Nonlocal Andreev Reflection Research Articles

Persistently negative: nonlocal Andreev reflection in a chiral mode

Persistently negative: nonlocal Andreev reflection in a chiral mode

Non-local Andreev reflection through Andreev molecular states in graphene Josephson junctions

We propose that a device composed of two vertically stacked monolayer graphene Josephson junctions can be used for Cooper pair splitting. The hybridization of the Andreev bound states of the two Josephson junction can facilitate non-local transport in this normal-superconductor hybrid structure, which we study by calculating the non-local differential conductance. Assuming that one of the graphene layers is electron and the other is hole doped, we find that the non-local Andreev reflection can dominate the differential conductance of the system. Our setup does not require the precise control of junction length, doping, or super conducting phase difference, which could be an important advantage for experimental realization.

Dynamic heat and charge transports through double-quantum-dot-interferometer modulated by Majorana bound states and time-oscillating Aharonov-Bohm flux

Dynamic properties of Majorana bound states (MBSs) coupled double-quantum-dot (DQD) interferometer threaded with ac magnetic flux are investigated, and the time-averaged thermal current formulas are derived. Photon-assisted local and nonlocal Andreev reflections contribute efficiently to the charge and heat transports. The modifications of source-drain electric, electric-thermal, thermal conductances (G, ξ, κ e ), Seebeck coefficient (S c ), and thermoelectric figure of merit (ZT) versus AB phase have been calculated numerically. These coefficients exhibit the shift of oscillation period from 2π to 4π distinctly due to attaching MBSs. The applied ac flux enhances the magnitudes of G, ξ, κ e obviously, and the detailed enhancing behaviors are relevant to the energy levels of DQD. The enhancements of S c and ZT are generated due to the coupling of MBSs, while the application of ac flux suppresses the resonant oscillations. The investigation provides a clue for detecting MBSs through measuring the photon-assisted S c and ZT versus AB phase oscillations.

Aharonov-Bohm oscillations in the Majorana fermion modulated charge and heat transports through a double-quantum-dot interferometer

The Aharonov-Bohm (AB) oscillation effects of electric and heat currents through a parallel double-quantum-dot (DQD) interferometer coupled with Majorana bound states (MBSs) have been investigated. Local and nonlocal Andreev reflections make efficient contributions to the transports, and the AB oscillation structures are modified by the coupled MBSs explicitly. Four linear electric and thermal conductance components are generated by electric potential and temperature differences. The Onsager relation is invalid due to the presence of Andreev reflections in linear response region. However, when the coupling of MBSs is removed, the Andreev reflections disappear, and the Onsager relation is recovered. The magnitudes of electric and thermal conductances are enhanced due to coupling MBSs, and the overall oscillation period changes from flux quantum Φ0 to 2Φ0. We obtain quite different oscillation structures of Seebeck coefficient for the equal-level and unequal-level DQD systems, where peak-valley and higher-lower double-peak structures appear. Meanwhile, the magnitude of Seebeck coefficient is enlarged obviously due to applying MBSs. The AB oscillation behaviors of these coefficients provide novel signatures for detecting Majorana fermions by employing DQD interferometer.

Double local and double nonlocal Andreev reflections in nodal-line semimetal-superconducting heterostructures

Double local and double nonlocal Andreev reflections in a nodal-line semimetal-superconductor-nodal-line semimetal junction were investigated. We find that double Andreev reflections (the specular Andreev reflection and the retro-Andreev reflection), double normal reflections (the specular normal reflection and the retronormal reflection), double electron transmissions (the specular electron transmission and the normal electron transmission), and double crossed Andreev reflections (the specular crossed Andreev reflection and the crossed Andreev reflection) exist simultaneously when an electron incidents from the semimetal side. The transport properties of the scattering process depend on the incident angle, incident energy, and barrier height. It is found that a finite incident angle and a finite barrier height are necessary conditions for the appearance of these local and nonlocal Andreev reflections. By adjusting these parameters, the reflections and transmissions can be controlled.

Differential conductance and shot noise of a graphene quantum dot coupled with Majorana bound states

Zigzag and armchair hexagonal graphene quantum dots (GQDs) are employed as tunneling spectrometers to detect the signature of Majorana bound states (MBSs) located at the ends of a hybrid nanowire-superconductor junction. The formulas of tunneling current and shot noise are derived to exhibit the local and non-local Andreev reflections induced by MBSs. When the energy splitting of MBSs in the nanowire (e M ) is zero, the hybridization of zero-bias state which comes from normal tunneling and MBSs appears in the zigzag hexagonal GQD system. For the armchair hexagonal GQD under the condition e M = 0, the zero-bias conductance peak emerges in the energy gap to show the absolute contribution of MBSs. When e M ≠ 0, the zero-bias conductance peak splits into symmetrically distributed multi-peaks. These signify that the connected MBSs open novel channels for electrons to tunnel through the metallic and semiconducting GQDs. Shot noise is enhanced by MBSs to form different appearances depending on concrete GQDs. The zigzag GQD system always displays sub-Poissonian shot noise, while super-Poissonian shot noise appears in the armchair GQD system due to the coupling of MBSs. We can distinguish the contributions of MBSs from that of normal tunneling electrons by shot noise and Fano factor at low energy region.

Nonlocal thermoelectricity in a Cooper-pair splitter

We investigate the nonlocal thermoelectric transport in a Cooper-pair splitter based on a double-quantum-dot-superconductor three-terminal hybrid structure. We find that the nonlocal coupling between the superconductor and the quantum dots gives rise to nonlocal thermoelectric effects which originate from the nonlocal particle-hole breaking of the system. We show that Cooper-pair splitting induces the generation of a thermo-current in the superconducting lead without any transfer of charge between the two normal metal leads. Conversely, we show that a nonlocal heat exchange between the normal leads is mediated by non-local Andreev reflection. We discuss the influence of finite Coulomb interaction and study under which conditions nonlocal power generation becomes possible, and when the Cooper-pair splitter can be employed as a cooling device.

Supercurrent carried by nonequilibrium quasiparticles in a multiterminal Josephson junction

We theoretically study coherent multiple Andreev reflections in a biased three-terminal Josephson junction. We demonstrate that the direct current flowing through the junction consists of supercurrent components when the bias voltages are commensurate. This dissipationless current depends on the phase in the superconducting leads and stems from the Cooper pair transfer processes induced by non-local Andreev reflections of the quasiparticles originating from the superconducting leads. We identify supercurrent-enhanced lines in the current and conductance maps of the recent measurement [Y. Cohen, et al., PNAS 115, 6991 (2018)] on a nanowire Josephson junction and show that the magnitude of the phase-dependent current components is proportional to the junction transparency with the power corresponding to the component order.

Nonlocal pairing as a source of spin exchange and Kondo screening

We report on a novel exchange mechanism, mediating the Kondo screening, in a correlated double quantum dot structure in the proximity of superconductor, stemming from nonlocal Andreev reflection processes. We estimate its strength perturbatively and corroborate analytical predictions with accurate numerical renormalization group calculations using an effective model for the superconductor-proximized nanostructure. We demonstrate that superconductor-induced exchange leads to a two-stage Kondo screening. We determine the dependence of the Kondo temperature on the coupling to superconductor and predict a characteristic modification of conventional low-temperature transport behavior, which can be used to experimentally distinguish this phenomenon from other Kondo effects.

Splitting efficiency and interference effects in a Cooper pair splitter based on a triple quantum dot with ferromagnetic contacts

We theoretically study the spin-resolved subgap transport properties of a Cooper pair splitter based on a triple quantum dot attached to superconducting and ferromagnetic leads. Using the Keldysh Green's function formalism, we analyze the dependence of the Andreev conductance, Cooper pair splitting efficiency, and tunnel magnetoresistance on the gate and bias voltages applied to the system. We show that the system's transport properties are strongly affected by spin dependence of tunneling processes and quantum interference between different local and nonlocal Andreev reflections. We also study the effects of finite hopping between the side quantum dots on the Andreev current. This allows for identifying the optimal conditions for enhancing the Cooper pair splitting efficiency of the device. We find that the splitting efficiency exhibits a nonmonotonic dependence on the degree of spin polarization of the leads and the magnitude and type of hopping between the dots. An almost perfect splitting efficiency is predicted in the nonlinear response regime when the energies of the side quantum dots are tuned to the energies of the corresponding Andreev bound states. In addition, we analyzed features of the tunnel magnetoresistance (TMR) for a wide range of the gate and bias voltages, as well as for different model parameters, finding the corresponding sign changes of the TMR in certain transport regimes. The mechanisms leading to these effects are thoroughly discussed.

Andreev reflections and the quantum physics of black holes

We establish an analogy between superconductor-metal interfaces and the quantum physics of a black hole, using the proximity effect. We show that the metal-superconductor interface can be thought of as an event horizon and Andreev reflection from the interface is analogous to the Hawking radiation in black holes. We describe quantum information transfer in Andreev reflection with a final state projection model similar to the Horowitz-Maldacena model for black hole evaporation. We also propose the Andreev reflection analogue of Hayden and Preskill's description of a black hole final state, where the black hole is described as an information mirror. The analogy between crossed Andreev reflections and Einstein-Rosen bridges is discussed: our proposal gives a precise mechanism for the apparent loss of quantum information in a black hole by the process of nonlocal Andreev reflection, transferring the quantum information through a wormhole and into another universe. Given these established connections, we conjecture that the final quantum state of a black hole is exactly the same as the ground state wave function of the superconductor/superfluid in the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity; in particular, the infalling matter and the infalling Hawking quanta, described in the Horowitz-Maldacena model, forms a Cooper pairlike singlet state inside the black hole. A black hole evaporating and shrinking in size can be thought of as the analogue of Andreev reflection by a hole where the superconductor loses a Cooper pair. Our model does not suffer from the black hole information problem since Andreev reflection is unitary. We also relate the thermodynamic properties of a black hole to that of a superconductor, and propose an experiment which can demonstrate the negative specific heat feature of black holes in a growing/evaporating condensate.

Nonlocal Andreev entanglements and triplet correlations in graphene with spin-orbit coupling

Using a wavefunction Dirac Bogoliubov-de Gennes method, we demonstrate that the tunable Fermi level of a graphene layer in the presence of Rashba spin orbit coupling (RSOC) allows for producing an anomalous nonlocal Andreev reflection and equal spin superconducting triplet pairing. We consider a graphene junction of a ferromagnet-RSOC-superconductor-ferromagnet configuration and study scattering processes, the appearance of spin triplet correlations, and charge conductance in this structure. We show that the anomalous crossed Andreev reflection is linked to the equal spin triplet pairing. Moreover, by calculating current cross-correlations, our results reveal that this phenomenon causes negative charge conductance at weak voltages and can be revealed in a spectroscopy experiment, and may provide a tool for detecting the entanglement of the equal spin superconducting pair correlations in hybrid structures.

Enhanced nonlocal Andreev reflection in F|S|F graphene spin-valve

Abstract In this paper crossed Andreev reflection (CAR) conductance is calculated in graphene-based Ferromagnetic-Superconductor-Ferromagnetic heterostructure. In this spin-valve system, the ferromagnetic semi-infinite layers act as leads. The leads are assumed to be half-metallic, i.e. the respective shift of the two spin sub-bands at each lead is such that the electronic states of just one spin sub-band are present near the Fermi level. In this graphene-based system, as in the corresponding metallic structures, if the leads are in anti-parallel configuration, direct Andreev reflection (AR) and electron cotunneling(CT) are weak while crossed Andreev reflection is considerable. The CAR reaches the maximum probability amplitude for thickness of the superconducting layer that is comparable to the superconducting coherence length. The behavior of the system at parallel configuration of the leads, contradicts with metallic FSF structures, so that an appreciable amount of CAR probability is obtained. This is provided in graphene by the combination of CAR and spin-dependent Klein tunneling through p-n barrier between different spin sub-bands of the two leads. In the case that the Fermi energy of the first lead is in Dirac point the result is the enhanced CAR due to blocking CT channels in both parallel and anti parallel configurations. The resulting nonlocal conductance oscillates with L exhibiting a π -phase shift between the two configurations.

Nonlocal spin-entangled Andreev reflection, fractional charge, and current-phase relations in topological bilayer-exciton-condensate junctions

We study Andreev reflection and Josephson currents in topological bilayer exciton condensates (TEC). These systems can create 100% spin entangled nonlocal currents with high amplitudes due to perfect nonlocal Andreev reflection. This Andreev reflection process can be gate tuned from a regime of purely retro reflection to purely specular reflection. We have studied the bound states in TEC-TI-TEC Josephson junctions and find a gapless dispersion for perpendicular incidence. The presence of a sharp transition in the supercurrent-phase relationship when the system is in equilibrium is a signature of fractional charge, which can be further revealed in ac measurements faster than relaxation processes via Landau-Zener processes.

Superconducting proximity effect in silicene: Spin-valley-polarized Andreev reflection, nonlocal transport, and supercurrent

We theoretically study the superconducting proximity effect in silicene, which features massive Dirac fermions with a tunable mass (band gap), and compute the conductance across a normal-superconductor (N-S) silicene junction, the nonlocal conductance of an N-S-N junction, and the supercurrent flowing in an S-N-S junction. It is demonstrated that the transport processes consisting of local and nonlocal Andreev reflection may be efficiently controlled via an external electric field owing to the buckled structure of silicene. In particular, we demonstrate that it is possible to obtain a fully spin-valley-polarized crossed Andreev reflection process without any contamination of elastic cotunneling or local Andreev reflection, in stark contrast to ordinary metals. It is also shown that the supercurrent flowing in the S-N-S junction can be fully spin-valley polarized and that it is controllable by an external electric field.

Resonant nonlocal Andreev reflection in a narrow quantum spin Hall system

Resonant nonlocal Andreev reflection in a narrow quantum spin Hall system

Nonlocal Andreev reflection with Rashba spin—orbital interaction in a triple-quantum-dot ring

We theoretically studied the nonlocal Andreev reflection with Rashba spin—orbital interaction in a triple-quantum-dot (QD) ring, which is introduced as Rashba spin—orbital interaction to act locally on one component quantum dot. It is found that the electronic current and spin current are sensitive to the systematic parameters. The interdot spin-flip term does not play a leading role in causing electronic and spin currents. Otherwise the spin precessing term leads to shift of the peaks of the the spin-up and spin-down electronic currents in different directions and results in the spin current. Moreover, the spin-orbital interaction suppresses the nonlocal Andreev reflection, so we cannot obtain the pure spin current.

Positive noise cross-correlation in hybrid superconducting and normal-metal three-terminal devices

Nonlocal entanglement is crucial for quantum information processes. While nonlocal entanglement has been realized for photons, it is much more difficult to demonstrate for electrons. One approach that has been proposed is to use hybrid superconducting/normal-metal devices, where a Cooper pair splits into spin-entangled electrons in two spatially separated normal-metal leads. This process of nonlocal Andreev reflection is predicted to lead to a negative nonlocal resistance and positive current-current correlation. By cross-correlation measurements as well as measurements of the local and nonlocal resistance, we present here experimental evidence showing that by independently controlling the energy of electrons at the superconductor/normal-metal interfaces, nonlocal Andreev reflction, the signature of spin-entanglement, can be maximized.

Local and nonlocal Andreev reflections in double barrier junctions

We study spin dependent charge transport through a junction, consisting of a superconductor connected to two normal or spin-polarized electrodes. Special attention is paid to a process known as a crossed or nonlocal Andreev reflection. The nonlocal reflection occurs when an incident electron (hole) from one electrode, injected with a subgap energy onto o superconducting layer, transforms into an Andreev hole (electron) in the second lead. When two electrons (holes), forming a Cooper pair, come from the same electrode, the process of the Andreev reflection is local. The coherent spin polarized transport in ferromagnet/superconductor/ferromagnet (F/S/F) double barrier junctions is analyzed using the Bogolubov-de Gennes (BdG) equation with appropriate boundary conditions. We calculate and discuss probabilities of the normal, local and crossed Andreev reflections, as well as probability of the elastic co-tunneling. These processes contribute to tunneling current when the distance between the magnetic electrodes is comparable to the superconducting coherence length. The dependence of the tunneling charge transport on the strength of the exchange field in the ferromagnetic electrodes, and on the height of the tunnel barriers are presented. We also discuss the local and nonlocal spin quantum entanglements of quasiparticle pairs induced by the Andreev reflections in the junction.

Nonlocal Andreev reflection and spin current in a three-terminal Aharonov–Bohm interferometer

This paper theoretically reports the nonlocal Andreev reflection and spin current in a normal metal-ferromagnetic metal-superconducting Aharonov–Bohm interferometer. It is found that the electronic current and spin current are sensitive to systematic parameters, such as the gate voltage of quantum dots and the external magnetic flux. The electronic current in the normal metal lead results from two competing processes: quasiparticle transmission and nonlocal Andreev reflection. The appearance of zero spin-up electronic current (or spin-down electronic current) signals the existence of nonlocal Andreev reflection, and the presence of zero electronic current results in the appearance of pure spin current.