Dynamical Mean-field Method Research Articles

Magnetic supersolid phases of two-dimensional extended Bose–Hubbard model with spin–orbit coupling

We investigate the quantum phases and phase transitions for spin–orbit coupled two-species bosons with nearest-neighbor (NN) interaction in a two-dimensional square lattice using inhomogeneous dynamical Guztwiller mean-field method. Under the effect of spin–orbit coupling and NN interaction, we uncover a rich variety of different magnetic supersolid (SS) phases. In the presence of intraspecies NN interaction, the phase diagram exhibits the phase-twisted double-checkerboard SS (PT-DCSS) and phase-striped double-checkerboard SS (PS-DCSS) phases. For both intra- and interspecies NN interactions, apart from the phase-twisted lattice SS (PT-LSS) and phase-striped lattice SS (PS-LSS) phases, some nontrivial SS phases with interesting properties occur. More importantly, we find that the emergences of these nontrivial SS phases are dependent of the interspecies on-site interaction. To further characterize the SS phases, we also discuss the spin-dependent momentum distributions and magnetic textures. The magnetic textures, such as antiferromagnetic, spiral and stripe orders are shown. Finally, we give the fully analytical insights into the numerical results.

Effect of localization–delocalization transition on thermoelectric properties of Bi2Te2Se topological insulator

The thermal transport properties of Bi2Te2Se topological insulators show a range of complex features. Large bulk resistivities coexisting with prominent Shubnikov–de Haas quantum oscillations and proximity to metallic states mark this p-band system as an unconventional topological quantum material. Here, using the density functional plus dynamical mean-field theory method, we show how localization–delocalization transition underpins the T-dependence of thermoelectric responses from room down to low temperatures. We present the implications of our many-particle analysis to resistivity, Seebeck coefficient, thermal conductivity, and Lorenz number and propose that related broadband systems close to electronic transitions could be of use in thermoelectrics.

Coulomb correlations in a germanide iron-based superconductor: The example of YFe2Ge2

We report results of a detailed theoretical study of the effect of Coulomb correlations on the structural, electronic, and magnetic properties of the non-pnictide iron-based superconductor ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$. In particular, by employing the density functional theory plus dynamical mean-field theory method ($\mathrm{DFT}+\mathrm{DMFT}$) we perform a structural optimization of the unit-cell volume and the internal coordinate ${z}_{\mathrm{Ge}}$ of Ge. Our results for the lattice parameters obtained by $\mathrm{DFT}+\mathrm{DMFT}$ show a much better agreement with experiment as compared to those calculated by DFT implying the importance of correlation effects. We show that ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$ is a moderately correlated system with electron-electron correlations being strong enough to renormalize the low-energy band structure but not sufficient to form Hubbard bands, in close analogy to the iron-based pnictides and chalcogenides. The analysis of the local spin-spin correlation function and momentum-dependent spin susceptibility $\ensuremath{\chi}(\mathbf{q})$ indicates the correlation induced formation of short-lived local moments in the Fe ${t}_{2g}$ states and the presence of competing spin fluctuations, in agreement with experiment.

Spin Seebeck coefficient and spin-thermal diffusion in the two-dimensional Hubbard model

We investigate the spin Seebeck coefficient $S_s$ in the square lattice Hubbard model at high temperatures of relevance to cold-atom measurements. We solve the model with the finite-temperature Lanczos and with the dynamical mean-field theory methods and find they give similar results in the considered regime. $S_s$ exceeds the atomic 'Heikes' estimates and the Kelvin entropic estimates drastically. We analyze the behavior in terms of a mapping onto the problem of a doped attractive model and derive an approximate expression that allows relating the enhancement of $S_s$ to distinct scattering of the spin-majority and the spin-minority excitations. Our analysis reveals the limitations of entropic interpretations of Seebeck coefficient even in the high-temperature regime. Large values of $S_s$ could be observed on optical lattices. We also calculate the full diffusion matrix. We quantify the spin-thermal diffusion, that is, the extent of the mixing between the spin and the thermal diffusion and discuss the results in the context of recent measurements of the spin-diffusion constant in cold atoms.

Quasiparticle multiplets and 5f electronic correlation in prototypical plutonium borides

In this paper, we investigate the electronic structures of plutonium borides (PuB x , x = 1, 2, 6, 12) to uncover the fascinating bonding behavior and orbital dependent correlations of 5f valence electrons by using the density functional theory combined with single-site dynamical mean-field method. We not only reproduce the correlated topological insulator of PuB6, but also predict the metallicity in PuB x (x = 1, 2, 12). It is found that the band structure, density of states, hybridization functions all indicate partially itinerant 5f states in PuB x (x = 1, 2, 6, 12). Especially, quasiparticle multiplets induced noteworthy valence state fluctuations implying the mixed-valence behavior of plutonium borides. Moreover, the itinerant degree of freedom for 5f electrons in PuB x (x = 1, 2, 12) is tuned by hybridization strength between 5f states and conduction bands, which is affected by atomic distance in plutonium borides. Lastly, 5f electronic correlations encoded in the electron self-energy functions demonstrate moderate 5f electronic correlations in PuB6 and orbital selective 5f electronic correlations in PuB x (x = 1, 2, 12). Consequently, the understanding of electronic structure and related crystal structure stability shall shed light on exploring novel 5f electrons states and ongoing experiment study.

Metal-insulator transition and local-moment collapse in negative charge transfer CaFeO3 under pressure

We compute the electronic structure, spin and charge state of Fe ions, and the structural phase stability of paramagnetic ${\mathrm{CaFeO}}_{3}$ under pressure using a fully self-consistent in charge density $\mathrm{DFT}+$ dynamical mean-field theory method. We show that at ambient pressure ${\mathrm{CaFeO}}_{3}$ is a negative charge transfer insulator characterized by strong localization of the Fe $3d$ electrons. It crystallizes in the monoclinic $P{2}_{1}/n$ crystal structure with a cooperative breathing mode distortion of the lattice. While the Fe $3d$ Wannier occupations and local moments are consistent with robust charge disproportionation of Fe ions in the insulating $P{2}_{1}/n$ phase, the physical charge density difference around the structurally distinct Fe $A$ and Fe $B$ ions with the ``contracted'' and ``expanded'' oxygen octahedra, respectively, is rather weak, of $\ensuremath{\sim}0.04$. This implies the importance of the Fe $3d$ and O $2p$ negative charge transfer and supports the formation of a bond-disproportionated state characterized by the Fe $A 3{d}^{5\ensuremath{-}\ensuremath{\delta}}{\underline{L}}^{2\ensuremath{-}\ensuremath{\delta}}$ and Fe $B 3{d}^{5}$ valence configurations with $\ensuremath{\delta}\ensuremath{\ll}1$, in agreement with strong hybridization between the Fe $3d$ and O $2p$ states. This complex interplay between electronic correlations, strong covalency, and lattice effects, resulting in bond disproportionation, is in many ways reminiscent of the behavior of rare-earth nickelates, $R{\mathrm{NiO}}_{3}$ $(R=\text{rare earth})$. Upon compression, ${\mathrm{CaFeO}}_{3}$ undergoes the metal-to-insulator phase transition (MIT) which is accompanied by a structural transformation into the orthorhombic $Pbnm$ phase. The phase transition is accompanied by suppression of the cooperative breathing mode distortion of the lattice and, hence, results in the melting of bond disproportionation of the Fe ions. Our analysis suggests that the MIT transition is associated with orbital-dependent delocalization of the Fe $3d$ electrons and leads to a remarkable collapse of the local magnetic moments. Our results imply the crucial importance of the interplay of electronic correlations and structural effects to explain the properties of ${\mathrm{CaFeO}}_{3}$.

Interface and bulk superconductivity in superconducting heterostructures with enhanced critical temperatures

We consider heterostructures obtained by stacking layers of two s-wave superconductors with significantly different coupling strengths, respectively in the weak- and strong-coupling regimes. The weak- and strong-coupling superconductors are chosen with similar critical temperatures for bulk systems. Using dynamical mean-field theory methods, we find an ubiquitous enhancement of the superconducting critical temperature for all the heterostructures where a single layer of one of the two superconductors is alternated with a thicker multilayer of the other. Two distinct physical regimes can be identified as a function of the thickness of the larger layer: (i) an inherently inhomogeneous superconductor characterized by the properties of the two isolated bulk superconductors where the enhancement of the critical temperature is confined to the interface and (ii) a bulk superconductor with an enhanced critical temperature extending to the whole heterostructure. We characterize the crossover between these regimes in terms of the competition between two length scales connected with the proximity effect and the pair coherence.

Metal-insulator and magnetic phase diagram of Ca2RuO4 from auxiliary field quantum Monte Carlo and dynamical mean field theory

Layered perovskite ruthenium oxides exhibit a striking series of metal-insulator and magnetic-nonmagnetic phase transitions easily tuned by temperature, pressure, epitaxy, and nonlinear drive. In this work, we combine results from two complementary state of the art many-body methods, Auxiliary Field Quantum Monte Carlo and Dynamical Mean Field Theory, to determine the low-temperature phase diagram of Ca$_2$RuO$_4$. Both methods predict a low temperature, pressure-driven metal-insulator transition accompanied by a ferromagnetic-antiferromagnetic transition. The properties of the ferromagnetic state vary non-monotonically with pressure and are dominated by the ruthenium $d_{xy}$ orbital, while the properties of the antiferromagnetic state are dominated by the $d_{xz}$ and $d_{yz}$ orbitals. Differences of detail in the predictions of the two methods are analyzed. This work is theoretically important as it presents the first application of the Auxiliary Field Quantum Monte Carlo method to an orbitally-degenerate system with both Mott and Hunds physics, and provides an important comparison of the Dynamical Mean Field and Auxiliary Field Quantum Monte Carlo methods.

Correlated electronic structure, orbital-dependent correlations, and Lifshitz transition in tetragonal FeS

Using density functional plus dynamical mean-field theory method (DFT+DMFT) with full self-consistency over the charge density, we study the effect of electronic correlations on the electronic structure, magnetic properties, orbital-dependent band renormalizations, and Fermi surface of the tetragonal phase of bulk FeS. We perform a direct structural optimization of the $P_4/nmm$ crystal structure of paramagnetic FeS, with respect to the lattice constant $a$ and the internal coordinate $z_\mathrm{S}$ of atom S. Our results show an anomalous sensitivity of the electronic structure and magnetic properties of FeS to fine details of its crystals structure. Upon expansion of the lattice volume, we observe a remarkable change of the electronic structure of FeS which is associated with a complete reconstruction of the Fermi surface topology (Lifshitz transition). This behavior is ascribed to a correlation-induced shift of the Van Hove singularity associated with the Fe $t_2$ orbitals at the $M$ point across the Fermi level. The Lifshitz phase transition is accompanied by a significant growth of local magnetic moments and emergence of strong orbital-selective correlations. It is seen as a pronounced anomaly (`kink') in the total energies upon expansion of the lattice, associated with a remarkable enhancement of compressibility. This behavior is accompanied by an orbital-dependent formation of local moments, a crossover from itinerant to localized orbital-selective moment behavior of the Fe $3d$ electrons. While exhibiting weak effective mass enhancement of the Fe $3d$ states $m^*/m \sim 1.3-1.4$, correlation effects reveal a strong impact on a position of the Van Hove singularity at the $M$ point, implying a complex interplay between electronic correlations and band structure effects in FeS.

Correlation strength, Lifshitz transition, and the emergence of a two-dimensional to three-dimensional crossover in FeSe under pressure

We report a detailed theoretical study of the electronic structure, spectral properties, and lattice parameters of bulk FeSe under pressure using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory method (DFT+DMFT). In particular, we perform a structural optimization and compute the evolution of the lattice parameters (volume, $c/a$ ratio, and the internal $z$ position of Se) and the electronic structure of the tetragonal (space group $P4/nmm$) paramagnetic FeSe. Our results for the lattice parameters are in good quantitative agreement with experiment. The $c/a$ ratio is slightly overestimated by about $3$~\%, presumably due to the absence of the van der Waals interactions between the FeSe layers in our calculations. The lattice parameters determined within DFT are off the experimental values by a remarkable $\sim$$6$-$15$~\%, implying a crucial importance of electron correlations. Upon compression to $10$~GPa, the $c/a$ ratio and the lattice volume show a decrease by $2$ and $10$~\%, respectively, while the Se $z$ coordinate weakly increases by $\sim$$2$~\%. Most importantly, our results reveal a topological change of the Fermi surface (Lifshitz transition) which is accompanied by a two- to three-dimensional crossover. Our results indicate a small reduction of the quasiparticle mass renormalization $m^*/m$ by about $5$~\% for the $e$ and less than $1$~\% for the $t_2$ states, as compared to ambient pressure. The behavior of the momentum-resolved magnetic susceptibility $\chi({\bf q})$ shows no topological changes of magnetic correlations under pressure, but demonstrates a reduction of the degree of the in-plane $(\pi,\pi)$ stripe-type nesting. Our results for the electronic structure and lattice parameters of FeSe are in good qualitative agreement with recent experiments on its isoelectronic counterpart FeSe$_{1-x}$S$_x$.

Generalized gradient expansion for inhomogeneous dynamical mean-field theory: Application to ultracold atoms in a harmonic trap

We develop a generalized gradient expansion of the inhomogeneous dynamical mean-field theory method for determining properties of ultracold atoms in a trap. This approach goes beyond the well-known local density approximation and at higher temperatures, in the normal phase, it shows why the local density approximation works so well, since the local density and generalized gradient approximations are essentially indistinguishable from each other (and from the exact solution within full inhomogeneous dynamical mean-field theory). But because the generalized gradient expansion only involves nearest-neighbor corrections, it does not work as well at low temperatures, when the systems enter into ordered phases. This is primarily due to the problem that ordered phases often satisfy some global constraints which determine the spatial ordering pattern, and the local density and generalized gradient approximations are not able to impose those kinds of constraints; they also overestimate the tendency to order. The theory is applied to phase separation of different mass fermionic mixtures represented by the Falicov-Kimball model and to determining the entropy per particle of a fermionic system represented by the Hubbard model. The generalized gradient approximation is a useful diagnostic for the accuracy of the local density approximation---when both methods agree, they are likely accurate, when they disagree, neither is likely to be correct.

Electronic correlation and magnetic frustration inLi2VOSiO4andVOMoO4

${\mathrm{Li}}_{2}{\mathrm{VOSiO}}_{4}$ and ${\mathrm{VOMoO}}_{4}$ have been proposed as realizations of the frustrated two-dimensional ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ quantum Heisenberg model. In this work, in order to test this picture, we study their electronic and magnetic properties by using the local-density approximation + dynamical mean-field theory method. We calculate the magnetic linear response function starting from material-specific Hubbard models and systematically map our results onto those from generalized quantum Heisenberg models. We obtain the effective local magnetic moments and the associated magnetic exchange couplings, in particular the ratio ${J}_{2}/{J}_{1}$, a measure of the frustration degree, and the ratio $2{J}_{\ensuremath{\perp}}/({J}_{1}+{J}_{2})$, measuring the three-dimensionality degree. Our results support a weak frustration picture for both materials, with small but non-negligible long-range interplane couplings, leading to three-dimensional order at low temperature. Implications for the physics of the two systems are discussed.

Effective doublon and hole temperatures in the photo-doped dynamic Hubbard model.

Hirsch's dynamic Hubbard model describes the effect of orbital expansion with occupancy by coupling the doublon operator to an auxiliary boson. In the Mott insulating phase, empty sites (holes) and doubly occupied orbitals (doublons) become charge carriers on top of the half-filled background. We use the nonequilibrium dynamical mean field method to study the properties of photo-doped doublons and holes in this model in the strongly correlated regime. In particular, we discuss how photodoping leads to doublon and hole populations with different effective temperatures, and we analyze the relaxation behavior as a function of the boson coupling and boson energy. In the polaronic regime, the nontrivial energy exchange between doublons, holes, and bosons can result in a negative temperature distribution for the holes.

Mechanism of magnetic moment collapse under pressure in ferropericlase

We propose a new scenario for the magnetic collapse under pressure in ferropericlase (FP) ()O without the presence of intermediate spin state, which contradicts the mechanism proposed in (2013 Phys. Rev. B 87 165113). This scenario is supported by results of combined local density approximation + dynamical mean-field theory method calculations for the paramagnetic phase at ambient and high pressures. At ambient pressure, calculation gave ()O as an insulator with Fe 3d-shell in high-spin state. Experimentally observed high-spin to low-spin state transition of the ion in the pressure range of 35–75 GPa is successfully reproduced in our calculations. The spin crossover is characterized by coexistence of ions in high and low spin state but intermediate spin state is absent in the whole pressure range. Moreover, the probability of Fe ion configuration with grows with pressure due to shortening of metal–oxygen distance. Also, no metal–insulator transition was obtained up to the pressure 140 GPa in agreement with experiment.

Quasiparticle properties of the superconducting state of the two-dimensional Hubbard model

Cluster dynamical mean field methods are used to calculate the normal and anomalous components of the electron self energy of the two dimensional Hubbard model. From these the evolution of the superconducting gap and the momentum dependent photoemission and inverse photoemission spectra across the phase diagram are determined. In the pseudogap regime, decreasing the temperature into the superconducting state leads to a decrease in the energy gap and the formation of a `peak-dip-hump' structure in the electronic density of states. The peak feature disperses very weakly. The calculated spectral functions are in good qualitative agreement with published data. The mathematical origin of the behavior is found to be the effect of the superconductivity on the pole structure giving rise to the normal state pseudogap. In particular the "hump" feature is found to arise from a zero crossing of the real part of the electron self energy rather than from an onset of scattering. The effect of superconductivity on the zone diagonal spectra is presented.

Nonlocal quantum fluctuations and fermionic superfluidity in the imbalanced attractive Hubbard model.

We study fermionic superfluidity in strongly anisotropic optical lattices with attractive interactions utilizing the cluster dynamical mean-field theory method, and focusing in particular on the role of nonlocal quantum fluctuations. We show that nonlocal quantum fluctuations impact the BCS superfluid transition dramatically. Moreover, we show that exotic superfluid states with a delicate order parameter structure, such as the Fulde-Ferrell-Larkin-Ovchinnikov phase driven by spin population imbalance, can emerge even in the presence of such strong fluctuations.

Orbital polarization in strainedLaNiO3: Structural distortions and correlation effects

Transition-metal heterostructures offer the fascinating possibility of controlling orbital degrees of freedom via strain. Here, we investigate theoretically the degree of orbital polarization that can be induced by epitaxial strain in LaNiO$_3$ films. Using combined electronic structure and dynamical mean-field theory methods we take into account both structural distortions and electron correlations and discuss their relative influence. We confirm that Hund's rule coupling tends to decrease the polarization and point out that this applies to both the $d^8\underline{L}$ and $d^7$ local configurations of the Ni ions. Our calculations are in good agreement with recent experiments, which revealed sizable orbital polarization under tensile strain. We discuss why full orbital polarization is hard to achieve in this specific system and emphasize the general limitations that must be overcome to achieve this goal.

Superconducting and pseudogap effects on the interplane conductivity and Raman scattering cross section in the two-dimensional Hubbard model

Cluster dynamical mean field methods are used to calculate the superconductivity-induced changes in the interplane conductivity and Raman scattering cross section of the two dimensional Hubbard model. When superconductivity emerges from the pseudogap, the superconducting response is found to be diminished in amplitude, broadened and, in the case of the interplane conductivity, shifted to higher frequency. The results are in agreement with data on high temperature copper-oxide superconductors indicating that the Hubbard model contains the essential low energy physics of the pseudogap and its interplay with superconductivity in the cuprates.

Theory of ferromagnetism in vanadium-oxide based perovskites

The conditions under which ferromagnetism may occur in transition metal oxides with partially filled $t_{2g}$ shells such as vanadium-based perovskites are studied using a combination of density functional and single-site dynamical mean field methods. For reasonable values of the correlation strength, rotations of the VO$_6$ octahedra play an important role in enabling ferromagnetism, with ferromagnetism typically occurring for rotations larger than a nonzero critical value. Ferromagnetism is suppressed near the Mott insulating phase but the phase boundary is otherwise only weakly dependent on carrier concentration. Design rules are suggested for new oxide systems exhibiting ferromagnetism.

Screening and nonlocal correlations in the extended Hubbard model from self-consistent combinedGWand dynamical mean field theory

We describe a recent implementation of the combined $GW$ and dynamical mean field method ($GW$+DMFT) for the two-dimensional Hubbard model with onsite and nearest-neighbor repulsion. We clarify the relation of the $GW$+DMFT scheme to alternative approaches in the literature, and discuss the corresponding approximations to the free-energy functional of the model. Furthermore, we describe a numerically exact technique for the solution of the $GW$+DMFT equations, namely, the hybridization expansion continuous-time algorithm for impurity models with retarded interactions. We compute the low-temperature phase diagram of the half-filled extended Hubbard model, addressing the metal-insulator transition at small intersite interactions and the transition to a charge-ordered state for stronger intersite repulsions. $GW$+DMFT introduces a nontrivial momentum dependence into the many-body self-energy and polarization. We find that the charge fluctuations included in the present approach have a larger impact on the latter than on the former. Finally, within the $GW$+DMFT framework, as in extended DMFT, the intersite repulsion translates into a frequency dependence of the local effective interaction. We analyze this dependence and show how it affects the local spectral function.