arXiv subjects

D. Chernyshov

Publications and source records attributed to D. Chernyshov.

At least 19 recordsLinked to original sources

arXiv2026

Symmetry-Selective Stabilization of Charge-Density Wave in ScV$_6$Sn$_6$

Charge-density-wave (CDW) order in kagome metals is highly sensitive to external tuning parameters such as chemical substitution and hydrostatic pressure, which generally suppress long-range order. Here, using high-resolution X-ray diffraction under controlled uniaxial strain, we show that anisotropic lattice deformation instead stabilizes and enhances the CDW state in ScV$_6$Sn$_6$. Compression along the [H00] and [HH0] directions lowers the crystal symmetry from hexagonal to orthorhombic, lifts the degeneracy between symmetry-equivalent in-plane CDW domains, and promotes long-range order while preserving the underlying trimer instability. Phonon calculations indicate only a moderate stabilization of the imaginary flat phonon mode, demonstrating that the increase in T$_\mathrm{CDW}$ is primarily driven by the in-plane ordering of the Sn$^\mathrm{T}$--Sc--Sn$^\mathrm{T}$ \textit{rattling} chains within the frustrated kagome lattice. A phenomenological model incorporating strain-dependent Ising couplings within a three-state Potts framework successfully reproduces the evolution of T$_\mathrm{CDW}$ under compression and captures the continuous nature of the transition. Our results establish uniaxial strain as a powerful symmetry-selective tuning parameter for order-disorder transformations in frustrated lattices.

cond-mat.str-el↗

arXiv2026

Flat band driven competing charge and spin instabilities in the altermagnet CrSb

The confinement of electronic wavefunctions in momentum space can give rise to flat electronic bands, where the quenching of kinetic energy enhances the density of states and amplifies interaction effects. Such conditions are fertile ground for emergent quantum phases, as spin, charge and lattice degrees of freedom become strongly entangled. In these regimes, subtle competitions between intertwined order parameters often dictate the macroscopic ground state, producing complex and sometimes unexpected collective behavior. Here we show that the altermagnet CrSb provides a realization of this scenario, and uncover short-range charge-order fluctuations at the M point of the Brillouin zone, q*=(1/2 0), persisting above the Neel temperature (TN). Remarkably, these fluctuations collapse upon entering the magnetically ordered phase, revealing a direct and robust competition between charge and spin order. At TN, the phonon dispersion at q* develops a pronounced Kohn-like anomaly, signaling strong electron-phonon coupling in the vicinity of the magnetic transition. Below TN, exchange striction dramatically renormalizes the associated soft phonon mode by approximately ~6 meV, the largest spin-phonon coupling ever reported. First-principles calculations attribute this behavior to a strong coupling between nearly dispersionless electronic states and a phonon branch that appears unstable at the harmonic level only when no magnetic order is considered, revealing the large sensitivity of the lattice to magnetic symmetry breaking. The competition between charge and spin order parameters, amplified by flat-band physics, drives the observed phonon anomaly and its abrupt reconstruction at TN. With its chemically simple structure and symmetry-protected altermagnetic state, CrSb emerges as a model platform to explore how flat electronic bands mediate giant spin-phonon coupling and competing broken symmetries.

cond-mat.str-el↗

arXiv2025

Unravelling disorder in kagome Yb$_{0.5}$Co$_3$Ge$_3$

The presence of phonon instabilities that leads to the formation of charge modulated states in kagome metals has allowed their classification based on the atomic displacements. Here, we use diffuse and inelastic x-ray scattering, backed by Monte Carlo simulations to describe a type-I instability in the kagome metal Yb$_{0.5}$Co$_3$Ge$_3$. We find that the in-plane distortion of Co in the kagome plane drives a structural transition with the appearance of new Bragg peaks at odd $L$, which are surrounded by a hexagonal diffuse signal. The anisotropic diffuse scattering, characteristic of a highly frustrated triangular lattice was simulated following a combination of Ising hamiltonian and Lennard-Jones potential, and demonstrate that the structural phase transition in Yb$_{0.5}$Co$_3$Ge$_3$ is of an order-disorder transformation type. The inelastic spectra reveals no softening but an anomalous broadening of the $\Gamma-A$ low energy acoustic mode. Our results highlight the critical role of the geometric frustration in promoting ordering from disorder in kagome lattices and power of diffuse scattering to disentangle the internal atomic displacements and correlated disorder.

cond-mat.str-el↗

arXiv2024

Frustrated charge density wave and quasi-long-range bond-orientational order in the magnetic kagome FeGe

The intrinsic frustrated nature of a kagome lattice is amenable to the realization of exotic phases of matter, such as quantum spin liquids or spin ices, and more recently the multiple-$\mathrm{\textbf{q}}$ charge density waves (CDW) in the kagome metals. Despite intense efforts to understand the mechanism driving the electronic modulations, its origin is still unknown and hindered by competing interactions and intertwined orders. Here, we identify a dimerization-driven 2D hexagonal charge-diffuse precursor in the antiferromagnetic kagome metal FeGe and demonstrate that the fraction of dimerized/undimerized states is the relevant order parameter of the multiple-$\mathrm{\textbf{q}}$ CDW of a continuous phase transition. The pretransitional charge fluctuations with propagation vector $\mathrm{\textbf{q}=\textbf{q}_M}$ at T$_{\mathrm{CDW}}$$<$T$<$T$^*$(125 K) are anisotropic, hence holding a quasi-long-range bond-orientational order. The broken translational symmetry emerges from the anisotropic diffuse precursor, akin to the Ising scenario of antiferromagnetic triangular lattices. The temperature and momentum dependence of the critical scattering show parallels to the stacked hexatic $\mathrm{B}$-phases reported in liquid crystals and transient states of CDWs and highlight the key role of the topological defect-mediated melting of the CDW in FeGe.

cond-mat.str-el↗

arXiv2023

Softening of a flat phonon mode in the kagome ScV$_6$Sn$_6$

The long range electronic modulations recently discovered in the geometrically frustrated kagome lattice have opened new avenues to explore the effect of correlations in materials with topological electron flat bands. The observation of the lattice response to the emergent new phases of matter, a soft phonon mode, has remained elusive and the microscopic origin of charge density waves (CDWs) is still unknown. Here, we show, for the first time, a complete melting of the ScV$_ 6$Sn$_ 6$ (166) kagome lattice. The low energy phonon with propagation vector $\frac{1}{3} \frac{1}{3} \frac{1}{2}$ collapses at 98 K, without the emergence of long-range charge order, which sets in with a propagation vector $\frac{1}{3} \frac{1}{3} \frac{1}{3}$. The CDW is driven (but locks at a different vector) by the softening of an overdamped phonon flat plane at k$_z$=$π$. We observe broad phonon anomalies in momentum space, pointing to (1) the existence of approximately flat phonon bands which gain some dispersion due to electron renormalization, and (2) the effects of the momentum dependent electron-phonon interaction in the CDW formation. Ab initio and analytical calculations corroborate the experimental findings to indicate that the weak leading order phonon instability is located at the wave vector $\frac{1}{3} \frac{1}{3} \frac{1}{2}$ of a rather flat collapsed mode. We analytically compute the phonon frequency renormalization from high temperatures to the soft mode, and relate it to a peak in the orbital-resolved susceptibility, obtaining an excellent match with both ab initio and experimental results, and explaining the origin of the approximately flat phonon dispersion. Our data report the first example of the collapse of a softening of a flat phonon plane and promote the 166 compounds of the kagome family as primary candidates to explore correlated flat phonon-topological flat electron physics.

cond-mat.str-el↗

arXiv2019

Common acoustic phonon lifetimes in inorganic and hybrid lead halide perovskites

The acoustic phonons in the organic-inorganic lead halide perovskites have been reported to have anomalously short lifetimes over a large part of the Brillouin zone. The resulting shortened mean free paths of the phonons have been implicated as the origin of the low thermal conductivity. We apply neutron spectroscopy to show that the same acoustic phonon energy linewidth broadening (corresponding to shortened lifetimes) occurs in the fully inorganic CsPbBr$_{3}$ by comparing the results on the organic-inorganic CH$_{3}$NH$_{3}$PbCl$_{3}$. We investigate the critical dynamics near the three zone boundaries of the cubic $Pm\overline{3}m$ Brillouin zone of CsPbBr$_{3}$ and find energy and momentum broadened dynamics at momentum points where the Cs-site ($A$-site) motions contribute to the cross section. Neutron diffraction is used to confirm that both the Cs and Br sites have unusually large thermal displacements with an anisotropy that mirrors the low temperature structural distortions. The presence of an organic molecule is not necessary to disrupt the low-energy acoustic phonons at momentum transfers located away from the zone center in the lead halide perovskites and such damping may be driven by the large displacements or possibly disorder on the $A$ site.

cond-mat.mtrl-sci↗

arXiv2018

Charge redistribution and the Magnetoelastic transition across the first-order magnetic transition in (Mn,Fe)2(P,Si,B)

We used temperature dependent high-resolution x-ray powder diffraction and magnetization measurements to investigate structural, magnetic and electronic degrees of freedom across the ferromagnetic magneto-elastic phase transition in Mn1Fe1P0.6-wSi0.4Bw (w = 0, 0.02, 0.04, 0.06, 0.08). The magnetic transition was gradually tuned from a strong first-order (w = 0) towards a second-order magnetic transition by substituting P by B. Increasing the B content leads to a systematic increase in the magnetic transition temperature and a decrease in thermal hysteresis, which completely vanishes for w = 0.08. Furthermore, the largest changes in lattice parameter across the magnetic transition occur for w = 0, which systematically becomes smaller approaching the samples with w = 0.08. Electron density plots show a strong directional preference of the electronic distribution on the Fe site, which indicates the forming of bonds between Fe atoms and Fe and P/Si in the paramagnetic phase. On the other hand, the Mn-site shows no preferred directions resembling the behaviour of a free electron gas. Due to the low B concentrations (w = 0 - 0.08), distortions of the lattice are limited. However, even small amounts of B strongly disturb the overall topology of the electron density across the unit cell. Samples containing B show a strongly reduced variation in the electron density compared to the parent compound without B.

cond-mat.mtrl-sci↗

arXiv2018

Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet MgCr2O4

In spinels ACr2O4 (A=Mg, Zn) realisation of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et. al (Pow. Diff. 17, 230( 2002)), while other features depend on sample or cooling protocol. Complex partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1=(1/2 1/2 0), k2=(1 0 1/2) appear. The ordered moment reaches 1.94(3) muB/Cr3+ for k1 and 2.08(3) muB/Cr3+ for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1-10] diagonals and a smaller c-component. By inelastic neutron scattering we investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of cluster-like excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

cond-mat.str-el↗

arXiv2017

Complex biphase nature of the superconducting dome of the FeSe phase diagram

Single crystal synchrotron X-ray diffraction as a function of temperature and pressure has revealed a complex biphase mixture in superconducting FeSe. Based on our experimental results we construct a phase diagram where structural behavior and superconducting properties of FeSe are found to be correlated. We show that below 6 GPa, where pressure promotes the superconducting critical temperature, the FeSe structure is composed of 2D layers of edge-shared FeSe4 tetrahedra, while above 6 GPa the superconductivity is strongly suppressed on formation of a new orthorhombic polymorph characterized by a 3D network of face sharing FeSe6 octahedra. Therefore changes in topology and connectivity of the FeSe structure are found to be detrimental for superconductivity to exist. This previously controversial crystal structure of the high pressure polymorph of FeSe was also unambiguously determined. High pressure FeSe adopts an orthorhombic MnP-type structure (Pnma) which corresponds to a slightly distorted hexagonal NiAs-type arrangement (P63/mmc). The structural transformation from the low- to high-pressure FeSe polymorph is first order in nature and is manifested as antiparallel displacements within the Fe and Se sublattices.

cond-mat.supr-con↗

arXiv2017

Reentrant Phase Coherence in Superconducting Nanowire Composites

The short coherence lengths characteristic of low-dimensional superconductors are associated with usefully high critical fields or temperatures. Unfortunately, such materials are often sensitive to disorder and suffer from phase fluctuations in the superconducting order parameter which diverge with temperature $T$, magnetic field $H$ or current $I$. We propose an approach to overcome synthesis and fluctuation problems: building superconductors from inhomogeneous composites of nanofilaments. Macroscopic crystals of quasi-one-dimensional Na$_{2-δ}$Mo$_6$Se$_6$ featuring Na vacancy disorder ($δ\approx$~0.2) are shown to behave as percolative networks of superconducting nanowires. Long range order is established via transverse coupling between individual one-dimensional filaments, yet phase coherence remains unstable to fluctuations and localization in the zero-($T$,$H$,$I$) limit. However, a region of reentrant phase coherence develops upon raising ($T$,$H$,$I$). We attribute this phenomenon to an enhancement of the transverse coupling due to electron delocalization. Our observations of reentrant phase coherence coincide with a peak in the Josephson energy $E_J$ at non-zero ($T$,$H$,$I$), which we estimate using a simple analytical model for a disordered anisotropic superconductor. Na$_{2-δ}$Mo$_6$Se$_6$ is therefore a blueprint for a future generation of nanofilamentary superconductors with inbuilt resilience to phase fluctuations at elevated ($T$,$H$,$I$).

cond-mat.supr-con↗

arXiv2016

Optically switched magnetism in photovoltaic perovskite CH$_3$NH$_3$(Mn:Pb)I$_3$

The demand for ever-increasing density of information storage and speed of manipulation boosts an intense search for new magnetic materials and novel ways of controlling the magnetic bit. Here, we report the synthesis of a ferromagnetic photovoltaic CH$_3$NH$_3$(Mn:Pb)I$_3$ material in which the photo-excited electrons rapidly melt the local magnetic order through the Ruderman-Kittel-Kasuya-Yosida interactions without heating up the spin system. Our finding offers an alternative, very simple and efficient way of optical spin control, and opens an avenue for applications in low power, light controlling magnetic devices.

cond-mat.mtrl-sci↗

arXiv2016

A disorder-enhanced quasi-one-dimensional superconductor

A powerful approach to analysing quantum systems with dimensionality $d>1$ involves adding a weak coupling to an array of one-dimensional (1D) chains. The resultant quasi-one-dimensional (q1D) systems can exhibit long-range order at low temperature, but are heavily influenced by interactions and disorder due to their large anisotropies. Real q1D materials are therefore ideal candidates not only to provoke, test and refine theories of strongly correlated matter, but also to search for unusual emergent electronic phases. Here we report the unprecedented enhancement of a superconducting instability by disorder in single crystals of Na$_{2-δ}$Mo$_6$Se$_6$, a q1D superconductor comprising MoSe chains weakly coupled by Na atoms. We argue that disorder-enhanced Coulomb pair-breaking (which usually destroys superconductivity) may be averted due to a screened long-range Coulomb repulsion intrinsic to disordered q1D materials. Our results illustrate the capability of disorder to tune and induce new correlated electron physics in low-dimensional materials.

cond-mat.supr-con↗

arXiv2015

Crystal structure and phonon softening in Ca3Ir4Sn13

We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide Ca3Ir4Sn13 using neutron and x-ray scattering techniques. For T > T* ~ 38 K the x-ray diffraction data can be satisfactorily refined using the space group Pm-3n. Below T* the crystal structure is modulated with a propagation vector of q = (1/2, 1/2, 0). This may arise from a merohedral twinning in which three tetragonal domains overlap to mimic a higher symmetry, or from a doubling of the cubic unit cell. Neutron diffraction and neutron spectroscopy results show that the structural transition at T* is of a second-order, and that it is well described by mean-field theory. Inelastic neutron scattering data point towards a displacive structural transition at T* arising from the softening of a low-energy phonon mode with an energy gap of Delta(120 K) = 1.05 meV. Using density functional theory the soft phonon mode is identified as a 'breathing' mode of the Sn12 icosahedra and is consistent with the thermal ellipsoids of the Sn2 atoms found by single crystal diffraction data.

cond-mat.str-el↗

arXiv2015

The Dzyaloshinskii-Moriya interaction is under control: an orchestrated flip of the chiral link between structure and magnetism for Fe$_{1-x}$Co$_x$Si

Monosilicides of 3d-metals frequently show a chiral magnetic ordering with the absolute configuration defined by the chirality of the crystal structure and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and magnetic chiralities are probed here for Fe$_{1-x}$Co$_x$Si series and their mutual relationship is found to be dependent on the chemical composition. The chirality of crystal structure was previously shown to be governed by crystal growth, and the value of the DMI is nearly the same for all monosilicides of Fe, Co and Mn. Our findings indicate that the sign of the DMI in Fe$_{1-x}$Co$_x$Si is controlled by the Co composition $x$, thus, opening a route towards controlled design of chiral spintronics devices.

cond-mat.mtrl-sci↗

arXiv2015

Structural disorder versus chiral magnetism in Cr$_{1/3}$NbS$_2$

The crystal structure of a disordered form of Cr$_{1/3}$NbS$_2$ has been characterized using diffraction and inelastic scattering of synchrotron radiation. In contrast to the previously reported symmetry (P6$_3$22), the crystal can be described by a regular twinning of an average P6$_3$ structure with three disordered positions of the Cr ions. Short-range correlations of the occupational disorder result in a quite intense and structured diffuse scattering; a static nature of the disorder was unambiguously attributed by the inelastic x-ray scattering. The diffuse scattering has been modeled using a reverse Monte-Carlo algorithm assuming a disorder of the Cr sub-lattice only. The observed correlated disorder of the Cr sub-lattice reduces the temperature of the magnetic ordering from 130 K to 88 K and drastically modifies the field dependence of the magnetization as it is evidenced by the SQUID magnetometery. We conclude, that in contrast to the helicoidal spin structure assumed for P6$_3$22 form, the compound under study is ferromagnetically ordered with a pronounced in-plane anisotropy.

cond-mat.mtrl-sci↗

arXiv2014

Lattice dynamics and antiferroelectricity in PbZrO$_3$ tested by X-ray and Brillouin light scattering

We report the results of comprehensive study of the critical dynamics of the prototype perovskite antiferroelectric PbZrO$_3$. The combination of inelastic X-ray and diffuse X-ray scattering techniques and Brillouin light scattering was used. It is found that the dispersion of the TA phonons is strongly anisotropic. The dispersion curve of the in-plane polarized TA phonons propagating in [1 1 0] direction demonstrates pronounced softening. Slowing down of the excitations at R-point is found, it is manifested in growing of the central peak. This slowing down is too weak to be considered as a primary origin of the corresponding order parameter. Obtained results are treated in terms of TA--TO flexoelectric mode coupling. It is demonstrated that the structural phase transformation in PbZrO$_3$ can be considered as the result of the only intrinsic instability associated with the ferroelectric soft mode.

cond-mat.mtrl-sci↗

arXiv2014

Compressibility and pressure-induced disorder in superconducting phase-separated Cs0.72Fe1.57Se2

Pressure-dependent diffraction response of the superconducting phase separated Cs0.72Fe1.57Se2 has been studied using synchrotron radiation up to the pressure of 19 GPa. The main and secondary phases of Cs0.72Fe1.57Se2 have been observed in the whole pressure range. The main ordered phase has been found to undergo an order-disorder transition in the Fe-sublattice at least at P = 11 GPa with the corresponding kinetics on the order of hours. Contrary to the analogous temperature induced transition, the secondary phase has not been suppressed suggesting that its stability pressure range is higher than 19 GPa or the corresponding transformation kinetics is slower at room temperature. Together with the previously reported pressure-dependent resistivity and magnetic susceptibility measurements, this work indicates that superconductivity in the AxFe2-ySe2 (A - alkali metals) phases could be related to the Fe-vacancy ordering in the main phase.

cond-mat.supr-con↗

arXiv2013

Crystal Structure of BaFe2Se3 as a Function of Temperature and Pressure: Phase Transition Phenomena and High-Order Expansion of Landau Potential

BaFe2Se3 (Pnma, CsAg2I3-type structure), recently assumed to show superconductivity at ~ 11 K, exhibits a pressure-dependent structural transition to the CsCu2Cl3-type structure (Cmcm space group) around 60 kbar, as evidenced from pressure-dependent synchrotron powder diffraction data. Temperature-dependent synchrotron powder diffraction data indicate an evolution of the room-temperature BaFe2Se3 structure towards a high symmetry CsCu2Cl3 form upon heating. Around 425 K BaFe2Se3 undergoes a reversible, first order isostructural transition, that is supported by the differential scanning calorimetry data. The temperature-dependent structural changes occur in two stages, as determined by the alignment of the FeSe4 tetrahedra and corresponding adjustments of the positions of Ba atoms. On further heating, a second order phase transformation into the Cmcm structure is observed at 660 K. A rather unusual combination of isostructural and second-order phase transformations is parameterized within phenomenological theory assuming high-order expansion of Landau potential. A generic phase diagram mapping observed structures is proposed on the basis of the parameterization.

cond-mat.supr-con↗