J. J. Hamlin

Electronic correlations in the iron pnictides

When electrons experience Coulomb repulsion, their kinetic energy becomes significantly reduced. This effect has now been measured in the pnictide superconductor LaFePO, and shows that correlations between electrons in these materials are just as strong as in some copper oxide and ruthenate superconductors. 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 spectroscopy1. 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 pnictides2,3,4 puts a spotlight on the relevance of correlation effects in these materials5. 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.

Pressure-induced unconventional superconducting phase in the topological insulator Bi2Se3.

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.

The dependence of T{sub c} on hydrostatic pressure in superconducting MgB{sub 2}.

The dependence of Tc on hydrostatic (He-gas) pressure for superconducting MgB2 has been determined to 0.7 GPa. We find that Tc decreases linearly and reversibly under pressure at the rate dTc/dP ≃ −1.11 ± 0.02 K/GPa. These studies were carried out on the same sample used in earlier structural studies under He-gas pressure which yielded the bulk modulus B = 147.2 ±0.7 GPa. The value of the logarithmic volume derivative of Tc is thus accurately determined, dln Tc/dln V = +4.16 ± 0.08, allowing quantitative comparison with theory. The present results support the emerging picture that MgB2 is a BCS superconductor with electron-phonon pairing interaction.

High pressure transport properties of the topological insulator Bi2Se3

We report x-ray diffraction, electrical resistivity, and magnetoresistance measurements on Bi2Se3 under high pressure and low temperature conditions. Pressure induces profound changes in both the room temperature value of the electrical resistivity as well as the temperature dependence of the resistivity. Initially, pressure drives Bi2Se3 toward increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance Δρ(xx) that is positive at low fields and becomes negative at higher fields. Our results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3.

Superconductivity in single crystals of LaFePO

Single crystals of the compound LaFePO were prepared using a flux growth technique at high temperatures. Electrical resistivity measurements reveal metallic behavior and a resistive transition to the superconducting state at a critical temperature Tc~6.6?K. Magnetization measurements also show the onset of superconductivity near 6?K. In contrast, specific heat measurements manifest no discontinuity at Tc. These results lend support to the conclusion that the superconductivity is associated with oxygen vacancies that alter the carrier concentration in a small fraction of the sample, although superconductivity characterized by an unusually small gap value cannot be ruled out. Under applied magnetic fields, Tc is suppressed anisotropically for fields perpendicular and parallel to the ab-plane, suggesting that the crystalline anisotropy strongly influences the superconducting state. Preliminary high pressure measurements show that Tc passes through a maximum of nearly 14?K at ~110?kbar, demonstrating that significantly higher Tc values may be achieved in the phosphorus-based oxypnictides.

Superconductivity at 17 K in yttrium metal under nearly hydrostatic pressures up to 89 GPa

In an experiment in a diamond anvil cell utilizing helium pressure medium, yttrium metal displays a superconducting transition temperature which increases monotonically from

Superconductivity in LnFePO (Ln= La, Pr and Nd) single crystals

{T}_{c}\ensuremath{\simeq}3.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Effect of pressure on the superconducting critical temperature of La[O0.89F0.11]FeAs and Ce[O0.88F0.12]FeAs

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Dependence of the superconducting transition temperature of single and polycrystalline MgB2 on hydrostatic pressure

30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}

Pressure-Induced Superconducting Phase in the Charge-Density-Wave Compound Terbium Tritelluride

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