Direct transport between superconducting subgap states in a double quantum dot (original) (raw)
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Physical Review B, 2010
We study low-temperature transport through a Coulomb blockaded quantum dot (QD) contacted by a normal (N), and a superconducting (S) electrode. Within an effective cotunneling model the conduction electron self energy is calculated to leading order in the cotunneling amplitudes and subsequently resummed to obtain the nonequilibrium T-matrix, from which we obtain the nonlinear cotunneling conductance. For even occupied dots the system can be conceived as an effective S/Ncotunnel junction with subgap transport mediated by Andreev reflections. The net spin of an odd occupied dot, however, leads to the formation of sub-gap resonances inside the superconducting gap which gives rise to a characteristic peak-dip structure in the differential conductance, as observed in recent experiments.
Nonequilibrium Transport through a Spinful Quantum Dot with Superconducting Leads
Physical Review Letters, 2011
We study the nonlinear cotunneling current through a spinful quantum dot contacted by two superconducting leads. Applying a general nonequilibrium Green function formalism to an effective Kondo model, we study the rich variation in the IV-characteristics with varying asymmetry in the tunnel coupling to source and drain electrodes. The current is found to be carried respectively by multiple Andreev reflections in the symmetric limit, and by spin-induced Yu-Shiba-Russinov bound states in the strongly asymmetric limit. The interplay between these two mechanisms leads to qualitatively different IV-characteristics in the cross-over regime of intermediate symmetry, consistent with recent experimental observations of negative differential conductance and re-positioned conductance peaks in sub-gap cotunneling spectroscopy.
Tunneling through the Quantum Dot Coupled between Normal and Superconducting Leads
Acta Physica Polonica A, 2007
We study the charge tunneling via the quantum dot coupled to normal and superconducting leads, where the superconducting electrode has either an isotropic or anisotropic (of d-wave symmetry) energy gap. We use the single impurity Anderson model and apply the nonequilibrium Green function formalism to determine the differential tunneling conductance. The influence of the proximity effect between the quantum dot and superconductor on the transport properties of the system manifests itself in the Andreev conductance.
Transport properties of proximitized double quantum dots
Physica E: Low-dimensional Systems and Nanostructures, 2021
We study the sub-gap spectrum and the transport properties of a double quantum dot coupled to metallic and superconducting leads. The coupling of both quantum dots to the superconducting lead induces a non-local pairing in both quantum dots by the Andreev reflection processes. Additionally, we obtain two channels of Cooper pair tunneling into a superconducting lead. In such a system, the direct tunneling process (by one of two dots) or the crossed tunneling process (by both quantum dots at the same time) is possible. We consider the dependence of the Andreev transmittance on an inter-dot tunneling amplitude and the coupling between a quantum dot and the superconducting lead. We also consider the occurrence of interferometric Fano-type line shapes in the linear Andreev conductance spectra.
Superconductor Science and Technology, 2003
We study the electron transport through the quantum dot coupled to the normal metal and BCS-like superconductor (N − QD − S) in the presence of the Kondo effect and Andreev scattering. The system is described by the single impurity Anderson model in the limit of strong on-dot interaction. We use recently proposed equation of motion technique for Keldysh nonequilibrium Green's function together with the modified slave boson approach to study the electron transport. We derive formula for the current which contains various tunneling processes and apply it to study the transport through the system. We find that the Andreev conductance is strongly suppressed and there is no zero-bias (Kondo) anomaly in the differential conductance. We discuss effects of the particle-hole asymmetry in the electrodes as well as the asymmetry in the couplings.
Physical Review B, 2010
We study the transport properties of a hybrid nanostructure composed of a ferromagnet, two quantum dots, and a superconductor connected in series. By using the non-equilibrium Green's function approach, we have calculated the electric current, the differential conductance and the transmittance for energies within the superconductor gap. In this regime, the mechanism of charge transmission is the Andreev reflection, which allows for a control of the current through the ferromagnet polarization. We have also included interdot and intradot interactions, and have analyzed their influence through a mean field approximation. In the presence of interactions, Coulomb blockade tend to localized the electrons at the double-dot system, leading to an asymmetric pattern for the density of states at the dots, and thus reducing the transmission probability through the device. In particular, for non-zero polarization, the intradot interaction splits the spin degeneracy, reducing the maximum value of the current due to different spin-up and spin-down densities of states. Negative differential conductance (NDC) appears for some regions of the voltage bias, as a result of the interplay of the Andreev scattering with electronic correlations. By applying a gate voltage at the dots, one can tune the effect, changing the voltage region where this novel phenomenon appears. This mechanism to control the current may be of importance in technological applications.
Quantum transport in superconductor–semiconductor junctions
Physica C: Superconductivity, 2001
Experimental and theoretical results of quantum transport in superconductor±semiconductor junctions are summarized. We will ®rst show the experimental results of reentrant behavior of the conductance as well as of giant Andreev backscattering. Then Andreev re¯ection in the quantum Hall regime is described. The charging eect on the proximity correction is ®nally discussed.
Multiterminal transport spectroscopy of subgap states in Coulomb-blockaded superconductors
2022
Subgap states are responsible for the low-bias transport features of hybrid superconductingsemiconducting devices. Here, we analyze the local and nonlocal differential conductance of Coulomb-blockaded multiterminal superconducting islands that host subgap states with different spatial structures. The emerging patterns of their transport spectroscopy are used to characterize the possible topological nature of these devices and offer the possibility of controlling their transport properties. We develop a next-to-leading order master equation to describe the multiterminal transport in superconductors with both strong Coulomb interactions and multiple subgap states, coupled with metallic leads. We show that the nonlocal differential conductance characterizes the spatial extension of the subgap states and signals the presence of degenerate bound states with a finite support on different parts of the device. Additionally, it displays sharp sign changes as a function of the induced charge of the superconductor, signaling energy crossings among its lowest excited states.
Quantum Transport and Photon-Assisted Andreev Reflection in Superconductor–Semiconductor Interface
Journal of Computational and Theoretical Nanoscience, 2008
We have investigated the quantum transport through the Superconductor-Semiconductor mesoscopic interface in the presence of an external radiation field. The current spectrum is analyzed as a function of the frequency and the temperature. The current-voltage (I-V) characteristics were found to be very sensitive to the photon frequency. Additionally, photon-assisted transport in our system is very robust: The one-photon channel remains up to low temperature, which implies that these structures support gain at THz frequencies even at 9 K. The resonances sit on a background current which it is deeply modified, as a result of photon assisted multiple Andreev reflections. The results render rigid support for the full quantum theory of transport between two superconductors based on the idea of Andreev bound states.
Transport through superconductor/magnetic dot/superconductor structures
Physica C: Superconductivity, 2002
The coupling of two s-wave superconductors through a small magnetic dot is discussed. Assuming that the dot charging energy is small compared to the superconducting gap, Ec ≪ ∆, and that the moment of the dot is classical, we develop a simple theory of transport through the dot. The presence of the magnetic dot will position Andreev bound states within the superconducting gap at energies tunable with the magnetic properties of the dot. Studying the Josephson coupling it is shown that the constructed junction can be tuned from a "0" to a "π"-junction via a degenerate two-level state either by changing the magnetic moment of the dot or by changing temperature. Furthermore, it is shown that details of the magnetic dot can be extracted from the sub-harmonic structure in the current-voltage characteristics of the junction.