly affect the superconducting properties [11]. In the present paper we study the impact of the substitution of Mn2+ impurity on the transport properties of NBCO superconductor. We have discussed issues related to the synthesis of Mn2+ substituted NBCO samples for the first time. Structural modifications induced in the layers parallel to the ab-plane have been investigated. Modifications in the intergranular networks by the Mn substitution have been studied. We have measured the resistivity of several Mn substituted Nd-123 samples as a function of temperature for studying the modifications of the transport
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properties for the first time. Comparisons of the present results with a few other related systems have been included in the discussion. The possibility of the several types of conduction mechanism including the variable range hopping (VRH) in samples with higher Mn concentration has been discussed. We have also estimated and compared the localization length and activation energy of samples showing metal to insulator transition. 2. Experimental details We have synthesized underdoped NdBa2Cu3xMnxO7y (x = 0, 0.1, 0.2, and 0.3) high-Tc materials by the usual solid state reaction [12]. Firstly, highly pure Nd2O3, BaCO3, CuO, and MnO are mixed in proper stoichiometric ratio. Mixing above compounds we have pressed it into pellet form and then calcinated pellets at 950 °C for 24 h. Pellets have been sintered at 950 °C in flowing oxygen for about 24 h. The annealing of the samples has been carried out at 600 °C in flowing oxygen for 72 h followed by the slow cooling. The characterization of the samples has been done with the help of the standard X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Energy Dispersive X-ray spectrometry (EDX) and Rutherford Backscattering Spectroscopy (RBS) method. The resistivity as a function of temperature has been measured using the standard four probe method [13,14]. The typical dc transport current used for transport measurements is in the range of 1.0 lA– 1.0 mA. 3. Results and discussions 3.1. XRD analysis The structural analysis of all samples has been done by using Xray diffraction (XRD) peaks. The XRD patterns have been shown in Fig. 1a. The undoped NBCO sample shows all peaks corresponding to the orthorhombic phase. In Pr doped series of Nd-123, it is observed that lattice constant were comparable to the parameters in our present samples [15]. All peaks have been identified as shown in the Fig. 1a. Comparing (2 0 0), (0 2 0) and (0 0 5) peaks we have extracted the lattice constants a, b, and c, respectively. The lattice constants extracted are shown in Fig. 1b. The lattice constants of pure (x = 0) NBCO sample are found to be
Fig. 1b. Lattice constants a and b of four samples as a function of the Mn concentration, x. In the inset the variation of another lattice constant, c with x has been plotted. We have used the extracted values obtained from the angles corresponding to (2 0 0), (0 2 0) and (0 0 5) XRD peaks.
a = 3.8683 Å, b = 3.8862 Å, and c = 11.596 Å. The minor modifications of lattice constants a and b have been observed in the Mnsubstituted samples (x = 0.1, 0.2, and 0.3) which is evident from the Fig. 1b. The most important fact obtained from the comparison of (0 0 l) peaks is the marginal structural modification of the planes parallel to the CuO planes. Studying other peaks the extracted lattice constants remain almost same. Lattice distortions by ionic radius mismatch arising out of the slight Ba substitution in Nd-site in pure sample may be one of the reasons for the modifications of the (0 0 l) planes in the orthorhombic phase of pure NBCO sample. The inclusion of Mn ion in Cu-site may have similar impact. The dependence of the lattice parameters support that Mn is indeed dissolved into the orthorhombic phase. Although, in the presence of the lower concentration of Mn, the scenario of such lattice modification may become even more complicated. However, in Mn-substituted NBCO samples the major peak positions remain unaltered though an inappreciable modification in the full width at half maxima (FWHM) has been observed. In Ca substituted NBCO an inappreciable change in lattice constant was reported earlier which is in fact comparable to the change in the lattice constants in Mn-substituted NBCO samples [16]. 3.2. SEM analysis We have used the Scanning Electron Microscope (SEM) to study the intergranular networks in pure Nd-123 and their
Fig. 1a. X-ray diffraction (XRD) pattern of NdBa2Cu3xMnxO7y superconductors corresponding to x = 0, 0.1, and 0.2 and 0.3. Several major (hkl) peaks have been identified and labeled in the graph.
Fig. 2. A typical SEM picture of NdBa2Cu3xMnxO7y (x = 0) shows weak link nature of the sample. See the text for the grain size distribution.
T.A. Jamadar, A.K. Ghosh / Physica C 469 (2009) 1971–1976
modifications by the Mn substitution. A typical SEM picture is shown in Fig. 2. The grain size is found to be unaltered in all studied samples. The grain size distribution is clearly observed. In fact, mainly two types of grains have been observed. The sizes of the longest grains are about 50–60l in length and about 10l in width. The smallest spherical shaped grains are also observed which have a typical size of about 5l However, the intergranular network is affected. The SEM picture shows a few numbers of voids at least in the shown Fig. 2 which reflects the compactness of the Nd-123 samples. During the synthesis the oxygen flowing at a temperature of 950 °C reduces the formation of the pores through the formation of the combination of the longer grains and a few disordered spherical grains embedded in all samples. It has been reported earlier that sintering condition strongly affects the distribution of grains and the intergranular matrix also in the YBCO superconductors [17]. We have also observed that the intergranular networks are well defined in all Mn substituted as well as undoped samples. A bending of the grains has been observed in all samples which is actually the cause of the weak link of being complicated in nature. 3.3. EDX and RBS results We have done EDX measurements using a typical scan area of 200 lm 200 lm. The integrated signal has been used for the determination of the atomic percentage of several elements. The result of the atomic percentage has been given in Table 1. Clear signature of the Mn substitution is observed in Cu-site with x = 0.1, 0.2, and 0.3 samples. All samples are found to be slight enriched with Ba atomic concentration. Interestingly, the Ba-rich phase has been achieved which is not very common in Nd-123 and related superconducting samples. Earlier, Ba-enriched NBCO compound has also been studied by using electron probe micro analysis [18]. It would be important to mention that the EDX is not very good method to estimate the oxygen concentration. In the present case we have a definite error bars not only for oxygen but also for all other elements. Actually the present data indicates the change in oxygen concentration from one sample to another very small. The summation of percentages of all elements is assumed to be approximately 100% in the iterative analysis process of the EDX which carries all the calculation. Other impurity elements are of negligible amount. Actually our aim is to keep oxygen constant as much as possible which is clearly achieved by the annealing process at 600 °C. Concerning the conversion of atomic percentage to stoichiometry we have used Nd as reference on the basis of which Ba is little higher than 2.0. In fact we do not see any Ba-rich phase which is detectable from XRD. In addition, we have not detected any other precipitate from XRD peaks. Several other features of Ba off-stoichiometric samples were carefully studied by several groups [18]. Most importantly EDX results at least reveal that oxygen remain almost constant in all samples having different Mn. That is most important for the present purpose. A systematic increase in the concentration of Mn has been observed in EDX whereas the oxygen content remains almost in the same level.
Table 1 Atomic concentrations in percentage of several elements obtained from the Energy Dispersive X-ray (EDX) measurements for the compositional analysis. Four samples have been identified in the sample column. Sample
Nd (%)
Ba (%)
Cu (%)
Mn (%)
O (%)
1 2 3 4
7.17 7.45 7.32 7.14
17.24 17.31 17.93 18.53
23.80 22.47 21.03 19.57
0 0.9 1.89 3.01
51.79 51.86 51.83 51.79
(x = 0) (x = 0.1) (x = 0.2) (x = 0.3)
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We have carried out Rutherford Backscattering Spectroscopy (RBS) measurements of samples with x = 0, 0.1, and 0.2 and 0.3. In RBS measurements we have used He ion beam which has an energy of 1.4 MeV. The backscattered ions of the target materials produced by the elastic collision are detected to determine the number of scattered ions and their energies. Channel number is associated with the backscattered ion and the yield to the number of atoms. A typical yield versus channel number corresponding to x = 0 has been shown in Fig. 3. In the inset of the figure, the steps corresponding to the Cu have been plotted for all four compounds. An inappreciable shifting of the step position may be related to the Mn concentration. The step at an energy of about 1.048 MeV shifts towards the higher energy channel which advocates the increase in Mn concentration. The steps corresponding to Ba and Nd are too close in energy scale to indicate any changes in the concentration. In fact, though the steps corresponding to Mn and Cu are close enough, the RBS spectra clearly indicates the Mn substitution in Cusite which is manifested in the slight shifting of the step corresponding to the (Mn and Cu) which has been indicated in the inset of Fig. 3. Both the EDX and RBS results reveal the substitution of transition element has been effective and successful in the present series of samples. 3.4. Transport properties The resistivity as a function of temperature of pure NBCO has been plotted in Fig. 4a. The onset critical temperature is found to 80.3 K which advocates the sample to be in the underdoped region. The processing condition has an impact in controlling the critical temperature as well as the broadening of the transition region in Nd-123 samples which is also obtained in the case of YBCO series samples. A little variation in the oxygen content from grain to grain may induce the broadening. It is reported earlier that the optimally doped Nd-123 has a Tc of 90 K which has been synthesized in Ar environment followed by cooling down slowly to the room temperature [19]. Our undoped NBCO sample shows lower Tc because of the different annealing condition which may be favorable for the sample to be in the underdoped region. In addition, the NBCO sample without Mn (x = 0) exhibits well metallic behavior (dq/dT > 0) from Tc (onset) to the ambient temperature. We have plotted resistivity as a function temperature corresponding to the Mn concentration of x = 0.10 in the Fig. 4b. The onset critical temperature is found to be 58.0 K. The Mn substitution induces strong suppression in the critical temperature. The
Fig. 3. A representative Rutherford Backscattering Spectroscopy (RBS) pattern of NdBa2Cu3xMnxO7y (x = 0) sample. Clear signatures of several elements have been observed from the steps corresponding to the Cu layers in the yield versus energy variation. In the inset we have plotted the same variation around the (Cu and Mn) step for all four samples. Black squares, green up triangles, blue down triangles and red squares are for x = 0, 0.1, 0.2, and 0.3, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4a. The resistivity as a function of temperature in NdBa2Cu3O7y (x = 0). The onset critical temperature is 80.3 K.
Fig. 4c. Variations of the resistivity versus temperature in NdBa2Cu2.8Mn0.2O7y, and NdBa2Cu2.7Mn0.3O7y. Nonsuperconducting behavior with very high resistivity has been observed in both cases down to 10 K. Also see Fig. 5a–c for fitting of these data related to several conductivity equations.
other samples remain nonsuperconducting down to about 10 K. Considering the nature of variations of resistivity of the samples with x = 0.1, 0.2, and 0.3, we explore several conductivity equations related to the normal state behavior. The general expression related to the variable range hopping (VRH) the resistivity is given by
q ¼ q0 expðT 0 =TÞm
Fig. 4b. The resistivity as a function of the temperature in the underdoped Mn substituted NdBa2Cu2.9Mn0.1O7y (x = 0.1). The onset critical temperature is 58.0 K.
reduction rate of critical temperature, dTc/dx, in NBCO is about 6.67 K per percentage of atomic concentration of Mn. In fact, in Ni doped YBCO the same degradation rate is about 2.58 K per Ni percentage [20]. Nonmagnetic element Zn in YBCO decreases superconducting transition temperature at the rate of 7.0 K per percentage which is comparable to the present rate [21]. Therefore, it would be enough justified to mention that Mn in NBCO acts as stronger pair breaker than some 3d elements (Ni for example) doped YBCO. However, in Mn doped LSCO superconductor, the rate of decrease in critical temperature is about 10 K per percentage of Mn concentration [22]. In compounds with the concentration of Mn, x = 0.2 and 0.3, the superconducting nature vanishes down to 10 K. Both samples (x = 0.2 and 0.3) show higher resistivity in comparison to the resistivities of other two samples (with x = 0 and 0.1). Clearly the metallic nature has been modified and the metal to insulator transition has been observed as shown in Fig. 4c. The important point to be noted here is the enhancement of the resistivity by an order of magnitude which is caused by the replacement of Cu by Mn. The dq/dT < 0 indicates the charge localization behavior over a wide range of temperature. In addition, the resistivity value becomes very high of the order of 1.0 X cm around 80.0 K for the Mn concentration of x = 0.2 which is around the onset of change the curvature. The localization of the holes in the superconducting plane by the magnetic moments may play the dominant role for the suppression of superconducting nature. All three compounds with x = 0.1, 0.2, and 0.3 have a common characteristic which is the increase in resistivity with the decrease in temperature at different temperature ranges (see Fig. 4b and 4c). Though the sample with x = 0.1 shows superconducting transition
where m = 1/3 and 1/4 represent two-dimensional (2DVRH) and three-dimensional VRH (3DVRH), respectively [23]. At a particular temperature the activation energy is proportional to T0. On the basis of the Efros–Shklovskii (ES) formulations, the conductivity can be expressed using m = 1/2 [24]. Therefore, to see if any of the above explains the normal state resistivity of the present samples we have tried all equations. All the equations have been tried to fit with the present data to explain the character of the conduction mechanism in case of metal to insulator transition induced by Mn substitution in NBCO. Next we discuss the normal state behavior of the samples with x = 0.2 and x = 0.3. We have plotted lnq as a function of T1/4 in Fig. 5a, lnq as a function of T1/3 in Fig. 5b and lnq as a function of T1/2 in Fig. 5c for samples with x = 0.2 and 0.3. Clearly these plots reveal that experimental data points deviate from the linearity in case of m = 1/2 strongly. It would be important to mention that ES conductivity (m = 1/2) mechanism shows maximum devia-
Fig. 5a. Variations of lnq as a function of T1/4 in NdBa2Cu2.8Mn0.2O7y (x = 0.2), and NdBa2Cu2.7Mn0.3O7y (x = 0.3). If we consider the entire temperature range (T1/ 4 = 0.25 K1/4 to T1/4 = 0.45 K1/4) slight deviations from the linear behavior have been observed. Solid and dotted lines are guide lines to the eye corresponding to the linear behavior for x = 0.2 and x = 0.3, respectively.
T.A. Jamadar, A.K. Ghosh / Physica C 469 (2009) 1971–1976
Fig. 5b. Variations of lnq as a function of T1/3 in NdBa2Cu2.8Mn0.2O7y (x = 0.2), and NdBa2Cu2.7Mn0.3O7y (x = 0.3). If we consider the entire temperature range clear deviation from the linear behavior is observed. The variation have different slope above T1/3 = 0.25 K1/3. Solid and dotted lines are guide lines to the eye corresponding to the linear behavior for x = 0.2 and x = 0.3, respectively.
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Fig. 6. The variation of lnq with T1/4 in NdBa2Cu2.7Mn0.3O7y. The solid line is the fitting line corresponding to 3D VRH and it has a slope such that T01/4 = 24.3 K1/4. The temperature range is from 27 K to 90 K in which the linear behavior have been observed indicating 3D VRH behavior.
Fig. 7. Plot of dlnqdlnT versus temperature for x = 0.2. Fig. 5c. Variations of lnq as a function of T1/2 in NdBa2Cu2.8Mn0.2O7y (x = 0.2), and NdBa2Cu2.7Mn0.3O7y (x = 0.3). If we consider the entire temperature range a curvature is clearly visible expressing that the ES relations is not suitable to explain the nonsuperconducting behavior in compounds with x = 0.2 and x = 0.3. Solid and dotted lines are guide to the eye corresponding to the linear behavior.
tion which indicates two samples do not have the conductivity variation as suggested by Efros-Shklovskii equation [24]. Comparing three plots, Fig. 5a corresponding to three-dimensional VRH case shows lowest standard deviation from the linear behavior. Considering 3DVRH in the entire temperature range it is found that slopes are 26.8 K1/4 and 29.3 K1/4 which are equivalent to T0 values of 5.2 105 K and 7.4 105 K for x = 0.2 and x = 0.3, respectively. In fact, VRH conduction has been reported to be valid in the limited range of temperature in several cuprate superconductors [25]. However, if we select data corresponding to lower temperature in the range of 90 K (T1/4 0.315 K1/4) to about 25 K (T1/4 0.435 K1/4), 3D VRH nicely fits. As a representative, we have plotted lnq as a function of T1/4 corresponding to x = 0.3 together with the fitting line which is linear in the temperature range corresponding to T1/4 0.325 K1/4 and T1/4 0.435 K1/4 in Fig. 6. We have extracted the slopes from which we have T0 values are 21.2 K1/4 and 24.3 K1/4 for the compounds with x = 0.2 and x = 0.3, respectively. These values are equivalent to T0 values of 2.0 105 K and 3.6 105 K for x = 0.2 and x = 0.3, respectively. Studying the variation of dlnq/dlnT with T in Fig. 7 for x = 0.2, it seems that the mechanism of conduction may be affected as a manifestation of which we may have different exponent values in different temperature ranges. Though the selection of temperature range definitely affects T0 values, the
enhancement of T0 in Mn-substituted samples is clearly manifested. In addition, the T0 values corresponding to x = 0.2 and 0.3 are comparable to several other cuprate materials such PrBa2Cu3Oy [25]. In addition, in Co doped samples the enhancement of T0 has been observed [26]. The activation energy is obtained by using the relation, D kBT(T0/T)1/4 which depends on temperature. We have estimated it with the help of T0 values taking the temperature value of 60 K. Typically the activation energies at 60 K are of the order of 39.4 meV and 45.14 meV for x = 0.2 and 0.3, respectively. Therefore, the Mn substitution causes an enormous increase in the activation energy. The paramagnetic moment may induce strong charge localization a consequence of which is the enhancement of the activation energy of the charge carriers. We have also tried to estimate localization length using the formula, T0 = 16a3/kBN(EF) where N(EF) is the density of states at the Fermi level. a1 is the localization length. Since the N(EF) of Nd-123, N(EF) 1.15 102 states/eV/A3 and assume that it does not change with the low concentration Mn substitution [26]. The localization lengths are 4.25 Å and 3.53 Å corresponding to x = 0.2 and 0.3, respectively which are comparable to several cuprate superconductors in the metal to insulator transition region [27]. Considering the limitations of 3D VRH in the limited range of temperature we have plotted lnq versus ln(T1) for samples having x = 0.2 and 0.3 which exhibit nonmetallic behaviors. Interestingly, for samples x = 0.2 and 0.3 we have obtained the linear behavior of lnq versus ln(T1) as shown in Fig. 8a and 8b, respectively. We have found that dlnq/dln(T1) are 2.30 X K and 2.39 X K corresponding to x = 0.2 and x = 0.3, respectively. However, the lnq versus ln(T1) corresponding to x = 0.3 shows a more linear behavior
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4. Conclusions
Fig. 8a. Plot of lnq as a function of ln(T1) for sample x = 0.2. The solid line is the fitting line.
In NBCO superconductor the rate of decrease of 6.67 K per atomic concentration of Mn in the critical temperature is weaker than that obtained by the Mn substitution in LSCO. Charge localization induced by the substitution of Mn causes enormous change in the resistivity of NBCO in x = 0.2 and x = 0.3. Metal to insulator transition has been observed in Mn-substituted samples with higher concentration. The lnq versus ln(T1) variations for x = 0.2 and x = 0.3 are linear over the entire range of temperature. However, using variable range hopping formula for x = 0.2 and 0.3 in lower temperature range we obtain an indication of the enhancement in the estimated activation energy in order of magnitude. The activation energy and the localization lengths of samples with x = 0.2 and 0.3 show compatibility with few other cuprate superconductors in which metal to insulator transition has been observed. Acknowledgements The work has been supported by a major research Project F.106/2004 (SR) funded by the University Grants Commission (UGC), India. AKG thanks M. Andersson and E. Hollmann for discussions and several measurements. References
Fig. 8b. Plot of lnq as a function of ln(T1) corresponding to x = 0.3. The solid line is fitting line.
in comparison to that corresponding to x = 0.2. Furthermore, since the activation energy is proportional to the slope dlnq/dln(T1) the enhancement is also manifested. The thermally activated conduction may be predominant and the possibility of polaron formation may be an important aspect to be studied in these Mn substituted compounds [23]. The Mn moment in NBCO superconductor is found to be weaker than that of the Zn in YBCO and Zn in LSCO superconductors in reducing the critical temperature. In Zn doped YBCO, the destruction of the antiferromagnetic correlation between Cu spin is said to be the possible origin of the charge localization [28]. In the underdoped LSCO, Zn is found to be effective in the localization process [29]. In NBCO, the charge localization has been induced by the Pr substitution in Nd-site [15]. In addition, the upward curvature in resistivity versus temperature curves in YBCO system have been very prominent in Zn and Co substituted YBCO system which has been attributed to the reduction in the hole density [30]. Recently, it has been reported that both Ni and Zn have a constant pair breaking energy in YBCO which is clearly in contrast with the calculation [31]. In NBCO, the pair breaking energy varies widely though it would be necessary to measure the pair breaking energy in presence of several magnetic and nonmagnetic impurities. The spin correlation in CuO plane of NBCO may be affected strongly by the introduction of the Mn moments. It would be interesting to mention that in Mn doped LSCO compound a Mn concentration of x 0.0211 has been reported to be the critical for changing the spin correlations in CuO plane in such a way that induces the change in resistivity which is analogous in several systems [22].
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Metal to insulator transition in Mn substituted underdoped NdBa2Cu3O7y: Evidence of strong charge localization T.A. Jamadar, Ajay Kumar Ghosh * Condensed Matter Physics Research Centre, Department of Physics, Jadavpur University, Kolkata 700 032, India
a r t i c l e
i n f o
Article history: Received 13 July 2009 Accepted 30 July 2009 Available online 11 August 2009 PACS: 74.25.Fy 74.62.c 74.72.h Keywords: Cuprate superconductor Metal to insulator transition Transport properties
a b s t r a c t We have synthesized underdoped NdBa2Cu3O7y (NBCO) and NdBa2Cu3xMnxO7y (x = 0.1, 0.2, and 0.3) samples. The analysis of the lattice parameters has been done by using the X-ray diffraction (XRD) method. Using the Scanning Electron Microscope (SEM) the granular nature as well as the intergranular networks has been studied. The Energy Dispersive X-ray (EDX) and Rutherford Backscattering Spectroscopy (RBS) studies confirm the substitution of Mn in the Cu-sites. The transport measurements in several undoped and Mn-substituted NBCO samples have been carried out. We have observed an indication of the metal to insulator transition as a result of the strong charge localization induced by Mn substitution. The applicability of various conductivity equations has been verified for comparison. Estimations of the activation energy and localization length have been carried out and discussed. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The degradation of the superconducting transition temperature in high-Tc materials has been observed to be induced by the several factors such as substitution, irradiation and application of the magnetic field [1–5]. Despite of the lowering in the critical temperature several other physical properties are drastically modified including the properties related to the critical current density. The enhancement in the critical current density involves optimization of both the granular and intergranular modifications in the sample. The degradation of superconducting properties has been studied in several systems such as YBCO and Bi-2212 [6]. The composite phases of the superconductors have been studied for investigation of the related issues [7]. Like other cuprate superconductors the structure of NdBa2Cu3O7y (NBCO) or Nd-123 reveals that it consists of layered structure with CuO2 planes. Substitution by divalent transition metals for Cu offers an attractive method of introducing structural disorder preserving the doping level in particular. The previous studies have revealed that the nonmagnetic Zn2+ induces a drastic reduction in Tc which is almost three times larger than that caused by the magnetic Ni2+ ions [1,2]. Magnetic impurities are said to be coupled via the exchange interaction and acts as weaker scattering centers. However, the nonmagnetic * Corresponding author. Tel.: +91 33 2413 8917; fax: +91 33 2414 6584. E-mail address: [email protected] (A.K. Ghosh). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.07.008
Zn doping exhibits that the superconducting features are strongly reduced. The observation is evident from the resonance feature studied by inelastic neutron scattering [3] and it is also consistent with the density of states at the Fermi level reported by others [4]. In addition to the finite density of states at Fermi level, the life time of quasi particle involved in superconducting pairing mechanism is strongly lowered [8]. Abrikosov and Gorkov (AG) have shown that the magnetic impurities act as pair breaking centers in superconductor using the field theoretical approach and it has been supported by several experiments in low-Tc superconductors [9]. The suppression of the critical temperature has also been found in cuprate high temperature superconductors [1,2]. A small amount of Cu around Zn may exhibit small magnetic moment in CuO2 plane which is able to act as pair breaking centre [10]. Both the magnetic and nonmagnetic impurities strongly affect the superconducting properties [11]. In the present paper we study the impact of the substitution of Mn2+ impurity on the transport properties of NBCO superconductor. We have discussed issues related to the synthesis of Mn2+ substituted NBCO samples for the first time. Structural modifications induced in the layers parallel to the ab-plane have been investigated. Modifications in the intergranular networks by the Mn substitution have been studied. We have measured the resistivity of several Mn substituted Nd-123 samples as a function of temperature for studying the modifications of the transport
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properties for the first time. Comparisons of the present results with a few other related systems have been included in the discussion. The possibility of the several types of conduction mechanism including the variable range hopping (VRH) in samples with higher Mn concentration has been discussed. We have also estimated and compared the localization length and activation energy of samples showing metal to insulator transition. 2. Experimental details We have synthesized underdoped NdBa2Cu3xMnxO7y (x = 0, 0.1, 0.2, and 0.3) high-Tc materials by the usual solid state reaction [12]. Firstly, highly pure Nd2O3, BaCO3, CuO, and MnO are mixed in proper stoichiometric ratio. Mixing above compounds we have pressed it into pellet form and then calcinated pellets at 950 °C for 24 h. Pellets have been sintered at 950 °C in flowing oxygen for about 24 h. The annealing of the samples has been carried out at 600 °C in flowing oxygen for 72 h followed by the slow cooling. The characterization of the samples has been done with the help of the standard X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Energy Dispersive X-ray spectrometry (EDX) and Rutherford Backscattering Spectroscopy (RBS) method. The resistivity as a function of temperature has been measured using the standard four probe method [13,14]. The typical dc transport current used for transport measurements is in the range of 1.0 lA– 1.0 mA. 3. Results and discussions 3.1. XRD analysis The structural analysis of all samples has been done by using Xray diffraction (XRD) peaks. The XRD patterns have been shown in Fig. 1a. The undoped NBCO sample shows all peaks corresponding to the orthorhombic phase. In Pr doped series of Nd-123, it is observed that lattice constant were comparable to the parameters in our present samples [15]. All peaks have been identified as shown in the Fig. 1a. Comparing (2 0 0), (0 2 0) and (0 0 5) peaks we have extracted the lattice constants a, b, and c, respectively. The lattice constants extracted are shown in Fig. 1b. The lattice constants of pure (x = 0) NBCO sample are found to be
Fig. 1b. Lattice constants a and b of four samples as a function of the Mn concentration, x. In the inset the variation of another lattice constant, c with x has been plotted. We have used the extracted values obtained from the angles corresponding to (2 0 0), (0 2 0) and (0 0 5) XRD peaks.
a = 3.8683 Å, b = 3.8862 Å, and c = 11.596 Å. The minor modifications of lattice constants a and b have been observed in the Mnsubstituted samples (x = 0.1, 0.2, and 0.3) which is evident from the Fig. 1b. The most important fact obtained from the comparison of (0 0 l) peaks is the marginal structural modification of the planes parallel to the CuO planes. Studying other peaks the extracted lattice constants remain almost same. Lattice distortions by ionic radius mismatch arising out of the slight Ba substitution in Nd-site in pure sample may be one of the reasons for the modifications of the (0 0 l) planes in the orthorhombic phase of pure NBCO sample. The inclusion of Mn ion in Cu-site may have similar impact. The dependence of the lattice parameters support that Mn is indeed dissolved into the orthorhombic phase. Although, in the presence of the lower concentration of Mn, the scenario of such lattice modification may become even more complicated. However, in Mn-substituted NBCO samples the major peak positions remain unaltered though an inappreciable modification in the full width at half maxima (FWHM) has been observed. In Ca substituted NBCO an inappreciable change in lattice constant was reported earlier which is in fact comparable to the change in the lattice constants in Mn-substituted NBCO samples [16]. 3.2. SEM analysis We have used the Scanning Electron Microscope (SEM) to study the intergranular networks in pure Nd-123 and their
Fig. 1a. X-ray diffraction (XRD) pattern of NdBa2Cu3xMnxO7y superconductors corresponding to x = 0, 0.1, and 0.2 and 0.3. Several major (hkl) peaks have been identified and labeled in the graph.
Fig. 2. A typical SEM picture of NdBa2Cu3xMnxO7y (x = 0) shows weak link nature of the sample. See the text for the grain size distribution.
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modifications by the Mn substitution. A typical SEM picture is shown in Fig. 2. The grain size is found to be unaltered in all studied samples. The grain size distribution is clearly observed. In fact, mainly two types of grains have been observed. The sizes of the longest grains are about 50–60l in length and about 10l in width. The smallest spherical shaped grains are also observed which have a typical size of about 5l However, the intergranular network is affected. The SEM picture shows a few numbers of voids at least in the shown Fig. 2 which reflects the compactness of the Nd-123 samples. During the synthesis the oxygen flowing at a temperature of 950 °C reduces the formation of the pores through the formation of the combination of the longer grains and a few disordered spherical grains embedded in all samples. It has been reported earlier that sintering condition strongly affects the distribution of grains and the intergranular matrix also in the YBCO superconductors [17]. We have also observed that the intergranular networks are well defined in all Mn substituted as well as undoped samples. A bending of the grains has been observed in all samples which is actually the cause of the weak link of being complicated in nature. 3.3. EDX and RBS results We have done EDX measurements using a typical scan area of 200 lm 200 lm. The integrated signal has been used for the determination of the atomic percentage of several elements. The result of the atomic percentage has been given in Table 1. Clear signature of the Mn substitution is observed in Cu-site with x = 0.1, 0.2, and 0.3 samples. All samples are found to be slight enriched with Ba atomic concentration. Interestingly, the Ba-rich phase has been achieved which is not very common in Nd-123 and related superconducting samples. Earlier, Ba-enriched NBCO compound has also been studied by using electron probe micro analysis [18]. It would be important to mention that the EDX is not very good method to estimate the oxygen concentration. In the present case we have a definite error bars not only for oxygen but also for all other elements. Actually the present data indicates the change in oxygen concentration from one sample to another very small. The summation of percentages of all elements is assumed to be approximately 100% in the iterative analysis process of the EDX which carries all the calculation. Other impurity elements are of negligible amount. Actually our aim is to keep oxygen constant as much as possible which is clearly achieved by the annealing process at 600 °C. Concerning the conversion of atomic percentage to stoichiometry we have used Nd as reference on the basis of which Ba is little higher than 2.0. In fact we do not see any Ba-rich phase which is detectable from XRD. In addition, we have not detected any other precipitate from XRD peaks. Several other features of Ba off-stoichiometric samples were carefully studied by several groups [18]. Most importantly EDX results at least reveal that oxygen remain almost constant in all samples having different Mn. That is most important for the present purpose. A systematic increase in the concentration of Mn has been observed in EDX whereas the oxygen content remains almost in the same level.
Table 1 Atomic concentrations in percentage of several elements obtained from the Energy Dispersive X-ray (EDX) measurements for the compositional analysis. Four samples have been identified in the sample column. Sample
Nd (%)
Ba (%)
Cu (%)
Mn (%)
O (%)
1 2 3 4
7.17 7.45 7.32 7.14
17.24 17.31 17.93 18.53
23.80 22.47 21.03 19.57
0 0.9 1.89 3.01
51.79 51.86 51.83 51.79
(x = 0) (x = 0.1) (x = 0.2) (x = 0.3)
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We have carried out Rutherford Backscattering Spectroscopy (RBS) measurements of samples with x = 0, 0.1, and 0.2 and 0.3. In RBS measurements we have used He ion beam which has an energy of 1.4 MeV. The backscattered ions of the target materials produced by the elastic collision are detected to determine the number of scattered ions and their energies. Channel number is associated with the backscattered ion and the yield to the number of atoms. A typical yield versus channel number corresponding to x = 0 has been shown in Fig. 3. In the inset of the figure, the steps corresponding to the Cu have been plotted for all four compounds. An inappreciable shifting of the step position may be related to the Mn concentration. The step at an energy of about 1.048 MeV shifts towards the higher energy channel which advocates the increase in Mn concentration. The steps corresponding to Ba and Nd are too close in energy scale to indicate any changes in the concentration. In fact, though the steps corresponding to Mn and Cu are close enough, the RBS spectra clearly indicates the Mn substitution in Cusite which is manifested in the slight shifting of the step corresponding to the (Mn and Cu) which has been indicated in the inset of Fig. 3. Both the EDX and RBS results reveal the substitution of transition element has been effective and successful in the present series of samples. 3.4. Transport properties The resistivity as a function of temperature of pure NBCO has been plotted in Fig. 4a. The onset critical temperature is found to 80.3 K which advocates the sample to be in the underdoped region. The processing condition has an impact in controlling the critical temperature as well as the broadening of the transition region in Nd-123 samples which is also obtained in the case of YBCO series samples. A little variation in the oxygen content from grain to grain may induce the broadening. It is reported earlier that the optimally doped Nd-123 has a Tc of 90 K which has been synthesized in Ar environment followed by cooling down slowly to the room temperature [19]. Our undoped NBCO sample shows lower Tc because of the different annealing condition which may be favorable for the sample to be in the underdoped region. In addition, the NBCO sample without Mn (x = 0) exhibits well metallic behavior (dq/dT > 0) from Tc (onset) to the ambient temperature. We have plotted resistivity as a function temperature corresponding to the Mn concentration of x = 0.10 in the Fig. 4b. The onset critical temperature is found to be 58.0 K. The Mn substitution induces strong suppression in the critical temperature. The
Fig. 3. A representative Rutherford Backscattering Spectroscopy (RBS) pattern of NdBa2Cu3xMnxO7y (x = 0) sample. Clear signatures of several elements have been observed from the steps corresponding to the Cu layers in the yield versus energy variation. In the inset we have plotted the same variation around the (Cu and Mn) step for all four samples. Black squares, green up triangles, blue down triangles and red squares are for x = 0, 0.1, 0.2, and 0.3, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4a. The resistivity as a function of temperature in NdBa2Cu3O7y (x = 0). The onset critical temperature is 80.3 K.
Fig. 4c. Variations of the resistivity versus temperature in NdBa2Cu2.8Mn0.2O7y, and NdBa2Cu2.7Mn0.3O7y. Nonsuperconducting behavior with very high resistivity has been observed in both cases down to 10 K. Also see Fig. 5a–c for fitting of these data related to several conductivity equations.
other samples remain nonsuperconducting down to about 10 K. Considering the nature of variations of resistivity of the samples with x = 0.1, 0.2, and 0.3, we explore several conductivity equations related to the normal state behavior. The general expression related to the variable range hopping (VRH) the resistivity is given by
q ¼ q0 expðT 0 =TÞm
Fig. 4b. The resistivity as a function of the temperature in the underdoped Mn substituted NdBa2Cu2.9Mn0.1O7y (x = 0.1). The onset critical temperature is 58.0 K.
reduction rate of critical temperature, dTc/dx, in NBCO is about 6.67 K per percentage of atomic concentration of Mn. In fact, in Ni doped YBCO the same degradation rate is about 2.58 K per Ni percentage [20]. Nonmagnetic element Zn in YBCO decreases superconducting transition temperature at the rate of 7.0 K per percentage which is comparable to the present rate [21]. Therefore, it would be enough justified to mention that Mn in NBCO acts as stronger pair breaker than some 3d elements (Ni for example) doped YBCO. However, in Mn doped LSCO superconductor, the rate of decrease in critical temperature is about 10 K per percentage of Mn concentration [22]. In compounds with the concentration of Mn, x = 0.2 and 0.3, the superconducting nature vanishes down to 10 K. Both samples (x = 0.2 and 0.3) show higher resistivity in comparison to the resistivities of other two samples (with x = 0 and 0.1). Clearly the metallic nature has been modified and the metal to insulator transition has been observed as shown in Fig. 4c. The important point to be noted here is the enhancement of the resistivity by an order of magnitude which is caused by the replacement of Cu by Mn. The dq/dT < 0 indicates the charge localization behavior over a wide range of temperature. In addition, the resistivity value becomes very high of the order of 1.0 X cm around 80.0 K for the Mn concentration of x = 0.2 which is around the onset of change the curvature. The localization of the holes in the superconducting plane by the magnetic moments may play the dominant role for the suppression of superconducting nature. All three compounds with x = 0.1, 0.2, and 0.3 have a common characteristic which is the increase in resistivity with the decrease in temperature at different temperature ranges (see Fig. 4b and 4c). Though the sample with x = 0.1 shows superconducting transition
where m = 1/3 and 1/4 represent two-dimensional (2DVRH) and three-dimensional VRH (3DVRH), respectively [23]. At a particular temperature the activation energy is proportional to T0. On the basis of the Efros–Shklovskii (ES) formulations, the conductivity can be expressed using m = 1/2 [24]. Therefore, to see if any of the above explains the normal state resistivity of the present samples we have tried all equations. All the equations have been tried to fit with the present data to explain the character of the conduction mechanism in case of metal to insulator transition induced by Mn substitution in NBCO. Next we discuss the normal state behavior of the samples with x = 0.2 and x = 0.3. We have plotted lnq as a function of T1/4 in Fig. 5a, lnq as a function of T1/3 in Fig. 5b and lnq as a function of T1/2 in Fig. 5c for samples with x = 0.2 and 0.3. Clearly these plots reveal that experimental data points deviate from the linearity in case of m = 1/2 strongly. It would be important to mention that ES conductivity (m = 1/2) mechanism shows maximum devia-
Fig. 5a. Variations of lnq as a function of T1/4 in NdBa2Cu2.8Mn0.2O7y (x = 0.2), and NdBa2Cu2.7Mn0.3O7y (x = 0.3). If we consider the entire temperature range (T1/ 4 = 0.25 K1/4 to T1/4 = 0.45 K1/4) slight deviations from the linear behavior have been observed. Solid and dotted lines are guide lines to the eye corresponding to the linear behavior for x = 0.2 and x = 0.3, respectively.
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Fig. 5b. Variations of lnq as a function of T1/3 in NdBa2Cu2.8Mn0.2O7y (x = 0.2), and NdBa2Cu2.7Mn0.3O7y (x = 0.3). If we consider the entire temperature range clear deviation from the linear behavior is observed. The variation have different slope above T1/3 = 0.25 K1/3. Solid and dotted lines are guide lines to the eye corresponding to the linear behavior for x = 0.2 and x = 0.3, respectively.
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Fig. 6. The variation of lnq with T1/4 in NdBa2Cu2.7Mn0.3O7y. The solid line is the fitting line corresponding to 3D VRH and it has a slope such that T01/4 = 24.3 K1/4. The temperature range is from 27 K to 90 K in which the linear behavior have been observed indicating 3D VRH behavior.
Fig. 7. Plot of dlnqdlnT versus temperature for x = 0.2. Fig. 5c. Variations of lnq as a function of T1/2 in NdBa2Cu2.8Mn0.2O7y (x = 0.2), and NdBa2Cu2.7Mn0.3O7y (x = 0.3). If we consider the entire temperature range a curvature is clearly visible expressing that the ES relations is not suitable to explain the nonsuperconducting behavior in compounds with x = 0.2 and x = 0.3. Solid and dotted lines are guide to the eye corresponding to the linear behavior.
tion which indicates two samples do not have the conductivity variation as suggested by Efros-Shklovskii equation [24]. Comparing three plots, Fig. 5a corresponding to three-dimensional VRH case shows lowest standard deviation from the linear behavior. Considering 3DVRH in the entire temperature range it is found that slopes are 26.8 K1/4 and 29.3 K1/4 which are equivalent to T0 values of 5.2 105 K and 7.4 105 K for x = 0.2 and x = 0.3, respectively. In fact, VRH conduction has been reported to be valid in the limited range of temperature in several cuprate superconductors [25]. However, if we select data corresponding to lower temperature in the range of 90 K (T1/4 0.315 K1/4) to about 25 K (T1/4 0.435 K1/4), 3D VRH nicely fits. As a representative, we have plotted lnq as a function of T1/4 corresponding to x = 0.3 together with the fitting line which is linear in the temperature range corresponding to T1/4 0.325 K1/4 and T1/4 0.435 K1/4 in Fig. 6. We have extracted the slopes from which we have T0 values are 21.2 K1/4 and 24.3 K1/4 for the compounds with x = 0.2 and x = 0.3, respectively. These values are equivalent to T0 values of 2.0 105 K and 3.6 105 K for x = 0.2 and x = 0.3, respectively. Studying the variation of dlnq/dlnT with T in Fig. 7 for x = 0.2, it seems that the mechanism of conduction may be affected as a manifestation of which we may have different exponent values in different temperature ranges. Though the selection of temperature range definitely affects T0 values, the
enhancement of T0 in Mn-substituted samples is clearly manifested. In addition, the T0 values corresponding to x = 0.2 and 0.3 are comparable to several other cuprate materials such PrBa2Cu3Oy [25]. In addition, in Co doped samples the enhancement of T0 has been observed [26]. The activation energy is obtained by using the relation, D kBT(T0/T)1/4 which depends on temperature. We have estimated it with the help of T0 values taking the temperature value of 60 K. Typically the activation energies at 60 K are of the order of 39.4 meV and 45.14 meV for x = 0.2 and 0.3, respectively. Therefore, the Mn substitution causes an enormous increase in the activation energy. The paramagnetic moment may induce strong charge localization a consequence of which is the enhancement of the activation energy of the charge carriers. We have also tried to estimate localization length using the formula, T0 = 16a3/kBN(EF) where N(EF) is the density of states at the Fermi level. a1 is the localization length. Since the N(EF) of Nd-123, N(EF) 1.15 102 states/eV/A3 and assume that it does not change with the low concentration Mn substitution [26]. The localization lengths are 4.25 Å and 3.53 Å corresponding to x = 0.2 and 0.3, respectively which are comparable to several cuprate superconductors in the metal to insulator transition region [27]. Considering the limitations of 3D VRH in the limited range of temperature we have plotted lnq versus ln(T1) for samples having x = 0.2 and 0.3 which exhibit nonmetallic behaviors. Interestingly, for samples x = 0.2 and 0.3 we have obtained the linear behavior of lnq versus ln(T1) as shown in Fig. 8a and 8b, respectively. We have found that dlnq/dln(T1) are 2.30 X K and 2.39 X K corresponding to x = 0.2 and x = 0.3, respectively. However, the lnq versus ln(T1) corresponding to x = 0.3 shows a more linear behavior
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4. Conclusions
Fig. 8a. Plot of lnq as a function of ln(T1) for sample x = 0.2. The solid line is the fitting line.
In NBCO superconductor the rate of decrease of 6.67 K per atomic concentration of Mn in the critical temperature is weaker than that obtained by the Mn substitution in LSCO. Charge localization induced by the substitution of Mn causes enormous change in the resistivity of NBCO in x = 0.2 and x = 0.3. Metal to insulator transition has been observed in Mn-substituted samples with higher concentration. The lnq versus ln(T1) variations for x = 0.2 and x = 0.3 are linear over the entire range of temperature. However, using variable range hopping formula for x = 0.2 and 0.3 in lower temperature range we obtain an indication of the enhancement in the estimated activation energy in order of magnitude. The activation energy and the localization lengths of samples with x = 0.2 and 0.3 show compatibility with few other cuprate superconductors in which metal to insulator transition has been observed. Acknowledgements The work has been supported by a major research Project F.106/2004 (SR) funded by the University Grants Commission (UGC), India. AKG thanks M. Andersson and E. Hollmann for discussions and several measurements. References
Fig. 8b. Plot of lnq as a function of ln(T1) corresponding to x = 0.3. The solid line is fitting line.
in comparison to that corresponding to x = 0.2. Furthermore, since the activation energy is proportional to the slope dlnq/dln(T1) the enhancement is also manifested. The thermally activated conduction may be predominant and the possibility of polaron formation may be an important aspect to be studied in these Mn substituted compounds [23]. The Mn moment in NBCO superconductor is found to be weaker than that of the Zn in YBCO and Zn in LSCO superconductors in reducing the critical temperature. In Zn doped YBCO, the destruction of the antiferromagnetic correlation between Cu spin is said to be the possible origin of the charge localization [28]. In the underdoped LSCO, Zn is found to be effective in the localization process [29]. In NBCO, the charge localization has been induced by the Pr substitution in Nd-site [15]. In addition, the upward curvature in resistivity versus temperature curves in YBCO system have been very prominent in Zn and Co substituted YBCO system which has been attributed to the reduction in the hole density [30]. Recently, it has been reported that both Ni and Zn have a constant pair breaking energy in YBCO which is clearly in contrast with the calculation [31]. In NBCO, the pair breaking energy varies widely though it would be necessary to measure the pair breaking energy in presence of several magnetic and nonmagnetic impurities. The spin correlation in CuO plane of NBCO may be affected strongly by the introduction of the Mn moments. It would be interesting to mention that in Mn doped LSCO compound a Mn concentration of x 0.0211 has been reported to be the critical for changing the spin correlations in CuO plane in such a way that induces the change in resistivity which is analogous in several systems [22].
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