arXiv subjects

Jakub Šebesta

Publications and source records attributed to Jakub Šebesta.

7 recordsLinked to original sources

arXiv2026

Ultralow-Field Triplon Condensation in a Spin-Ladder Magnet

We realise the first ultralow-field Bose-Einstein condensation of triplons in a spin-ladder magnet, uncovering a quantum critical point at only $μ_0 H_{c1}=0.17$ T in Henmilite ($\mathrm{Ca_2Cu(OH)_4[B(OH)_4]_2}$). Unlike dimer magnets, a ladder retains extended one-dimensional correlations in its gapped parent state, making this limit strongly fluctuation dominated. Thermodynamic, magnetoelastic, $μ$SR, and neutron-diffraction measurements overturn the previous assignment of zero-field antiferromagnetic order, establishing a quantum-disordered coupled-ladder parent state with persistent low-energy dynamics. The weak low-temperature anomaly instead marks a gap-controlled crossover from the correlated ladder regime into the activated quantum-disordered state. These measurements further reveal an exceptionally asymmetric ordered dome extending to $μ_0 H_{c2}\simeq 8.2$ T. Quantum Monte Carlo simulations for the relevant spin Hamiltonian place Henmilite just on the gapped side of the zero-field ladder-ordering instability, naturally accounting for the strong separation between the exchange and residual-gap scales and the tiny critical field. Our findings extend ultralow-field triplon condensation beyond the dimer paradigm and establish Henmilite as a platform for controlled tuning across quantum criticality in a fluctuation-dominated spin ladder.

cond-mat.str-el↗

arXiv2026

Spin-lattice model simulations of tetragonal FePt system

Magnetic materials play a key role in the contemporary industry, providing unique features with a wide application potential. To study physical phenomena and design new materials, it is important to possess an appropriate tool, a model allowing simulation of desired behavior. Spin-lattice model simulations can be used to investigate spacious systems containing thousands of atoms, while complex phenomena arising from the interplay of lattice and spin dynamics can be modeled. One of the important phenomena that can be modeled in the spin lattice simulation is magnetoelastic behavior, offering direct conversion between the mechanical and magnetic energy. However, so far, only models for systems with cubic symmetry have been introduced. Therefore, here, a spin-lattice model for a system with tetragonal symmetry is proposed, where its strength is manifested by simulation of magnetoelastic properties of a characteristic representative L1$_{0}$ FePt system.

cond-mat.mtrl-sci↗

arXiv2026

Higher order magnetoelasticity energy corrections in bcc and fcc systems

Magnetoelastic properties play a vital role in industrial applications. Despite being hidden behind either purely magnetic or elastic behavior, magnetoelasticity takes place in a wide range of devices as transducers, acoustic actuators, or fast response sensors. In this work, we inspect the impact of higher-order terms on the anisotropic magnetostriction behavior. Regarding ab-initio calculations, the anisotropic magnetostriction can be related to the strain dependence of the magnetocrystaline energy. Commonly, the description is restricted to a linear strain dependence in the magnetoelastic energy. Here, we derive higher-order terms in strain for bcc and fcc crystal structures. Using a simple parametrization, we show that the influence of the higher-order strain terms is negligible for the studied cubic systems.

cond-mat.mtrl-sci↗

arXiv2025

Magnetoelasticity - magnetic structure interrelation - tetragonal MnPt system study

Magnetic materials represent an essential ingredient for the contemporary industry. Apart from common material parameters such as magnetocrystalline anisotropy, coercivity, or saturation magnetization, magnetoelastic behavior is vital for applications serving in various devices, e.g., in acoustic actuators, transducers, or sensors providing a desirable fast response and high efficiency with respect to applied magnetic field. Magnetoelastic properties have been studied for ferromagnetic 3d elements, or especially in high symmetry systems containing rare-earth elements to achieve higher values. Since, unlike for rare earth Laves phases, in the transition metals or alloys, these effects are very weak. Here, in contrast, we analyze the magnetoelastic behavior of antiferromagnetic tetragonal system MnPt, explaining the experimentally measured data based on the theoretical calculations and discussing the influence of the magnetic structure. Particularly, we inspect the origin of magnetocrystalline anisotropy energy, as well as the size and source of the isotropic and anisotropic parts of magnetoelastic (magnetostriction) coefficients.

cond-mat.mtrl-sci↗

arXiv2023

Photo-induced modification and relaxation dynamics of Weyl-semimetals

The use of ultrashort laser pulses to investigate the response of materials on femtosecond time-scales enables detailed tracking of charge, spin and lattice degrees of freedom. When pushing the limits of the experimental resolution, connection to theoretical modeling becomes increasingly important in order to infer causality relations. Weyl-semimetals is particular class of materials of recent focus due to the topological protection of the Weyl-nodes, resulting in a number of fundamentally interesting phenomena. In this work, we provide a first-principles framework based on time-dependent density-functional theory for tracking the distribution of Weyl-nodes in the Brillouin-zone following an excitation by a laser pulse. For the material TaAs, we show that residual shifts in the Weyl-Nodes' position and energy distribution is induced by a photo-excitation within femto-seconds, even when the laser-frequency is off-resonant with the Weyl-node. Further, we provide information about the relaxation pathway of the photoexcited bands through lattice vibrations.

cond-mat.mtrl-sci↗

arXiv2023

Structural regulation of mechanical gating in molecular junctions

In contrast to silicon-based transistors, single molecule junctions can be gated by simple mechanical means. Specifically, charge can be transferred between the junction's electrodes and its molecular bridge when the interelectrode distance is modified, leading to variations in the electronic transport properties of the junction. While this effect has been studied extensively, the influence of the molecule orientation on mechanical gating has not been addressed, despite its potential influence on the gating effectiveness. Here, we show that the same molecular junction can experience either clear mechanical gating or none, depending on the molecule orientation in the junctions. The effect is found in silver-ferrocene-silver break junctions, and analyzed in view of ab-initio and transport calculations, where the influence of molecular orbitals geometry on charge transfer to or from the molecule is revealed. The molecule orientation is thus a new degree of freedom that can be used to optimize mechanically-gated molecular junctions.

cond-mat.mes-hall↗

arXiv2020

Twin Domain Structure in Magnetically Doped Topological Insulators

Twin domains are naturally present in the topological insulator \BiSe{} and affect strongly its properties. While studies of its behavior for ideal \BiSe{} structure exist, little is known about their possible interaction with other defects. Extra information are needed especially for the case of artificial perturbation of topological insulator states by magnetic doping, which has attracted a lot of attention recently. Employing ab initio calculations based on layered Green's function formalism, we study the interaction between twin planes in \BiSe{}. We show the influence of various magnetic and non-magnetic chemical defects on the twin plane formation energy and discuss the related modification of their distribution. Furthermore, we examine the change of dopants' magnetic properties at sites in the vicinity of a twin plane, and the dopants' preference to occupy such sites. Our results suggest that twin planes repel each other at least over distance of $3-4$~nm. However, in the presence of magnetic Mn and Fe defects a close TP placement is preferred. Furthermore, calculated twin plane formation energies indicate that in this situation their formation becomes suppressed. Finally, we discuss the influence of twin planes on the surface band gap.

cond-mat.mtrl-sci↗