arXiv subjects

Asako Terasawa

Publications and source records attributed to Asako Terasawa.

5 recordsLinked to original sources

arXiv2020

First-principles study of exchange coupling constants in Nd$_x$Fe$_{1-x}$ amorphous alloy

We investigate the exchange coupling constant $J_{ij}$ in Nd$_x$Fe$_{1-x}$ amorphous alloys with different compositions within the framework of first-principles calculation. We observed a strong atomic-dependence of $J_{ij}$ and its fluctuations. We show that the composition strongly affects the distance dependence of $J_{ij}$. Composition dependence of calculated Curie temperatures is modest for $x<0.5$. To examine the effect of the local environment on the exchange couplings, we demonstrate combined analyses of the coordination structure and exchange coupling constants using the Gabriel graph. Our study reveals that the Curie temperatures are mostly dominated by the averaged $J_{ij}$ and coordination numbers determined by the pairs of neighboring atoms. We also observed that the exchange couplings between Fe--Fe and Fe--Nd become stronger with increasing the number of surrounding Nd atoms.

cond-mat.mtrl-sci↗

arXiv2020

Single-site orthogonalization for first-principles computations of exchange coupling constants

For accurate first-principles computations of exchange coupling constants $J_{ij}$ by the Liechtenstein method with localized basis sets, we developed a scheme using the single-site orthogonalization (SO). In contrast to the non-orthogonal (NO) scheme, where the basis set is used to compute $J_{ij}$ without modification, and the Löwdin orthogonalization (LO) scheme, the SO scheme exhibits much less dependence of $J_{ij}$ on the choice of the basis set. The SO scheme achieves convergence of $J_{ij}$ for bcc Fe, hcp Co, and fcc Ni with an increase in the number of the basis set, while the NO and LO schemes result in the fluctuation depending on the basis set. This improvement by the SO scheme is attributed to the removal of orbital overlaps with avoiding ill-defined single-site effective potentials. We further improve the SO scheme by introducing appropriate spin population, so that the SO with spin-population scaling (SOS) scheme can provide converged Curie temperatures for transition metals. Moreover, negative values of $J_{ij}$ for dhcp Nd and rhombohedral Sm obtained by the SOS scheme can coincide with the experimentally-found magnetic order that cannot be reproduced by positive sets of $J_{ij}$.

cond-mat.mtrl-sci↗

arXiv2019

Efficient algorithm based on Liechtenstein method for computing exchange coupling constants using localized basis set

For large-scale computation of the exchange coupling constants $J_{ij}$, we reconstruct the Liechtenstein formula for localized orbital representation and simplify the energy integrations by adopting the finite pole approximation of the Fermi function proposed by Ozaki [Phys. Rev. B 75, 035123 (2007)]. We calculate the exchange coupling constant $J_{\mathrm{1NN}}$ of the first-nearest-neighbor sites in body-centered-cubic Fe systems of various sizes to estimate the optimal computational parameters that yield appropriate values at the lowest computational cost. It is shown that the number of poles needed for a computational accuracy of 0.05 meV is determined as $\sim$ 60, whereas the number of necessary Matsubara poles needed to obtain similar accuracy, which was determined in previous studies, is on the order of 1000. Finally, we show $J_{ij}$ as a function of atomic distance, and compared it with one derived from Korringa-Kohn-Rostoker Green's function formalism. The distance profile of $J_{ij}$ derived by KKR formalism agrees well with that derived by our study, and this agreement supports the reliability of our newly derived formalism.

cond-mat.mtrl-sci↗

arXiv2018

Hidden orders in amorphous structures: extraction of nearest neighbor networks of amorphous Nd-Fe alloys with Gabriel graph analyses

Using the scheme of Delaunay and Gabriel graphs, we analyzed the amorphous structures of computationally created Nd-Fe alloys for several composition ratios based on melt quench simulations with finite temperature first-principles molecular dynamics.By the comparison of the radial distribution functions of the whole system and those derived from the Delaunay and Gabriel graphs, it was shown that the Gabriel graphs represent the first nearest neighbor networks well in the examined amorphous systems. From the Gabriel graph analyses, we examined the coordination structures of amorphous Nd-Fe alloys statistically. We found that the ranges of distributions of coordination numbers vary depending on the composition ratio. The angular distributions among three adjacent atoms were also analyzed, and it was found that the angular distributions behave differently in the Nd-rich and Fe-rich samples. We found that the orders in the amorphous system becomes stronger as increasing the Nd ratio, which corresponds to the appearance of crystalline grain boundary phases at high Nd composition ratio [T. T. Sasaki et al., Acta Mater. 115, 269-277 (2016)].

cond-mat.mtrl-sci↗

arXiv2012

An ignition key for atomic-scale engines

A current-carrying resonant nanoscale device, simulated by non-adiabatic molecular dynamics, exhibits sharp activation of non-conservative current-induced forces with bias. The result, above the critical bias, is generalized rotational atomic motion with a large gain in kinetic energy. The activation exploits sharp features in the electronic structure, and constitutes, in effect, an ignition key for atomic-scale motors. A controlling factor for the effect is the non-equilibrium dynamical response matrix for small-amplitude atomic motion under current. This matrix can be found from the steady-state electronic structure by a simpler static calculation, providing a way to detect the likely appearance, or otherwise, of non-conservative dynamics, in advance of real-time modelling.

cond-mat.mes-hall↗