arXiv subjects

Oleg I. Utesov

Publications and source records attributed to Oleg I. Utesov.

At least 19 recordsLinked to original sources

arXiv2026

Steady reservoir-mediated self-trapping of polariton condensates

Nonequilibrium bosonic condensates of exciton-polaritons generated by tightly focused incoherent optical beams are typically expected to form a stationary ballistically expanding state with the notable exception of experimentally observed compact self-trapped states [Phys. Rev. Lett. 123, 047401 (2019)]. We show that the back-action of the condensate on the optically pumped excitonic reservoir via bosonic stimulated scattering can cause local reservoir depletion, producing a steady condensate confined in a self-induced potential trap. Notably, this self-trapped state is formed by persistent converging rather than radially expanding polariton currents and exists even below the conventionally defined condensation threshold. This state remains steady over nanosecond-scale times and is eventually ejected from the self-induced trap due to a bullet-type instability, enabling spiking neuromorphic dynamics in networks of polariton condensates.

cond-mat.mes-hall↗

arXiv2026

Subcritical bifurcation and on-off bistability in ballistic polariton condensates

Dynamics of exciton-polariton condensates under continuous-wave incoherent Gaussian optical pumping is considered. It is shown that the conventional supercritical Stuart-Landau picture is invalid in a certain domain of the parameter space. For strong polariton repulsion from the reservoir and relatively small pump spots, the dynamics is adequately described by the quintic Stuart-Landau equation. The corresponding subcritical pitchfork bifurcation leads to condensate formation, accompanied by bistability between the trivial and nontrivial states over a finite pump-power range and a one-bit memory. Further increase of the repulsion parameter or decrease of the spot size breaks down the perturbative approach and leads to a peculiar self-trapping regime with complex dynamics. Experimental evidence of the emergence of the proposed behavior is provided. Our findings can be used to design polaritonic setups that exploit the predicted memory effect.

cond-mat.mes-hall↗

arXiv2026

Nematic Phase Transitions in 1D Flat Band Condensates

We investigate the ground-state properties of one-dimensional Gross-Pitaevskii flat-band lattices which are parametrized through their compact localized state (CLS) amplitudes. We uncover a CLS geometry-driven phase transition into a macroscopically degenerate nematic state with broken time-reversal symmetry. The transition is marked by the appearance of constant-density flat-band states and a vanishing sound velocity. We demonstrate that even infinitesimal onsite interactions can destabilize a $k=0$ plane wave condensate, driving the system into a nematic manifold, which persists for any interaction strength. For the particular choice of constant density CLSs which can tile the lattice, we identify additional families of continuously degenerate ground states characterized by vanishing phase stiffness. Utilizing Bogoliubov-de Gennes excitations and parallel tempering, we show that these tiling phases are thermally selected at low temperatures via an order-by-disorder mechanism. We exemplify our findings for different classes of flat bands. Our findings also reveal that the sound velocity in flat-band condensates is a sensitive probe of the underlying nematic phase transitions.

cond-mat.stat-mech↗

arXiv2026

Vector-field control and emergent basal-plane anisotropy of magnetic textures in noncentrosymmetric (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P

(Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P is a room-temperature magnet with $S_4$ symmetry that hosts a rich variety of topological spin textures. Here, we report a combined resonant small-angle x-ray scattering and ptychography study of (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P in vector magnetic fields over a broad temperature range. We demonstrate deterministic vector-field control of magnetic stripe domains, where in-plane fields continuously rotate their orientation via a transition from a chiral stripe to an achiral fan configuration. Furthermore, at 50 K and below, the stripe orientation becomes metastably pinned and retains its field-trained direction. While the magnitude of the wavevector is nearly isotropic within the basal plane at room temperature, a pronounced temperature evolution of anisotropic interactions emerges upon cooling. In particular, non-trivial anisotropy axes develop at 20-50 K reflecting the combined effects of magnetocrystalline anisotropy, anisotropic exchange, and Dzyaloshinskii-Moriya interaction (DMI), whose effective orientation is found to rotate with temperature. These results establish (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P as a model system for vector-field control of chiral spin textures and reveal a previously unrecognized temperature-driven evolution of the effective DMI landscape in a noncentrosymmetric magnet.

cond-mat.mes-hall↗

arXiv2026

Frustration of harmonic and solitonic helimagnetism on the body-centered tetragonal lattice of GdAlSi

The triangular lattice antiferromagnet (TLAF) with nearest-neighbor exchange interaction is a model platform in the field of frustrated magnetism. Here, anharmonic (`up-up-down') and harmonic (`120 degree') magnetic states compete, because the fundamental helimagnetic wave and its higher harmonic are degenerate in energy. We show that a body-centered tetragonal lattice (BCTL) can realize a similar frustration of harmonic and anharmonic helimagnetic states, and that the tetragonal magnetic Weyl semimetal GdAlSi realizes this scenario. In an applied magnetic field, resonant elastic X-ray scattering reveals a competition of harmonic cycloidal and solitonic double-Q states, well consistent with mean-field calculations. Our work provides a new paradigm for frustration physics in BCTL materials.

cond-mat.str-el↗

arXiv2026

A Surface-confined Spiral State With the Double Period in the Cubic Chiral Helimagnet Cu$_2$OSeO$_3$

The chiral magnetoelectric insulator Cu$_2$OSeO$_3$ hosts a rich and anisotropic magnetic phase diagram that includes helical, conical, field-polarised, tilted conical, and skyrmion lattice phases. Using resonant elastic x-ray scattering (REXS), we uncover a new spiral state confined to the surface of Cu$_2$OSeO$_3$. This surface-confined spiral state (SSS) displays a real-space pitch of $\sim$120 nm, which remarkably is twice the length of the incommensurate structures observed to-date in Cu$_2$OSeO$_3$. The SSS phase emerges at temperatures below 30~K when the magnetic field is applied between $3^\circ$ to $18^\circ$ away from the $\langle\text{110}\rangle$ crystallographic axes. Its surface localisation is demonstrated through a combination of REXS in reflection and transmission geometries, with complementary small-angle neutron scattering measurements suggesting its absence from the bulk. We attribute the stabilisation of the SSS to competing anisotropic interactions at the crystal surface. The discovery of a robust, surface-confined spiral paves the way for engineering energy-efficient, nanoscale spin-texture platforms for next-generation devices.

cond-mat.str-el↗

arXiv2026

Localization--non-ergodic transition in controllable-dimension fractal networks from diffusion-limited aggregation

Our study connects the physics of disordered integer-dimensional systems and regular self-similar objects by studying spectral properties of fractal agglomerates with tunable dimension. The latter is controlled by parameter $α$ of the algorithm that generates the agglomerates. We consider the nearest-neighbor tight-binding model on the agglomerates embedded in 2D and 3D, and observe that all eigenstates are localized in the 2D case, whereas in the 3D case, there is a localization--non-ergodic transition upon increasing $α$,i.e., going from sparse to dense fractals: a sub-extensive number of critical states emerge in the spectrum at a certain critical value of $α$. The complex geometry of the agglomerates is also responsible for a peculiar hierarchy of compact localized states and singularities in the density of states, which are typical for ordered fractals.

cond-mat.dis-nn↗

arXiv2025

Rapid Determination of Nanodiamond Size Distribution and Impurity Concentration from Raman Spectra Using an Open Machine-Learning Toolbox

Ready-to-use numerical toolbox for nanodiamond Raman spectra calculation and fit is presented. The developed theoretical approach allows accounting for arbitrary nanoparticle size-distribution and the microscopic line broadening mechanisms for the optical phonons. The two tools for solving the inverse problem of the nanodiamond properties reconstruction using a known Raman spectrum are provided. The first one utilizes a dense neural network trained on a vast array of synthetic Raman spectra. The second approach is based on the stochastic Metropolis algorithm, which updates the ensemble parameters by small quantities, tending to the state with minimal error. Both methods are available thanks to the computationally instant elasticity theory-like model for optical phonon modes in diamond nanocrystals that accurately reproduces the results of the atomistic approaches. Using experimental Raman spectra for nanodiamonds prepared by various techniques, we tested our tools and observed a faithful agreement with the data as well as between the two methods. The open and documented software is accessible online (nanoraman.pythonanywhere.com) and as a Python module (github.com/KoniakhinSV/Nanoparticle_Raman).

cond-mat.mes-hall↗

arXiv2025

Thermodynamic theory of square skyrmion lattice in tetragonal frustrated antiferromagnets

High-temperature part of the phase diagram of tetragonal frustrated antiferromagnets is discussed within the framework of the mean-field approach. Based on recent experimental findings, we generalize previous theoretical studies by considering a model that includes frustrated Heisenberg exchange, biquadratic exchange, magnetodipolar interaction, anisotropic exchange, and single-ion anisotropy. It is analytically demonstrated that a subtle interplay among these interactions results in a variety of phase diagrams in the temperature-magnetic field plane. We argue that one of the proposed diagrams reproduces all crucial features of the phase diagram experimentally observed for~\gdru~compound. Besides magnetodipolar interaction and additional easy-axis contribution, it requires moderate biquadratic exchange. We show that despite the remarkable square skyrmion lattice being stable even if only magnetodipolar interaction/compass anisotropy or biquadratic exchange is included, their ``symbiosis'' allows for greatly enhancing its stability region. It is also demonstrated that higher-order harmonics play an important role in the stabilization of the square skyrmion lattice. The developed analytical approach can be useful for the refinement of the microscopic model parameters when comparing its predictions with experimental findings.

cond-mat.str-el↗

arXiv2025

Helical spin dynamics in Cu$_2$OSeO$_3$ as measured with small-angle neutron scattering

The insulating chiral magnet Cu$_2$OSeO$_3$ exhibits a rich array of low-temperature magnetic phenomena, making it a prime candidate for the study of its spin dynamics. Using spin wave small-angle neutron scattering (SWSANS), we systematically investigated the temperature-dependent behavior of the helimagnon excitations in the field-polarized phase of Cu$_2$OSeO$_3$. Our measurements, spanning 5-55 K, reveal the temperature evolution of spin-wave stiffness and damping constant with unprecedented resolution, facilitated by the insulating nature of Cu$_2$OSeO$_3$. These findings align with theoretical predictions and resolve discrepancies observed in previous studies, emphasizing the enhanced sensitivity of the SWSANS method. The results provide deeper insights into the fundamental magnetic properties of Cu$_2$OSeO$_3$, contributing to a broader understanding of chiral magnets.

cond-mat.str-el↗

arXiv2025

Mobility edges and fractal states in quasiperiodic Gross-Pitaevskii chains

We explore properties of a Gross-Pitaevskii chain subject to an incommensurate periodic potential, i.e., a nonlinear Aubry-Andre model. We show that the condensate crucially impacts the properties of the elementary excitations. In contrast to the conventional linear Aubry-Andre model, the boundary between localized and extended states (mobility edge) exhibits nontrivial branching. For instance, in the high-density regime, tongues of extended phases at intermediate energies penetrate the domain of localized states. In the low-density case, the situation is opposite, and tongues of localized phases emerge. Moreover, intermediate critical (fractal) states are observed. The low-energy phonon part of the spectrum is robust against the incommensurate potential. Our study shows that accounting for interactions, already at the classical level, lead to highly nontrivial behavior of the elementary excitation spectrum.

cond-mat.dis-nn↗

arXiv2025

Observation of magnetic skyrmion lattice in Cr$_{0.82}$Mn$_{0.18}$Ge by small-angle neutron scattering

Incommensurate magnetic phases in chiral cubic crystals are an established source of topological spin textures such as skyrmion and hedgehog lattices, with potential applications in spintronics and information storage. We report a comprehensive small-angle neutron scattering (SANS) study on the $B20$-type chiral magnet Cr$_{0.82}$Mn$_{0.18}$Ge, exploring its magnetic phase diagram and confirming the stabilization of a skyrmion lattice under low magnetic fields. Our results reveal a helical ground state with a decreasing pitch from 40 nm to 35 nm upon cooling, and a skyrmion phase stable in applied magnetic fields of 10-30 mT, and over an unusually wide temperature range for chiral magnets of 6 K ($\sim T_\textrm{C}/2 < T < T_\textrm{C}$, $T_\textrm{C}=13$ K). The skyrmion lattice forms a standard two-dimensional hexagonal coordination that can be trained into a single domain, distinguishing it from the three-dimensional hedgehog lattice observed in MnGe-based systems. Additionally, we demonstrate the persistence of a metastable SkL at 2 K, even at zero field. These findings advance our understanding of magnetic textures in Cr-based $B20$ compounds, highlighting Cr$_{0.82}$Mn$_{0.18}$Ge~as a promising material for further exploration in topological magnetism.

cond-mat.mes-hall↗

arXiv2024

Competing anisotropies in the chiral cubic magnet Co$_8$Zn$_8$Mn$_4$ unveiled by resonant x-ray magnetic scattering

The cubic $β$-Mn-type alloy Co$_8$Zn$_8$Mn$_4$ is a chiral helimagnet that exhibits a peculiar temperature-dependent behavior in the spiral pitch, which decreases from 130 nm at room temperature to 70 nm below 20 K. Notably, this shortening is also accompanied by a structural transition of the metastable skyrmion texture, transforming from a hexagonal lattice to a square lattice of elongated skyrmions. The underlying mechanism of these transformations remain unknown, with interactions potentially involved including temperature-dependent Dzyaloshinskii-Moriya interaction, magnetocrystalline anisotropy, and exchange anisotropy. Here, x-ray resonant magnetic small-angle scattering in vectorial magnetic fields was employed to investigate the temperature dependence of the anisotropic properties of the helical phase in Co$_8$Zn$_8$Mn$_4$. Our results reveal quantitatively that the magnitude of the anisotropic exchange interaction increases by a factor of 4 on cooling from room temperature to 20 K, leading to a 5% variation in the helical pitch within the (001) plane at 20 K. While anisotropic exchange interaction contributes to the shortening of the spiral pitch, its magnitude is insufficient to explain the variation in the spiral periodicity from room to low temperatures. Finally, we demonstrate that magnetocrystalline and exchange anisotropies compete, favoring different orientations of the helical vector in the ground state.

cond-mat.mes-hall↗

arXiv2023

Small-angle neutron scattering in fully polarized phase of non-collinear magnets with interfacial-like DMI

Spin waves in non-collinear magnets with $C_{nv}$ symmetry are discussed in the context of inelastic neutron scattering in small-angle scattering geometry. In the framework of a minimal model including exchange coupling, Dzayloshinskii-Moriya interaction (DMI), and single-ion anisotropy, we consider the system properties at moderate external magnetic fields and in the high-field fully polarized phase. In the latter case, the magnon spectrum is gapped and non-reciprocal due to DMI with the minimum perpendicular to the field direction for the in-plane field and is ferromagnetic-like for the field along the high-symmetry axis. Inelastic small-angle neutron scattering in the fully polarized phase (SWSANS) is considered in four different geometries. It is shown that analysis of the field dependence of the SWSANS cutoff feature in these geometries allows determining all important parameters of the model. The possibility of the proposed method utilization in thin films with interfacial DMI is also discussed.

cond-mat.str-el↗

arXiv2023

Ferromagnetic monolayer with interfacial Dzyaloshinskii-Moriya interaction: magnon spectrum and effect of quenched disorder

We discuss theoretically a ferromagnetic monolayer with an interfacial Dzyaloshinskii-Moriya interaction (iDMI) and a small axial anisotropy. It is shown that the system has a long-period cycloid magnetic order slightly distorted by the anisotropy whose modulation vector $\bf k$ can have several orientations. We find that due to iDMI-induced umklapp terms in the Hamiltonian, the spectrum of long-wavelength magnons is essentially anisotropic: it is linear and quadratic for momenta directed along and perpendicular to $\bf k$, respectively. Due to such a quasi-1D spectrum, the temperature correction to the mean spin value has a power-law singularity hampering the magnetic ordering at $T\ne0$. We demonstrate that the umklapps lead to a peculiar band structure of the magnon spectrum similar to electronic bands in solids. We discuss also the effect of vacancies and defect bonds on system properties at $T=0$. Owing to the quasi-1D character of the spectrum, the distortion of the cycloid structure by a single defect bond is described by the field of 1D electric dipole. As a result, even infinitesimal concentrations of defects $c$ destroy the long-range order and establish a short-range order whose correlation length shows a power-law dependence on $1/c$. Our findings should also be applicable to ultra-thin films with strong enough iDMI.

cond-mat.str-el↗

arXiv2023

Direct observation of the exchange anisotropy in the helimagnetic insulator Cu$_2$OSeO$_3$

The helical magnetic structures of cubic chiral systems are well-explained by the competition among Heisenberg exchange, Dzyaloshinskii-Moriya interaction, cubic anisotropy, and anisotropic exchange interaction (AEI). Recently, the role of the latter has been argued theoretically to be crucial for the low-temperature phase diagram of the cubic chiral magnet Cu$_2$OSeO$_3$, which features tilted conical and disordered skyrmion states for a specific orientation of the applied magnetic field ($μ_0 \vec{\mathrm{H}} \parallel [001]$). In this study, we exploit transmission resonant x-ray scattering ($t-$REXS) in vector magnetic fields to directly quantify the strength of the AEI in Cu$_2$OSeO$_3$, and measure its temperature dependence. We find that the AEI continuously increases below 50\,K, resulting in a conical spiral pitch variation of $10\%$ in the (001) plane. Our results contribute to establishing the interaction space that supports tilted cone and low-temperature skyrmion state formation, facilitating the goals for both a quantitative description and eventual design of the diverse spiral states existing amongst chiral magnets.

cond-mat.str-el↗

arXiv2023

Magnons in the fan phase of anisotropic frustrated antiferromagnets

Elementary excitations spectrum of the fan phase of anisotropic frustrated antiferromagnets is discussed analytically. In the linear spin-wave approximation, the spectrum is determined by the Hamiltonian, including normal, anomalous, and umklapp terms. The latter mix states with momenta which differ by two modulation vectors of the fan structure. This mixing leads to essential rearrangement of the low-energy part of the spectrum, which is shown to consist of a gapless phason branch with linear dispersion and a gapped ``optical'' branch, which corresponds to the fan structure amplitude oscillations. In the high-energy part of the spectrum, the effect of the umklapps is negligible, and the excitations are similar to the magnons of the fully polarized phase.

cond-mat.str-el↗

arXiv2022

Easy-plane antiferromagnet in tilted field: gap in magnon spectrum and susceptibility

Motivated by recent experimental data on dichloro-tetrakis thiourea-nickel (DTN) [T.A. Soldatov $\textit{et al}$, Phys. Rev. B ${\bf 101}$, 104410 (2020)], a model of antiferromagnet on a tetragonal lattice with single-ion easy-plane anisotropy in the tilted external magnetic field is considered. Using the smallness of the in-plane field component, we analytically address field dependence of the energy gap in ``acoustic'' magnon mode and transverse uniform magnetic susceptibility in the ordered phase. It is shown that the former is non-monotonic due to quantum fluctuations, which was indeed observed experimentally. The latter is essentially dependent on the ``optical'' magnon rate of decay on two magnons. At magnetic fields close to the one which corresponds to the center of the ordered phase, it leads to experimentally observed dynamical diamagnetism phenomenon.

cond-mat.str-el↗