Two-orbital System Research Articles

Hund Physics Landscape of Two-Orbital Systems.

Motivated by the recent discovery of superconductivity in infinite-layer nickelates RE_{1-δ}Sr_{δ}NiO_{2} (RE=Nd, Pr), we study the role of Hund coupling J in a quarter-filled two-orbital Hubbard model, which has been on the periphery of the attention. A region of negative effective Coulomb interaction of this model is revealed to be differentiated from three- and five-orbital models in their typical Hund metal active fillings. We identify distinctive regimes including four different correlated metals, one of which stems from the proximity to a Mott insulator, while the other three, which we call "intermediate" metal, weak Hund metal, and valence-skipping metal, from the effect of J being away from Mottness. Defining criteria characterizing these metals is suggested, establishing the existence of Hund metallicity in two-orbital systems.

Observation of spin-exchange dynamics between itinerant and localized Yb171 atoms

We report on the observation of the spin-exchange dynamics of $^{171}\mathrm{Yb}$ atoms in the ground state $^1\mathrm{S}_0$ and in the metastable state $^3\mathrm{P}_0$. We implement the mixed-dimensional two-orbital system using a near-resonant and magic-wavelength optical lattices, where the $^1\mathrm{S}_0$ and $^3\mathrm{P}_0$ atoms are itinerant in a one-dimensional tube and localized in three dimensions, respectively. By exploiting an optical Stern-Gerlach method, we observe the spin depolarization of the $^1\mathrm{S}_0$ atoms induced by the spin-exchange interaction with the $^3\mathrm{P}_0$ atom. Our work could open the way to the quantum simulation of the Kondo effect.

Antiferromagnetic interorbital spin-exchange interaction of Yb171

We report on the investigation of the scattering properties between the ground state $^1S_0$ and the metastable state $^3P_0$ of the fermionic isotope of $^{171}$Yb. We successfully measure the $s$-wave scattering lengths in the two-orbital collision channels as $a_{eg}^+=225(13)a_0$ and $a_{eg}^-=355(6)a_0$, using the clock transition spectroscopy in a three-dimensional optical lattice. The result shows that the interorbital spin-exchange interaction is antiferromagnetic, indicating that $^{171}$Yb atom is a promising isotope for the quantum simulation of the Kondo effect with the two-orbital system.

Cavity-assisted mesoscopic transport of fermions: Coherent and dissipative dynamics

We study the interplay between charge transport and light-matter interactions in a confined geometry, by considering an open, mesoscopic chain of two-orbital systems resonantly coupled to a single bosonic mode close to its vacuum state. We introduce and benchmark different methods based on self-consistent solutions of non-equilibrium Green's functions and numerical simulations of the quantum master equation, and derive both analytical and numerical results. It is shown that in the dissipative regime where the cavity photon decay rate is the largest parameter, the light-matter coupling is responsible for a steady-state current enhancement scaling with the cooperativity parameter. We further identify different regimes of interest depending on the ratio between the cavity decay rate and the electronic bandwidth. Considering the situation where the lower band has a vanishing bandwidth, we show that for a high-finesse cavity, the properties of the resonant Bloch state in the upper band are transfered to the lower one, giving rise to a delocalized state along the chain. Conversely, in the dissipative regime with low cavity quality factors, we find that the current enhancement is due to a collective decay of populations from the upper to the lower band.

Basic Properties of Conductivity and Normal Hall Effect in the Periodic Anderson Model

Exact formulas of diagonal conductivity $\sigma_{xx}$ and Hall conductivity $\sigma_{xy}$ are derived from the Kubo formula in hybridized two-orbital systems with arbitrary band dispersions. On the basis of the theoretical framework for the Fermi liquid based on these formulas, the ground-state properties of the periodic Anderson model with electron correlation and weak impurity scattering are studied on the square lattice. It is shown that imbalance of the mass-renormalization factors in $\sigma_{xx}$ and $\sigma_{xy}$ causes remarkable increase in the valence-fluctuation regime as the f level increases while the cancellation of the renormalization factors causes slight increase in $\sigma_{xx}$ and $\sigma_{xy}$ in the Kondo regime. The Hall coefficient $R_{\rm H}$ shows almost constant behavior in both the regimes. Near half filling, $R_{\rm H}$ is expressed by the total hole density as $R_{\rm H}=1/(\bar{n}_{\rm hole}e)$ while $R_{\rm H}$ approaches zero near quarter filling, which reflects the curvature of the Fermi surface. These results hold as far as the damping rate for f electrons is less than about $10~\%$ of the renormalized hybridization gap. From these results we discuss pressure dependence of residual resistivity and normal Hall effect in Ce- and Yb-based heavy electron systems.

Room temperature quantum spin Hall insulators with a buckled square lattice.

Two-dimensional (2D) topological insulators (TIs), also known as quantum spin Hall (QSH) insulators, are excellent candidates for coherent spin transport related applications because the edge states of 2D TIs are robust against nonmagnetic impurities since the only available backscattering channel is forbidden. Currently, most known 2D TIs are based on a hexagonal (specifically, honeycomb) lattice. Here, we propose that there exists the quantum spin Hall effect (QSHE) in a buckled square lattice. Through performing global structure optimization, we predict a new three-layer quasi-2D (Q2D) structure, which has the lowest energy among all structures with the thickness less than 6.0 Å for the BiF system. It is identified to be a Q2D TI with a large band gap (0.69 eV). The electronic states of the Q2D BiF system near the Fermi level are mainly contributed by the middle Bi square lattice, which are sandwiched by two inert BiF2 layers. This is beneficial since the interaction between a substrate and the Q2D material may not change the topological properties of the system, as we demonstrate in the case of the NaF substrate. Finally, we come up with a new tight-binding model for a two-orbital system with the buckled square lattice to explain the low-energy physics of the Q2D BiF material. Our study not only predicts a QSH insulator for realistic room temperature applications but also provides a new lattice system for engineering topological states such as quantum anomalous Hall effect.

Multiorbital cluster dynamical mean-field theory with an improved continuous-time quantum Monte Carlo algorithm

We implement a multi-orbital cluster dynamical mean-field theory (DMFT), by improving a sample-update algorithm in the continuous-time quantum Monte Carlo method based on the interaction expansion. The proposed sampling scheme for the spin-flip and pair-hopping interactions in the two-orbital systems mitigates the sign problem, giving an efficient way to deal with these interactions. In particular, in the single-site DMFT, we see that the negative signs vanish. We apply the method to the two-dimensional two-orbital Hubbard model at half filling, where we take into account the short-range spatial correlation effects within a four-site cluster. We show that, compared to the single-site DMFT results, the critical interaction value for the metal-insulator transition decreases and that the effects of the spin-flip and pair-hopping terms are less significant in the parameter region we have studied. The present method provides a firm starting point for the study of inter-site correlations in multi-orbital systems. It also has a wide applicable scope in terms of realistic calculations in conjunction with density functional theory.

Two-orbital physics of high-spin fermionic alkaline-earth atoms confined in a one-dimensional chain

We study the effect of the coupling between the electronic ground state of high spin alkaline-earth fermionic atoms and their metastable optically excited state, when the system is confined in a one-dimensional chain, and show that the system provides a possible realization of a finite momentum pairing (Fulde-Ferrell-Larkin-Ovchinnikov-like) state without spin- or bare mass imbalance. We determine the $\beta$-functions of the renormalization group trajectories for general spin and analyze the structure of the possible gapped and gapless states in the hydrodynamic limit. Due to the SU(N) symmetry in the spin space, complete mode separation can not be observed even in the fully gapless 2N-component Luttinger liquid state. Contrary, 4 velocities characterize the system. We solve the renormalization group equations for spin-9/2 strontium-87 isotope and analyze in detail its phase diagram. The fully gapless Luttinger liquid state does not stabilize in the two-orbital system of the $^{87}$Sr atoms, instead, different gapped non-Gaussian fixed points are identified either with dominant density or superconducting fluctuations. The superconducting states are stable in a nontrivial shaped region in the parameter space as a consequence of the coupling between the two electronic states.

Coherence Lengths for Superconductivity in the Two-Orbital Negative-U Hubbard Model

Multi-component superconductivity models, developed more than fty years starting from the papers [1 3], include rather varied physics. In connection with the presence of interacting order parameters the main properties of the multi-band systems are quite di erent from the corresponding characteristics in single-band superconductors. The examination of theses peculiarities has been an object of growing interest. The various multi-component theoretical scenarios have been applied for a number of superconducting materials (see [4 7] and references therein). The research activity in this direction has been especially stimulated by the acceptance of the multi-gap superconductivity in MgB2 [8], cuprates [9] and iron arsenic compounds [10]. In particular, the derivation of high-quality superconducting regions from oxygen ordering, observed recently in La2CuO4+y [11], supports the multi-band theoretical scenario of superconductivity in cuprates. In the present contribution we study the coherency of the superconducting ordering of a two-band (two-orbital) system with the negative-U Hubbard intra-orbital pairing and inter-orbital pair-transfer interaction. One can distinguish here two characteristic length scales [12] in the spatial behaviour of superconducting uctuations. One of these lengths as a function of temperature behaves critically diverging at the phase transition point. The other one remains nite and its temperature dependence is weaker. The formation of these length scales is caused by the interband interaction mixing the superconducting order parameters of initially non-interacting bands. The critical and non-critical coherence lengths associate also with critical and non-critical uctuations (see e.g. Refs. [13, 14]) which appear as the certain linear combinations of the deviations from the equilibrium band superconducting orders. Consequently, these length scales cannot be attributed to di erent bands involved [15] (see also Refs. [16 18]). Our results have been obtained using the superconducting negative-U Hubbard model [19] for a two-orbital system on a two-dimensional lattice.

Thermoelectric power of single-orbital and two-orbital Hubbard models on triangular lattices

The thermoelectric power of the single-orbital and two-orbital correlated electron systems on a two-dimensional triangular lattice is investigated by the dynamical mean field theory with the exact diagonalization solver at finite temperature. It reveals that for the hole doping case, the geometrical frustration together with the Coulomb interaction greatly enhances thermoelectric power, in comparison with the electron doping case. In an anisotropic triangular lattice, we find a large thermoelectric power in the direction with small hopping integral in the anisotropic systems. In the two-orbital systems, we show that though the orbital entropy contributes to thermoelectric power, Hund's rule coupling may suppress the large thermoelectric power.

Intramolecular charge ordering in the multi molecular orbital system (TTM-TTP)I3

Starting from the structure of the (TTM-TTP)I(3) molecular-based material, we examine the characteristics of frontier molecular orbitals using ab initio (CASSCF/CASPT2) configurations interaction calculations. It is shown that the singly occupied and second-highest-occupied molecular orbitals are close to each other, i.e., this compound should be regarded as a two-orbital system. By dividing virtually the [TTM-TTP] molecule into three fragments, an effective model is constructed to rationalize the origin of this picture. In order to investigate the low-temperature, symmetry breaking experimentally observed in the crystal, the electronic distribution in a pair of [TTM-TTP] molecules is analyzed from CASPT2 calculations. Our inspection supports and explains the speculated intramolecular charge ordering which is likely to give rise to low-energy magnetic properties.

Two-orbital systems with crystal field splitting and interorbital hopping

The nondegenerate two-orbital Hubbard model is studied within the dynamic mean-field theory to reveal the influence of two important factors, i.e. crystal field splitting and interorbital hopping, on orbital selective Mott transition (OSMT) and realistic compound Ca$_{2-x}$Sr$_{x}$RuO$_{4}$. A distinctive feature of the optical conductivity of the two nondegenerate bands is found in OSMT phase, where the metallic character of the wide band is indicated by a nonzero Drude peak, while the insulating narrow band has its Drude peak drop to zero in the mean time. We also find that the OSMT regime expands profoundly with the increase of interorbital hopping integrals. On the contrary, it is shown that large and negative level splitting of the two orbitals diminishes the OSMT regime completely. Applying the present findings to compound Ca$_{2-x}$Sr$_{x}$RuO$_{4}$, we demonstrate that in the doping region from $x=0.2$ to 2.0, the negative level splitting is unfavorable to the OSMT phase.

OPTICAL CONDUCTIVITY AND ORBITAL SUSCEPTIBILITY OF THE TWO-ORBITAL HUBBARD MODEL: EXTENDED LINEARIZED DYNAMICAL MEAN-FIELD THEORY

We study the optical conductivity and orbital susceptibility of the two-orbital Hubbard model within the extended linearized dynamical mean-field theory. In the orbital-selective Mott phase (OSMP), the optical conductivity of the wide band presents a non-zero Drude peak, while the optical conductivity of the narrow band drops to zero in the lower energy region. It is shown that the OSMP is constrained by the negative crystal-field splitting, while it can survive in the region with positive crystal-field splitting. We also find that the orbital susceptibility presents a reversion at Fermi surface in the OSMP regime, which implies that orbital ordering can coexist with OSMP in two-orbital systems.

Spectroscopic evidence for the low-temperature charge-separated state of [Pd(dmit) 2] salts

We have measured the temperature dependence of reflectivity spectra of a single crystal of (C 2H 5) 2(CH 3) 2Sb[Pd(dmit) 2] 2 (dmit = 1,3-dithiol-2-thione-4,5-dithiolate), to elucidate the origin of its phase transition at 70 K. A remarkable splitting has been found for the bonding–antibonding excitation of the dimer, [Pd(dmit) 2] 2, in the low-temperature phase. This manifests that the phase transition is due to the charge separation, 2 dimer − → dimer 0 + dimer 2−. This result, supporting our quantum chemical model for a strongly dimerized two-orbital system, gives a clue to incorporate a new spin–charge–orbital interplay in a molecular assembly.

Valence instability in a dimer of two-orbital system: possible charge separation due to `negative U' effect

A novel mechanism of charge separation in a dimer of two-orbital molecular system is proposed. The effective electron–electron repulsive energy in the dimer, U eff, has been estimated by means of valence-bond (VB) model calculations for the ground state of the dimer. It has been found that the HOMO–LUMO interplay leads to negative U eff in the strong dimerization condition, indicating spontaneous separation of charges on dimers (2 dimer − → dimer 0 + dimer 2−). Possible relevance to experimental features in some [Pd(dmit) 2] salts (dmit=1,3-dithiol-2-thione-4,5-dithiolate) is discussed.