arXiv subjects

Sergey Smirnov

Publications and source records attributed to Sergey Smirnov.

At least 19 recordsLinked to original sources

arXiv2026

Fluctuation response of a minimal Kitaev chain in nonequilibrium states

Minimal Kitaev chains provide a unique platform to engineer Majorana states in quantum dots interacting via normal tunneling and crossed Andreev reflection specified by their amplitudes $|\eta_{n,a}|$. Here we analyze fluctuations of electric currents in a double quantum dot Kitaev chain using the differential effective charge $q$, that is the ratio of the differential shot noise and conductance. At low bias voltages $V$ we find that $q=e/2$ in a very narrow vicinity of the point $|\eta_n|=|\eta_a|$ whereas $q=3e/2$ almost in the whole sweet spot region and marks the range where the poor man's Majorana states largely govern the fluctuations. At high $V$ we show that the sweet spot region is still characterized by $q=3e/2$ uniquely identifying the poor man's Majorana states using the high voltage tails. For $|\eta_n|=0$ or $|\eta_a|=0$ we obtain $q=e$ at any $V$. Remarkably, before the asymptotic value $q=e$ is reached for very high $V$, the maximal value $q=2e$ is formed at $|eV|=2\sqrt{|\eta_n|^2+|\eta_a|^2}$. The unique nature and potentially rich fluctuation behavior revealed in this work provide a stimulating ground for the next generation experiments on nonequilibrium shot noise in minimal Kitaev chains.

cond-mat.mes-hall↗

arXiv2025

Thermoelectric fluctuations of interfering Majorana bound states

Nonequilibrium states produced by electric and thermal voltages ($V$, $V_T$) provide a straightforward insight into underlying degrees of freedom of composite nanostructures and are of particular interest to probe Majorana bound states. Here we explore fluctuations of thermoelectric currents at finite frequencies $ω$ in a quantum dot coupled to two interfering Majorana bound states. At small $V$ we find that in the emission spectra the differential thermoelectric quantum noise $\partial S^>/\partial V_T$ shows an antiresonance whereas in the absorption spectra Majorana interference induces an antiresonance-resonance pair. At large $V$ this pair is preserved whereas the emission antiresonance turns into an antiresonance-resonance pair identical to the absorption one making $\partial S^>/\partial V_T$ antisymmetric in the frequency $ω$. This antisymmetry distinguishes Majorana behavior from the one induced by Andreev bound states and does not break at higher temperatures making it attractive for experiments on Majorana interference via thermoelectric fluctuation response.

cond-mat.mes-hall↗

arXiv2024

Nonequilibrium finite frequency resonances in differential quantum noise driven by Majorana interference

Nonequilibrium quantum noise $S^>(ω,V)$ measured at finite frequencies $ω$ and bias voltages $V$ probes Majorana bound states in a host nanostructure via fluctuation fingerprints unavailable in average currents or static shot noise. When Majorana interference is brought into play, it enriches nonequilibrium states and makes their nature even more unique. Here we demonstrate that an interference of two Majorana modes via a nonequilibrium quantum dot gives rise to a remarkable finite frequency response of the differential quantum noise $\partial S^>(ω,V,Δϕ)/\partial V$ driven by the Majorana phase difference $Δϕ$. Specifically, at low bias voltages there develops a narrow resonance of width $\hbarΔω\sim\sin^2Δϕ$ at a finite frequency determined by $V$, whereas for high bias voltages there arise two antiresonances at two finite frequencies controlled by both $V$ and $Δϕ$. We show that the maximum and minimum of these resonance and antiresonances have universal fractional values, $3e^3/4h$ and $-e^3/4h$. Moreover, detecting the frequencies of the antiresonances provides a potential tool to measure $Δϕ$ in nonequilibrium experiments on Majorana finite frequency quantum noise.

cond-mat.mes-hall↗

arXiv2023

Majorana differential shot noise and its universal thermoelectric crossover

Nonequilibrium states driven by both electric bias voltages $V$ and temperature differences $ΔT$ (or thermal voltages $eV_T\equiv k_BΔT$) are unique probes of various systems. Whereas average currents $I(V,V_T)$ are traditionally measured in majority of experiments, an essential part of nonequilibrium dynamics, stored particularly in fluctuations, remains largely unexplored. Here we focus on Majorana quantum dot devices, specifically on their differential shot noise $\partial S^>(V,V_T)/\partial V$, and demonstrate that in contrast to the differential electric or thermoelectric conductance, $\partial I(V,V_T)/\partial V$ or $\partial I(V,V_T)/\partial V_T$, it reveals a crossover from thermoelectric to pure thermal nonequilibrium behavior. It is shown that this Majorana crossover in $\partial S^>(V,V_T)/\partial V$ is induced by an interplay of the electric and thermal driving, occurs at an energy scale determined by the Majorana tunneling amplitude, and exhibits a number of universal characteristics which may be accessed in solely noise experiments or in combination with measurements of average currents.

cond-mat.mes-hall↗

arXiv2022

Revealing universal Majorana fractionalization using differential shot noise and conductance in nonequilibrium states controlled by tunneling phases

Universal fractionalization of quantum transport characteristics in Majorana quantum dot devices is expected to emerge for well separated Majorana bound states. We show that the Majorana universality of the differential shot noise $\partial S^>/\partial V$ and conductance $\partial I/\partial V$ at low bias voltages $V$ arises only in ideal setups with only one Majorana mode entangled with the quantum dot. In realistic devices, where both Majorana modes are entangled with the quantum dot, $\partial S^>/\partial V$ and $\partial I/\partial V$ become very sensitive to tunneling phases and their universal fractional values are hard to observe even for well separated Majorana bound states. In contrast, as revealed here, the ratio $(\partial S^>/\partial V)/(\partial I/\partial V)$ weakly depends on the tunneling phases and is fractional when the Majorana bound states are well separated or integer when they significantly overlap. Importantly, for very large $V$ we demonstrate that this ratio becomes fully independent of the tunneling phases and its universal fractional Majorana value may be observed in state-of-the-art experiments.

cond-mat.mes-hall↗

arXiv2021

Majorana ensembles with fractional entropy and conductance in nanoscopic systems

Quantum thermodynamics is a promising route to unambiguous detections of Majorana bound states. Being fundamentally different from quantum transport, this approach reveals unique Majorana thermodynamic behavior and deepens our insight into Majorana quantum transport itself. Here we demonstrate that a nanoscopic system with topological superconductors produces a remarkable accumulation of Majorana thermodynamic states in wide ranges of Majorana tunneling phases by means of increasing its temperature $T$. Revealing this physical behavior is twofold beneficial. First, it significantly reduces the dependence of the entropy on the tunneling phases which become almost irrelevant in experiments. Second, the fractional Majorana entropy $S_M^{(2)}=k_B\ln(2^\frac{3}{2})$ may be observed at high temperatures substantially facilitating experiments. Analyzing quantum transport, we predict that when the temperature increases, the above thermodynamic behavior will induce an anomalous increase of the linear conductance from vanishing values up to the unitary fractional Majorana plateau $G_M=e^2/2h$ extending to high temperatures.

cond-mat.mes-hall↗

arXiv2021

Majorana entropy revival via tunneling phases

Measuring the Majorana entropy $S_M=k_B\log(2^\frac{1}{2})$ may uniquely reveal whether an initial equilibrium state of a nanoscale device is of Majorana nature and subsequent operations deal with an essentially nonlocal pair of non-Abelian Majorana bound states and not with trivial or other accidental non-Abelian states. However, in realistic setups both Majorana modes are inevitably involved in tunneling processes. We show that even when the tunneling amplitude of one Majorana mode is significantly suppressed, the Majorana entropy ruins and straightforward experiments will in general detect entropy $S\ll S_M$. To avoid this general problem we present a mechanism of the Majorana entropy revival via the tunneling phases of the Majorana modes and demonstrate that to successfully observe the universal Majorana plateau $S=S_M$ one should intelligently tune the tunneling phases instead of leaving them uncontrolled. Practical feasibility of appropriate Majorana entropy measurements is supported by an example with parameters well achievable in modern labs.

cond-mat.mes-hall↗

arXiv2020

Walk-off controlled self-starting frequency combs in $χ^{(2)}$ optical microresonators

Investigations of frequency combs in $χ^{(3)}$ optical microresonators are burgeoning nowadays. Changeover to $χ^{(2)}$ resonators promises further advances and brings new challenges. Here, the comb generation entails not only coupled first and second harmonics (FHs and SHs) and two dispersion coefficients, but also a substantial difference in the group velocities - the spatial walk-off. We predict walk-off controlled highly stable comb generation, drastically different from that known in the $χ^{(3)}$ case. This includes the general notion of antiperiodic state, formation of coherent antiperiodic steady states (solitons), where the FH and SH envelopes move with a common velocity without shape changes, characterization of the family of antiperiodic steady states, and the dependence of comb spectra on the pump power and the group velocity difference.

physics.optics↗

arXiv2020

Dual Majorana universality in thermally induced nonequilibrium

We demonstrate that nonequilibrium nanoscopic systems with Majorana zero modes admit special kind of universality which cannot be classified as of strictly transport or strictly thermodynamic nature. To reveal such kind of Majorana universality we explore purely thermal nonequilibrium states of a quantum dot whose low-energy degrees of freedom are governed by Majorana zero modes. Specifically, the quantum dot is coupled to a topological superconductor, supporting Majorana zero modes, as well as to two normal metallic contacts with the same chemical potentials but different temperatures. It is shown that the Majorana universality in this setup is dual: it is stored inside both the response of the electric current, excited by exclusively the temperature difference, and the quantum dot compressibility. The latter is defined as the derivative of the quantum dot particle number with respect to the chemical potential and forms a universal Majorana ratio with a proper derivative of the electric current that flows in nonequilibrium states of purely thermal nature.

cond-mat.mes-hall↗

arXiv2019

Dynamic Majorana resonances and universal symmetry of nonequilibrium thermoelectric quantum noise

Nonequilibrium states of a nanoscopic system may be achieved by both applying a bias voltage $V$ to its contacts and producing a difference $ΔT$ in their temperatures. Then the total current results from two competing flows, induced by $V$ and $ΔT$, respectively. Here we explore finite frequency quantum noise of this thermoelectric current flowing through a quantum dot whose low-energy dynamics is governed by Majorana degrees of freedom. We demonstrate that at finite frequency $ω$ Majorana zero modes induce a perfect universal symmetry between photon emission and absorption spectra and produce universal thermoelectric resonances in their frequency dependence in contrast to non-Majorana quantum noise which is either asymmetric or non-universal. In particular, at low temperatures the differential thermoelectric quantum noise induced by Majorana zero modes shows resonances with a nontrivial maximum $\frac{e^3}{h}\log(2^{1/4})$ at $ω=\mp\frac{|eV|}{\hbar}$ when $k_\text{B}ΔT\ll|eV|$. Our results challenge cutting-edge experiments using quantum noise detectors to reveal the universal spectral symmetry and resonant structure of the Majorana thermoelectric finite frequency quantum noise.

cond-mat.mes-hall↗

arXiv2019

Nonlinear solutions for χ^(2) frequency combs in optical microresonators

Experimental and theoretical studies of nonlinear frequency combs in χ^(3) optical microresonators attracted tremendous research interest during the last decade and resulted in prototypes of solitonbased steadily working devices. Realization of similar combs owing to χ^(2) optical nonlinearity promises new breakthroughs and is a big scientific challenge. We analyze the main obstacles for realization of the χ^(2) frequency combs in high-Q microresonators and propose two families of steadystate nonlinear solutions, including soliton and periodic solutions, for such combs. Despite generic periodicity of light fields inside microresonators, the nonlinear solutions can be topologically different and relevant to periodic and antiperiodic boundary conditions. The found particular solutions exist owing to a large difference in the group velocities between the first and second harmonics, typical of χ^(2) microresonators, and to the presence of the pump. They have no zero-pump counterparts relevant to conservative solitons. Stability issue for the found comb solutions remains open and requires further numerical analysis.

physics.optics↗

arXiv2019

Unhiding a concealed resonance by multiple Kondo transitions in a quantum dot

Kondo correlations are responsible for the emergence of a zero-bias peak in the low temperature differential conductance of Coulomb blockaded quantum dots. In the presence of a global SU(2)$\otimes$SU(2) symmetry, which can be realized in carbon nanotubes, they also inhibit inelastic transitions which preserve the Kramers pseudospins associated to the symmetry. We report on magnetotransport experiments on a Kondo correlated carbon nanotube where resonant features at the bias corresponding to the pseudospin-preserving transitions are observed. We attribute this effect to a simultaneous enhancement of pseudospin-non-preserving transitions occurring at that bias. This process is boosted by asymmetric tunneling couplings of the two Kramers doublets to the leads and by asymmetries in the potential drops at the leads. Hence, the present work discloses a fundamental microscopic mechanisms ruling transport in Kondo systems far from equilibrium.

cond-mat.str-el↗

arXiv2019

Majorana finite frequency nonequilibrium quantum noise

Quantum finite frequency noise is one of fundamental aspects in quantum measurements performed during quantum information processing where currently Majorana bound states offer an efficient way to implement fault-tolerant quantum computation via topological protection from decoherence or unitary errors. Thus a detailed exploration of Majorana finite frequency noise spectra, preferably in a nonequilibrium device, is a timely challenge of fundamental importance. Here we present results on finite frequency differential noise that is the derivative of the noise with respect to the frequency. This quantity has universal units of $e^2$ and scans in high detail all peculiarities of the Majorana noise clearly demonstrating its universal finite frequency features. In particular, we provide photon absorption spectra on all energy scales and reveal a rich structure including universal Majorana plateaus as well as universal Majorana resonances and antiresonances at characteristic frequencies. Our results are of immediate interest to state-of-the-art experiments involving quantum noise mesoscopic detectors able to separately measure photon absorption and emission spectra.

cond-mat.mes-hall↗

arXiv2018

Universal Majorana thermoelectric noise

Thermoelectric phenomena resulting from an interplay between particle flows induced by electric fields and temperature inhomogeneities are extremely insightful as a tool providing substantial knowledge about the microscopic structure of a given system. Tuning, e.g., parameters of a nanoscopic system coupled via tunneling mechanisms to two contacts one may achieve various situations where the electric current induced by an external bias voltage competes with the electric current excited by the temperature difference of the two contacts. Even more exciting physics emerges when the system's electronic degrees freedom split to form Majorana fermions which make the thermoelectric dynamics universal. Here we propose revealing this unique universal signatures of Majorana fermions in strongly nonequilibrium quantum dots via noise of the thermoelectric transport beyond linear response. It is demonstrated that whereas mean thermoelectric quantities are only universal at large bias voltages, the noise of the electric current excited by an external bias voltage and the temperature difference of the contacts is universal at any bias voltage. We provide truly universal, i.e. independent of the system's parameters, thermoelectric ratios between nonlinear response coefficients of the noise and mean current at large bias voltages where experiments may easily be performed to uniquely detect these truly universal Majorana thermoelectric signatures.

cond-mat.mes-hall↗

arXiv2017

Nonequilibrium Majorana fluctuations

Nonequilibrium physics of random events, or fluctuations, is a unique fingerprint of a given system. Here we demonstrate that in noninteracting systems, whose dynamics is driven by Majorana states, the effective charge $e^*$, characterizing the electric current fluctuations, is fractional. This is in contrast to noninteracting Dirac systems with the trivial electronic charge, $e^*=e$. Quite the opposite, in the Majorana state we predict two different fractional effective charges at low and high energies, $e^*_l=e/2$ and $e^*_h=3e/2$, accessible at low and high bias voltages, respectively. We show that while the low energy effective charge $e^*_l$ is sensitive to thermal fluctuations of the current, the high energy effective charge $e^*_h$ is robust against thermal noise. A unique fluctuation signature of Majorana fermions is, therefore, encoded in the high voltage tails of the electric current noise easily accessible in experiments on strongly nonequilibrium systems even at high temperatures.

cond-mat.mes-hall↗

arXiv2015

Majorana tunneling entropy

In thermodynamics a macroscopic state of a system results from a number of its microscopic states. This number is given by the exponent of the system's entropy $\exp(S)$. In non-interacting systems with discrete energy spectra, such as large scale quantum dots, $S$ as a function of the temperature has usually a plateau shape with integer values of $\exp(S)$ on these plateaus. Plateaus with non-integer values of $\exp(S)$ are fundamentally forbidden and would be thermodynamically infeasible. Here we investigate the entropy of a non-interacting quantum dot coupled via tunneling to normal metals with continuum spectra as well as to topological superconductors. We show that the entropy may have non-integer plateaus if the topological superconductors support weakly overlapping Majorana bound states. This brings a fundamental change in the thermodynamics of the quantum dot whose specific heat $c_V$ acquires low temperature Majorana peaks which should be absent according to the conventional thermodynamics. We also provide a fundamental thermodynamic understanding of the transport properties, such as the linear conductance. In general our results show that the thermodynamics of systems coupled to Majorana modes represents a fundamental physical interest with diverse applications depending on versatility of possible coupling mechanisms.

cond-mat.mes-hall↗

arXiv2015

Orthogonal Cherenkov sound in spin-orbit coupled systems

Conventionally the Cherenkov sound is governed by {\it orbital} degrees of freedom and is excited by {\it supersonic} particles. Additionally, it usually has a {\it forward} nature with a conic geometry known as the Cherenkov cone whose axis is oriented {\it along} the {\it supersonic} particle motion. Here we predict Cherenkov sound of a unique nature entirely resulting from the electronic {\it spin} degree of freedom and demonstrate a fundamentally distinct Cherenkov effect originating from essentially {\it subsonic} electrons in two-dimensional gases with both Bychkov-Rashba and Dresselhaus spin-orbit interactions. Specifically, we show that the axis of the conventional {\it forward} Cherenkov cone gets a nontrivial {\it quarter-turn} and at the same time the sound distribution strongly localizes around this rotated axis being now {\it orthogonal} to the {\it subsonic} particle motion. Apart from its fundamentally appealing nature, the orthogonal Cherenkov sound could have applications in planar semiconductor technology combining spin and acoustic phenomena to develop, {\it e.g.}, acoustic amplifiers or sound sources with a flexible spin dependent orientation of the sound propagation.

cond-mat.mes-hall↗

arXiv2014

Asymmetric Cherenkov acoustic reverse in topological insulators

A general phenomenon of the Cherenkov radiation known in optics or acoustics of conventional materials is a formation of a forward cone of, respectively, photons or phonons emitted by a particle accelerated above the speed of light or sound in those materials. Here we suggest three-dimensional topological insulators as a unique platform to fundamentally explore and practically exploit the acoustic aspect of the Cherenkov effect. We demonstrate that applying an in-plane magnetic field to a surface of a three-dimensional topological insulator one may suppress the forward Cherenkov sound up to zero at a critical magnetic field. Above the critical field the Cherenkov sound acquires pure backward nature with the polar distribution differing from the forward one generated below the critical field. Potential applications of this asymmetric Cherenkov reverse are in design of low energy electronic devices such as acoustic ratchets or, in general, in low power design of electronic circuits with a magnetic field control of the direction and magnitude of the Cherenkov dissipation.

cond-mat.mes-hall↗