Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, ...the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.
The temperature-dependent evolution of the Kondo lattice is a long-standing topic of theoretical and experimental investigation and yet it lacks a truly microscopic description of the relation of the ...basic f-c hybridization processes to the fundamental temperature scales of Kondo screening and Fermi-liquid lattice coherence. Here, the temperature dependence of f-c hybridized band dispersions and Fermi-energy f spectral weight in the Kondo lattice system CeCoIn₅ is investigated using f-resonant angle-resolved photoemission spectroscopy (ARPES) with sufficient detail to allow direct comparison to first-principles dynamical mean-field theory (DMFT) calculations containing full realism of crystalline electric-field states. The ARPES results, for two orthogonal (001) and (100) cleaved surfaces and three different f-c hybridization configurations, with additional microscopic insight provided by DMFT, reveal f participation in the Fermi surface at temperatures much higher than the lattice coherence temperature, T* ≈ 45 K, commonly believed to be the onset for such behavior. The DMFT results show the role of crystalline electric-field (CEF) splittings in this behavior and a T-dependent CEF degeneracy crossover below T* is specifically highlighted. A recent ARPES report of low T Luttinger theorem failure for CeCoIn₅ is shown to be unjustified by current ARPES data and is not found in the theory.
In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electron's ...kinetic energy, leading to remarkable experimental manifestations in optical spectroscopy. The high-transition-temperature (Tc) superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-Tc superconductivity in the iron pnictides puts a spotlight on the relevance of correlation effects in these materials. Here, we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. We deduce that electronic many-body effects are important in the iron pnictides despite the absence of a Mott transition.
•Electronic and optical properties of PbCrO4 are studied under compression.•Band-gap collapses are observed and correlated with structural phase transitions.•PbCrO4 band-gap is reduced from 2.3 to ...0.8eV in a 20GPa range.•PbCrO4 is an n-type semiconductor with donor levels associated to Frenkel defects.•A deep-to-shallow donor transformation at HP induces a large resistivity decrease.
The electronic transport properties and optical properties of lead(II) chromate (PbCrO4) have been studied at high pressure by means of resistivity, Hall-effect, and optical-absorption measurements. Band-structure first-principle calculations have been also performed. We found that the low-pressure phase is a direct band-gap semiconductor (Eg=2.3eV) that shows a high resistivity. At 3.5GPa, associated to a structural phase transition, a band-gap collapse takes place, becoming Eg=1.8eV. At the same pressure the resistivity suddenly decreases due to an increase of the carrier concentration. In the HP phase, PbCrO4 behaves as an n-type semiconductor, with a donor level probably associated to the formation of oxygen vacancies. At 15GPa a second phase transition occurs to a phase with Eg=1.2eV. In this phase, the resistivity increases as pressure does probably due to the self-compensation of donor levels and the augmentation of the scattering of electrons with ionized impurities. In the three phases the band gap red shifts under compression. At 20GPa, Eg reaches a value of 0.8eV, behaving PbCrO4 as a narrow-gap semiconductor.
Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi2Se3 under applied pressures up to 50 GPa. As a function of pressure, ...superconductivity is observed to onset above 11 GPa with a transition temperature Tc and upper critical field Hc2 that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, Tc remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where Tc remains constant. Together with a quasilinear temperature dependence of Hc2 that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc points to an unconventional pressure-induced pairing state in Bi2Se3 that is unique among the superconducting topological insulators.
Abstract
The neutron spin resonance is a collective magnetic excitation that appears in the unconventional copper oxide, iron pnictide and heavy fermion superconductors. Although the resonance is ...commonly associated with a spin-exciton due to the
d
(
s
±
)-wave symmetry of the superconducting order parameter, it has also been proposed to be a magnon-like excitation appearing in the superconducting state. Here we use inelastic neutron scattering to demonstrate that the resonance in the heavy fermion superconductor Ce
1−
x
Yb
x
CoIn
5
with
x
=0, 0.05 and 0.3 has a ring-like upward dispersion that is robust against Yb-doping. By comparing our experimental data with a random phase approximation calculation using the electronic structure and the momentum dependence of the "Equation missing"
-wave superconducting gap determined from scanning tunnelling microscopy (STM) for CeCoIn
5
, we conclude that the robust upward-dispersing resonance mode in Ce
1−
x
Yb
x
CoIn
5
is inconsistent with the downward dispersion predicted within the spin-exciton scenario.
A comprehensive study of the electronic structure, thermodynamic and electrical transport properties reveals the existence of inhomogeneous superconductivity due to structural disorder in Ca3Rh4Sn13 ...doped with La (Ca3−xLaxRh4Sn13) or Ce (Ca3−xCexRh4Sn13) with superconducting critical temperatures T c higher than those (Tc) observed in the parent compounds. The T − x diagrams and the entropy S(x)T isotherms document well the relation between the degree of atomic disorder and separation of the high-temperature T c and Tc-bulk phases. In these dirty superconductors, with the mean free path much smaller than the coherence length, the Werthamer-Helfand-Hohenber theoretical model does not fit well the Hc2(T) data. We demonstrate that this discrepancy can result from the presence of strong inhomogeneity or from two-band superconductivity in these systems. Both the approaches very well describe the H − T dependencies, but the present results as well as our previous studies give stronger arguments for the scenario based on the presence of nanoscopic inhomogeneity of the superconducting state. A comparative study of La-doped and Ce-doped Ca3Rh4Sn13 showed that in the disordered Ca3−xCexRh4Sn13 alloys the presence of spin-glass effects is the cause of the additional increase of T c in respect to the critical temperatures of disordered Ca3−xLaxRh4Sn13. We also revisited the nature of structural phase transition at T ∼ 130 170 K and documented that there might be another precursor transition at higher temperatures. Raman spectroscopy and thermodynamic properties suggest that this structural transition may be associated with a CDW-type instability.
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