Fluctuation-induced diamagnetism in type II superconductors above H c2

The fluctuation-induced diamagnetism in strong magnetic fields in type II superconductors is studied theoretically. It is shown that the fluctuation-induced diamagnetism at low temperatures (i.e., TT _c0) is described in terms of a universal function of [B \s- H _c2(T)]/H_s(T) for both dirty and pure superconductors, where H _s(T), the scaling field, is proportional to (T/T _c0)H_c2(0).Work supported by the National Science Foundation under Grant DMR 76-21032.     

Electronic specific heat of overdoped Tl2Ba2CuO6+δ in a magnetic field

We have measured the magnetic field dependence of the electronic specific heat ^el (=C^el/T) of polycrystalline Tl₂Ba₂CuO₆₊ from 8 K to above T_c, for samples with T_c = 24 K to 72 K using a high resolution, differential technique. We find that in applied fields of up to 13 T the position of the specific heat jump does not shift significantly for any T_c. This is in strong disagreement with H_C2(T) inferred from magnetoresistance experiments on the same compound, where the resistive transition is shifted down considerably by similar fields. However, there is a dramatic collapse of the specific heat anomaly height in field — signifying a loss in entropy around T_c — compensated by a corresponding increase in ^el at lower temperatures. The superconducting anomaly is almost completely suppressed in 13 T for the lowest T_c sample, compared to a reduction of 65 % for the sample with T_c = 72 K. We find simple vertical scaling of ^el(T) — _n suggesting that the field forces a proportion of the superconductor into the normal state, while the remainder in the superconducting state is unaffected.     

Quantum oscillations and reversible and irreversible superconducting phase transitions in indium microcylinders

Using a sensitive mutual inductance method, measurements of first- and second-order phase transitions in indium microcylinders (2.8–7.6 µm in diameter) have been made. Quantum oscillations in the supercooling field have been observed. The values of _SC (T/T _c )=₁(T/T _c ) derived fromH _SC =H _C3 have been corrected for both the temperature and the size dependence in the relationship betweenH _c2 andH _c3 . The results for ₁(T/T _c ) from the 7.6-µm sample then agree reasonably well with the temperature dependence of ₁ observed for niobium. However, for the smallest cylinders anomalously low values of _SC (T/T _c ) were obtained nearT _c . Very close to the transition temperature second-order (nonhysteretic) phase transitions have been observed and are interpreted in terms of the Ginzburg-Landau theory.     

Upper critical field anisotropy in the Chevrel phase compound PbMo6S8

The anisotropy of the upper critical field,H _c2, was measured on deoxygenated single crystals of the Chevrel phase compound PbMo₆S₈. The superconducting transition temperature,T _c, was determined with the applied field pointing in various directions in the hexagonal plane. At a fixed value of the external field,T _c is an oscillating function of direction. Since theH _c2 versusT curve is linear in the field region we probed, the shape of the anisotropy inT _c at constantH is the same as the anisotropy ofH _c2 at constantT. The data reflect the low-temperature symmetry of the crystal structure. Two-fold symmetry was observed in the plane. The data were fitted to the effective mass model with uniaxial symmetry. The anisotropy ofH _c2 is more complex than that previously observed by us in Cu_1.8Mo₆S₈. We relate this observation to the structural differences of these compounds.     

Magnetic Field Influence on the Low Temperature Heat Capacity and Heat Release of (TMTSF)2PF6

We have measured under applied magnetic field the heat capacity c _p and heat release of the quasi-1D conductor (TMTSF)₂PF₆ in its spin density wave (SDW) ground state. The low-temperature heat capacity (T < 0.5 K) is dominated by a Schottky anomaly contribution. A corresponding term was also found for the heat release. In this T-range both properties are strongly sensitive to moderate magnetic fields (i.e., below 0.5 T): their amplitudes have a sharp maximum for H _c=0.2 T. We show that the corresponding density of two-level states N _S undergoes a sharp maximum at H _c, which rules out an extrinsic origin for the two-level states. Instead we suppose an intrinsic origin, such as defects of commensurability of the SDW, as the (slow) excitations related to the domain walls. This effect is found to be independent on the field orientations H a and H a.     

The anisotropy of the upper critical field of the Chevrel-phase compound Cu1.8Mo6S8

The anisotropy of the upper critical field has been measured on deoxygenated single crystals of the Chevrel-phase Cu_1.8Mo₆S₈. Data were taken with the applied magnetic field pointing in various directions in the hexagonal and (001) planes. The data reflect the low-temperature, triclinic symmetry of the crystal structure. At each of a series of field values, oscillations in the superconducting transition temperature were observed. Since theH _c2 versusT curve is linear in the field region probed, the shape of the anisotropy inT _c at constantH is the same as the anisotropy of the upper critical fieldH _c2 at constantT. Oscillations with pseudo-threefold symmetry have been observed for the first time in the (001) plane. The data are fitted with an expansion of spherical harmonics with triclinic symmetry.     

Vortex Lattice Melting in YBa2Cu4O8 with H∥ab

We report c-axis resistivity measurements in the mixed state of YBa₂Cu₄O₈ with the magnetic field applied parallel to the a-, b-, and c-axes. For all orientations of the magnetic field, a kink is observed in the resistive transition, associated with the first-order melting of an anisotropic three-dimensional vortex lattice. Whereas the melting lines for Hb and Hc obey the expected relation H _m = H ₀(1 – T/T _c ) ⁿ with n = 1.5, H _m (T) for Ha follows a different temperature dependence with a lower exponent. This result is consistent with a suppression of the melting line due to a reduction in the dimensionality of YBa₂Cu₄O₈ in this field geometry, as observed in normal-state magnetoresistance measurements.     

Phonon anomalies in YNi2 11B2C

Recently we have discovered a new peak with phonon character which appears only below T_c in an intermetallic superconductor, YNi₂ ¹¹B₂C (Tc14-2K) by neutron inelastic scattering measurements. The peak intensity of the new peak shows the order-parameter-like temperature dependence of the superconductivity. The new peak grows by absorbing the spectral weight from the above-lying soft phonon mode. In the present study we investigate the magnetic field dependence of the new peak. With increasing magnetic field, the new peak is gradually suppressed and disappears by H=H_c2 (4.7T at T=5.5K). Beyond H_c2, the profile of the scattering function recovers that of the non-superconducting state above T_c These results strongly indicate that this new peak is undoubtedly associated with the superconducting state in YNi₂ ¹¹B₂C.     

Critical fields and generalized Ginzburg-Landau parameters for Mo-Re alloys containing paramagnetic impurities

We have investigated the temperature dependence of the upper critical fieldH _c2 and the thermodynamical fieldH _c for superconducting Mo-Re alloys containing magnetic impurities. In addition, the temperature dependence of the Ginzburg-Landau parameters ₁(T) and ₂(T) has been determined by measuring the magnetization and the specific heat. Our findings are compared with the theory of Fulde and Maki. In agreement with theory, our measurements show that the two pair-breaking mechanisms involved (magnetic field and magnetic impurities) are nonadditive if one departs from the linearized Ginzburg-Landau equation.     

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.     

A New Method for the Estimation of Hc2 Anisotropy in Polycrystalline MgB2 Samples

By using isothermal magnetization measurements in polycrystalline MgB₂ samples, we estimate the H ^c _c2 in the interval [0, T _c]. By combining these measurements to the estimated H ^ab _c2 from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H ^c _c2 values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the (T ) = H ^ab _c2/H^c2 with (T _c) 3.     

Specific Heat Near Hmetamag=18 T in UPd2Al3

We have measured the specific heat vs. temperature in fields to 20 T of UPd₂Al₃ in order to try to determine the reason for the disagreement in the magnetic phase diagram as determined by magnetization and _ac vs. resistivity measurements, with the resistivity measurements showing the magnetic anomaly trending towards T=0 as HH _metamag. Our data show a slight monotonic suppression of T _N with increasing field in agreement with both previous magnetoresistance and magnetization data until HH _metamag. However, as field exceeds 17 T the specific heat shows a peak (presumably in a narrow field range connected with both antiferromagnetism and the H>H _metamag magnetization anomaly) that is suppressed with further increases in field no lower in temperature than 11.7 K (in contrast to the feature in the magnetoresistivity which, e.g., occurs at 8.4 K in 18 T) before it starts to move up in temperature with increasing field at H18 T (H _metamag). Possible causes of the specific heat anomaly for H>H _metamag, as well as for the discrepancy between the specific heat and the resistivity as HH _metamag, are discussed.     

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.     

Anisotropy of the Ginzburg-Landau parameter κ in NbSe2

On small single crystals of 2H NbSe ₂ we have measured the temperature and angular dependence ofH _c2 as well as the temperature dependence ofH _c1 close toT _c. Using an effective mass approach to the Ginzburg-Landau equations we have analyzed our data in terms of an anisotropic and obtained values for the coherence distances and penetration depths parallel and perpendicular to the layers. The connection between these parameters and normal-state properties is discussed and shown to be more complex than in the isotropic case.     

Transport Properties of the Iron-based Superconductor SrFe2(As,P)2 in High Magnetic Fields

We have investigated the resistive upper critical field (μ ₀ H _c2) of the iron-based superconductor SrFe₂(As_1?x P _x )₂ (x=0.35, T _c～29 K) in pulsed high magnetic fields of up to 52 T. For H∥ab, $\mu_{0}H_{\mathrm{c}2}^{ab}(T)$ exhibits the Werthamer-Helfand-Hohenberg (WHH) like behavior. While, for H∥c, $\mu_{0}H_{\mathrm {c}2}^{c}(T)$ is almost linear against T and exhibits a slightly upward curvature near T _c. The anisotropy of μ ₀ H _c2 decreases from 2.5 near T _c to ～1.2 at T=0 K monotonically. Similar isotropic behavior of μ ₀ H _c2 at low temperatures has also been observed in the Fe_1+δ (Te,Se). We demonstrate the results of the two-band analysis for μ ₀ H _c2(T) on the present sample and discuss the anisotropy of μ ₀ H _c2 by comparing with those in some kinds of iron-based superconductors.     

Pressure Effect in High-Temperature Superconductors and Fullerides

The explanation for the pressure effect in high-temperature superconductors and fullerides is offered. Besides the dependence of the pressure derivative of the critical temperature T _c on doping, the direct dependence of T _c on pressure and the universal dependence of the relative change of T _c ^max–T _c with pressure are obtained for high-temperature superconductors. The unity of the model of the pressure effect in high-temperature superconductors and fullerides is justified. The dependence of T _c on the lattice constant and the connection between the pressure effect and the chemical pressure effect in fullerides are discussed.     

Magnetic screening and specific heat of an anisotropic Coulomb Bose-gas as a model for the superconducting state

The thermodynamics of the Coulomb Bose-gas (CBG) is studied using the Bogoliubov approximation. Within this approximation the condensate density did not tend to zero as the transition temperature, T_c, was approached, but remained finite, resulting in a first order phase transition. By the use of the condensate density as the only temperature dependent feature of the electromagnetic screening the London penetration depth is derived. The free energy of the system and the specific heat are calculated. We report a well defined -point transition in the specific heat. Comparisons are made with the experimental results for YBCO, a high temperature superconductor.We also report recient findings of the Bose-Einstein condensation of CBG in a magnetic field at ultra-low temperatures and ultra-high magnetic fields. A low-temperature dependence of the upper critical field H_c2(T) is obtained both for the particle-impurity and particle-particle scattering.     

Doping dependence of the critical field Hc2 and the transition temperature in Zn doped YBa2(Cu1−xZnx)3O7−δ

We present data on the doping dependence of T_c in Zn doped YBa₂(Cu_1−xZn_x)₃O_6.94, for x up to 10.6%. We find re-entrant behaviour in the range between 9.0 and 10.6% Zn. Data on the magneto-resistance of a sample with extremely low T_c (9% Zn, zero field resistive onset near 3.5 K) show linear normal resistance in all fields down to 1.8 K in 17.5 T. A resistively defined critical field H^*, related to H_c2, shows divergent behaviour. The temperature dependence of H^* is well represented by expressions derived for the Bose condensation of pre-formed pairs.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.