Tsuyoshi Miyazaki

We present the approach needed to calculate stress within density functional theory (DFT) using a localized orbital basis, both for exact diagonalisation and linear scaling approaches, and demonstrate our implementation within the large scale DFT code Conquest . For the linear scaling approach, we test the rate of convergence of stress with density matrix range, and compare it to the convergence of energy and forces for different materials with a range of band gaps. We show that excellent convergence is found for modest cutoffs (below 0.1 GPa error for range of 20 a 0 ), and show that large-scale isothermal-isobaric molecular dynamics is stable and accurate, showing negligible drift over picoseconds of dynamics.

Vacancy-type defects in GaN can migrate and aggregate during post-growth annealing. Here, we perform large-scale first-principles calculations to clarify the stability of (VGaVN)n aggregates (n = 1–6) in bulk GaN. For several structure models at each n, the defect formation energy of (VGaVN)n and the energy gain for the aggregation of (VGaVN)n−1 and VGaVN are calculated to investigate the stability during the stepwise growth. The calculated results show that the aggregation is always favorable, and the energy gain for this process is significantly large at n = 3 and 6. This result is consistent with the recently reported positron annihilation experiments. The structural stability and the electronic structure of the various models are discussed with respect to the number of dangling bonds and lattice relaxations, depending on the morphology of (VGaVN)n.

Stability and electronic structure of (0001) interfaces between inversion domains in GaN with Mg incorporation are investigated by first-principles calculations. We consider basic models of the interface consisting of 1.5 layers of Mg, as well as several different models made from basic models. When one Mg atom moves away from the interface of the basic model, it is revealed that the system becomes less stable as the distance increases. The same tendency is also valid for other stacking models. Although the electronic structure of the basic model shows no significant contributions from Mg atoms in the valence band top region, it is found that features near the valence band maximum characteristic of Mg in bulk GaN appear when the displaced Mg atom is two or three layers away from the interface, as in the case of a single Mg atom in bulk GaN.

We theoretically study the piezomagnetic effect on the altermagnetic state in $κ$-type molecular conductors, focusing on its nonrelativistic mechanism. By introducing shear stress as a monoclinic distortion, we evaluate variations in the effective tight-binding model using first-principles calculations. Using the derived parameters, we investigate the Hubbard model and its effective Heisenberg model on the two-dimensional (distorted) $κ$-type lattice within mean-field approximation. We show that the system exhibits the piezomagnetic effect, i.e., a net magnetization induced at finite temperatures in the undoped insulating state and both in the ground state and at finite temperatures upon doping. In a real-space picture, this uniform magnetization arises from the ferrimagnetic spin structure due to inequivalent spin sites induced by lattice distortion. Meanwhile, in a momentum-space picture, it stems from the {\it s}-wave spin splitting of the electron and magnon bands, independent of spin-orbit coupling. We find that this nonrelativistic piezomagnetism remains finite, but becomes smaller in the limit of strong dimerization where the energy gap between the bonding and antibonding orbitals is infinitely large and the {\it d}-wave altermagnetic spin splitting is absent, highlighting the importance of the multi-orbital nature.

The molecular triangular lattice system, β^{'}-EtMe_{3}Sb[Pd(dmit)_{2}]_{2}, is considered as a candidate material for the quantum spin liquid state, although ongoing debates arise from recent controversial results. Here, the results of electron spin resonance and muon-spin relaxation measurements on β^{'}-EtMe_{3}Sb[Pd(dmit)_{2}]_{2} are presented. Both results indicate characteristic behaviors related to quasi-one-dimensional spin dynamics, whereas the direction of anisotropy found in electron spin resonance is in contradiction with previous theories. We succeed in interpreting the experiments by combining density-functional theory calculations and analysis of the effective model taking into account the multiorbital nature of the system. While the quantum-spin-liquid-like origin of β^{'}-EtMe_{3}Sb[Pd(dmit)_{2}]_{2} was initially attributed to the magnetic frustration of the triangular lattice, it appears that the primary origin is a 1D spin liquid resulting from the dimensional reduction effect.

Size control of Pt nanoparticle catalysts is one of the key challenges in fuel cell development. Miniaturization of Pt nanoparticles is expected to increase activity and save the amount of Pt, while very small particles (e.g. sub-nano particles or clusters) have different electronic structures and stabilities from “nano” particles. To investigate the atomic and electronic structures of materials, first-principles density functional theory (DFT) calculation is a powerful tool. However, because of its high computational cost, the DFT applications have been limited mainly for small particles and clusters of tens to hundreds of atoms. In this study, by using our own large-scale DFT calculation method, we investigate the size and site dependences of atomic and electronic structures in Pt nanoparticles with the diameters of several nanometers, which are comparable to the sizes of the particles in practical use. The computational cost of the conventional DFT calculation methods scale cubically to the system size, i.e., the number of the atoms in the target system and the number of local functions for each atom, which are used to express electronic density. We have developed the multi-site support function (MSSF) method [1,2], in which local functions are constructed as the linear combinations of the atomic orbital functions belonging to not only the target atom but also its neighboring atoms. The MSSF method enables us to reduce the number of local functions to be the minimal-basis size, leading the dramatical computational cost reduction. The MSSF method is implemented in our large-scale DFT code CONQUEST [3,4]. We optimized the structures of Pt nanoparticles with the diameters of 0.5 nm (13 atoms) - 5.5 nm (3871 atoms) and found that the Pt-Pt bond lengths inside the nanoparticle converge to those in bulk Pt when the nanoparticle size becomes large. The similar trend is found also in the electronic structures as shown in the figure. In the presentation, we compare the size dependence of the stability and atomic and electronic structures of Pt nanoparticle with other metallic nanoparticles. Site dependence of the electronic structures and reactivity with molecules are also investigated by using large-scale DFT together with statistical analysis. [1] A. Nakata, D. R. Bowler, T. Miyazaki, J. Phys. Soc. Jpn., 91, 091011 (2022). [2] A. Nakata, D. R. Bowler, T. Miyazaki, Phys. Chem. Chem. Phys. 17, 31427 (2015). [3] http://www.order-n.org/ [4] A. Nakata, J. S. Baker, S. Y. Mujahed, J. T. L. Poulton, S. Arapan, J. Lin, Z. Raza, S. Yadav, L. Truflandier, T. Miyazaki, D. R. Bowler, J. Chem. Phys., 152, 164112 (2020). Figure 1