Motivated by the remarkable discovery of superconductivity in elemental Bismuth at ambient pressure, we study its normal state in detail using a combination of tight-binding (TB) band-structure ...supplemented by dynamical mean-field theory (DMFT). We show that a two-fluid model composed of preformed and dynamically fluctuating excitons coupled to a tiny number of carriers provides a unified rationalization of a range of ill-understood normal state spectral and transport data. Based on these, we propose that resonant scattering involving a very low density of renormalized carriers and the excitonic liquid drives logarithmic enhancement of vertex corrections, boosting superconductivity in Bi. A confirmatory test for our proposal would be the experimental verification of an excitonic semiconductor with electronic nematicity as a 'competing order' on inducing a semi-metal-to semiconductor transition in Bi by an external perturbation like pressure.
The interplay between multiple bands, sizable multi-band electronic correlations and strong spin-orbit coupling may conspire in selecting a rather unusual unconventional pairing symmetry in layered ...Sr
RuO
. This mandates a detailed revisit of the normal state and, in particular, the T-dependent incoherence-coherence crossover. Using a modern first-principles correlated view, we study this issue in the actual structure of Sr
RuO
and present a unified and quantitative description of a range of unusual physical responses in the normal state. Armed with these, we propose that a new and important element, that of dominant multi-orbital charge fluctuations in a Hund's metal, may be a primary pair glue for unconventional superconductivity. Thereby we establish a connection between the normal state responses and superconductivity in this system.
Using a combination of local density approximation and dynamical mean-field theory calculations, we explore the correlated electronic structure of a member of the layered iron oxychalcogenides group, ...Na2Fe2OSe2. We find that the parent compound is a multi-orbital Mott insulator. Surprisingly, and somewhat reminiscently of the underdoped high-Tc cuprate scenario, carrier localization is found to persist upon hole doping because the chemical potential lies in a gap structure with almost vanishing density of states. On the other hand, in remarkable contrast, electron doping drives an orbital-selective metallic phase with coexisting pseudogapped (Mott-localized) and itinerant carriers. These remarkably contrasting behaviors in a single system thus stem from drastic electronic reconstruction caused by large-scale transfer of dynamical spectral weight involving states with distinct orbital character at low energies, fitting the oxychalcogenides neatly into the increasingly visible pattern for Fe-based systems of having orbital-selective Mott phases. We detail the implications that follow from our analysis, and discuss the nature and symmetries of the superconductive states that may arise upon appropriately doping or pressurizing Na2Fe2OSe2.
The fate of exotic spin liquid states with fractionalized excitations at finite temperature (T) is of great interest, since signatures of fractionalization manifest in finite-temperature (T) dynamics ...in real systems, above the tiny magnetic ordering scales. Here, we study a Jordan–Wigner (JW) fermionized Kitaev spin liquid at finite T employing combined exact diagonalization and Monte Carlo simulation methods. We uncover (i) checkerboard or stripy-ordered flux crystals depending on density of flux, and (ii) establish, surprisingly, that: (a) the finite-T version of the T=0 transition from a gapless to gapped phases in the Kitaev model is a Mott transition of the fermions, belonging to the two-dimensional Ising universality class. These transitions correspond to a topological transition between a string condensate and a dilute closed string state (b) the Mott “insulator” phase is a precise realization of Laughlin’s gossamer (here, p-wave) superconductor (g-SC), and (c) the Kitaev Toric Code phase (TC) is adiabatically connected to the g-SC, and is a fully Gutzwiller-projected fermi sea of JW fermions. These findings establish the finite-T quantum spin liquid phases in the d=2 to be hidden Fermi liquid(s) of neutral fermions.
•Kitaev QSL gapless-phase is described through partially Gutzwiller projected p-wavesuperfluid•Gapless-to-gapped QSL transition analogous to MIT of JW fermions in D=2 Ising class•Gapped QSL (gossamer spin liquid) to Toric Code phase (JW fermi gas) at large Jz
Tetragonal FeSe, a prototype iron-chalcogenide superconductor, shows signatures of a strange incoherent normal state. Motivated thereby, we use LDA+DMFT to show how multi-band correlations generate a ...low-energy pseudogap in the normal state, giving an incoherent metal in good semi-quantitative agreement with observations. Anomalous responses in the normal state, including orbital-dependent effective mass enhancement and photoemission lineshape, are consistently understood.
PDF
