Global perspectives of the bulk electronic structure of URu2Si2 from angle-resolved photoemission

Previous high-resolution angle-resolved photoemission (ARPES) studies of URu2Si2 have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level (E F) at low photon energies near the zone center, with an emphasis on electronic reconstruction due to Brillouin zone folding. A substantial challenge to a proper description is that these states interact with other hole-band states that are generally absent from bulk-sensitive soft x-ray ARPES measurements. Here we provide a more global k-space context for the presence of such states and their relation to the bulk Fermi surface topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure using photon energies intermediate between the low-energy regime and the high-energy soft x-ray regime. Small-spot spatial dependence, f-resonant photoemission, Si 2p core-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of the E F-crossing bands and their relation to specific U- or Si-terminations of the cleaved surface. The bulk Fermi surface topology is critically compared to density functional theory and to dynamical mean field theory calculations. In addition to clarifying some aspects of the previously measured high symmetry Γ, Z and X points, incommensurate 0.6a* nested Fermi-edge states located along Z-N-Z are found to be distinctly different from the density functional theory Fermi surface prediction. The temperature evolution of these states above THO, combined with a more detailed theoretical investigation of this region, suggests a key role of the N-point in the hidden order transition.

First principles band structure investigation of cubic spinel CuMn2Te4Compound for thermoelectric applications

Spinel CuMn2Te4 material is investigated for its nonmagnetic and ferromagnetic phases by first principles calculation using the FP-LAPW method. Exchange and correlation are treated with GGA. T otal energy calculations reveal that CuMn2T e4 compound tend to stabilise at ferromagnetic phase. Non vanishing bands at the Fermi Energy level illustrates the metallic nature of CuMn2Te4at both of its nonmagnetic and ferromagnetic phases. Transport properties are studied using the BoltzTraP code at 325K. Thermo power of -26.22μv/k is estimated for nonmagnetic CuMn2Te4. At ferromagnetic phase, these values are predicted as 84.85μv/kand 53.39μv/k respectively for spin-up and spin-down states. Computed power factor values are 2.29 x1014Wm-1s-1K-2for NM phase and 2.33×1017Wm−1s−1K−2/1.001×1010Wm−1s-−1K-−2respectivelyfor FM up-spin/down-spin phases. A total magnetic moment of 7.11003 μB/F.U. is obtained for the energetically favourable ferromagnetic phase of CuMn2Te4 compound. High pressure investigations reveal the possibility of electronic topological transition that may lead to changes in the Fermi Surface topology and hence changes in the physical properties of nonmagnetic CuMn2Te4.

Ultrafast dynamical Lifshitz transition

Fermi surface is at the heart of our understanding of metals and strongly correlated many-body systems. An abrupt change in the Fermi surface topology, also called Lifshitz transition, can lead to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. While Lifshitz transitions have been demonstrated for a broad range of materials by equilibrium tuning of macroscopic parameters such as strain, doping, pressure, and temperature, a nonequilibrium dynamical route toward ultrafast modification of the Fermi surface topology has not been experimentally demonstrated. Combining time-resolved multidimensional photoemission spectroscopy with state-of-the-art TDDFT+U simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated type-II Weyl semimetal Td-MoTe2. We demonstrate that this nonequilibrium topological electronic transition finds its microscopic origin in the dynamical modification of the effective electronic correlations. These results shed light on a previously unexplored ultrafast scheme for controlling the Fermi surface topology in correlated quantum materials.