Denis Feinberg

Coupling superconducting-ferromagnetic point contacts by Andreev reflections

The properties of a device made of two point contacts between normal (N) or ferromagnetic (F) tips, and a superconductor (S), are discussed as a function of the spin polarization and the distance L between the contacts. When L is smaller than the superconductor coherence length ξ, nonlocal Andreev reflections occur: for opposite spin polarizations of the contacts, “mixed” Cooper pairs made of electrons coming one from each tip can be injected into the superconductor. This leads to unusual properties, for instance, the parallel resistance of two S/F contacts goes from infinity for full and equal polarizations, to a finite Andreev value for opposite ones.

Correlated tunneling into a superconductor in a multiprobe hybrid structure

We consider tunneling in a hybrid system consisting of a superconductor with two or more probe electrodes which can be either normal metals or polarized ferromagnets. In particular we study transport at subgap voltages and temperatures. Besides Andreev pair tunneling at each contact, in multi-probe structures subgap transport involves additional channels, which are due to coherent propagation of two particles (electrons or holes), each originating from a different probe electrode. The relevant processes are electron cotunneling through the superconductor and conversion into a Cooper pair of two electrons stemming from different probes. These processes are non-local and decay when the distance between the pair of involved contacts is larger than the superconducting coherence length. The conductance matrix of a three-terminal hybrid structure is calculated. The multi-probe processes enhance the conductance of each contact. If the contacts are magnetically polarized the contribution of the various conduction channels can be separately detected.

DYNAMICAL MEAN-FIELD THEORY OF THE SMALL POLARON

A dynamical mean-field theory of the small polaron problem is presented, which becomes exact in the limit of infinite dimensions. The ground-state properties and the one-electron spectral function are obtained for a single electron interacting with Einstein phonons by a mapping of the lattice problem onto a polaronic impurity model. The one-electron propagator of the impurity model is calculated through a continued fraction expansion, at both zero and finite temperature, for any electron-phonon coupling and phonon energy. In contrast to the ground-state properties, such as the effective polaron mass, which show a continuous behavior as the coupling is increased, spectral properties exhibit a sharp qualitative change at low enough phonon frequency: beyond a critical coupling, one energy gap and then more open in the density of states at low energy, while the high-energy part of the spectrum is broad and can be qualitatively explained by a strong coupling adiabatic approximation. As a consequence, narrow and coherent low-energy subbands coexist with an incoherent featureless structure at high energy. The subbands denote the formation of quasiparticle polaron states. Also, divergencies of the self-energy may occur in the gaps. At finite temperature such an effect triggers an important damping and broadening of the polaron subbands. On the other hand, in the large phonon frequency regime such a separation of energy scales does not exist and the spectrum always has a multipeaked structure.

SQUEEZING PHENOMENA IN INTERACTING ELECTRON-PHONON SYSTEMS

The molecular crystal model of electrons coupled to Einstein phonons is studied as a function of the two parameters: the coupling constant A and the ratio of the electron-phonon coupling energy to the phonon energy, denoted by α. Both the one-electron and the many-electron models are studied, starting (for the former) from the adiabatic limit and (for the latter) from the anti-adiabatic one. In the “multiphonon” regime α>1, the sharp crossover between quasi-free electrons (λ≪1) and small polarons (λ≫1) is investigated, emphasizing the anomalous lattice fluctuations which occur in the intermediate regime (λ≈1). These fluctuations are due to the band motion of the electrons strongly coupled to the lattice and are shown in turn to weaken the electron mass renormalization inherent to self-trapping. In a relevant part of the intermediate region the effective electron mass slowly increases with λ, due to a competition between the phonon dressing effect and the reduction of lattice momentum fluctuations. This re...

Theory of the tilted vortex lattice in Josephson-coupled layered superconductors

Abstract Field penetration is studied for arbitrary directions in Josephson-coupled layered superconductors, in the quasi-2D regime described by the Lawrence-Doniach model with transverse coherence length ξ ⊥ = ξ ‖/ Γ ⪡ d , the interlayer distance. Flux lines appear as oblique lines of 2D vortices piercing the layers. In the absence of Josephson coupling, the normal field component creates an Abrikosov vortex lattice normal to the layers, while the parallel component penetrates freely. For finite Josephson coupling, the vortex lattice structure may have two distinct behaviours: for moderate fields it is dominated by anisotropic London 3D currents, while in high fields the lattice has 2D character. In the 3D regime the usual core is replaced by an effective core made of 2D cores together with a region where screening is essentially 2D. Moreover, when tan θ > Γ, where θ is the field angle with respect to the normal to the layers, the effective cores are made of 2D vortices connected with pieces of Josephson vortices.

Spin-Current Shot Noise as a Probe of Interactions in Mesoscopic Systems

It is shown that the spin-resolved current shot noise can probe attractive or repulsive interactions in mesoscopic systems. This is illustrated in two physical situations: (i) a normal-superconducting junction where the spin-current noise is found to be zero, and (ii) a single-electron transistor where the spin-current noise is found to be Poissonian. Repulsive interactions may also lead to weak attractive correlations (bunching of opposite spins) in conditions far from equilibrium. Spin-current shot noise can also be used to measure the spin relaxation time T1, and a setup is proposed in a quantum dot geometry.

Anomalous Josephson Current in Junctions with Spin Polarizing Quantum Point Contacts

We consider a ballistic Josephson junction with a quantum point contact in a two-dimensional electron gas with Rashba spin-orbit coupling. The point contact acts as a spin filter when embedded in a circuit with normal electrodes. We show that with an in-plane external magnetic field an anomalous supercurrent appears even for zero phase difference between the superconducting electrodes. In addition, the external field induces large critical current asymmetries between the two flow directions, leading to supercurrent rectifying effects.

ELECTROMAGNETIC PINNING OF VORTICES BY NON-SUPERCONDUCTING DEFECTS AND THEIR INFLUENCE ON SCREENING

Abstract An image method is developed in order to analyze the contribution of screening currents to the interaction of vortices with cylindrical defects. In the case of layered superconductors the interaction between elementary (pancake) vortices and a circular dielectric region is calculated. The method can be easily generalized to an arbitrary configuration of vortices, and the deformation of the vortex lattice near a single defect is considered as an example. It also shows that the global effect of a collection of defects is to weaken screening of the magnetic field and renormalize the London penetration length for long-range field components. More generally, the analogy between the London equations at short distances and electrostatics is discussed in detail.

Andreev scattering and cotunneling between two superconductor-normal metal interfaces: the dirty limit

Abstract.Crossed Andreev reflections and cotunneling occur between two neighbouring superconductor- normal metal or superconducting-ferromagnet interfaces. Previous works assumed a clean BCS superconductor. Here the calculation of the corresponding crossed conductance terms is generalized to a dirty superconductor. The range of the effect is shown to be the coherence length

PARALLEL FIELD PENETRATION IN A LAYERED SUPERCONDUCTOR

\tilde{\xi} = \sqrt{\hbar D/\Delta}