Upper Critical Field and Specific Heat of Cuprates

A representative set of magnetotransport measurements in novel superconductors is analyzed. The resistive upper critical field, H _c2 (T) of many cuprates, of superconducting spin-ladders, and organic (TMTSF)2X systems has a universal nonlinear temperature dependence H _c2 (T_c – T)^3/2 in a wide temperature interval near T _c, while its low-temperature behavior depends on the chemical formula and sample quality. The unusual H _c2(T) is described as the Bose–Einstein condensation field of preformed pairs. Its universal temperature dependence follows from the scaling arguments. Controversy in the determination of H _c2 (T) from the resistivity and specific heat measurements is resolved in the framework of the charged Bose-gas model with the impurity scattering. It is shown that specific heat shows two anomalies. The high-temperature anomaly is strong and shows only weak shift with applied field. The low-temperature anomaly corresponds to resistive transition and is very weak in agreement with the experiments. Both anomalies coincide at H = 0.     

Comparative Study of Dense Bulk MgB2 Materials Prepared by Different Methods

We report on the results of a comparative investigation of highly dense bulk MgB₂ samples prepared by three methods: (i) hot deformation; (ii) high pressure sintering; and (iii) mechanical alloying of Mg and B powders with subsequent hot compaction. All types of samples were studied by AC susceptibility, DC magnetization, and resistivity measurements in magnetic fields up to ₀ H = 160 kOe. A small but distinct anisotropy of the upper critical field connected with some texture of MgB₂ grains was found for the hot deformed samples. The samples prepared by high pressure sintering as well as by mechanical alloying show improved superconducting properties, including high upper critical fields H _c2 (₀ H _c2 (0) 23 T), irreversibility fields H _irr which are strongly shifted towards higher values H _irr(T) 0.8 H _c2(T) and high critical current J _c (J _c = 10⁵ A/cm² at 20 K and 1 T).     

Upper critical field of Bi1·5-Pb0·5-Ca2-Sr2-Cu3-O x superconductor

The highT _c superconductor of nominal composition Bi_1·5-Pb_0·5-Ca₂-Sr₂-Cu₃-O _x was prepared by solid-state reaction method. The upper critical fieldH _c2 of the material with a zero-resistance transition at 110K was investigated by observing variations in resistance with temperature down to 90 K and with a magnetic field up to7T. The slope ofH _c2 with temperature was about −0·41T/K for zero resistance transition and −6·86T/K for onset of superconductivity.H _c2 values at 0K were estimated to be 31·3T and 563T for zero-resistance transition and onset of superconductivity respectively.     

Implications of Tc-Variation in UBe13 for a Possible Fulde–Ferrell–Larkin–Ovchinnikov Phase

We measure the heat capacity of a UBe₁₃ sample with an unusually low T _c for a polycrystal. We find an upturn in the upper critical field H _c2(T) below about T _c/2, much as for higher-T _c samples. Comparing the critical fields in our sample and in samples with higher T _c's shows that the low temperature limit of H _c2 is proportional to T _c(H = 0), as expected if the upturn comes from an FFLO phase and strong coupling.     

Upper Critical Field Hc2 on c Axis of Ag/Bi-2223 Polycrystal Tape

The upper critical field H _c2, based on Ginzburg–Landau theory at medial magnetic fields, can be calculated from the relationship of the magnetization intensity M(H) versus magnetic field H if M is linear with ln₀ H. For Ag/Bi-2223 tapes, the measured M(H) was found to be linear with ln₀ H. In this case, the values of H _c2(T) may in principle be determined. To do this, we will meet another problem in that the obliquity of crystal planes is not equal for different grains in tapes, and values of grain-oblique angle will appear in the calculated H _c2. Obviously, for Ag/Bi-2223 tapes is an uncertain parameter and hard to determine. To avoid the effects of , we only study the H _c2 in the c axis direction and the projections of H onto the c axis is taken as the actual applied fields. Thus, the effects of grain-oblique angle may be counteracted when measuring critical current density J, if the external magnetic fields are applied to the tape along both the parallel to and perpendicular to the c axes directions (on the narrow surfaces of the tape). On medial fields (H = 0 – 3 T), the upper critical fields H _c2(T) on c axis for the Ag/Bi-2223 tape are obtained and fitted as ₀ H _c2(T) = 830 e ^–0.07T . The average slope d[₀ H _c2(T)]/dT – 8T/K is found to be as large as that of Bi-2212. On extrapolating the relation to T = 0 K, the value of ₀ H _c2(0) 800T. The coherence length _ab is determined from H _c2(c), and _ab (0) = 0.63 nm at T = 0 K.     

Layered Superconductors in an External Magnetic Field

Presence of localized magnetic moments and inhomogeneites leads to an unconventional behavior of the upper critical field for layered superconductors. In addition, one can observe a large diamagnetic moment at H H _c2 and in the normal resistive state.     

The upper critical field of thin superconducting films with large resistance

The influence of the electron-electron interaction on the upper critical fieldH _c2 of thin superconducting films with large resistance and of layered superconductors is investigated theoretically. The orbital effect only is taken into account. It is shown that the electron-electron interaction in the diffusion and Cooper channels influencesH _c2 in different manners. The interaction in the diffusion channel (dynamically screened Coulomb interaction) leads to essential deviations from the standard BCS dirty limitH _c2 (T) curve at low temperatures and as a result upward curvature inH _c2 (T) is possible. The interaction in the Cooper channel is significant only at temperaturesT _c–TT_c and enlarges the slopedH _c2 (T)/dt atT=T _c.     

Superconducting gap function and upper critical field in the presence of inhomogeneous magnetic order

Rare earth ternary superconductors are known to exhibit oscillatory magnetic orders below their superconducting transition temperatures. The study of behaviour of superconducting electrons in an inhomogeneous magnetic field is therefore important for such systems. We report here the results of our theoretical study of superconducting gap function Δ(T) and upper critical fieldH _c2 (T). The results are applied to analyse and explain the variation of Δ(T) andH _c2 (T) in case of NdRh₄B₄.     

Critical field and impurity scattering: Application to layered and 3D superconducting systems

An efficient and rigorous method of evaluating the critical fieldH _c2 is developed. The method allows one to consider in a regular way the angular dependence of the scattering cross-section. The method is applied to layered and 3D superconductors.     

Irreversible Magnetic Properties of Ceramic Hgba2Ca2Cu3O8 at Very High Pulsed Magnetic Fields

The magnetic properties of ceramic HgBa₂Ca₂Cu₃O₈ have been investigated using pulsed magnetic fields up to 50 tesla. The irreversibility line H _irr (T) and the magnetic critical current J _c,m (H,T) have been extracted from the high field hysteresis loops. Irreversibility fields exceed available fields at temperatures below 15 K. The magnetic critical current J _c,m is strongly dependent on the granular structure of the ceramic superconductor. This is reflected in the moderate value of the critical current density and its significant field dependence.     

Measurements ofHc2(T) in Bi-Sr-Cu-O

H _c2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T.H _c2(T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor.     

Upper critical fields of the superconducting layered compounds Nb1−x Ta x Se2

The upper critical fieldsH _c2 (T) of the superconducting layered compounds Nb_1–x Ta _x Se₂ (0x0.20) have been measured with the magnetic field oriented either perpendicular or parallel to the layer plane. Just belowT _c , positive (upward) curvature is seen inH _c2 (T) for both field directions and for all compositions. The amount of positive curvature is not a function of crystal quality. At low temperatures,H _c2 (T) displays enhanced linearity as compared to the prediction of the isotropic theory, and shows no evidence of Pauli paramagnetic limiting for either field orientation. The critical field slopesdH _c2 /dT nearT _c change only slightly with composition.H _c2 (T) has been calculated for two Fermi surface models for NbSe₂ using the band structure calculated by Wexler and Woolley. TheseH _c2 (T) calculations indicate that Fermi surface anisotropy can explain the positive curvature and enhanced linearity seen in the experimental data. The calculations also show that theH _c2 (T) data are better explained with a Fermi surface that consists of open, undulating cylinders with an additional smaller closed piece, than with a Fermi surface model that consists only of the open, undulating cylinders of the original Wexler and Woolley model.     

Upper critical and irreversibility fields in selectively doped YBa2Cu3O7

The electrical resistivity of (Y_1–xPr_x)Ba₂Cu₃O_7–gd and YBa₂(Cu_1–xZn_x)₃O_7– thin films and (Y_1–x. Tb_x)Ba₂Cu₃O_7– thin films and single crystals has been measured as a function of 0xx_crit, 2KT300K and OTH20T. The samples were oriented withc-axis parallel to applied fields. Upper critical field H_c2 and irreversibility field H_irr values have been determined from these measurements. Increased Tb doping appears to shift H_irr to higher temperatures. This coupled with observed twin peaks in magnetization measurements reflects an enhancement of flux pinning. Unlike Tb which does not appear to alterT _c , Pr and Zn doping of this system tends to depress bothT _c and the slope of the mean field normal phase-mixed phase boundary line (dH _c2/dT).     

Upper critical field of high-T c oxides: Evidence for 2e Bose liquid

We measure and theoretically describe the upper critical field, H_c ²(T), of BSCCO — 2212 crystals with T_c > 92K by the use of the out-of-plane resistivity in magnetic fields (H ab) up to 15T. The empirical procedure is proposed allowing us to extrapolate the H_c ²(T) curve up to H_c ² 230T and t/t _c 0.35 which is independent of the choice of the R/R_N ratio. We found that H_c2(T) does not follow the conventional theory with or without fluctuations and the magnetic scattering, but is consistent with the prediction based on the Bose-Einstein condensation of charged bosons formed aboveT _c . Our results together with several other measurements of H _c 2 and with the heat capacity data provide an evidence for the possibility of 2 _e Base liquid ground state of high-T _c oxides.We highly appreciate the enlightening discussions with Sir Nevill Mott, P. Edwards, J. Cooper, N. Hussey, J. Loram, A. Mackenzie, and K. Ziebeck.     

Strong coupling effects on the upper critical field of the heavy-fermion superconductor UBe13

The upper critical fieldH _c2 (T) of a high quality single crystal of UBe₁₃ is studied with very low noise resistivity measurements. It shows a large but finite slope of —45 T/K, an unusual temperature dependence with an inflexion point atT/T _C 0.5 and a large saturated limit forT 0 ofH _c2 (0) = 14 T. The complete temperature dependence ofH _c2 (T) can be described by a simple model of strong coupling superconductor, assuming a full Pauli limitation and the occurence of a non-uniform superconducting state (FFLO state) at low temperature.     

Critical currents in YBaCuO ceramics

Magnetization measurements on a ceramic YBaCuO sample were carried out. TheM(H, T) dependence has demonstrated some peculiarities which we assume are connected withH _j1,H _j2,H _c1 fields. By analyzing the magnetization in the field region betweenH _j1 andH _j2 within the framework of the critical state model, one obtains the dependence of the intergranular critical current density on field and temperature.     

Pair-breaking effects in the superconducting Kondo alloy LaCe

The depression of the superconducting transition temperature of LaCe alloys and the upper critical fieldH _c2 (T) of La and LaCe was measured, down to 0.05 K. The results disagree with calculations, based on recent theories, including the temperature dependence of the spin-flip amplitude.     

On the influence of Fermi surface anisotropy onH c2 of weak and strong coupling superconductors

The influence of Fermi surface anisotropy on the upper critical fieldH _c2(T) of bulk type-II strong and weak coupling superconductors is investigated. It is found that anisotropic superconducting parameters can cause an enhancement ofH _c2(0) of more than 30% and can thus explain the 30% difference between measured values ofH _c2(0) and the prediction of weak coupling theories which recently has been found in the system Nb _75±x Pt _25x .Research supported by Sonderforschungsbereich 125.     

Helium wetting and prewetting phenomena at finite temperatures

Helium wetting and prewetting phenomena at temperatures T>0 are addressed. Emphasis is given to the weak-binding substrates Cs and H₂, which have been predicted and observed to be nonwet and wet, respectively, by⁴He at low temperatures. Calculations of the wetting temperature T_W and the prewetting line near T_W for Cs, as well as the prewetting line near T=0 for H₂, are given. Predictions concerning the interplay between superfluid onset and prewetting are made, and prewetting critical temperatures are estimated.     

Effect of structural relaxation on critical fieldsH c andH c2 and on resistivity in sputtered amorphous alloys Zr76Cu24 and Zr76Ni24

We report critical field results on sputtered amorphous Zr₇₆Cu₂₄ and Zr₇₆Ni₂₄ alloys. TheH _c (T) obtained from calorimetric measurements is well described by assuming that both systems are weak coupling superconductors. A similar good agreement is not found forH _c2 (T) obtained by a resistive method; tentative explanations of this discrepancy are suggested. We also report resistivity measurements in the normal state on Zr₇₆Ni₂₄ alloys. Results are well interpreted in the framework of the Faber-Ziman diffraction model. An alternative explanation, the Kondo-like model for two-level systems, is also discussed.Also at the Service National des Champs Intenses.