arXiv subjects

R. Mélin

Publications and source records attributed to R. Mélin.

At least 19 recordsLinked to original sources

arXiv2018

Nonadiabatic Josephson current pumping by microwave irradiation

Irradiating a Josephson junction with microwaves can operate not only on the amplitude but also on the phase of the Josephson current. This requires breaking time inversion symmetry, which is achieved by introducing a phase lapse between the microwave components acting on the two† sides of the junction. General symmetry arguments and the solution of a specific single level quantum dot model show that this induces chirality in the Cooper pair dynamics, due to the topology of the Andreev bound state wavefunction. Another essential condition is to break electron-hole symmetry within the junction. A shift of the current-phase relation is obtained, which is controllable in sign and amplitude with the microwave phase and an electrostatic gate, thus producing a "chiral" Josephson transistor. The dot model is solved in the infinite gap limit by Floquet theory and in the general case with Keldysh nonequilibrium Green's functions. The chiral current is nonadiabatic: it is extremal and changes sign close to resonant chiral transitions between the Andreev bound states.

cond-mat.mes-hall↗

arXiv2018

Positive noise cross-correlations in superconducting hybrids: Roles of interfaces and interactions

Shot noise cross-correlations in normal metal-superconductor-normal metal structures are discussed at arbitrary interface transparencies using both the scattering approach of Blonder, Tinkham and Klapwik and a microscopic Green's function approach. Surprisingly, negative crossed conductance in such set-ups [R. Melin and D. Feinberg, Phys. Rev. B 70, 174509 (2004)] does not preclude the possibility of positive noise cross-correlations for almost transparent contacts. We conclude with a phenomenological discussion of interactions in the one dimensional leads connected to the superconductor, which induce sign changes in the noise cross-correlations.

cond-mat.mes-hall↗

arXiv2015

Closing the proximity gap in a metallic Josephson junction between three superconductors

We describe the proximity effect in a short disordered metallic junction between three superconducting leads. Andreev bound states in the multi-terminal junction may cross the Fermi level. We reveal that for a quasi-continuous metallic density of states, crossings at the Fermi level manifest as closing of the proximity-induced gap. We calculate the local density of states for a wide range of transport parameters using quantum circuit theory. The gap closes inside an area of the space spanned by the superconducting phase differences. We derive an approximate analytic expression for the boundary of the area and compare it to the full numerical solution. The size of the area increases with the transparency of the junction and is sensitive to asymmetry. The finite density of states at zero energy is unaffected by electron-hole decoherence present in the junction, although decoherence is important at higher energies. Our predictions can be tested using tunneling transport spectroscopy. To encourage experiments, we calculate the current-voltage characteristic in a typical measurement setup. We show how the structure of the local density of states can be mapped out from the measurement.

cond-mat.mes-hall↗

arXiv2014

Quartets and the Current-Phase Structure of a Double Quantum Dot Superconducting Bijunction at Equilibrium

The equilibrium current-phase structure of a tri-terminal superconducting Josephson junction (bijunction) is analyzed as a function of the two relevant phases. The bijunction is made of two noninteracting quantum dots, each one carrying a single level. Nonlocal processes coupling the three terminals are described in terms of quartet tunneling and pair cotunneling. These couplings are due to nonlocal Andreev and cotunneling processes through the central superconductor $S_0$, as well as direct interdot coupling. In some cases, two degenerate midgap Andreev states appear, symmetric with respect to the ($π,π$) point. The lifting of this degeneracy by interdot couplings induces a strong non-local inductance at low enough temperatures. This effect is compared to the mutual inductance of a two-loop circuit.

cond-mat.mes-hall↗

arXiv2014

D.C. Josephson transport by quartets and other Andreev resonances in superconducting bijunctions

Bijunctions are three-terminal Josephson junctions where three superconductors are connected by a single weak link made of a metallic region or of quantum dots. Biasing two of the superconductors with commensurate voltages yields Andreev resonances that produce d.c. Josephson currents made of correlated Cooper pairs. For instance with applied voltages (0, V,-V), quartets formed by two entangled Cooper pairs are emitted by one reservoir towards the two others. Theory involving non-equilibrium Green's functions reveal the microsopic mechanism at play, e.g. multiple coherent Andreev reflections that provide an energy-conserving and fully coherent channel. Recent experiments on diffusive Aluminum-Copper bijunctions show transport anomalies that are interpreted in terms of quartet resonances.

cond-mat.mes-hall↗

arXiv2014

Proposal for the observation of nonlocal multipair production: the biSQUID

We propose an all-superconducting three-terminal setup consisting in a carbon nanotube (or semiconducting nanowire) contacted to three superconducting leads. The resulting device, referred to as a "biSQUID", is made of four quantum dots arranged in two loops of different surface area. We show how this biSQUID can prove a useful tool to probe nonlocal quantum phenomena in an interferometry setup. We study the measured critical current as a function of the applied magnetic field, which shows peaks in its Fourier spectrum, providing clear signatures of multipair Josephson processes. The device does not require any specific fine-tuning as these features are observed for a wide range of microscopic parameters -- albeit with a non-trivial dependence. Competing effects which may play a significant role in actual experimental realizations are also explored.

cond-mat.mes-hall↗

arXiv2013

Multipair DC-Josephson Resonances in a biased all-superconducting Bijunction

An all-superconducting bijunction consists of a central superconductor contacted to two lateral superconductors, such that non-local crossed Andreev reflection is operating. Then new correlated transport channels for the Cooper pairs appear in addition to those of separated conventional Joseph- son junctions. We study this system in a configuration where the superconductors are connected through gate-controllable quantum dots. Multipair phase-coherent resonances and phase-dependent multiple Andreev reflections are both obtained when the voltages of the lateral superconductors are commensurate, and they add to the usual local dissipative transport due to quasiparticles. The two-pair resonance (quartets) as well as some other higher order multipair resonances are π-shifted at low voltage. Dot control can be used to dramatically enhance the multipair current when the voltages are resonant with the dot levels.

cond-mat.mes-hall↗

arXiv2009

Self-consistent microscopic calculations for non-local transport through nanoscale superconductors

We implement self-consistent microscopic calculations in order to describe out-of-equilibrium non-local transport in normal metal-superconductor-normal metal hybrid structures in the presence of a magnetic field and for arbitrary interface transparencies. A four terminal setup simulating usual experimental situations is described by means of a tight-binding model. We present results for the self-consistent order parameter and current profiles within the sample. These profiles illustrate a crossover from a quasi-equilibrium to a strong non-equilibrium situation when increasing the interface transparencies and the applied voltages. We analyze in detail the behavior of the non-local conductance in these two different regimes. While in quasi-equilibrium conditions this can be expressed as the difference between elastic cotunneling and crossed Andreev transmission coefficients, in a general situation additional contributions due to the voltage dependence of the self-consistent order parameter have to be taken into account. The present results provide a first step towards a self-consistent theory of non-local transport including non-equilibrium effects and describe qualitatively a recent experiment [Phys. Rev. Lett. 97, 237003 (2006)].

cond-mat.supr-con↗

arXiv2007

Non local Andreev reflection in a carbon nanotube superconducting quantum interference device

We investigate a superconducting quantum interference device (SQUID) based on carbon nanotubes in a fork geometry [J.-P. Cleuziou {\it et al.}, Nature Nanotechnology {\bf 1}, 53 (2006)], involving tunneling of evanescent quasiparticles through a superconductor over a distance comparable to the superconducting coherence length, with therefore ``non local'' processes generalizing non local Andreev reflection and elastic cotunneling. Non local processes induce a reduction of the critical current and modify the current-phase relation. We discuss arbitrary interface transparencies. Such devices in fork geometries are candidates for probing the phase coherence of crossed Andreev reflection.

cond-mat.mes-hall↗

arXiv2007

Subgap tunneling via quantum-interference effect: insulators and charge density waves

A quantum interference effect is discussed for subgap tunneling over a distance comparable to the coherence length, which is a consequence of ``advanced-advanced'' and ``retarded-retarded'' transmission modes [Altland and Zirnbauer, Phys. Rev. B 55, 1142 (1997)]. Effects typical of disorder are obtained from the interplay between multichannel averaging and higher order processes in the tunnel amplitudes. Quantum interference effects similar to those occurring in normal tunnel junctions explain magnetoresistance oscillations of a CDW pierced by nanoholes [Latyshev et al., Phys. Rev. Lett. 78, 919 (1997)], having periodicity h/2e as a function of the flux enclosed in the nanohole. Subgap tunneling is coupled to the sliding motion by charge accumulation in the interrupted chains. The effect is within the same trend as random matrix theory for normal metal-CDW hybrids [Visscher et al., Phys. Rev. B 62, 6873 (2000)]. We suggest that the experiment by Latyshev et al. probes weak localization-like properties of evanescent quasiparticles, not an interference effect related to the quantum mechanical ground state.

cond-mat.mes-hall↗

arXiv2007

Equilibrium Low Temperature Heat Capacity of the Spin Density Wave compound (TMTTF)2 Br: effect of a Magnetic Field

We have investigated the effect of the magnetic field (B) on the very low-temperature equilibrium heat capacity ceq of the quasi-1 D organic compound (TMTTF)2Br, characterized by a commensurate Spin Density Wave (SDW) ground state. Below 1K, ceq is dominated by a Schottky-like contribution, very sensitive to the experimental time scale, a property that we have previously measured in numerous DW compounds. Under applied field (in the range 0.2- 7 T), the equilibrium dynamics, and hence ceq extracted from the time constant, increases enormously. For B = 2-3 T, ceq varies like B2, in agreement with a magnetic Zeeman coupling. Another specific property, common to other Charge/Spin density wave (DW) compounds, is the occurrence of metastable branches in ceq, induced at very low temperature by the field exceeding a critical value. These effects are discussed within a generalization to SDWs in a magnetic field of the available Larkin-Ovchinnikov local model of strong pinning. A limitation of the model when compared to experiments is pointed out.

cond-mat.dis-nn↗

arXiv2007

Nonlocal transport mediated by nonlocal Hikami boxes: Condensation of evanescent quasiparticles injected into the superconducting gap

Evanescent quasiparticles entering a superconductor and propagating over distances larger than the coherence length give rise to intermediate states induced by double Andreev scattering on disorder.The resulting effective attractive interaction between evanescent quasiparticles is retarded at the extremely slow frequency of the applied bias voltage. The non-BCS out-of-equilibrium mesoscopic condensate is compatible with a recent experiment [S. Russo et al., Phys. Rev. Lett. 95, 027002 (2005)]. Microscopic theory is discussed in the random phase approximation.

cond-mat.mes-hall↗

arXiv2006

Dephasing of Andreev pairs entering a charge density wave

An Andreev pair from a s-wave superconductor (S) entering a conventional gapless charge density wave (CDW) below the Peierls gap dephases on the Fermi wavelength while one particle states are localized on the CDW coherence length. The paths following different sequences of impurities interfere destructively, due to the different electron and hole densities in the CDW. The same conclusion holds for averaging over the conduction channels in the ballistic system. We apply two microscopic approaches to this phenomenon: i) a Blonder, Tinkham, Klapwijk (BTK) approach for a single highly transparent S-CDW interface; and ii) the Hamiltonian approach for the Josephson effect in a clean CDW and a CDW with non magnetic disorder. The Josephson effect through a spin density wave (SDW) is limited by the coherence length, not by the Fermi wave-length.

cond-mat.mes-hall↗

arXiv2006

Thouless energy of a superconductor from non local conductance fluctuations

We show that a spin-up electron from a normal metal entering a superconductor propagates as a composite object consisting of a spin-down hole and a pair in the condensate. This leads to a factorization of the non local conductance as two local Andreev reflections at both interfaces and one propagation in the superconductor, which is tested numerically within a one dimensional toy model of reflectionless tunneling. Small area junctions are characterized by non local conductance fluctuations. A treatment ignoring weak localization leads to a Thouless energy inverse proportional to the sample size, as observed in the numerical simulations. We show that weak localization can have a strong effect, and leads to a coupling between evanescent quasiparticles and the condensate by Andreev reflections ``internal'' to the superconductor.

cond-mat.mes-hall↗

arXiv2006

Strongly disordered Hubbard model in one dimension: spin and orbital infinite randomness and Griffiths phases

We study by the strong disorder renormalization group (RG) method the low-energy properties of the one-dimensional Hubbard model with random-hopping matrix-elements $t_{min} t_{max}$) and to an orbital infinite randomness fixed point for vanishing interactions ($U_{max}/t_{max} \to 0$). To each critical infinite randomness fixed point is connected a Griffiths phase, the correlation length and dynamical exponent of which have well defined asymptotic dependences on the corresponding quantum control parameter. The theoretical predictions for the scaling in the vicinity of the critical points compare well to numerical RG simulations.

cond-mat.dis-nn↗

arXiv2006

Interplay between phase defects and spin polarization in the specific heat of the spin density wave compound (TMTTF)_2Br in a magnetic field

Equilibrium heat relaxation experiments provide evidence that the ground state of the commensurate spin density wave (SDW) compound (TMTTF)$_2$Br after the application of a sufficient magnetic field is different from the conventional ground state. The experiments are interpreted on the basis of the local model of strong pinning as the deconfinement of soliton-antisoliton pairs triggered by the Zeeman coupling to spin degrees of freedom, resulting in a magnetic field induced density wave glass for the spin carrying phase configuration.

cond-mat.dis-nn↗

arXiv2006

Contribution of weak localization to non local transport at normal metal / superconductor double interfaces

In connection with a recent experiment [Russo {\it et al.}, Phys. Rev. Lett. {\bf 95}, 027002 (2005)], we investigate the effect of weak localization on non local transport in normal metal / insulator / superconductor / insulator / normal metal (NISIN) trilayers, with extended interfaces. The negative weak localization contribution to the crossed resistance can exceed in absolute value the positive elastic cotunneling contribution if the normal metal phase coherence length or the energy are large enough.

cond-mat.mes-hall↗

arXiv2006

Possibility of reflectionless tunneling crossed transport at normal metal / superconductor double interfaces

We investigate one dimensional models (the Blonder, Tinkham, Klapwijk model and a tight-binding model) of non local transport at normal metal / superconductor (NS) double interfaces. We find a negative elastic cotunneling crossed conductance, strongly enhanced by additional scatterers away from the interfaces, suggesting the possibility of reflectionless tunneling non local transport at double NS interfaces with contacts having a sufficiently small extension.

cond-mat.mes-hall↗