arXiv subjects

Andrei D. Zaikin

Publications and source records attributed to Andrei D. Zaikin.

At least 37 records · Page 2Linked to original sources

arXiv2019

Quantum phase slips and voltage fluctuations in superconducting nanowires

We argue that quantum phase slips (QPS) may generate non-equilibrium voltage fluctuations in superconducting nanowires. In the low frequency limit we evaluate all cumulants of the voltage operator which obey Poisson statistics and show a power law dependence on the external bias. We specifically address quantum shot noise which power spectrum $S_Ω$ may depend non-monotonously on temperature. In the long wire limit $S_Ω$ decreases with increasing frequency $Ω$ and vanishes beyond a threshold value of $Ω$ at $T \to 0$. Our predictions can be directly tested in future experiments with superconducting nanowires.

cond-mat.mes-hall↗

arXiv2019

Full counting statistics of quantum phase slips

We work out a microscopic theory describing complete statistics of voltage fluctuations generated by quantum phase slips (QPS) in superconducting nanowires. We evaluate the cumulant generating function and demonstrate that shot noise of the voltage as well as the third and all higher voltage cumulants differ from zero only due to the presence of QPS. In the zero-frequency limit voltage fluctuations in superconducting nanowires are described by Poisson statistics just as in a number of other tunneling-like problems. However, at non-zero frequencies quantum voltage fluctuations in superconducting nanowires become much more complicated and are not anymore accounted for by Poisson statistics. In the case of short superconducting nanowires we explicitly evaluate all finite-frequency voltage cumulants and establish a non-trivial relation between these cumulants and the current-voltage characteristics of our system.

cond-mat.mes-hall↗

arXiv2018

Intrinsic dissipation in superconducting junctions probed by qubit spectroscopy

We propose to study frequency dependent intrinsic dissipation in a highly transparent Josephson junction by means of qubit spectroscopy. The spectral density of the effective dissipative bath may contain significant contributions from Andreev bound states coupled to fluctuations of the Josephson phase. Varying either the bias current applied to the junction or magnetic flux through a superconducting ring in the rf-SQUID setup, one can tune the level splitting value close to the bottom of the Josephson potential well. Monitoring the qubit energy relaxation time one can probe the spectral density of the effective dissipative bath and unambiguously identify the contribution emerging from Andreev levels.

cond-mat.supr-con↗

arXiv2018

Topology-controlled thermopower oscillations in multiterminal Andreev interferometers

We theoretically investigate coherent oscillations of the thermopower $\mathcal{S}$ as a function of the magnetic flux $Φ$ in six-terminal Andreev interferometers. We demonstrate that the thermopower behavior is determined by a number of contributions originating from the Josephson-like and Aharonov-Bohm-like effects as well as from electron-hole asymmetry. The relative weight of these contributions depends on the relation between temperature, voltage bias and an effective Thouless energy of our setup. We particularly emphasize the role of the system topology that may have a dramatic impact on the behavior of $\mathcal{S}(Φ)$.

cond-mat.mes-hall↗

arXiv2018

Interplay between Josephson and Aharonov-Bohm effects in Andreev interferometers

Proximity induced quantum coherence of electrons in multi-terminal voltage-driven hybrid normal-superconducting nanostructures may result in a non-trivial interplay between topology-dependent Josephson and Aharonov-Bohm effects. We elucidate a trade-off between stimulation of the voltage-dependent Josephson current due to non-equilibrium effects and quantum dephasing of quasiparticles causing reduction of both Josephson and Aharonov-Bohm currents. We also predict phase-shifted quantum coherent oscillations of the induced electrostatic potential as a function of the externally applied magnetic flux. Our results may be employed for engineering superconducting nanocircuits with controlled quantum properties.

cond-mat.mes-hall↗

arXiv2018

Current-phase relation and flux-dependent thermoelectricity in Andreev interferometers

We predict a novel $(I_0,ϕ_0)$-junction state of multi-terminal Andreev interferometers that emerges from an interplay between long-range quantum coherence and non-equilibrium effects. Under non-zero bias $V$ the current-phase relation $I_S(ϕ)$ resembles that of a $ϕ_0$-junction differing from the latter due to a non-zero average $I_0(V) = \left< I_S(ϕ)\right>_ϕ$. The flux-dependent thermopower ${\mathcal S}(Φ)$ of the system exhibits features similar to those of a $(I_0,ϕ_0)$-junction and in certain limits it can reduce to either odd or even function of $Φ$ in the agreement with a number of experimental observations.

cond-mat.supr-con↗

arXiv2017

Shot noise in ultrathin superconducting wires

Quantum phase slips (QPS) may produce non-equilibrium voltage fluctuations in current-biased superconducting nanowires. Making use of the Keldysh technique and employing the phase-charge duality arguments we investigate such fluctuations within the four-point measurement scheme and demonstrate that shot noise of the voltage detected in such nanowires may essentially depend on the particular measurement setup. In long wires the shot noise power decreases with increasing frequency $Ω$ and vanishes beyond a threshold value of $Ω$ at $T \to 0$

cond-mat.supr-con↗

arXiv2017

Quantum phase fluctuations and density of states in superconducting nanowires

We argue that quantum fluctuations of the phase of the order parameter may strongly affect the electron density of states (DOS) in ultrathin superconducting wires. We demonstrate that the effect of such fluctuations is equivalent to that of a quantum dissipative environment formed by sound-like plasma modes propagating along the wire. We derive a non-perturbative expression for the local electron DOS in superconducting nanowires which fully accounts for quantum phase fluctuations. At any non-zero temperature these fluctuations smear out the square-root singularity in DOS near the superconducting gap and generate quasiparticle states at subgap energies. Furthermore, at sufficiently large values of the wire impedance this singularity is suppressed down to $T=0$ in which case DOS tends to zero at subgap energies and exhibits the power-law behavior above the gap. Our predictions can be directly tested in tunneling experiments with superconducting nanowires.

cond-mat.mes-hall↗

arXiv2017

Large thermoelectric effect in ballistic Andreev interferometers

Employing quasiclassical theory of superconductivity combined with Keldysh technique we investigate large thermoelectric effect in multiterminal ballistic normal-superconducting (NS) hybrid structures. We argue that this effect is caused by electron-hole asymmetry generated by coherent Andreev reflection of quasiparticles at interfaces of two different superconductors with non-zero phase difference. Within our model we derive a general expression for thermoelectric voltages $V_{T1,2}$ induced in two different normal terminals exposed to a thermal gradient. Our results apply at any temperature difference in the subgap regime and allow to explicitly analyze both temperature and phase dependencies of $V_{T1,2}$ demonstrating that in general there exists no fundamental relation between these voltages and the equilibrium Josephson current in SNS junctions.

cond-mat.supr-con↗

arXiv2016

Quantum Phase Slip Noise

Quantum phase slips (QPS) generate voltage fluctuations in superconducting nanowires. Employing Keldysh technique and making use of the phase-charge duality arguments we develop a theory of QPS-induced voltage noise in such nanowires. We demonstrate that quantum tunneling of the magnetic flux quanta across the wire yields quantum shot noise which obeys Poisson statistics and is characterized by a power law dependence of its spectrum $S_Ω$ on the external bias. In long wires $S_Ω$ decreases with increasing frequency $Ω$ and vanishes beyond a threshold value of $Ω$ at $T \to 0$. Quantum coherent nature of QPS noise yields non-monotonous dependence of $S_Ω$ on $T$ at small $Ω$.

cond-mat.mes-hall↗

arXiv2015

Effective action theory of Andreev level spectroscopy

With the aid of the Keldysh effective action technique we develop a microscopic theory describing Andreev level spectroscopy experiments in non-tunnel superconducting contacts. We derive an effective impedance of such contacts which accounts for the presence of Andreev levels in the system. At subgap bias voltages and low temperatures inelastic Cooper pair tunneling is accompanied by transitions between these levels resulting in a set of sharp current peaks. We evaluate the intensities of such peaks, establish their dependence on the external magnetic flux piercing the structure and estimate thermal broadening of these peaks. We also specifically address the effect of capacitance renormalization in a non-tunnel superconducting contact and its impact on both the positions and heights of the current peaks. At overgap bias voltages the $I-V$ curve is determined by quasiparticle tunneling and contains current steps related to the presence of discrete Andreev states in our system.

cond-mat.supr-con↗

arXiv2015

Diffusive superconductors beyond Usadel approximation: Electron-hole asymmetry and large photoelectric effect

We extend the quasiclassical formalism for diffusive superconductors by deriving anisotropic (gradient) corrections to the Usadel equation. We demonstrate that in a number of physical situations such corrections may play a crucial role being responsible for the effects which cannot be recovered within the standard Usadel approximation. One of them is the so-called photoelectric effect in superconductors and superconducting-normal (SN) hybrid structures. Provided a superconducting part of the system is irradiated by an external ac electromagnetic field the charge imbalance develops and a non-vanishing dc voltage is induced across the SN interface. In the presence of magnetic impurities in a superconductor the magnitude of this effect becomes large and can easily be detected in modern experiments.

cond-mat.supr-con↗

arXiv2014

Supercurrent dephasing by electron-electron interactions

We demonstrate that in sufficiently long diffusive superconducting-normal-superconducting ($SNS$) junctions dc Josephson current is exponentially suppressed by electron-electron interactions down to zero temperature. This suppression is caused by the effect of {\it Cooper pair dephasing} which occurs in the normal metal and defines a new fundamental length scale $L_φ$ in the problem. Provided the temperature length exceeds $L_φ$ this dephasing length can be conveniently extracted from equilibrium measurements of the Josephson current.

cond-mat.mes-hall↗

arXiv2014

Enhancement of thermoelectric effect in diffusive superconducting bilayers with magnetic interfaces

We demonstrate that thermoelectric currents in superconducting bilayers with a spin-active interface are controlled by the two competing processes. On one hand, spin-sensitive quasiparticle scattering at such interface generates electron-hole imbalance and yields orders-of-magnitude enhancement of the thermoelectric effect in the system. On the other hand, this electron-hole imbalance gets suppressed in the superconductor bulk due to electron scattering on non-magnetic impurities. As a result, large thermoelectric currents can only flow in the vicinity of the spin-active interface and decay away from this interface at a distance exceeding the electron elastic mean free path $\ell$. The magnitude of the thermoelectric effect reaches its maximum provided $\ell$ becomes of order of the total bilayer thickness.

cond-mat.supr-con↗

arXiv2014

Decoherence of Cooper pairs and subgap magnetoconductance of superconducting hybrids

We demonstrate that electron-electron interactions fundamentally restrict the penetration length of superconducting correlations into a diffusive normal metal (N) attached to a superconductor (S). We evaluate the subgap magnetoconductance $G$ of SN hybrids in the presence of electron-electron interactions and demonstrate that the effect of the magnetic field on $G$ is twofold: It includes ($i$) additional temperature independent dephasing of Cooper pairs and ($ii$) Zeeman splitting between the states with opposite spins. The dephasing length of Cooper pairs can be directly extracted from measurements of the subgap magnetoconductance in SN systems at low temperatures.

cond-mat.mes-hall↗

arXiv2014

Giant thermopower in superconducting heterostructures with spin-active interfaces

We predict parametrically strong enhancement of the thermoelectric effect in metallic bilayers consisting of two superconductors separated by a spin-active interface. The physical mechanism for such an enhancement is directly related to electron-hole imbalance generated by spin-sensitive quasiparticle scattering at the interface between superconducting layers. We explicitly evaluate the thermoelectric currents flowing in the system and demonstrate that they can reach maximum values comparable to the critical ones for superconductors under consideration.

cond-mat.supr-con↗

arXiv2014

Electron-hole imbalance and large thermoelectric effect in superconducting hybrids with spin-active interfaces

We argue that spin-sensitive quasiparticle scattering may generate electron-hole imbalance in superconducting structures, such as, e.g., superconducting-normal hybrids with spin-active interfaces. We elucidate a transparent physical mechanism for this effect demonstrating that scattering rates for electrons and holes at such interfaces differ from each other. Explicitly evaluating the wave functions of electron-like and hole-like excitations in superconducting-normal bilayers we derive a general expression for the thermoelectric current and show that -- in the presence of electron-hole imbalance -- this current can reach maximum values as high as the critical current of a superconductor.

cond-mat.supr-con↗

arXiv2013

Persistent currents in quantum phase slip rings

We investigate the effect of interacting quantum phase slips on persistent current and its fluctuations in ultrathin superconducting nanowires and nanorings pierced by the external magnetic flux. We derive the effective action for these systems and map the original problem onto an effective sine-Gordon theory on torus. We evaluate both the flux dependent persistent current and the critical radius of the ring beyond which this current gets exponentially suppressed by quantum fluctuations. We also analyze fluctuations of persistent current caused by quantum phase slips. At low temperatures the supercurrent noise spectrum has the form of coherent peaks which can be tuned by the magnetic flux. Experimental observation of these peaks can directly demonstrate the existence of plasma modes in superconducting nanorings.

cond-mat.mes-hall↗