arXiv subjects

R. I. Shekhter

Publications and source records attributed to R. I. Shekhter.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear nanoelectromechanics of a movable Cooper-pair box

We theoretically study the dynamics of a movable Cooper-pair box coupled to a normal-metal pillar using a semiclassical approach. We analyze the dynamical stability induced by the nonlinear nanoelectromechanical coupling between the mechanical motion and an inelastic Andreev tunneling through linear stability and bifurcation analyses. As a function of $η$, defined as the ratio of electrostatic energy to Josephson coupling energy, the system exhibits reentrant stability. At small $η$, the fixed point loses stability through a supercritical Hopf bifurcation, giving rise to self-sustained vibrations. With a further increase of $η$, a second critical point appears, at which the fixed point regains stability. We show that this second transition corresponds to an inverse subcritical Hopf bifurcation in the adiabatic regime and to an inverse supercritical Hopf bifurcation in the nonadiabatic regime. These results extend previous studies of adiabatic self-vibrations to the nonadiabatic regime and reveal a rich nonlinear dynamical phase diagram arising from the interplay between electronic and mechanical degrees of freedom in superconducting devices.

cond-mat.supr-con↗

arXiv2025

Magnetizing weak links by time-dependent spin-orbit interactions: momentum conserving and non-conserving processes

Rashba spin-orbit interactions generated by time-dependent electric fields acting on weak links (that couple together non-magnetic macroscopic leads) can magnetize the junction. The Rashba spin-orbit interaction that affects the spins of electrons tunneling through the weak links changes their momentum concomitantly. We establish the connection between the magnetization flux induced by processes that conserve the momentum and the magnetization created by tunneling events that do not. Control of the induced magnetization can be achieved by tuning the polarization of the AC electric field responsible for the spin-orbit Rashba interaction (e.g., from being circular to linear), by changing the applied bias voltage, and by varying the degree of a gate voltage-induced asymmetry of the device.

cond-mat.mes-hall↗

arXiv2023

Magnetization near a constriction between BCS superconductors by spin-dependent tunneling

Spin-dependent electron tunneling through a voltage-biased micro-constriction between two bulk superconductors is shown to create a static magnetization near the constriction and an ac Josephson-like spin current. Although spin-dependent tunneling generates quantum spin fluctuations even in the absence of a bias, the formation of spin-triplet Cooper pairs necessary for the creation of magnetization is blocked by destructive interference between different quasi-electron and quasi-hole tunneling channels, unless there is an asymmetry between the tunneling densities of states for electrons and holes. Breaking the symmetry in the electron-hole tunnel density of states and creating electron-hole tunneling imbalance by biasing the device destroys the destructive interference and enables triplet Cooper-pair formation. As a result, magnetizing the superconductor becomes possible. The role of the voltage in lifting the blockade hindering the spin-triplet Cooper pair formation is an example of an electrically-controlled dissipation-less spintronic phenomenon.

cond-mat.supr-con↗

arXiv2023

Andreev probing of Cooper-pair flying qubit

We propose a nanomechanical device which can actuate and probe a flying qubit that can be used to facilitate quantum information transfer over a long distance. The flying qubit is formed by a movable Cooper pair box (CPB) consisting of a superconducting dot and a bulk superconductor which are entangled by removing the Coulomb blockade of Cooper pair tunneling electrostatically. We suggest that flying qubit states formed on movable CPB can be observed in electron transport to a normal electrode via Andreev reflections. The charge transfer due to periodic mechanical motion of CPB leads to nonzero current at zero voltage and its coherence can be identified through oscillatory dependence of the current on a gate voltage.

cond-mat.mes-hall↗

arXiv2023

Transduction of quantum information from charge qubit to nanomechanical cat-state

We suggest a nanoelectromechanical setup and corresponding time protocol of its manipulation by which we transduce quantum information from charge qubit to nanomechanical cat state. The setup is based on the AC Josephson effect between bulk superconductors and mechanically vibrating mesoscopic superconducting island in the regime of the Cooper pair box. Starting with a pure state with quantum information initially encoded into superposition of the Cooper pair box states, applying a specially tailored time protocol upon bias voltage and gate electrodes, we obtain a new pure state with information finally encoded into superposition of nanomechanical coherent states constituting the cat state. This performance is achieved using quantum entanglement between electrical and mechanical states. Nanomechanical cat states serve as a storage of quantum information, motivated by significantly longer decoherence time with respect to the charge qubit states, from which the information can be transdued back to the charge qubit applying the reverse time protocol.

cond-mat.mes-hall↗

arXiv2022

Magnetoconductance Anisotropies and Aharonov-Casher Phases

The spin-orbit interaction is a key tool for manipulating and functionalizing spin-dependent electron transport. However, the precise value of the spin-orbit coupling constant is hard to determine and control. Here we propose a measurement of the geometric (Aharonov-Casher) phase of the electron wavefunction generated by the spin-orbit coupling as a means to deduce the spin-orbit coupling strength in weak links. Unlike other proposed measurements of geometric phases, which rely on interference, we show that the anisotropy of the magnetoconductance, measured at different directions of an external magnetic field,is determined by the geometric phase. Specifically we consider weak links in which the Rashba interaction is caused by an external electric field, but our method is expected to apply also for other forms of the spin-orbit coupling. Measuring this magnetoconductance anisotropy thus allows calibrating Rashba spintronic devices by an external electric field that tunes the spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2022

Pumping and cooling of nanomechanical vibrations generated by Cooper pair exchange

We consider a nanoelectromechanical system composed of a carbon nanotube suspended between two normal leads and coupled to a superconducting scanning tunneling microscope (STM) tip via vacuum tunnel barrier. Treating the nanotube as a single-level quantum dot, it is shown that an applied voltage between the superconducting STM tip and normal leads gives rise to a pumping or a cooling of the mechanical subsystem depending on the direction of the electronic flow. It is also demonstrated that the transition between these two regimes is controlled by the strength of the tunnel coupling between the nanotube and superconducting STM tip and the relative position of the electronic level. Such phenomena are realized due to a specific electromechanical coupling that is fully governed by the quantum dynamics of the Cooper pairs. The amplitude of the self-sustained oscillations in the pumping regime is analyzed numerically, and the effective temperature of the mechanical subsystem in the cooling regime is obtained.

cond-mat.mes-hall↗

arXiv2022

Cooling of nanomechanical vibrations by Andreev injection

A nanoelectromechanical weak link composed of a carbon nanotube suspended between two normal electrodes in a gap between two superconducting leads is considered. The nanotube is treated as a movable single-level quantum dot in which the position-dependent superconducting order parameter is induced due to the Cooper pair tunneling. We show that electron tunneling processes significantly affect the state of the mechanical subsystem. We found that at a given direction of the applied voltage between the electrodes, the stationary state of the mechanical subsystem has a Boltzmann form with an effective temperature depended on the parameters of the device. As this takes place, the effective temperature can reach significantly small values (cooling effect). We also demonstrate that nanotube fluctuations strongly affect the dc current through the system. The latter can be used to probe the predicted effects in an experiment.

cond-mat.mes-hall↗

arXiv2021

Nanomechanics driven by the superconducting proximity effect

We consider a nanoelectromechanical weak link composed of a carbon nanotube suspended above a trench in a normal metal electrode and positioned in a gap between two superconducting leads. The nanotube is treated as a movable single-level quantum dot in which the position-dependent superconducting order parameter is induced as a result of Cooper pair tunneling. We show that in such a system, self-sustained bending vibrations can emerge if a bias voltage is applied. The occurrence of this effect crucially depends on the direction of the bias voltage and the relative position of the quantum dot level. We also demonstrate that the nanotube vibrations strongly affect the dc current through the system, a characteristic that can be used for the direct experimental observation of the predicted phenomenon.

cond-mat.mes-hall↗

arXiv2020

Effects of magnetic fields on the Datta-Das spin field-effect transistor

A Datta-Das spin field-effect transistor is built of a heterostructure with a Rashba spin-orbit interaction (SOI) at the interface (or quantum well) separating two possibly magnetized reservoirs. The particle and spin currents between the two reservoirs are driven by chemical potentials that are (possibly) different for each spin direction. These currents are also tuned by varying the strength of the SOI, which changes the amount of the rotation of the spins of electrons crossing the heterostructure. Here we investigate the dependence of these currents on additional Zeeman fields on the heterostructure and on variations of the reservoir magnetizations. In contrast to the particle current, the spin currents are not necessarily conserved; an additional spin polarization is injected into the reservoirs. If a reservoir has a finite (equilibrium) magnetization, then we surprisingly find that the spin current into that reservoir can only have spins which are parallel to the reservoir magnetization, independent of all the other fields. This spin current can be enhanced by increasing the magnetization of the other reservoir, and can also be tuned by the SOI and the various magnetic fields. When only one reservoir is magnetized then the spin current into the other reservoir has arbitrary tunable size and direction. In particular, this spin current changes as the magnetization of the other reservoir is rotated. The optimal conditions for accumulating spin polarization on an unpolarized reservoir are to either apply a Zeeman field in addition to the SOI, or to polarize the other reservoir.

cond-mat.mes-hall↗

arXiv2020

Nano-mechanics driven by Andreev tunneling

We predict and analyze mechanical instability and corresponding self-sustained mechanical oscillations occurring in a nanoelectromechanical system composed of a metallic carbon nanotube (CNT) suspended between two superconducting leads and coupled to a scanning tunneling microscope (STM) tip. We show that such phenomena are realized in the presence of both the coherent Andreev tunneling between the CNT and superconducting leads, and an incoherent single electron tunneling between the voltage biased STM tip and CNT. Treating the CNT as a single-level quantum dot, we demonstrate that the mechanical instability is controlled by the Josephson phase difference, relative position of the electron energy level, and the direction of the charge flow. It is found numerically that the emergence of the self-sustained oscillations leads to a substantial suppression of DC electric current.

cond-mat.mes-hall↗

arXiv2020

Magnetization generated by microwave-induced Rashba interaction

We show that a controllable dc magnetization is accumulated in a junction comprising a quantum dot coupled to non-magnetic reservoirs if the junction is subjected to a time-dependent spin-orbit interaction. The latter is induced by an ac electric field generated by microwave irradiation of the gated junction. The magnetization is caused by inelastic spin-flip scattering of electrons that tunnel through the junction, and depends on the polarization of the electric field: a circularly polarized field leads to the maximal effect, while there is no effect in a linearly polarized field. Furthermore, the magnetization increases as a step function (smoothened by temperature) as the microwave photon energy becomes larger than the absolute value of the difference between the single energy level on the quantum dot and the common chemical potential in the leads.

cond-mat.mes-hall↗

arXiv2020

Photovoltaic effect generated by spin-orbit interactions

An AC electric field applied to a junction comprising two spin-orbit coupled weak links connecting a quantum dot to two electronic terminals is proposed to induce a DC current and to generate a voltage drop over the junction if it is a part of an open circuit. This photovoltaic effect requires a junction in which mirror reflection-symmetry is broken. Its origin lies in the different fashion inelastic processes modify the reflection of electrons from the junction back into the two terminals, which leads to uncompensated DC transport. The effect can be detected by measuring the voltage drop that is built up due to that DC current. This voltage is an even function of the frequency of the AC electric field and is {\em not} related to quantum pumping.

cond-mat.mes-hall↗

arXiv2019

Kondo effect in a Aharonov-Casher interferometer

We consider a model describing a spin field-effect transistor based on a quantum nanowire with a tunable spin-orbit interaction embedded between two ferromagnetic leads with anticollinear magnetization. We investigate a regime of a strong interplay between resonance Kondo scattering and interference associated with the Aharonov-Casher effect. Using the Keldysh technique at weak coupling regime we calculate perturbatively the charge current. It is predicted that the effects of the spin-orbit interaction result in a non-vanishing current for any spin polarization of the leads including the case of fully polarized anti-collinear contacts. We analyze the influence of the Aharonov-Casher phase and degree of spin polarization in the leads onto a Kondo temperature.

cond-mat.mes-hall↗

arXiv2019

DC spin generation by junctions with AC driven spin-orbit interaction

An unbiased one-dimensional weak link between two terminals, subjected to the Rashba spin-orbit interaction caused by an AC electric field which rotates periodically in the plane perpendicular to the link, is shown to inject spin-polarized electrons into the terminals. The injected spin-polarization has a DC component along the link and a rotating transverse component in the perpendicular plane. In the adiabatic, low rotation-frequency regime, these polarization components are proportional to the frequency. The DC component of the polarization vanishes for a linearly-polarized electric field.

cond-mat.mes-hall↗

arXiv2019

Effects of different lead magnetizations on the Datta-Das spin field-effect transistor

A Datta-Das spin field effect transistor is built of a one-dimensional weak link, with Rashba spin orbit interactions (SOI), which connects two magnetized reservoirs. The particle and spin currents between the two reservoirs are calculated to lowest order in the tunneling through the weak link and in the wide-band approximation, with emphasis on their dependence on the origins of the `bare' magnetizations in the reservoirs. The SOI is found to generate magnetization components in each reservoir, which rotate in the plane of the electric field (generating the SOI) and the weak link, only if the `bare' magnetization of the other reservoir has a non-zero component in that plane. The SOI affects the charge current only if both reservoirs are polarized. The charge current is conserved, but the transverse rotating magnetization current is not conserved since the SOI in the weak link generates extra spin polarizations which are injected into the reservoirs.

cond-mat.mes-hall↗

arXiv2018

Mechanically Induced Thermal Breakdown in Magnetic Shuttle Structures

A theory of a thermally induced single-electron "shuttling" instability in a magnetic nanomechanical device subject to an external magnetic field is presented in the Coulomb blockade regime of electron transport. The model magnetic shuttle device considered comprises a movable metallic grain suspended between two magnetic leads, which are kept at different temperatures and assumed to be fully spin polarized with antiparallel magnetizations. For a given temperature difference shuttling is found to occur for a region of external magnetic fields between a lower and an upper critical field strength, which separate the shuttling regime from normal small-amplitude "vibronic" regimes. We find that (i) the upper critical magnetic field saturates to a constant value in the high temperature limit and that the shuttle instability domain expands with a decrease of the temperature, (ii) the lower critical magnetic field depends not only on the temperature independent phenomenological friction coefficient used in the model but also on intrinsic friction (which vanishes in the high temperature limit) caused by magnetic exchange forces and electron tunneling between the quantum dot and the leads. The feasibility of using thermally driven magnetic shuttle systems to harvest thermal breakdown phenomena is discussed.

cond-mat.mes-hall↗

arXiv2018

Electric and Magnetic Gating of Rashba-Active Weak Links

In a one-dimensional weak-link wire the spin-orbit interaction (SOI) alone cannot generate a nonzero spin current. We show that a Zeeman field acting in the wire in conjunction with the Rashba SOI there does yield such a current, whose magnitude and direction depend on the direction of the field. When this field is not parallel to the effective field due to the SOI, both the charge and the spin currents oscillate with the length of the wire. Measuring the oscillating anisotropic magnetoresistance can thus yield information on the SOI strength. These features are tuned by applying a magnetic and/or an electric field, with possible applications to spintronics.

cond-mat.mes-hall↗