G. W. Crabtree

The hydrogen economy

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Solar energy conversion

If solar energy is to become a practical alternative to fossil fuels, we must have efficient ways to convert photons into electricity, fuel, and heat. The need for better conversion technologies is a driving force behind many recent developments in biology, materials, and especially nanoscience.

Two-band superconductivity in MgB{sub 2}.

The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at Delta(1)=2.3 meV and Delta(2)=7.1 meV at 4.2 K. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multiband system. Temperature evolution of the tunneling spectra follows the BCS scenario [Phys. Rev. Lett. 3, 552 (1959)]] with both gaps vanishing at the bulk T(c). The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [Phys. Rev. Lett. 87, 087005 (2001)]].

Superconductivity in YBa2−xSrxCu3O7−δ

We report structure, resistivity, and Meissner effect measurements on YBa2−xSrxCu3O7−δ for 0<x<2.0. We find a region of solid solubility extending at least to x=1.0 and a monotonic depression of Tc with x. Using arguments based on structural changes with Sr doping, we speculate that the depression of Tc is due to the local distortion of the lattice in the neighborhood of the Sr site and the introduction of additional oxygen vacancies.

Self-assembled monolayer-enhanced hydrogen sensing with ultrathin palladium films

Resistive-type palladium structures for hydrogen sensing remains as a research focus for their simplicity in device construction. We demonstrate that a siloxane self-assembled monolayer placed between a substrate and an evaporated ultrathin Pd film promotes the formation of small Pd nanoclusters and reduces the stiction between the palladium and the substrate. The resulting Pd nanocluster film can detect 2%H2 with a rapid response time of ∼70ms and is sensitive to 25 ppm hydrogen, detectable by a 2% increase in conductance due to the hydrogen-induced palladium lattice expansion.

Vortex physics in high-temperature superconductors

The discovery of high‐temperature superconductors has stimulated dramatic growth in our understanding of the physics of quantized vortex lines. These superconductors exclude magnetic fields weaker than a lower critical field Hc1≤10−2 tesla. Stronger fields penetrate as an array of vortices, each consisting of exactly one quantum of flux (φ0 = hc/2e) surrounded in the plane perpendicular to the field by circulating supercurrents that extend radially a few hundred nanometers. The behavior of vortices dominates many physical properties of high‐temperature superconductors up to the upper critical field Hc2∼102 tesla, where superconductivity gives way to normal metallic behavior and magnetic fields penetrate uniformly.

Electronic and magnetic properties of rare-earth ions in REBa2Cu3O7-x (RE=Dy, Ho, Er)☆

Abstract Heat capacity, resistivity, and magnetic susceptability data have been obtained for the compounds REBa2Cu3O7-x, where RE = Dy, Ho or Er. Neutron diffraction data on the Ho compound show a structure identical to that of YBa2Cu3O7-x. Magnetic transitions are observed at Tm=0.95, 0.17 and 0.59 K for Dy, Ho and Er compounds, respectively. It is argued that these are due predominantly to dipolar interactions. Resistivity data show that the magnetic state is coexistent with superconductivity in all cases. From the heat capacity data, the degeneracies of the crystal field ground states are determined, and estimates are given for the magnetic moment in the ground state and the energy separation of the first excited crystal field state.

Scanning Tunneling Spectroscopy inMgB2

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39 K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter gamma. The value of the superconducting gap is 5 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair breaking as seen in the conductance spectra in fields up to 6 T.

Nickel antidot arrays on anodic alumina substrates.

Large-area nickel antidot arrays with a density up to 1010/cm2 have been fabricated by depositing nickel onto anodic aluminum oxide membranes that contain lattices of nanopores. Electron microscopy images show a high degree of order of the antidot arrays. Various sizes and shapes of the antidots were observed with increasing thickness of the deposited nickel. New features appear in the antidot arrays in both magnetization and transport measurements when the external magnetic field is parallel to the current direction, including an enhancement and a nonmonotonous field dependence of the magnetoresistance, larger values of the coercive field and remanence moment, and smaller saturation field.

Oxygen vacancy ordering and superconductivity in YBa2Cu3O7−x

Abstract The superconducting properties of YBa 2 Cu 3 O 7−x have been shown to depend markedly on the overall oxygen stoichiometry and on the ordering of oxygen vacancies on the available sites. YBa 2 Cu 3 O 7−x has been studied in situ by high-temperature neutron powder diffraction in controlled oxygen atmospheres in order to learn the structural properties of the system in thermodynamic equilibrium. Additionally, metastable, oxygen-deficient samples produced by quenching into liquid nitrogen from high temperature and various oxygen partial pressures have been studied at room temperature or low temperature in order to correlate the structural and superconducting properties. Such experiments have shown that a systematic relationship exists between the superconducting transition temperature, T c , the number of oxygen vacancies, and the degree of oxygen vacancy ordering. This brief paper reviews our work in these areas.