arXiv subjects

Matej Komelj

Publications and source records attributed to Matej Komelj.

12 recordsLinked to original sources

arXiv2025

Quantum computing of magnetic-skyrmion-like patterns in Heisenberg ferromagnets

We diagonalize the quantum two-dimensional spin-1/2 Heisenberg model with Dzyaloshinskii-Moriya interaction (DMI) by applying the variational quantum eigensolver, running on a quantum-computer simulator, which turns out to be a more efficient approach than a classical direct diagonalization for systems with more than 17 sites. The calculated external-magnetic-field dependence of the total energy, of the magnetization, as well as of the topological charge exhibits a distinctive discontinuity which hints for the existence of zero-temperature magnetic skyrmions-like structures at the quantum level, controlled by the combination of the exchange-coupling and the DMI parameters. The potentially measurable jump in the magnetization upon changing the field indicates the investigated objects as stable enough for eventual applications in spintronics or even as information carriers.

quant-ph↗

arXiv2015

Electron-phonon coupling and exchange-correlation effects in superconducting ${\rm H_3S}$ under high pressure

We investigate the ${\rm H_3S}$ phase of sulphur hydride under high pressure $\simeq$ 200 GPa by means of {\it ab-initio} calculations within the framework of the density-functional theory (DFT) with the PBE0 hybrid exchange-correlation ($E_{\rm xc}$) approximation. The choice of $E_{\rm xc}$ has the largest effect on the calculated electron-phonon coupling (EPC) matrix elements; the high pressure equation of state and phonon frequencies are only slightly modified. Mode-dependent EPC correction factors are determined from PBE0 using a frozen-phonon supercell approach, while standard density-functional perturbation theory is used to determine the EPC with PBE generalized-gradient approximation $E_{\rm xc}$. Our principle finding is that the calculated PBE0 $T_c$ is enhanced by 25\% compared to PBE. This is similar in magnitude, but in opposite direction, to the proposed suppression of $T_c$ by anharmonic effects [Errea {\em et al., Phys. Rev. Lett.} {\bf 114}, 157004 (2015)]. Our calculations demonstrate the importance of considering exchange-correlation approximations for calculations of superconducting properties for this class of materials.

cond-mat.supr-con↗

arXiv2015

Influence of strain on the properties of CeRuPO and CeOsPO Kondo systems

We investigated the influence of isotropic strain on the type of the magnetic ground-state and the Fermi surface for the structurally-equivalent CeRuPO and CeOsPO crystals with slightly different lattice parameters. The two systems exhibit different magnetic orderings at zero strain. According to the phase diagram of CeRuPO under pressure the difference might be due to the different Ce-Ce inter-atomic distances. We applied {\it ab-initio} calculations based on the density-functional theory and the generalized-gradient approximation with an additional Coulomb repulsion (GGA+$U$), which indeed reveal a significant impact of the strain and the effective $U$ parameter on the magnetic ground state. However, it is demonstrated that the difference is more likely related to the details in the Fermi surface.

cond-mat.str-el↗

arXiv2014

Frustration-induced nanometre-scale inhomogeneity in a triangular antiferromagnet

Phase inhomogeneity of otherwise chemically homogenous electronic systems is an essential ingredient leading to fascinating functional properties, such as high-$T_c$ superconductivity in cuprates, colossal magnetoresistance in manganites, and giant electrostriction in relaxors. In these materials distinct phases compete and can coexist due to intertwined ordered parameters. Charge degrees of freedom play a fundamental role, although phase-separated ground states have been envisioned theoretically also for pure spin systems with geometrical frustration that serves as a source of phase competition. Here we report a paradigmatic magnetostructurally inhomogenous ground state of the geometrically frustrated $α$-NaMnO$_2$ that stems from the system's aspiration to remove magnetic degeneracy and is possible only due to the existence of near-degenerate crystal structures. Synchrotron X-ray diffraction, nuclear magnetic resonance and muon spin relaxation show that the spin configuration of a monoclinic phase is disrupted by magnetically short-range ordered nanoscale triclinic regions, thus revealing a novel complex state of matter.

cond-mat.str-el↗

arXiv2012

Orthorhombic fulleride (CH3NH2)K3C60 close to Mott-Hubbard instability: Ab initio study

We study the electronic structure and magnetic interactions in methylamine-intercalated orthorhombic alkali-doped fullerene (CH3NH2)K3C60 within the density functional theory. As in the simpler ammonia intercalated compound (NH3)K3C60, the orthorhombic crystal-field anisotropy Δlifts the t1u triple degeneracy at the Γpoint and drives the system deep into the Mott-insulating phase. However, the computed Δand conduction electron bandwidth W cannot alone account for the abnormally low experimental Néel temperature, T_N = 11 K of the methylamine compound, compared to the much higher value T_N = 40 K of the ammonia one. Significant interactions between CH3NH2 and C60^{3-} are responsible for the stabilization of particular pseudo-Jahn-Teller fullerene-cage distortions and the ensuing low-spin S = 1/2 state. These interactions also seem to affect the magnetic properties, as interfullerene exchange interactions depend on the relative orientation of pseudo-Jahn-Teller distortions of neighboring C60^{3-} molecules. For the ferro-orientational order of CH3NH2-K^+ groups we find an apparent reduced dimensionality in magnetic exchange interactions, which may explain the suppressed Néel temperature. The disorder in exchange interactions caused by orientational disorder of CH3NH2-K^+ groups could further contribute to this suppression.

cond-mat.str-el↗

arXiv2007

Magnetic coupling in CoCr_2O_4 and MnCr_2O_4: an LSDA+U study

We present a first principles LSDA+U study of the magnetic coupling constants in the spinel magnets CoCr_2O_4 and MnCr_2O_4. Our calculated coupling constants highlight the possible importance of AA interactions in spinel systems with magnetic ions on both A and B sites. Furthermore, we show that a careful analysis of the dependence of the magnetic coupling constants on the LSDA+U parameters provides valuable insights in the underlying coupling mechanisms, and allows to obtain a quantitative estimate of the magnetic coupling constants. We discuss in detail the capabilities and possible pitfalls of the LSDA+U method in determining magnetic coupling constants in complex transition metal oxides.

cond-mat.mtrl-sci↗

arXiv2004

An ab-initio investigation of magnetism in two-dimensional uranium systems

The orbital and spin magnetic moments, and the X-ray-magnetic circular-dichroism (XMCD) spectra at the $M_{4,5}$ edges of the U atoms in a UAs/Co multilayer and in an $α$-U film are calculated within the framework of the density-functional theory in combination with the local-spin-density approximation (LSDA), the generalized-gradient approximation (GGA) and the LDA+$U$ method. The antiparallel arrangement between the U and Co spin magnetic moments at the interface results in the vanishing of ferromagnetism for the case of very thin Co layers. The U moments decay rapidly with the distance from the film surface. The magnitude of the magnetic-dipole term $ $, which appears in the spin XMCD sum rule, is small. The different exchange-correlation treatments do not yield qualitatively different results.

cond-mat.str-el↗

arXiv2004

A test of the spin-orbit sum rule for actinides using an ab-initio calculation

The expectation value of the angular part of the spin-orbit-coupling (SOC) operator $ $ and the branching ratio for the $N_{4,5}$ X-ray-absorption spectroscopy in $δ$-Pu, $α$-U and face-centered-cubic (fcc) Th are calculated within the framework of the density-functional theory in combination with the local-spin-density approximation (LSDA), the generalized-gradient approximation (GGA) and the LDA+$U$ method. The SOC contribution is calculated in terms of the first- and the second- variational schemes. A strong variation in the magnitudes of the calculated $ $ operator along the actinide series is found. The results show that the SOC sum rule for $δ$-Pu and $α$-U is reasonably valid, whereas there is a sign mismatch for the case of fcc Th. The calculated branching ratios for the case without SOC in the valence shell strongly deviate from the statistical value.

cond-mat.str-el↗

arXiv2003

Anisotropy of the orbital methods and the magnetic dipole term $T_z$ in ${\rm CrO_2}$: An {\it ab-initio} studt

A systematic study is performed by the {\it ab-initio} density functional theory of the anisotropy of the orbital moments $ $ and the magnetic dipole term $ $ in bulk ${\rm CrO_2}$. Two different band-structure techniques are used (FLAPW and LMTO-ASA), and the electronic correlations are treated by the local-spin-density approximation (LSDA), the LSDA+ orbital polarization method, and the LSDA+$U$ method. The calculated anisotropies of $ $ and $ $ are very large compared to Fe, Ni and Co but still a factor of 5 and 2 smaller than the anisotropies obtained from a recently suggested analysis of the X-ray magnetic circular dichroism spectra for a thick layer of ${\rm CrO_2}$.

cond-mat.mtrl-sci↗

arXiv2003

Magnetism in systems with various dimensionality: A comparison between Fe and Co

A systematic ab initio study is performed for the spin and orbital moments and for the validity of the sum rules for x-ray magnetic circular dichroism for Fe systems with various dimensionality (bulk, Pt-supported monolayers and monatomic wires, free-standing monolayers and monatomic wires). Qualitatively, the results are similar to those for the respective Co systems, with the main difference that for the monatomic Fe wires the term in the spin sum rule is much larger than for the Co wires. The spin and orbital moments induced in the Pt substrate are also discussed.

cond-mat.mtrl-sci↗

arXiv2002

From the bulk to monatomic wires: An ab-initio study of magnetism in Co systems with various dimensionality

A systematic ab-initio study within the framework of the local-spin-density approximation including spin-orbit coupling and an orbital-polarization term is performed for the spin and orbital moments and for the X-ray magnetic circular dichroism (XMCD) spectra in hcp Co, in a Pt supported and a free standing Co monolayer, and in a Pt supported and a free standing monatomic Co wire. When including the orbital-polarization term, the orbital moments increase drastically when going to lower dimensionality, and there is an increasing asymmetry between the L_2 and L_3 XMCD signal. It is shown that spin and orbital moments can be obtained with good accuracy from the XMCD spectra via the sum rules. The term of the spin sum rule is surprisingly small for the wires, and the reason for this is discussed.

cond-mat.mtrl-sci↗

arXiv2000

Magnetoelastic coupling in epitaxial magnetic films: An ab-initio study

A method is developed which allows to determine the first-order and the second-order magnetoelastic coefficients of a magnetic bulk material from the ab-initio calculation of the magnetocrystalline anisotropy energy as function of a prestrain. Excplicit results are given for bcc Fe, and they agree well with experimental data obtained from the magnetostrictive stress measurements for epitaxial Fe films.

cond-mat.mtrl-sci↗