D. A. Zocco

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

Single crystals of the compounds LaFePO, PrFePO and NdFePO have been prepared by means of a flux growth technique and studied by electrical resistivity, magnetic susceptibility and specific heat measurements. We have found that PrFePO and NdFePO display superconductivity with values of the superconducting critical temperature Tc of 3.2 and 3.1 K, respectively. The effect of annealing on the properties of LaFePO, PrFePO and NdFePO is also reported. The LnFePO (Ln=lanthanide) compounds are isostructural with the LnFeAsO1−xFx compounds that become superconducting with Tc values as high as 55 K for Ln=Sm. A systematic comparison of the occurrence of superconductivity in the series LnFePO and LnFeAsO1−xFx points to a possible difference in the origin of the superconductivity in these two series of compounds.

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

We have performed several high-pressure resistivity experiments on the recently discovered superconductors La[O0.89F0.11]FeAs and Ce[O0.88F0.12]FeAs. At ambient pressure, these materials have superconducting onset temperatures Tc of 28 K and 44 K, respectively. While the Tc of La[O0.89F0.11]FeAs goes through a maximum between 10 and 68 kbar, in qualitative agreement with a recent report by Takahashi et al., the Tc of Ce[O0.88F0.12]FeAs decreases monotonically over the measured pressure range. At 265 kbar, the Tc of the cerium-based compound has been suppressed below 1.1 K.

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

A series of high-pressure electrical resistivity measurements on single crystals of TbTe3 reveal a complex phase diagram involving the interplay of superconducting, antiferromagnetic and charge-density-wave order. The onset of superconductivity reaches a maximum of almost 4 K (onset) near approximately 12.4 GPa.

Persistent non-metallic behavior in Sr2IrO4 and Sr3Ir2O7 at high pressures.

Iridium-based 5d transition-metal oxides are attractive candidates for the study of correlated electronic states due to the interplay of enhanced crystal-field, Coulomb and spin-orbit interaction energies. At ambient pressure, these conditions promote a novel Jeff = 1/2 Mott-insulating state, characterized by a gap of the order of ~0.1 eV. We present high-pressure electrical resistivity measurements of single crystals of Sr2IrO4 and Sr3Ir2O7. While no indications of a pressure-induced metallic state up to 55 GPa were found in Sr2IrO4, a strong decrease of the gap energy and of the resistance of Sr3Ir2O7 between ambient pressure and 104 GPa confirm that this compound is in the proximity of a metal-insulator transition.

Ferromagnetic quantum critical point in UCo 1 − x Fe x Ge

We have carried out a comprehensive study of the UCo1-xFexGe series across the entire range of compositions 0 <= x <= 1, and report the results of x-ray diffraction, magnetization, specific heat, and electrical resistivity to uncover the T-x phase diagram. Substitution of Fe into UCoGe initially results in an increase in the Curie temperature and a rapid destruction of the superconductivity. Near x = 0.22, the ferromagnetic transition is suppressed to zero temperature at an apparent ferromagnetic itinerant electron quantum critical point, where the temperature dependence of the electrical resistivity and specific heat in this region reveal non-Fermi liquid behavior.

Pressure dependence of the charge-density-wave and superconducting states in GdTe3, TbTe3, and DyTe3

We present electrical resistivity and ac-susceptibility measurements of

Search for pressure-induced superconductivity in CeFeAsO and CeFePO iron pnictides

{\text{GdTe}}_{3}, {\text{TbTe}}_{3}

Probing the superconductivity of PrPt4Ge12 through Ce substitution

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