We perform a systematic ab initio study on phonon-mediated superconductivity in the transition-metal-based superconductors Sc₆M Te₂ (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 Sc₆ FeTe₂ exhibits strong lattice anharmonicity. Moreover, for M=Fe and Co, we find low-frequency soft phonon bands of M, which can be interpreted as “rattling phonons” 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 phonon-mediated superconductors by strategically combining nonsuperconducting and magnetic transition-metal elements.

Inspired by the recent discovery of a new polar metal LiReO₃ by K. Murayama et al, we calculate the temperature (T)-dependent crystal structures of LiBO₃ with B=Ta, W, Re, Os, using the self-consistent phonon (SCPH) theory. We have reproduced the experimentally observed polar-nonpolar structural phase transitions and the transition temperatures (T_c) of LiTaO₃, LiReO₃, and LiOsO₃. From the calculation, we predict that LiWO₃ is a polar metal, which is yet to be tested experimentally. Upon doping electrons to the insulating LiTaO₃, 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 long-range Li-O interaction, which is caused by the presence of the itinerant electrons. The quantitative finite-temperature calculations do not show signs of unscreened long-range interactions by the weak electron-phonon coupling or enhancement of polar instabilities by carrier doping, as expected in some previous works.

We investigate the functionality of the Cr-doped RuO₂ as an electrode of the magnetic tunnel junction (MTJ), motivated by a recent experiment showing that Cr doping into rutile-type RuO₂ is an effective tool to easily control its antiferromagnetic order and the resultant magnetotransport phenomena. We perform first-principles calculation of the tunnel magnetoresistance (TMR) effect in the MTJ based on the Cr-doped RuO₂ electrodes. We find that a finite TMR effect appears in the MTJ originating from the momentum-dependent spin splitting in the electrodes, which suggests that RuO₂ 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.

We develop an efficient real-time simulation method for the spin-charge coupled system in the velocity gauge. This method enables us to compute the real-time simulation for the two-dimensional 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 real-time evolution of the Kondo lattice model on the triangular lattice, which hosts an all-in/all-out (triple-Q) 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 off-resonant 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 multiple-spin texture, pointing out the mode-dependent electrical activity. We show the complex interplay between the complex spin texture and the itinerant electrons in the two-dimensional spin-charge coupled system.

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 low-energy effective Hamiltonian with one antibonding (AB) Cu 3d_x2-y2/O 2p_σ orbital per Cu atom in the CuO₂ plane, notably the nearest-neighbor hopping amplitude |t₁| and the ratio u=U/|t₁|, where U is the onsite effective Coulomb repulsion. However, the CF dependence of |t₁| and u is a highly nontrivial question. In this paper, we propose the universal dependence of |t₁| and u on the CF and structural features in hole doped cuprates with a single CuO₂ layer sandwiched between block layers. To do so, we perform extensive ab initio calculations of |t₁| and u and analyze the results by employing a machine-learning method called hierarchical dependence extraction (HDE). The main results are (a) |t₁| has a main-order dependence on the radii R_X and R_A of the apical anion X and cation A in the block layer. (|t₁| increases when R_X or R_A decreases.) (b) u has a main-order 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.

Recent studies identified spin-order-driven phenomena such as spin-charge interconversion without relying on the relativistic spin-orbit 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 spin-translation 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 spin-charge 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.

We develop a high-throughput computational scheme based on cluster multipole theory to identify new functional antiferromagnets (AFMs). This approach is applied to 228 magnetic compounds listed in the AtomWork-Adv database, known for their elevated N莨�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 cross-referencing 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.

Exciton condensation is a phenomenon that indicates the spontaneous formation of electron-hole 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 finite-temperature density functional theory and many-body perturbation theory to calculate the critical behavior of exciton condensation. Utilizing our methodology on monolayer TiSe₂, we identify a phase transition involving lattice distortion and nontrivial electron-hole 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 electron-phonon interaction, is the underlying cause of the charge-density-wave 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.

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 mean-field 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 two-dimensional 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.

Heating effects in Floquet-engineered 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 single-band Hubbard model and study the effects of multicolor driving with nonequilibrium dynamical mean-field 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 three-photon absorption of the main excitation, which is a parity-odd process, we consider auxiliary electric-field excitations and a combination of electric-field excitations and hopping modulations. On the other hand, to suppress the two-photon absorption, which is a parity-even process, we consider hopping modulations. The conditions for an efficient suppression of heating are well captured by the Floquet effective Hamiltonian derived with the high-frequency 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 Floquet-induced change of the low energy physics.

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.

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 first-principles 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 Wannier-based tight-binding 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.

Recent developments in quantum hardware and quantum algorithms have made it possible to utilize the capabilities of current noisy intermediate-scale quantum devices for addressing problems in quantum chemistry and condensed-matter physics. Here we report a demonstration of solving the dynamical mean-field theory (DMFT) impurity problem for the Hubbard-Holstein model on the IBM Quantum Processor Kawasaki, including self-consistency 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 frequency-dependent 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 Trotter-expansion approaches. Our results provide an essential building block to study electronic correlations and plasmonic excitations on future quantum computers with modern ab initio techniques.

Motivated by cuprate and nickelate superconductors, we perform a comprehensive study of the superconducting instability in the single-band 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 single-band description. Instead, we identify some palladates, notably RbSr₂PdO₃ and A'₂PdO₂Cl₂ (A'=Ba_0.5La_0.5), to be virtually optimal, while others, such as NdPdO₂, are too weakly correlated.

We formulate a first-principle scheme for structural optimization at finite temperature (T) based on the self-consistent 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 three-step structural phase transitions in BaTiO₃ and its pressure-temperature (p-T) 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.

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 interstitial-electron-induced 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 high-efficiency thermoelectricity as jointly contributed by the non-parabolic 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. First-principles calculations are utilized to demonstrate these properties with the representative candidate K₄Ba₂[SnBi₄]. This work suggests a pathway of realizing topological phases in bulk molecular systems, which may advance the interdisciplinary research between topological and molecular materials.

Materials carrying topological surface states (TSS) provide a fascinating platform for the hydrogen evolution reaction (HER). Based on systematic first-principles calculations for A₃B (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 Pt₃Sn and Pd₃Sn are just -0.066 and -0.092 eV, respectively, which are almost half of the value of Pt.

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 Aubry-André-Harper model, we first reveal that the electron-charge 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 translational-symmetry 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 charge-distribution histogram, and the hyperuniformity class. The change to a different hyperuniformity class occurs as a first-order phase transition except for an electron-hole 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.

Doping represents one of the most promising avenues for optimizing superconductors, such as high-pressure conventional superconductors with record-breaking critical temperatures. In this work, we perform an extensive search for substitutional dopants in LaH₁₀, looking for elements that enhance its electronic structure. In total, 70 elements were investigated as possible substitutions of La-sites 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 harmonic-level phonon calculations and performing first-principles calculation of T_c, Ca-doped LaH₁₀ shows T_c which is 15% higher than the one of LaH₁₀. It provides a promising route to reach room-temperature superconductivity in pressurized hydrides by doping.

By classical and path integral molecular dynamics simulations, we study the pressure-temperature (P-T) phase diagram of LaH₁₀ to clarify the impact of temperature and atomic zero-point 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 Fm篆�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 Fm篆�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 Fm篆�3m structure and those with lower symmetries.

Using a cluster extension of the dynamical mean-field 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 strong-coupling regime. The resulting significantly momentum-dependent (spatially nonlocal) electron correlations induce an electronic structure reconstruction involving a Fermi surface volume change and a redistribution of the momentum-space occupation. Our finding will give a deeper insight into the physics of Hund's ferromagnets above T_C.