Recent studies on high-Tc superconductors have aroused new interest in tunnelling effects in unconventional superconductors. Unlike in conventional s-wave superconductors, the d-wave pairing state in these materials has an internal phase of the pair potential. The internal phase as a function of the wavevector of the Cooper pairs has a large influence on the electric properties of tunnelling junctions. Important effects of the internal phase on the Josephson current were first predicted theoretically. The idea has been established through several experiments using high-Tc Josephson junctions, which detect ?-phase shift between the a- and b-axis directions and fractional flux quanta. These results give convincing evidence for d-wave symmetry in high-Tc superconductors. In addition, the existence of new interference effects in the quasiparticle states near surfaces and boundaries has been suggested through theoretical predictions. Experimentally, a large number of tunnelling spectroscopy data showed zero-bias conductance peaks (ZBCPs), the origin of which cannot be explained in terms of the classical concept that a tunnelling conductance spectrum is a phase-insensitive probe of the electronic states. It is clarified theoretically that the observed ZBCPs reflect the formation of zero-energy states on the surface due to the ?-phase shift of internal phase in the d-wave pairing symmetry. The formulation developed for tunnelling spectroscopy suggests that tunnelling spectroscopy is essentially phase sensitive. In addition, the formation of the bound states has been shown to have a serious influence on the electrical properties of Josephson junctions. Several anomalous properties including strong enhancement of the Josephson current in the low-temperature region have been predicted theoretically. In this report, recent developments in tunnelling effects on surface bound states in unconventional superconductors are reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/63/10/202/pdf

Tunnelling effects on surface bound states in unconventional superconductors

The case for dx2 − y2 pairing in the cuprate superconductors

The paramagnetic effect found experimentally in some granular high temperature superconductors is explained by a model of a Josephson network of d -wave superconductors. The unusual microwave absorption characteristics of these systems is also explained in a consistent way. Finally, a new experiment is proposed to give a decisive test for d -wave superconductivity in high T c superconductors.

Paramagnetic Effect in High T c Superconductors -A Hint for d-Wave Superconductivity

The effects of on-site Coulomb interaction in two-dimensional organic conductors, κ-(BEDT-TTF) 2 X, α-(BEDT-TTF) 2 I 3 and (BEDT-TTF) 2 MHg(SCN) 4 , have been studied within the Hartree-Fock approximation. If the strength of Coulomb interaction exceeds some critical value, various kinds of antiferromagnetic states appear and these phases have been classified on a phase diagram with two key parameters, which leads to a unified view on the ground states of this unique family.

Phase Diagram of Two-Dimensional Organic Conductors: (BEDT-TTF) 2X

https://repository.library.northeastern.edu/files/neu:331186/fulltext.pdf

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

For certain ceramic samples of the Bi-based high-temperture superconductors, the dc-field-cooling signal becomes paramagnetic in fields below a few 100 mOe This effect correlates with an anomaly in the low-field microwave absorption The data are consistent with orbital paramagnetic moments due to the appearance of spontaneous supercurrents in fields smaller than the lower critical field ${\mathit{H}}_{\mathit{c}1\mathit{a}}$ parallel to the CuO planes

Paramagnetic Meissner effect in Bi high-temperature superconductors.

We have studied the low-field Meissner effect of polycrystalline Bi high-temperature superconductors using a special superconducting-quantum-interference-device magnetometer. In certain samples a surprising feature was observed: Instead of the usual diamagnetic moment a paramagnetic moment develops in the field cooling mode below ${\mathit{T}}_{\mathit{c}}$ for fields H1 Oe. The data are consistent with orbital paramagnetic moments due to spontaneous currents. Such currents may originate in so-called \ensuremath{\pi} contacts in the weak-link network of polycrystalline material. In some of these samples also an anomaly in the low-field microwave absorption was observed, which is obviously correlated with the existence of spontaneous currents.

Paramagnetic Meissner effect in high-temperature superconductors

We identify the preparation procedures to obtain polycrystalline pellets of Bi-2212 and of Bi-2223 high-temperature superconductors, which slow only the superconducting properties and the X-ray diffraction pattern of one or the other phase (“single-phase” pellets). These procedures were found by systematic variation of the preparation, with substitution of PbO for Bi2O3 as well as with addition of PbO to the stoichiometric mixture of starting powders. Bi-2223 in single-phase form has a TZR of 107.5 ± 0.5 K, whereas single-phase pellets of “Bi-2212” can have sharp transitions with TZR's between 55–95 K, depending on the oxygen partial pressure of the atmosphere in the final annealing step.

Preparation and characterisation of single-phase Bi-Pb-Sr-Ca-Cu-O high temperature superconductors

We present measurements of the electric resistivity, the Hall effect, the Nernst effect and the thermopower in the mixed state of Bi 1.76 Pb 0.24 Sr 2 Ca 2 Cu 3 O δ and of Tl 2 Ba 2 Ca 2 Cu 3 O δ . We find a large broadening of the resistive transition independent of the current direction with respect to the magnetic field, thermally activated behavior of the resistivity ϱ at low temperatures and a strongly non-linear dependence of ϱ on the magnetic field. The Hall coefficient R H is strongly temperature-dependent and shows a pronounced sign change in small magnetic fields. We find a large Nernst effect, from which the entropy/heat transported by the vortices is estimated. The thermopower is larger by a factor of 1000 compared to what is expected from the conventional theory of flux motion. It is remarkably similar to the resistivity.

Resistivity, Hall effect, Nernst effect and thermopower in the mixed state of Bi- and Tl-based high-Tc superconductors

By analyzing the Korringa contribution to the EPR linewidth of ${\mathrm{Gd}}^{3+}$ ions doped to high-quality ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ samples with various Sr content (x=0.08--0.35) a dependence of the density of states N(${\mathit{E}}_{\mathit{F}}$) on x has been extracted. The analysis of the observed dependence together with that for the critical temperature ${\mathit{T}}_{\mathit{c}}$(x) enabled us to study the relation between the coupling constant \ensuremath{\lambda} and N(${\mathit{E}}_{\mathit{F}}$) in course of Sr doping. We found that up to the optimum Sr concentration of x=0.15 the coupling constant \ensuremath{\lambda} increases correspondingly to increasing of the density of states. At the same time \ensuremath{\lambda} rapidly decreases in the overdoped region (xg0.15) while N(${\mathit{E}}_{\mathit{F}}$) continuously grows. The possible reasons for such a behavior are discussed.

N. Knauf

Papers

Paramagnetic Meissner effect in Bi high-temperature superconductors.

Paramagnetic Meissner effect in high-temperature superconductors

Preparation and characterisation of single-phase Bi-Pb-Sr-Ca-Cu-O high temperature superconductors

Resistivity, Hall effect, Nernst effect and thermopower in the mixed state of Bi- and Tl-based high-Tc superconductors

Coupling constant versus density of states in La2-xSrxCuO4 as revealed by EPR of Gd spin probes.