The vortex-like excitations detected by Nernst signals in high-Tc superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

Surface Superconductivity¶in Applied Magnetic Fields Above HC2

In this paper we study the surface superconductivity phenomenon of type 2 superconductors under applied magnetic fields above the critical field . We show that, for a cylindrical superconductor of infinite height placed in a homogeneous applied magnetic field, H e ₃, with H lying in between and keeping away from and , superconductivity persists uniformly at a thin sheath surrounding the entire lateral surface of the sample. As the applied field decreases gradually, superconductivity in the surface sheath becomes strong and develops into a surface superconducting state, while the interior of the sample remains close to the normal state. Received: 21 May 2001 / Accepted: 11 February 2002     

Topological properties of superconducting junctions

Motivated by recent developments in the field of one-dimensional topological superconductors, we investigate the topological properties of s-matrix of generic superconducting junctions where dimension should not play any role. We argue that for a finite junction the s-matrix is always topologically trivial. We resolve an apparent contradiction with the previous results by taking into account the low-energy resonant poles of s-matrix. Thus no common topological transition occurs in a finite junction. We reveal a transition of a different kind that concerns the configuration of the resonant poles.     

Novel route to a finite center-of-mass momentum pairing state for superconductors: A current-driven Fulde-Ferrell-Larkin-Ovchinnikov state

The previously studied Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is stabilized by a magnetic field via the Zeeman coupling in spin-singlet superconductors. Here we suggest a novel route to achieve nonzero center-of-mass momentum pairing states in superconductors with Fermi surface nesting. We investigate two-dimensional superconductors under a uniform external current, proportional to a finite pair momentum of q(e). We find that an FFLO state with a spontaneous pair momentum of q(s) is stabilized above a certain critical current that depends on the direction of the external current. A finite q(s) arises in order to make the total pair-momentum of q(t)(=q(s) + q(e)) perpendicular to the nesting vector, which is independent of spin states of Cooper pairs. We also discuss experimental signatures of the FFLO state.     

De Haas-van Alphen effect in unconventional superconductors

A theory of the de Haas-van Alphen effect in type-II p-wave and D-wave superconductors (the latter corresponds to the B _1g one-dimensional representation of group D _4h ) has been developed. Solutions for the order parameter and density of quasiparticle states near the upper critical field have been calculated. If the curve enclosing the extremal cross section of the Fermi surface in the plane perpendicular to the external magnetic field coincides with the line of nodes of the superconducting order parameter, the effect of the transition to the superconducting state on the amplitude of magnetization oscillations is negligible. If the line of nodes is oriented differently with respect to the applied magnetic field, the de Haas-van Alphen oscillations are suppressed in a manner qualitatively similar to the case of conventional superconductors. Zh. éksp. Teor. Fiz. 113, 2174–2192 (June 1998)     

Gapless Fermi surfaces of anisotropic multiband superconductors in a magnetic field

We propose that a new state with a fully gapless Fermi surface appears in quasi-2D multiband superconductors in magnetic field applied parallel to the plane. It is characterized by a paramagnetic moment caused by a finite density of states on the open Fermi surface. We calculate thermodynamic and magnetic properties of the gapless state for both s-wave and d-wave cases, and discuss the details of the first order metamagnetic phase transition that accompanies the appearance of the new phase in s-wave superconductors. We suggest possible experiments to detect this state both in the s-wave (2-H NbSe2) and d-wave (CeCoIn5) superconductors.     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Very high upper critical fields of F-doped Fe-based layered superconductors NdO0.88F0.12FeAs and CeO0.88F0.12FeAs

Using the solid state reaction method, we have synthesized the polycrystalline F-doped NdO_0.88F_0.12FeAs and CeO_0.88F_0.12FeAs with the superconducting transition temperatures at about 48 and 40 K, respectively. To obtain the upper critical field H _c2 of Nd(Ce)O_0.88F_0.12FeAs samples, we measured the electrical resistivity under magnetic field up to 14 T. Based on the Werthamer-Helfand-Hohenberg (WHH) relation together with the H _c2(T) curves in a relatively high field, we estimated that these superconductors have a rather high upper critical field of about 115 T for Nd-based and 107 T for Ce-based samples, indicating the similarities between these ReO_1−x F _x FeAs (Re = rare earth element) superconductors and high T _c cuprate superconductors. Recommended by Prof. Nie Yuxin, Executive Editor of Science in China Series G-Physics, Mechanics & Astronomy Supported by the National Basic Research Program of China (973 Program)(Grant No. 2006CB9213001) and the National Natural Science Foundation of China (Grant No. 10774181)     

Self-consistent theory of pairbreaking in kondo superconductors

A theory is presented for the calculation of equilibrium properties of superconductors containing magnetic impuritiesof finite concentration. The theory is based on the Nagaoka-Suhl approach to thes — d model and treats finite impurity concentrations within a self-consistentT-matrix-approximation. An integral equation is derived for the pairbreaking parameter which describes de-pairing of Cooper pairs by spin flip scattering. Numerical solutions of this equation are used to compute the phase diagram in the temperature-concentration-plane and the transition-temperature-dependence of the upper critical field. These calculations are in good agreement with experimental data on LaCeAl₂-samples. In addition, the recent discovery of three normal superconducting transitions in LaCeY is discussed quantitatively.Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Jülich/Köln     

Electronic and electron-phonon phenomena in low-dimensional BEDT-TTF-based organic conductors and superconductors

Two related groups of k-phase ion-radical salts (BEDT-TTF) with different electrical properties, namely, superconductors with different transition temperatures and conductors, which transfer to insulating state with decreasing temperature, have been studied by micro-optic spectroscopy. Polarized reflectance spectra of microcrystals have been measured for the three principal crystallographic directions within the 700–40000 cm⁻¹ region, and the corresponding spectra of the optical functions obtained. The anisotropy of the electronic system in the crystals has been established as two-dimensional. The spectra obtained were quantitatively analyzed, the key parameters of the electronic structure and the vibronic coupling constants determined. It is concluded that the conductors have smaller vibronic coupling constants, more narrow allowed electronic bands, and stronger electron-electron interaction compared to those of the superconductors, and that vibronic coupling is the necessary condition for the onset of superconductivity in the superconductors studied. Fiz. Tverd. Tela (St. Petersburg) 41, 897–899 (May 1999)     

Flux pinning by a barrier on the surface of YBaCuO grains studied in dynamic regime

The model of a surface barrier was found to explain well the observed behaviour in quasi-static magnetization experiments on highT_csuperconductors. We study this phenomenon in dynamic conditions in an AC magnetic field with frequencies ranging from 3 Hz up to 115 kHz. The experimental data recorded on a polycrystalline YBaCuO sample are explained by a model supposing the existence of an intergrain critical state and the barrier on the surface of the grains. The most striking feature we found is the decrease of the surface barrier with increasing frequency.     

Effect of transparency on Josephson junction between s+g-wave superconductors

In this paper, a dc Josephson junction between borocarbide superconductors has been studied theoretically. The s+g-wave pairing symmetry which is observed in rare earth complex of borocarbides has a huge anisotropy and is an interesting form of unconventional superconductivity. We calculate the Josephson current in a superconductor--insulator--superconductor (SIS) Josephson junction with s+g-wave superconducting pairing symmetry. In our planar junction c-axis is parallel to an interface with finite transparency but ab-planes of two tetragonal superconductors are misorientated by angle α. We obtain that the Josephson current is strongly dependent on mis-orientation between the left and the right ab-planes. An insulator sandwiched between two superconductors which acts as a potential barrier is demonstrated by a transparency coefficient. The effects of the potential barrier and the mis-orientation on the current are studied analytically and numerically. Occurrence of 0--π transition in this s+g-wave junction is investigated in this paper. A comparison between d-wave Josephson junction and s+g-wave one is also made in the present paper.     

Magnetization studies in niobium and YBa2Cu3O7 samples

The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−H _max to +H _max) and reverse (+H _max to −H _max) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asH _peak,H _I,H _II etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (M _rem(H)−M _FC(H)+M _ZFC(H)) being equal to, less than or greater than zero, whereM _rem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.     

The flux distribution and hysteresis losses in highT c superconductors including viscous forces on flux lines

By comparing the contributions of pinning and viscous forces to the restoring force on flux ines in type II and highT _c superconductors, it is shown that the flux flow in highT _c superconducto rs should play an important role in determining the magnetic flux distribution and hence the hyste resis losses in a.c. fields. Both quantities are calculated in the extreme case of very large viscous forces with respect to the pinning force. The magnetic field and frequency dependence of the losses are changed with respect to the results obtained from the critical state model. The theoretical results are qualitatively confirmed by a.c. susceptibility measurements at different magnetic field amplitudes and frequencies. The quantitative differences indicate that the flux flow effects in highT _c superconductors are by far not so strong as expected and supposed by some theories. The reasons for these discrepancies are discussed.     

Magnetic flux structures of composite superconducting structures with d- and s-waves superconductors (d-dots)

Composite superconducting structures with d- and s-wave superconductors, d-dots, can be used as two state devices. Their functions depend on structures of the spontaneous magnetic field, which appears because of the anisotropy of d-wave superconductivity. Solving two-components Ginzburg–Landau equation, we have investigated magnetic field structures for d-dots with smaller and larger holes around the corners of d-wave superconducting region. And we argued the effect of holes on the magnetic structures.     

Superconductivity in weakly correlated electron systems

We study the influence of the short-ranged Hubbard correlation U between the conduction electrons on the Cooper pair formation in normal (s-wave) superconductors. The Coulomb correlation is considered within the standard second order perturbation theory, which becomes exact in the weak coupling limit but goes beyond the simple Hartree-Fock treatment by yielding a finite lifetime of the quasiparticles at finite temperature. An attractive pairing interaction V, which may be mediated by the standard electron-phonon mechanism, is considered between nearest neighbor sites. A critical value for the attractive interaction is required to obtain a superconducting state. For finite temperature a gapless superconductivity is obtained due to the finite lifetime of the quasiparticles, i.e. the Coulomb correlation has a pair-breaking influence. The energy gap and depend very sensitively on U, V and band filling n and develop a maximum away from half filling as function of n. The ratio varies with n, being higher than the BCS value near half filling and reaching the BCS value for lower n. Received 17 February 1999     

Experimental observation of the finite relaxation time at the zero field transition point of magnetically impure superconductors

From the measurements of surface microwave resistance of superconductors in the vortex state, we have observed that the relaxation time of the order parameter remains finite at the zero field phase transition point for magnetically impure superconductors. Experimental data are well explained by the multiple pair breaking theory of Fulde and Maki, and the flux flow resistivity calculation of Baba and Maki.     

A thermodynamic description of the irreversibility line in strongly type-II superconductors

The one-component dense vortex plasma theory, which describes thermal motion of the magnetic vortices in strongly type-II superconductors in a magnetic field, is used to study the irreversibility line in high-T _c superconductors. We propose a possible universal form for the irreversibility line, and examine some recent experiments.