First-Principles Study of the Optical Dipole Trap for Two-Dimensional Excitons in Graphane.

Recent studies on excitons in two-dimensional materials have been widely conducted for their potential usages for novel electronic and optical devices. Especially, sophisticated manipulation techniques of quantum degrees of freedom of excitons are in demand. In this Letter we propose a technique of forming an optical dipole trap for excitons in graphane, a two-dimensional wide gap semiconductor, based on first-principles calculations. We develop a first-principles method to evaluate the transition dipole matrix between excitonic states and combine it with the density functional theory and GW+BSE calculations. We reveal that in graphane the huge exciton binding energy and the large dipole moments of Wannier-like excitons enable us to induce the dipole trap of the order of meV depth and µm width. This Letter opens a new way to control light-exciton interacting systems based on newly developed numerically robust ab initio calculations.

A noise-robust data assimilation method for crystal structure determination using powder diffraction intensity.

Crystal structure prediction for a given chemical composition has long been a challenge in condensed-matter science. We have recently shown that experimental powder x-ray diffraction (XRD) data are helpful in a crystal structure search using simulated annealing, even when they are insufficient for structure determination by themselves [Tsujimoto et al., Phys. Rev. Mater. 2, 053801 (2018)]. In the method, the XRD data are assimilated into the simulation by adding a penalty function to the physical potential energy, where a crystallinity-type penalty function, defined by the difference between experimental and simulated diffraction angles was used. To improve the success rate and noise robustness, we introduce a correlation-coefficient-type penalty function adaptable to XRD data with significant experimental noise. We apply the new penalty function to SiO2 coesite and ɛ-Zn(OH)2 to determine its effectiveness in the data assimilation method.

Quartic Anharmonicity of Rattlers and Its Effect on Lattice Thermal Conductivity of Clathrates from First Principles.

We investigate the role of the quartic anharmonicity in the lattice dynamics and thermal transport of the type-I clathrate Ba_{8}Ga_{16}Ge_{30} based on ab initio self-consistent phonon calculations. We show that the strong quartic anharmonicity of rattling guest atoms causes the hardening of vibrational frequencies of low-lying optical modes and thereby affects calculated lattice thermal conductivities κ_{L} significantly, resulting in an improved agreement with experimental results including the deviation from κ_{L}∝T^{-1} at high temperature. Moreover, our static simulations with various different cell volumes shows a transition from crystal-like to glasslike κ_{L} around 20 K. Our analyses suggest that the resonance dip of κ_{L} observed in clathrates with large guest free spaces is attributed mainly to the strong three-phonon scattering of acoustic modes along with the presence of higher-frequency dispersive optical modes.

Neural-network Kohn-Sham exchange-correlation potential and its out-of-training transferability.

We incorporate in the Kohn-Sham self-consistent equation a trained neural-network projection from the charge density distribution to the Hartree-exchange-correlation potential n → VHxc for a possible numerical approach to the exact Kohn-Sham scheme. The potential trained through a newly developed scheme enables us to evaluate the total energy without explicitly treating the formula of the exchange-correlation energy. With a case study of a simple model, we show that the well-trained neural-network VHxc achieves accuracy for the charge density and total energy out of the model parameter range used for the training, indicating that the property of the elusive ideal functional form of VHxc can approximately be encapsulated by the machine-learning construction. We also exemplify a factor that crucially limits the transferability-the boundary in the model parameter space where the number of the one-particle bound states changes-and see that this is cured by setting the training parameter range across that boundary. The training scheme and insights from the model study apply to more general systems, opening a novel path to numerically efficient Kohn-Sham potential.

Correlated Band Structure of a Transition Metal Oxide ZnO Obtained from a Many-Body Wave Function Theory.

Obtaining accurate band structures of correlated solids has been one of the most important and challenging problems in first-principles electronic structure calculation. There have been promising recent active developments of wave function theory for condensed matter, but its application to band-structure calculation remains computationally expensive. In this Letter, we report the first application of the biorthogonal transcorrelated (BITC) method: self-consistent, free from adjustable parameters, and systematically improvable many-body wave function theory, to solid-state calculations with d electrons: wurtzite ZnO. We find that the BITC band structure better reproduces the experimental values of the gaps between the bands with different characters than several other conventional methods. This study paves the way for reliable first-principles calculations of the properties of strongly correlated materials.

Universal two-dimensional characteristics in perovskite-type oxyhydrides ATiO2H (A = Li, Na, K, Rb, Cs).

A series of unsynthesized perovskite-type oxyhydrides ATiO2H (A = Li, Na, K, Rb, Cs) are investigated by the density functional calculations. These oxyhydrides are stable in the sense of the formation energies for some possible synthesis reactions. They are crystallized into quite similar crystal structures with the long c-axis, and the corner-sharing TiO4H2 octahedra of the ideal perovskite-type structure are deformed into the 5-fold coordinated titanium atoms with the OH plane and the apical oxygen atoms. All of these oxyhydrides exhibit two-dimensional electronic states at the valence band maximum characterized by the in-plane oxygen 2p and the hydrogen 1s orbitals. While the c-axis becomes short as the ionic radius of the A atom becomes small and the two-dimensional characteristics are weakened, the electronic state at the valence band maximum is still characterized as the O-H in-plane state. Additionally, the Born effective charge tensors, spontaneous electric polarizations, dielectric tensors, and piezoelectric tensors are evaluated. It is found that the spontaneous electric polarizations of these oxyhydrides are much larger than that of tetragonal BaTiO3.

Possible "Magnéli" Phases and Self-Alloying in the Superconducting Sulfur Hydride.

We theoretically give an infinite number of metastable crystal structures for the superconducting sulfur hydride H_{x}S under pressure. Previously predicted crystalline phases of H_{2}S and H_{3}S have been thought to have important roles for experimentally observed low and high T_{c}, respectively. The newly found structures are long-period modulated crystals where slablike H_{2}S and H_{3}S regions intergrow on a microscopic scale. The extremely small formation enthalpy for the H_{2}S-H_{3}S boundary indicated by first-principles calculations suggests possible alloying of these phases through the formation of local H_{3}S regions. The modulated structures and gradual alloying transformations between them not only explain the peculiar pressure dependence of T_{c} in sulfur hydride observed experimentally, but also could prevail in the experimental samples under various compression schemes.

Iterative diagonalization of the non-Hermitian transcorrelated Hamiltonian using a plane-wave basis set: Application to sp-electron systems with deep core states.

We develop an iterative diagonalization scheme in solving a one-body self-consistent-field equation in the transcorrelated (TC) method using a plane-wave basis set. Non-Hermiticity in the TC method is well handled with a block-Davidson algorithm. We verify that the required computational cost is efficiently reduced by our algorithm. In addition, we apply our plane-wave-basis TC calculation to some simple sp-electron systems with deep core states to elucidate an impact of the pseudopotential approximation to the calculated band structures. We find that a position of the deep valence bands is improved by an explicit inclusion of core states, but an overall band structure is consistent with a regular setup that includes core states into the pseudopotentials. This study offers an important understanding for the future application of the TC method to strongly correlated solids.

Impact of rattlers on thermal conductivity of a thermoelectric clathrate: a first-principles study.

We investigate the role of rattling guest atoms on the lattice thermal conductivity of a type-I clathrate Ba_{8}Ga_{16}Ge_{30} by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause tenfold reductions in the relaxation time of phonons by increasing the phonon-phonon scattering probability. Contrary to the resonant scattering scenario, the reduction in the relaxation time occurs in a wide frequency range, which is crucial for explaining the unusually low thermal conductivities of clathrates. We also find that the impact of rattlers on the group velocity of phonons is secondary because the flattening of phonon dispersion occurs only in a limited phase space in the Brillouin zone.

Optimization of the Jastrow factor using the random-phase approximation and a similarity-transformed Hamiltonian: application to band-structure calculation for some semiconductors and insulators.

Based on the random-phase approximation and the transcorrelated (TC) method, we optimize the Jastrow factor together with one-electron orbitals in the Slater determinant in the correlated wave function with a new scheme for periodic systems. The TC method is one of the promising wave function theories for first-principles electronic structure calculation, where the many-body wave function is approximated as a product of a Slater determinant and a Jastrow factor, and the Hamiltonian is similarity-transformed by the Jastrow factor. Using this similarity-transformed Hamiltonian, we can optimize the one-electron orbitals without evaluating 3N-dimensional integrations for the N-electron system. In contrast, optimization of the Jastrow factor within the framework of the TC method is computationally much more expensive and has not been performed for solid-state calculations before. In this study, we also benefit from the similarity-transformation in optimizing the Jastrow factor. Our optimization scheme is tested in applications to some solids from narrow-gap semiconductors to wide-gap insulators, and it is verified that the band gap of a wide-gap insulator and the lattice constants of some solids are improved by this optimization with reasonable computational cost.

Trimer effects in fragment molecular orbital-linear combination of molecular orbitals calculation of one-electron orbitals for biomolecules.

The fragment molecular orbital (FMO)-linear combination of molecular orbitals (LCMO) method incorporates as an efficient post-process calculation of one-electron orbitals of the whole system after the FMO total energy calculation. A straightforward way to increase the accuracy is inclusion of the trimer effect. Here, we derive a comprehensive formulation called the FMO3-LCMO method. To keep the computational costs of the trimer term low enough, we use a matrix-size reduction technique. We evaluated the accuracy and efficiency of the FMO3-LCMO scheme in model biological systems (alanine oligomer and chignolin). The results show that delocalized electronic orbitals with covalent and hydrogen bonds are better described at the trimer level, and the FMO3-LCMO method is applicable to quantitative evaluations of a wide range of frontier orbitals in large biosystems.

Efficient algorithm of the transcorrelated method for periodic systems.

The transcorrelated (TC) method is one of the promising wave-function-based approaches for the first-principles electronic structure calculations. In this method, the many-body wave function is approximated as the Jastrow-Slater type and one-electron orbitals in the Slater determinant are optimized with a one-body self-consistent-field equation such as that in the Hartree-Fock (HF) method. Although the TC method has yielded good results for both molecules and solids, its computational cost in solid-state calculations, being of order O(N(k)(3)N(b)(3)) with N(k) and N(b) the respective numbers of k-points and bands, has for some years hindered its wide application in condensed matter physics. Although an efficient algorithm was proposed for a Gaussian basis set, that algorithm is not applicable to a plane-wave basis that is suited to and widely used in solid-state calculations. In this paper, we present a new efficient algorithm of the TC method for the plane-wave basis or an arbitrary basis function set expanded in terms of plane waves, with which the computational cost of the TC method scales as O(N(k)(2)N(b) (2)). This is the same as that of the HF method. We applied the TC method with the new algorithm to obtain converged band structure and cell parameters of some semiconductors.

Development of the temperature-dependent interatomic potential for molecular dynamics simulation of metal irradiated with an ultrashort pulse laser.

Laser ablation is often explained by a two-temperature model (TTM) with different electron and lattice temperatures. To realize a classical molecular dynamics simulation of the TTM, we propose an extension of the embedded atom method to construct an interatomic potential that is dependent on the electron temperature. This method is applied to copper, and its validity is demonstrated by comparison of several physical properties, such as the energy-volume curve, phonon dispersion, electronic heat capacity, ablation threshold, and mean square displacement of atoms, with those of finite-temperature density functional theory.

Two-electron reduction of a Rh-Mo-Rh dithiolato complex to form a triplet ground state associated with a change in CO coordination mode.

We synthesized a dithiolato-bridged heterometal trinuclear complex [{(eta(5)-C(5)Me(5))Rh(S(2)C(6)H(4))}(2)Mo(CO)(2)] (1) in which two rhodadithiolene complex units are bridged by a Mo(CO)(2) moiety. Complex 1 with a Rh(III)-Mo(0)-Rh(III) bond exhibits reversible one-step two-electron reduction with potential inversion. This redox process between 1 and 1(2-) accompanies a reversible structural change, which is an alternation in the CO coordination mode between semibridging and bridging. The ground state of dianion 1(2-) with a Rh(II)-Mo(0)-Rh(II) bond is assigned to spin triplet. These alternations of CO coordination mode and spin state are fully consistent with the density functional theory calculation results. This is the first example of multinuclear metalladithiolene complex which was successful in elucidating a reversible multielectron redox process associated with structural change and spin state change.

Ferromagnetism above 1000 K in a highly cation-ordered double-perovskite insulator Sr3OsO6.

Magnetic insulators have wide-ranging applications, including microwave devices, permanent magnets and future spintronic devices. However, the record Curie temperature (TC), which determines the temperature range in which any ferri/ferromagnetic system remains stable, has stood still for over eight decades. Here we report that a highly B-site ordered cubic double-perovskite insulator, Sr3OsO6, has the highest TC (of ~1060 K) among all insulators and oxides; also, this is the highest magnetic ordering temperature in any compound without 3d transition elements. The cubic B-site ordering is confirmed by atomic-resolution scanning transmission electron microscopy. The electronic structure calculations elucidate a ferromagnetic insulating state with Jeff = 3/2 driven by the large spin-orbit coupling of Os6+ 5d2 orbitals. Moreover, the Sr3OsO6 films are epitaxially grown on SrTiO3 substrates, suggesting that they are compatible with device fabrication processes and thus promising for spintronic applications.

Interfacial transmetallation synthesis of a platinadithiolene nanosheet as a potential 2D topological insulator.

The construction of two-dimensional metal complex materials is fascinating because of the structural and functional diversity of these materials. Previously, we have reported the synthesis of electroconductive nickelladithiolene (NiDT) and palladadithiolene (PdDT) nanosheets using benzenehexathiol (BHT). Down the group from Ni, Pd to Pt, there is a distinct positive shift in the reduction potential; as a result, it becomes synthetically more challenging to stabilize Pt2+ than to form metallic Pt(0) in the presence of BHT as a reducing agent. Herein, a novel synthetic strategy for the preparation of platinadithiolene nanosheet (PtDT) using a dibutyltin-protected BHT ligand is reported, leading to transmetallation in the presence of dioxygen. Both free-standing stacked sheets and atomic layer sheets were obtained and characterized by microscopic techniques such as AFM, SEM, and TEM. To study the morphology of the sheets and determine their charge neutrality, X-ray photoelectron (XP) and infrared (IR) spectroscopic techniques were used. Powder X-ray diffraction analysis of the multilayer PtDT indicates a half-way slipped hexagonal configuration in the P3[combining macron]1m space group. The band structure of this PtDT exhibits a band gap at the Fermi level, which is different from that of NiDT in the staggered configuration, and a Dirac gap, indicating the possibility of 2D topological insulation at room temperature. PtDT is insulating but chemically activated by oxidation with I2 to increase the conductivity by more than 106 folds up to 0.39 S cm-1. The MDT sheets exhibit electrocatalytic activity for the hydrogen evolution reaction, and the activity order is NiDT < PdDT < PtDT.

Optical Absorption Spectra Calculated from a First-Principles Wave Function Theory for Solids: Transcorrelated Method Combined with Configuration Interaction Singles.

We present a new ab initio method for excited-state calculations based on wave function theory: transcorrelated (TC) method combined with configuration interaction singles (CIS). Conventional CIS with the Hartree-Fock wave function, while a popular method for excited-state calculations of molecular systems, cannot describe electron correlation effects, such as the screening effect in solids, resulting in inaccurate results such as overestimation of the band gap and exciton binding energy. Here, we adopt the TC method, which takes electron correlations into account through a similarity transformation of the Hamiltonian using the Jastrow factor, and combine it with the CIS approximation. We calculate the optical absorption spectra of solid LiF and GaAs as a test, and verify that the present method reproduces the spectra more accurately than the conventional HF-CIS. The excitonic effect is well described with our method.

Anomalous phase diagram of ferroelectric (Ba,Ca)TiO3 single crystals with giant electromechanical response.

We report the anomalous phase evolution in ferroelectric single crystals Ba1-xCaxTiO3 (0.02

Regioselective cycloaddition reaction of alkene molecules with the asymmetric dimer on Si(100)c(4 x 2).

We investigated the adsorption states of 2-methylpropene and propene on Si(100)c(4 x 2) using low-temperature scanning tunneling microscopy. We have found that regioselective cycloaddition reactions (di-sigma bond formation) occur between the asymmetric alkene molecules and the asymmetric dimers on Si(100)c(4 x 2). First-principles calculations have elucidated that the regioselectivity is closely related to the structures of precursor species and these precursor species have carbocation-like features. Thus, we conclude that Markovnikov's rule is applicable for the cycloaddition of asymmetric alkene with the asymmetric dimer on Si(100)c(4 x 2).

Effect of through-space electron transfer on infrared spectrum of amorphous selenium.

In this paper we present theoretical analyses on an infrared (IR) spectrum of amorphous selenium. The system is described by a 216-atom-chain model, and a set of molecular-dynamics simulations is performed to generate vitreous structures and vibrational modes. To describe an electronic structure of the system we employ a complete neglect of differential overlap model parametrized by ab initio cluster calculations. An IR intensity is evaluated with the Berry-phase formula for an electronic polarization. The effect of the through-space electron transfer on the IR spectrum is studied by artificially changing the magnitude of matrix elements associated with the electron transfer between nonbonded atoms in the chain. We find that the through-space electron transfer leads to (i) the enhancement of the bending IR peak at 135 cm(-1) and (ii) the appearance of a new low-frequency peak around 50 cm(-1), thus resulting in a good agreement with the experiment. The mechanism is discussed by a simple dipole model.