arXiv subjects

Tomonori Shirakawa

Publications and source records attributed to Tomonori Shirakawa.

51 records · Page 3Linked to original sources

arXiv2017

Ground state phase diagram of the triangular lattice Hubbard model by density matrix renormalization group method

Two-dimensional density-matrix renormalization group method is employed to examine the ground state phase diagram of the Hubbard model on the triangular lattice at half filling. The calculation reveals two discontinuities in the double occupancy with increasing the repulsive Hubbard interaction U at Uc1 = 7.55 t and Uc2 = 9.65 t (t being the hopping integral), indicating that there are three phases separated by first order transitions. The absence of any singularity in physical quantities for 0 < U < Uc1 implies that this phase corresponds to a metallic phase. The local spin density induced by an applied pinning magnetic field for U > Uc2 exhibits a three sublattice feature, which is compatible with the Neel ordered state realized in the strong coupling limit. For Uc1 < U < Uc2, a response to the applied pinning magnetic field is comparable to that in the metallic phase but a relatively large spin correlation length is found with neither valence bond nor chiral magnetic order, suggesting a paramagnetic nature which resembles gapless spin liquid. The calculation also finds that the pair- ing correlation function monotonically decreases with increasing U and thus the superconductivity is unlikely in the intermediate phase.

cond-mat.str-el↗

arXiv2017

Unravelling local spin polarization of Zhang-Rice singlet in lightly hole-doped cuprates using high-energy optical conductivity

Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holes in lightly hole-doped La$_{1.95}$Sr$_{0.05}$Cu$_{0.95}$Zn$_{0.05}$O$_{4}$. The changes of the local spin polarization manifest strongly in the temperature-dependent optical conductivity at ~7.2 eV, with an anomaly at the magnetic stripe phase (~25 K), accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoretical calculations, we also assign high-energy optical transitions and their corresponding temperature dependence, particularly at ~2.5 ~8.7, ~9.7, ~11.3 and ~21.8 eV. Our result shows the importance of a strong mixture of spin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magnetic correlations using high- energy optical conductivity in correlated electron systems.

cond-mat.str-el↗

arXiv2016

From a quasimolecular band insulator to a relativistic Mott insulator in $t_{2g}^5$ systems with a honeycomb lattice structure

The $t_{2g}$ orbitals of an edge-shared transition-metal oxide with a honeycomb lattice structure form dispersionless electronic bands when only hopping mediated by the edge-sharing oxygens is accessible. This is due to the formation of isolated quasimolecular orbitals (QMOs) in each hexagon, introduced recently by Mazin et al. [Phys. Rev. Lett. 109, 197201 (2012)], which stabilizes a band insulating phase for $t_{2g}^5$ systems. However, with help of the exact diagonalization method to treat the electron kinetics and correlations on an equal footing, we find that the QMOs are fragile against not only the spin-orbit coupling (SOC) but also the Coulomb repulsion. We show that the electronic phase of $t_{2g}^5$ systems can vary from a quasimolecular band insulator to a relativistic $J_{\rm eff}=1/2$ Mott insulator with increasing the SOC as well as the Coulomb repulsion. The different electronic phases manifest themselves in electronic excitations observed in optical conductivity and resonant inelastic x-ray scattering. Based on our calculations, we assert that the currently known Ru$^{3+}$- and Ir$^{4+}$-based honeycomb systems are far from the quasimolecular band insulator but rather the relativistic Mott insulator.

cond-mat.str-el↗

arXiv2016

Density matrix renormalization group study in energy space for a single-impurity Anderson model and an impurity quantum phase transition

The density matrix renormalization group method is applied to obtain the ground state phase diagram of the single impurity Anderson model on the honeycomb lattice at half filling. The calculation of local static quantities shows that the phase diagram contains two distinct phases, the local moment (LM) phase and the asymmetric strong coupling (ASC) phase. These results are supported by the local spin and charge excitation spectra, which exhibit qualitatively different behavior in these two phases and also reveal the existence of the valence fluctuating point at the phase boundary. For comparison, we also study the low-energy effective pseudogap Anderson model. Although the high-energy excitations are obviously different, we find that the ground state phase diagram and the asymptotically low-energy excitations are in good quantitative agreement with those for the single impurity Anderson model on the honeycomb lattice, thus providing the first quantitative justification for the previous studies based on low-energy approximate approaches. Furthermore, we find that the lowest entanglement level is doubly degenerate for the LM phase, whereas it is singlet for the ASC phase and is accidentally three fold degenerate at the valence fluctuating point. Our results therefore clearly demonstrate that the low-lying entanglement spectrum can be used to determine with high accuracy the phase boundary of the impurity quantum phase transition.

cond-mat.str-el↗

arXiv2016

Emergence of massless Dirac quasiparticles in correlated hydrogenated graphene with broken sublattice symmetry

Using the variational cluster approximation (VCA) and the cluster perturbation theory, we study the finite temperature phase diagram of a half-depleted periodic Anderson model on the honeycomb lattice at half filling for a model of graphone, i.e., single-side hydrogenated graphene. The ground state of this model is found to be ferromagnetic (FM) semi-metal. The spin wave dispersion in the FM state is linear in momentum at zero temperature but becomes quadratic at finite temperatures, implying that the FM state is fragile against thermal fluctuations. Indeed, our VCA calculations find that the paramagnetic (PM) state dominates the finite temperature phase diagram. More surprisingly, we find that massless Dirac quasiparticles with the linear energy dispersion emerge at the Fermi energy upon introducing the electron correlation $U$ at the impurity sites in the PM phase. The Dirac Fermi velocity is found to be highly correlated to the quasiparticle weight of the emergent massless Dirac quasiparticles at the Fermi energy and monotonically increases with $U$. These unexpected massless Dirac quasiparticles are also examined with the Hubbard-I approximation and the origin is discussed in terms of the spectral weight redistribution involving a large energy scale of $U$. Considering an effective quasiparticle Hamiltonian which reproduces the single-particle excitations obtained by the Hubbard-I approximation, we argue that the massless Dirac quasiparticles are protected by the electron correlation. Our finding is therefore the first example of the emergence of massless Dirac quasiparticles due to dynamical electron correlations without breaking any spatial symmetry. The experimental implications of our results for graphone as well as a graphene sheet on transition metal substrates are also briefly discussed.

cond-mat.str-el↗

arXiv2016

Spin-Orbital Entangled Excitonic Insulators in $(t_{2g})^4$ Correlated Electron Systems

We employ the multi-orbital dynamical mean-field theory to examine the ground state of a three-orbital Hubbard model with a relativistic spin-orbit coupling (SOC) at four electrons per site. We demonstrate that the interplay between the strong electron correlations and the SOC induces a Van Vleck-type nonmagnetic insulator and its magnetic exciton condensation. We also find in the moderate electron correlation regime that the SOC induces another type of a nonmagnetic excitonic insulator, in addition to a relativistic band insulator. The characteristic features among these insulators are manifested in the momentum resolved single-particle excitations, thus accessible by angle-resolved photoemission spectroscopy experiments.

cond-mat.str-el↗

arXiv2016

Slave rotor theory of the Mott transition in the Hubbard model: a new mean field theory and a new variational wave function

A new mean field theory is proposed for the Mott transition in the Hubbard model based on the slave rotor representation of the electron operator. This theory provides a better description of the role of the long range charge correlation in the Mott insulating state and offers a good estimation of the critical correlation strength for the Mott transition. We have constructed a new variational wave function for the Mott insulating state based on this new slave rotor mean field theory. We find this new variational wave function outperforms the conventional Jastrow type wave function with long range charge correlator in the Mott insulating state. It predicts a continuous Mott transition with non-divergent quasiparticle mass at the transition point. We also show that the commonly used on-site mean field decoupling for the slave rotor corresponds to the Gutzwiller approximation for the Gutzwiller projected wave function with only on-site charge correlator, which can not describe the Mott transition in any finite dimensional system.

cond-mat.str-el↗

arXiv2015

Spin-orbit-induced exotic insulators in a three-orbital Hubbard model with $(t_{2g})^5$ electrons

On the basis of the multi-orbital dynamical mean field theory, a three-orbital Hubbard model with a relativistic spin-orbit coupling (SOC) is studied at five electrons per site. The numerical calculations are performed by employing the continuous-time quantum Monte Carlo (CTQMC) method based on the strong coupling expansion. We find that appropriately choosing bases, i.e., the maximally spin-orbit-entangled bases, drastically improve the sign problem in the CTQMC calculations, which enables us to treat exactly the full Hund's coupling and pair hopping terms. This improvement is also essential to reach at low temperatures for a large SOC region where the SOC most significantly affects the electronic structure. We show that a metal-insulator transition is induced by the SOC for fixed Coulomb interactions. The insulating state for smaller Coulomb interactions is antiferromagnetically ordered with the local effective total angular momentum $j=1/2$, in which the $j=1/2$ based band is essentially half-filled while the $j=3/2$ based bands are completely occupied. More interestingly, for larger Coulomb interactions, we find that an excitonic insulating state emerges, where the condensation of an electron-hole pair in the $j=1/2$ and $j=3/2$ based bands occurs. The origin of the excitonic insulator as well as the experimental implication is discussed.

cond-mat.str-el↗

arXiv2014

Block Lanczos density-matrix renormalization group method for general Anderson impurity models: Application to magnetic impurity problems in graphene

We introduce a block Lanczos (BL) recursive technique to construct quasi-one-dimensional models, suitable for density-matrix renormalization group (DMRG) calculations, from single- as well as multiple-impurity Anderson models in any spatial dimensions. This new scheme, named BL-DMRG method, allows us to calculate not only local but also spatially dependent static and dynamical quantities of the ground state for general Anderson impurity models without losing elaborate geometrical information of the lattice. We show that the BL-DMRG method can be easily extended to treat a multi-orbital Anderson impurity model. We also show that the symmetry adapted BL bases can be utilized, when it is appropriate, to reduce the computational cost. As a demonstration, we apply the BL-DMRG method to three different models for graphene: (i) a single adatom on the honeycomb lattice, (ii) a substitutional impurity in the honeycomb lattice, and (iii) an effective model for a single carbon vacancy in graphene. Our analysis reveals that, for the particle-hole symmetric case at half filling of electron density, the ground state of model (i) behaves as an isolated magnetic impurity with no Kondo screening while the ground state of the other two models forms a spin singlet state. We also calculate the real-space dependence of the spin-spin correlation functions between the impurity site and the conduction sites for these three models. Our results clearly show that, reflecting the presence of absence of unscreened magnetic moment at the impurity site, the spin-spin correlation functions decay as $r^{-3}$, differently from the non-interacting limit ($r^{-2}$), for model (i) and as $ r^{-4}$, exactly the same as the non-interacting limit, for models (ii) and (iii) in the asymptotic $r$, where $r$ is the distance between the impurity site and the conduction site.

cond-mat.str-el↗

arXiv2014

Theoretical study of insulating mechanism in multi-orbital Hubbard models with a large spin-orbit coupling: Slater versus Mott scenario in Sr$_2$IrO$_4$

To examine the insulating mechanism of 5$d$ transition metal oxide Sr$_2$IrO$_4$, we study the ground state properties of a three-orbital Hubbard model with a large relativistic spin-orbit coupling on a square lattice. Using a variational Monte Carlo method, we find that the insulating state appearing in the ground state phase diagram for one hole per site varies from a weakly correlated to a strongly correlated antiferromagnetic (AF) state with increasing Coulomb interactions. This crossover is characterized by the different energy gain mechanisms of the AF insulating state, i.e., from an interaction-energy driven Slater-type insulator to a band-energy driven Mott-type insulator with increasing Coulomb interactions. Our calculations reveal that Sr$_2$IrO$_4$ is a "moderately correlated" AF insulator located in the intermediate coupling region between a Slater-type and a Mott-type insulators.

cond-mat.str-el↗

arXiv2013

Rock-salt SnS and SnSe: Native Topological Crystalline Insulators

Unlike time-reversal topological insulators, surface metallic states with Dirac cone dispersion in the recently discovered topological crystalline insulators (TCIs) are protected by crystal symmetry. To date, TCI behaviors have been observed in SnTe and the related alloys Pb$_{1-x}$Sn$_{x}$Se/Te, which incorporate heavy elements with large spin-orbit coupling (SOC). Here, by combining first-principles and {\it ab initio} tight-binding calculations, we report the formation of a TCI in the relatively lighter rock-salt SnS and SnSe. This TCI is characterized by an even number of Dirac cones at the high-symmetry (001), (110) and (111) surfaces, which are protected by the reflection symmetry with respect to the ($\bar{1}$10) mirror plane. We find that both SnS and SnSe have an intrinsically inverted band structure and the SOC is necessary only to open the bulk band gap. The bulk band gap evolution upon volume expansion reveals a topological transition from an ambient pressure TCI to a topologically trivial insulator. Our results indicate that the SOC alone is not sufficient to drive the topological transition.

cond-mat.mtrl-sci↗

arXiv2012

Monte Carlo study of an unconventional superconducting phase in Ir-oxide J_{eff}=1/2 Mott insulators induced by carrier doping

Based on a microscopic theoretical study, we show that novel superconductivity is induced by carrier doping in layered perovskite Ir oxides where a strong spin-orbit coupling causes an effective total angular momentum J_{eff}=1/2 Mott insulator. Using a variational Monte Carlo method, we find an unconventional superconducting state in the ground state phase diagram of a t_{2g} three-orbital Hubbard model on the square lattice. This superconducting state is characterized by a d_{x^2-y^2}-wave "pseudospin singlet" formed by the J_{eff}=1/2 Kramers doublet, which thus contains inter-orbital as well as both singlet and triplet components of t_{2g} electrons. The superconducting state is found stable only by electron doping, but not by hole doping, for the case of carrier doped Sr_2IrO_4. We also study an effective single-orbital Hubbard model to discuss the similarities to high-T_c cuprate superconductors and the multi-orbital effects.

cond-mat.supr-con↗

arXiv2012

Large spectral weight transfer in optical conductivity of SrTiO$_{3}$ induced by intrinsic vacancies

The optical conductivity ($σ_{1}$) of SrTiO$_{3}$ for various vacancies has been systematically studied using a combination of ultraviolet - vacuum ultraviolet (UV-VUV) reflectivity and spectroscopic ellipsometry. For cation (Ti) vacancies, $σ_{1}$ shows large spectral weight transfer over a wide range of energy from as high as 35 eV to as low as 0.5 eV and the presence of mid-gap states, suggesting that strong correlations play an important role. While for anion (O) vacancies $σ_{1}$ shows changes from 7.4 eV up to 35 eV. These unexpected results can be explained in terms of orbital reconstruction.

cond-mat.str-el↗

arXiv2011

Microscopic Study of Electronic and Magnetic Properties for Ir Oxide

The exact diagonalization and the variational cluster approximation (VCA) are used to study the nature of a novel Mott insulator induced by a strong spin-orbit coupling for a two-dimensional three-band Hubbard model consisting of the $t_{2g}$ manifold of $5d$ orbitals. To characterize the ground state, we introduce a local Kramer's doublet which can represent a state with effective angular momentum $J_{\rm eff}=|{\bm S}-{\bm L}|=1/2$ as well as spin $S=1/2$. Our systematic study of the pseudo-spin structure factor defined by the Kramer's doublet shows that the $J_{\rm eff}=1/2$ Mott insulator is smoothly connected to the $S=1/2$ Mott insulator. Using the Kramer's doublet as a variational state for the VCA, we examine the one-particle excitations for the Mott insulating phase. These results are compared with recent experiments on Sr$_2$IrO$_4$.

cond-mat.str-el↗

arXiv2010

Microscopic study of spin-orbit-induced Mott insulator in Ir oxides

Motivated by recent experiments of a novel 5$d$ Mott insulator in Sr$_2$IrO$_4$, we have studied the two-dimensional three-orbital Hubbard model with a spin-orbit coupling $λ$. The variational Monte Carlo method is used to obtain the ground state phase diagram with varying a on-site Coulomb interaction $U$ as well as $λ$. It is found that the transition from a paramagnetic metal to an antiferromagnetic (AF) insulator occurs at a finite $U=U_{\mathrm{MI}}$, which is greatly reduced by a large $λ$, characteristic of 5$d$ electrons, and leads to the "spin-orbit-induced" Mott insulator. It is also found that the Hund's coupling induces the anisotropic spin exchange and stabilizes the in-plane AF order. We have further studied the one-particle excitations using the variational cluster approximation, and revealed the internal electronic structure of this novel Mott insulator. These findings are in agreement with experimental observations on Sr$_2$IrO$_4$, and qualitatively different from those of extensively studied 3$d$ Mott insulators.

cond-mat.str-el↗