Pair-breaking Effect Research Articles

Coexistence of spin triplet superconductivity and antiferromagnetism in a quasi-one-dimensional system (TMTSF) 2PF 6

Simultaneous nuclear magnetic resonance (NMR) and electrical transport measurements were carried out on a quasi-one-dimensional organic superconductor, (TMTSF) 2PF 6, in a narrow pressure range between 5.5 and 6.4 kbar. Data indicate that there exist macroscopic domains of spin density wave (SDW) and metallic/superconducting regions with little mutual interaction between the adjacent phases. The previously proposed domain model, self-consistent reconstruction of superconducting domains in magnetic fields and temperatures, is likely responsible for the unusually weak orbital superconducting pair-breaking effects observed in the system.

Growth of [110] La2∕3Sr1∕3MnO3–YBa2Cu3O7 heterostructures

Y Ba 2 Cu 3 O 7 – La 2 ∕ 3 Sr 1 ∕ 3 Mn O 3 heterostructures of [110] orientation are grown to allow direct injection of spin polarized holes from the La2∕3Sr1∕3MnO3 into the CuO2 superconducting planes. The magnetic response of the structure at T<Tsc shows both diamagnetic and ferromagnetic moments with [001] direction as magnetic easy axis. While the superconducting transition temperature (Tsc) of these structures is sharp (ΔTsc≃2.5K), the critical current density (Jc) follows a dependence of the type Jc=Jo(1−T∕Tsc)3∕2 with highly suppressed Jo(≃2×104A∕cm2), indicating strong pair breaking effects of the ferromagnetic boundary.

Nonuniform Spin Triplet Superconductivity due to Antisymmetric Spin–Orbit-Coupling in Noncentrosymmetric Superconductor CePt3Si

We show that the nonuniform state [Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state] of the spin triplet superconductivity in noncentrosymmetric systems is stabilized by antisymmetric spin–orbit coupling even if the magnetic field is absent. The transition temperature of the spin triplet superconductivity is reduced by the antisymmetric spin–orbit coupling in general. This pair breaking effect is shown to be similar to the Pauli pair breaking effect due to magnetic field for the spin singlet superconductivity, in which FFLO state is stabilized near the Pauli limit (or Chandrasekhar–Clogston limit) of external magnetic field. Since there are gapless excitations in nonuniform superconducting state, some physical quantities such as specific heat and penetration depth should obey different temperature-dependences from those in the uniform superconductivity. We discuss the possibility of the realization of nonuniform state in CePt 3 Si.

Magnetic-Field Enhancement of Superconductivity in Ultranarrow Wires

We study the effect of an applied magnetic field on sub-10-nm wide MoGe and Nb superconducting wires. We find that magnetic fields can enhance the critical supercurrent at low temperatures, and do so more strongly for narrower wires. We conjecture that magnetic moments are present, but their pair-breaking effect, active at lower magnetic fields, is suppressed by higher fields. The corresponding microscopic theory, which we have developed, quantitatively explains all experimental observations, and suggests that magnetic moments have formed on the wire surfaces.

Antiferromagnetic superconductors with effective-mass anisotropy in magnetic fields

We derive critical field H_c2 equations for antiferromagnetic \textit{s}-wave, d_{x^2-y^2}-wave, and d_{xy}-wave superconductors with effective mass anisotropy in three dimensions, where we take into account (i) the Jaccarino-Peter mechanism of magnetic-field-induced superconductivity (FISC) at high fields, (ii) an extended Jaccarino-Peter mechanism that reduces the Pauli paramagnetic pair-breaking effect at low fields where superconductivity and an antiferromagnetic long-range order with a canted spin structure coexist, and (iii) the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) state. As an example, experimental phase diagrams observed in organic superconductor kappa-(BETS)_2FeBr_4 are theoretically reproduced. In particular, the upper critical field of low-field superconductivity is well reproduced without any additional fitting parameter other than those determined from the critical field curves of the FISC at high fields. Therefore, the extended Jaccarino-Peter mechanism seems to occur actually in the present compound. It is predicted that the FFLO state does not occur in the FISC at high fields in contrast to the compound lambda-(BETS)_2FeCl_4, but it may occur in low-field superconductivity for s-wave and d_{x^2-y^2}-wave pairings. We also briefly discuss a possibility of compounds that exhibit unconventional behaviors of upper critical fields.

Strong magnetic pair breaking in Mn-substitutedMgB2single crystals

Magnetic ions (Mn) were substituted in $\mathrm{Mg}{\mathrm{B}}_{2}$ single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, ${T}_{c}$, in ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{B}}_{2}$ has been found to be rapidly suppressed at an initial rate of $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕%\mathrm{Mn}$, leading to a complete suppression of superconductivity at about 2% Mn substitution. This reflects the strong coupling between the conduction electrons and the $3d$ local moments, predominantly of magnetic character, since the nonmagnetic ion substitutions, e.g., with Al or C, suppress ${T}_{c}$ much less effectively (e.g., $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕%\mathrm{Al}$). The magnitude of the magnetic moment ($\ensuremath{\simeq}1.7\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per Mn), derived from normal state susceptibility measurements, uniquely identifies the Mn ions to be divalent, and to be in the low-spin state $(S=1∕2)$. This has been found also in x-ray absorption spectroscopy measurements. Isovalent ${\mathrm{Mn}}^{2+}$ substitution for ${\mathrm{Mg}}^{2+}$ mainly affects superconductivity through spin-flip scattering reducing ${T}_{c}$ rapidly and lowering the upper critical field anisotropy ${H}_{c2}^{ab}∕{H}_{c2}^{c}$ at $T=0$ from 6 to 3.3 ($x=0.88%$ Mn), while leaving the initial slope $\mathrm{d}{H}_{c2}∕\mathrm{d}T$ near ${T}_{c}$ unchanged for both field orientations.

Direct Urca neutrino rate in color superconducting quark matter

If deconfined quark matter exists inside compact stars, the primary cooling mechanism is neutrino radiation via the direct Urca processes d->u+e+antinu_e and u+e->d+nu_e. Below a critical temperature, T_c, quark matter forms a colour superconductor, one possible manifestation of which is a condensate of <ud> quark Cooper pairs in an electric-charge neutralising background of electrons. We compute the neutrino emission rate from such a phase, including charged pair-breaking and recombination effects, and find that on a material temperature domain below T_c the pairing-induced suppression of the neutrino emission rate is not uniformly exponential. If gapless modes are present in the condensed phase, the emissivity at low temperatures is moderately enhanced above that of completely unpaired matter. The importance of charged current pair-breaking processes for neutrino emission both in the fully gapped and partially gapped regimes is emphasised.

Magnetic Pair-Breaking in Y1−xHoxNi2B2C(x = 0, 0.25, 0.5, 0.75) Single Crystals

Temperature-dependent resistivity and magneticsusceptibility were studied for quaternary borocarbide intermetalliccompounds Y1−xHoxNi2B2C (x = 0, 0.25,0.5, 0.75), which show coexistence of superconductivity with magnetism.In a normal state, the compounds exhibit conventional metallicbehaviour. The Debye temperature θD is derived by fittingthe temperature dependence of resistivity to the Bloch–Gruneisenexpression, i.e. θD scales with M−0.5 (M is theaveraged atomic mass on the Y3+ site), which meansthat the acoustic mode of the lattice vibrating spectrum is influencedby the Y3+ site atoms. Fitting the temperature-dependentmagnetic susceptibility above TN to the Curie–Weiss law,effective magnetic moment μeff is deduced, and thende Gennes factor dG is calculated. It is found that as Ho contentincreases, μeff as well as dG increases and TCdecreases. Moreover, the decrease of TC scaleswith dG, i.e., ΔTC∝−nI2N(εF)dG, which is consistent with the prediction of theAbrikosov–Gor'kov theory. We suggest that the depression of TCcould be mainly ascribed to the magnetic pair-breaking effect ofmagnetic Ho3+ ions. The change of Debye temperature with Ho contentmay not have significant impact on TC.

Antivortices due to competing orbital and paramagnetic pair-breaking effects

Thermodynamically stable vortex—antivortex structures in a quasi-twodimensional superconductor in a tilted magnetic field are predicted. For this geometry, both orbital and spin pair-breaking effects exist, with their relative strength depending on the tilt angle θ. The spectrum of possible states contains the ordinary vortex state (for large θ) and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state (for θ = 0) as limits. The quasi-classical equations are solved nearH c2 for arbitrary θ and it is shown that stable states with co-existing vortices and antivortices exist in a small interval close to θ= 0. The results are compared with recent predictions of antivortices in mesoscopic samples.

Bound states at the interface between antiferromagnets and superconductors

We present a detailed theoretical investigation of interfaces and junctions involving itinerant antiferromagnets. By solving the Bogoliubov--de Gennes equations with a tight-binding model on a square lattice, we study both the self-consistent order parameter fields proximate to interfaces between antiferromagnets (AF) and $s$-wave (sSC) or $d$-wave (dSC) superconductors, the dispersion of quasiparticle subgap states at interfaces and interlayers, and the local density of states (LDOS) as a function of distance from the interface. In addition, we present the quasiclassical approach to interfaces and junctions involving itinerant antiferromagnets developed in an earlier paper. Analytical results are in excellent agreement with what we obtain numerically. Strong effects of pair breaking in the presence of low-energy interface Andreev states are found in particular for AF/sSC interfaces when interface potentials are not too high. Potential barriers induce additional extrema in the dispersive quasiparticle spectra with corresponding peaks in the LDOS. Discrete quasiparticle dispersive levels in AF--normal metal $(N)$--AF systems are found to depend strongly on the misorientation angle of the magnetizations in the two antiferromagnets.

Superconducting Spin Valve Effect of a V Layer Coupled to an Antiferromagnetic[Fe/V]Superlattice

We studied superconducting V layers deposited on an antiferromagnetically coupled [Fe(2)V(11)](20) superlattice. The parallel upper critical magnetic field exhibits an anomalous T dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature T(S) decreases when rotating the relative sublattice magnetization directions of the superlattice from antiparallel to parallel. This proves that the pair breaking effect of a Fe2 layer is reduced if at a distance of 1.5 nm a second Fe2 layer with antiparallel spin orientation exists.

Andreev reflection and pair-breaking effects at the superconductor/magnetic semiconductor interface

We investigate the applicability of spin-polarization measurements using Andreev reflection in a point-contact geometry in heavily doped dilute magnetic semiconductors, such as (Ga,Mn)As. Although we observe conventional Andreev reflection in nonmagnetic (Ga,Be)As epilayers, our measurements indicate that in ferromagnetic (Ga,Mn)As epilayers with comparable hole concentration the conductance spectra can only be adequately described by a broadened density of states and a reduced superconducting gap. We suggest that these pair-breaking effects stem from inelastic scattering in the metallic impurity band of (Ga,Mn)As and can be explained by introducing a finite quasiparticle lifetime or a higher effective temperature. For (Ga,Mn)As with 8% Mn concentration and $140\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ Curie temperature we evaluate the spin polarization to be $83\ifmmode\pm\else\textpm\fi{}17%$.

SUPERCONDUCTING PROPERTY IN Mn DOPED La1.85Sr0.15CuO4 SYSTEM

In this paper the superconducting properties in La 1.85S Sr 0.15 Cu 1-x Mn x O 4 and La 1.85-2x Sr 0.15+2x Cu 1-x Mn x O 4 samples are discussed. The superconducting transition temperature keeps constantly at 37 K for all superconducting samples. The suppression of superconductivity in these systems does not fit the Arbrikosov and Gork'ov theory which predicts that Tc should decrease in the presence of magnetic impurities in the lattice. The magnetic pair-breaking effect is not the reason for the suppression of superconductivity in Mn doping systems. The localization of carriers in the CuO 2 plane induced by Mn doping should be responsible for the suppression of superconductivity.

Irradiation-induced suppression of the critical temperature in high-T c superconductors: Pair breaking versus phase fluctuations

Experiments on the irradiation-induced suppression of the critical temperature in high-T c superconductors are analyzed within the mean-field Abrikosov-Gor’kov-like approach. It is shown that the experimental data for YBa2Cu3O7-δ single crystals can be quantitatively explained by the pair-breaking effects under the assumption of the combined effect of potential and spin-flip scattering on the critical temperature and with an accounting for a nonpure d-wave superconducting order parameter.

The influence of local structure on superconductivity in Fe 0.5Cu 0.5Ba 2YCu 2O 7.41: a Mössbauer effect study

In this paper, the Mössbauer effect of the Fe 0.5Cu 0.5Ba 2YCu 2O 7.41 superconductor with T c = 80 K synthesized at 1000 °C at high pressure of 6 GPa was reported. Results indicate that the high-pressure synthesis makes only a slight amount of Fe located at Cu(2) sites migrate to Cu(1) sites and there is no obvious magnetic pair-breaking effect induced by Fe located at Cu(2) sites. In addition, the high-pressure synthesis changed fourfold coordinated Fe to fivefold coordinated Fe in the chains. We argued that the fivefold coordinated Fe in CuO x chains favors the occurrence of superconductivity in Fe 0.5Cu 0.5Ba 2YCu 2O 7.41 superconductor.

Reduction of Pauli Paramagnetic Pair-breaking Effect in Antiferromagnetic Superconductors

Antiferromagnetic superconductors in a magnetic field are studied. We examine a mechanism which significantly reduces the Pauli paramagnetic pair-breaking effect. The mechanism is realized even in the presence of the orbital pair-breaking effect. We illustrate it using a three-dimensional model with an intercalated magnetic subsystem. The upper critical field is calculated for various parameters. It is shown that the upper critical field can reach several times the pure Pauli paramagnetic limit. The possible relevance to the large upper critical field observed in the heavy fermion antiferromagnetic superconductor CePt_3Si discovered recently is briefly discussed. We try to understand the large upper critical field in the compound CePt_3Si and field-induced superconductivity in the compound CePb_3 within a unified framework.

Superconductivity inNaxCoO2·yH2O: Protection of ad1+id2state by spin-charge separation

Superconductivity in ${\text{Na}}_{x}{\text{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}y{\mathrm{H}}_{2}\mathrm{O}$ is likely to be a $p$ or $d$ wave; however, experiments are unable to pinpoint the symmetry. A simple estimate of pair breaking effects from an unavoidable ``${\text{Na}}^{+}$ vacancy disorder'' in an ordered ${\text{Na}}^{+}$ lattice, at an optimal ${x}_{\text{opt}}\ensuremath{\approx}0.30$ is shown to destroy a Fermi liquid-based $p$- or $d$-wave superconductivity. However, a robustness of superconducting and normal states, seen in experiments, is pointed out and argued to imply the presence of a ``quantum protectorate,'' possibly a spin-charge decoupling that protects a ${d}_{1}+i{d}_{2}$ and not a $p$ state. A calculation of Knight shift and $1∕{T}_{1}$ in the framework of resonating valence bond mean-field theory and a fit to the data of Kobayashi et al. (e-print, cond-mat/0305649; cond-mat/0306264) is made.

Two-dimensional superconductor in a tilted magnetic field: States with finite Cooper-pair momentum

Varying the angle $\ensuremath{\theta}$ between applied field and the conducting planes of a layered superconductor in a small interval close to the plane-parallel field direction, a large number of superconducting states with unusual properties may be produced. For these states, the pair breaking effect of the magnetic field affects both the orbital and the spin degree of freedom. This leads to pair wave functions with finite momentum, which are labeled by Landau quantum numbers $0&lt;n&lt;\ensuremath{\infty}.$ The stable order-parameter structure and magnetic-field distribution for these states is found by minimizing the quasiclassical free energy near ${H}_{c2}$ including nonlinear terms. One finds states with coexisting linelike and pointlike order-parameter zeros and states with coexisting vortices and antivortices. The magnetic response may be diamagnetic or paramagnetic depending on the position within the unit cell. The structure of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states at $\ensuremath{\theta}=0$ is reconsidered. The transition $\stackrel{\ensuremath{\rightarrow}}{n}\ensuremath{\infty}$ of the paramagnetic vortex states to the FFLO limit is analyzed and the physical reason for the occupation of higher Landau levels is pointed out.

Pair-breaking effect due to the long-range antiferromagnetic order in high- Tc superconductors RE123 (RE=Dy, Gd)

Here, we examine some low field effects that allow us to study the possible interaction between antiferromagnetism and superconductivity in high- T c REBa 2Cu 3O 7− δ (RE123, RE=Dy, Gd). For Dy123 and Gd123, the screening effect hides the interaction between magnetic (4f) and conduction electrons and no sign of mutual influence has been observed near T N. For Gd 1+ x Ba 2− x Cu 3O 7− δ , where T N increases and T c decreases with x, a peak in the temperature dependence of the ac susceptibility has been found for x=0.2. This peak is attributed to the suppression of superconducting persistent currents due to the pair-breaking effect resulting from the enhanced magnetic fluctuations near the phase transition temperature.

Andreev reflection spectroscopy of MgB 2 in the vortex state

Spectroscopy based on the Andreev reflection (AR) process at the interface between the normal metal tip and the superconductor has become one of the very successful methods for studies in novel exotic superconductors. The method is capable to address the size, symmetry as well as multiplicity of the superconducting order parameter. The method provided one of the first evidences of the two-gap superconductivity in MgB 2 with a detailed temperature dependence of the both gaps. A theory treating the Andreev reflection spectroscopy in the mixed state is missing. We analyse the AR spectra of MgB 2 in the mixed state via modelling the magnetic pair-breaking by the increasing spectral broadening parameter Γ. As a result a non-trivial pair breaking effect in the π-band is found.