Carlos Chaparro

Sr flux stability against oxidation in oxide-molecular-beam-epitaxy environment: Flux, geometry, and pressure dependence

Maintaining stable fluxes for multiple source elements is a challenging task when the source materials have significantly different oxygen affinities in a complex-oxide molecular-beam-epitaxy (MBE) environment. Considering that Sr is one of the most easily oxidized and widely used elements in various complex oxides, we took Sr as a probe to investigate the flux-stability problem in a number of different conditions. Source oxidation was less for higher flux, extended port geometry, and unmelted source shape. The extended port geometry also eliminated the flux transient after opening a source shutter as observed in the standard port. We also found that the source oxidation occurred more easily on the crucible wall than on the surface of the source material. Atomic oxygen, in spite of its stronger oxidation effectiveness, did not make any difference in source oxidation as compared to molecular oxygen in this geometry. Our results may provide a guide for solutions to the source oxidation problem in oxide-MBE ...

Anisotropic phase diagram and superconducting fluctuations of single-crystalline SmFeAsO(0.85)F(0.15)

We report on the specific-heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO{sub 1-x}F{sub x}. In zero field, we find a clear cusplike anomaly in C/T with {Delta}C/T{sub c} = 24 mJ/mol K{sup 2} at T{sub c} = 49.5 K. In magnetic fields along the c axis, pronounced superconducting fluctuations induce broadening and suppression of the specific-heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of -3.5 T/K and an anisotropy of {Lambda} = 8. The small value of {Delta}C/T{sub c} yields a Sommerfeld coefficient {gamma} {approx} 8 mJ/mol K{sup 2}, indicating that SmFeAsO{sub 1-x}F{sub x} is characterized by a modest density of states and strong coupling.

Anisotropic phase diagram and superconducting fluctuations in SmFeAsO{sub 0.85}F{sub 0.15}.

We report on the specific-heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO{sub 1-x}F{sub x}. In zero field, we find a clear cusplike anomaly in C/T with {Delta}C/T{sub c} = 24 mJ/mol K{sup 2} at T{sub c} = 49.5 K. In magnetic fields along the c axis, pronounced superconducting fluctuations induce broadening and suppression of the specific-heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of -3.5 T/K and an anisotropy of {Lambda} = 8. The small value of {Delta}C/T{sub c} yields a Sommerfeld coefficient {gamma} {approx} 8 mJ/mol K{sup 2}, indicating that SmFeAsO{sub 1-x}F{sub x} is characterized by a modest density of states and strong coupling.

Anisotropic Phase Diagram and Superconducting Fluctuations in SmFeAsO0.85F0.15

We report on the specific-heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO{sub 1-x}F{sub x}. In zero field, we find a clear cusplike anomaly in C/T with {Delta}C/T{sub c} = 24 mJ/mol K{sup 2} at T{sub c} = 49.5 K. In magnetic fields along the c axis, pronounced superconducting fluctuations induce broadening and suppression of the specific-heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of -3.5 T/K and an anisotropy of {Lambda} = 8. The small value of {Delta}C/T{sub c} yields a Sommerfeld coefficient {gamma} {approx} 8 mJ/mol K{sup 2}, indicating that SmFeAsO{sub 1-x}F{sub x} is characterized by a modest density of states and strong coupling.

Normal-state conductance used to probe superconducting tunnel junctions for quantum computing

Here we report normal-state conductance measurements of three different types of superconducting tunnel junctions that are being used or proposed for quantum computing applications: p-Al/a-AlO/p-Al, e-Re/e-AlO/p-Al, and e-V/e-MgO/p-V, where p stands for polycrystalline, e for epitaxial, and a for amorphous. All three junctions exhibited significant deviations from the parabolic behavior predicted by the WKB approximation models. In the p-Al/a-AlO/p-Al junction, we observed enhancement of tunneling conductances at voltages matching harmonics of Al–O stretching modes. On the other hand, such Al–O vibration modes were missing in the epitaxial e-Re/e-AlO/p-Al junction. This suggests that absence or existence of the Al–O stretching mode might be related to the crystallinity of the AlO tunnel barrier and the interface between the electrode and the barrier. In the e-V/e-MgO/p-V junction, which is one of the candidate systems for future superconducting qubits, we observed suppression of the density of states at zero bias. This implies that the interface is electronically disordered, presumably due to oxidation of the vanadium surface underneath the MgO barrier, even if the interface was structurally well ordered, suggesting that the e-V/e-MgO/p-V junction will not be suitable for qubit applications in its present form. This also demonstrates that the normal-state conductance measurement can be effectively used to screen out low quality samples in the search for better superconducting tunnel junctions.