arXiv subjects

Anna Pomyalov

Publications and source records attributed to Anna Pomyalov.

At least 19 recordsLinked to original sources

arXiv2026

Breakdown of the classical rupture theory and earthquake propagation in the "forbidden" super-Rayleigh range

Earthquakes propagating faster than the shear wave-speed are commonly thought to undergo a super-shear transition upon which they discontinuously jump from the sub-Rayleigh regime to the super-shear one. The super-Rayleigh regime, i.e., the range of propagation speeds between the Rayleigh and shear wave-speeds, is regarded as "forbidden" by the two-dimensional classical rupture theory. Here, we revisit the assumptions underlying the classical theory and develop a rupture theory that takes into account the dependence of the fault strength (frictional resistance) on the slip rate. The theory quantitatively agrees with numerical simulations nearly up to the Rayleigh wave-speed. Yet, very close to the latter, two-dimensional rupture solutions change their character due to frictional rate nonlinearity and rupture continuously propagates through the "forbidden" super-Rayleigh range into the super-shear regime, without a sharp super-shear transition. These results demonstrate that frictional rate dependence, generically observed in experiments, can have profound implications for fast earthquake propagation.

physics.geo-ph↗

arXiv2025

Unified theory of magnetization temperature dependence in ferrimagnets

Recent advancements in spintronics and fundamental physical research have brought increased attention to the rare-earth-based magnetically ordered materials. One of the important properties of these materials is the temperature dependence of the spontaneous magnetization $M(T)$. Recently, a successful framework was proposed for the theoretical description of M(T) across the entire temperature range from zero to the Curie temperature in simple cubic ferromagnets, EuO and EuS. We extend this approach to compute and analyze $M(T)$ for multi-sublattice collinear ferrimagnets such as Yttrium Iron Garnet $Y_3Fe_5 O_{12}$. We analyzed and generalized for multi-sublattice collinear ferrimagnets two well-known approximations describing $M(T)$. The first approach is the Bloch-3/2 law, which describes the suppression of $M(T)$ due to spin-wave excitation, and is valid in the low-temperature limit $T << T_c$. The second one is Weiss's mean-field approximation, which provides a reasonable description of $M(T)$ near $T_c$. Using a single tuning parameter, we combine these two approaches to describe $M(T)$ for any $0<T<T_c$. The theoretical result for $M(T)$ aligns well with our measurements and the previously available experimental data across the entire temperature range. We also demonstrate that experimental and theoretical dependences $M(T)$ follow the mean-field prediction $\sqrt{T_c - T }$ for almost all temperatures.

cond-mat.mtrl-sci↗

arXiv2025

Precritical anomalous scaling and magnetization temperature dependence in cubic ferromagnetic crystals

Recent developments in spintronics have drawn renewed attention to the spin dynamics of cubic ferromagnetic crystals EuO and EuS. These ferromagnets have the simplest possible magnetic structure, making them the most suitable systems for testing various theoretical models of magnetic materials. A commonly used Weiss mean-field approximation (MFA) provides only a qualitative description of the magnetization temperature dependence $M(T)$. We develop a consistent theory for $M(T)$ based on the perturbation diagrammatic technique for spin operators. Our theory is in excellent quantitative agreement with the experimental dependence of $M(T)$ for EuO and EuS throughout the entire temperature range from $T=0$ to Curie temperature $T_{C}$. In particular, our theoretical dependence $M(T)$ demonstrates a scaling behavior $M(T)\propto (T_{C}-T)^{β_*}$ with the scaling index $β_*\approx 1/3$ in a wide range of temperatures in agreement with the experimentally observed apparent scaling in EuO and EuS. The scaling behavior with $β_*\approx 1/3$ is manifested in the temperature range $T\lesssim T_{C}$ where corrections to the magnetization due to its fluctuations $δM (T)\lesssim M(T)$. To distinguish it from the narrow ``critical" range $T\approx T_{C}$ with $δM (T) > M(T)$, we term this $T$-range ``precritical". The precritical corrections $δM (T)$ are still large enough to affect the $M(T)$ behavior. The index $β_*$ fundamentally differs from the ``normal" scaling index $β_{MFA}=1/2$ predicted by the MFA, which neglects the magnetization fluctuations. We refer to the emerging in our theory apparent magnetization scaling with $β_*\approx 1/3$ as ``precritical anomalous".

cond-mat.mtrl-sci↗

arXiv2024

Unsteady slip pulses under spatially-varying prestress

It was recently established that self-healing slip pulses under uniform prestress $τ_b$ are unstable frictional rupture modes, i.e., they either slowly expand/decay with time t. Furthermore, their dynamics were shown to follow a reduced-dimensionality description corresponding to a special $L(c)$ line in a plane defined by the pulse propagation velocity $c(t)$ and size $L(t)$. Yet, uniform prestress is rather the exception than the rule in natural faults. We study the effects of a spatially-varying prestress $τ_b(x)$ on 2D slip pulses, initially generated under a uniform $τ_b$ along a rate-and-state friction fault. We consider periodic and constant-gradient prestress $τ_b(x)$ around the reference uniform $τ_b$. For a periodic $τ_b(x)$, pulses either sustain and form quasi-limit cycles in the $L-c$ plane or decay predominantly monotonically along the $L(c)$ line, depending on the instability index of the initial pulse and the properties of the periodic $τ_b(x)$. For a constant-gradient $τ_b(x)$, expanding/decaying pulses closely follow the $L(c)$ line, with systematic shifts determined by the sign and magnitude of the gradient. We also find that a spatially-varying $τ_b(x)$ can revert the expanding/decaying nature of the initial reference pulse. Finally, we show that a constant-gradient $τ_b(x)$, of sufficient magnitude and specific sign, can lead to the nucleation of a back-propagating rupture at the healing tail of the initial pulse, generating a bilateral crack-like rupture. This pulse-to-crack transition, along with the above-described effects, demonstrate that rich rupture dynamics merge from a simple, nonuniform prestress. Furthermore, we show that as long as pulses exist, their dynamics are related to the special $L(c)$ line, providing an effective, reduced-dimensionality description of unsteady slip pulses under spatially-varying prestress.

cond-mat.mtrl-sci↗

arXiv2023

Dynamic Screening by Plasticity in Amorphous Solids

In recent work, it was shown that elasticity theory can break down in amorphous solids subjected to nonuniform {\em static} loads. The elastic fields are screened by geometric dipoles; these stem from gradients of the quadrupole field associated with plastic responses. Here we study the dynamical responses induced by {\em oscillatory} loads. The required modification to classical elasticity is described. Exact solutions for the displacement field in circular geometry are presented, demonstrating that dipole screening results in essential departures from the expected predictions of classical elasticity theory. Numerical simulations are conducted to validate the theoretical predictions and to delineate their range of validity.

cond-mat.dis-nn↗

arXiv2023

Local temperature control of magnon frequency and direction of supercurrents in a magnon Bose-Einstein condensate

The creation of temperature variations in magnetization, and hence in the frequencies of the magnon spectrum in laser-heated regions of magnetic films, is an important method for studying Bose-Einstein condensation of magnons, magnon supercurrents, Bogoliubov waves, and similar phenomena. In our study, we demonstrate analytically, numerically, and experimentally that, in addition to the magnetization variations, it is necessary to consider the connected variations of the demagnetizing field. In case of a heat induced local minimum of the saturation magnetization, the combination of these two effects results in a local increase in the minimum frequency value of the magnon dispersion at which the Bose-Einstein condensate emerges. As a result, a magnon supercurrent directed away from the hot region is formed.

cond-mat.quant-gas↗

arXiv2023

Bose-Einstein condensation in systems with flux equilibrium

We consider flux equilibrium in dissipative nonlinear wave systems subject to external energy pumping. In such systems, the elementary excitations, or quasiparticles, can create a Bose-Einstein condensate. We develop a theory on the Bose-Einstein condensation of quasiparticles for various regimes of external excitation, ranging from weak and stationary to ultra-strong pumping, enabling us to determine the number of quasiparticles near the bottom of the energy spectrum and their distribution along wave vectors. We identify physical phenomena leading to condensation in each of the regimes. For weak stationary pumping, where the distribution of quasiparticles deviates only slightly from thermodynamic equilibrium, we define a range of pumping parameters where the condensation occurs and estimate the density of the condensate and the fraction of the condensed quasiparticles. As the pumping amplitude increases, a powerful influx of injected quasiparticles is created by the Kolmogorov-Zakharov scattering cascade, leading to their Bose-Einstein condensation. With even stronger pumping, kinetic instability may occur, resulting in a direct transfer of injected quasiparticles to the bottom of the spectrum. For the case of ultra-strong parametric pumping, we have developed a stationary nonlinear theory of kinetic instability. The theory agrees qualitatively with experimental data obtained using Brillouin light scattering spectroscopy during parametric pumping of magnons in room-temperature films of yttrium-iron garnet.

cond-mat.quant-gas↗

arXiv2023

The dynamics of unsteady frictional slip pulses

Self-healing slip pulses are major spatiotemporal failure modes of frictional systems, featuring a characteristic size $L(t)$ and a propagation velocity $c_{\rm p}(t)$ ($t$ is time). Here, we develop a theory of slip pulses in realistic rate-and-state dependent frictional systems. We show that slip pulses are intrinsically unsteady objects -- in agreement with previous findings -- yet their dynamical evolution is closely related to their unstable steady-state counterparts. In particular, we show that each point along the time-independent $L^{\mbox{(0)}}(τ_{\rm d})\!-\!c^{\mbox{(0)}}_{\rm p}(τ_{\rm d})$ line, obtained from a family of steady-state pulse solutions parameterized by the driving shear stress $τ_{\rm d}$, is unstable. Nevertheless, and remarkably, the $c^{\mbox{(0)}}_{\rm p}[L^{\mbox{(0)}}]$ line is a dynamic attractor such that the unsteady dynamics of slip pulses (when they exist) -- whether growing ($\dot{L}(t)\!>\!0$) or decaying ($\dot{L}(t)\!<\!0$) -- reside on the steady-state line. The unsteady dynamics along the line are controlled by a single slow unstable mode. The slow dynamics of growing pulses, manifested by $\dot{L}(t)/c_{\rm p}(t)\!\ll\!1$, explain the existence of sustained pulses, i.e.~pulses that propagate many times their characteristic size without appreciably changing their properties. Our theoretical picture of unsteady frictional slip pulses is quantitatively supported by large-scale, dynamic boundary-integral method simulations.

cond-mat.mtrl-sci↗

arXiv2023

Correlation-enhanced interaction of a Bose-Einstein condensate with parametric magnon pairs and virtual magnons

Nonlinear interactions are crucial in science and engineering. Here, we investigate wave interactions in a highly nonlinear magnetic system driven by parametric pumping leading to Bose--Einstein condensation of spin-wave quanta -- magnons. Using Brillouin light scattering spectroscopy in yttrium-iron garnet films, we found and identified a set of nonlinear processes resulting in off-resonant spin-wave excitations -- virtual magnons. In particular, we discovered a dynamically-strong, correlation-enhanced four-wave interaction process of the magnon condensate with pairs of parametric magnons having opposite wavevectors and fully correlated phases.

cond-mat.quant-gas↗

arXiv2022

Self-healing (solitonic) slip pulses in frictional systems

A prominent spatiotemporal failure mode of frictional systems is self-healing slip pulses, which are propagating solitonic structures that feature a characteristic length. Here, we numerically derive a family of steady state slip pulse solutions along generic and realistic rate-and-state dependent frictional interfaces, separating large deformable bodies in contact. Such nonlinear interfaces feature a non-monotonic frictional strength as a function of the slip velocity, with a local minimum. The solutions exhibit a diverging length and strongly inertial propagation velocities, when the driving stress approaches the frictional strength characterizing the local minimum from above, and change their character when it is away from it. An approximate scaling theory quantitatively explains these observations. The derived pulse solutions also exhibit significant spatially-extended dissipation in excess of the edge-localized dissipation (the effective fracture energy) and an unconventional edge singularity. The relevance of our findings for available observations is discussed.

cond-mat.mtrl-sci↗

arXiv2021

Experimental observation of Josephson oscillations in a room-temperature Bose-Einstein magnon condensate

The alternating current (ac) Josephson effect in a time-independent spatially-inhomogeneous setting is manifested by the occurrence of Josephson oscillations - periodic macroscopic phase-induced collective motions of the quantum condensate. So far, this phenomenon was observed at cryogenic temperatures in superconductors, in superfluid helium, and in Bose-Einstein condensates (BECs) of trapped atoms. Here, we report on the discovery of the ac Josephson effect in a magnon BEC carried by a room-temperature ferrimagnetic film. The BEC is formed in a parametrically populated magnon gas in the spatial vicinity of a magnetic trench created by a dc electric current. The appearance of the Josephson effect is manifested by oscillations of the magnon BEC density in the trench, caused by a coherent phase shift between this BEC and the BEC in the nearby regions. Our findings advance the physics of room-temperature macroscopic quantum phenomena and will allow for their application for data processing in magnon spintronics devices.

cond-mat.quant-gas↗

arXiv2021

Generic Mechanism for Remote Triggering of Earthquakes

``Remote triggering" refers to the inducement of earthquakes by weak perturbations that emanate from far away sources, typically intense earthquakes that happen at much larger distances than their nearby aftershocks, sometimes even around the globe. Here, we propose a mechanism for this phenomenon; the proposed mechanism is generic, resulting from the breaking of Hamiltonian symmetry due to the existence of friction. We allow a transition from static to dynamic friction. {\em Linearly stable} stressed systems display giant sensitivity to small perturbations of arbitrary frequency (without a need for resonance), which trigger an instability with exponential oscillatory growth. Once nonlinear effects kick in, the blow up in mean-square displacements can reach 15-20 orders of magnitude. Analytic and numerical results of the proposed model are presented and discussed.

physics.geo-ph↗

arXiv2021

Theory of anisotropic superfluid He-4 counterflow turbulence

We develop an analytic theory of strong anisotropy of the energy spectra in the thermally-driven turbulent counterflow of superfluid He-4. The key ingredients of the theory are the three-dimensional differential closure for the vector of the energy flux and the anisotropy of the mutual friction force. We suggest an approximate analytic solution of the resulting energy-rate equation, which is fully supported by the numerical solution. The two-dimensional energy spectrum is strongly confined in the direction of the counterflow velocity. In agreement with the experiment, the energy spectra in the direction orthogonal to the counterflow exhibit two scaling ranges: a near-classical non-universal cascade-dominated range and a universal critical regime at large wavenumbers. The theory predicts the dependence of various details of the spectra and the transition to the universal critical regime on the flow parameters. This article is a part of the theme issue 'Scaling the turbulence edifice'.

cond-mat.other↗

arXiv2021

Evolution of room-temperature magnon gas toward coherent Bose-Einstein condensate

The appearance of spontaneous coherence is a fundamental feature of a Bose-Einstein condensate and an essential requirement for possible applications of the condensates for data processing and quantum computing. In the case of a magnon condensate in a magnetic crystal, such computing can be performed even at room temperature. So far, the process of coherence formation in a magnon condensate was inaccessible. We study the evolution of magnon radiation spectra by direct detection of microwave radiation emitted by magnons in a parametrically driven yttrium iron garnet crystal. By using specially shaped bulk samples, we show that the parametrically overpopulated magnon gas evolves to a state, whose coherence is only limited by the natural magnon relaxation into the crystal lattice.

cond-mat.mtrl-sci↗

arXiv2018

Tunable space-time crystal in room-temperature magnetodielectrics

We report the experimental realization of a space-time crystal with tunable periodicity in time and space in the magnon Bose-Einstein Condensate (BEC), formed in a room-temperature Yttrium Iron Garnet (YIG) film by radio-frequency space-homogeneous magnetic field. The magnon BEC is prepared to have a well defined frequency and non-zero wavevector. We demonstrate how the crystalline "density" as well as the time and space textures of the resulting crystal may be tuned by varying the experimental parameters: external static magnetic field, temperature, thickness of the YIG film and power of the radio-frequency field. The proposed space-time crystals provide a new dimension for exploring dynamical phases of matter and can serve as a model nonlinear Floquet system, that brings in touch the rich fields of classical nonlinear waves, magnonics and periodically driven systems.

cond-mat.quant-gas↗

arXiv2018

Long-distance supercurrent transport in a room-temperature Bose-Einstein magnon condensate

The term supercurrent relates to a macroscopic dissipation-free collective motion of a quantum condensate and is commonly associated with such famous low-temperature phenomena as superconductivity and superfluidity. Another type of motion of quantum condensates is second sound - a wave of the density of a condensate. Recently, we reported on an enhanced decay of a parametrically induced Bose-Einstein condensate (BEC) of magnons caused by a supercurrent outflow of the BEC phase from the locally heated area of a room temperature magnetic film. Here, we present the direct experimental observation of a long-distance spin transport in such a system. The condensed magnons being pushed out from the potential well within the heated area form a density wave, which propagates through the BEC many hundreds of micrometers in the form of a specific second sound pulse - Bogoliubov waves - and is reflected from the sample edge. The discovery of the long distance supercurrent transport in the magnon BEC further advances the frontier of the physics of quasiparticles and allows for the application of related transport phenomena for low-loss data transfer in perspective magnon spintronics devices.

cond-mat.quant-gas↗

arXiv2018

Theory of energy spectra in superfluid He-4 counterflow turbulence

In the thermally driven superfluid He-4 turbulence, the counterflow velocity $U_{\rm ns}$ partially decouples the normal and superfluid turbulent velocities. Recently we suggested [J. Low Temp. Phys. 187, 497 (2017)] that this decoupling should tremendously increase the turbulent energy dissipation by mutual friction and significantly suppress the energy spectra. Comprehensive measurements of the apparent scaling exponent nexp of the 2nd-order normal fluid velocity structure function $S_2(r)\propto r^{n_{\rm exp}}$ in the counterflow turbulence [Phys.Rev.B 96, 094511 (2017)] confirmed our scenario of gradual dependence of the turbulence statistics on the flow parameters. We develop an analytical theory of the counterflow turbulence, accounting for a twofold mechanism of this phenomenon: i) a scale-dependent competition between the turbulent velocity coupling by the mutual friction and the $U_{\rm ns}$-induced turbulent velocity decoupling and ii) the turbulent energy dissipation by the mutual friction enhanced by the velocity decoupling. The suggested theory predicts the energy spectra for a wide range of flow parameters. The mean exponents of the normal fluid energy spectra $\langle m_n\rangle_{10}$, found without fitting parameters, qualitatively agree with the observed $n_{\rm exp} + 1$ for $T\gtrsim 1.85$K

cond-mat.other↗

arXiv2018

From Kinetic Instability to Bose-Einstein Condensation and Magnon Supercurrents

Evolution of an overpopulated gas of magnons to a Bose-Einstein condensate and excitation of a magnon supercurrent, propelled by a phase gradient in the condensate wave function, can be observed at room-temperature by means of the Brillouin light scattering spectroscopy in an yttrium iron garnet material. We study these phenomena in a wide range of external magnetic fields in order to understand their properties when externally pumped magnons are transferred towards the condensed state via two distinct channels: A multistage Kolmogorov-Zakharov cascade of the weak-wave turbulence or a one-step kinetic-instability process. Our main result is that opening the kinetic instability channel leads to the formation of a much denser magnon condensate and to a stronger magnon supercurrent compared to the cascade mechanism alone.

cond-mat.quant-gas↗