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The irreversibility line of superconductors is most usually established from magnetization curves. However, many low-${T}_{c}$ materials show extremely reversible magnetization curves, while still having a finite critical current. Confirmation of a reversibility line requires other measurements. We have made measurements of dc magnetization, ac susceptibility, and magnetoresistivity as a function of applied field and temperature on Nb alloy samples in order to investigate the irreversibility line in low-${T}_{c}$ superconductors. The results show that there exists an observable field region below the mean-field critical field ${B}_{c2},$ where the magnetization is reversible during a cycle of increasing and decreasing field, which is in agreement with a previous report by Suenaga et al. In addition to dc magnetization, ac susceptibility and magnetoresistivity measurements were also carried out on the same sample as alternative techniques to probe the irreversibility line to determine the best way of distinguishing a genuine thermally activated reversibility from a finite, but low, critical current density. The results showed that the collapse of the dc magnetic hysteresis, the onset of the diamagnetic ac susceptibility (or the peak of the ac loss) and the zero resistance occur at nearly the same field, namely, the irreversibility field ${B}_{\mathrm{irr}}.$ These experimental observations indicate that the irreversibility line is not unique to high-${T}_{c}$ oxides but also exists in conventional superconducting metallic alloys although much closer to ${B}_{c2}.$ However, it is difficult to reconcile these results with measurements on other low-${T}_{c}$ materials which show zero resistance up to the surface critical field ${B}_{c3}.$

The dc magnetization of a superconducting ${\mathrm{BaPb}}_{0.75}{\mathrm{Bi}}_{0.25}{\mathrm{O}}_{3}$ sample has been measured as a function of magnetic field and temperature. The results showed that an irreversibility line lies considerably below the superconducting normal phase transition boundary ${B}_{c2}(T),$ similar to that commonly observed in high-temperature superconductors, even though ${\mathrm{BaPb}}_{0.75}{\mathrm{Bi}}_{0.25}{\mathrm{O}}_{3}$ displays a very long coherence length. We attributed the observed large magnetically reversible region to a very small thermodynamic field, which leads to low condensation energies. The reversible magnetization data obtained in the region above the irreversibility line enable us to determine a full set of superconducting parameters for the material.

Magnetic measurements have been carried out on the hot-isostatically-pressed Chevrel-phase superconductor ${\mathrm{PbMo}}_{6}$${\mathrm{S}}_{8}$ at temperatures from 4.2 K to ${\mathit{T}}_{\mathit{c}}$ and for magnetic fields up to 12 T. The results show that for the ${\mathrm{PbMo}}_{6}$${\mathrm{S}}_{8}$ compound there is a wide magnetically reversible region, between the irreversibility field ${\mathit{B}}_{\mathrm{}\mathrm{irr}}$ and the upper critical field ${\mathit{B}}_{\mathit{c}2}$, on the isothermal magnetic hysteresis curves. The ${\mathit{B}}_{\mathrm{irr}}$(T) line, i.e., the irreversibility line, was found to obey a power-law expression: ${\mathit{B}}_{\mathrm{irr}}$=${\mathit{B}}^{\mathrm{*}}$(1-T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathrm{\ensuremath{\alpha}}}$ with \ensuremath{\alpha}\ensuremath{\approxeq}1.5. Magnetic relaxation measurements revealed that the flux-creep effect in the material studied is substantial and is greater than those observed in conventional metallic alloys, but smaller than in high-temperature superconductors. The existence of the irreversibility line and pronounced flux-creep effect in ${\mathrm{PbMo}}_{6}$${\mathrm{S}}_{8}$ is attributed to the short coherence length of the material. From the reversible magnetization data, the values of the penetration depth, the coherence length, and the critical fields are obtained together with the Ginzburg-Landau parameter \ensuremath{\kappa}. At 4.2 K, the critical current density ${\mathit{J}}_{\mathit{c}}$ is ${10}^{9}$ A ${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$ at zero field, and decreases to 2\ifmmode\times\else\texttimes\fi{}${10}^{8}$ A ${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$ at 10 T. Pinning force curves measured at different temperatures obey a Kramer-scaling law of the form: ${\mathit{F}}_{\mathit{p}}$(=${\mathit{J}}_{\mathit{c}}$\ifmmode\times\else\texttimes\fi{}B)\ensuremath{\propto}${\mathit{b}}^{1/2}$(1-b${)}^{2}$, which indicates that the ${\mathit{J}}_{\mathit{c}}$ is limited by one predominant flux-pinning mechanism.

We present a tunneling study of the superconducting gap and pseudogap from near optimally doped submicron Bi${}_{2}$Sr${}_{2\ensuremath{-}x}$La${}_{x}$CuO${}_{6+\ensuremath{\delta}}$ intrinsic Josephson junctions whose self-heating is significantly suppressed. We find that as temperature decreases a pseudogap develops around ${T}^{\ensuremath{\star}}$ $=$ 165 K while the superconducting gap starts to open at ${T}_{c0}$ $=$ 40 K, which is well above the superconducting transition temperature ${T}_{c}$ of 30 K. We discuss the temperature-dependent properties of the superconducting phase and pseudogap phase. Our results demonstrate a clear two-gap feature of the system with precursor pairing up to ${T}_{c0}$.

We use interfacial transport and transmission electron microscopy to study the YBa2Cu3O7-delta(YBCO)/La0.67Ca0.33MnO3(LCMO) junction that form a d(x)2(-y)2-wave superconductor and a ferromagnet (FM) heterostructure. Zero bias conductance peaks (ZBCPs) caused by Andreev reflection at zero bias were observed in the interfacial differential conductance spectra for the sample with an orderly mixed c- and a-axis [with (001) and (100)/(110)] orientation of YBCO in the interface. When the YBCO film is oriented with the c-axis normal to the interface, a zero bias conductance dip (ZBCD) is observed. These results evidence the existence of surface Andreev bound states (midgap states) in the {110} surface of YBCO, which can lead ZBCP to appear in all directions except the {100} surface of YBCO where ZBCD occurs. Furthermore, the absence of any obvious influence of spin-polarization of LCMO on ZBCP in zero magnetic field implies that the LCMO is not half-metallic in the interface of YBCO/LCMO, while strong magnetic field dependence of ZBCP shows that the applied magnetic field enhances the spin polarization of LCMO and suppresses ZBCPs in YBCO.

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with a significant change in interboron layer distance as reported previously by Slusky Moreover, transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is probably related to the structural transition. The modifications of the superconducting transition temperature T-c, the normal-state resistivity, and the upper critical field B-c2(0) by Al doping are discussed in terms of Al-substitution-induced changes in the electronic structure at the Fermi energy.