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Register a new userThermodynamic Properties of Ferromagnetic Mott- Insulators GaV4S8
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ChandraShekhar Yadav
Publication date
2007
fields
Physics
and research's language is
English
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We present the results of the magnetic and specific heat measurements on V4 tetrahedral-cluster compound GaV4S8 between 2 to 300K. We find two transitions related to a structural change at 42K followed by ferromagnetic order at 12K on cooling. Remarkably similar properties were previously reported for the cluster compounds of Mo4. These compounds show an extremely high density of low energy excitations in their electronic properties. We explain this behavior in a cluster compound as due to the reduction of coulomb repulsion among electrons that occupy highly degenerate orbits of different clusters.
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We study the electronic properties of GaV4S8 (GVS) and GaTaSe8 (GTS), two distant members within the large family of chalcogenides AM4X8, with A={Ga, Ge}, M={V, Nb, Ta, Mo} and X={S, Se}. While all these compounds are Mott insulators, their ground state show many types of magnetic order, with GVS being ferromagnetic and GTS non-magnetic. Based on their bandstructures, calculated with Density Functional Theory methods, we compute an effective tight binding Hamiltonian in a localised Wannier basis set, for each one of the two compounds. The localised orbitals provide a very accurate representation of the bandstructure, with hopping amplitudes that rapidly decrease with distance. We estimate the super-exchange interactions and show that the Coulomb repulsion with the Hunds coupling may account the for the different ground states observed in GVS and GTS. Our localised Wannier basis provides a starting point for realistic Dynamical Mean Field Theory studies of strong correlation effects in this family compounds.
The inverse Faraday effect (IFE), where a static magnetization is induced by circularly polarized light, offers a promising route to ultrafast control of spin states. Here we study the inverse Faraday effect in Mott insulators using the Floquet theory. In the Mott insulators with inversion symmetry, we find that the effective magnetic field induced by the IFE couples ferromagnetically to the neighboring spins. While for the Mott insulators without inversion symmetry, the effective magnetic field due to IFE couples antiferromagnetically to the neighboring spins. We apply the theory to the spin-orbit coupled single- and multi-orbital Hubbard model that is relevant for the Kitaev quantum spin liquid materials and demonstrate that the magnetic interactions can be tuned by light.
Angle-resolved photoemission spectroscopy (ARPES) has revealed peculiar properties of mobile dopants in correlated anti-ferromagnets (AFMs). But describing them theoretically, even in simplified toy models, remains a challenge. Here we study ARPES spectra of a single mobile hole in the $t-J$ model. Recent progress in the microscopic description of mobile dopants allows us to use a geometric decoupling of spin and charge fluctuations at strong couplings, from which we conjecture a one-to-one relation of the one-dopant spectral function and the spectrum of a constituting spinon in the emph{undoped} parent AFM. We thoroughly test this hypothesis for a single hole doped into a 2D Heisenberg AFM by comparing our semi-analytical predictions to previous quantum Monte Carlo results and our large-scale time-dependent matrix product state (td-MPS) calculations of the spectral function. Our conclusion is supported by a microscopic trial wavefuntion describing spinon-chargon bound states, which captures the momentum and $t/J$ dependence of the quasiparticle residue. Our conjecture suggests that ARPES measurements in the pseudogap phase of cuprates can directly reveal the Dirac-fermion nature of the constituting spinons. Specifically, we demonstrate that our trial wavefunction provides a microscopic explanation for the sudden drop of spectral weight around the nodal point associated with the formation of Fermi arcs, assuming that additional frustration suppresses long-range AFM ordering. We benchmark our results by studying the cross-over from two to one dimension, where spinons and chargons are confined and deconfined respectively.
In high-resolution core-valence-valence (CVV) Auger electron spectroscopy from the surface of a solid at thermal equilibrium, the main correlation satellite, visible in the case of strong valence-electron correlations, corresponds to a bound state of the two holes in the final state of the CVV Auger process. We discuss the physical significance of this satellite in nonequilibrium pump-probe Auger spectroscopy by numerical analysis of a single-band Hubbard-type model system including core states and a continuum of high-energy scattering states. It turns out that the spectrum of the photo-doped system, due to the increased double occupancy, shares features with the equilibrium spectrum at higher fillings. The pumping of doublons can be watched when working with overlapping pulses at short $Delta t$. For larger pump-probe delays $Delta t$ and on the typical femtosecond time scale for electronic relaxation processes, spectra are hardly $Delta t$-dependent, reflecting the high stability of bound two-hole states for strong Hubbard-$U$. We argue that taking into account the spatial expansion of single-particle orbitals when these are doubly occupied, as described by the dynamical Hubbard model, produces an oscillation of the barycenter of the satellite as a function of $Delta t$. Pump-probe Auger-electron spectroscopy is thus highly sensitive to dynamical screening of the Coulomb interaction.
We argue that in lightly hole doped perovskite-type Mn oxides the holes (Mn$^{4+}$ sites) are surrounded by nearest neighbor Mn$^{3+}$ sites in which the occupied $3d$ orbitals have their lobes directed towards the central hole (Mn$^{4+}$) site and with spins coupled ferromagnetically to the central spin. This composite object, which can be viewed as a combined orbital-spin-lattice polaron, is accompanied by the breathing type (Mn$^{4+}$) and Jahn-Teller type (Mn$^{3+}$) local lattice distortions. We present calculations which indicate that for certain doping levels these orbital polarons may crystallize into a charge and orbitally ordered ferromagnetic insulating state.
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