K. Conder

Surface and bulk electronic structure of the strongly correlated system SmB6 and implications for a topological Kondo insulator

Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB6 may be a realization of a topological Kondo insulator. We have performed an angle-resolved photoemission spectroscopy study on SmB6 in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. The obtained electronic structure in the whole three-dimensional momentum space reveals one electron-like 5d bulk band centered at the X point of the bulk Brillouin zone that is hybridized with strongly correlated f electrons, as well as the opening of a Kondo band gap (Delta(B) similar to 20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the center Gamma point and boundary (X) over bar point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB6 is likely to be related to the pronounced surface states sitting inside the band hybridization gap and/or the presence of a topological Kondo insulating state.

Pressure induced static magnetic order in superconducting FeSe1-x.

We report on a detailed investigation of the electronic phase diagram of FeSe1-x under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure approximately 0.8 GPa the nonmagnetic and superconducting FeSe1-x enters a region where static magnetic order is realized above T{c} and bulk superconductivity coexists and competes on short length scales with the magnetic order below T{c}. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.

Direct Observation of the Oxygen Isotope Effect on the In-Plane Magnetic Field Penetration Depth in Optimally Doped YBa~2Cu~3O~7~-~d~e~l~t~a

We report the first direct observation of the oxygen-isotope ((16)O/(18)O) effect on the in-plane penetration depth lambda(ab) in a nearly optimally doped YBa(2)Cu(3)O(7-delta) film using the novel low-energy muon-spin rotation technique. Spin-polarized low-energy muons are implanted in the film at a known depth z beneath the surface and process in the local magnetic field B(z). This feature allows us to measure directly the profile B(z) of the magnetic field inside the superconducting film in the Meissner state and to make a straightforward determination of lambda(ab). A substantial isotope shift Delta lambda(ab)/lambda(ab)=2.8(1.0)% at 4 K is observed, implying that the in-plane effective supercarrier mass m*(ab) is oxygen-isotope dependent with Delta m*(ab)/m*(ab)=5.5(2.0)%. These results are in good agreement with magnetization measurements on powder samples.

Tuning the superconducting and magnetic properties of Fe y Se 0.25 Te 0.75 by varying the iron content

The superconducting and magnetic properties of FeySe0.25Te0.75 single crystals (0.9≤y≤1.1) were studied by means of x-ray diffraction, superconducting quantum interference device magnetometry, muon-spin rotation, and elastic neutron diffraction. The samples with y<1 exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for y≥1 but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the FeySe1−xTex can be tuned not only by varying the Se/Te ratio but also by changing the Fe content. ©2010 The American Physical Society

Evolution of two-gap behavior of the superconductor FeSe1-x.

The superfluid density, rho{s}, of the iron chalcogenide superconductor, FeSe1-x, was studied as a function of pressure by means of muon-spin rotation. The analysis of rho{s}(T) within the two-gap scheme reveals that the effect on both, the transition temperature T{c} and rho{s}(0), is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.

The ratio of small polarons to free carriers in derived from susceptibility measurements

The normal-state spin susceptibility of was measured as a function of doping x. For , is well described by small-polaron theory. For , a temperature-independent Pauli component is present in as well. These results indicate the coexistence of small polarons and free carriers above the transition concentration . The concentration of the free carriers increases rapidly with , reaching a maximum for optimal doping (x = 0.15). The large mass anisotropy of the polarons is obtained quantitatively from the -measurements.

Effect of oxygen ordering on the structural and magnetic properties of the layered perovskites PrBaCo2O5+δ

The influence of oxygen ordering on the structural and magnetic properties of the layered Co-based perovskites PrBaCo2O5+δ (0.16 0.7), could thus be arising from the percolation of oxygen-rich ferromagnetic clusters.

Oxygen isotope effects in : evidence for polaronic charge carriers and their condensation

Magnetization measurements have been performed on the oxygen-isotope exchanged samples ( and ) of the one-layer cuprate superconductors . From magnetization measurements on the fine-grained and decoupled samples in the Meissner state, we find substantial oxygen-isotope effects on both the penetration depth and . From normal-state susceptibility measurements, we are able to show that there is a negligible oxygen-isotope effect on the carrier density n. The combined results strongly suggest that there is an oxygen-isotope effect on the effective supercarrier mass , which is huge for x = 0.06, and reduced to a smaller value for x = 0.15. We discuss the isotope effects, supercarrier mass anisotropy, normal-state gap, in-plane penetration depth, and mid-infrared spectra for on the basis of the small polaron theory of superconductivity. We find that the agreement between the calculated and experimental results is excellent without any adjustable parameters.

Evolution of two-gap behavior of the superconductor FeSe_{1-x}

The superfluid density, rho{s}, of the iron chalcogenide superconductor, FeSe1-x, was studied as a function of pressure by means of muon-spin rotation. The analysis of rho{s}(T) within the two-gap scheme reveals that the effect on both, the transition temperature T{c} and rho{s}(0), is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.

Oxygen isotope exchange kinetics and site-selective oxygen substitution in YBa2Cu318O7−χ

Abstract Studies of the oxygen isotope exchange kinetics in 18 O substituted YBa 2 Cu 3 O 7 (123) have been performed. As a weight decrease is observed during the 18 O→ 16 O exchange, high resolution thermogravimetric measurements (both isothermal and non-isothermal) were used to investigate the kinetics. The exchange process takes place at relatively low temperatures, 225–430°C, and was found to be non-isokinetic in this temperature range. Two ranges with different activation energies have been found: (1) 1.88 eV both in the ranges 433-398°C (total exchange of 18 O in the planes+apex+chains) and 353-307°C (exchange mainly in apex+chains) but with different pre-exponential factors and (2) 0.71 eV at t