Taken together and viewed holistically, recent theory, low temperature (T) transport, photoelectron spectroscopy and quantum oscillation experiments have built a very strong case that the paradigmatic mixed valence insulator SmB6 is currently unique
as a three-dimensional strongly correlated topological insulator (TI). As such, its many-body T-dependent bulk gap brings an extra richness to the physics beyond that of the weakly correlated TI materials. How will the robust, symmetry-protected TI surface states evolve as the gap closes with increasing T? For SmB6 exploiting this opportunity first requires resolution of other important gap-related issues, its origin, its magnitude, its T-dependence and its role in bulk transport. In this paper we report detailed T-dependent angle resolved photoemission spectroscopy (ARPES) measurements that answer all these questions in a unified way.
The electronic structure of (Ce,Yb)CoIn5 has been studied by a combination of photoemission, x-ray absorption and bulk property measurements. Previous findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for x>0.3 were confirmed.
One new result of this study is that the Yb valence for x<0.2 increases rapidly with decreasing x from 2.3+ toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around x=0.2. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of about 50% cross sectional area reductions of the alpha- and beta-sheets for x=1 compared to x=0, and a smaller, essentially proportionate, size change of the alpha-sheet for x=0.2. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing x in this alloy system are discussed.
Temperature dependent photoemission spectroscopy in Li0.9Mo6O17 contributes to evidence for one dimensional physics that is unusually robust. Three generic characteristics of the Luttinger liquid are observed, power law behavior of the k-integrated s
pectral function down to temperatures just above the superconducting transition, k-resolved lineshapes that show holon and spinon features, and quantum critical (QC) scaling in the lineshapes. Departures of the lineshapes and the scaling from expectations in the Tomonaga Luttinger model can be partially described by a phenomenological momentum broadening that is presented and discussed. The possibility that some form of 1d physics obtains even down to the superconducting transition temperature is assessed.
Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe $L$ edges is investigated in Fe pnictide materials, and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra fo
r 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters $Ulesssim 2$eV and $Japprox 0.8$eV.
