arXiv subjects

M. I. Katsnelson

Publications and source records attributed to M. I. Katsnelson.

At least 19 recordsLinked to original sources

arXiv2026

Suppression of local magnetic moment formation and paramagnetic exchange interactions in monolayer Fe$_3$GeTe$_2$

We study the electronic and magnetic properties of monolayer Fe$_3$GeTe$_2$ within the DFT+DMFT approach in the paramagnetic phase. We argue that this compound is sufficiently far from the local magnetic moment limit, demonstrating non-linear temperature dependencies of the partial inverse local and uniform magnetic susceptibilities in a broad temperature range. We find that in the regime of moderate Coulomb interactions ($U=3-4$ eV), the iron atoms located above and below the Ge plane carry a substantial local magnetic moment ($μ\gtrsim 4.5 μ_B$), while the iron atom located within the Ge plane does not exhibit any pronounced magnetic moment. At the same time, the RKKY-type exchange interactions between these two symmetry-nonequivalent types of atoms turn out to be crucial for stabilizing long-range ferromagnetic order in Fe$_3$GeTe$_2$. The estimated spin-wave stiffness and Curie temperature are in good agreement with the experimental data, indicating that a dynamical treatment of electron correlations in Fe$_3$GeTe$_2$ is essential to properly describe its partially itinerant magnetic behavior.

cond-mat.str-el↗

arXiv2025

Spin-lattice couplings in $3d$ ferromagnets: analysis from first-principles

Magnetoelasticity plays a crucial role in numerous magnetic phenomena, including magnetocalorics, magnon excitation via acoustic waves, and ultrafast demagnetization/Einstein-de Haas effect. Despite a long-standing discussion on anisotropy-mediated magnetoelastic interactions of relativistic origin, the exchange-mediated magnetoelastic parameters within an atomistic framework have only recently begun to be investigated. As a result, many of their behaviors and values for real materials remain poorly understood. Therefore, by using a proposed simple modification of the embedded cluster approach that reduces the computational complexity, we critically analyze the properties of exchange-mediated spin-lattice coupling parameters for elemental $3d$ ferromagnets (bcc Fe, fcc Ni, and fcc Co), comparing methods used for their extraction and relating their realistic values to symmetry considerations and orbitally-decomposed contributions. Additionally, we investigate the effects of noncollinearity (spin temperature) and applied pressure on these parameters. For Fe, we find that single-site rotations, associated with spin temperatures around $\sim100$ K, induce significant modifications, particularly in Dzyaloshinskii-Moriya-type couplings; in contrast, such interactions in Co and Ni remain almost configuration independent. Moreover, we demonstrate a notable change in the exchange-mediated magnetoelastic constants for Fe under isotropic contraction. Finally, the conversion between atomistic, quantum-mechanically derived parameters and the phenomenological magnetoelastic theory is discussed, which can be an useful tool towards larger and more realistic dynamics simulations involving coupled subsystems.

cond-mat.mtrl-sci↗

arXiv2025

Plasmonic waveguides in two-dimensional materials: a quantum mechanical description using semiclassical techniques

Plasmons are likely to play an important role in integrated photonic ciruits, because they strongly interact with light and can be confined to subwavelength scales. These plasmons can be guided and controlled by plasmonic waveguides, which can be created by patterning different materials or by structuring the dielectric environment. We have constructed a semi-analytical theory to describe plasmonic waveguides, and, more generally, plasmons in spatially inhomogeneous systems. Our theory employs techniques from semiclassical analysis, and is therefore applicable when the electron wavelength is much smaller than the characteristic length scale of changes in the system parameters. We obtain an effective classical Hamiltonian that describes the dynamics of quantum plasmons, given by the Lindhard function with spatially varying parameters. Adding the wave-like character of the plasmons to the classical trajectories generated by this Hamiltonian, we find two different mechanisms for waveguiding. In the first case, a localized plasmonic state arises due to total internal reflection similar to photonic waveguides. The second mechanism relies on a varying dielectric environment, which locally modifies the screening of the electrons. Here, a quasi-localized state arises due to local changes in the amplitude of the plasmonic excitation. Our results provide a solid basis to understand previous numerical studies.

cond-mat.mes-hall↗

arXiv2025

Superconductivity of Bad Fermions: Origin of Two Gaps in HTSC Cuprates

We investigate the spectral properties of the doped ${t-t'}$ Hubbard model with parameters typical for high-temperature cuprate superconductors. Our approach is based on a novel strong-coupling Green's function expansion around a reference system -- the exactly solvable undoped particle-hole symmetric Hubbard lattice -- that possesses a large antiferromagnetic Mott-Hubbard-Slater gap in the electron spectrum. The electron spectral function in the case of a large next-nearest-neighbor hopping ${t'=-0.3t}$, which is characteristic of the ${T_c \approx 100\,\text{K}}$ family of cuprates, reveals a strongly renormalized flat band feature with a pseudogap around the antinodal point. The superconducting response of this system to a small ${d_{x^2-y^2}}$-like external field exhibits a very unusual form. It features a pseudogap at the antinodal point in the normal part of the Nambu Green's function, related to a ``bad-fermion'' behavior in a normal phase, as well as a ${d}$-wave-like structure in the anomalous (Gorkov's) Green's function, with zero response at the nodal point of the Brillouin zone. Remarkably, we find that the anomalous part of the response deviates essentially from the simplest ${(\cos{k_x}-\cos{k_y})}$ form in momentum space. Specifically, its extrema are shifted away from the ${(π,0)}$ and ${(0,π)}$ points due to suppression of the response by the pseudogap. The observed two-gap structure of the electron spectra in a generic strong-coupling model of cuprates can serve as a basis for phenomenological treatment of different physical properties of high-temperature superconductors within two-fluid model.

cond-mat.str-el↗

arXiv2025

Chiral Spin and Orbital Angular Momentum Textures in Mn Chains on W(110): Interplay of Spin-Orbit Coupling and Crystal Field Effects

Stabilization of unusual spin-orbit driven magnetic orderings are achieved for chains of Mn atoms deposited on a W(110) substrate. First-principles electronic structure calculations show that the ground state spin configuration is non-collinear, forming chiral spiral-like structures, driven by competing nearest and next-nearest neighbor interactions. The orbital magnetic moments are also found to exhibit non-collinear ordering that, interestingly, tend to align in-plane for some systems with an orientation distinctly differently from that of the spin moment. We analyse the mechanism behind such behaviour, and find that it is due to the competition between the spin-orbit interaction and crystal-field splitting. Model calculations based on this assumption reproduce the main findings observed in our first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2024

Dynamic heterogeneity in the self-induced spin glass state of elemental neodymium

Spin glasses are magnetic materials exhibiting numerous magnetization patterns, that randomly vary both in real space and in time. To date, it is still not well understood what the nature of these spatiotemporal dynamics is, namely if they are completely random or if there are links between given time and length scales. Here we show the ubiquitous behavior of dynamic heterogeneity in the self-induced spin glass state of elemental neodymium. We used spin-polarized scanning tunneling microscopy in combination with atomistic spin dynamics simulations to image the locally ordered magnetic patterns in the glass state, and tracked the induced spatiotemporal dynamics in response to external perturbations. We observed that the real space magnetization exhibited a coexistence of slow and fast dynamics reminiscent of dynamic heterogeneity in structural glasses. Furthermore, we found that zero-field cooling imprints a specific set of metastable periodicities into the spin glass, which evolved during aging and could be thermally reinitialized. These results demonstrate the importance of local length scales for the understanding of aging dynamics in spin glasses and provide a link to the more general picture of true glasses.

cond-mat.mtrl-sci↗

arXiv2024

Semi-Dirac Fermions in a Topological Metal

Topological semimetals with massless Dirac and Weyl fermions represent the forefront of quantum materials research. In two dimensions (2D), a peculiar class of fermions that are massless in one direction and massive in the perpendicular direction was predicted sixteen years ago. These highly exotic quasiparticles - the semi-Dirac fermions - ignited intense theoretical and experimental interest but remain undetected. Using magneto-optical spectroscopy, we demonstrate the defining feature of semi-Dirac fermions - $B^{2/3}$ scaling of Landau levels - in a prototypical nodal-line metal ZrSiS. In topological metals, including ZrSiS, nodal-lines extend the band degeneracies from isolated points to lines, loops or even chains in the momentum space. With $\textit{ab initio}$ calculations and theoretical modeling, we pinpoint the observed semi-Dirac spectrum to the crossing points of nodal-lines in ZrSiS. Crossing nodal-lines exhibit a continuum absorption spectrum but with singularities that scale as $B^{2/3}$ at the crossing. Our work sheds light on the hidden quasiparticles emerging from the intricate topology of crossing nodal-lines and highlights the potential to explore quantum geometry with linear optical responses.

cond-mat.mtrl-sci↗

arXiv2024

Two-level atom witness of thermalization of multimode optical fibers

In the present project, we study the dynamics of the two-level system coupled with the multimode optical system. In particular, we considered a square lattice of optical fibers. We aimed to answer whether we can infer information about the thermalization of optical modes through the thermalization of two-level atoms. After averaging over the set of modes, the dynamic of the two-level system is free of quantum revivals, and that is the signature of thermalization. We showed that the temperature of the two-level system increases with the temperature of optical modes and mean photon number. In the high-temperature limit of optical modes, the temperature of the level system tends to be infinity, and level populations are equal.

physics.optics↗

arXiv2024

Electronic correlations arising from anti-Stoner spin excitations: an ab initio study of itinerant ferro- and antiferromagnet

The anti-Stoner excitations are a spin-flips in which, effectively, an electron is promoted from a minority to a majority spin state, i.e., complementary to Stoner excitations and spin-waves. Since their spectral power is negligible in strong itinerant ferromagnets and they are identically absent in the ferromagnetic Heisenberg model, their properties and role in correlating electrons were hardly investigated so far. On the other hand, they are present in weak ferromagnets, fcc Ni being a prominent example, and both types of spin-flips (down-to-up and up-to-down) must be treated on the equal footing in systems with the degenerate spin up and down bands, in particular antiferromagnets in which case we choose CrSb as a model system. For these two materials we evaluate the strength of the effective interaction between the quasiparticles and the gas of virtual spin-flip excitations. To this end, we compute the corresponding self-energy taking advantage of our novel efficient \textit{ab initio} numerical scheme. We find that in Ni the band-structure renormalization due to the anti-Stoner processes is weaker than the one due to Stoner-type magnons in the majority spin channel but the two become comparable in the minority one. The effect can be traced back primarily to the spectral strength of the respective spin excitations and the densities of the final available quasiparticle states in the scattering process. For the antiferromagnet, the situation is more complex and we observe that the electron-magnon interaction is sensitive not only to these densities of states but critically to the spatial shapes of the coupling magnonic modes as well.

cond-mat.mtrl-sci↗

arXiv2023

Plasmonic skyrmion quantum thermodynamics

The primary obstacle in the field of quantum thermodynamics revolves around the development and practical implementation of quantum heat engines operating at the nanoscale. One of the key challenges associated with quantum working bodies is the occurrence of "quantum friction," which refers to irreversible wasted work resulting from quantum inter-level transitions. Consequently, the construction of a reversible quantum cycle necessitates the utilization of adiabatic shortcuts. However, the experimental realization of such shortcuts for realistic quantum substances is exceedingly complex and often unattainable. In this study, we propose a quantum heat engine that capitalizes on the plasmonic skyrmion lattice. Through rigorous analysis, we demonstrate that the quantum skyrmion substance, owing to its topological protection, exhibits zero irreversible work. Consequently, our engine operates without the need for adiabatic shortcuts. We checked by numerical calculations and observed that when the system is in the quantum skyrmion phase, the propagated states differ from the initial states only by the geometricl and dynamical phases. The adiabacit evoluation leads to the zero transition matrix elements and zero irreversible work. By employing plasmonic mods and an electric field, we drive the quantum cycle. The fundamental building blocks for constructing the quantum working body are individual skyrmions within the plasmonic lattice. As a result, one can precisely control the output power of the engine and the thermodynamic work accomplished by manipulating the number of quantum skyrmions present.

quant-ph↗

arXiv2023

Topological entanglement entropy to identify topological order in quantum skyrmions

We study the topological entanglement entropy and scalar chirality of a topologically ordered skyrmion formed in a two-dimensional triangular lattice. Scalar chirality remains a smooth function of the magnetic field in both helical and quantum skyrmion phases. In contrast, topological entanglement entropy remains almost constant in the quantum skyrmion phase, whereas it experiences enhanced fluctuations in the helical phase. Therefore, topological entanglement entropy is an effective tool to distinguish between the two phases and pinpoint the quantum phase transition in the system.

cond-mat.str-el↗

arXiv2023

Semiclassical theory for plasmons in two-dimensional inhomogeneous media

The progress in two-dimensional materials has led to rapid experimental developments in quantum plasmonics, where light is manipulated using plasmons. Although numerical methods can be used to quantitatively describe plasmons in spatially inhomogeneous systems, they are limited to relatively small setups. Here, we present a novel semi-analytical method to describe plasmons in two-dimensional inhomogeneous media within the framework of the Random Phase Approximation (RPA). Our approach is based on the semiclassical approximation, which is formally applicable when the length scale of the inhomogeneity is much larger than the plasmon wavelength. We obtain an effective classical Hamiltonian for quantum plasmons by first separating the in-plane and out-of-plane degrees of freedom and subsequently employing the semiclassical Ansatz for the electrostatic plasmon potential. We illustrate this general theory by considering scattering of plasmons by radially symmetric inhomogeneities. We derive a semiclassical expression for the differential scattering cross section and compute its numerical values for a specific model of the inhomogeneity.

cond-mat.mes-hall↗

arXiv2023

Observation of Kekulé vortices induced in graphene by hydrogen adatoms

Fractional charges are one of the wonders of the fractional quantum Hall effect, a liquid of strongly correlated electrons in a large magnetic field. Fractional excitations are also anticipated in two-dimensional crystals of non-interacting electrons under time-reversal symmetry, as bound states of a rotating bond order known as Kekulé vortex. However, the physical mechanisms inducing such topological defects remain elusive, preventing experimental realisations. Here, we report the observation of Kekulé vortices in the local density of states of graphene under time-reversal symmetry. The vortices result from intervalley scattering on chemisorbed hydrogen adatoms and have a purely electronic origin. Their 2π winding is reminiscent of the Berry phase π of the massless Dirac electrons. Remarkably, we observe that point scatterers with different symmetries such as divacancies can also induce a Kekulé bond order without vortex. Therefore, our local-probe study further confirms point defects as versatile building blocks for the control of graphene's electronic structure by kekulé order.

cond-mat.mes-hall↗

arXiv2023

Quantum simulator to emulate lower dimensional molecular structure

Bottom-up quantum simulators have been developed to quantify the role of various interactions, dimensionality, and structure in creating electronic states of matter. Here, we demonstrated a solid-state quantum simulator emulating molecular orbitals, based solely on positioning individual cesium atoms on an indium antimonide surface. Using scanning tunneling microscopy and spectroscopy, combined with ab initio calculations, we showed that artificial atoms could be made from localized states created from patterned cesium rings. These artificial atoms served as building blocks to realize artificial molecular structures with different orbital symmetries. These corresponding molecular orbitals allowed us to simulate 2D structures reminiscent of well known organic molecules. This platform could further be used to monitor the interplay between atomic structures and the resulting molecular orbital landscape with sub-molecular precision.

cond-mat.mes-hall↗

arXiv2023

Exchange interactions in iron and nickel: DFT+DMFT study in paramagnetic phase

We analyze possible ways to calculate magnetic exchange interactions within the density functional theory plus dynamical mean-field theory (DFT+DMFT) approach in the paramagnetic phase. Using the susceptibilities obtained within the ladder DMFT approach together with the random phase approximation result for the Heisenberg model, we obtain bilinear exchange interactions. We show that the earlier obtained result of Stepanov et al. [Phys. Rev. Lett. 121, 037204 (2018); Phys. Rev. B 105, 155151 (2022)] corresponds to considering individual magnetic moments in each orbital in the leading-order approximation in the non-local correlations. We consider a more general approach and apply it to evaluate the effective magnetic parameters of iron and nickel. We show that the analysis, based on the inverse orbital-summed susceptibilities, yields reasonable results for both, weak and strong magnets. For iron we find, in the low-temperature limit, the exchange interaction $J_0\simeq 0.20$ eV, while for nickel we obtain $J_0\simeq 1.2$ eV. The considered method also allows one to describe the spin-wave dispersion at temperatures $T\sim T_C$, which is in agreement with the experimental data.

cond-mat.str-el↗

arXiv2023

Topological dynamical quantum phase transition in a quantum skyrmion phase

Quantum skyrmionic phase is modelled in a 2D helical spin lattice. This topological skyrmionic phase retains its nature in a large parameter space before moving to a ferromagnetic phase. Next nearest-neighbour interaction improves the stability and it also causes a shift of the topological phase in the parameter space. Nonanalytic behaviour of the rate function observed, when the system which is initially in a quantum skyrmion phase is quenched to a trivial quantum ferromagnetic phase, indicates a dynamical quantum phase transition. Dynamical quantum phase transition is absent when the system initially in a skyrmion phase is quenched to a helical phase.

cond-mat.str-el↗

arXiv2023

Non-local correlation effects due to virtual spin-flip processes in itinerant electron ferromagnets

We present an ab initio method for eletcronic structure calculations, which accounts for the interaction of electrons and magnons in ferromagnets. While it is based on a many body perturbation theory we approximate numerically complex quantities with quantities from time dependent density functional theory. This results in a simple and affordable algorithm which allows us to consider more complex materials than those usually studied in this context ($3d$ ferromagnets) while still being able to account for the non-locality of the self energy. Furthermore, our approach allows for a relatively simple way to incorporate self-consistency. Our results are in a good agreement with experimental and theoretical findings for iron and nickel. Especially the experimental exchange splitting of nickel is predicted accurately within our theory. Additionally, we study the halfmetallic ferromagnet NiMnSb concerning its non-qusiparticle states appearing in the bandgap due to spin-flip excitations.

cond-mat.mtrl-sci↗

arXiv2022

Level-spectra Statistics in Planar Fractal Tight-Binding Models

In this communication, we study the level-spectra statistics when a noninteracting electron gas is confined in \textit{Sierpiński Carpet} (\textit{SC}) lattices. These \textit{SC} lattices are constructed under two representative patterns of the $self$ and $gene$ patterns, and classified into two subclass lattices by the area-perimeter scaling law. By the singularly continuous spectra and critical traits using two level-statistic tools\iffalse the nearest spacing distribution and alternative gap-ratio distribution\fi, we ascertain that both obey the critical phase due to broken translation symmetry and the long-range order of scaling symmetry. The Wigner-like conjecture is confirmed numerically since both belong to the Gaussian orthogonal ensemble. An analogy was observed in a quasiperiodic lattice~\cite{Zhong1998Level}. In addition, this critical phase isolates the crucial behavior near the metal-insulator transition edge in Anderson model. The lattice topology of the self-similarity feature can induce level clustering behavior.

cond-mat.mes-hall↗