arXiv subjects

Yu-ichiro Matsushita

Publications and source records attributed to Yu-ichiro Matsushita.

At least 55 records · Page 3Linked to original sources

arXiv2023

Atomic scale localization of Kohn-Sham wavefunction at SiO2/4H-SiC interface under electric field, deviating from envelope function by effective mass approximation

To clarify the cause of the low channel conductivity at the SiO2/4H-SiC interface, the wavefunction at the SiC conduction band minimum was calculated using density functional theory under an applied electric field. We found that the wavefunction for a 4H-SiC (0001) slab tends to be localized at the cubic site closest to the interface. Importantly, because the conduction electrons are distributed closer to the interface (< 5 angstroms) than expected from the effective mass approximation (EMA), they are more frequently scattered by interface defects. This is expected to be the reason why the channel conductivity for the (0001) face is particularly low compared with that for other faces, such as (11-20). The breakdown of the EMA for the (0001) interface is related to the long structural periodicity along the [0001] direction in 4H-SiC crystals.

cond-mat.mtrl-sci↗

arXiv2022

Acceleration of probabilistic imaginary-time evolution method combined with quantum amplitude amplification

A probabilistic imaginary-time evolution (PITE) method was proposed as a nonvariational method to obtain a ground state on a quantum computer. In this formalism, the success probability of obtaining all imaginary-time evolution operators acting on the initial state decreases as the imaginary time proceeds. To alleviate the undesirable nature, we propose quantum circuits for PITE combined with the quantum amplitude amplification (QAA) method. We reduce the circuit depth in the combined circuit with QAA by introducing a pre-amplification operator. We successfully demonstrated that the combination of PITE and QAA works efficiently and reported a case in which the quantum acceleration is achieved. Additionally, we have found that by optimizing a parameter of PITE, we can reduce the number of QAA operations and that deterministic imaginary-time evolution (deterministic ITE) can be achieved which avoids the probabilistic nature of PITE. We applied the deterministic ITE procedure to multiple imaginary-time steps and discussed the computational cost for the circuits. Finally, as an example, we demonstrate the numerical results of the PITE circuit combined with QAA in the first- and second-quantized Hamiltonians.

quant-ph↗

arXiv2022

Systematic study on the dependence of the warm-start quantum approximate optimization algorithm on approximate solutions

Quantum approximate optimization algorithm (QAOA) is a promising hybrid quantum-classical algorithm to solve combinatorial optimization problems in the era of noisy intermediate-scale quantum computers. Recently warm-start approaches have been proposed to improve the performance of QAOA, where approximate solutions are obtained by classical algorithms in advance and incorporated into the initial state and/or unitary ansatz. In this work, we study in detail how the accuracy of approximate solutions affect the performance of the warm-start QAOA (WS-QAOA). We numerically find that in typical MAX-CUT problems, WS-QAOA tends to outperform QAOA as approximate solutions become closer to the exact solutions in terms of the Hamming distance. We reveal that this could be quantitatively attributed to the initial state of the ansatz. We also solve MAX-CUT problems by WS-QAOA with approximate solutions obtained via QAOA, having a better result than QAOA especially when the circuit is relatively shallow. We believe that our study may deepen understanding of the performance of WS-QAOA and also provide a guide as to the necessary quality of approximate solutions.

quant-ph↗

arXiv2022

Skyrmions in van der Waals centrosymmetric materials with Dzyaloshinskii-Moriya interactions

Skyrmions can appear in non-centrosymmetric material because of the non-vanishing Dzyaloshinskii-Moriya interactions (DMI). In this paper, we study the magnetic properties of the rhombohedral MX$_{3}$ (M: V, Cr, Mn, and Fe. X: Cl, Br, and I) van der Waals materials with centrosymmetric lattice by combining first-principles calculations and Monte Carlo simulations. We found that the Dzyaloshinskii-Moriya vector acting between the second nearest neighbor sites of the intra-layer is non-zero and quite large even in MX$_{3}$ due to the breaking of the local inversion symmetry. We have found that the large DMI causes nano-scale magnetic vortexes, so-called skyrmions in MX$_{3}$. The diameter of skyrmions, e.g., in CrCl$_{3}$, is small, i.e., 2 nm, which is promising for the application of high-density magnetic memory devices. We have also found that not only conventional skyrmions in CrCl$_{3}$ and VCl$_{3}$ but also antiferromagnetic skyrmions in FeCl$_{3}$ and meron in MnCl$_{3}$ appear. Furthermore, we have found that the skyrmions in ClCl$_{3}$ and VCl$_{3}$ have a different helicity, which shows the possibility of controlling the helicity by electron/hole doping in MX$_{3}$ materials. Van der Waals materials, which have a great advantage of high degrees of freedom in structures such as heterostructures and twisted structures, exhibit high potential as skyrmion materials.

cond-mat.mtrl-sci↗

arXiv2022

Probabilistic imaginary-time evolution by using forward and backward real-time evolution with a single ancilla: first-quantized eigensolver of quantum chemistry for ground states

Imaginary-time evolution (ITE) on a quantum computer is a promising formalism for obtaining the ground state of a quantum system. As a kind of it, the probabilistic ITE (PITE) takes advantage of measurements to implement the nonunitary operations. We propose a new approach of PITE which requires only a single ancillary qubit. Under a practical approximation, the circuit is constructed from the forward and backward real-time evolution (RTE) gates as black boxes, generated by the original many-qubit Hamiltonian. All the efficient unitary quantum algorithms for RTE proposed so far and those in the future can thus be transferred to ITE exactly as they are. Our approach can also be used for obtaining the Gibbs state at a finite temperature and the partition function. We apply the approach to several systems as illustrative examples to see its validity. We also discuss the application of our approach to quantum chemistry by focusing on the scaling of computational cost, leading to a novel framework denoted by first-quantized quantum eigensolver.

quant-ph↗

arXiv2022

Dzyaloshinskii-Moriya interaction in Nd$_{2}$Fe$_{14}$B as the origin of spin reorientation and rotating magnetocaloric effect

The mechanism of spin reorientation in Nd$_{2}$Fe$_{14}$B, which is a host crystal of a well-known neodymium permanent magnet, is studied by combining first-principles calculations and Monte Carlo simulations. The spin reorientation is thought to be derived from crystal field effects and gets less attention because of the undesirable property for hard magnet application. Dzyaloshinskii-Moriya interactions are usually less attractive or often ignored in rare-earth bulk systems, including permanent magnets such as Nd$_{2}$Fe$_{14}$B since people believe that the magnetic anisotropy is more dominant than the Dzyaloshinskii-Moriya interactions. However, in this study, we have found, for the first time, that the spin reorientation in Nd$_{2}$Fe$_{14}$B is attributed to Dzyaloshinskii-Moriya interactions. We have found, furthermore, the spin reorientation in Nd$_{2}$Fe$_{14}$B yields a great stage of rotating magnetocaloric effect at practical application level. We have found that the Dzyaloshinskii-Moriya interactions definitely contributes to the physical properties as a non-negligible effect in magnetic materials.

cond-mat.mtrl-sci↗

arXiv2021

Effect of magnetocrystalline anisotropy on magnetocaloric properties of AlFe$_{2}$B$_{2}$ compound

It is well known that the temperature dependence of the effective magnetocrystalline anisotropy energy obeys the $l(l+1)/2$ power law of magnetization in the Callen-Callen theory. Therefore, according to the Callen-Callen theory, the magnetocrystalline anisotropy energy is assumed to be zero at the critical temperature where the magnetization is approximately zero. This study estimates the temperature dependence of the magnetocrystalline anisotropy energy by integrating the magnetization versus magnetic field ($M$--$H$) curves, and found that the magnetocrystalline anisotropy is still finite even above the Curie temperature in the uniaxial anisotropy, whereas this does not appear in the cubic anisotropy case. The origin is the fast reduction of the anisotropy field, which is the magnetic field required to saturate the magnetization along the hard axis, in the case of cubic anisotropy. Therefore, the magnetization anisotropy and anisotropic magnetic susceptibility, those are the key factors of magnetic anisotropy, could not be established in the case of cubic anisotropy. In addition, the effect of magnetocrystalline anisotropy on magnetocaloric properties, as the difference between the entropy change curves of AlFe$_{2}$B$_{2}$ appears above the Curie temperature, which is in good agreement with a previous experimental study. This is proof of magnetic anisotropy at slightly above Curie temperature.

cond-mat.mtrl-sci↗

arXiv2021

Anisotropic magnetocaloric effect of CrI$_{3}$: A theoretical study

CrI$_{3}$ is considered to be a promising candidate for spintronic devices and data storage. We derived the Heisenberg Hamiltonian for CrI$_{3}$ from density functional calculations using the Liechtenstein formula. Moreover, the Monte--Carlo simulations with the Sucksmith--Thompson method were performed to analyze the effect of magnetic anisotropy energy on the thermodynamic properties. Our method successfully reproduced the negative sign of isothermal magnetic entropy changes when a magnetic field was applied along the hard plane. We found that the temperature dependence of the magnetocrystalline anisotropy energy is not negligible at temperatures slightly above the Curie temperature. We clarified that the origin of this phenomenon is attributed to anisotropic magnetic susceptibility and magnetization anisotropy. The difference between the entropy change of the easy axis and the hard plane is proportional to the temperature dependence of the magnetic anisotropy energy, implying that the anisotropic entropy term is the main source of the temperature dependence of the free energy difference when magnetizing in a specific direction other than the easy axis. We also investigated the magnetic susceptibility that can be used for the characterization of the negative sign of the entropy change in the case of a hard plane. The competition of magnetocrystalline anisotropy energy and external magnetic field at low temperature and low magnetic field region causes a high magnetic susceptibility as the fluctuation of magnetization. Meanwhile, the anisotropy energy is suppressed at a sufficient magnetic field applied along the hard axis, the magnetization is fully rotated to the direction of the external magnetic field.

cond-mat.mtrl-sci↗

arXiv2020

Implementation of quantum imaginary-time evolution method on NISQ devices: Nonlocal approximation

The imaginary-time evolution method is widely known to be efficient for obtaining the ground state in quantum many-body problems on a classical computer. A recently proposed quantum imaginary-time evolution method (QITE) faces problems of deep circuit depth and difficulty in the implementation on noisy intermediate-scale quantum (NISQ) devices. In this study, a nonlocal approximation is developed to tackle this difficulty. We found that by removing the locality condition or local approximation (LA), which was imposed when the imaginary-time evolution operator is converted to a unitary operator, the quantum circuit depth is significantly reduced. We propose two-step approximation methods based on a nonlocality condition: extended LA (eLA) and nonlocal approximation (NLA). To confirm the validity of eLA and NLA, we apply them to the max-cut problem of an unweighted 3-regular graph and a weighted fully connected graph; we comparatively evaluate the performances of LA, eLA, and NLA. The eLA and NLA methods require far fewer circuit depths than LA to maintain the same level of computational accuracy. Further, we developed a ``compression'' method of the quantum circuit for the imaginary-time steps as a method to further reduce the circuit depth in the QITE method. The eLA, NLA, and the compression method introduced in this study allow us to reduce the circuit depth and the accumulation of error caused by the gate operation significantly and pave the way for implementing the QITE method on NISQ devices.

quant-ph↗

arXiv2020

Charge and spin response functions on a quantum computer: applications to molecules

We propose a scheme for the construction of charge and spin linear-response functions of an interacting electronic system via quantum phase estimation and statistical sampling on a quantum computer. By using the unitary decomposition of electronic operators for avoiding the difficulty due to their non-unitarity, we provide the circuits equipped with ancillae for probabilistic preparation of qubit states on which the necessary non-unitary operators have acted. We perform simulations of such construction of the response functions for C2 and N2 molecules by comparing with the accurate ones based on the full configuration interaction calculations. It is found that the accurate detection of subtle structures coming from the weak poles in the response functions requires a large number of measurements.

quant-ph↗

arXiv2020

Construction of Green's functions on a quantum computer: applications to molecular systems

We propose a scheme for the construction of one-particle Green's function (GF) of an interacting electronic system via statistical sampling on a quantum computer. Although the non-unitarity of creation and annihilation operators for the electronic spin orbitals prevents us from preparing specific states selectively, probabilistic state preparation is demonstrated to be possible for the qubits. We provide quantum circuits equipped with at most two ancillary qubits for obtaining all the components of GF. We perform simulations of such construction of GFs for LiH and H$_2$O molecules based on the unitary coupled-cluster (UCC) method to demonstrate the validity of our scheme by comparing the spectra exact within UCC and those from full configuration interaction calculations. We also examine the accuracy of sampling method by exploiting the Galitskii--Migdal formula, which gives the total energy only from the GF.

quant-ph↗

arXiv2019

A Parallel Computing Method for the Coupled-Cluster Singles and Doubles

In this paper, we present a parallel computing method for the coupled-cluster singles and doubles (CCSD) in periodic systems. The CCSD in periodic systems solves simultaneous equations for single-excitation and double-excitation amplitudes. In the simultaneous equations for double-excitation amplitudes, each equations are characterized by four spin orbitals and three independent momentums of electrons. One of key ideas of our method is to use process numbers in parallel computing to identify two indices which represent momentum of an electron. When momentum of an electron takes $N_{\bm{k}}$ values, $N_{\bm{k}}^2$ processes are prepared in our method. Such parallel distribution processing and way of distribution of known quantities in the simultaneous equations reduces orders of computational cost and required memory scales by $N_{\bm{k}}^2$ compared with a sequential method. In implementation of out method, communication between processes in parallel computing appears in the outmost loop in a nested loop but does not appear inner nested loop.

cond-mat.str-el↗

arXiv2019

Energetics and electronic structure of native point defects in $α$-Ga2O3

We report first-principles calculations that clarify the formation energies and charge transition levels of native point defects (Ga and O vacancies, interstitials, and a Ga vacancy-O vacancy pair) in corundum structured $α$-Ga2O3. Either under a Ga- or O-rich growth condition, the negatively-charged Ga vacancy and the positively-charged Ga interstitial on a site surrounded by six O atoms are dominant when the Fermi level approaches the conduction and valence band edges, respectively. These defects would compensate carrier electrons and holes, respectively. Ga-rich conditions relatively suppress the formation of the Ga vacancy and, therefore, are suited for extrinsic n-type doping of $α$-Ga2O3.

cond-mat.mtrl-sci↗

arXiv2019

Structure and energetics of carbon-related defects in SiC (0001)/SiO$_{\rm 2}$ systems revealed by first-principles calculations: Defects in SiC, SiO$_{\rm 2}$, and just at their interface

We report first-principles calculations that reveal the atomic forms, stability, and energy levels of carbon-related defects in SiC (0001)/SiO$_{\rm 2}$ systems. We clarify the stable position (SiC side, SiO$_{\rm 2}$ side, or just at the SiC/SiO$_{\rm 2}$ interface) of defects depending on the oxidation environment. Under an O-rich condition, the di-carbon antisite ((C$_{\rm 2}$)$_{\rm Si}$) in the SiC side is stable and critical for $n$-channel MOSFETs, whereas the di-carbon defect (Si-C-C-Si) at the interface becomes critical under an O-poor condition. Our results suggest that the oxidation of SiC under a high-temperature O-poor condition is favorable in reducing the defects, in consistent with recent experimental reports.

cond-mat.mtrl-sci↗

arXiv2019

Wannier interpolation of one-particle Green's functions from coupled-cluster singles and doubles (CCSD)

We propose two schemes for interpolation of the one-particle Green's function (GF) calculated within coupled-cluster singles and doubles (CCSD) method for a periodic system. They use Wannier orbitals for circumventing huge cost for a large number of sampled k points. One of the schemes is the direct interpolation, which obtains the GF straightforwardly by using Fourier transformation. The other is the self-energy-mediated interpolation, which obtains the GF via the Dyson equation. We apply the schemes to a LiH chain and trans-polyacetylene and examine their validity in detail. It is demonstrated that the direct-interpolated GFs suffer from numerical artifacts stemming from slow convergence of CCSD GFs in real space, while the self-energy-mediated interpolation provides more physically appropriate GFs due to the localized nature of CCSD self-energies. Our schemes are also applicable to other explicitly correlated methods capable of providing GFs.

cond-mat.mtrl-sci↗

arXiv2019

Native point defects and carbon clusters in 4H-SiC: A hybrid functional study

We report first-principles calculations that clarify the formation energies and charge transition levels of native point defects and carbon clusters in the 4H polytype of silicon carbide (4H-SiC) under a carbon-rich condition. We applied a hybrid functional that reproduces the experimental bandgap of SiC well and offers reliable defect properties. For point defects, we investigated single vacancies, antisites, and interstitials of Si and C on relevant sites. For carbon clusters, we systematically introduced two additional C atoms into the perfect 4H-SiC lattice with and without removing Si atoms and performed structural optimization to identify stable defect configurations. We found that neutral Si antisites are energetically favorable among Si-point defects in a wide range of the Fermi level position around the intrinsic regime, whereas negatively-charged Si vacancies and a positively-charged Si interstitial on a site surrounded by three Si and three C atoms become favorable under n- and p-type conditions, respectively. For C-point defects, neutral C antisites are favorable under intrinsic and n-type conditions, whereas positively-charged C vacancies become favorable under p-type conditions. We also found that a di-carbon antisite is more favorable than a C-split interstitial, which is the most stable form of single C interstitials.

cond-mat.mtrl-sci↗

arXiv2018

Structural stability and energy levels of carbon-related defects in amorphous SiO$_2$ and its interface with SiC

We report the density-functional calculations that systematically clarify the stable forms of carbon-related defects and their energy levels in amorphous SiO$_2$ using the melt-quench technique in molecular dynamics. Considering the position dependence of the O chemical potential near and far from the SiC/SiO$_2$ interface, we determine the most abundant forms of carbon-related defects: Far from the interface, the CO$_2$ or CO in the internal space in SiO$_2$ is abundant and they are electronically inactive; near the interface, the carbon clustering is likely and a particular mono-carbon defect and a di-carbon defect induce energy levels near the SiC conduction-band bottom, thus being candidates for the carrier traps.

cond-mat.mtrl-sci↗

arXiv2018

One-particle Green's function of interacting two electrons using analytic solutions for a three-body problem: comparison with exact Kohn--Sham system

For a three-electron system with finite-strength interactions confined to a one-dimensional harmonic trap, we solve the Schroedinger equation analytically to obtain the exact solutions, from which we construct explicitly the simultaneous eigenstates of the energy and total spin for the first time. The solutions for the three-electron system allow us to derive analytic expressions for the exact one-particle Green's function (GF) for the corresponding two-electron system. We calculate the GF in frequency domain to examine systematically its behavior depending on the electronic interactions. We also compare the pole structure of non-interacting GF using the exact Kohn--Sham (KS) potential with that of the exact GF to find that the discrepancy of the energy gap between the KS system and the original system is larger for a stronger interaction. We perform numerical examination on the behavior of GFs in real space to demonstrate that the exact and KS GFs can have shapes quite different from each other. Our simple model will help to understand generic characteristics of interacting GFs.

cond-mat.mes-hall↗