First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa4Se8

The nature of the Mott transition in the absence of any symmetry braking remains a matter of debate. We study the correlation-driven insulator-to-metal transition in the prototypical 3D Mott system GaTa4Se8, as a function of temperature and applied pressure. We report novel experiments on single crystals, which demonstrate that the transition is of first order and follows from the coexistence of two states, one insulating and one metallic, that we toggle with a small bias current. We provide support for our findings by contrasting the experimental data with calculations that combine local density approximation with dynamical mean-field theory, which are in very good agreement.Comment: 5 pages and 4 figures. Supplemental material: 2 pages, 2 figure

Quantum and thermal fluctuations in the SU(N) Heisenberg spin-glass model near the quantum critical point

We solve for the SU(N) Heisenberg spin-glass in the limit of large N focusing on small S and T. We study the effect of quantum and thermal fluctuations in the frequency dependent response function and observed interesting transfers of spectral weight. We compute the T-dependence of the order parameter and the specific heat and find an unusual T^2 behavior for the latter at low temperatures in the spin-glass phase. We find a remarkable qualitative agreement with various experiments on the quantum frustrated magnet SrCr_{9p}Ga_{12-9p}O_{19}.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let

Specific heat of the quantum Bragg Glass

We study the thermodynamics of the vibrational modes of a lattice pinned by impurity disorder in the absence of topological defects (Bragg glass phase). Using a replica variational method we compute the specific heat $C_v$ in the quantum regime and find $C_v \propto T^3$ at low temperatures in dimension three and two. The prefactor is controlled by the pinning length. The non trivial cancellation of the linear term in $C_v$ arises from the so-called marginality condition and has important consequences for other mean field models.Comment: 5 pages, RevTex, strongly revised versio

Thermoelectrics Near the Mott Localization-Delocalization Transition

We give an overview on current status of the theoretical research on Thermoelectricity for correlated materials. We derive the theoretical formulas which become exact at low and high temperature and discuss the intermediate temperature results. In particular, we show that within Dynamical Mean Field Theory the low temperature sign of the thermopower is not necessary the same as in LDA, and that significant non-universality is expected due to strong correlations.Comment: appeared in "Properties and Applications of Thermoelectric Materials", Edited by V. Zlatic and A.C. Hewson, Springe

Localised Wannier orbital basis for the Mott insulators GaV

We study the electronic properties of GaV4S8 (GVS) and GaTa4Se8 (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 states show many types of magnetic order, with GVS being ferromagnetic and GTS non-magnetic. Based on their band structures, calculated with density functional theory methods, we compute an effective tight-binding Hamiltonian in a localised Wannier basis set, for each of the two compounds. The localised orbitals provide a very accurate representation of the band structure, with hopping amplitudes that rapidly decrease with distance. We estimate the superexchange interactions and show that the Coulomb repulsion with Hund's 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

Heavy quasiparticles and cascades without symmetry breaking in twisted bilayer graphene

Abstract Among the variety of correlated states exhibited by twisted bilayer graphene, cascades in the spectroscopic properties and in the electronic compressibility occur over larger ranges of energy, twist angle and temperature compared to other effects. This suggests a hierarchy of phenomena. Using a combined dynamical mean-field theory and Hartree calculation, we show that the spectral weight reorganisation associated with the formation of local moments and heavy quasiparticles can explain the cascade of electronic resets without invoking symmetry breaking orders. The phenomena reproduced here include the cascade flow of spectral weight, the oscillations of remote band energies, and the asymmetric jumps of the inverse compressibility. We also predict a strong momentum differentiation in the incoherent spectral weight associated with the fragile topology of twisted bilayer graphene