Synthesis and investigation of reentrant oxide superconductors

The conditions for the occurrence of reentrant superconductivity in granular superconductors are studied. The parameters of synthesis of the ceramic samples of CaLaBaCu₃O _x , SrLaBaCu₃O _x and YBa_2–x Sr _x Cu₃O _y Se _z compositions, the results of their microstructure research, the elemental composition and the temperature dependence of the resistance are given. Samples are obtained, which reproducibly exhibit the reentrant superconductivity after the tens of thermal cycling. The relationship between the occurrence of reentrant superconductivity and the oxygen content in the samples was not established.     

Paramagnetism and reentrant behavior in quasi-one-dimensional superconductors at high magnetic fields

The thermodynamics of quasi-one-dimensional superconductors in the presence of large magnetic fields is studied. When the quantum effects of the magnetic field are taken into account, several reentrant superconducting phases persist at very high fields. In the last reentrant phase the free-energy change, the specific-heat jump and the excess magnetization are estimated near the critical temperature. In particular, the excess magnetization is found to be paramagnetic as opposed to diamagnetic in weak fields and its sign is controlled by the slope of H_c2(T). We further generalize this result to the entire phase diagram (including all quantum phases) and to different physical systems using general thermodynamic relations which show that the sign of the excess magnetization ΔM of the superconducting state near H_c2(T) follows dH_c2(T)/dT. These relations provide a scenario for the evolution of the sign of ΔM from weak fields to strong fields.     

Doping dependent tunneling conductance in SDW ordered copper oxide superconductors

It is observed that doping suppresses the long range anti-ferromagnetic order and induces superconducting phase for a suitable doping. In order to study this effect, we present a model study of the doping dependence of the tunneling conductance in high-T_c systems. The system is described by the Hamiltonian consisting of spin density wave (SDW) and s-wave type superconducting interaction in presence of varying impurity concentrations. The gap equations are calculated by using Green’s functions technique of Zubarev. The gap equations and the chemical potential are solved self-consistently. The imaginary part of the electron Green’s functions shows the quasi-particle density of states which represent the tunneling conductance observed by the scanning tunneling microscopy (STM). We investigate the effect of impurity on the gap equations as well as on the tunneling conductance. The results will be discussed based on the experimental observations.     

Role of plasmons in high transition temperature copper oxide superconductors

We present a model calculation of the effective interaction between the charge carriers in YBa₂Cu₃O_7−δ and other related copper oxide superconductors. Our calculation shows that the plasmons (electronic excitations) play a dominant role in producing high superconducting transition temperatures in these materials.     

Effect of anisotropy in the Abrikosov phase of copper oxide superconductors

A flux liquid serves as a model for the Abrikosov phase of a type II superconductor with short coherence length and strong fluctuations. Here we discuss the effect of anisotropy due to the planes in the material and disorder due to impurities in terms of the density-density correlation function. We find that both effects change the qualitative behavior of the correlation lengths.     

NMR studies of the superconducting state of copper oxide superconductors

The authors review and up-date their work on Knight shifts, spin-lattice relaxation, and indirect nuclear spin-spin coupling for YBa₂Cu₃O₇ in the superconducting state. The data are analyzed in particular to show what it may indicate about the orbital and spin pairing of the superconducting state.     

Self-heating-induced phenomena in long reentrant ferromagnetic thin film superconductors

We predict several novel self-heating-induced features of the current-voltage characteristics at bath temperatures below the lower critical temperature as well as the temperature dependence of the various critical currents of long reentrant ferromagnetic thin film superconductors on the basis of existing theories and available experimental results. Further we point out how the experimental investigation of the above can lead to a better understanding of reentrant superconductivity and magnetic Kapitza resistance.     

Pseudogap formation in copper oxide superconductors with inclusion of spin and charge fluctuations

The influence of spin and charge fluctuations on the pseudogap formation in cuprate superconductors has been studied using the diagram technique for Hubbard operators. It has been shown that the joint inclusion of the spin and charge fluctuations leads to the formation of “shadow” bands with a strong modulation of the spectral intensity and to a decrease in the density of electronic states at the Fermi level.