David G. Hinks

Structure of the single‐phase high‐temperature superconductor YBa2Cu3O7−δ

We have determined the crystal structure of the single‐phase stoichiometric high‐temperature superconductor in the Y‐Ba‐Cu‐O system using high‐resolution neutron powder diffraction. This compound has an orthorhombic structure with space group Pmmm and lattice constants a=3.8231 A, b=3.8864 A, and c=11.6807 A. The structure consists of ‘‘dimpled’’ CuO2 layers in the a‐b planes loosely bonded by one‐dimensional fencelike square‐planar CuO3 chains in the b‐c planes.

Evolution of the pseudogap from Fermi arcs to the nodal liquid

The pseudogap phase in the cuprates is a most unusual state of matter: it is a metal, but its Fermi surface is broken up into disconnected segments known as Fermi arcs. Using angle resolved photoemission spectroscopy, we show that the anisotropy of the pseudogap in momentum space and the resulting arcs depend only on the ratio T/T*(x), where T*(x) is the temperature below which the pseudogap first develops at a given hole doping x. In particular, the arcs collapse linearly with T/T* and extrapolate to zero extent as T goes to 0. This suggests that the T = 0 pseudogap state is a nodal liquid, a strange metallic state whose gapless excitations are located only at points in momentum space, just as in a d-wave superconductor.

Electronic Spectra and Their Relation to the ( π,π) Collective Mode in High- Tc Superconductors

The photoemission line shape near (pi, 0) in Bi(2)Sr(2)CaCu(2)O(8+delta) below T(c) is characterized by a sharp peak, followed at higher energy by a dip and hump. We study the evolution of this line shape as a function of momentum, temperature, and doping. We find the hump scales with the peak and persists above T(c) in the pseudogap state. We present strong evidence that the peak-dip-hump structure arises from the interaction of electrons with a collective mode of wave vector (pi, pi). The inferred mode energy and its doping dependence agree well with a magnetic resonance observed by neutron scattering.

Phase diagram and superconductivity in the Y‐Ba‐Cu‐O system

We have determined the phase diagram of the Y‐Ba‐Cu‐O system through structural, superconducting critical temperature and critical current density characterization. Our results show that a single‐phase compound with a stoichiometry YBa2Cu3Oy is responsible for the high‐temperature superconductivity (92.5 K) in this system.

Preparation of Bi‐Sr‐Ca‐Cu‐O superconductors from oxide‐glass precursors

A superconductor and precursor therefor from oxide mixtures of Ca, Sr, Bi and Cu. Glass precursors quenched to elevated temperatures result in glass free of crystalline precipitates having enhanced mechanical properties. Superconductors are formed from the glass precursors by heating in the presence of oxygen to a temperature below the melting point of the glass.

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.

Experimental study of MgB2 decomposition

The thermal stability of MgB2 has been studied experimentally to determine the role of thermodynamic and kinetic barriers in the decomposition process. The MgB2 decomposition rate approaches one monolayer per second at 650 °C and has an activation energy of 2.0 eV. The evaporation coefficient is inferred to be ∼10−4, indicating that this process is kinetically limited. These values were inferred from in situ measurements using a quartz crystal microbalance and a residual gas analyzer, in conjunction with ex situ measurements of redeposited material by Rutherford backscattering spectroscopy and secondary ion mass spectroscopy. The presence of a large kinetic barrier to decomposition indicates that the synthesis of MgB2 thin films conditions may be possible with vacuum processing, albeit within a narrow window in the reactive growth conditions.

Observation of a d -wave nodal liquid in highly underdoped Bi 2 Sr 2 CaCu 2 O 8+ δ

High-temperature superconductivity in the cuprates arises when charge carriers are added to an insulator. Between these states lies the so-called nodal liquid at low temperature. Photoemission spectroscopy suggests that superconductivity evolves smoothly from this nodal-liquid state.

Scanning tunneling spectroscopy in MgB2

Abstract We present extensive scanning tunneling spectroscopy measurements at low temperatures in the multiband superconductor MgB 2 . We find a similar behavior in single crystalline samples and in single grains, which clearly shows the partial superconducting density of states of both the π and σ bands of this material. The superconducting gaps corresponding to both bands are not single valued. Instead, we find a distribution of superconducting gaps centered around 1.9 and 7.5 mV, corresponding respectively to each set of bands. Interband scattering effects, leading to a single gap structure at 4 mV and a smaller critical temperature can be observed in some locations on the surface. S–S junctions formed by pieces of MgB 2 attached to the tip clearly show the subharmonic gap structure associated with this type of junctions. We discuss future developments and possible new effects associated with the multiband nature of superconductivity in this compound.

Variable-range hopping conduction in Ba1−xKxBiO3−y system

Abstract Normal state resistivity of the Ba1−xKxBiO3−y system is reported for various K concentrations. In the concentration range where the material is superconducting, phonon-assisted variable-range hopping [exp(T0/T) 1 4 ] is observed between 160 and 300 K.