arXiv subjects

Seiji Yunoki

Publications and source records attributed to Seiji Yunoki.

At least 91 records · Page 5Linked to original sources

arXiv2020

Photoinduced $η$-pairing at finite temperatures

We numerically prove photoinduced $η$-pairing in a half-filled fermionic Hubbard chain at both zero and finite temperature. The result, obtained by combining the matrix-product-state based infinite time-evolving block decimation technique and the purification method, applies to the thermodynamic limit. Exciting the Mott insulator by a laser electric field docked on via the Peierls phase, we track the time-evolution of the correlated many-body system and determine the optimal parameter set for which the nonlocal part of the $η$-pair correlation function becomes dominant during the laser pump at zero and low temperatures. These correlations vanish at higher temperatures and long times after pulse irradiation. In the high laser frequency strong Coulomb coupling regime we observe a remnant enhancement of the Brillouin-zone boundary pair-correlation function also at high temperatures, if the Hubbard interaction is about a multiple of the laser frequency, which can be attributed to an enhanced double occupancy in the virtual Floquet state.

cond-mat.str-el↗

arXiv2020

Superconductivity and charge density wave under a time-dependent periodic field in the one-dimensional attractive Hubbard model

We investigate the competition between superconductivity (SC) and charge density wave (CDW) under a time-dependent periodic field in the attractive Hubbard model. By employing the time-dependent exact diagonalization method, we show that the driving frequency and amplitude of the external field can control the enhancement of either the superconducting pair or the CDW correlations, which are degenerate in the ground state of the half-filled attractive Hubbard model in the absence of the field. In the strong-coupling limit of the attractive Hubbard interaction, the controllability is characterized by the anisotropic interaction of the effective model. The anisotropy is induced by the external field and lifts the degeneracy of SC and CDW. We find that the enhancement or suppression of the superconducting pair and CDW correlations in the periodically-driven attractive Hubbard model can be well interpreted by the quench dynamics of the effective model derived in the strong-coupling limit.

cond-mat.str-el↗

arXiv2020

Thermodynamic properties of an $S=1/2$ ring-exchange model on the triangular lattice

By using a numerically exact diagonalization technique and a block-extended version of the finite-temperature Lanczos method, we study thermodynamic properties of an $S=1/2$ Heisenberg model on the triangular lattice with an antiferromagnetic nearest-neighbor interaction $J$ and a four-spin ring-exchange interaction $J_{\rm c}$. Calculations are performed on small clusters under the periodic-boundary conditions. In contrast to the purely triangular case with $J_{\rm c}=0$, the specific heat exhibits a characteristic double-peak structure for $J_{\rm c}/J \gtrsim 0.04$. From the calculation of the entropy and the uniform magnetic susceptibility, it is shown that non-magnetic excitations exist below the magnetic excitation for $J_{\rm c}/J \gtrsim 0.04$.

cond-mat.str-el↗

arXiv2020

Symmetry-adapted variational quantum eigensolver

We propose a scheme to restore spatial symmetry of Hamiltonian in the variational-quantum-eigensolver (VQE) algorithm for which the quantum circuit structures used usually break the Hamiltonian symmetry. The symmetry-adapted VQE scheme introduced here simply applies the projection operator, which is Hermitian but not unitary, to restore the spatial symmetry in a desired irreducible representation of the spatial group. The entanglement of a quantum state is still represented in a quantum circuit but the nonunitarity of the projection operator is treated classically as postprocessing in the VQE framework. By numerical simulations for a spin-$1/2$ Heisenberg model on a one-dimensional ring, we demonstrate that the symmetry-adapted VQE scheme with a shallower quantum circuit can achieve significant improvement in terms of the fidelity of the ground state and has a great advantage in terms of the ground-state energy with decent accuracy, as compared to the non-symmetry-adapted VQE scheme. We also demonstrate that the present scheme can approximate low-lying excited states that can be specified by symmetry sectors, using the same circuit structure for the ground-state calculation.

quant-ph↗

arXiv2020

Characterization of photoexcited states in the half-filled one-dimensional extended Hubbard model assisted by machine learning

Photoinduced nonequilibrium states can provide new insight into dynamical properties of strongly correlated electron systems. One of the typical and extensively studied systems is the half-filled one-dimensional extended Hubbard model (1DEHM). Here, we propose that the supervised machine learning (ML) can provide useful information for characterizing photoexcited states in 1DEHM. Using entanglement spectra as a training dataset, we construct neural network. Judging from the trained network, we find that bond-spin-density wave (BSDW) order can be enhanced in photoexcited states if the frequency of a driving pulse nearly resonates with gap. We separately calculate the time evolution of local and non-local order parameters and confirm that the correlation functions of BSDW are actually enhanced by photoexcitation as predicted by ML. The successful prediction of BSDW demonstrates the advantage of ML to assist characterizing photoexcited quantum states.

cond-mat.str-el↗

arXiv2020

Finite-$m$ scaling analysis of Berezinskii-Kosterlitz-Thouless phase transitions and entanglement spectrum for the six-state clock model

We investigate the Berezinskii-Kosterlitz-Thouless transitions for the square-lattice six-state clock model with the corner-transfer matrix renormalization group (CTMRG). Scaling analyses for effective correlation length, magnetization, and entanglement entropy with respect to the cutoff dimension $m$ at the fixed point of CTMRG provide transition temperatures consistent with a variety of recent numerical studies. We also reveal that the fixed point spectrum of the corner transfer matrix in the critical intermediate phase of the six-state clock model is characterized by the scaling dimension consistent with the $c=1$ boundary conformal field theory associated with the effective $Z_6$ dual sine-Gordon model.

cond-mat.stat-mech↗

arXiv2020

Photoinduced $η$ pairing in the Kondo lattice model

The previous theoretical study has shown that pulse irradiation to the Mott insulating state in the Hubbard model can induce the enhancement of superconducting correlation due to the generation of $η$ pairs. Here, we show that the same mechanism can be applied to the Kondo lattice model, an effective model for heavy electron systems, by demonstrating that the pulse irradiation indeed enhances the $η$-pairing correlation. As in the case of the Hubbard model, the non-linear optical process is essential to increase the number of photoinduced $η$ pairs and thus the enhancement of the superconducting correlation. We also find the diffusive behavior of the spin dynamics after the pulse irradiation, suggesting that the increase of the number of $η$ pairs leads to the decoupling between the conduction band and the localized spins in the Kondo lattice model, which is inseparably related to the photodoping effect.

cond-mat.str-el↗

arXiv2020

Photoinduced $η$-pairing in One-dimensional Mott Insulators

Employing the density-matrix renormalization group technique in the matrix-product-state representation, we investigate the photoexcited superconducting correlations induced by the $η$-pairing mechanism in the half-filled Hubbard chain. We estimate the characteristic pair correlation function and verify the accuracy of our numerical results by comparison with exact-diagonalization data for small systems. The optimal parameter set of the pump that most enhances the $η$-pair correlations, is calculated in the strong-coupling regime. For such a pump, we explore the possibility of quasi-long-range order.

cond-mat.str-el↗

arXiv2019

Phase diagram of the two-dimensional Hubbard-Holstein model: enhancement of $s$-wave pairing between charge and magnetic orders

We investigate the role of electron-electron and electron-phonon interactions in strongly correlated systems by performing unbiased quantum Monte Carlo simulations in the square lattice Hubbard-Holstein model at half-filling. We study the competition and interplay between antiferromagnetism (AFM) and charge-density wave (CDW), establishing its very rich phase diagram. In the region between AFM and CDW phases, we have found an enhancement of superconducting pairing correlations, favoring (nonlocal) $s$-wave pairs. Our study sheds light over past inconsistencies in the literature, in particular the emergence of CDW in the pure Holstein model case.

cond-mat.str-el↗

arXiv2019

Non-Fermi-liquid behavior and doping asymmetry in an organic Mott insulator interface

High-$T_{\rm C}$ superconductors show anomalous transport properties in their normal states, such as the bad-metal and pseudogap behaviors. To discuss their origins, it is important to speculate whether these behaviors are material-dependent or universal phenomena in the proximity of the Mott transition, by investigating similar but different material systems. An organic Mott transistor is suitable for this purpose owing to the adjacency between the two-dimensional Mott insulating and superconducting states, simple electronic properties, and high doping/bandwidth tunability in the same sample. Here we report the temperature dependence of the transport properties under electron and hole doping in an organic Mott electric-double-layer transistor. At high temperatures, the bad-metal behavior widely appears except at half filling regardless of the doping polarity. At lower temperatures, the pseudogap behavior is observed only under hole doping, while the Fermi-liquid-like behavior is observed under electron doping. The bad-metal behavior seems a universal high-energy scale phenomenon, while the pseudogap behavior is based on lower energy scale physics that can be influenced by details of the band structure.

cond-mat.str-el↗

arXiv2019

Mechanism of superconductivity and electron-hole doping asymmetry in $κ$-type molecular conductors

Unconventional superconductivity in molecular conductors is observed at the border of metal-insulator transitions in correlated electrons under the influence of geometrical frustration. The symmetry as well as the mechanism of the superconductivity (SC) is highly controversial. To address this issue, we theoretically explore the electronic properties of carrier-doped molecular Mott system $κ$-(BEDT-TTF)$_2$X. We find significant electron-hole doping asymmetry in the phase diagram where antiferromagnetic (AF) spin order, different patterns of charge order, and SC compete with each other. Hole-doping stabilizes AF phase and promotes SC with "$d_{xy}$"-wave symmetry, which has similarities with high-$T_c$ cuprates. In contrast, in the electron-doped side, geometrical frustration destabilizes the AF phase and the enhanced charge correlation induces another SC with "extended-$s$+$d_{x^2-y^2}$"-wave symmetry. Our results disclose the mechanism of each phase appearing in filling-control molecular Mott systems, and elucidate how physics of different strongly-correlated electrons are connected, namely, molecular conductors and high-$T_c$ cuprates.

cond-mat.supr-con↗

arXiv2019

Photoinduced electron-electron pairing in the extended Falicov-Kimball model

By employing the time-dependent exact diagonalization method, we investigate the photoexcited states of the excitonic insulator in the extended Falicov-Kimball model (EFKM). We here show that the pulse irradiation can induce the interband electron-electron pair correlation in the photoexcited states, while the excitonic electron-hole pair correlation in the initial ground state is strongly suppressed. We also show that the photoexcited states contains the eigenstates of the EFKM with a finite number of interband electron-electron pairs, which are responsible for the enhancement of the electron-electron pair correlation. The mechanism found here is due to the presence of the internal SU(2) pairing structure in the EFKM and thus it is essentially the same as that for the photoinduced $η$-pairing in the repulsive Hubbard model reported recently [T. Kaneko et al., Phys. Rev. Lett. ${\bf 122}$, 077002 (2019)]. This also explains why the nonlinear optical response is effective to induce the electron-electron pairs in the photoexcited states of the EFKM. Furthermore, we show that, unlike the $η$-pairing in the Hubbard model, the internal SU(2) structure is preserved even for a nonbipartite lattice when the EFKM has the direct-type band structure, in which the pulse irradiation can induce the electron-electron pair correlation with momentum ${\it {\bf q}}$ = ${\textbf 0}$ in the photoexcited states. We also discuss briefly the effect of a perturbation that breaks the internal SU(2) structure.

cond-mat.str-el↗

arXiv2019

Novel MAB phases and insights into their exfoliation into 2D MBenes

Considering the recent breakthroughs in the synthesis of novel two-dimensional (2D) materials from layered bulk structures, ternary layered transition metal borides, known as MAB phases, have come under scrutiny as a means of obtaining novel 2D transition metal borides, so-called MBene. Here, based on a set of phonon calculations, we show the dynamic stability of many Al-containing MAB phases, MAlB (M = Ti, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc), M$_2$AlB$_2$ (Sc, Ti, Zr, Hf, V, Cr, Mo, W, Mn, Tc, Fe, Rh, Ni), M$_3$Al$_2$B$_2$ (M = Sc, T, Zr, Hf, Cr, Mn, Tc, Fe, Ru, Ni), M$_3$AlB$_4$ (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe), and M$_4$AlB$_6$ (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo). By comparing the formation energies of these MAB phases with those of their available competing binary M$-$B and M$-$Al, and ternary M$-$Al$-$B phases, we find that some of the Sc-, Ti-, V-, Cr-, Mo-, W-, Mn-, Tc-, and Fe-based MAB phases could be favorably synthesized in an appropriate experimental condition. In addition, by examining the strengths of various bonds in MAB phases via crystal orbital Hamilton population and spring constant calculations, we find that the B$-$B and then M$-$B bonds are stiffer than the M$-$Al and Al$-$B bonds. The different strength between these bonds implies the etching possibility of Al atoms from MAB phases, consequently forming various 2D MB, M$_2$B$_3$, and M$_3$B$_4$ MBenes. Furthermore, we employ the nudged elastic band method to investigate the possibility of the structural phase transformation of the 2D MB MBenes into graphene-like boron sheets sandwiched between transition metals and find that the energy barrier of the transformation is less than $0.4$ eV/atom.

cond-mat.mtrl-sci↗

arXiv2019

Machine Learning Phase Diagram in the Half-filled One-dimensional Extended Hubbard Model

We demonstrate that supervised machine learning (ML) with entanglement spectrum can give useful information for constructing phase diagram in the half-filled one-dimensional extended Hubbard model. Combining ML with infinite-size density-matrix renormalization group, we confirm that bond-order-wave phase remains stable in the thermodynamic limit.

cond-mat.str-el↗

arXiv2019

Two-dimensional ground-state mapping of a Mott-Hubbard system in a flexible field-effect device

A Mott insulator sometimes induces unconventional superconductivity in its neighbors when doped and/or pressurized. Because the phase diagram should be strongly related to the microscopic mechanism of the superconductivity, it is important to obtain the global phase diagram surrounding the Mott insulating state. However, the parameter available for controlling the ground state of most Mott insulating materials is one-dimensional owing to technical limitations. Here we present a two-dimensional ground-state mapping for a Mott insulator using an organic field-effect device by simultaneously tuning the bandwidth and bandfilling. The observed phase diagram showed many unexpected features such as an abrupt first-order superconducting transition under electron doping, a recurrent insulating phase in the heavily electron-doped region, and a nearly constant superconducting transition temperature in a wide parameter range. These results are expected to contribute toward elucidating one of the standard solutions for the Mott-Hubbard model. Theoretical calculations reproducing the main features of our experimental results have also been conducted.

cond-mat.str-el↗

arXiv2019

Topological property of a $t_{2g}^5$ system with a honeycomb lattice structure

A $t_{2g}^5$ system with a honeycomb lattice structure such as Na$_2$IrO$_3$ was firstly proposed as a topological insulator even though Na$_2$IrO$_3$ and its isostructural materials in nature have been turned out to be a Mott insulator with magnetic order. Here we theoretically revisit the topological property based on a minimal tight-binding Hamiltonian for three $t_{2g}$ bands incorporating a strong spin orbit coupling and two types of the first nearest neighbor (NN) hopping channel between transition metal ions, i.e., the hopping ($t_1$) mediated by edge-shared ligands and the direct hopping ($t_1'$) between $t_{2g}$ orbitals via $ddσ$ bonding. We demonstrate that the topological phase transition takes place by varying only these hopping parameters with the relative strength parametrized by $θ$, i.e., $t_1=t\cosθ$ and $t_1'=t\sinθ$. We also explore the effect of the second and third NN hopping channels, and the trigonal distortion on the topological phase for the whole range of $θ$. Furthermore, we examine the electronic and topological phases in the presence of on-site Coulomb repulsion $U$. Employing the cluster perturbation theory, we show that, with increasing $U$, a trivial or topological band insulator in the absence of $U$ can be transferred into a Mott insulator with nontrivial or trivial band topology. We also show that the main effect of the Hund's coupling can be understood simply as the renormalization of $U$. We briefly discuss the relevance of our results to the existing materials.

cond-mat.str-el↗

arXiv2019

Possible superconductivity induced by a large spin-orbit coupling in carrier doped iridium oxide insulators: A weak coupling approach

We study possible superconductivity in a carrier-doped iridium oxide insulator Sr$_{2}$IrO$_{4}$ based on an effective $t_{2g}$ three-orbital Hubbard model on the square lattice with a large spin-orbit coupling (SOC). Numerically solving the linearized Eliashberg equation for the superconducting (SC) gap function with the random phase approximation, we systematically examine both singlet and triplet SC gap functions with possible pairing symmetries and the parameter dependence of the superconductivity. For the realistic SOC $λ$ and Hund's coupling $J/U$ relevant to Sr$_{2}$IrO$_{4}$, namely, for a large $λ$ and small $J/U$ region, we find that the intra-band antiferromagnetic (AF) pseudospin $\bm{j}_{\text{eff}} = -\bm{l}+\bm{s}$ fluctuations favor a $d_{x^{2}-y^{2}}$-wave pseudospin $j_{\text{eff}}=1/2$ singlet pairing in the electron-doping. We also find that the $d_{x^{2}-y^{2}}$-wave pairing is more stabilized with increasing the SOC and decreasing the Hund's coupling. Furthermore, we show for a small $λ$ and large $J/U$ region that an $s_{\pm}$-wave singlet pairing is favored in the hole-doped region. The origin of the $s_{\pm}$-wave pairing is due to the inter-band pair scattering arising from the intra-orbital AF spin $\bm s$ fluctuations. Although the possibility of a pseudospin triplet pairing is considered, we find it always unfavorable for all parameters studied here. The experimental consequences for other strongly correlated materials with a large SOC are also discussed.

cond-mat.supr-con↗

arXiv2019

Photoinduced $η$ Pairing in the Hubbard Model

By employing unbiased numerical methods, we show that pulse irradiation can induce unconventional superconductivity even in the Mott insulator of the Hubbard model. The superconductivity found here in the photoexcited state is due to the $η$-pairing mechanism, characterized by staggered pair-density-wave oscillations in the off-diagonal long-range correlation, and is absent in the ground-state phase diagram; i.e., it is induced neither by a change of the effective interaction of the Hubbard model nor by simple photocarrier doping. Because of the selection rule, we show that the nonlinear optical response is essential to increase the number of $η$ pairs and thus enhance the superconducting correlation in the photoexcited state. Our finding demonstrates that nonequilibrium many-body dynamics is an alternative pathway to access a new exotic quantum state that is absent in the ground-state phase diagram and also provides an alternative mechanism for enhancing superconductivity.

cond-mat.str-el↗