arXiv subjects

A. L. Chernyshev

Publications and source records attributed to A. L. Chernyshev.

At least 37 records · Page 2Linked to original sources

arXiv2020

Rethinking $α$-RuCl$_3$

We argue that several empirical constraints strongly restrict parameters of the effective microscopic spin model describing $α$-RuCl$_3$. In particular, such constraints dictate a substantial \emph{positive} off-diagonal anisotropic coupling, $Γ^\prime\!>\!0$, not anticipated previously. The renormalization by quantum fluctuations allows to reconcile larger values of the advocated bare parameters with their earlier assessments and provides a consistent description of the field evolution of spin excitations in the paramagnetic phase. We assert that large anisotropic terms inevitably result in strong anharmonic coupling of magnons, necessarily leading to broad features in their spectra due to decays, in accord with the observations in $α$-RuCl$_3$. Using duality transformations, we explain the origin of the pseudo-Goldstone mode that is ubiquitous to the studied parameter space and is present in $α$-RuCl$_3$. Our analysis offers a description of $α$-RuCl$_3$ as an easy-plane ferromagnet with antiferromagnetic further-neighbor and strong off-diagonal couplings, which is in a fluctuating zigzag ground state proximate to an incommensurate phase that is continuously connected to a ferromagnetic one.

cond-mat.str-el↗

arXiv2020

Magnon damping in the zigzag phase of the Kitaev-Heisenberg-$Γ$ model on a honeycomb lattice

We calculate magnon dispersions and damping in the Kitaev-Heisenberg model with an off-diagonal exchange $Γ$ and isotropic third-nearest-neighbor interaction $J_3$ on a honeycomb lattice. This model is relevant to a description of the magnetic properties of iridium oxides $α$-Li$_2$IrO$_3$ and Na$_2$IrO$_3$, and Ru-based materials such as $α$-RuCl$_3$. We use an unconventional parametrization of the spin-wave expansion, in which each Holstein-Primakoff boson is represented by two conjugate hermitian operators. This approach gives us an advantage over the conventional one in identifying parameter regimes where calculations can be performed analytically. Focusing on the parameter regime with the zigzag spin pattern in the ground state that is consistent with experiments, we demonstrate that one such region is $Γ= K>0$, where $K$ is the Kitaev coupling. Within our approach we are able to obtain explicit analytical expressions for magnon energies and eigenstates and go beyond the standard linear spin-wave theory approximation by calculating magnon damping and demonstrating its role in the dynamical structure factor. We show that the magnon damping effects in both Born and self-consistent approximations are very significant, underscoring the importance of non-linear magnon coupling in interpreting broad features in the neutron-scattering spectra.

cond-mat.str-el↗

arXiv2018

Topography of Spin Liquids on a Triangular Lattice

Spin systems with frustrated anisotropic interactions are of significant interest due to possible exotic ground states. We have explored their phase diagram on a nearest-neighbor triangular lattice using the density-matrix renormalization group and mapped out the topography of the region that can harbor a spin liquid. We find that this spin-liquid phase is continuously connected to a previously discovered spin-liquid phase of the isotropic $J_1\!-\!J_2$ model. The two limits show nearly identical spin correlations, making the case that their respective spin liquids are isomorphic to each other.

cond-mat.str-el↗

arXiv2017

Disorder-Induced Mimicry of a Spin Liquid in YbMgGaO$_4$

We suggest that a randomization of the pseudo-dipolar interaction in the spin-orbit-generated low-energy Hamiltonian of YbMgGaO$_4$ due to an inhomogeneous charge environment from a natural mixing of Mg$^{2+}$ and Ga$^{3+}$ can give rise to orientational spin disorder and mimic a spin-liquid-like state. In the absence of such quenched disorder, $1/S$ and density matrix renormalization group calculations both show robust ordered states for the physically relevant phases of the model. Our scenario is consistent with the available experimental data and further experiments are proposed to support it.

cond-mat.str-el↗

arXiv2017

Field induced spontaneous quasiparticle decay and renormalization of quasiparticle dispersion in a quantum antiferromagnet

The notion of a quasiparticle, such as a phonon, a roton, or a magnon, is used in modern condensed matter physics to describe an elementary collective excitation. The intrinsic zero-temperature magnon damping in quantum spin systems can be driven by the interaction of the one-magnon states and multi-magnon continuum. However, detailed experimental studies on this quantum many-body effect induced by an applied magnetic field are rare. Here we present a high-resolution neutron scattering study in high fields on an S=1/2 antiferromagnet C9H18N2CuBr4. Compared with the non-interacting linear spin-wave theory, our results demonstrate a variety of phenomena including field-induced renormalization of one-magnon dispersion, spontaneous magnon decay observed via intrinsic linewidth broadening, unusual non-Lorentzian two-peak structure in the excitation spectra, and a dramatic shift of spectral weight from one-magnon state to the two-magnon continuum.

cond-mat.mtrl-sci↗

arXiv2016

Damped Topological Magnons in the Kagomé-Lattice Ferromagnets

We demonstrate that interactions can substantially undermine the free-particle description of magnons in ferromagnets on geometrically frustrated lattices. The anharmonic coupling, facilitated by the Dzyaloshinskii-Moriya interaction, and a highly-degenerate two-magnon continuum yield a strong, non-perturbative damping of the high-energy magnon modes. We provide a detailed account of the effect for the $S=1/2$ ferromagnet on the kagomé lattice and propose further experiments.

cond-mat.str-el↗

arXiv2016

Field-induced decays in $XXZ$ triangular-lattice antiferromagnets

We investigate field-induced transformations in the dynamical response of the $XXZ$ model on the triangular lattice that are associated with the anharmonic magnon coupling and decay phenomena. A set of concrete theoretical predictions is made for a close physical realization of the spin-1/2 $XXZ$ model, Ba$_3$CoSb$_2$O$_9$. We demonstrate that dramatic modifications in magnon spectrum must occur in low out-of-plane fields that are easily achievable for this material. The hallmark of the effect is a coexistence of the clearly distinct well-defined magnon excitations with significantly broadened ones in different regions of the ${\bf k}-ω$ space. The field-induced decays are generic for this class of models and become more prominent at larger anisotropies and in higher fields.

cond-mat.str-el↗

arXiv2016

Spontaneous decays of magneto-elastic excitations in noncollinear antiferromagnet (Y,Lu)MnO3

When magnons and phonons, the fundamental quasiparticles of the solid, are coupled to one another, they form a new hybrid quasi-particle, leading to novel phenomena and interesting applications. Despite its wide-ranging importance, however, detailed experimental studies on the underlying Hamiltonian is rare for actual materials. Moreover, the anharmonicity of such magnetoelastic excitations remains largely unexplored although it is essential for a proper understanding of their diverse thermodynamic behaviour as well as intrinsic zero-temperature decay. Here we show that in noncollinear antiferromagnets, a strong magnon-phonon coupling can significantly enhance the anharmonicity, resulting in the creation of magnetoelastic excitations and their spontaneous decay. By measuring the spin waves over the full Brillouin zone and carrying out anharmonic spin wave calculations using a Hamiltonian with an explicit magnon-phonon coupling, we have identified a hybrid magnetoelastic mode in (Y,Lu)MnO3 and quantified its decay rate and the exchange-striction coupling term required to produce it. Our work has wide implications for understanding of the spin-phonon coupling and the resulting excitations of the broad classes of noncollinear magnets.

cond-mat.mtrl-sci↗

arXiv2016

Dynamical structure factor of triangular-lattice antiferromagnet

We elucidate the role of magnon interaction and spontaneous decays in the spin dynamics of the triangular-lattice Heisenberg antiferromagnet by calculating its dynamical structure factor within the spin-wave theory. Explicit theoretical results for neutron-scattering intensity are provided for spins S=1/2 and S=3/2. The dynamical structure factor exhibits unconventional features such as quasiparticle peaks broadened by decays, non-Lorentzian lineshapes, and significant spectral weight redistribution to the two-magnon continuum. This rich excitation spectrum illustrates the complexity of the triangular-lattice antiferromagnet and provides distinctive qualitative and quantitative fingerprints for experimental observation of decay-induced magnon dynamics.

cond-mat.str-el↗

arXiv2016

Heat Transport in Spin Chains with Weak Spin-Phonon Coupling

The heat transport in a system of $S=1/2$ large-$J$ Heisenberg spin chains, describing closely Sr$_2$CuO$_3$ and SrCuO$_2$ cuprates, is studied theoretically at $T\ll J$ by considering interactions of the bosonized spin excitations with optical phonons and defects. Treating rigorously the multi-boson processes, we derive a microscopic spin-phonon scattering rate that adheres to an intuitive picture of phonons acting as thermally populated defects for the fast spin excitations. The mean-free path of the latter exhibits a distinctive $T$-dependence reflecting a critical nature of spin chains and gives a close description of experiments. By the naturalness criterion of realistically small spin-phonon interaction, our approach stands out from previous considerations that require large coupling constants to explain the data and thus imply a spin-Peierls transition, absent in real materials.

cond-mat.str-el↗

arXiv2015

Field-induced dynamical properties of the $XXZ$ model on a honeycomb lattice

We present a comprehensive $1/S$ study of the field-induced dynamical properties of the nearest-neighbor $XXZ$ antiferromagnet on a honeycomb lattice using the formalism of the nonlinear spin-wave theory developed for this model. The external magnetic field controls spin frustration in the system and induces non-collinearity of the spin structure, which is essential for the two-magnon decay processes. Our results include an intriguing field-evolution of the regions of the Brillouin zone wherein decays of spin excitations are prominent, a detailed classification of the decay channels involving magnons from both excitation branches, and a thorough analysis of the singularities in the magnon spectra due to coupling to the two magnon continuum, all of which are illustrated for several field and anisotropy values. We highlight a number of features related to either the non-Bravais nature of the lattice, or the existence of the Dirac-like points in the spectrum. In addition, the asymptotic behavior of the decay rates near high-symmetry points is analyzed in detail. The inelastic neutron-scattering spin-spin structure factor is obtained in the leading $1/S$ order and is shown to exhibit qualitatively distinct fingerprints of the decay-induced magnon dynamics such as quasiparticle peaks broadened by decays and strong spectral weight redistribution.

cond-mat.str-el↗

arXiv2015

Order and excitations in large-$S$ kagomé-lattice antiferromagnets

We systematically investigate the ground-state and the spectral properties of antiferromagnets on a kagomé lattice with several common types of the planar anisotropy: $XXZ$, single-ion, and out-of-plane Dzyaloshinskii-Moriya. Our main focus is on the role of nonlinear, anharmonic terms, which are responsible for the quantum order-by-disorder effect and for the corresponding selection of the ground-state spin structure in many of these models. The $XXZ$ and the single-ion anisotropy models exhibit a quantum phase transition between the ${\bf q}\!=\!0$ and the $\sqrt{3}\times\!\sqrt{3}$ states as a function of the anisotropy parameter, offering a rare example of the quantum order-by-disorder fluctuations favoring a ground state which is different from the one selected by thermal fluctuations. The nonlinear terms are also shown to be crucial for a very strong near-resonant decay phenomenon leading to the quasiparticle breakdown in the kagomé-lattice antiferromagnets whose spectra are featuring flat or weakly dispersive modes. The effect is shown to persist even in the limit of large spin values and should be common to other frustrated magnets with flat branches of excitations. Model calculations of the spectrum of the $S=5/2$ Fe-jarosite with Dzyaloshinskii-Moriya anisotropy provide a convincing and detailed characterization of the proposed scenario.

cond-mat.str-el↗

arXiv2015

Strong quantum effects in an almost classical antiferromagnet on a kagome lattice

Two ubiquitous features of frustrated spin systems stand out: massive degeneracy of their ground states and flat, or dispersionless, excitation branches. In real materials, the former is frequently lifted by secondary interactions or quantum fluctuations, in favor of an ordered or spin-liquid state, but the latter often survive. We demonstrate that flat modes may precipitate remarkably strong quantum effects even in the systems that are otherwise written off as almost entirely classical. The resultant spectral features should be reminiscent of the quasiparticle breakdown in quantum systems, only here the effect is strongly amplified by the flatness of spin-excitation branches, leading to the damping that is not vanishingly small even at $S\!\gg\!1$. We provide a theoretical analysis of excitation spectrum of the $S=5/2$ iron-jarosite to illustrate our findings and to suggest further studies of this and other frustrated spin systems.

cond-mat.str-el↗

arXiv2015

Thermal conductivity in large-$J$ two-dimensional antiferromagnets: Role of phonon scattering

Motivated by the recent heat transport experiments in 2D antiferromagnets, such as La$_2$CuO$_4$, where the exchange coupling $J$ is larger than the Debye energy $Θ_{\rm D}$, we discuss different types of relaxation processes for magnon heat current with a particular focus on coupling to 3D phonons. We study thermal conductivity by these in-plane magnetic excitations using two distinct techniques, Boltzmann formalism within the relaxation-time approximation and memory-function approach. Within these approaches, a close consideration is given to the scattering of magnons by both acoustic and optical branches of phonons. A remarkable accord between the two methods with regards to the asymptotic behavior of the effective relaxation rates is demonstrated. Additional scattering mechanisms, due to grain boundaries, impurities, and finite correlation length in the paramagnetic phase, are discussed and included in the calculations of the thermal conductivity $κ(T)$. Again, we demonstrate a close similarity of the results from the two techniques of calculating $κ(T)$. Our complementary approach strongly suggests that scattering from optical or zone-boundary phonons is important for magnon heat current relaxation in a high temperature window of $Θ_D\lesssim T \ll J$.

cond-mat.str-el↗

arXiv2015

Spin-waves in triangular lattice antiferromagnet: decays, spectrum renormalization, and singularities

We present a comprehensive study of the dynamical properties of the quantum Heisenberg antiferromagnet on a triangular lattice within the framework of spin-wave theory. The distinct features of spin-wave excitations in the triangular-lattice antiferromagnet are (i) finite lifetime at zero temperature due to spontaneous two-magnon decays, (ii) strong renormalization of magnon energies with respect to the harmonic result, and (iii) logarithmic singularities in the decay rate Gamma_k. Quantum corrections to the magnon spectrum are obtained using both the on-shell and off-shell solutions of the Dyson equation with the lowest-order magnon self-energy. At low-energies magnon excitations remain well-defined albeit with the anomalous decay rate Gamma_k ~ k^2 at k->0 and Gamma_k ~ |k-Q_AF|^7/2 at k->Q_AF. At high energies, magnons are heavily damped with the decay rate reaching (2 Gamma_k/E_k) ~ 0.3 for the case S=1/2. The on-shell solution shows logarithmic singularities in Gamma_k with the concomitant jump-like discontinuities in E_k along certain contours in the momentum space. Such singularities are even more prominent in the magnon spectral function A(k,w). Although the off-shell solution removes such log-singularities, the decay rates remain strongly enhanced. We also discuss the role of higher-order corrections and show that such singularities may lead to complete disappearance of the spectrum in the vicinity of certain k-points. The kinematic conditions for two-magnon decays are analyzed for various generalizations of the triangular-lattice antiferromagnet as well as for the XXZ model on a kagome lattice. Our results suggest that decays and singularities in the spin-wave spectra must be ubiquitous in all these systems.

cond-mat.str-el↗

arXiv2014

Quantum Selection of Order in an $XXZ$ Antiferromagnet on a Kagomé Lattice

Selection of the ground state of the kagomé-lattice $XXZ$ antiferromagnet by quantum fluctuations is investigated by combining non-linear spin-wave and real-space perturbation theories. The two methods unanimously favor ${\bf q}=0$ over $\sqrt{3}\times\sqrt{3}$ magnetic order in a wide range of the anisotropy parameter $0\leq Δ\leq 0.72$. Both approaches are also in an accord on the magnitude of the quantum order-by-disorder effect generated by topologically non-trivial, loop-like spin-flip processes. A tentative $S-Δ$ phase diagram of the model is proposed.

cond-mat.str-el↗

arXiv2013

Spin pseudogap in Ni-doped SrCuO2

The S=1/2 spin chain material SrCuO2 doped with 1% S=1 Ni-impurities is studied by inelastic neutron scattering. At low temperatures, the spectrum shows a pseudogap Δ~ 8 meV, absent in the parent compound, and not related to any structural phase transition. The pseudogap is shown to be a generic feature of quantum spin chains with dilute defects. A simple model based on this idea quantitatively accounts for the exprimental data measured in the temperature range 2-300 K, and allows to represent the momentum-integrated dynamic structure factor in a universal scaling form.

cond-mat.str-el↗

arXiv2013

Impurity-induced frustration: low-energy model of diluted oxides

We provide a detailed derivation of the low-energy model for Zn-diluted La2CuO4 in the limit of low doping together with a study of the ground-state properties of that model. We consider Zn-doped La2CuO4 within a framework of the three-band Hubbard model, which closely describes high-Tc cuprates on the energy scale of the most relevant atomic orbitals. Qualitatively, we find that the hybridization of zinc and oxygen orbitals can result in an impurity state with the energy \varepsilon, which is lower than the effective Hubbard gap U. The low-energy, spin-only Hamiltonian includes terms of the order t^2/U and t^4/\varepsilon^3. That is, besides the usual nearest-neighbor superexchange J-terms of order t^2/U, the low-energy model contains impurity-mediated, further-neighbor frustrating interactions among the Cu spins surrounding Zn-sites in an otherwise unfrustrated antiferromagnetic background. These terms can be substantial when \varepsilon ~ U/2, the latter value corresponding to the realistic CuO2 parameters. In order to verify this spin-only model, we subsequently apply the T-matrix approach to study the effect of impurities on the antiferromagnetic order parameter. Previous theoretical studies, which include only the dilution effect of impurities, show a large discrepancy with experimental data in the doping dependence of the staggered magnetization at low doping. We demonstrate that this discrepancy is eliminated by including impurity-induced frustrations into the effective spin model with realistic CuO2 parameters. Recent experimental study shows a significantly stronger suppression of spin stiffness in the case of Zn-doped La2CuO4 compared to the Mg-doped case and thus gives a strong support to our theory. We argue that the proposed impurity-induced frustrations should be important in other strongly correlated oxides and charge-transfer insulators.

cond-mat.str-el↗