arXiv subjects

László Oroszlány

Publications and source records attributed to László Oroszlány.

At least 19 recordsLinked to original sources

arXiv2026

Proximity-Induced Spin Reorientation in Monolayer CrI$_3$ on Hexagonal WTe$_2$

Magnetic anisotropy controls the orientation and thermal stability of two-dimensional magnetic order. Predicting proximity-induced changes in anisotropy requires linking the electronic structure to microscopic magnetic interactions and finite-temperature behavior. Here we study monolayer CrI$_3$ on hexagonal WTe$_2$ using a first-principles-to-finite-temperature workflow centered on relativistic spin-Hamiltonian mapping. We find that WTe$_2$ reorients the CrI$_3$ magnetization from out-of-plane to in-plane and substantially enhances the magnetic ordering scale within the extracted spin models. Analysis of the extracted spin Hamiltonians shows that the reorientation is driven by a substrate-induced change in the balance between symmetric anisotropic exchange and onsite anisotropy. We establish a transferable workflow for proximity-controlled magnetism in two-dimensional van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Strain induced magnetic phase transitions in Fe3GeTe2 monolayer

We investigate the magnetic properties of a monolayer of Fe3GeTe2 as a function of the lattice constant by combining first-principles calculations with atomistic spin dynamics simulations. The calculated magnetic exchange interactions reveal a competition between ferromagnetic and antiferromagnetic couplings, with the latter being significantly strengthened under compressive strain. Stochastic Landau-Lifshitz-Gilbert simulations reveal a substantial decrease in the Curie temperature with decreasing lattice constant, and predict a transition of the magnetic ground state from a ferromagnetic configuration to a conical spin-spiral state. We introduce a simple spin-model which explains the stabilization of the spiral phase due to competing exchange interactions. We found multiple magnetic phase transitions involving ferromagnetic, conical spin-spiral, and planar Neel states, depending on both the lattice constant and the temperature. The absence of Dzyaloshinskii-Moriya interactions is found to significantly reduce the Neel temperature, while leaving the Curie temperature largely unaffected. Our findings reveal the importance of lattice distortions in controlling complex magnetic phases and their evolution with temperature.

cond-mat.mtrl-sci↗

arXiv2026

Shuttling of $\mathbb{Z}_4$ parafermions in an electronic ladder model

Parafermions with non-Abelian statistics have been proposed as a promising platform for quantum computation, potentially enabling a broader set of topologically protected gates than Majorana fermions. The experimental and theoretical exploration of these exotic quasiparticles remains challenging, as their stability is linked to strong electron-electron interactions. A key step toward practical applications is the controlled shuttling of parafermionic modes, which is required for implementing geometric braiding operations. In the present work, we investigate the real-time dynamics of the elementary shuttling process by applying a combination of the density matrix renormalization group and the time-dependent variational principle approaches. We analyze the transport of $\mathbb{Z}_4$ parafermion edge states and assess the corresponding adiabatic speed limit under experimentally relevant conditions.

cond-mat.str-el↗

arXiv2026

AKLT Hamiltonian from Hubbard tripods

We investigate how the spin-1 Affleck-Kennedy-Lieb-Tasaki (AKLT) Hamiltonian can emerge from a microscopic fermionic model based on half-filled Hubbard tripods. We first show that a single tripod hosts a robust threefold-degenerate low-energy manifold corresponding to an effective $S = 1$ degree of freedom. This manifold prevails over a broad range of interactions and remains stable against moderate disorder. We then combine exact diagonalization with fourth-order quasi-degenerate perturbation theory to derive an effective bilinear-biquadratic spin model for a pair of coupled tripods and identify coupling regimes where the target ratio is approached. In particular, tuning leg-center hopping together with two symmetry-inequivalent leg-leg hoppings yields the characteristic singlet-triplet degeneracy associated with a biquadratic-to-bilinear ratio close to 1/3. Extending the analysis to three tripods, we compare nonequivalent coupling geometries and find a strategy that suppresses unwanted longer-range and multispin terms while preserving the target nearest-neighbor couplings in the weak-coupling regime. These results establish a concrete bottom-up route from Hubbard clusters to valence-bond-solid spin physics in tunable quantum-dot arrays.

cond-mat.str-el↗

arXiv2025

Graphene-based quantum heterospin graphs

We investigate from first principles a variety of low-dimensional open quantum spin systems based on magnetic nanographene structures that contain spin-1/2 and spin-1 triangulenes and/or olympicenes. These graphene nanostructures behave as localized spins and can be effectively described by a quantum bilinear-biquadratic Heisenberg Hamiltonian, for which we will compute the energy spectrum and the quantum numbers associated with the low-energy eigenstates. We propose the experimental realization of antiferromagnetic alternating spin chains using these graphene nanostructures, which result in ferrimagnetic systems whose ground state spin and degeneracy depend on the length of the chain. We identify a double degeneracy in the total spin quantum number $S$ of the first excited state in three-leg spin graphs (3-LSGs) and other heterospin nanostructures, which depends on both the number of sites and the spin species, and originates from the swapping transformation symmetry of the Hamiltonian. Numerical simulations indicate that this degeneracy remains largely robust for $N=7$ spin-1 3-LSGs under realistic perturbations present in experimental conditions.

cond-mat.mes-hall↗

arXiv2025

Non-unitary Time Evolution via the Chebyshev Expansion Method

The Chebyshev expansion method is a well-established technique for computing the time evolution of quantum states, particularly in Hermitian systems with a bounded spectrum. Here, we show that the applicability of the Chebyshev expansion method extends well beyond this constraint: It remains valid across the entire complex plane and is thus suitable for arbitrary non-Hermitian matrices. We identify that numerical rounding errors are the primary source of errors encountered when applying the method outside the conventional spectral bounds, and they are not caused by fundamental limitations. By carefully selecting the spectral radius and the time step, we show how these errors can be effectively suppressed, enabling accurate time evolution calculations in non-Hermitian systems. We derive an analytic upper bound for the rounding error, which serves as a practical guideline for selecting time steps in numerical simulations. As an application, we illustrate the performance of the method by computing the time evolution of wave packets in the Hatano-Nelson model.

cond-mat.mes-hall↗

arXiv2024

Tuning magnetic exchange interactions in 2D magnets: the case of CrGeX$_3$ (X = Se, Te) and Janus Cr$_2$Ge$_2$(Se,Te)$_3$ monolayers

We present a computational study to explore the potential of different experimental approaches to tune the magnetic interactions in two-dimensional van der Waals magnets. We selected CrGeSe$_3$, CrGeTe$_3$, and Janus Cr$_2$Ge$_2$(Se,Te)$_3$ monolayers as case studies and calculated the full exchange tensors among all relevant atomic pairs and analyze their dependence on different external parameters, such as biaxial and uniaxial strain, as well as gate voltage. We pay special attention to interactions that emerge or vanish due to changes of the symmetry of the system. We find that biaxial and uniaxial strains significantly modify isotropic exchange couplings, which can lead to a transition from a ferromagnetic to an antiferromagnetic phase, while a gate voltage induces Dzyaloshinskii-Moriya interactions, forming a vortex pattern whose chirality is determined by the sign of the electric field. The electric dipole moment of the Janus material is large, raising the possibility of multiferroic behaviour. The polarizability is similar for the three compounds.

cond-mat.mes-hall↗

arXiv2024

Signature of pressure-induced topological phase transition in ZrTe$_5$

The layered van der Waals material ZrTe$_5$ is known as a candidate topological insulator (TI), however its topological phase and the relation with other properties such as an apparent Dirac semimetallic state is still a subject of debate. We employ a semiclassical multicarrier transport (MCT) model to analyze the magnetotransport of ZrTe$_5$ nanodevices at hydrostatic pressures up to 2 GPa. The temperature dependence of the MCT results between 10 and 300 K is assessed in the context of thermal activation, and we obtain the positions of conduction and valence band edges in the vicinity of the chemical potential. We find evidence of the closing and re-opening of the band gap with increasing pressure, which is consistent with a phase transition from weak to strong TI. This matches expectations from ab initio band structure calculations, as well as previous observations that CVT-grown ZrTe$_5$ is a weak TI in ambient conditions.

cond-mat.mes-hall↗

arXiv2024

Snell's law in multirefringent systems

In anisotropic crystals, Maxwell's equations permit only birefringence for the propagation of light. Notwithstanding, multirefringent systems comprising more than two propagating modes exist, such as in electron optics and photonic crystals. It has been demonstrated that in such systems, the propagation of waves in the short wavelength limit results in the formation of anomalous caustics. To calculate these caustic curves, we generalized Snell's law valid for reflection and refraction in multirefringent systems possessing more than one propagating mode. The emergence of anomalous caustics in the wave function patterns obtained from rigorous quantum mechanical calculations in electron-optical systems is confirmed. These calculations were performed on electron scattering from circularly gated potential regions applied to multilayer rhombohedral graphene. Our results may generate further work to explore more complex phenomena in multirefringent systems.

cond-mat.mes-hall↗

arXiv2024

Surface reconstruction limited magnetism of the nodal loop semimetal Ca$_3$P$_2$

Nodal loop semimetals are topological materials with drumhead surface states characterized by reduced kinetic energy. If the Fermi energy of such a system is near these nondispersive states interaction among charge carriers substantially impacts their electronic structure. The emergence of magnetism in these surface states is one of the possible consequences. Ca3P2 an already synthesized material possesses a remarkably large nodal loop which is situated exactly at the Fermi level of the bulk system. In the present work, we investigate how surface magnetism is impacted by surface reconstruction and lattice termination in finite slabs in this material. We show that a slight deviation from the stoichiometric occupation of Ca sites results in the stabilization of magnetic patterns.

cond-mat.mtrl-sci↗

arXiv2024

A simple electronic ladder model harboring $\mathbb{Z}_4$ parafermions

Parafermions are anyons with the potential for realizing non-local qubits that are resilient to local perturbations. Compared to Majorana zero modes, braiding of parafermions implements an extended set of topologically protected quantum gates. This, however, comes at the price that parafermionic zero modes can not be realized in the absence of strong interactions whose theoretical description is challenging. In the present work, we construct a simple lattice model for interacting spinful electrons with parafermionic zero energy modes. The explicit microscopic nature of the considered model highlights new realization avenues for these exotic excitations in recently fabricated quantum dot arrays. By density matrix renormalization group calculations, we identify a broad range of parameters, with well-localized zero modes, whose parafermionic nature is substantiated by their unique $8π$ periodic Josephson spectrum.

cond-mat.str-el↗

arXiv2023

Relativistic magnetic interactions from non-orthogonal basis sets

We propose a method to determine the magnetic exchange interaction and on-site anisotropy tensors of extended Heisenberg spin models from density functional theory including relativistic effects. The method is based on the Liechtenstein-Katsnelson-Antropov-Gubanov torque formalism, whereby energy variations upon infinitesimal rotations are performed. We assume that the Kohn-Sham Hamiltonian is expanded in a non-orthogonal basis set of pseudo-atomic orbitals. We define local operators that are both hermitian and satisfy relevant sum rules. We demonstrate that in the presence of spin-orbit coupling a correct mapping from the density functional total energy to a spin model that relies on the rotation of the exchange field part of the Hamiltonian can not be accounted for by transforming the full Hamiltonian. We derive a set of sum rules that pose stringent validity tests on any specific calculation. We showcase the flexibility and accuracy of the method by computing the exchange and anisotropy tensors of both well-studied magnetic nanostructures and of recently synthesized two-dimensional magnets. Specifically, we benchmark our approach against the established Korringa-Kohn-Rostoker Green's function method and show that they agree well. Finally, we demonstrate how the application of biaxial strain on the two-dimensional magnet T-CrTe2 can trigger a magnetic phase transition.

cond-mat.mtrl-sci↗

arXiv2023

Surface magnon spectra of nodal loop semimetals

In this paper we establish a connection between the bulk topological structure and the magnetic properties of drumhead surface states of nodal loop semimetals. We identify the magnetic characteristics of the surface states and compute the system's magnon spectrum by treating electron-electron interactions on a mean-field level. We draw attention to a subtle connection between a Lifshitz-like transition of the surface states driven by mechanical distortions and the magnetic characteristics of the system. Our findings may be experimentally verified e.g. by spin polarized electron energy loss spectroscopy of nodal semimetal surfaces.

cond-mat.mes-hall↗

arXiv2023

Electrically driven singlet-triplet transition in triangulene spin-1 chains

Recently, graphene triangulene chains have been synthesized and their magnetic response has been analyzed by STM methods by Mishra and coworkers (Nature 598, 287 (2021)). Motivated by this study, we determine the exchange bilinear and biquadratic constants of the triangulene chains by calculating two-spin rotations in the spirit of the magnetic force theorem. We then analyze open-ended, odd-numbered chains, whose edge states pair up forming a triplet ground state. We propose three experimental approaches that enable us to trigger and control a singlet-triplet spin transition. Two of these methods are based on applying a mechanical distortion to the chain. We finally show that the transition can be controlled efficiently by the application of an electric field.

cond-mat.mes-hall↗

arXiv2023

Efficient qudit based scheme for photonic quantum computing

Linear optics is a promising alternative for the realization of quantum computation protocols due to the recent advancements in integrated photonic technology. In this context usually qubit based quantum circuits are considered, however, photonic systems naturally allow also for d-ary, i.e., qudit based, algorithms. This work investigates qudits defined by the possible photon number states of a single photon in d > 2 optical modes. We demonstrate how to construct locally optimal non-deterministic many-qudit gates using linear optics and photon number resolving detectors, and explore the use of qudit cluster states in the context of a d-ary optimization problem. We find that the qudit cluster states require less optical modes and are encoded by a fewer number of entangled photons than the qubit cluster states with similar computational capabilities. We illustrate the benefit of our qudit scheme by applying it to the k-coloring problem.

quant-ph↗

arXiv2023

Revealing the band structure of ZrTe$_5$ using Multicarrier Transport

The layered material ZrTe$_5$ appears to exhibit several exotic behaviors which resulted in significant interest recently, although the exact properties are still highly debated. Among these we find a Dirac/Weyl semimetallic behavior, nontrivial spin textures revealed by low temperature transport, and a potential weak or strong topological phase. The anomalous behavior of resistivity has been recently elucidated as originating from band shifting in the electronic structure. Our work examines magnetotransport behavior in ZrTe$_5$ samples in the context of multicarrier transport. The results, in conjunction with ab-initio band structure calculations, indicate that many of the transport features of ZrTe$_5$ across the majority of the temperature range can be adequately explained by the semiclassical multicarrier transport model originating from a complex Fermi surface.

cond-mat.mes-hall↗

arXiv2022

Revealing the topological phase diagram of ZrTe$_5$ using the complex strain fields of microbubbles

Topological materials host robust properties, unaffected by microscopic perturbations, owing to the global topological properties of the bulk electron system. Materials in which the topological invariant can be changed by easily tuning external parameters are especially sought after. Zirconium pentatelluride (ZrTe$_5$) is one of a few experimentally available materials that reside close to the boundary of a topological phase transition, allowing the switching of its invariant by mechanical strain. Here, we unambiguously identify a topological insulator - metal transition as a function of strain, by a combination of ab initio calculations and direct measurements of the local charge density. Our model quantitatively describes the response to complex strain patterns found in bubbles of few layer ZrTe$_5$ without fitting parameters, reproducing the mechanical deformation dependent closing of the band gap observed using scanning tunneling microscopy. We calculate the topological phase diagram of ZrTe$_5$ and identify the phase at equilibrium, enabling the design of device architectures which exploit the unique topological switching characteristics of the system.

cond-mat.mtrl-sci↗

arXiv2021

Time dependent electric transport in nodal-loop semimetals

Close to the Fermi energy, nodal loop semimetals have a torus-shaped, strongly anisotropic Fermi surface which affects their transport properties. Here we investigate the non-equilibrium dynamics of nodal loop semimetals by going beyond linear response and determine the time evolution of the current after switching on a homogeneous electric field. The current grows monotonically with time for electric fields perpendicular to the nodal loop plane however it exhibits non-monotonical behavior for field orientations aligned within the plane. After an initial non-universal growth $\sim Et$, the current first reaches a plateau $\sim E$. Then, for perpendicular directions, it increases while for in-plane directions it decreases with time to another plateau, still $\sim E$. These features arise from interband processes. For long times or strong electric fields, the current grows as $\sim E^{3/2}t$ or $\sim E^3t^2$ for perpendicular or parallel electric fields, respectively. This non-linear response represents an intraband effect where the large number of excited quasiparticles responds to the electric field. Our analytical results are benchmarked by the numerical evaluation of the current from continuum and tight-binding models of nodal loop semimetals.

cond-mat.mes-hall↗