Linear T-term Research Articles

Physical, electrical, transport and magnetic properties of Nd(Ba,Nd)2.1Cu3O7−δ system

In this study, the superconducting Nd(Ba,Nd)2.1Cu3O7−δ system has been prepared using conventional solid-state reaction technique. Transport properties including structural/microstructural evolution, electrical, magnetic and critical current density properties were investigated. After high temperature heat treatments at over 1,000 °C, large and strongly connected grains were obtained but weakly connected and small in size granular formation were obtained for the low temperature heat treated samples at around 900 °C. The best T c and T 0 values were obtained as 93 and 89 K respectively for the sample prepared at 1,020 °C for 24 h, which is very close to peritectic temperature of YBCO material. Magnetization of the sample heat treated at 1,020 °C was investigated in detail. The magnetization hysteresis loops are expounded to be the product of Nd-123 grains and unscreened Nd3+ ions within intergranular boundaries and vortex cores. The peak effect on the magnetization curves was described by the extended critical state model. Temperature dependencies of the irreversibility field, the peak field and the full penetration field correlate and there is scaling behavior of the pinning force as well. Thermoelectric power data was analyzed by “Modified two band model with linear T-term for superconductors”. Temperature dependence of thermal conductivity of the samples showed small peak with broad maximum just below the T c value. Thermal conductivity of samples prepared was calculated by using “The Modified Callaway Model and Wiedermann–Franz law” and results obtained discussed.

Investigation of thermoelectric power with modification of two band model with linear T term for superconductors and thermal conductivity study of Se added YBCO samples

Abstract In this study, the electrical and thermal transport properties such as thermoelectric power and thermal conductivity of the Se added YBCO system was investigated. The samples fabricated under different heat treatment conditions exhibited the T c value between 90.1 K and 80.6 K. Positive thermoelectric power for all the samples was obtained over all measurement temperature range which indicates the hole type conductivity. Thermoelectric power data was analyzed by replacing term ( T – T * ) by T in “ Modified two band model with linear T-term ” which is good agreement with experimental TEP data for all the samples. T * obtained by fitting corresponds to a temperature close to T 0 obtained from resistivity measurement. Temperature dependence of thermal conductivity of the samples showed small peak with broad maximum just below T c . Thermal conductivity of the samples prepared was analyzed by the lattice thermal conductivity component by “ The modified Callaway model ” and the electronic component by “ Wiedemann –Franz law ”.

Thermoelectric power and thermal conductivity study of the Y3Ba5Cu8Ox system

In the present study, the effect of heat treatment conditions on the structural/microstructural, electrical and transport properties, such as thermoelectric power and thermal conductivity, of the Y-358 system was investigated. The Y-123 phase instead of Y-358 were obtained with heat treatments. XRD and SEM analyses showed that the best structural formation was obtained for the sample prepared at 900°C for 24h. All the samples exhibited the Tc value around 91K. Positive S(T) value was obtained in all the samples. S(T) data was analyzed by “Two band model with linear T-term”. In thermal conductivity of the samples, small peak with broad maximum was found just below Tc, unlike that of the HTc cuprates.

The single crystal superconducting Bi-2212 whiskers fabrication and their thermal transport properties

In this study, we have fabricated the superconducting single crystal whiskers using glass–ceramic BiSrCaCuO material. The crystallization kinetic studies (crystallization activation energy, E a , and the Avrami parameter, n,) were carried out using DTA data. The oxygen diffusion energy was calculated using TG data. XRD and SEM analysis of the whiskers showed Bi-2212 phase formation and the c-axis orientation. The in-plane resistivity, ρ ab, and the out-of-plane resistivity, ρ c , properties of the whiskers were investigated. The thermoelectric power, S( T), value between 10 and 300 K is found to be positive, indicating that charge carriers in the whiskers are holes. We analyzed the thermoelectric power as a function of temperature with “ Two band model with linear T-term” Linear temperature dependence in the thermal conductivity down to T c and then a sharp rise just below T c were obtained.

Magnetic phase transitions in R5NiPb3 (R=Ce, Nd, and Gd)

We report magnetic and thermodynamic measurements for R5NiPb3 (R=Ce, Nd, Gd). Gd5NiPb3 has a transition at 68K, apparently ferrimagnetic from specific heat and magnetization measurements. For Ce5NiPb3, a phase transition is observed in heat capacity at 48 K, with the field dependence of heat capacity and magnetization also indicating ferrimagnetic behavior. Ce5NiPb3 shows metamagnetism at low temperatures, behavior rather similar to Nd5NiPb3 with its previously reported magnetic ordering below 42 K. Using nonmagnetic La5NiPb3, we isolated the magnetic heat capacity for all three materials. The results include a relatively large linear-T term in the specific heat and integrated magnetic entropies consistently smaller than expected.

Thermoelectric power and thermal conduction studies on the Nd substituted BPSCCO (2234) superconductors

In this study, we have carried out thermoelectric power (S–T) and thermal conductivity (κ–T) measurements from room temperature down to 15K on Bi1.7Pb0.3−xNdxSr2Ca3Cu4O12+y samples, where 0.0⩽x⩽0.1. The thermoelectric powers, S(T), of the samples have both negative (at high T) and positive values (at low T), and are found to increase with decreasing temperature, reaching maxima (peaks) around their Tc values and to drop rapidly to zero below Tc. The magnitude of the peaks decreases with increasing Nd-content. The results obtained have been analyzed in terms of the “two-band model with linear T-term” and “Xin’s two-band model”. A very good agreement between the first model and our thermoelectric power data is obtained, but the fit to the second model is poor. The fittings to the models indicate that while the electrons are effective at higher temperatures, the holes become dominant at lower temperatures, the number of holes and the Fermi energies decrease with increasing Nd-content. Nd substitution also has a considerable effect on the thermal conductivity, κ. A broad maximum is obtained on κ versus T plot, just below the Tc value of each sample. However, the peaks decrease in intensity and also broaden with increasing Nd concentration. This behavior is believed to be due to the scattering of the carriers.

Structural characterization and transport properties of the H Tc Bi 2Sr 2(Ca,Cd)Cu 2O 8+ δ glass-ceramic rods

In this study, structural and transport properties including electrical resistance ( R– T) thermoelectric power ( S– T) and thermal conductivity ( κ– T), for the glass-ceramic rods of Bi 2Sr 2Ca 1− x Cd x Cu 2O 8+ δ with 0 ≤ x ≤ 0.8 were investigated. XRD and SEM-EDX investigations showed multiphase, complex and deformed structure in the Cd-substituted samples. An increase in the a-axis and decrease in the c-axis were obtained. All the samples showed metallic trend between 300 K and T c but the value of T c decreased significantly with increasing the Cd concentration in the system. The hole concentration per Cu was calculated by using Presland method and increased with increasing the substitution level. The thermoelectric power values, S, of the samples are obtained to be positive and found to increase almost linearly with decreasing temperature, reaching a maximum value around T c. With reducing temperature further, the thermoelectric power is found to fall rapidly to zero. Magnitude of S decreases with increasing the substitution level. The results obtained have been analyzed in terms of “ Two band model with linear T-term” and “ Xin's two band model”. An excellent agreement between Two band model with linear T-term and thermoelectric power data was obtained. The substitution has considerable effect on the thermal conductivity, κ. The magnitude of κ was suppressed when the Cd concentration was increased. A sharp rising was obtained just below T c in all the samples, suggesting that the impurities play an important role on heat transport due to increase of the electron–phonon-impurity scattering rate.

Thermal and superconducting properties of glass–ceramic HTc BiSrCa(CuPr)O system

Abstract In this study, the glass samples with nominal composition of Bi 2 Sr 2 Ca 2 Cu 3− x Pr x O 10+ y , where x = 0.5, were fabricated using conventional melt quenching technique. After appropriate heat treatments, the glass–ceramic samples obtained showed a deformed and multiphase structure. While the a (= b )-axis was nearly unchanged, the c -axis was obtained to be smaller than that of the unsubstituted Bi-2223 system. From the resistance versus temperature measurements, a semiconducting type behavior down to T c was observed. The best T c value was obtained to be 46 K and T zero 29 K for the sample heat treated at 1123 K (850 °C) for 60 h. The Pr-substituted Bi-2223 glass–ceramic samples showed negative thermoelectric power, indicating that the dominant carriers are electrons in the samples. “ Two-band model with linear T-term ” was used to analyze the TEP data. A rapid rise in thermal conductivity, κ , was obtained just below T c . This suggests an enhancement of the quasiparticle contribution to the heat conductivity and so an increase of the quasiparticle mean free path.

The interplay between weak ferromagnetism and superconductivity in RuSr2EuCu2O8

The magnetic field dependence of the low-temperature specific heat (LTSH) and I–V characteristics measurements on RuSr2EuCu2O8 were systematically carried out. It is found that the linear-T term γ decreases with increasing H, in contrast to the predicted result for the clean d-wave superconductivity. This finding suggests that the interaction between quasiparticles and magnons be weakened by magnetic field. In addition, the LTSH data exhibits a remarkably large T2 term for both H=0 and H≠0. The T2 term might be predominately associated with weak ferromagnetism in the Ru sublattice. Furthermore, the effective flux pinning energy E(H,T) determined from fitting the nonlinear region of the I–V curves follows a power-law dependence on magnetic field with an exponent close to −0.5 at a fixed temperature, indicating that the vortex lines of RuSr2EuCu2O8 is quasi-two-dimensional.

Influence of sintering temperature on thermopower and hardness ofRuSr2GdCu2O8

The structural, mechanical and transport properties ofRuSr2GdCu2O8 sintered between1000 and 1065 °C in O2 atmosphere have been studied by XRD, microhardness and Seebeck coefficientmeasurements. The pellets have a tetragonal perovskite-type structure. Microhardnessand bulk density of the considered samples increase with increasing sinteringtemperature. TEP is positive for all samples and decreases with increasingsintering temperature. The normal state TEP data are discussed in view ofsome theoretical models, namely a two-band model with an additional linearT term and variable range hopping models.

Thermoelectric Power and Thermal Conduction Studies on the Gd Substituted BPSCCO (2234) Superconductors

Measurements of the electrical resistance (R-T), thermoelectric power (S-T) and thermal conductivity (κ-T) have been carried out on the superconductor Bi1.7Pb0.3-x Gd x Sr2Ca3Cu4O12+y , (0.01 ≤ x ≤ 0.1). According to the XRD patterns the volume fraction of the Bi-2223 phase decreases in favor of Bi-2212. All the samples show normal metallic behavior down to their relevant transition temperature, T c . T c -values decrease significantly with increasing Gd concentration in the system. The hole concentration per Cu has been calculated by using the Presland method and found to decrease with increasing Gd content. The thermoelectric power values of the samples are positive and increase in magnitude with increasing the substitution level. The results obtained have been analyzed in terms of “Two band model with linear T-term” and “Xin’ s two band model”. A very good agreement between the first model and our thermoelectric power data was obtained, but the fit to the second model was poor. The substitution has considerable effect on the thermal conductivity, κ. The magnitude of κ is suppressed and a peak appears just below their T c , values but becomes weaker and broader when the Gd concentration is increased.

THERMAL CONDUCTIVITY OF YBa2Cu3O7: s-WAVE INTERPRETATION

Previous interpretations of the thermal conductivity data of YBa 2 Cu 3 O 7 in terms of d-wave superconductivity have created the impression that this experiment can only be successfully explained in terms of a d-wave model. Here we show that thermal conductivity data can be (better) explained in terms of s-wave hole-pairing, which also handles some outstanding problems with the d-wave interpretation, namely, muon spectra that are s-wave and specific heat data that exhibit a zero-field linear-T term. The basic notion is that a high-temperature superconductor such as YBa 2 Cu 2 O 7 contains both superconducting and non-superconducting bands. The main superconducting bands are associated with BaO , and the CuO 2 bands are non-superconducting.

Synthesis and characterization of Er-substituted Bi-2223 H-Tc glass–ceramic superconductors

The Bi2Sr2Ca2Cu3−xErxO10+δ (x = 0.5 and 1.0) H-Tc system was prepared by glass–ceramic technique to investigate the microstructural formation, thermal and transport properties. We have found that Er+3 ions have a solid solubility limit in the Bi-2223 system and this limit was passed at x = 1.0. Below this limit glass samples were prepared and converted to the glass–ceramic form easily, but after the solid solubility limit glass formation was diminished and crystallized samples were formed after melt quenching. The substitution had considerable effect on the electrical properties comparing with the unsubstituted Bi-2223 material. The Tc values of the x = 0.5 and 1.0 sample were found to be 71 ± 0.1K. The thermoelectric power data were analyzed in view of “two band model with linear T-term” and “Xin’s two band model”. Thermoelectric power was obtained to be positive for the both samples, indicating that the hole type conductivity is dominant in the materials prepared. The magnitude of thermal conductivity, κ, was also influenced by the Er concentration. When the Er concentration was increased the magnitude of κ was suppressed. It strongly indicates that the impurities play a significant role in limiting the heat transport due to a strong increase of the electron–phonon-impurity scattering rate.

Low-temperature specific heat of overdoped Bi2201 single crystals

Low-temperature specific heat of heavily overdoped Bi2Sr2CuO6 single crystals was studied. The specific heat under zero magnetic field included a linear-T term, suggesting the presence of a large pair-breaking effect, which we attribute to impurities because their influence on d-wave superconductors increases when the energy gap decreases with carrier overdoping. The field dependence of the electronic specific heat followed well the prediction for an impure d-wave superconductor.

Specific heat ofYBa2Cu3O7−δ

We present measurements of the specific heat of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals from 2 to 10 K and from 0 to 10 T, including both twinned and untwinned crystals with various oxygen doping (7-\ensuremath{\delta}=6.95, 6.97, and 6.99). The zero-field specific heat includes a linear-T term which is small compared to other ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ samples, but large compared to the ${\mathrm{T}}^{2}$ electronic specific heat predicted for a superconductor with clean lines of nodes in the gap function. This zero-field linear-T term is found to decrease with decreasing concentration of twin boundaries and oxygen vacancies \ensuremath{\delta}. The specific heat increases in a magnetic field and has a strong dependence on field orientation. Several possible field-dependent contributions are compared to the data, including a Schottky anomaly, vortex-vortex interactions, a traditional mixed-state HT term associated with vortex cores, and a $\sqrt{H}$T term predicted by G. Volovik for the mixed state in superconductors with lines of nodes in the gap function. The field dependence of the specific heat, according to a comparison of fits based on these possibilities, supports the existence of lines of nodes and suggests the presence of quasiparticle excitations outside the vortex core.

Detailed Lanczos study of one- and two-hole band structure and finite-size effects in the t-J model.

We present accurate numerical results for low-lying one- and two-hole states in the t-J model on a 4\ifmmode\times\else\texttimes\fi{}4 lattice. We find six level crossings in the one-hole ground state for 0t/J\ensuremath{\infty}; accurate t/J values of these crossings and the associated ground-state quantum numbers are given. A degeneracy of k=(0,0) S=1/2, and S=3/2 one-hole levels at t/J=1/2 is noted, which is consistent with a recent analytical result. For small t/J, the S=1/2 one-hole and S=0 two-hole bandwidths on the 4\ifmmode\times\else\texttimes\fi{}4 lattice are ${\mathit{W}}_{\mathit{h}}$=[1.190 445 7(1)]t and ${\mathit{W}}_{\mathrm{hh}}$=[2.575(4)]${\mathit{t}}^{2}$/J, respectively. The origin of these qualitatively different behaviors is discussed, and a simple relation is found between the small-(t/J) one-hole bandwidth and a static-hole ground-state matrix element. The linear-t term in ${\mathit{W}}_{\mathit{h}}$ is apparently a finite-lattice artifact. As a measure of finite-size effects we determined the rms hole-hole separation in the two-hole ground states; we find evidence of important finite-size effects for t/J\ensuremath{\gtrsim}1, for which the rms hole-hole separation is clearly constrained by the 4\ifmmode\times\else\texttimes\fi{}4 lattice. Intermediate-(t/J) hole separations and binding energies for 0.3\ensuremath{\lesssim}t/J\ensuremath{\lesssim}1, however, scale approximately as powers of t/J, and can be used to give bulk-limit estimates for t/J=3. In particular, we estimate that the bulk-limit ground-state rms hole-hole separation at t/J=3 is \ensuremath{\approxeq}1.8${\mathit{a}}_{0}$, corresponding to 7 \AA{} in the high-temperature superconductors. The similarity to the observed in-plane coherence length of ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$\ensuremath{\approxeq}14 \AA{} supports the identification of t-J model hole pairs with the Cooper pairs of high-temperature superconductivity.

Preparation, resistivity, magnetic properties and specific heat of the 95 K superconductor YBa 2Cu 3.5O 7.5+x (“247”)

Under suitable preparation conditions, Y 2Ba 4Cu 7O 15 (“247”) has the highest critical temperature of the YBCO superconductor family. We present an extensive characterization of 247 samples with T c onsets as high as 95K in bulk ceramic form. The experiments performed include metallographic examinations, X-ray diffraction, microprobe analysis, resistivity, AC susceptibility, Meissner and normal-state susceptibility, magnetic determinations of the slope of the critical field at T c and of the critical current density, and specific heat from 1 to 330 K. Several properties (e.g., normal-state susceptibility, specific heat jump at T c, slope of H c2( T) at T c, resistivity) may be considered as an approximate average between those of YBa 2Cu 4O 8 (“124”) and those of YBa 2Cu 3O 7 (“123”). The signature of fluctuations in magnetic properties is enhanced with respect to both 124 and 123. The behavior of the specific heat close to T = 0 is similar to that of the other members of the family, with the presence of a quasi linear T-term and the absence of any upturn in C/ T. The maximum T c appears to be related to optimal doping, consistent with the behavior observed in 123 as a function of the oxygen concentration.

Inelastic scattering time in three-dimensional nearly magnetic disordered fermion systems.

The inelastic electron-electron scattering time ${\ensuremath{\tau}}_{\mathrm{in}{}^{\mathrm{\ensuremath{-}}1}}$ due to paramagnon exchange in disordered metals was recently calculated perturbatively in two dimensions by Chang and Abrahams for systems near a magnetic instability [(1-\ifmmode \bar{I}\else \={I}\fi{}${)}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\gg}1 where (1-\ifmmode \bar{I}\else \={I}\fi{}${)}^{\mathrm{\ensuremath{-}}1}$ is the Stoner enhancement]. In the present paper ${\ensuremath{\tau}}_{\mathrm{in}{}^{\mathrm{\ensuremath{-}}1}}$ is reexamined in three dimensions. When T\ensuremath{\tau}<1-\ifmmode \bar{I}\else \={I}\fi{} (\ensuremath{\tau} is the elastic lifetime), the usual (T\ensuremath{\tau}${)}^{3/2}$ law, well known in unenhanced systems, is recovered, multiplied however by (1-\ifmmode \bar{I}\else \={I}\fi{}${)}^{\mathrm{\ensuremath{-}}3/2}$. When 1-\ifmmode \bar{I}\else \={I}\fi{}<T\ensuremath{\tau}<1, a regime more accessible to experiments when \ifmmode \bar{I}\else \={I}\fi{} is close to 1, ${\ensuremath{\tau}}_{\mathrm{in}{}^{\mathrm{\ensuremath{-}}1}}$ switches to linear behavior (1-\ifmmode \bar{I}\else \={I}\fi{}${)}^{\mathrm{\ensuremath{-}}1}$(T\ensuremath{\tau}). Another linear-T term should also arise even at very low temperature from diagrams neglected by Chang and Abrahams and considered by Isawa. In our case these extra terms will be enhanced by a factor (1-\ifmmode \bar{I}\else \={I}\fi{}${)}^{\mathrm{\ensuremath{-}}1}$. The resulting temperature dependence of the localization contribution ${\ensuremath{\sigma}}_{L}$ to the conductivity \ensuremath{\sigma}\ensuremath{\propto}1- \ensuremath{\surd}\ensuremath{\tau}/${\ensuremath{\tau}}_{\mathrm{in}\phantom{\rule{0ex}{0ex}}}$, may thus become nonnegligible compared to (and in competition with) the comtribution due to interactions, ${\ensuremath{\sigma}}_{I}$,L. .AE