 Ab initio study on magnetism suppression, anharmonicity, rattling mode, and superconductivity in Sc_{6}M Te_{2} (M=Fe,Co,Ni)

Phys. Rev. B 110, 104505 (2024)
We perform a systematic ab initio study on phononmediated superconductivity in the transitionmetalbased superconductors Sc_{6}M Te_{2} (M=Fe,Co,Ni). Firstly, our charge analysis reveals significant electron transfer from Sc to because of the substantial difference in the electronegativity, filling the 3 d orbitals of M and suppressing magnetic instability. Secondly, we show that Sc_{6} FeTe_{2} exhibits strong lattice anharmonicity. Moreover, for M=Fe and Co, we find lowfrequency soft phonon bands of M, which can be interpreted as ¡Èrattling phonons¡É in the framework formed by Sc. While not observed in the case of M=Ni, the rattling phonons give rise to a prominent peak or plateau in the Eliashberg spectral function and enhance the pairing instability. By reproducing the experimental trend of superconducting transition temperatures, our study underscores the potential of designing phononmediated superconductors by strategically combining nonsuperconducting and magnetic transitionmetal elements.
 Continuous crossover between insulating ferroelectrics and polar metals: Ab initio calculation of structural phase transitions of LiBO_{3} (B=Ta, W, Re, Os)

Phys. Rev. B 110, 094102 (2024)
Inspired by the recent discovery of a new polar metal LiReO_{3} by K. Murayama et al, we calculate the temperature (T)dependent crystal structures of LiBO_{3} with B=Ta, W, Re, Os, using the selfconsistent phonon (SCPH) theory. We have reproduced the experimentally observed polarnonpolar structural phase transitions and the transition temperatures (T_{c}) of LiTaO_{3}, LiReO_{3}, and LiOsO_{3}. From the calculation, we predict that LiWO_{3} is a polar metal, which is yet to be tested experimentally. Upon doping electrons to the insulating LiTaO_{3}, the predicted T_{c} is quickly suppressed and approaches those of the polar metals. Thus, there is a continuous crossover between ferroelectric insulators and polar metals if we dope electrons to the ferroelectric insulators. Investigating the detailed material dependence of the interatomic force constants (IFCs), we explicitly show that the suppression of T_{c} in polar metals can be ascribed to the screening of the longrange LiO interaction, which is caused by the presence of the itinerant electrons. The quantitative finitetemperature calculations do not show signs of unscreened longrange interactions by the weak electronphonon coupling or enhancement of polar instabilities by carrier doping, as expected in some previous works.
 Firstprinciples study of the tunnel magnetoresistance effect with Crdoped RuO_{2} electrode

Phys. Rev. B 110, 064433 (2024)
We investigate the functionality of the Crdoped RuO_{2} as an electrode of the magnetic tunnel junction (MTJ), motivated by a recent experiment showing that Cr doping into rutiletype RuO_{2} is an effective tool to easily control its antiferromagnetic order and the resultant magnetotransport phenomena. We perform firstprinciples calculation of the tunnel magnetoresistance (TMR) effect in the MTJ based on the Crdoped RuO_{2} electrodes. We find that a finite TMR effect appears in the MTJ originating from the momentumdependent spin splitting in the electrodes, which suggests that RuO_{2} with Cr doping will function effectively as the electrode of the MTJ. We also show that this TMR effect can be qualitatively captured by the local density of states inside the tunnel barrier.
 Effect of collective spin excitations on electronic transport in topological spin textures

Phys. Rev. B 110, 014425 (2024)
We develop an efficient realtime simulation method for the spincharge coupled system in the velocity gauge. This method enables us to compute the realtime simulation for the twodimensional system with a complex spin texture. We focus on the effect of the collective excitation of the localized spins on the electronic transport properties of the nontrivial topological state in real space. To investigate this effect, we calculate the linear optical conductivity by calculating the realtime evolution of the Kondo lattice model on the triangular lattice, which hosts an allin/allout (tripleQ) magnetic structure. In the linear conductivity spectra, we observe multiple peaks below the band gap regime, attributed to the resonant contributions of collective modes similar to the skyrmionic system, alongside broadband modifications resulting from offresonant spin dynamics. This result shows that the collective excitation, similar to the skyrmionic system, influences the optical response of the electron systems based on symmetry analysis. We elucidate the interference between the contributions from the different spin excitations to the optical conductivity in the multiplespin texture, pointing out the modedependent electrical activity. We show the complex interplay between the complex spin texture and the itinerant electrons in the twodimensional spincharge coupled system.
 Universal chemical formula dependence of ab initio lowenergy effective Hamiltonian in singlelayer carrierdoped cuprate superconductors: Study using a hierarchical dependence extraction algorithm

Phys. Rev. B 110, 014502 (2024)
We explore the possibility to control the superconducting transition temperature at optimal hole doping T^{opt}_{c} in cuprates by tuning the chemical formula (CF). T^{opt}_{c} can be theoretically predicted from the parameters of the ab initio lowenergy effective Hamiltonian with one antibonding (AB) Cu 3d_{x2y2}/O 2p_{σ} orbital per Cu atom in the CuO_{2} plane, notably the nearestneighbor hopping amplitude t_{1} and the ratio u=U/t_{1}, where U is the onsite effective Coulomb repulsion. However, the CF dependence of t_{1} and u is a highly nontrivial question. In this paper, we propose the universal dependence of t_{1} and u on the CF and structural features in hole doped cuprates with a single CuO_{2} layer sandwiched between block layers. To do so, we perform extensive ab initio calculations of t_{1} and u and analyze the results by employing a machinelearning method called hierarchical dependence extraction (HDE). The main results are (a) t_{1} has a mainorder dependence on the radii R_{X} and R_{A} of the apical anion X and cation A in the block layer. (t_{1} increases when R_{X} or R_{A} decreases.) (b) u has a mainorder dependence on the ionic charge Z_{X} of X and the hole doping δ of the AB orbital. (u decreases when Z_{X} increases or δ increases.) We elucidate and discuss the microscopic mechanism of items (a) and (b). We demonstrate the predictive power of the HDE by showing the consistency between items (a) and (b) and results from previous works. The present results provide a basis for optimizing superconducting properties in cuprates and possibly akin materials. Also, the HDE method offers a general platform to identify dependencies between physical quantities.
 Symmetry analysis with spin crystallographic groups: Disentangling effects free of spinorbit coupling in emergent electromagnetism

Phys. Rev. B 109, 094438 (2024) (Editors' suggestion)
Recent studies identified spinorderdriven phenomena such as spincharge interconversion without relying on the relativistic spinorbit interaction. Those physical properties can be prominent in systems containing light magnetic atoms due to sizable exchange splitting and may pave the way for realization of giant responses correlated with the spin degree of freedom. In this paper, we present a systematic symmetry analysis based on the spin crystallographic groups and identify the physical property of a vast number of magnetic materials up to 1500 in total. By decoupling the spin and orbital degrees of freedom, our analysis enables us to take a closer look into the relation between the dimensionality of spin structures and the resultant physical properties and to identify the spin and orbital contributions separately. In stark contrast to the established analysis with magnetic space groups, the spin crystallographic group manifests richer symmetry including spintranslation symmetry and leads to emergent responses. For representative examples, we discuss the geometrical nature of the anomalous Hall effect and magnetoelectric effect and classify the spin Hall effect arising from the nonrelativistic spincharge coupling. Using the power of computational analysis, we apply our symmetry analysis to a wide range of magnets, encompassing complex magnets such as those with noncoplanar spin structures as well as collinear and coplanar magnets. We identify emergent multipoles relevant to physical responses and argue that our method provides a systematic tool for exploring sizable electromagnetic responses driven by spin order.
 Highthroughput calculations of antiferromagnets hosting anomalous transport phenomena

Phys. Rev. B 109, 094435 (2024)
We develop a highthroughput computational scheme based on cluster multipole theory to identify new functional antiferromagnets (AFMs). This approach is applied to 228 magnetic compounds listed in the AtomWorkAdv database, known for their elevated Nè¾¿el temperatures. We conduct systematic investigations of both stable and metastable magnetic configurations of these materials. Our findings reveal that 34 of these compounds exhibit AFM structures with zero propagation vectors and magnetic symmetries identical to conventional ferromagnets, rendering them potentially invaluable for spintronics applications. By crossreferencing our predictions with the existing MAGNDATA database and published literature, we verify the reliability of our findings for 26 out of 28 compounds with partially or fully elucidated magnetic structures in the experiments. These results not only affirm the reliability of our scheme but also point to its potential for broader applicability in the ongoing quest for the discovery of functional magnets.
 Development of an ab initio method for exciton condensation and its application to TiSe_{2}

Phys. Rev. Research 5, 043183 (2023)
Exciton condensation is a phenomenon that indicates the spontaneous formation of electronhole pairs, which can lead to a phase transition from a semimetal to an excitonic insulator by opening a gap at the Fermi surface. Although the idea of an excitonic insulator has been proposed for several decades, current theoretical approaches can only provide qualitative descriptions, and a quantitative predictive tool is still lacking. To shed light on this issue, we developed an ab initio method based on finitetemperature density functional theory and manybody perturbation theory to calculate the critical behavior of exciton condensation. Utilizing our methodology on monolayer TiSe_{2}, we identify a phase transition involving lattice distortion and nontrivial electronhole correlation at a temperature exceeding the critical temperature of phonon softening. By breaking down the components within the gap equation, we demonstrate that exciton condensation, mediated by electronphonon interaction, is the underlying cause of the chargedensitywave state observed in this compound. Overall, the methodology introduced in this work is general and sets the stage for searching for potential excitonic insulators in natural material systems.
 Natural orbital impurity solver for realfrequency properties at finite temperature

Phys. Rev. B 108, 195124 (2023)
We extend the natural orbital impurity solver [Y. Lu, M. Hoeppner, O. Gunnarsson, and M. W. Haverkort, Phys. Rev. B 90, 085102 (2014)] to finite temperatures and apply it to calculate spectral and transport properties of correlated electrons within the dynamical meanfield theory. First, we benchmark our method against the exact diagonalization result for small clusters, finding that the natural orbital scheme works well not only for zero temperature but for low finite temperatures. The method yields smooth and sufficiently accurate spectra, which agree well with the results of the numerical renormalization group. Using the smooth spectra, we calculate the electric conductivity and Seebeck coefficient for the twodimensional Hubbard model at low temperatures which are within the scope of many experiments and practical applications. These results demonstrate the usefulness of the natural orbital framework for obtaining the real frequency information of correlated electron systems.
 Suppression of heating by multicolor driving protocols in Floquetengineered strongly correlated systems

Phys. Rev. B 108, 035151 (2023)
Heating effects in Floquetengineered systems are detrimental to the control of physical properties. In this paper, we show that the heating of periodically driven strongly correlated systems can be suppressed by multicolor driving, i.e., by applying auxiliary excitations which interfere with the absorption processes from the main drive. We focus on the Mott insulating singleband Hubbard model and study the effects of multicolor driving with nonequilibrium dynamical meanfield theory. The main excitation is a periodic electric field with frequency &Omega smaller than the Mott gap, while for the auxiliary excitations, we consider additional electric fields and/or hopping modulations with a higher harmonic of &Omega. To suppress the threephoton absorption of the main excitation, which is a parityodd process, we consider auxiliary electricfield excitations and a combination of electricfield excitations and hopping modulations. On the other hand, to suppress the twophoton absorption, which is a parityeven process, we consider hopping modulations. The conditions for an efficient suppression of heating are well captured by the Floquet effective Hamiltonian derived with the highfrequency expansion in a rotating frame. As an application, we focus on the exchange couplings of the spins (pseudospins) in the repulsive (attractive) model and demonstrate that the suppression of heating allows us to realize and clearly observe a significant Floquetinduced change of the low energy physics.
 Local density of states as a probe for tunneling magnetoresistance effect: Application to ferrimagnetic tunnel junctions

Phys. Rev. B 107, 214442 (2023)
We investigate the tunneling magnetoresistance (TMR) effect using the lattice models which describe the magnetic tunnel junctions (MTJ). First, taking a conventional ferromagnetic MTJ as an example, we show that the product of the local density of states (LDOS) at the center of the barrier traces the TMR effect qualitatively. The LDOS inside the barrier has the information on the electrodes and the electron tunneling through the barrier, which enables us to easily evaluate the tunneling conductance more precisely than the conventional Julliere's picture. We then apply this method to the MTJs with collinear ferrimagnets including antiferromagnets. The TMR effect in the ferrimagnetic MTJs changes depending on the interfacial magnetic structures originating from the sublattice structure, which can also be captured by the LDOS. Our findings will reduce the computational cost for the qualitative evaluation of the TMR effect and be useful for a broader search for the materials which work as the TMR devices showing high performance.
 Magnetic interactions in intercalated transition metal dichalcogenides: A study based on ab initio model construction

Phys. Rev. B 107, 184429 (2023)
Transition metal dichalcogenides (TMDs) are known to have a wide variety of magnetic structures by hosting other transition metal atoms in the van der Waals gaps. To understand the chemical trend of the magnetic properties of the intercalated TMDs, we perform a systematic firstprinciples study for 48 compounds with different hosts, guests, and composition ratios. Starting with calculations based on spin density functional theory, we derive classical spin models by applying the local force method to the ab initio Wannierbased tightbinding model. We show that the calculated exchange couplings are overall consistent with the experiments, and the chemical trend can be understood in terms of the occupation of the 3d orbital in the intercalated transition metal. The present results give us a useful guiding principle to predict the magnetic structure of compounds that are yet to be synthesized.
 Dynamical meanfield theory for the HubbardHolstein model on a quantum device

Phys. Rev. B 107, 165155 (2023) (Editors' suggestion)
Recent developments in quantum hardware and quantum algorithms have made it possible to utilize the capabilities of current noisy intermediatescale quantum devices for addressing problems in quantum chemistry and condensedmatter physics. Here we report a demonstration of solving the dynamical meanfield theory (DMFT) impurity problem for the HubbardHolstein model on the IBM Quantum Processor Kawasaki, including selfconsistency of the DMFT equations. This opens up the possibility to investigate strongly correlated electron systems coupled to bosonic degrees of freedom and impurity problems with frequencydependent interactions. The problem involves both fermionic and bosonic degrees of freedom to be encoded on the quantum device, which we solve using a recently proposed Krylov variational quantum algorithm to obtain the impurity Green's function. We find the resulting spectral function to be in good agreement with the exact result, exhibiting both correlation and plasmonic satellites and significantly surpassing the accuracy of standard Trotterexpansion approaches. Our results provide an essential building block to study electronic correlations and plasmonic excitations on future quantum computers with modern ab initio techniques.
 Optimizing Superconductivity: From Cuprates via Nickelates to Palladates

Phys. Rev. Lett. 130 166002 (2023)(Editors' suggestion), featured in Physics
Motivated by cuprate and nickelate superconductors, we perform a comprehensive study of the superconducting instability in the singleband Hubbard model. We calculate the spectrum and superconducting transition temperature T_{c} as a function of filling and Coulomb interaction for a range of hopping parameters, using the dynamical vertex approximation. We find the sweet spot for high T_{c} to be at intermediate coupling, moderate Fermi surface warping, and low hole doping. Combining these results with first principles calculations, neither nickelates nor cuprates are close to this optimum within the singleband description. Instead, we identify some palladates, notably RbSr_{2}PdO_{3} and A'_{2}PdO_{2}Cl_{2} (A'=Ba_{0.5}La_{0.5}), to be virtually optimal, while others, such as NdPdO_{2}, are too weakly correlated.
 Ab initio structural optimization at finite temperatures based on anharmonic phonon theory: Application to the structural phase transitions of BaTiO_{3}

Phys. Rev. B, 106 224104 (2022)
We formulate a firstprinciple scheme for structural optimization at finite temperature (T) based on the selfconsistent phonon (SCP) theory, which accurately takes into account the effect of strong phonon anharmonicity. The T dependence of the shape of the unit cell and internal atomic configuration is determined by minimizing the variational free energy in the SCP theory. At each optimization step, the interatomic force constants in the new structure are calculated without running additional electronic structure calculations, which makes the method dramatically efficient. We demonstrate that the thermal expansion of silicon and the threestep structural phase transitions in BaTiO_{3} and its pressuretemperature (pT) phase diagram are successfully reproduced. The present formalism will open the way to the nonempirical prediction of physical properties at finite T of materials having a complex structural phase diagram.
 InterstitialElectronInduced Topological Molecular Crystals

Adv. Phys. Research, 2 2200041 (2022)
Topological phases are usually unreachable in molecular solids, which are characterized by weakly dispersed energy bands with a large gap, in contrast to topological materials. In this work, however, it is proposed that nontrivial electronic topology may ubiquitously emerge in a class of molecular crystals that contain interstitial electronic states, the bands of which are prone to be inverted with those of molecular orbitals. Guidelines are provided to hunt for such interstitialelectroninduced topological molecular crystals, especially in the topological insulating state. They exhibit a variety of exceptional qualities, as brought about by the intrinsic interplay of molecular crystals, interstitial electrons, and topological nature: 1) They may host cleavable surfaces along multiple orientations, with pronounced topological boundary states free from dangling bonds. 2) Strong response to moderate mechanical perturbations, whereby topological phase transition would occur under relatively low pressure. 3) Inherent highefficiency thermoelectricity as jointly contributed by the nonparabolic band structure (therewith high thermopower), highly mobile interstitial electrons (high electrical conductivity), and soft phonons (small lattice thermal conductivity). 4) Ultralow work function owing to the active interstitial electrons. Firstprinciples calculations are utilized to demonstrate these properties with the representative candidate K_{4}Ba_{2}[SnBi_{4}]. This work suggests a pathway of realizing topological phases in bulk molecular systems, which may advance the interdisciplinary research between topological and molecular materials.
 Efficient hydrogen evolution reaction due to topological polarization

Phys. Rev. B, 106 165120 (2022)
Materials carrying topological surface states (TSS) provide a fascinating platform for the hydrogen evolution reaction (HER). Based on systematic firstprinciples calculations for A_{3}B (A = Ni, Pd, Pt; B = Si, Ge, Sn), we propose that topological electric polarization characterized by the Zak phase can be crucial to designing efficient catalysts for the HER. For A3B, we show that the Zak phase takes a nontrivial value of π in the whole (111) projected Brillouin zone, which causes quantized electric polarization charges at the surface. There, depending on the adsorption sites, the hydrogen (H) atom hybridizes with the TSS rather than with the bulk states. When the hybridization has an intermediate character between the covalent and ionic bonds, the H states are localized in the energy spectrum, and the change in the Gibbs free energy (ΔG) due to the H adsorption becomes small. Namely, the interaction between the H states and the substrate becomes considerably weak, which is a highly favorable situation for the HER. Notably, we show that ΔG for Pt_{3}Sn and Pd_{3}Sn are just 0.066 and 0.092 eV, respectively, which are almost half of the value of Pt.
 Quantum phase transition between hyperuniform density distributions

Phys. Rev. Res., 4 033241 (2022)
We study an electron distribution under a quasiperiodic potential in light of hyperuniformity, aiming to establish a classification and analysis method for aperiodic but orderly density distributions realized in, e.g., quasicrystals. Using the AubryAndréHarper model, we first reveal that the electroncharge distribution changes its character as the increased quasiperiodic potential alters the eigenstates from extended to localized ones. While these changes of the charge distribution are characterized by neither multifractality nor translationalsymmetry breaking, they are characterized by hyperuniformity class and its order metric. We find a nontrivial relationship between the density of states at the Fermi level, a chargedistribution histogram, and the hyperuniformity class. The change to a different hyperuniformity class occurs as a firstorder phase transition except for an electronhole symmetric point, where the transition is of the third order. Moreover, we generalize the hyperuniformity order metric to a function, to capture more detailed features of the density distribution, in some analogy with a generalization of the fractal dimension to a multifractal one.
 Optimal alloying in hydrides: Reaching roomtemperature superconductivity in LaH_{10}

Phys. Rev. B, 105 174516 (2022)
Doping represents one of the most promising avenues for optimizing superconductors, such as highpressure conventional superconductors with recordbreaking critical temperatures. In this work, we perform an extensive search for substitutional dopants in LaH_{10}, looking for elements that enhance its electronic structure. In total, 70 elements were investigated as possible substitutions of Lasites at doping ratio of 12.5% under high pressure. To accelerate the screening of the ternary phases, our protocol to scan the chemical space is 1) to be constrained to highly symmetric patterns of hydrogen atoms, 2) focusing on phases with compact basis of lanthanum atoms (minimize enthalpy) and 3) to choose candidate dopants that preserve the van Hove singularity around the Fermi level. We found Ca as the best candidate dopants, which shift the van Hove singularity and increase the electronic DOS at the Fermi level. By using harmoniclevel phonon calculations and performing firstprinciples calculation of T_{c}, Cadoped LaH_{10} shows T_{c} which is 15% higher than the one of LaH_{10}. It provides a promising route to reach roomtemperature superconductivity in pressurized hydrides by doping.
 Quantum and temperature effects on the crystal structure of superhydride LaH_{10}: A path integral molecular dynamics study

Phys. Rev. B, 105 174111 (2022)
By classical and path integral molecular dynamics simulations, we study the pressuretemperature (PT) phase diagram of LaH_{10} to clarify the impact of temperature and atomic zeropoint motions. We calculate the XRD pattern and analyze the space group of the crystal structures. For 125 GPa≤P≤150 GPa and T=300K, we show that a highly symmetric Fmä¿—3m structure, for which superconductivity is particularly favored, is stabilized only by the temperature effect. On the other hand, for T=200K, the interplay between the temperature and quantum effects is crucial to realize the Fmä¿—3m structure. For P=100GPa and T=300K, we find that the system is close to the critical point of the structural phase transition between the Fmä¿—3m structure and those with lower symmetries.
 Fermi Surface Expansion above Critical Temperature in a Hund Ferromagnet

Phys. Rev. Lett., 128 206401 (2022)
Using a cluster extension of the dynamical meanfield theory, we show that strongly correlated metals subject to Hund's physics exhibit significant electronic structure modulations above magnetic transition temperatures. In particular, in a ferromagnet having a large local moment due to Hund's coupling (Hund's ferromagnet), the Fermi surface expands even above the Curie temperature (T_{C}) as if a spin polarization occurred. Behind this phenomenon, effective Hund's physics works in momentum space, originating from ferromagnetic fluctuations in the strongcoupling regime. The resulting significantly momentumdependent (spatially nonlocal) electron correlations induce an electronic structure reconstruction involving a Fermi surface volume change and a redistribution of the momentumspace occupation. Our finding will give a deeper insight into the physics of Hund's ferromagnets above T_{C}.