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Register a new userPhotoinduced Hund excitons in the breakdown of a two-orbital Mott insulator
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We study the photoinduced breakdown of a two-orbital Mott insulator and resulting metallic state. Using time-dependent density matrix renormalization group, we scrutinize the real-time dynamics of the half-filled two-orbital Hubbard model interacting with a resonant radiation field pulse. The breakdown, caused by production of doublon-holon pairs, is enhanced by Hunds exchange, which dynamically activates large orbital fluctuations. The melting of the Mott insulator is accompanied by a high to low spin transition with a concomitant reduction of antiferromagnetic spin fluctuations. Most notably, the overall time response is driven by the photogeneration of excitons with orbital character that are stabilized by Hunds coupling. These unconventional Hund excitons correspond to bound spin-singlet orbital-triplet doublon-holon pairs. We study exciton properties such as bandwidth, binding potential, and size within a semiclassical approach. The photometallic state results from a coexistence of Hund excitons and doublon-holon plasma.
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Motivated by the recent discovery of superconductivity in infinite-layer nickelates RE$_{1-delta}$Sr$_delta$NiO$_2$ (RE$=$Nd, Pr), we study the role of Hunds 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 Hunds 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 Hunds metal, and valence-skipping metal, from the effect of $J$ being away from Mottness. Defining criteria characterizing these metals are suggested, establishing the existence of Hunds metallicity in two-orbital systems.
We have studied the electronic structure of the two-dimensional Heisenberg antiferromagnet VOCl using photoemission spectroscopy and density functional theory including local Coulomb repulsion. From calculated exchange integrals and the observed energy dispersions we argue that the degree of one-dimensionality regarding both the magnetic and electronic properties is noticeably reduced compared to the isostructural compounds TiOCl and TiOBr. Also, our analysis provides conclusive justification to classify VOCl as a multi-orbital Mott insulator. In contrast to the titanium based compounds density functional theory here gives a better description of the electronic structure. However, a quantitative account of the low-energy features and detailed line shapes calls for further investigations including dynamical and spatial correlations.
Orbital differentiation is a common theme in multiorbital systems, yet a complete understanding of it is still missing. Here, we consider a minimal model for orbital differentiation in Hund metals with a highly accurate method: We use the numerical renormalization group as a real-frequency impurity solver for a dynamical mean-field study of three-orbital Hubbard models, where a crystal field shifts one orbital in energy. The individual phases are characterized with dynamic correlation functions and their relation to diverse Kondo temperatures. Upon approaching the orbital-selective Mott transition, we find a strongly suppressed spin coherence scale and uncover the emergence of a singular Fermi liquid and interband doublon-holon excitations. Our theory describes the diverse polarization-driven phenomena in the $t_{2g}$ bands of materials such as ruthenates and iron-based superconductors, and our methodological advances pave the way towards real-frequency analyses of strongly correlated materials.
We study the effects of hole doping on one-dimensional Mott insulators with orbital degrees of freedom. We describe the system in terms of a generalized t-J model. At a specific point in parameter space the model becomes integrable in analogy to the one-band supersymmetric t-J model. We use the Bethe ansatz to derive a set of nonlinear integral equations which allow us to study the thermodynamics exactly. Moving away from this special point in parameter space we use the density-matrix renormalization group applied to transfer matrices to study the evolution of various phases of the undoped system with doping and temperature. Finally, we study a one-dimensional version of a realistic model for cubic titanates which includes the anisotropy of the orbital sector due to Hunds coupling. We find a transition from a phase with antiferromagnetically correlated spins to a phase where the spins are fully ferromagnetically polarized, a strong tendency towards phase separation at large Hunds coupling, as well as the possibility of an instability towards triplet superconductivity.
In condensed-matter physics, electronic Mott insulators have triggered considerable research due to their intricate relation with high-temperature superconductors. However, unlike atomic systems for which Mott phases were recently shown for both bosonic and fermionic species, in the solid-state the fingerprint of a Mott insulator implemented with bosons is yet to be found. Here we unveil such signature by exploring the Bose-Hubbard hamiltonian using semiconductor excitons confined in two-dimensional lattices. We emphasise the regime where on-site interactions are comparable to the energy separation between lattice confined states. We then observe that Mott phases are accessible, with at most two excitons uniformly filling lattice sites. The technology introduced here allows us to program on-demand the geometry of the lattice confining excitons. This versatility, combined with the long-range nature of dipolar interactions between excitons, provide a new route to explore many-body phases spontaneously breaking the lattice symmetry.
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