arXiv subjects

H. Jónsson

Publications and source records attributed to H. Jónsson.

6 recordsLinked to original sources

arXiv2021

Lifetime of skyrmions in discrete systems with infinitesimal lattice constant

Topological protection of chiral magnetic structures is investigated by taking a two-dimensional magnetic skyrmion as an example. The skyrmion lifetime is calculated based on harmonic transition state theory for a discrete lattice model using various values of the ratio of the lattice constant and the skyrmion size. Parameters of the system corresponding to exchange, anisotropy and Dzyaloshinsky-Moriya interaction are chosen in such a way as to keep the energy and size of the skyrmion unchanged for small values of the lattice constant, using scaling relations derived from continuous micromagnetic description. The number of magnetic moments included in the calculations reaches more than a million. The results indicate that in the limit of infinitesimal lattice constant, the energy barrier for skyrmion collapse approaches the Belavin-Polyakov lower bound of the energy of a topological soliton in the $σ$-model, the entropy contribution to the pre-exponential factor in the Arrhenius rate expression for collapse approaches a constant and the skyrmion lifetime can, for large enough number of spins, correspond to thermally stable skyrmion at room temperature even without magnetic dipole-dipole interaction.

cond-mat.mtrl-sci↗

arXiv2020

Stability of Long-lived Antiskyrmions in Mn-Pt-Sn Material

The lifetime of antiskyrmions at room temperature in a Mn-Pt-Sn tetragonal Heusler material has been calculated using an atomic scale representation including nearly a million spins. The evaluation of the pre-exponential factor in the Arrhenius rate expression for this large system is made possible by an implementation of harmonic transition state theory that avoids evaluation of the eigenvalues of the Hessian matrix.The parameter values in the extended Heisenberg Hamiltonian, including anisotropic Dzyaloshinskii-Moriya interaction, are chosen to reproduce experimental observations [A. K. Nayak $\it{et\,al.}$, Nature $\textbf{548}$, 561 (2017)], in particular the 150 nm diameter. The calculated results are consistent with the long lifetime observed in the laboratory and this exceptional stability of the antiskyrmions is found to result from large activation energy for collapse due to strong exchange coupling while the pre-exponential factor in the Arrhenius expression for the lifetime is found to have a typical magnitude of 10$^{-12}$ s, despite the large number of spins. The long lifetime is, therefore, found to result from energetic effects rather than entropic effects in this system.

cond-mat.mes-hall↗

arXiv2020

Localized and delocalized states of a diamine cation: A critical test of wave function methods

The relative stability of a localized and delocalized electronic state in the same molecule, the N,N' -Dimethylpiperazine cation, is calculated at various levels of theory up to multireference configuration interaction (MRCI+Q). This system has received a great deal of attention because of recent experimental studies of corresponding Rydberg states of the molecule and the failure of most density functional approximations to produce a metastable localized state. A cut through the energy surface involving two dihedral angles is generated at the level of MRCI+Q as well as Hartree-Fock (HF), Möller-Plesset second order perturbation theory (MP2), coupled cluster theory with and without perturbative triple excitations (CCSD and CCSD(T)) and complete active space self-consistent field calculations with and without perturbative correction (CASSCF and NEVPT2). Remarkably, while CCSD produces a localized state, CCSD(T) does not, and similarly, large active-space CASSCF does while NEVPT2 does not. The inclusion of dynamic correlation in a perturbative way thus adversely affects the accuracy of the calculation, most notably for CCSD(T), the 'golden standard'. The MRCI+Q results are in close correspondence with the experimental results, as well as CAS(19,20) DMRG-CASSCF calculations. The results presented here establish a benchmark system for the study of electronic state localization.

physics.chem-ph↗

arXiv2020

NWChem: Past, Present, and Future

Specialized computational chemistry packages have permanently reshaped the landscape of chemical and materials science by providing tools to support and guide experimental efforts and for the prediction of atomistic and electronic properties. In this regard, electronic structure packages have played a special role by using first-principledriven methodologies to model complex chemical and materials processes. Over the last few decades, the rapid development of computing technologies and the tremendous increase in computational power have offered a unique chance to study complex transformations using sophisticated and predictive many-body techniques that describe correlated behavior of electrons in molecular and condensed phase systems at different levels of theory. In enabling these simulations, novel parallel algorithms have been able to take advantage of computational resources to address the polynomial scaling of electronic structure methods. In this paper, we briefly review the NWChem computational chemistry suite, including its history, design principles, parallel tools, current capabilities, outreach and outlook.

physics.chem-ph↗

arXiv2019

Coupled quasimonopoles in chiral magnets

Magnetic singularities, also known as magnetic monopoles or Bloch points, represent intriguingphenomena in nanomagnetism. We show that a pair of coupled Bloch points may appear as alocalized, stable state in cubic chiral magnets. Detailed analysis is presented of the stability of suchobjects in the interior of crystals and in geometrically confined systems.

cond-mat.mes-hall↗

arXiv2017

Lifetime of racetrack skyrmions

The skyrmion racetrack is a promising concept for future information technology. There, binary bits are carried by nanoscale spin swirls -- skyrmions -- driven along magnetic strips. Stability of the skyrmions is a critical issue for the realization of this technology. Here we demonstrate that the racetrack skyrmion lifetime can be calculated from first principles as a function of temperature, magnetic field and track width. Our method combines harmonic transition state theory extended to include Goldstone modes, with an atomistic spin Hamiltonian parametrized from density functional theory calculations. We demonstrate that two annihilation mechanisms contribute to the skyrmion stability: At low external magnetic field, escape through the track boundary prevails, but a crossover field exists, above which the collapse in the interior becomes dominant. Considering a Pd/Fe bilayer on an Ir(111) substrate as a well-established model system, the calculated lifetime is found to be consistent with reported experimental measurements. Our results open the door for predictive simulations, free from empirical parameters, to aid the design of skyrmion-based information technology.

cond-mat.mes-hall↗