Pair-breaking Effect Research Articles

Effect of controlled disorder on thermal conductivity of Bi2212

In-plane thermal conductivity of optimally-doped Bi2212 single crystals were studied before and after irradiation by high-energy electrons. Introduction of point defects by this procedure decreases the quasi-particle mean-freepath and weakens the superconductivity due to pair-breaking effects. We report on thermal conductivity data on a wide temperature range (100 mK to 100 K) performed in parallel with resistivity and thermpower at higher temperatures. The introduction of disorder leads to a decrease in thermal conductivity in the whole explored range but leaves the zero-temperature linear term, associated with nodal quasiparticles, virtually unchanged.

Transport mechanisms in Zn, Ni and Mg doped La 1.85Sr 0.15CuO 4+δ systems

Transport mechanisms have been comparatively studied for La 1.85Sr 0.15Cu 1- x M x O 4- δ (M=Zn, Ni, Mg) systems with a wide dopant range (0⩽ x⩽0.3)by means of the resistivity. The analysis indicates that the normal-state transport properties can be well interpreted based on the localization mechanism. The suppressions of the superconductivity are closely related to magnetic pair-breaking effects induced by Zn, Ni and Mg ions.

Interplay between magnetism and superconductivity in Nb/Co multilayers

We have used torque magnetometry to investigate the influence of magnetic spacer layers on the superconductivity in Nb/Co multilayers. Compared to similar systems which have nonmagnetic spacer layers, the samples exhibit two dimensional superconductivity for relatively thin magnetic layer thicknesses ${d}_{\mathrm{Co}},$ reflecting the effects of magnetic pair breaking. Moreover, the transition temperature to the superconducting state ${T}_{c}$ shows a nonmonatonic dependence on ${d}_{\mathrm{Co}},$ which we examine within the framework of recent theories.

Enhancement from surface pair breaking of nonequilibrium Josephson current

The combined effect of surface pair breaking, barrier transmission and the phase difference on quasiparticle interface Andreev bound states in junctions between d-wave superconductors is examined. An approach, based on the quasiclassical theory of superconductivity, is developed for describing bound states. Zaitsev's boundary conditions at a bound state energy simplify and admit new analytical results. Interface bound states for quasiparticle trajectories along the interface normal turn out to split in the tunneling limit on account of both the phase difference and surface pair breaking. With nonequilibrium occupation of the split bound states, the Josephson current in a tunnel junction can significantly exceed its conventional value.

Flux structures in two-dimensional superconductors for nearly plane–parallel magnetic field

The recent discovery of several classes of layered superconducting compounds such as cuprate compounds, organic superconductors, and ruthenate–cuprate compounds has renewed the interest in an experimental verification of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. For an external magnetic field nearly parallel to the layers one has competiton between spin and orbital pair-breaking effects (between FFLO and vortex state). This leads to superconducting states with pairing in Landau levels n>0. Recently, a nonlinear expansion of the quasiclassical equations has been performed in order to find the stable flux structures below the upper critical field. Here, the states with Landau quantum numbers n=1, 3 , the so-called “quasi-one-dimensional” states, are discussed in detail.

Comment on “Electrical transport in junctions between unconventional superconductors: Application of the Green’s-function formalism”

Effects of surface pair breaking, entirely neglected by M. P. Samanta and S. Datta [Phys. Rev. B $57,$ 10 972 (1998)], is quite important in considering surface (or interface) quasiparticle bound states and associated characteristics of junctions involving unconventional superconductors. The whole class of bound states with nonzero energy is simply omitted within the framework of the approach, using uniform spatial profile of the order parameter up to the interface. The contribution of these bound states (as well as midgap states) to current-voltage characteristics of the SIS tunnel junctions were studied in our earlier article. Dependence of midgap state contribution to the Josephson critical current upon crystal to interface orientations is shown as well to be fairly sensitive to the effect of surface pair breaking.

Anomalous magnetic properties of Heusler superconductor YbPd 2Sn

We have investigated the magnetic properties of the Heusler compound YbPd 2Sn, which exhibits coexistence of superconductivity ( T c=2.3 K) and antiferromagnetism ( T N=0.22 K). In the temperature dependence of the specific heat in applied magnetic fields, we found an anomalous broadening of the Schottky peak caused by the Zeeman splitting of the Γ 7 crystalline–electric-field (CEF) ground state of the Yb 3+ ions. This fact indicates the existence of antiferromagnetic (AF) short-range correlations, which are hardly destroyed by magnetic field. An anomalous temperature dependence of the upper critical field B c 2 with a maximum around 0.8 K can be explained by taking into account the pair breaking effect of Cooper pairs caused by the Yb-moment fluctuation through an exchange interaction. We propose that a weak magnetic coupling between the cubic Pd sublattice responsible for superconductivity and the NaCl-type Yb–Sn sublattice responsible for the AF magnetic ordering play an essential role for the coexistence.

Coexistence of Singlet and Triplet Attractive Channels in the Pairing Interactions Mediated by Antiferromagnetic Fluctuations

We propose a phase diagram of quasi-low-dimensional type II superconductors in parallel magnetic fields, when antiferromagnetic fluctuations contribute to the pairing interactions. We point out that pairing interactions mediated by antiferromagnetic fluctuations necessarily include both singlet channels and triplet channels as attractive interactions. Usually, a singlet pairing is favored at zero field, but a triplet pairing occurs at high fields where the singlet pairing is suppressed by the Pauli paramagnetic pair-breaking effect. As a result, the critical field increases divergently at low temperatures. A possible relation to experimental phase diagrams of a quasi-one-dimensional organic superconductor is briefly discussed. We also discuss a possibility that a triplet superconductivity is observed even at zero field.

Magnetic scattering and unusual behavior of the critical fields and other parameters: Application to borocarbides

The pair-breaking effect and its impact on the properties of borocarbides is studied. The pair-breaking effect caused by localized magnetic moments drastically affects the superconducting properties. Interaction between the magnetic moments and the resulting ordering trend lead to a behavior entirely different from the conventional picture. The main focus is on the behavior of the upper ( H c2 ) and lower ( H c1 ) critical fields. In addition, the temperature dependence of several quantities (penetration depth, coherence length, etc.) has been calculated. The theory has been applied to the borocarbide LuNi 2 B 2 C and is in very good agreement with the recent experimental data.

Organic superconductors in the Pauli paramagnetic limit region

The low-dimensionality of the organic superconductors leads the pronounced anisotropy in the upper critical field. In the vicinity of the field direction parallel to the superconducting plane, the critical field shoots out due to the suppression of the orbital pair-breaking effect and is determined by the spin magnetic effect. The organic superconductors are suitable for the study of the high field state related to the spin effect under aligned field, since they are of high crystalline quality. The reported experimental results covering the behavior at low temperature are reviewed first and the breakthrough of the BCS Pauli paramagnetic limit is argued. The in-plane anisotropy of the upper critical field is also touched upon.

Quasiparticle interface states in junctions involvingd-wave superconductors

Influence of surface pair breaking, barrier transmission and phase difference on quasiparticle bound states in junctions with d-wave superconductors is examined. Based on the quasiclassical theory of superconductivity, an approach is developed to handle interface bound states. It is shown in SIS' junctions that low energy bound states get their energies reduced by surface pair breaking, which can be taken into account by introducing an effective order parameter for each superconductor at the junction barrier. More interestingly, for the interface bound states near the continuous spectrum the effect of surface pair breaking may result in a splitting of the bound states. In the tunneling limit this can lead to a square root dependence of a nonequilibrium Josephson current on the barrier transmision, which means an enhancement as compared to the conventional critical current linear in the transmission. Reduced broadening of bound states in NIS junctions due to surface pair breaking is found.

Organic Superconductors as Stages for Exploration of Superconductivity in High Magnetic Fields

The low dimensionality of organic superconductors leads to pronounced anisotropy in the upper critical field. In the vicinity of the field direction parallel to the superconducting plane, the critical field shoots out due to the suppression of the orbital pair-breaking effect. Organic superconductors are suitable for the study of the high-field state related to the spin effect under an aligned field, since they are of a high crystalline quality. The reported experimental results covering the behavior at low temperatures are reviewed first, and the breakthrough of the BCS Pauli paramagnetic limit is discussed. The potential of the low-dimensional organic superconductor for study of the effect of electronic spectrum quantization is argued.

Impurities in d-Wave Superconductors: Effect of Singular Density of States

We analyzed the effect of nonmagnetic impurities in d-wave superconductors with an energy dependent density of states. Two singular forms for the density of states have been considered. The strength of singularity affect the degradation of the critical temperature and reduces the pair-breaking effect.

Anisotropic impurities in anisotropic superconductors

The physical properties of anisotropic superconductors such as the critical temperature and other important properties depend sensitively on the electron mean free path. The sensitivity to impurity scattering and the resulting anomalies are considered a characteristic feature of strongly anisotropic pairing. These anomalies are usually analyzed in terms of s-wave impurity scattering which leads to universal pair-breaking effects depending on only two scattering parameters, the mean free path and the impurity cross section. We investigate here corrections coming from anisotropies in the scattering cross section, and find not only quantitative but also qualitative deviations from universal s-wave isotropic pair breaking. The properties we study are the transition temperature, quasiparticle bound states at impurities, and pinning of single- and double-flux lines by impurities.

Vertex corrections in the gap equation for high-temperature superconductors

We show that the Migdal theorem is obviously violated in the high Tc cuprates and the vertex correction should be included, in particular, in the gap equation, in order to be consistent with the anomalously strong inelastic scattering in the ``hot spots'', which is observed from the various normal state experiments. The vertex correction is obtained by utilizing the generalized Ward identity, which is shown to hold in the important scattering channel for the pairing interaction in the high Tc cuprates. As a result, we find a strong enhancement of Tc from the vertex correction despite of the strong pair breaking effect due to the inelastic scattering.

Influence of magnetic order in the superconducting properties of Nb/Fe/Cu multilayers

The superconducting properties of a Nb film can be strongly affected by a ferromagnetic layer in contact with it due to the magnetic proximity effect. In order to study the efficiency of this pair-breaking effect as a function of the magnetic order present in the ferromagnetic layer, [Fe/Cu]/Nb multilayers have been grown by DC sputtering and MBE. As the Fe layer thickness is varied in the range 0–20 Å, a phase transition is induced from FCC γ-Fe to BCC α-Fe so that the Fe Curie temperature is abruptly increased. An enhancement in the superconducting transition temperature of the Nb film is found for the samples with BCC α-Fe in comparison with those with FCC γ-Fe. These results imply a more efficient proximity effect for the material with the lower Curie temperature.

Suppression of superconductivity inSr2RuO4caused by defects

We have investigated depairing effects in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4},$ the unconventional superconductor in the layered perovskite structure. We prepared crystals of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ with very low levels of impurity elements, and systematically controled their superconducting transition temperature ${T}_{c}$ ranging from 1.5 to 0.6 K by adjustments of crystal growth conditions. The dependence of ${T}_{c}$ on the residual resistivity ${\ensuremath{\rho}}_{0}$ in these crystals suggests that the defects are strong pair breakers, in addition to impurities. We further characterized the effects of pair breaking in this unconventional superconductor. We found that the in-plane coherence length ${\ensuremath{\xi}}_{\mathrm{ab}}(0)$ evaluated from ${H}_{c2}(T)$ is inversely proportional to ${T}_{c},$ and decreases with increasing mean free path l; the latter behavior is opposite to that of conventional superconductors. In addition, we examined the temperature dependence of ${H}_{c2\ensuremath{\parallel}c}$ which substantially deviates from the BCS theory and even from a recent theory of a p-wave superconductor.

Microwave properties of RE–Ni 2B 2C (RE=Y, Er) superconducting thin films

A comparative study of the microwave properties of YNi 2B 2C and ErNi 2B 2C superconducting films is reported. The measurements were performed using a microstrip resonator ( f=2 GHz) and a microwave cavity ( f=87 GHz). The analysis of the temperature dependence of the surface impedance Z s confirms that YNi 2B 2C compound is a conventional BCS (s-wave) superconductor. On the contrary, ErNi 2B 2C samples show an anomalous behaviour in both the surface resistance and penetration depth. This is tentatively ascribed to pair breaking effects on the superconducting density of states. A simplified generalisation of the Abrikosov–Gorkov theory, proposed by Machida, is successfully applied to describe the penetration depth data in the antiferromagnetic compound, while for the surface resistance granular effects cannot be ruled out.

Experimental investigation of a local mixed state induced by a small ferroparticle in YBaCuO films

Direct local measurements of the characteristics of a mixed state induced by a magnetic field of a small ferromagnetic particle have been carried out in thin film YBaCuO samples, using a scanning Hall probe. The results provide evidence for an extremely low energy barrier for entry of vortex half-loops into high quality films and further breaking of these half-loops into vortex-antivortex pairs. The observed phenomena are consistent with the Josephson medium model. We obtained the temperature dependence of the intergrain critical current density, which points to strong pair breaking effects at the grain boundaries.

The peierls transition in per 2[au xpt 1− x(mnt) 2]: pair-breaking field effects

Microwave experiments have been carried out on the two-chain organic compounds Per 2[Au x Pt 1− x (mnt) 2] (x=0;0.02; 1) as a function of temperature and magnetic field. A theoretical mean-field theory is proposed to show how pair-breaking effects due to either alloying, intrinsic disorder or localized spins modify the variation of the Peierls critical temperature of the Perylene stacks with the magnetic field.