Pair-breaking Effect Research Articles

Phase dependent differential thermopower ofs-wave-superconductor–normal-metal–d-wave-superconductor junctions: Pair-breaking effects and Gaussian fluctuations

We start with revisiting our previous results on thermoelectric response of s-wave-superconductor--normal-metal--$s$-wave-superconductor $(\mathrm{SNS})$ configuration in a C-shaped ${\mathrm{Bi}}_{x}{\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{y}$ sample in order to include strong fluctuation effects. Then, by appropriate generalization of the Ginzburg-Landau theory based on admixture of s-wave $(S)$ and d-wave $(D)$ superconductors, we consider a differential thermoelectric power (TEP) of s-wave-superconductor--normal-metal--$d$-wave-superconductor $(\mathrm{SND})$ junction. In addition to its strong dependence on the relative phase $\ensuremath{\theta}={\ensuremath{\varphi}}_{s}\ensuremath{-}{\ensuremath{\varphi}}_{d}$ between the two superconductors, two major effects are shown to influence the behavior of the predicted TEP. One, based on the chemical imbalance at $\mathrm{SD}$ interface, results in a pronounced maximum of the TEP peak near $\ensuremath{\theta}=\ensuremath{\pi}/2$ (where the so-called $s+id$ mixed pairing state is formed) for two identical superconductors with ${T}_{\mathrm{cd}}{=T}_{\mathrm{cs}}\ensuremath{\equiv}{T}_{c}.$ Another effect, which should manifest itself at $\mathrm{SD}$ interface comprising an s-wave low-${T}_{c}$ superconductor and a d-wave high-${T}_{c}$ superconductor with ${T}_{\mathrm{cd}}\ensuremath{\ne}{T}_{\mathrm{cs}},$ predicts ${S}_{p}\ensuremath{\propto}{T}_{\mathrm{cd}}\ensuremath{-}{T}_{\mathrm{cs}}$ for the TEP peak value. The experimental conditions under which the predicted behavior of the induced differential TEP can be measured are discussed.

Current-carrying states in superconductor/insulator and superconductor/semiconductor superlattices in the mesoscopic regime

We discuss some of the basic theoretical aspects of current-carrying states in superconducting superlattices with tunnel barriers in the mesoscopic regime, when p0−1⪡a⪡ξ0(a is the superconducting layer thickness, p0is the Fermi momentum, ξ0is the BCS coherence length and ℏ= 1). We establish the necessary conditions for the observation of the classical Josephson effect (with sinusoidal current–phase dependence) and derive self-consistent analytical expressions for the critical Josephson current. These expressions are proportional to the small factor a/ξ0and have unusual temperature dependence as compared with the single-junction case. For certain parameter values, the superconducting gap exhibits an exponential decrease due to pair-breaking effect of the supercurrent. The supercurrent can completely destroy the superconductivity of the system above a certain characteristic temperature T*. In this paper, we also study the effect of intrabarrier exchange interactions. We show that this effect is strongly enhanced compared with the single-junction case and can manifest itself in an exponential decrease of the critical temperature.

Neutron-Irradiation Effects on the Magnetic-Field-Induced Electronic Phase Transitions in Graphite

The effects of fast-neutron irradiation on the magnetic-field-induced electronic phase transition in graphite have been investigated; the fast-neutron irradiation introduces lattice defects in graphite, some of which act as acceptors. It has been found that the transition temperature is lowered with increasing concentration of lattice vacancies introduced by neutron irradiation. The suppression of the transition temperature can be interpreted as a manifestation of the pair-breaking effect by charged lattice vacancies, similar to magnetic impurities in a superconductor. This supports the existing theoretical interpretation of the field-induced transition being a density-wave transition.

Disorder and chain superconductivity inYBa2Cu3O7−δ

The effects of chain disorder on superconductivity in YBa_2Cu_3O_{7-\delta} are discussed within the context of a proximity model. Chain disorder causes both pair-breaking and localization. The hybridization of chain and plane wavefunctions reduces the importance of localization, so that the transport anisotropy remains large in the presence of a finite fraction $\delta$ of oxygen vacancies. Penetration depth and specific heat measurements probe the pair-breaking effects of chain disorder, and are discussed in detail at the level of the self-consistent T-matrix approximation. Quantitative agreement with these experiments is found when chain disorder is present.

Reply to the “Comment on ‘Thermal conductivity ofYBa2(Cu1−xZnx)3O7−δ:Relation betweenxand δ ’ ”

In the preceding Comment, Williams and Tallon do not agree with our interpretation of the thermal conductivity peak of ${\mathrm{YBa}}_{2}({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{)}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ compounds as a function of x and \ensuremath{\delta} in terms of a van Hove scenario of high-${T}_{c}$ superconductivity. Their main claim is that Zn doping does not lead to the variation of hole concentration in the ${\mathrm{CuO}}_{2}$ planes, but rather to pair-breaking effects, i.e., the strong scattering limit of the d-wave energy gap parameter. Williams and Tallon quote in their Comment various experimental data in order to support this latter interpretation. In this Reply, we argue that the van Hove scenario picture discussed in our commented paper seems consistent for oxygen- and zinc-doped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}.$

Gap Symmetry and a Revival of Superconductivity in High Parallel Magnetic Fields in Q2D and Q1D Organic, High-Tc, and Sr2RuO4 Compounds

From the BCS theory, we derive an equation determining the upper critical field, Hc2(T), parallel to conducting planes of both layered Q2D and Q1D superconductors. It extends the quasiclassical Ginzburg–Landau–Abrikosov–Gor'kov (GLAG) and Lawrence–Doniach descriptions of Hc2(T) to the case of high magnetic fields where the quantum nature of an electron motion along open Fermi surfaces is important. This equation demonstrates two new phenomena: (1) a complete restoration of superconductivity at H ≃ Hc5 > Hc2(0) in the case of p-wave pairing of electrons; (2) a surviving of superconductivity at Hc2(0) < H < H*p in the case of s(d)-wave pairing, where H*p ≠ Hp [Here, Hc2(0) and Hp are the quasiclassical GLAG upper critical field and the paramagnetic Clogston–Chandrasekhar limiting field, correspondingly; Hc5 and H*p are defined in the text]. Our analysis of recent experimental data on (TMTSF)2PF6 by I. J. Lee et al. and M. J. Naughton et al. shows that superconductivity significantly exceeds H*p. This demonstrates a strong suppression of the Pauli pair-breaking effects and may reflect a p-wave pairing of electrons. We propose to measure Hc2(T) in Sr2RuO4 (which is believed to be a p-wave superconductor) to prove a symmetry of a superconducting order parameter.

Exactly Aligned Magnetic Field and the Fulde–Ferrell–Larkin–Ovchinnikov State in Quasi-Low-Dimensional Superconductors

In quasi-low-dimensional type II superconductors, the orbital pair-breaking effect is considerably reduced when the magnetic field is exactly aligned in a direction parallel to the highly conducting layer. Then, since the superconducting phase survives up to a very high magnetic field comparable to the Pauli-paramagnetic limit, we could expect some drastic phenomena, such as the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. In this paper, we discuss the condition of the tilt angle for the appearance of the FFLO state, and also discuss the stable spatial structure of the FFLO state. Further, we show an example of a quasi-two-dimensional system in which the FFLO state is remarkably enhanced, that is, a two-dimensional tight binding system of appropriate hole concentrations.

A magnetic pair-breaking effect in rare earth-doped manganites

The magnetic properties of La 0.60R 0.07Di 0.33MnO 3 ferromagnetic manganites (Di = Sr, Ba) are studied, where La is partly replaced by magnetic rare earths R. It is shown that (i) there is a ferromagnetic coupling between Mn and R spins, (ii) the Curie temperature is lowered compared to the parent La compound and (iii) its depression is correlated with the effective moment of the rare earth ion. This last relation is tentatively explained by a magnetic pair-breaking effect, where fluctuating R moments lower the double-exchange coupling between Mn atoms.

Observation of bolometric optical response in superconducting microwave meander lines

The change in the resonant frequency of YBa2Cu3O7−x meander lines under thermal and optical perturbations was used to determine whether pair breaking effects are induced either by bolometric or nonbolometric responses. The frequency-domain measurements were performed by monitoring the change of both the microwave resonance mode frequency and frequency width. The variation of the resonant frequency induced by two external perturbations shows a quadratic dependence on each perturbation in the range of 0.28Xc–0.56Xc. A perturbation to the fourth dependence is observed above 0.63Xc, where Xc is either the critical temperature or the critical optical pulse energy at which resonance can no longer be distinguished. The results suggest that the photoresponse of the superconducting meander line is bolometric due to laser heating.

Experimental investigation of a local mixed state induced by a small ferromagnetic particle in Y-Ba-Cu-O films: Extremely low energy barrier for formation of vortex-antivortex pairs

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 Y-Ba-Cu-O 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.

Optical study of pair-breaking effect in Zn-substituted YBa 2Cu 3O 7− δ

The Zn-doping effects on the a-, b- and c-axis optical conductivity spectra have been investigated for highly oxygenated YBa 2Cu 3O 7− δ crystals. The absolute values of the diagonal components of penetration depth tensor were obtained. The measurement reveals that Zn substitution leads to very strong but anisotropic suppression of the condensate density along the three crystallographic axes.

STM investigation of BSCCO 2212 and borocarbide materials

We present spectroscopic Scanning Tunneling Microscope (STM) measurements performed at 4.2 K on BSCCO 2212 single crystals and Y, Lu and Er based borocarbide compounds. The conductance versus voltage spectra on BSCCO 2212 reveal a reproducible dip feature near e| V|=2Δ, for both voltage polarities pointing to a strong coupling origin of the feature. The conductance spectra obtained on YNi 2B 2C thin films and LuNi 2B 2C single crystals are similar and correspond to a BCS ratio of 3.2 indicating weak coupling superconductivity in these compounds. On Er Ni 2B 2C single crystals, the conductance spectra show a pronounced broadening compared to the spectra obtained on LuNi 2B 2C and YNi 2B 2C, which is attributed to the pair-breaking effect due to the Er magnetic ions.

Bose-Einstein condensation with a BCS model interaction

A simple model of a boson-fermion mixture of unpaired fermions plus linear-dispersion-relation Cooper pairs that includes pair-breaking effects leads to Bose-Einstein condensation for dimensions greater than unity, at critical temperatures substantially greater than those of the BCS theory of superconductivity, for the same BCS model interaction between the fermions.

Effect of Spatial Variations of the Superconducting Gap on Suppression of the Transition Temperature by Impurities

We calculate correction to the critical temperature of a dirty superconductor, which results from the local variations of the gap function near impurity sites. This correction is of the order of ${T}_{c}/{E}_{F}$ and becomes important for short-coherence length superconductors. It generally reduces a pair-breaking effect. In $s$-wave superconductors small amounts of nonmagnetic impurities can increase the transition temperature.

Theory of ac Josephson Effect in Superconducting Constrictions

We have developed a microscopic theory of the ac Josephson effect within the general model of a high-transparency Josephson junction: short superconducting constriction with either ballistic or diffusive electron transport. Applications of the theory were studied, including smearing of the subgap current singularities by pair-breaking effects in the superconducting electrodes of the constriction, and the structure of these singularities in constrictions between the composite superconducting-normal electrodes with the proximity-induced gap in the normal layer.

Effect of magnetic impurity correlations on Josephson tunneling

The ordering trend of magnetic impurities at low temperature results in the frustration of the pair-breaking effect and induces a `recovery' of superconducting properties. We show that this effect manifests itself in the deviation of the Josephson current amplitude from the values obtained within the Ambegaokar–Baratoff and the Abrikosov–Gor'kov models. We consider both weak and strong-coupling cases. The theory is applied to describe the experimental data obtained for the low- T c superconductor SmRh 4B 4. We further predict a `recovery' effect of the Josephson current in high-temperature superconductors.

Doping dependence of the spatially modulated dynamical spin correlations and the superconducting-transition temperature inLa2−xSrxCuO4

Systematic low-energy neutron-scattering studies have been performed on float-zone-grown single crystals of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ with $x$ extending from zero doping, $x=0$, to the overdoped, weakly superconducting regime, $x=0.25$. For $x$ beyond a critical doping value of ${x}_{c}\ensuremath{\approx}0.05$ the low-energy spin-fluctuation peak position shifts from ($\frac{1}{2}$, $\frac{1}{2}$) to ($\frac{1}{2}$\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}, $\frac{1}{2}$), and ($\frac{1}{2}$, $\frac{1}{2}$\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}); ${x}_{c}$ also represents the onset concentration for superconductivity. For $0.06l~xl~0.12$ the incommensurability $\ensuremath{\delta}$ follows approximately the quantitative relation $\ensuremath{\delta}=x$. However, beyond $x\ensuremath{\approx}0.12$ the incommensurability tends to saturate around $\ensuremath{\delta}\ensuremath{\approx}1/8$. The superconducting-transition temperature ${T}_{c}(x)$ for stoichiometric samples at a given doping scales linearly with $\ensuremath{\delta}$ up to the optimal doping value of $x$. The peak momentum width of the spin fluctuations at low energies is small throughout the superconducting concentration region except in the strongly overdoped region. An anomalously small width is observed for $x=\frac{1}{8}$. The incommensurate spatial modulation is found to be robust with respect to pair-breaking effects that lower ${T}_{c},$ such as deoxygenation of the sample or replacement of Cu by Zn.

Magnetism of V, Mo, and Co surface impurities on Pb measured by their pair-breaking effect

Thin films of Pb are covered in situ with submonolayers of V, Mo, and Co in the range between 0.01 and 1 monolayers. If the surface impurities are magnetic they will reduce the superconducting transition temperature of the Pb film. From the reduction of ${T}_{c}$ the magnetic dephasing rate of the surface impurities $1/{\ensuremath{\tau}}_{s}$ and their magnetic cross section ${\ensuremath{\sigma}}_{s}$ are calculated. We find that single V surface impurities are magnetic, while single Mo and Co impurities are nonmagnetic. Co surface clusters are magnetic.

Pinhole junctions in d-wave superconductors

We present a self consistent treatment of pinhole junctions in d x 2− y2 superconductors. The current–phase relation j s( χ) is studied at different temperatures and at different angles α between the crystal â-axis and the junction (surface) normal. We show that the critical current of a junction can be reduced by pair-breaking effects at the separation. We also study the Josephson energy of a pinhole as a function of the phase difference across the junction. In particular are mapped the positions of the energy minima χ min at different temperatures as functions of ( α L, α R), the crystal orientations of the left and right superconductors. With decreasing temperature there is an increasing range of crystal orientations where χ min varies continuously from 0 to π.

Superconductivity in (Bi,Pb)2Ca2Sr2Cu3−xVxOy

The superconducting properties of V-doped (Bi,Pb)2Sr2Ca2Cu3Oy (2223) samples have been investigated by means of X-ray diffraction and electrical resistivity measurements. The volume fraction of the 2223 phase decreases with a decrease in the c lattice parameter. The onset of superconductivity in the samples remains almost constant while the temperature at which the resistivity becomes zero decreases with increasing dopant content. The electrical resistivity data suggest that the substitution of copper by vanadium suppresses the superconductivity in (Bi,Pb)2Sr2Ca2Cu3−xVxOy by destroying the phase coherence and by pair-breaking effects. The suppression of superconductivity in the system could also be due to a decrease in the carrier concentration.