C.C. Tsuei

Anomalous electrical conduction in disordered and non-crystalline metallic conductors

Abstract Many disordered and non-crystalline metallic conductors are characterized by both a negative temperature coefficient (α = ϱ-1 dϱ/dT) of resistivity ϱ over a wide range of temperatures T and a gradual leveling-off of ϱ at low temperatures. Experimental results will be presented to show that ϱ varies as -ln T (for T ⪸ the Debye temperature) in contrast to the prediction of existing theories. This anomalous electron transport can be understood in terms of an attractive interaction between conduction electrons and localized excitations arising from a structural indeterminacy in the atomic arrangement. The possibility of using this scattering mechanism to explain the unusual deviation from linear T dependence of resistivity (the bulge effect) in many structurally unstable superconductors such as A-15 Nb3Ge, V3Si, bcc Nb and alloys containing the ω-phase is also discussed.

Upper critical fields of amorphous transition metal based alloys

Abstract The upper critical fields [ H c 2 ( T )] for several superconducting amorphous transition metal alloys have been measured. At low temperatures and high fields, the behavior of the critical field cannot be described by existing theory. Corrections to the theory for electron spin and spin-orbit scattering effects, strong-coupling, purity, and anisotropy effects are not adequate to bring the theory into agreement with experiment even when an unphysical choice of correction parameters is assumed. Using the present and all other available H c 2 ( T ) data on three dimensional amorphous superconductors, we show that the magnitude of the deviation from theory is systematically related to the normal state electrical resistivity of the samples. We therefore argue that the breakdown of the theory is due to its failure to account for strong electron scattering effects. Such effects are associated with the disorder induced tendency toward Anderson localization.

The ratio of energy gap to transition temperature in amorphous superconductors

Abstract Using results of tunneling measurements, values of 2Δ(o)/kT c for a number of amorphous transition-metal based superconductors are found to be 3.5±0.1 in agreement with the BCS theory in the weak-coupled limit unlike previously reported results suggesting strong electron-phonon coupling in amorphous non-transition metals such as Ga and Pb-Bi alloys. A comparison of our results with existing data for amorphous soft-metal alloys and A-15 compounds suggests that non-crystallinity alone does not give rise to a significantly enhanced electron-phonon interaction.

The critical specific heat of a glassy ferromagnet

Abstract Using standard ac calorimetry, we have measured the specific heat of the amorphous ferromagnet Fe75P15C10 in the neighborhood of its Curie temperature TC ≅ 600 K. Even though this material is structurally disordered, a sharp lambda-like cusp was observed, typical of a pure crystalline substance. We report the first determination of the critical specific heat exponent α = δ′ = -.18 ± .04 and the amplitude ratio A+/A- = 1.2 ± .3 for an amorphous ferromagnet and find that these values are typical of a Heisenberg system. Our results are in agreement with recent renormalization group calculations which indicate that the fixed point which characterizes the critical behavior will be stable against the addition of disorder to the ordered system if α

Amorphous Heusler alloys: The effect of annealing on structure and magnetic and transport properties

Abstract Alloys with the Heusler compositions Cu2MnIn, Cu2MnAl and Cu2MnSn have been prepared as amorphous films for the first time. The structural disorder results in as-deposited films which are not ferromagnetic and, in fact, exhibit spin-glass properties due to the distribution of MnMn distances and co-existence of positive and negative exchange interactions. Annealing of the In and Al alloys can restore single phase ordered Heusler structures which are ferromagnets with Curie temperatures typical of bulk alloys. The calculated Mn moments remain lower than the bulk values of 4 μB probably due to disorder at grain boundaries. Films annealed under non-optimum conditions crystallized into several non-magnetic structures as well as the magnetic L21 Heusler phase. Lattice constants of all phases were determined. The temperature dependence of resistivity through the amorphous-crystalline transformation was found to confirm the results of thermal annealing on structure and magnetization.

Topography and local conductance images of YBa2Cu3O7 crystals fractured in ultra high vacuum

Abstract Atomic-scale topography and local dynamic conductance images of polycrystalline YBa2Cu3O7 samples fractured in ultra high vacuum are obtained with a cross-sectional scanning tunneling microscope. Topographic features are found which consist of strips with a 12 A periodicity. Simultaneous measurements of normalized conductance (d In I/dV) on these surfaces exhibit a sharp contrast from strip to strip. The corresponding tunneling spectra at these areas show an alternation of metallic and semiconducting characteristics. Possible implication of the results on the basic understanding of high temperature superconductivity is discussed.

Hybridization and bonding in transition metal compound superconductors: X-ray photoemission from crystalline and amorphous Nb3Ge.

Abstract An XPS (or ESCA) study of the high T c crystalline and the low T c amorphous phases of Nb 3 Ge indicates a change of the Nb-Ge bonding from covalent to metallic upon the amorphous to crystalline transition. The high T c superconductivity of Nb 3 Ge does not appear to stem only from an unusually high electronic density of states at the Fermi level but rather from another property such as a resonant enhancement of the electron-phonon coupling resulting from cation-anion hybridization at E F .

31P NMR in the ferromagnetic state of amorphous Fe75P15C10

Abstract The first observation of a metalloid site hyperfine field (hf) distribution in a ferromagnetic amorphous alloy is reported using a spin-echo NMR technique. The 31P nuclei in amorphous Fe75P15C10 show a hf distribution with a maximum at about 27 kOe. The sample was prepared with Fe enriched to 99.93% in 56Fe. A comparison of the NMR spectra on samples containing natural Fe and 56Fe also provides the 57Fe hf distribution whose peak value agrees with Mossbauer results.

X-ray scattering study of the temperature dependence of the structure of the high Tc superconductor La1.8Sr0.2CuO4

Abstract We present data from powder scans of the High Tc (≈40 K) superconductor La1.8Sr0.2CuO4 as a function of temperature from room temperature to 10 K. We observe a K2NiF4-type structure, with no evidence for a structural phase transition or anomalous changes in the lattice constants. For T ≳ 190 K we find αa  (1/a)da/dT and αc  (1/c)dc/dT to be 1.4 ± 0.5 × 10−5K−1 and 1.7 ± 0.5 × 10−5K−1, respectively.

A comparative study of amorphous and crystalline superconducting molybdenum films by ultraviolet photoemission spectroscopy

Abstract Ultraviolet photoemission measurements at two photon energies are carried out, to understand the strong enhancement of the superconducting transition temperature T c in the nitrogen stabilized amorphous molybdenum films. The rise of T c can be attributed to a 40% increase of the electronic density of states at the Fermi level N(E F ) and a 15% softening of the phonon spectrum.