Energy exchange calculations in a simple mechanical system to investigate the origin of friction

The microscopic origin of friction is an important topic in science and technology. To date, noteworthy aspects of it remain unsolved. In an effort to shed some light on the possible mechanisms that could give rise to the macroscopic emergence of friction, a very simple 1D system of two particles is considered, one of them is free but moving with an initial velocity, and the other confined by a harmonic potential. The two particles interact via a repulsive Gaussian potential. While it represents in a straightforward manner a tip substrate system in the real world, no analytic solutions can be found for its motion. Because of the interaction, the free particle (tip) may overcome the bound particle (substrate) losing part of its kinetic energy. We solve Newton’s equations of the two particles numerically and calculate the net exchange of energy in the asymptotic state in terms of the relevant parameters of the problem. The effective dissipation that emerges from this simple, classical model with no ad hoc terms shows, surprisingly, a range of rich, nontrivial, behavior. We give theoretical reasoning which provides a satisfactory qualitative description. The essential ingredient of that reasoning is that the transfer of energy from the incoming particle to the confined one can be regarded as the source of the emergent dissipation force the friction experienced by the incoming particle.

Stability of the global climate system against strong perturbations

<p>The global climate system is an aggregate of a huge number of interacting components, each having an intrinsic time scale. Such a complex dynamical system demonstrates nontrivial behavior and can exhibit a variety of possible modes of evolution. Gradual change of the parameters of the global climate system can lead to transitions (e.g., the Mid-Pleistocene Transition or to abrupt climate changes) from the observed to a new mode.<br>In this work, we investigate the stability of the global climate system against strong sudden perturbations in the last 2.5 million years. This case is fundamentally different from the small perturbations case: in particularly, the system response cannot be described by a linearized evolution operator. To estimate the climate system’s nonlinear stability during the last 2.5 million years, we use a nonlinear data-driven model of climate dynamics in Pleistocene [1] and basin stability criterion [2]. Our results indicate that the stabilityof the Pleistocene climate to large perturbations decreases with time: past climates being much more stable compared to the present one.<br>This work was supported by RFBR grant 19-02-00502.</p><p>1. D. Mukhin, A. Gavrilov, E. Loskutov, J. Kurths, A. Feigin. “Bayesian Data Analysis for Revealing Causes of the Middle Pleistocene Transition”. ScientificReports, 9 7328 (2019).<br>2. V. Klinshov, S. Kirillov, J. Kurths, V. Nekorkin. “Interval stability for complex systems”. New Journal of Physics, v. 20, p. 043040.</p>

Механизм переноса заряда в новом магнитном топологическом изоляторе MnBi-=SUB=-0.5-=/SUB=-Sb-=SUB=-1.5-=/SUB=-Te-=SUB=-4-=/SUB=-

New layered magnetic topological insulator with the composition MnBi0.5Sb1.5Te4 is obtained. In-layer plane electrical resistivity and electrical resistivity in the direction perpendicular to the layers have been studied over the wide temperature range 1.4-300K. It is found that the temperature dependence of the resistivity ρ(Т) exhibits "metallic" behavior in both directions in a temperature range above 50K. Below 50K ρ is growing with decreasing temperature and shows nontrivial behavior with a peculiarity near the critical temperature Tc=23K. The growth of resistivity in the temperature range 50-23K is caused by spin fluctuations which precede magnetic phase transition at 23K. Below Тc and down to 1.4 K, the behavior of ρ(Т) represents a typical manifestation of the effect of weak localization, as confirmed by the analysis of the obtained data on magnetoresistance.

Anisotropic Ferroelectric Distortion Effects on the RKKY Interaction in Topological Crystalline Insulators

Manipulation of electronic and magnetic properties of topological materials is a topic of much interest in spintronic and valleytronic applications. Perturbation tuning of multiple Dirac cones on the (001) surface of topological crystalline insulators (TCIs) is also a related topic of growing interest. Here we show the numerical evidence for the ferroelectric structural distortion effects on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurity moments on the SnTe (001) and related alloys. The mirror symmetry breaking between Dirac cones induced by the ferroelectric distortion could be divided into various possible configurations including the isotropically gapped, coexistence of gapless and gapped, and anisotropically gapped phases. Based on the retarded perturbed Green’s functions of the generalized gapped Dirac model, we numerically find the RKKY response for each phase. The distortion-induced symmetry breaking constitutes complex and interesting magnetic responses between magnetic moments compared to the pristine TCIs. In the specific case of coexisted gapless and gapped phases, a nontrivial behavior of the RKKY interaction is observed, which has not been seen in other Dirac materials up until now. For two impurities resided on the same sublattices, depending on the distortion strength, magnetic orders above of a critical impurity separation exhibit irregular ferromagnetic $ antiferromagnetic phase transitions. However, independent ofthe impurity separation and distortion strength, no phase transition emerges for two impurities resided on different sublattices. This essential study sheds light on magnetic properties of Dirac materials with anisotropic mass terms and also makes TCIs applications relatively easy to understand.

Test of the unitary coupled-cluster variational quantum eigensolver for a simple strongly correlated condensed-matter system

The variational quantum eigensolver has been proposed as a low-depth quantum circuit that can be employed to examine strongly correlated systems on today’s noisy intermediate-scale quantum computers. We examine details associated with the factorized form of the unitary coupled-cluster variant of this algorithm. We apply it to a simple strongly correlated condensed-matter system with nontrivial behavior — the four-site Hubbard model at half-filling. This work show some of the subtle issues one needs to take into account when applying this algorithm in practice, especially to condensed-matter systems.

Topology and Broken Symmetry in Floquet Systems

Floquet systems are governed by periodic, time-dependent Hamiltonians. Prima facie they should absorb energy from the external drives involved in modulating their couplings and heat up to infinite temperature. However, this unhappy state of affairs can be avoided in many ways. Instead, as has become clear from much recent work, Floquet systems can exhibit a variety of nontrivial behavior—some of which is impossible in undriven systems. In this review, we describe the main ideas and themes of this work: novel Floquet drives that exhibit nontrivial topology in single-particle systems, the existence and classification of exotic Floquet drives in interacting systems, and the attendant notion of many-body Floquet phases and arguments for their stability to heating.

Phase time and transmission probability in the traversal of a PT-symmetric potential: The case of an electromagnetic waveguide

We study the unconventional transmission properties of a wave packet through a PT-symmetric potential region as describing the actual electromagnetic wave propagation along a waveguide filled with gain and loss media. The nontrivial behavior of the transmission probability manifests in the giant amplification of the incident electromagnetic signal of given wavelengths for well-defined configurations, depending on the gain/loss contrast. Maximum transmission peaks are related to spectral singularities and a strict correlation exists between the “resonant” wavelengths and the gain/loss contrast. The transit times are as well calculated, showing their surprising vanishing in the opaque barrier limit, independently of the gain/loss contrast, which is reminiscent of some sort of Hartman effect. Also, nonlocal effects manifest in the presence of negative delay times for given configurations, while a correlation is apparent between maximum delay times and transmission probability peaks, though appreciably depending on the gain/loss contrast.

ON THE ROBUSTNESS OF MAGNETISM IN ZIGZAG GRAPHENE NANORIBBONS

In this work, using the single-band Hubbard model, we numerically study the magnetic ordering in zigzag graphene nanoribbons (ZGNRs). We calculate density of states and charge density distribution in the ZGNR, and show the nontrivial behavior of its electronic band structure in the presence of an external transverse electric field. Then, the robustness of such edge magnetic ordering and consequent half-metallicity is investigated for nanoribbons defected with single-atom vacancies. Our results show that the nontrivial magnetic properties of ZGNRs are robust to an acceptable percentage of single atom vacancies.