Pair-breaking Effect Research Articles

Pair breaking in rotating Fermi gases

We study the pair-breaking effect of rotation on a cold Fermi gas in the BCS-BEC crossover region. In the framework of BCS theory, which is supposed to be qualitatively correct at zero temperature, we find that in a trap rotating around a symmetry axis, three regions have to be distinguished: (A) a region near the rotational axis where the superfluid stays at rest and where no pairs are broken, (B) a region where the pairs are progressively broken with increasing distance from the rotational axis, resulting in an increasing rotational current, and (C) a normal-fluid region where all pairs are broken and which rotates like a rigid body. Due to region B, density and current do not exhibit any discontinuities.

Study of superconducting gap in presence of lattice strain

A theoretical model for the band Jahn-Teller (BJT) distortion versus s -wave superconductivity competition is proposed for the hole and electron doped cuprates like R 2− x M x CuO 4 (R=Nd, La,… and M=Sr,Ce) in presence of rare-earth impurity f level electron and hybridization using Zubarev’s technique. The temperature dependent superconducting gap equation, the mixed pairing parameter and the Kondo excitations are analyzed microscopically. The lattice strain shows spontaneous suppression at temperatures below the superconducting transition temperature. The superconducting pair breaking effect is explained from the self-consistent model calculations.

Comparing the physical properties of Pr/Gd and Pr/Ce substitutions in Ru(Gd1.5Ce0.5)Sr2Cu2O10−δ

We have compared the electrical and magnetic properties of Ru(Gd1.5−xPrx)Ce0.5Sr2Cu2O10−δ (Pr/Gd samples) with x = 0.0, 0.01, 0.03, 0.033, 0.035, 0.04, 0.05, 0.06, 0.1 and RuGd1.5(Ce0.5−x Prx) Sr2 Cu2O10−δ (Pr/Ce samples) with x = 0.0, 0.01, 0.03, 0.05, 0.08, 0.1, 0.15, 0.2 prepared by the standard solid-state reaction technique. We obtained the XRD patterns for different samples with various x. The lattice parameters versus x for different substitutions have been obtained from Rietveld analysis. To determine how the magnetic and superconducting properties of these layered cuprate systems can be affected by Pr substitution, the resistivity, and magnetoresistivity, with Hext varying from 0.0 to 15 kOe, have been measured at various temperatures. Superconducting transition temperature Tc and magnetic transition Tirr, have been obtained through resistivity and ac susceptibility measurements. The Tc suppression due to Pr/Gd and Pr/Ce substitutions show competition between pair breaking by magnetic impurities, hole doping due to different valances of ions, difference in ionic radii, and oxygen stoichiometry. Pr/Gd substitution suppresses superconductivity more rapidly than for Pr/Ce, showing that the effect of hole doping and magnetic impurity pair breaking is stronger than the difference in ionic radii. In Pr/Gd substitution, the small difference between the ionic radii of Pr3+,4+ and Gd3+, and absorption of more oxygen due to the higher valence of Pr with respect to Gd, decreases the mean Ru-Ru distance, and as a result, the magnetic exchange interaction becomes stronger with the increase of x. However, Pr/Ce substitution has the opposite effect. The magnetic parameters such as Hc, obtained through magnetization measurements versus applied magnetic field isotherm at 77 K and room temperatures, become stronger with x in Pr/Gd and weaker with x in Pr/Ce substitution.

Field Dependence of π-Band Superconducting Gap in MgB2 Thin Films from Point-Contact Spectroscopy

We present the results of point-contact spectroscopy measurements on high-quality epitaxial MgB2 thin films with injected current along the c-axis. The temperature and field dependences of π-band properties with the field parallel to (H‖) or perpendicular to (H⊥) the c-axis are investigated in detail. When a magnetic field is applied, either parallel or perpendicular to the c-axis, the density of the quasiparticle state (DOS) of the π-band proliferates quickly with increasing field, while the gap amplitude of the π-band decreases slowly, which is different from the recent theoretical calculations, showing a field dependent competition between the interband scattering and the pair-breaking effects.

Magnetic-Field Variations of the Pair-Breaking Effects of Superconductivity in (TMTSF)2ClO4

We have studied the onset temperature of the superconductivity Tc_onset of the organic superconductor (TMTSF)2ClO4, by precisely controlling the direction of the magnetic field H. We compare the results of two samples with nearly the same onset temperature but with different scattering relaxation time tau. We revealed a complicated interplay of a variety of pair-breaking effects and mechanisms that overcome these pair-breaking effects. In low fields, the linear temperature dependences of the onset curves in the H-T phase diagrams are governed by the orbital pair-breaking effect. The dips in the in-plane field-angle phi dependence of Tc_onset, which were only observed in the long-tau sample, provides definitive evidence that the field-induced dimensional crossover enhances the superconductivity if the field direction is more than about 19-degrees away from the a axis. In the high-field regime for H//a, the upturn of the onset curve for the long-tau sample indicates a new superconducting state that overcomes the Pauli pair-breaking effect but is easily suppressed by impurity scatterings. The Pauli effect is also overcome for H//b' by a realization of another state for which the maximum of Tc_onset(phi) occurs in a direction different from the crystalline axes. The effect on Tc_onset of tilting the applied field out of the conductive plane suggests that the Pauli effect plays a significant role in determining Tc_onset. The most plausible explanation of these results is that (TMTSF)2ClO4 is a singlet superconductor and exhibits Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in high fields.

Effect of screening current induced pair breaking on magnetization of superconducting lead nanoparticles

Magnetization measurements on lead nanoparticles in the size range 35–45 nm are presented. It is shown that the critical fields in these nanoparticles are enhanced significantly above their bulk values with temperature dependence also distinct from that of bulk. The observed “type II” like shape of the magnetization curves is explained on the basis of the Ginzburg–Landau phenomenology by invoking the pair breaking effect of the London screening currents, which makes the effective penetration depth an increasing function of the field. The temperature dependence of critical field is found to be consistent with our explanation.

Gd-substituted Bi-2223 superconductor

The effects of gadolinium doping at calcium site on the normal and superconducting properties of Bi-2223 system were studied. The Gd-doped (BiPb)-2223 series of specimens, namely Batch I, II and III were sintered at three different sintering temperatures 830, 850 and 895°C respectively. The properties investigated are (1) the normal state resistivity with a view to study metal-to-insulator transition, (2) the XRD patterns of the specimens with a view to study the relative composition of (BiPb)-2212 and (BiPb)-2223 phases and (3) the superconducting fluctuation behaviour (SFB) with a view to determine the effect of doping, if any, on the dimensionality of the fluctuation conductivity of the system. The normal state resistivity of Gd-substituted Bi-2223 specimens shows metallic, semiconducting and insulating behaviour. The T c (R = 0) values indicate that (BiPb)-2223 phase is responsible for the observed superconducting transitions in Batch I and Batch II specimens with Gd concentrations x ≤ 0.7. This observation is further confirmed in the analysis of XRD patterns of these specimens. Gadolinium, being a magnetic impurity, has pair breaking effect near the Fermi level and decreases T c (R = 0). The analysis of the superconducting fluctuation behaviour (SFB) shows a 2D dimensionality without any cross-over.

Dimensional crossover and flux pinning of decoupled Cu50Ni50∕Nb multilayers

The behaviors of superconducting transition temperature Tc and upper critical field Hc2 as a function of different superconductor thicknesses have been investigated in Cu50Ni50∕Nb trilayers and multilayers. We deduced superconductor critical thickness, below which superconductivity vanishes, by analyzing the data in terms of proximity theory. The temperature dependence of Hc2 measurement reveals the spatial dimensional crossover and the flux pinning mechanism in the superconductor. A strong pair-breaking effect was observed in the weak ferromagnetic Cu50Ni50 layers from both measurements. We attribute our observation to the high interface transparency between Cu50Ni50 and Nb.

Andreev reflection study of the new magnetic superconductor Mo3Sb7 in magnetic field

Andreev reflection in contacts based on the magnetic superconductor Mo3Sb7 in a magnetic field is investigated by the point-contact method. It is found that the behavior of the order parameter in the magnetic field Δ(H) is essentially dependent on the value of Δ, which varies widely (Δ(0,0)≈0.01–0.31meV) for different contacts at practically invariant Tc. At high Δ the dependence Δmax(H) deviates slightly from the theoretical prediction applicable under point-contact conditions. As the order parameter decreases, the deviation starts to increase and becomes very large when Δ approaches its minimum. Proceeding from the results obtained in this study and from the temperature measurements on Mo3Sb7 performed previously, we are inclined as before to assign the compound to the class of superconductors with an anisotropic gap function. Therefore, in no way can it be considered as a conventional BCS-type superconductor. The upper critical field Hc2≃16.5kOe found here is close to that obtained from magnetization measurements in other studies. According to an estimate made, the pair-breaking effect of the Pauli paramagnetism is rather weak in Mo3Sb7.

Magnetic instabilities along the superconducting phase boundary of Nb∕Ni multilayers

We report vibrating reed and superconducting quantum interference device magnetometer data that exhibit prominent dips or oscillations of the superconducting (SC) onset temperature, ΔTC(H)≈0.01–0.7K, for a [Nb(23nm)∕Ni(5nm)]5 multilayer (ML) in dc magnetic fields applied nearly parallel to the ML plane. The vibrating reed data exhibit reproducible structures below TC that may reflect multiple SC transitions, but they are sensitive to ac field amplitude and dc field orientation. This striking behavior poses challenges for theoretical and experimental investigations of interfaces between SC and ferromagnetic layers that involve magnetic pair breaking effects, “pi phase shifts” of the SC order parameter, and exotic (“LOFF”) pairing states. Alternatively, the anomalies may mark dynamical instabilities within a confined, strongly anisotropic Abrikosov vortex lattice.

Effect of magnetic pair breaking on Andreev bound states and resonant supercurrent in quantum dot Josephson junctions

We propose a model for resonant Josephson tunneling through quantum dots that accounts for Cooper pair-breaking processes in the superconducting leads caused by a magnetic field or spin-flip scattering. The pair-breaking effect on the critical supercurrent ${I}_{c}$ and the Josephson current-phase relation $I(\ensuremath{\varphi})$ is largely due to the modification of the spectrum of Andreev bound states below the reduced (Abrikosov-Gorkov) quasiparticle gap. For a quantum dot formed in a quasi-one-dimensional channel, both ${I}_{c}$ and $I(\ensuremath{\varphi})$ can show a significant magnetic-field dependence induced by pair breaking despite the suppression of the orbital magnetic-field effect in the channel. This case is relevant to recent experiments on quantum dot Josephson junctions in carbon nanotubes. Pair-breaking processes are taken into account via the relation between the Andreev scattering matrix and the quasiclassical Green functions of the superconductors in the Usadel limit.

Laser-photoemission study of antiferromagnetic superconductor ErNi2B2C

We have investigated the electronic structures of antiferromagnetic superconductor ErNi2B2C by using laser-photoemission spectroscopy. The observed spectra below the superconducting transition temperature (Tc=9.0K) show a very broad coherent peak and finite density of states at the Fermi level due to strong magnetic pair breaking effects. In addition, we find a dip structure in the temperature dependence of the superconducting order parameter just below the antiferromagnetic ordering temperature (TN=6.0K). This behavior can be well interpreted as a result of competition between the rapid evolution of the antiferromagnetic molecular field and the increase of the superconducting condensation energy with decreasing temperature.

Proximity effect of superconductivity and magnetism in the Nd0.7Ca0.3MnO3∕YBa2Cu3O7 bilayer

Temperature and current dependent resistivity are investigated in the single YBa2Cu3O7 layer and the YBa2Cu3O7∕Nd0.7Ca0.3MnO3 (YBCO∕NCMO) bilayer with the thickness of each layer being 200nm. Our results show that the onset temperature (Tcon) of the superconducting transition in YBCO layer is linearly suppressed by current with a rate of 0.1K∕mA. However, Tcon of the NCMO∕YBCO bilayer is reduced effectively from 54to30K with increasing the applied current from 1to40mA, which indicates an enhanced effect of pair breaking by the polarized quasiparticles.

Study of the superconducting and magnetic properties of niobium doped RuSr2Gd1.5Ce0.5Cu2O10−δ ruthenocuprates

Polycrystalline samples of Ru1−xNbxSr2Gd1.5Ce0.5Cu2O10−δ, 0⩽x⩽0.5, have been synthesized and studied by resistivity, magnetization and ac susceptibility measurements. It was identified a strong suppression of the spin glass (SG) transition, which was totally suppressed in samples for x⩾0.2. The hysteresis loops at low temperatures are the result of the contribution of two distinct magnetic phases: a canted AFM phase and the SG phase. More importantly, the significant changes in the magnetic response of the material affect the superconducting properties of the samples. It was found that both Tc and the superconducting fraction are reduced in samples that show the spin glass phase, possibly due to the magnetic pair breaking effect.

Specific-heat of Na0.35CoO2 · 1.3H2O: Effects of sample age and pair breaking on two-gap superconductivity

Specific-heat measurements on Na0.35CoO2·1.3H2O show an evolution of the superconductivity, and its ultimate disappearance, with increasing sample age. An increase in pair breaking action, which occurs preferentially in an electron band with a small energy-gap, produces a shift in the relative contributions of two electron bands to the superconducting condensation. The similarity of the time scale for these changes to that recently reported for structural changes suggests a relation between the two effects and a basis for understanding the sample dependence of the properties of this material in general.

Andreev reflection measurements of spin polarization

We discuss aspects of Andreev reflection (AR) measurements in normal metal-superconductor (N-S) and ferromagnet-superconductor (F-S) devices. We describe the analytical model used to quantify spin polarization from the conductance measurements and discuss the validity of this simple model using parabolic bands as simple surrogates for real band structures. We present (AR) measurements of spin polarization in a $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Pt}\text{\ensuremath{-}}\mathrm{Pb}$ and $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Pt}\text{\ensuremath{-}}\mathrm{Pb}$ lithographically fabricated nanocontact systems where a scattering layer of has been deliberately added to the interface to enable the study of the effect of pair-breaking scattering on AR conductance and spin polarization. We compare these results to the previously published results from clean $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Pb}$ and $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Pb}$ devices and argue that the measurements in devices with the Pt layer can be explained by the presence of inelastic-scattering-induced pair-breaking effects. We modify the analytical model to include this effect and show that in some instances, it may be impossible to distinguish between the effects of a finite spin polarization and inelastic scattering. This has implications for AR measurements of spin polarization at disordered or poorly formed F-S interfaces.

Pair breaking effects on upper critical fields in ferromagnet/superconductor layered structures

The influence of magnetic and spin–orbit impurity scattering on the upper critical fields Hc2 in ferromagnet/superconductor (F/S) hybrid structures is theoretically investigated. The generalized Usadel equations which are a pair of coupled equations containing additional spin–orbit and magnetic impurity scattering are shown to be decoupled in spite of the coupling interaction between the spin exchange field and the spin–orbit interaction in the F layer. The temperature dependence of the parallel upper critical field in the F/S bilayer is shown to be less prominent in the presence of the spin–orbit and the spin–flip scattering processes. The interplay between the zero- and pi-phases is analyzed in the case of the F/S multilayers. The pi-phase formation is found to be feasible through the strong oscillation of the zero temperature upper critical fields versus the thickness of the ferromagnetic layer. Our results which are valid in wide range of parameters may be very important for comparison with experimental data.

Magnetoresistance in asymmetric ferromagnet/superconductor/ferromagnet double tunnel junctions

We investigate theoretically the spin-dependent transport in a ferromagnet/superconductor/ferromagnet double tunnel junction taking into account the junction asymmetry. The spin-polarized tunneling current induces spin imbalance in the superconductor giving rise to suppression of superconductivity. We find that this pair breaking effect exhibits sensitive dependence on the asymmetric natures of the tunnel junction. We show, in particular, that the junction asymmetry can result in substantial changes in the magnetoresistive behavior.

Fluctuation Magneto-Conductivity in La–Ba–Ca–Cu–O Superconductors

Temperature variation of magneto-resistivity measurements have been carried out at 1 T magnetic field on (La1.5Gd0.5)Ba2CaCu5Oz and, on pure and 5 wt.% Ag doped (La1.6Y0.4)Ba2Ca0.8Cu4.8Oz superconductors. These data were analyzed in terms of fluctuation magneto-conductivity by using Aslamazov—Larkin and Maki—Thompson models for layered superconductors. These materials exhibit strong pair breaking effect and the typical phase breaking time at 100 K is found to be in the order of 10−16 s. The typical values of coherence lengths along ab plane and along c-direction at 0 K are found to be 16.6 and 0.5 A.

Mn and Fe impurities in MgB 2

Based on first principles calculations, we show that Mn impurities are magnetic in MgB 2 due to exchange-splitting of d 3 z 2 − 1 band. Thus, Mn impurities could act as strong magnetic scattering centres leading to pair-breaking effects in MgB 2. In contrast, we find Fe impurities in MgB 2 to be nearly non-magnetic.