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In this paper, we find the new gap-open mechanism for massless Dirac electrons in molecular solids α-(BETS)2I3 . The gap-opening mechanism in correlated Dirac in α-(BETS)2I3 has been an unresolved mystery for over 25 years. Using a state-of-the-art ab initio method for correlated electron systems,  we show that the one-dimensional spin correlations induced by the electronic correlations can open the gap in the massless Dirac electrons in α-(BETS)2I3. Our finding paves a new way for generating massive Dirac electrons utilizing electronic correlations.

In this paper, we propose a silver-analog of cuprates hight-Tc superconductors such as Sr2AgO2F2. Our ab initio calculations suggest that the microscopic parameters of the Ag-based oxides are similar to those of cuprates. We also show that the ground state of the low-energy effective Hamiltonian for Sr2AgO2F2 is an antiferromagnetic Mott insulator. This result indicates that Ag-based oxides are promising candidates for high-Tc superconductors.

In this paper, we find new quantities, zeros of Green functions, for detecting a wide range of topological insulators. By utilizing the recently rediscovered eigenvector-eigenvalue identity, we prove that the traverses of the zeros appear in the topological phases due to the band inversions (exchange between the occupied and the unoccupied bands). We have shown that the traverses of the zeros universally appear in all six classes of the topological insulators and the higher-order topological insulators. In addition to the conventional topological insulators, we have also shown that the zeros are useful for identifying the higher-order topological insulators.

In this paper, based on the ab initio calculations,  we have shown that  1D anisotropic quantum spin liquid appears in the effective Hamiltonians of β'-EtMe3Sb[Pd(dmit)2]2, where quantum spin liquid behaviors are experimentally reported.  We have also demonstrated that the 1D nature of the quantum spin liquid can explain the thermodynamic quantities observed in EtMe3Sb such as large thermal conductivity.

In this paper, we have systematically derived and analyzed ab initio low-energy effective Hamiltonians for dmit salts (β'-X[Pd(dmit)2]2, X represents a cation) with available room- and low-temperature structures. We find that the the changes in the lattice constant associated with lowering temperature largely affect the microscopic parameter in the effective Hamiltonians. We also analyzed the obtained effective Hamiltonians using the exact diagonalization method with the boundary-condition average. As a result, we find that the magnetic ordered moment is significantly reduced at X=EtMe3Sb. The reduction is consistent with the quantum spin liquid behavior observed in experiment.  Our comprehensive derivation and numerically exact analysis of the ab initio effective Hamiltonians will clarify the microscopic origin of the enigmatic quantum spin liquid behavior observed in dmit salts.

In this paper, we develop a theoretical method for predicting superconducting critical temperatures (Tc) in the correlated electron systems, which combines modern data-science techniques with the ab initio derivation of effective Hamiltonians. We derive the microscopic ab initio effective Hamiltonians for more the 30 iron-based superconductors. Then, we constructed a regression model that reproduces experimental Tc from the microscopic parameters in the effective Hamiltonians.  We also propose a way to enhance Tc based on the regression model. The developed method inspires a new direction in the search for new high-Tc superconductors as well as other exotic materials.

In this paper, we analyze how the precessing magnetization induces the spin current in the topological Dirac semimetals. We show that the spin current takes the universal semi-quantized value when the exchange coupling is large enough. We demonstrate that the spin current is robust against disorder and propagate long-range distance with low dissipations. We also find that the spin current generates the semi-quantized torque on the opposite side of the ferromagnetic insulators. Our analysis demonstrates that the topological Dirac semimetal is a promising candidate for a building block of the next-generation spintronics device.

In this paper (arXiv:2102.04665), we find new quantities, zeros of Green functions, for detecting a wide range of topological insulators. We show that the crosses of the zeros universally appear in all six classes of the topological insulators and the higher-order topological insulators. We prove that the crosses of the zeros are guaranteed by the gapless surface/edge states. 

RESPACK is a software package for deriving low-energy effective Hamiltonians of solids. Using RESPACK, one can obtain the maximally localized Wannier functions and calculate the screened interactions by the constrained random phase approximation (cRPA) method. In the paper, we explain how to derive low-energy effective Hamiltonian for a cubic perovskite SrVO3.  This paper is OPEN ACCESS. Source codes and tutorials are available on the home page of RESPACK. 

In this paper, we systematically derive ab initio low-energy effective Hamiltonians for molecular solids Pd(dmit)2 salts. By solving the Hamiltonians, we show that the obtained Hamiltonians reproduce the compound dependence of magnetic properties including quantum spin liquid behavior observed in EtMe3Sb[Pd(dmit)2]2. In the ab initio calculations,  we use several open-source software packages, i.e., Quantum ESPRESSO, RESPACK, and HΦ.

In this paper, we analyze the finite-temperature magnetization process of the kagome quantum antiferromagnets using the thermal pure quantum state (TPQ) method. As a result, we show that finite-temperature fluctuations asymmetrically melt the one-third plateau, which is reported in the zero-temperature calculations. We also show that the asymmetric melting is caused by the large density of states below the plateau, and it induces the enhancement of the specific heat. In the calculations, we use HΦ, which an open-source software package for the exact diagonalization.

Kω is a library for efficiently obtaining the dynamical Green functions using the shifted Krylov subspace method. In the paper, we explain how to use Kω. We also present several applications such as the implementation of the contour-integral method, the calculation of the dynamical spin structure factors, and the calculation of the optical conductivity. Paper is available here (OPEN ACCESS) and the tutotials are available in KOmega-Gallery.

In this paper, we study the spin-to-charge conversion at the edge of a quantum spin Hall insulator (QSHI) attach to an insulating ferromagnet (FM). Based on the Floquet-Keldysh formalism, we obtain the fomula of the conversion efficiency and cralify how to maximize the conversion efficiency.