Proximity effects in a superconductor/ferromagnet junction

A proximity effect in an s-wave superconductor/ferromagnet (SC/F) junction is theoretically studied using the second order perturbation theory for the tunneling Hamiltonian and Green's function method. We calculate a pair amplitude induced by the proximity effect in a weak ferromagnetic metal (FM) and a half-metal (HM). In the SC/FM junction, it is found that a spin-singlet pair amplitude (Ψ_s) and spin-triplet pair amplitude (Ψ_t) are induced in FM and both amplitudes depend on the frequency in the Matsubara representation. Ψ_s is an even function and Ψ_t is an odd function with respect to the Matsubara frequency (ω_n). In the SC/HM junction, we examine the proximity effects by taking account of magnon excitations in HM. It is found that the triplet-pair correlation is induced in HM. The induced pair amplitude in HM shows a damped oscillation as a function of the position and contains the terms of even and odd functions of ω_n as in the case of the SC/FM junction. We discuss that in our tunneling model the pair amplitude of even function of ω_n only contributes to a Josephson current.     

Superconducting pi qubit with a ferromagnetic Josephson junction

Solid-state qubits have the potential for the large-scale integration and for the flexibility of layout for quantum computing. However, their short decoherence time due to the coupling to the environment remains an important problem to be overcome. We propose a new superconducting qubit which incorporates a spin-electronic device: the qubit consists of a superconducting ring with a ferromagnetic pi junction which has a metallic contact and a normal Josephson junction with an insulating barrier. Thus, a quantum coherent two-level state is formed without an external magnetic field. This feature and the simple structure of the qubit make it possible to reduce its size leading to a long decoherence time.     

Spin Josephson current in a ferromagnet/noncentrosymmetric superconductor junction

We investigate the equilibrium spin transport in a ferromagnet/noncentrosymmetric superconductor (FM/NCS) junction where the NCS has a dominant triplet order parameter and helical edge state. Based on the symmetry analysis and numerical calculation, we demonstrate that there is a nonzero spin supercurrent flowing in the junction, which stems from the exchange coupling between the FM magnetization and triplet Cooper-pair spin. It is also found that a transverse spin current other than the helical edge spin current is flowing along the interface of the junction, and its polarization is related to the longitudinal spin supercurrent. Besides, an equilibrium Hall current is also shown to flow along the junction’s interface due to the broken time-reversal symmetry from the FM.     

Experimental evidence for a collective insulating state in two-dimensional superconductors

We present the results of an experimental study of the current-voltage characteristics in a strong magnetic field (B) of disordered, superconducting, thin films of amorphous indium oxide. As the B strength is increased superconductivity degrades, until a critical field (B(c)) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high B.     

Scattering matrix for a junction of two horns

The asymptotic behavior of the scattering matrix for a junction of two horns is studied. Dedicated to the memory of Vladimir Borovikov     

Experimental evidence for a light and broad scalar resonance in D(+) --> pi(-)pi(+)pi(+) decay

From a sample of 1172 +/- 61 D(+)-->pi(-)pi(+)pi(+) decays, we find gamma(D(+)-->pi(-)pi(+)pi(+))/gamma(D(+)-->K-pi(+)pi(+)) = 0.0311 +/- 0.0018(+0.0016)(-0.0026). Using a coherent amplitude analysis to fit the Dalitz plot of these decays, we find strong evidence that a scalar resonance of mass 478(+24)(-23) +/- 17 MeV/c(2) and width 324(+42)(-40) +/- 21 MeV/c(2) accounts for approximately half of all decays.     

The three-dimensional three-state Potts ferromagnet exposed to random fields: evidence for a second order transition

Using large scale Monte Carlo simulations, the ordering of the three-dimensional three state Potts ferromagnet exposed to random fields is investigated. Studies of the order parameter probability distribution and of various of its moments suggest that the order of the transition depends on the strength of the random field: i.e., the first order transition of the pure ferromagnetic model persists for weak random fields, but turns into a second order transition for a range of random fields of medium strength. For large random fields the transition seems to be first order again. In this range large domains of strongly aligned Potts spins occur already in the disordered phase and the associated slow relaxation hampers significantly the Monte Carlo study of thermodynamic equilibrium phenomena. These results are discussed in the light of current theoretical concepts. Possible applications to experiments on diluted anisotropic molecular crystals and orientational glasses are briefly mentioned.     

Supercooling in a system of two superconductors

Supercooling in the transition of a type I superconductor to the superconducting state in contact with another superconductor whose critical temperature is higher has been measured. Using aluminum as a test material, it has been demonstrated that at temperatures below the critical temperature T _c and magnetic fields below the critical field H _c(T), aluminum remains in a metastable normal state, in spite of its contact with another superconductor. This means that it is not possible to generate a thermodynamic instability in a superconductor’s electronic system through the “proximity effect” with another superconductor whose critical temperature is higher. This experimental observation demonstrates a radical difference between surface superconductivity, which certainly generates instability in normal electronic states, and superconductivity induced by the proximity effect near a junction with another superconductor. Zh. éksp. Teor. Fiz. 112, 1119–1131 (September 1997)     

Magnetic gap effect on the tunneling conductance in a topological insulator ferromagnet/superconductor junction

The tunneling conductance on the surface of a topological-insulator-based ferromagnet/superconductor (F/S) structure is studied where S is an s-wave superconductor with superconducting order parameter ∼Δ. The conductance is calculated based on the BTK formalism. The magnetization in F is applied along the z-direction () in order to induce the energy-mass gaps (m) for the Dirac electrons in the F-region. In this work, the influence of energy gap due to the magnetic field in the F-region on the conductance is emphasized. The Fermi energy mismatch between F (EFF=EF) and S (EFS=EF+U), where the gate potential U is applied to the electrode on top of S, is also considered. As a result, a biased voltage V can cause the conductance switch at eV=Δ, depending on the value of the magnetic field. The conductance is found to be linearly dependent on either m or U. The slope of the curve can also be adjusted. This linear behavior in a topological-insulator-based F/S structure may be valuable for electronic applications of the linear-control-current devices. The tunneling conductances of the quasi-Dirac-particle in a topological-insulator-based F/S junction are quite different from those of a graphene-based F/S junction.     

Josephson junction through a thin ferromagnetic layer: negative coupling

We investigate Josephson coupling through a ferromagnetic thin film using superconductor-insulator-ferromagnet-superconductor planar junctions. Damped oscillations of the critical current are observed as a function of the ferromagnetic layer thickness. We show that they result from the exchange energy gained or lost by a quasiparticle Andreev-reflected at the ferromagnet-superconductor interface. The critical current cancels out at the transition from positive ("0") to negative ("pi") coupling, in agreement with theoretical calculations.     

Charge transport through a flexible molecular junction

Vibrationally inelastic electron transport through a flexible molecular junction is investigated. The study is based on a mechanistic model for a biphenyl molecule between two metal electrodes. Employing methods from electron-molecule scattering theory, which allow a numerically exact treatment, we study the effect of vibrational excitation on the transmission probability for different parameter regimes. The current-voltage characteristic is analyzed for different temperatures, based on a Landauer-type formula. Furthermore, the process of electron assisted tunneling between adjacent wells in the torsional potential of the molecule is discussed and the validity of approximate methods to describe the transmission probability is investigated.This paper is dedicated to Jií Horaáek our friend and scientific mentor of one of us (M)on the occasion of his sixtieth birthday.     

On the relation between two models for superconductors with a structural distortion

Two three-dimensional models exhibiting coexistence of superconductivity and lattice distortion are discussed. One of the models, studied in detail by Rusinov, Do Chan Kat and Kopayev, takes into account the superconductivity inter- and intra-band pairing phenomenologically. The other has been derived by Mattis and Langer from a Fröhlich-type hamiltonian by eliminating the linear electron-phonon coupling in the presence of a “condensing” phonon mode. Both approaches are shown to agree in the limit ω_D å μ, where ω_D is the cut-off energy and ω is the deviation of the chemical potential from the band centre.     

Photon-assisted transport through a quantum dot coupled to superconductors

Photon-assisted electron transport for resonant tunneling has been investigated by using a current formula developed based on the nonequilibrium Green’s function technique. We have studied the external frequency dependence as well as the energy level position dependence for the resonant ac tunneling through the quantum dot coupled to two superconducting reservoirs.     

Effect of normal electrons in superconductors on the current-voltage characteristic of a Josephson junction

It is established that the conductivity of normal electrons in superconductors plays a large role in the current-voltage characteristic of a Josephson junction having a high critical current density, described on the basis of a nonlocal electrodynamics. It is shown in the resistive limit that the normal-electron contribution increases the current through the Josephson junction, while in the capacitive limit it decreases the contribution of Cherenkov resonances. Fiz. Tverd. Tela (St. Petersburg) 41, 582–587 (April 1999)     

Lorenz number in high T(c) superconductors: evidence for bipolarons

The strong electron-phonon interaction in cuprates has gathered support over the last decade in a number of experiments. While phonons remain almost unrenormalized, electrons are transformed into itinerant bipolarons and thermally excited polarons when the electron-phonon interaction is strong. We calculate the Lorenz number of the system to show that the Wiedemann-Franz law breaks down because of the interference of polaron and bipolaron contributions in the heat flow. The model fits numerically the experimental Hall Lorenz number, which provides direct evidence for bipolarons in the cuprates.     

0-pi Transitions in a superconductor/chiral ferromagnet/superconductor junction induced by a homogeneous cycloidal spiral

We study the pi phase in a superconductor-ferromagnet-superconductor Josephson junction, with a ferromagnet showing a cycloidal spiral spin modulation with in-plane propagation vector. Our results reveal a high sensitivity of the junction to the spiral order and indicate the presence of 0-pi quantum phase transitions as function of the spiral wave vector. We find that the chiral magnetic order introduces chiral superconducting triplet pairs that strongly influence the physics in such Josephson junctions, with potential applications in nanoelectronics and spintronics.     

Theoretical study of photoinduced superconductors in a two band model

A study of photoinduced high-T_c superconductivity is presented by canonical two-band BCS model containing Fermi surfaces of p and d holes. We have obtained two superconducting gaps from this model. Studies of chemical potential and hole concentration dependences on critical temperature (T_c) are made. The enhancement of T_c is found due to doping.The study of specific heat and density of states based on this model is also presented. The dependence T_c(n_h) for the system YBa₂Cu₃O_7?x (1 2 3) obtained theoretically agrees with the available experimental data.