Excitonic optical spectra and energy structures in a one-dimensional Mott insulator demonstrated by applying a many-body Wannier functions method to a charge model
T. Yamaguchi, K. Iwano, T. Miyamoto, N. Takamura, N. Kida, Y. Takahashi, T. Hasegawa, H. Okamoto
We have applied a many-body Wannier functions method to theoretically calculate an excitonic optical conductivity spectrum and energy structure in a one-dimensional (1D) Mott insulator at absolute zero temperature with large system size. Focusing on full charge fluctuations associated with pairs of a holon and doublon, we employ a charge model, which is interpreted as a good effective model to investigate photoexcitations of a 1D extended Hubbard model at half-filling in the spin-charge separation picture. As a result, the theoretical spectra with appropriate broadenings qualitatively reproduce the recent experimental data of ET-F$_{2}$TCNQ at 294 K with and without a modulated electric field. Regarding the excitonic energy structure, we have found that the excitons, especially for even-parity, are weakly bound by many-body effects. This is also consistent with the fitting parameters reported in the recent experiment. Thus, our theoretical method presented in this paper is practically useful to understand physical roles of charge fluctuations in many-body excited states of a 1D Mott insulator.
This letter extends my recent paper on antiferromagnetic NiO [Structural Distortion Stabilizing the Antiferromagnetic and Insulating Ground State of NiO, Symmetry 2020, 12(1), 56] by including also the paramagnetic phase of this compound. I report evidence that paramagnetic NiO possesses a narrow, roughly half-filled energy band that produces a nonadiabatic atomic-like motion providing the basis for a Mott insulator in the paramagnetic phase. While the atomic-like motion operating in the antiferromagnetic phase is adapted to the symmetry of the antiferromagnetic state, in the paramagnetic phas…
To study excitonic effects on high-harmonic generation (HHG) in Mott insulators, we investigate pumped nonequilibrium dynamics in the one-dimensional extended Hubbard model. By employing time-dependent calculations based on the exact diagonalization and infinite time-evolving block decimation methods, we find the strong enhancement of the HHG intensity around the exciton energy. The subcycle analysis in the sub-Mott-gap regime shows that the intensity region of the time-resolved spectrum around the exciton energy splits into two levels and oscillates following the driving electric field. This …
We show how strongly correlated materials could be described within the framework of an excitonic insulator formalism, and delineate the relationship between inter- and intra-band ordering phenomena. Our microscopic model of excitons clarifies the fundamental role of Van-Hove-singularity-nesting in driving both inter- and intra-band ordering transitions, and uncovers an interesting connection with resonating-valence-bond physics.
We investigate the effect of nonlocal interactions on the photo-doped Mott insulating state of the two-dimensional Hubbard model using a nonequilibrium generalization of the dynamical cluster approximation. In particular, we compare the situation where the excitonic states are lying within the continuum of doublon-holon excitations to a set-up where the excitons appear within the Mott gap. In the first case, the creation of nearest-neighbor doublon-holon pairs by excitations across the Mott gap results in enhanced excitonic correlations, but these excitons quickly decay into uncorrelated doubl…
Extended Hubbard model with renormalized Wannier wave functions in the correlated state II: Quantum critical scaling of the wave function near the Mott-Hubbard transition
我们给出了一个由Slater s轨道组成的重正化单粒子Wannier函数的量子临界行为的模型例子，并在一个可调的高斯STO-7 G基表示，这是计算立方晶格的Gutzwiller关联态附近的金属-绝缘体转变（MIT）。讨论是在扩展的哈伯德模型和早先提出的方法[cf. EUR. J. B { \bf 66}，385（2008）]。组分原子波函数大小、Wannier函数最大值以及系统能量都随着晶格参数$R$的增加而成比例，如$[(R-R_{c})/R_{c}]^{s}$与$s$在间隔$[0.9,1.0]$中。这种标度律被解释为粒子间库仑排斥的主导作用的证据，这对于$R>R_c$是intersite字符。还得到了绝缘体-金属过渡晶格参数值$R=R_{c}$与原{ \em Mott判据}的关系。方法的可行性进行了测试，通过比较我们的结果与哈伯德链，其中Mott-Hubbard过渡是缺席的确切方法。鉴于我们的结果的独特功能，在定性方面也提供了广泛的讨论。
The real part of optical conductivity $\text{Re}σ(ω)$ of the Mott insulators has a large amount of information on how spin and charge degrees of freedom interact with each other. By using the time-dependent density-matrix renormalization group, we study $\text{Re}σ(ω)$ of the two-dimensional Hubbard model on a square lattice at half filling. We find an excitonic peak at the Mott-gap edge of $\text{Re}σ(ω)$ not only for the two-dimensional square lattice but also for two- and four-leg ladders. For the square lattice, however, we do not clearly find a gap between an excitonic peak and continuum …
We point out the universal presence of the excitonic superexchange in spin-orbit-coupled Mott insulators. It is observed that, the restriction to the lowest spin-orbit-entangled "$J$" states may sometimes be insufficient to characterize the microscopic physics, and the virtual excitonic processes via the upper "$J$" states provide an important correction to the superexchange. We illustrate this excitonic superexchange from a two-dimensional $5d$ iridate Sr$_2$IrO$_4$ and explain its physical consequences such as the orbital-like coupling to the external magnetic flux and the nonlinear magnetic…
A three-orbital model derived from the two-dimensional projection of the $ab$ $initio$ Hamiltonian for alkaline doped fullerene A$_3$C$_{60}$ with A=Cs,Rb,K is studied by a variational Monte Carlo method. We correctly reproduce the experimental isotropic s-wave superconductivity around the $ab$ $initio$ parameters. With narrowing the bandwidth, the transition to an insulator is also reproduced, where orbital symmetry is found to be spontaneously broken with emergence of an excitonic Mott insulator for two orbitals and an antiferromagnetic insulator nearly degenerate with a spin liquid for the …
Observation of excitonic Floquet states in a one-dimensional organic Mott insulator using mid-infrared pump near-infrared probe reflection spectroscopy
We define, compute and analyze the nonequilibrium differential optical conductivity of the one-dimensional extended Hubbard model at half-filling after applying a pump pulse, using the time-dependent density matrix renormalization group method. The melting of the Mott insulator is accompanied by a suppression of the local magnetic moment and ensuing photogeneration of doublon-holon pairs. The differential optical conductivity reveals $(i)$ mid-gap states related to parity-forbidden optical states, and $(ii)$ strong renormalization and hybridization of the excitonic resonance and the absorption…
Time-resolved soft-X-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe$_2$. We present a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels reveals a delayed core-hole renormalization due to screening by excited quasi-free carriers, revealing an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectr…
Extended Hubbard model with renormalized Wannier wave functions in the correlated state III: Statistically consistent Gutzwiller approximation and the metallization of atomic solid hydrogen
Using the variational cluster approach based on the self-energy functional theory, we study the possible occurrence of excitonic order and superconductivity in the two-orbital Hubbard model with intra- and inter-orbital Coulomb interactions. It is known that an antiferromagnetic Mott insulator state appears in the regime of strong intra-orbital interaction, a band insulator state appears in the regime of strong inter-orbital interaction, and an excitonic insulator state appears between them. In addition to these states, we find that the $s^{\pm}$-wave superconducting state appears in the small…
Oxygen vacancies are a common source of excess electrons in complex oxides. In Mott insulators these additional electrons can induce a metal-insulator transition (MIT), fundamentally altering the electronic properties of the system. Here we study the effect of oxygen vacancies in LaTiO3, a prototypical Mott insulator close to the MIT. We show that the introduction of oxygen vacancies creates a vacancy-related band immediately below the partially filled Ti-t2g bands. We study the effect of this additional band on the Mott MIT using a combination of density functional theory and dynamical mean-f…
Many-Body Renormalization of Semiconductor Quantum Wire Excitons: Absorption, Gain, Binding, Unbinding, and Mott Transition
通过求解动态屏蔽的Bethe-Salpeter库仑相互作用方程，从理论上研究了GaAs量子线结构中准一维多体激子在外部光激发条件下的形成和稳定性.与最近的几个实验结果一致，计算的激子峰显示出非常弱的载流子密度依赖性高达（甚至高于）莫特跃迁密度，$n_c\sim3\times10^5$ cm $^{-1}$。在$n_c$之上，我们发现了相当大的光学增益，有力地证明了一维量子线激光器运行的可能性。
Many-body Effects on Excitonic Optical Properties of Photoexcited Semiconductor Quantum Wire Structures
通过求解动态屏蔽的Bethe-Salpeter方程，研究了载流子相互作用引起的多体效应对高度光激发的一维半导体量子线系统激子光学性质的影响.包括电子/空穴自能和电子-空穴相互作用顶点函数中的动态屏蔽效应，我们发现对于大范围的光激发密度（$n= 0-6\times 10^5$ cm $^{-1}$），激子吸收基本上在恒定能量处达到峰值，高于该峰值吸收峰消失而没有明显的增益，即，观察到\textit{no}激子到自由电子-空穴等离子体的Mott跃迁，与以前的理论结果相反，但与最近的实验结果一致。这种增益的缺乏（或莫特跃迁的不存在）源于一维等离子体激元或电荷密度激发的强非弹性散射，与一维系统的非费米液体性质密切相关。我们的理论工作证明了一维光激发电子-空穴系统从低密度区（$n<10^5$ cm $^{-1}$）的非相互作用激子稀气体的有效费米液体行为到高密度区（$n>10^5$ cm $^{-1}$）的非费米液体行为的转变或交叉.对该问题进行了常规的准静态近似，并与全动力学结果进行了比较。激子结合能和吸收光谱的数值结果作为载流子密度和温度的函数。
Theory of doped Mott insulators is revisited in the light of recent understanding on the singular self-energy structure of the single-particle Green's function. The unique pole structure in the self-energy induces the high-temperature superconductivity in the anomalous part, while it generates Mott gap and pseudogap in the normal part. Here, we elucidate that fractionalization of electrons, which is exactly hold in the Mott insulator in the atomic limit, more generally produces the emergent Mott-gap fermion and dark (hidden) fermions. It does not require any spontaneous symmetry breaking. The …
Optical responses in an excitonic insulating (EI) system with strong electron correlation are studied. We adopt the two-orbital Hubbard model with a finite energy difference between the two orbitals where the spin state degree of freedom exists. This model is analyzed by the variational cluster approach. In order to include the local electron correlation effect, the vertex correction is taken into account in the formulation of the optical conductivity spectra. We calculate a finite-temperature phase diagram, in which an EI phase appears between a low-spin band insulating state and a high-spin …
We study the effects of electron doping in Mott insulators containing d^4 ions such as Ru4+, Os4+, Rh5+, and Ir5+ with J=0 singlet ground state. Depending on the strength of the spin-orbit coupling, the undoped systems are either nonmagnetic or host an unusual, excitonic magnetism arising from a condensation of the excited J=1 triplet states of t_2g^4. We find that the interaction between J-excitons and doped carriers strongly supports ferromagnetism, converting both the nonmagnetic and antiferromagnetic phases of the parent insulator into a ferromagnetic metal, and further to a nonmagnetic me…
Excitons with large binding energies $\sim$2-3 eV in CrX$_{3}$ are historically characterized as being localized (Frenkel) excitons that emerge from the atomic $d{-}d$ transitions between the Cr-3$d$-$t_{2g}$ and $e_{g}$ orbitals. The argument has gathered strength in recent years as the excitons in recently made monolayers are found at almost the same energies as the bulk. The Laporte rule, which restricts such parity forbidden atomic transitions, can relax if, at least, one element is present: spin-orbit coupling, odd-parity phonons or Jahn-Teller distortion. While what can be classified as …
We study the effects of electronic correlations on fragile topology using dynamical mean-field theory. Fragile topological insulators (FTIs) offer obstruction to the formation of exponentially localized Wannier functions, but they can be trivialized by adding certain trivial degrees of freedom. For the same reason, FTIs do not host symmetry-protected flow of edge states between bulk bands in cylindrical boundary conditions but are expected to have a spectral flow between the fragile bands and other bands under certain twisted boundary conditions. We here analyze commonly observed effects of st…
Two-dimensional (2D) moire materials provide a new solid-state platform with unprecedented controllability for studies of correlated quantum phenomena. To date, experimental studies have focused on the correlated electronic states; the correlated bosonic states in moire materials have remained practically unexplored. Here, we report a correlated dipolar excitonic insulator, a charge insulating state driven by the formation of excitons, in a Coulomb-coupled WSe2 monolayer and WSe2/WS2 moire bilayer at total hole doping density equal to the moire density. The system is a Mott insulator when all …
Spin-orbit Mott insulators composed of $t_{2g}^4$ transition metal ions may host excitonic magnetism due to the condensation of spin-orbital $J=1$ triplons. Prior experiments suggest that the $4d$ antiferromagnet Ca$_{2}$RuO$_{4}$ embodies this notion, but a $J = 0$ nonmagnetic state as a basis of the excitonic picture remains to be confirmed. We use Ru $L_3$-edge resonant inelastic x-ray scattering to reveal archetypal $J$ multiplets with a $J=0$ ground state in the cubic compound K$_{2}$RuCl$_{6}$, which are well described within the $LS$-coupling scheme. This result highlights the critical …
We study the spectral and transport properties of a Mott insulator driven by a static electric field into a non-equilibrium steady state. For the dissipation, we consider two mechanisms: Wide-band fermion reservoirs and phonons included within the Migdal approximation. The electron correlations are treated via non-equilibrium dynamical mean field theory with an impurity solver suitable for strong correlations. We find that dissipation via phonons is limited to restricted ranges of field values around Wannier-Stark resonances. To cover the full range of field strengths, we allow for a small cou…
We propose a new mechanism for quantum anomalous Hall (QAH) effect in the AB stacked MoTe$_2$/WSe$_2$ system. Based on the observation that the inter-layer tunneling is suppressed in the AB stacking, we consider a model with two layers coupled through the Coulomb interaction. The moiré lattices of the two layers are shifted to form a honeycomb lattice together. Initially, the system is in a layer-polarized Mott insulator at $ν_T=1$. But with a displacement field, an equal number of holes and electrons are doped into the two layers respectively and they form inter-layer exciton condensation. Th…
