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Register a new userQuasiparticles of periodically driven quantum dot coupled between superconducting and normal leads
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We investigate subgap quasiparticles of a single level quantum dot coupled to the superconducting and normal leads, whose energy level is periodically driven by external potential. Using the Floquet formalism we determine the quasienergies and analyze redistribution of their spectral weights between individual harmonics upon varying the frequency and amplitude of the driving potential. We also propose feasible spectroscopic methods for probing the in-gap quasiparticles observable in the differential conductance of the charge current averaged over a period of oscillations.
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Dynamical processes induced by the external time-dependent fields can provide valuable insight into the characteristic energy scales of a given physical system. We investigate them here in a nanoscopic heterostructure, consisting of the double quantum dot coupled in series to the superconducting and the metallic reservoirs, analyzing its response to (i)~abrupt bias voltage applied across the junction, (ii) sudden change of the energy levels, and imposed by (iii)~their periodic driving. We explore subgap properties of this setup which are strictly related to the in-gap quasiparticles and discuss their signatures manifested in the time-dependent charge currents. The characteristic multi-mode oscillations, their beating patters and photon-assisted harmonics reveal a rich spectrum of dynamical features that might be important for designing the superconducting qubits.
Sub-gap transport properties of a quantum dot (QD) coupled to two superconducting and one metallic leads are studied theoretically, solving the time-dependent equation of motion by the Laplace transform technique. We focus on time-dependent response of the system induced by a sudden switching on the QD-leads couplings, studying the influence of initial conditions on the transient currents and the differential conductance. We derive analytical expressions for measurable quantities and find that they oscillate in time with the frequency governed by the QD-superconducting lead coupling and acquire damping, due to relaxation driven by the normal lead. Period of these oscillations increases with the superconducting phase difference $phi$. In particular, for $phi=pi$ the QD occupancy and the normal current evolve monotonically (without any oscillations) to their stationary values. In such case the induced electron pairing vanishes and the superconducting current is completely blocked. We also analyze time-dependent development of the Andreev bound states. We show, that the measurable conductance peaks do not appear immediately after sudden switching of the QD coupling to external leads but it takes some finite time-interval for the system needs create these Andreev states. Such time-delay is mainly controlled by the QD-normal lead coupling.
Crossed Andreev reflection (cAR) is a scattering process that happens in a quantum transport set-up consisting of two normal metals (NM) attached to a superconductor (SC), where an electron incident from one NM results in a hole emerging in the other. Typically, an electron tunnelling through the superconductor from one NM to the other (ET) competes with cAR and masks the signature of cAR in the conductance spectrum. We propose a novel scheme to enhance cAR, in which SC part of the NM-SC-NM is side-coupled to another SC having a different SC phase to form a Josephson junction in the transverse direction. At strong enough coupling and adequate phase difference, one can smoothly traverse between highly ET-dominant to highly cAR-dominant transport regimes by tuning chemical potential, due to the appearance of subgap Andreev states that are extended in the longitudinal direction. We also discuss connections to realistic systems.
We study the transient phenomena appearing in a subgap region of the double quantum dot coupled in series between the superconducting and normal metallic leads, focusing on the development of the superconducting proximity effect. For the uncorrelated nanostructure we derive explicit expressions of the time-dependent occupancies in both quantum dots, charge currents, and electron pairing induced on individual dots and between them. We show that the initial configurations substantially affect the dynamical processes, in which the in-gap bound states emerge upon coupling the double quantum dot to superconducting reservoir. In particular, the superconducting proximity effect would be temporarily blocked whenever the quantum dots are initially singly occupied. Such {it triplet}/{it Andreev blockade} has been recently reported experimentally for double quantum dots embedded in the Josephson [D. Bouman et al., Phys. Rev. B 102, 220505 (2020)] and Andreev [P. Zhang et al., arXiv:2102.03283 (2021)] junctions. We also address the role of correlation effects within the lowest-order decoupling scheme and by the time-dependent numerical renormalization group calculations. Competition of the repulsive Coulomb interactions with the superconducting proximity effect leads to renormalization of the in-gap quasiparticles, speeding up the quantum oscillations and narrowing a region of transient phenomena, whereas the dynamical Andreev blockade is well pronounced in the weak inter-dot coupling limit. We propose feasible methods for detecting the characteristic time-scales that could be observable by the Andreev spectroscopy.
We consider a quantum dot coupled to both superconducting and spin-polarized electrodes, and study the triad interplay of the Kondo effect, superconductivity, and ferromagnetism, any pair of which compete with and suppress each other. We find that the interplay leads to a mixed-valence quantum phase transition, which for other typical sysmstems is merely a crossover rather than a true transition. At the transition, the system changes from the spin doublet to singlet state. The singlet phase is adiabatically connected (through crossovers) to the so-called charge Kondo state and to the superconducting state. We analyze in detail the physical characteristics of different states and propose that the measurement of the cross-current correlation and the charge relaxation resistance can clearly distinguish between them.
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