arXiv subjects

N. V. Gnezdilov

Publications and source records attributed to N. V. Gnezdilov.

16 recordsLinked to original sources

arXiv2025

Dynamic thermalization on noisy quantum hardware

Emulating thermal observables on a digital quantum computer is essential for quantum simulation of many-body physics. However, thermalization typically requires a large system size due to incorporating a thermal bath, whilst limited resources of near-term digital quantum processors allow for simulating relatively small systems. We show that thermal observables and fluctuations may be obtained for a small closed system without a thermal bath. Thermal observables occur upon classically averaging quantum mechanical observables over randomized variants of their time evolution that run independently on a digital quantum processor. Using an IBM quantum computer, we experimentally find thermal occupation probabilities with finite positive and negative temperatures defined by the initial state's energy. Averaging over random evolutions facilitates error mitigation, with the noise contributing to the temperature in the simulated observables. This result fosters probing the dynamical emergence of equilibrium properties of matter at finite temperatures on noisy intermediate-scale quantum hardware.

quant-ph↗

arXiv2023

Ultrafast dynamics of cold Fermi gas after a local quench

We consider non-equilibrium dynamics of two initially independent reservoirs $A$ and $B$ filled with a cold Fermi gas coupled and decoupled by two quantum quenches following one another. We find that the von Neumann entropy production induced by the quench is faster than thermal transport between the reservoirs and defines the short-time dynamics of the system. We analyze the energy change in the system which adds up the heat transferred between $A$ and $B$ and the work done by the quench to uncouple the reservoirs. In the case when $A$ and $B$ interact for a short time, we notice an energy increase in both reservoirs upon decoupling. This energy gain results from the quench's work and does not depend on the initial temperature imbalance between the reservoirs. We relate the quench's work to the mutual correlations of $A$ and $B$ expressed through their von Neumann entropies. Utilizing this relation, we show that once $A$ and $B$ become coupled, their entropies grow (on a timescale of the Fermi time) faster than the heat flow within the system. This result may provide a track of quantum correlations' generation at finite temperatures which one may probe in ultracold atoms, where we expect the characteristic timescale of correlations' growth to be $\sim 0.1 {\rm ms}$.

cond-mat.quant-gas↗

arXiv2022

Energy dynamics, information and heat flow in quenched cooling and the crossover from quantum to classical thermodynamics

The dynamics when a hot many-body quantum system is brought into instantaneous contact with a cold many-body quantum system can be understood as a combination of early time quantum correlation (von Neumann entropy) gain and late time energy relaxation. We show that at the shortest timescales there is an energy increase in each system linked to the entropy gain, even though equilibrium thermodynamics does not apply. This energy increase is of quantum origin and results from the collective binding energy between the two systems. Counter-intuitively, this implies that also the hotter of the two systems generically experiences an initial energy increase when brought into contact with the other colder system. In the limit where the energy relaxation overwhelms the (quantum) correlation build-up, classical energy dynamics emerges where the energy in the hot system decreases immediately upon contact with a cooler system. We use both strongly correlated SYK systems and weakly correlated mixed field Ising chains to exhibit these characteristics, and comment on its implications for both black hole evaporation and quantum thermodynamics.

hep-th↗

arXiv2021

Quantum tunneling dynamics in a complex-valued Sachdev-Ye-Kitaev model quench-coupled to a cool bath

The Sachdev-Ye-Kitaev (SYK) model describes interacting fermionic zero modes in zero spatial dimensions, e.g. quantum dot, with interactions strong enough to completely washout quasiparticle excitations in the infrared. In this paper, we consider the complex-valued SYK model at initial temperature $T$ and chemical potential $μ$ coupled to a large reservoir by a quench at time $t=0$. The reservoir is kept at zero temperature and charge neutrality. We find that the dynamics of the discharging process of the SYK quantum dot reveals a distinctive characteristic of the SYK non-Fermi liquid (nFl) state. In particular, we focus on the tunneling current induced by the quench. We show that the temperature dependent contribution to the current's half-life scales linearly in $T$ at low temperatures for the SYK nFl state, while for the Fermi liquid it scales as $T^2$.

cond-mat.str-el↗

arXiv2020

Planckian superconductor

The Planckian relaxation rate $\hbar/t_\mathrm{P} = 2πk_\mathrm{B} T$ sets a characteristic time scale for both equilibration of quantum critical systems and maximal quantum chaos. In this note, we show that at the critical coupling between a superconducting dot and the complex Sachdev-Ye-Kitaev model, known to be maximally chaotic, the pairing gap $Δ$ behaves as $η\,\, \hbar/t_\mathrm{P}$ at low temperatures, where $η$ is an order one constant. The lower critical temperature emerges with a further increase of the coupling strength so that the finite $Δ$ domain is settled between the two critical temperatures.

cond-mat.str-el↗

arXiv2018

Low-high voltage duality in tunneling spectroscopy of the Sachdev-Ye-Kitaev model

The Sachdev-Ye-Kitaev (SYK) model describes a strongly correlated metal with all-to-all random interactions (average strength $J$) between $N$ fermions (complex Dirac fermions or real Majorana fermions). In the large-$N$ limit a conformal symmetry emerges that renders the model exactly soluble. Here we study how the non-Fermi liquid behavior of the closed system in equilibrium manifests itself in an open system out of equilibrium. We calculate the current-voltage characteristic of a quantum dot, described by the complex-valued SYK model, coupled to a voltage source via a single-channel metallic lead (coupling strength $Γ$). A one-parameter scaling law appears in the large-$N$ conformal regime, where the differential conductance $G=dI/dV$ depends on the applied voltage only through the dimensionless combination $ξ=eVJ/Γ^2$. Low and high voltages are related by the duality $G(ξ)=G(π/ξ)$. This provides for an unambiguous signature of the conformal symmetry in tunneling spectroscopy.

cond-mat.mes-hall↗

arXiv2018

Valley switch in a graphene superlattice due to pseudo-Andreev reflection

Dirac electrons in graphene have a valley degree of freedom that is being explored as a carrier of information. In that context of "valleytronics" one seeks to coherently manipulate the valley index. Here we show that reflection from a superlattice potential can provide a valley switch: Electrons approaching a pristine-graphene--superlattice-graphene interface near normal incidence are reflected in the opposite valley. We identify the topological origin of this valley switch, by mapping the problem onto that of Andreev reflection from a topological superconductor, with the electron-hole degree of freedom playing the role of the valley index. The valley switch is ideal at a symmetry point of the superlattice potential, but remains close to 100% in a broad parameter range.

cond-mat.mes-hall↗

arXiv2017

Dipolar phase transitions in the cavity with Majorana populated Josephson junctions

We found that QED in the lowest eigenstate of the microwave-cavity, capacitively coupled with Majorana populated Josephson junctions array, is described with an effective $ϕ^{4}$ model emerging in the finite domain of the system parameters. Strong anharmonicity of the cavity mode arises due to electric-dipole phase transition facilitated by the single-electron zero mode and Cooper pair tunneling between superconducting islands. An instantonic meandering of the electromagnetic field potential at finite temperatures is predicted.

cond-mat.str-el↗

arXiv2017

Valley-momentum locking in a graphene superlattice with Y-shaped Kekulé bond texture

Recent experiments by Gutiérrez $\textit{et al.}$ [Nature Phys. $\textbf{12}$, 950 (2016)] on a graphene-copper superlattice have revealed an unusual Kekulé bond texture in the honeycomb lattice --- a Y-shaped modulation of weak and strong bonds with a wave vector connecting two Dirac points. We show that this so-called "Kek-Y" texture produces two species of massless Dirac fermions, with valley isospin locked parallel or antiparallel to the direction of motion. In a magnetic field $B$ the valley degeneracy of the $B$-dependent Landau levels is removed by the valley-momentum locking --- but a $B$-independent and valley-degenerate zero-mode remains.

cond-mat.mes-hall↗

arXiv2016

Phonon contributions to ab initio double mass differences of magic nuclei

Odd-even double mass differences (DMD) of magic nuclei are found within the approach starting from the free $NN$ interaction with account for particle-phonon coupling (PC) effects. We consider three PC effects: the phonon induced effective interaction, the renormalization of the "ends" due to the $Z$-factor corresponding to the pole PC contribution to the nucleon mass operator and the change of the single-particle energies. The perturbation theory in $g^2_L$, where $g_L$ is the vertex of the $L$-phonon creation, is used for PC calculations. PC corrections to single-particle energies are found self-consistently with an approximate account for the tadpole diagram. Results for magic $^{40,48}$Ca, $^{56,78}$Ni, $^{100,132}$Sn and $^{208}$Pb nuclei are presented. For lighter part of this set of nuclei, from $^{40}$Ca till $^{56}$Ni, the cases divide approximately in half between those where the PC corrections to DMD values make agreement with the data better and the ones with the opposite result. In the major part of the cases of worsening of description of DMD, a poor applicability of the perturbation theory for the induced interaction is the most probable reason of the phenomenon. For intermediate nuclei, $^{78}$Ni and $^{100}$Sn, there is no sufficiently accurate data on masses of nuclei necessary for finding DMD values. Finally, for heavier nuclei, $^{132}$Sn and $^{208}$Pb, PC corrections always make agreement with the experiment better.

nucl-th↗

arXiv2016

Phonon-particle coupling effects in odd-even double mass differences of magic nuclei

A method is developed to consider the particle-phonon coupling (PC) effects in the problem of finding odd-even double mass differences (DMD) of magic nuclei within the approach starting from the free $NN$-potential. Three PC effects are taken into account, the phonon induced interaction, the renormalization of the "ends" due to the $Z$-factors and the change of the single-particle energies. We use the perturbation theory in $g^2_L$, where $g_L$ is the vertex of the $L$-phonon creation. PC corrections to single-particle energies are found self-consistently with an approximate account for the so-called tadpole diagram. Results for double-magic $^{132}$Sn and $^{208}$Pb nuclei show that the PC corrections make agreement with the experimental data better.

nucl-th↗

arXiv2016

Wiedemann-Franz-type relation between shot noise and thermal conduction of Majorana surface states in a three-dimensional topological superconductor

We compare the thermal conductance $G_{\rm thermal}$ (at temperature $T$) and the electrical shot noise power $P_{\rm shot}$ (at bias voltage $V\gg k_{\rm B}T/e$) of Majorana fermions on the two-dimensional surface of a three-dimensional topological superconductor. We present analytical and numerical calculations to demonstrate that, for a local coupling between the superconductor and metal contacts, $G_{\rm thermal}/P_{\rm shot}= {\cal L}T/eV$ (with ${\cal L}$ the Lorenz number). This relation is ensured by the combination of electron-hole and time-reversal symmetries, irrespective of the microscopics of the surface Hamiltonian, and provides for a purely electrical way to detect the charge-neutral Majorana surface states. A surface of aspect ratio $W/L\gg 1$ has the universal shot-noise power $P_{\rm shot}=(W/L)\times (e^2/h)\times (eV/2π)$

cond-mat.mes-hall↗

arXiv2015

Topologically protected charge transfer along the edge of a chiral $\textit{p}$-wave superconductor

The Majorana fermions propagating along the edge of a topological superconductor with $p_x+ip_y$ pairing deliver a shot noise power of $\frac{1}{2}\times e^2/h$ per eV of voltage bias. We calculate the full counting statistics of the transferred charge and find that it becomes trinomial in the low-temperature limit, distinct from the binomial statistics of charge-$e$ transfer in a single-mode nanowire or charge-$2e$ transfer through a normal-superconductor interface. All even-order correlators of current fluctuations have a universal quantized value, insensitive to disorder and decoherence. These electrical signatures are experimentally accessible, because they persist for temperatures and voltages large compared to the Thouless energy.

cond-mat.mes-hall↗

arXiv2014

Spectroscopic factors of magic and semimagic nuclei within the self-consistent theory of finite Fermi systems

A scheme is presented to find single-particle spectroscopic factors (SF) of magic and semimagic nuclei within the self-consistent theory of finite Fermi systems (TFFS). In addition to the energy dependence of the mass operator $Σ$ induced by the surface-phonon coupling effects which are commonly considered in this problem, the in-volume energy dependence of the operator $Σ$ inherent in the self-consistent TFFS is also taken into account. This dependence arises due to the effect of high-lying particle-hole excitations and persists in nuclear matter. The self-consistent basis of the energy density functional method by Fayans {\it et al.} is used. Both the surface and in-volume contributions to the SFs turned out to be of comparable magnitude. The results for magic $^{40,48}$Ca and $^{208}$Pb nuclei and semimagic lead isotopes are presented.

nucl-th↗

arXiv2014

Self-consistent description of single-particle levels of magic nuclei

Single-particle levels of seven magic nuclei are calculated within the Energy Density Functional (EDF) method by Fayans et al. Three versions of the EDF are used, the initial Fayans functional DF3 and its two variations, DF3-a and DF3-b, with different values of spin-orbit parameters. Comparison is made with predictions of the Skyrme-Hartree-Fock method with the HFB-17 functional. For the DF3-a functional, phonon coupling (PC) corrections to single-particle energies are found self-consistently with an approximate account for the tadpole diagram. Account for the PC corrections improves agreement with the data for heavy nuclei, e.g. for 208 Pb. On the other hand, for lighter nuclei, e.g. 40,48 Ca, PC corrections make the agreement a little worse. As estimations show, the main reason is that the approximation we use for the tadpole term is less accurate for the light nuclei.

nucl-th↗

arXiv2012

An ab initio theory of double odd-even mass differences in nuclei

Two aspects of the problem of evaluating double odd-even mass differences D_2 in semi-magic nuclei are studied related to existence of two components with different properties, a superfluid nuclear subsystem and a non-superfluid one. For the superfluid subsystem, the difference D_2 is approximately equal to 2Δ, the gap Δbeing the solution of the gap equation. For the non-superfluid subsystem, D_2 is found by solving the equation for two-particle Green function for normal systems. Both equations under consideration contain the same effective pairing interaction. For the latter, the semi-microscopic model is used in which the main term calculated from the first principles is supplemented with a small phenomenological addendum containing one phenomenological parameter supposed to be universal for all medium and heavy atomic nuclei.

nucl-th↗