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We introduce a non-phenomenological framework based on the influence functional method to incorporate simultaneous interactions of particles with fermionic and bosonic thermal reservoirs. In the slow-motion limit, the electronic friction kernel becomes Markovian, enabling an analytical expression for the friction coefficient. The framework is applied to a prototypical electrochemical system, where the metal electrode and solvent act as fermionic and bosonic reservoirs, respectively. We investigate quantum vibrational relaxation of hydrogen on metal surfaces, showing that dissipation to electron-hole pairs reduces the relaxation time. Additionally, in solvated proton discharge, electronic friction prolongs charge transfer by delaying proton transitions between potential wells. This study provides new insights into the interplay of solvent and electronic dissipation effects, with direct relevance to electrochemical processes and other systems involving multiple thermal reservoirs.

We explore the possibility of obtaining new superconductor candidates from functionalized MXene compounds Zr$_2$NS$_2$, Zr$_2$NCl$_2$, and Sc$_2$NCl$_2$ based on $\textit{ab initio}$ calculations with density functional theory for superconductors (SCDFT). The predicted superconducting transition temperature $T_c$ at ambient pressure may reach up to 9.48 K (Zr$_2$NS$_2$). Strain-induced $T_c$ enhancement is demonstrated with an example case for Zr$_2$NCl$_2$. We note a correlation between the profiles of superconducting gap $\Delta$ and the electron-phonon coupling $\lambda$ across the Fermi surface for all compounds, which are likely influenced by their modified electronic bandstructure components., Comment: 7 pages, 5 figures. Corrections and improvements are made

We study a moving adsorbate interacting with a metal electrode immersed in a solvent using the time-dependent Newns-Anderson-Schmickler model Hamiltonian. We have adopted a semiclassical trajectory treatment of the adsorbate to discuss the electron and energy transfers that occur between the adsorbate and the electrode. Using the Keldysh Green's function scheme, we found a non-adiabatically suppressed electron transfer caused by the motion of the adsorbate and coupling with bath phonons that model the solvent. The energy is thus dissipated into electron-hole pair excitations, which are hindered by interacting with the solvent modes and facilitated by the applied electrode potential. The average energy transfer rate was discussed in terms of the electron friction coefficient and given an analytical expression in the slow-motion limit.

The magnetic phases of electron-doped infinite-layer Sr$_{1-x}$La$_x$CuO$_2$ are elucidated by first-principles density functional calculations. The antiferromagnetic parent state, metallic transition, as well as lattice evolution with doping and pressure are found to be consistent with experiments. The specific heat coefficient $\gamma$, magnetic exchange coupling $J$, as well as the density of states at Fermi level $N(0)$ of low-energy states with multiple magnetic configurations are investigated. We highlight a subset of such states in which we note an increase in $N(0)$ to suggest the interesting effects of magnetic fluctuations and La substitution on the electronic structure of this material., Comment: 11 pages, 10 figures. This arXiv update is self-prepared to look similar to its published version (see journal reference and DOI below). All citations should refer to the published version

Zr oxides and oxynitrides are promising candidates to replace precious metal cathodes in polymer electrolyte fuel cells. Oxygen reduction reaction activity in this class of materials has been correlated with the amount of oxygen vacancies, but a microscopic understanding of this correlation is still lacking. To address this, we simulate a defective Zr$_7$O$_8$N$_4$/H$_2$O interface model and compare it with a pristine ZrO$_2$/H$_2$O interface model. First, ab initio replica exchange Monte Carlo sampling was performed to determine defect segregation at the surface in the oxynitride slab model, then molecular dynamics accelerated by neural network potentials was used to perform 1000 of 500 ps-long simulations to attain sufficient statistical accuracy of the solid/liquid interface structure. The presence of oxygen vacancies on the surface was found to clearly modify the local adsorption structure: water molecules were found to adsorb preferentially on Zr atoms surrounding oxygen vacancies, but not on the oxygen vacancies themselves. The fact that oxygen vacancy sites are free from poisoning by water molecules may explain the activity enhancement in defective systems. The layering of water molecules was also modified considerably, which should influence the proton and O$_2$ transport near the interfaces which is another parameter that determines the overall activity., Comment: 64 pages, 30 figures

We perform first-principles calculation to study the electronic structure of HgBa$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{2n+2+x}$ copper oxides up to $n = 6$ for the undoped parent compound $(x = 0)$ and up to $n = 3$ for the doped compound $(x > 0)$ by means of the SCAN meta-GGA density functional. Our calculations predict an antiferromagnetic insulator ground state for the parent compounds with an energy gap that decreases with the number of CuO$_{2}$ planes. We report structural, electronic and magnetic order evolution with $x$ which agree with experiments. We find an enhanced density of states at Fermi level at $x \approx 0.25$ for the single-layered compound manifesting in a peak of the Sommerfeld parameter, which recently has been discussed as a possible signature of quantum criticality generic to all cuprates., Comment: (version 1): 28 pages, 15 figures; (version 2): 29 pages, 15 figures. Improved figure clarity. Corrected typos; (version 3): 29 pages, 15 figures. corrections in introduction and references section

Density functional theory is the standard theory for computing the electronic structure of materials, which is based on a functional that maps the electron density to the energy. However, a rigorous form of the functional is not known and has been heuristically constructed by interpolating asymptotic constraints known for extreme situations, such as isolated atoms and uniform electron gas. Recent studies have demonstrated that the functional can be effectively approximated using machine learning (ML) approaches. However, most ML models do not satisfy asymptotic constraints. In this study, by applying a novel ML model architecture, we demonstrate a neural network-based exchange-correlation functional satisfying physical asymptotic constraints. Calculations reveal that the trained functional is applicable to various materials with an accuracy higher than that of existing functionals, even for materials whose electronic properties are different from the properties of materials in the training dataset. Our proposed approach thus improves the accuracy and generalization performance of the ML-based functional by combining the advantages of ML and analytical modeling.

The bandgap of insulating materials is renormalized in various ways by the electron-phonon interaction owing to the dynamical and quantum fluctuations of nuclei. These fluctuation effects are considered in the perturbative Allen-Heine-Cardona theory using the formulae for the Fan-Migdal and Debye-Waller terms. However, the material dependence is not clear in the formulae. Thus, in this study, we focus on the analytical form of the Debye-Waller term and find that the term can be reformulated using the momentum matrix. In addition, the optical selection rule is found to play a role. For diamond-type materials, the Debye-Waller term can be approximately decomposed into a product of the optical transition energy, the mean square displacement of nuclei, and the dipole transition probability. The decomposition can also be applied with an additional approximation to zinc-blende-type materials, as revealed by our first-principles calculation. The magnitudes of the Debye-Waller term of several materials can thus be estimated using basic physical quantities prior to performing the calculation of the electron-phonon interaction.

The renormalization-group approaches for classical liquids in previous works require a repulsive reference such as a hard-core one when applied to systems with short-range repulsion. The need for the reference is circumvented here by using a functional renormalization group approach for integrating the hierarchical flow of correlation functions along a path of variable interatomic coupling. We introduce the cavity distribution functions to avoid the appearance of divergent terms and choose a path to reduce the error caused by the decomposition of higher order correlation functions. We demonstrate using an exactly solvable one-dimensional models that the resulting scheme yields accurate thermodynamic properties and interatomic distribution at various densities when compared to integral-equation methods such as the hypernetted chain and the Percus-Yevick equation, even in the case where our hierarchical equations are truncated with the Kirkwood superposition approximation, which is valid for low-density cases., Comment: 17 pages, 7 figures, v2: some sentences added, some subsections added, minor modifications for figures and sentences made, references added

Thermal electron emission process of a hydrogen impurity is an important topic of fundamental semiconductor physics. Despite of decades-long study, theory is not established yet. Here, we study the process of $\mathrm{H}^{-}$ in silicon, $\mathrm{H^{-}} \to \mathrm{H^{0}} + e^{-}$, using a first-principles calculation. Our calculation indicates that the process consists of two steps: slow diffusion of H$^{-}$ from a tetrahedral site to a bond-center site, which is the rate-limiting step, and faster nonradiative transition from $\mathrm{H}^{-}$ to $\mathrm{H}^{0} + e^{-}$ that occurs subsequently at the body-center site. The calculated rate is consistent with a deep level transient spectroscopy experiment

Kohn-Sham density functional theory is the base of modern computational approaches to electronic structures. Their accuracy vitally relies on the exchange-correlation energy functional, which encapsulates electron-electron interaction beyond the classical one. The functional provides a way to obtain the density and energy without solving the many-body equation and can, in principle, be determined to reproduce the exact ones universally. However, the past approaches are dependent on the theoretical development, which limits the possibility of the functional to human's intuition. Here, we demonstrate a systematic way to machine-learn a functional from a database, without complicated assumptions. The density and energy are related with a flexible feed-forward neural network, which is trained to reproduce accurate dataset, and the KS-DFT is solved by taking the functional derivatives with the back-propagation technique. Surprisingly, a trial functional, trained for just a few molecules, has been shown to be applicable to hundreds of molecular systems with comparable accuracy to the standard functionals. Also, by adding the nodes connected to the hidden layers, a non-local term is straightforwardly included to improve accuracy, which has been hitherto impractically difficult. Utilizing the strategy of rapidly advancing machine learning techniques, this novel approach is expected to enrich the DFT framework by constructing a functional just from a database for materials conventionally difficult to calculate accurately.

We demonstrate the feasibility of performing sufficient configurational sampling of disordered oxides directly from first principles without resorting to the use of fitted models such as cluster expansion. This is achieved by harnessing the power of modern-day cluster supercomputers using the replica exchange Monte Carlo method coupled directly with structural relaxation and energy calculation performed by density functional codes. The idea is applied successfully to the calculation of the temperature-dependence of the degree of inversion in the cation sublattice of MgAl$_2$O$_4$ spinel oxide. The possibility of bypassing fitting models will lead to investigation of disordered systems where cluster expansion is known to perform badly: for example, systems with large lattice deformation due to defects, or systems where long-range interactions dominate such as electrochemical interfaces., Comment: 6 pages, 4 figures

We analyze wave functions constructed as a sum of product of two-electron functions, or as a polynomial of geminals, to investigate their ability to represent the ground state of a strongly correlated few-body system. The known difficulty associated with variational determination of the total energy is overcome by applying a tensor decomposition method called Waring decomposition. Convergence speed of the total energy is compared for various polynomial types. The result provides information bridging between geminal product wave functions and the full-CI in the strongly correlated regime, thereby enriching knowledge on the hierarchy of molecular orbital theories of electron pairs., Comment: 14 pages, 2 figures

We report first-principles simulation of polarization mechanisms in hydroxyapatite to explain the underlying mechanism behind the reported ion conductivities and polarization under electrical poling at elevated temperatures. It is found that ion conduction occurs mainly in the column of OH$^-$ ions along the $c$-axis through a combination of the flipping of OH$^-$ ions, exchange of proton vacancies between OH$^-$ ions, and the hopping of the OH$^-$ vacancy. The calculated activation energies are consistent with those found in conductivity measurements and thermally stimulated depolarization current measurements.

Honeycomb structures of group IV elements can host massless Dirac fermions with non-trivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials especially in monolayer structures. We present a detailed investigation of the \beta 12 boron sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that the lattice of the \beta 12-sheet could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calculations. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices., Comment: accepted for publication in Physical Review Letters

Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form two concentric loops centred around the {\Gamma} point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a new platform to study the novel physical properties in two-dimensional Dirac materials and provide new opportunities to realize high-speed low-dissipation devices., Comment: Accepted in Nature Communications

We extend the Coleman's antisymmetrized geminal power (AGP) to develop a wave function theory that can incorporate up to four-body correlation in a region of strong correlation. To facilitate the variational determination of the wave function, the total energy is rewritten in terms of the traces of geminals. This novel trace formula is applied to a simple model system consisting of one dimensional Hubbard ring with a site of strong correlation. Our scheme significantly improves the result obtained by the AGP-CI scheme of Uemura et al. and also achieves more efficient compression of the degrees of freedom of the wave function. We regard the result as a step toward a first-principles wave function theory for a strongly correlated point defect or adsorbate embedded in an AGP-based mean-field medium., Comment: 17 pages, 5 figures

By paying special attention to the fact that the doped holes induce deformation of CuO6 octahedrons (or CuO5 pyramids) in cuprate superconductors, we develop a non-rigid band theory treating doping-induced alterations of energy-band structures in copper oxide superconductors. Thanks to this theory, we obtain a complete picture of the doping-induced alteration in the electronic structure of La2CuO4, from the spin-disordered insulating phase to the metallic phase. We conclude that the Fermi surface structure of this cuprate in the underdoped region consists of Fermi pockets in the antinodal region and Fermi arcs in the nodal region, and thus that the origin of a so-called pseudogap is closely related to the existence of Fermi pockets. Moreover, we show that the carriers on the Fermi pockets contribute to the phonon mechanism in d-wave superconductivity. Finally, we discuss how one will be able to find higher Tc materials, based on the conclusions mentioned above.

The description of the molecular solid phase of O$_2$, especially its ground-state antiferromagnetic insulating phase, is known to be quite unsatisfactory within the conventional local and semilocal density functional approximations used in the Kohn-Sham formalism of density functional theory. The recently-developed van der Waals functionals that take into account nonlocal correlations have also shown subpar performance in this regard. The difficulty lies in the subtle balance between the van der Waals interactions and the exchange coupling between the antiferromagnetic and ferromagnetic molecule pairs in the molecular crystal. Here, we report that the DFT$+U$ approach used in combination with the vdW-DF functional performs surprisingly well in this regard, and discuss the reasoning behind this behavior. We also apply this approach to study the recently-reported magnetic field-induced $\theta$ phase of solid O$_2$., Comment: Final published version

To avoid the combinatorial computational cost of configuration interaction (CI), we have previously introduced the symmetric tensor decomposition CI (STD-CI) method, where we take advantage of the antisymmetric nature of the electronic wave function and express the CI coefficients compactly as a series of Kronecker product states (STD series) [W. Uemura and O. Sugino, Phys. Rev. Lett. 109, 253001 (2012)]. Here we extend the variational degrees of freedom by using different molecular orbitals for different terms in the STD series. This scheme is equivalent to the linear combination of the Hartree-Fock-Bogoliubov state or the antisymmetrized geminal powers (AGP). The total energy converges very rapidly within 0.72 $\mu$Hartree taking only 10 terms for the water molecule, and the convergence is likewise fast for Hubbard tetramers. The computational cost scales as the fifth power of the number of electrons and the square of the number of terms in the STD series, indicating the promise of this AGP-based scheme for highly accurate and efficient computation of quantum systems., Comment: 14 pages, 5 figures

The lattice relaxation in oxygen-deficient Ta2O5 is investigated using first-principles calculations. The presence of a charge-neutral oxygen vacancy can result in a long-ranged lattice relaxation which extends beyond 18 {\AA} from the vacancy site. The lattice relaxation has significant effects on the vacancy formation energy as well as the electronic structures. The long-ranged behavior of the lattice relaxation is explained in terms of the Hellmann-Feynman forces and the potential energy surface related to the variation of Ta-O bond lengths., Comment: 25 pages, 6 figures, 1 table

We provide a first-principles description of the crystalline and oxygen-deficient Ta2O5 using refined computational methods and models. By performing calculations on a number of candidate structures, we determined the low-temperature phase and several stable oxygen vacancy configurations, which are notably different from the previous results. The most stable charge-neutral vacancy site induces a shallow level near the bottom of conduction band. Stability of different charge states is studied. Based on the results, we discuss the implications of the level structures on experiments, including the leakage current in Ta2O5-based electronic devices and catalysts., Comment: 23 pages, 4 figures and 2 tables in the body texts plus 4 figures in the supplementary material

The configuration interaction (CI) is a versatile wavefunction theory for interacting fermions but it involves an extremely long CI series. Using a symmetric tensor decomposition (STD) method, we convert the CI series into a compact and numerically tractable form. The converted series encompasses the Hartree-Fock state in the first term and rapidly converges to the full-CI state, as numerically tested using small molecules. Provided that the length of the STD-CI series grows only moderately with the increasing complexity of the system, the new method will serve as one of the alternative variational methods to achieve full-CI with enhanced practicability., Comment: 10 pages, 6 figures

We studied the band gap of {\beta}-PtO2 using first-principles calculations based on density functional theory (DFT). The results are obtained within the framework of generalized gradient approximation (GGA), GGA+U, GW and the hybrid functional method. For different types of calculations, the calculated band gap increases from ~ 0.46 eV to 1.80 eV. In particular, the band gap by GW (conventional and self-consistent) calculation shows a tendency of converging to ~ 1.25 \pm 0.05 eV. Effect of the on-site Coulomb interaction on the bonding characteristics is also analyzed., Comment: 19 pages, 6 figures

For a rigorous quantum simulation of nonadiabatic dynamics of electrons and nuclei, knowledge of not only first-order but also second-order nonadiabatic couplings (NAC), is required. Here we propose a method to efficiently calculate second-order NAC from time-dependent density functional theory (TDDFT), on the basis of the Casida ansatz adapted for the computation of first-order NAC, which has been justified in our previous work and can be shown to be valid for calculating second-order NAC between ground state and singly excited states within the Tamm-Dancoff approximation. Test calculations of second-order NAC in the immediate vicinity of Jahn-Teller and Renner-Teller intersections show that calculation results from TDDFT, combined with modified linear response theory, agree well with the prediction from the Jahn-Teller / Renner-Teller models. Contrary to the diverging behavior of first-order NAC near all types of intersection points, the Cartesian components of second-order NAC are shown to be negligibly small near Renner-Teller glancing intersections, while they are significantly large near the Jahn-Teller conical intersections. Nevertheless, the components of second-order NAC can cancel each other to a large extent in Jahn-Teller systems, indicating the background of neglecting second-order NAC in practical dynamics simulations. On the other hand, it is shown that such a cancellation becomes less effective in an elliptic Jahn-Teller system and thus the role of second-order NAC needs to be evaluated in the rigorous framework. Our study shows that TDDFT is promising to provide accurate data of NAC for full quantum mechanical simulation of nonadiabatic processes., Comment: 27 pages, 10 figures

We studied the electronic properties of beta-platinum dioxide ({\beta}-PtO2), a catalytic material, based on density functional theory. Using the GGA+U method which reproduces the GW band structures and the experimental structural parameters, we found that the creation of an oxygen vacancy will induce local magnetic moment on the neighboring Pt and O atoms. The magnetism originates not only from the unpaired electrons that occupy the vacancy induced gap state, but also from the itinerant valence electrons. Because of antiferromagnetic (AF) coupling and the localized nature of gap states, the total magnetic moment is zero for charge-neutral state (V_o^0) and is ~ 1 \mu B for singly-charged states (V_o^\mu). Calculation of grand potential shows that, the three charge states (V_o^0, V_o^\pm) are of the same stability within a small region, and the negatively charged state (V_o^-) is energetically favored within a wide range of the band gap. On this basis we discussed the implication on catalytic behavior., Comment: 45 pages, 11 figures, 3 tables