Amorphous Bi Research Articles

Ratio between energy gap and transition temperature in ultrathin superconducting films

The ratio 2Δ(0) k B T C increases with decreasing film thickness (down to 5 nm) for vapour-quenched micro-crystalline Pb, while it is almost independent of thickness for amorphous Ga and Bi.

Evidence for Covalent Bonding in Crystalline and Amorphous As, Sb, and Bi from Valence-Band Photoelectron Spectra

X-ray photoelectron spectra (XPS) of the valence bands of crystalline and amorphous As, Sb, and Bi have been measured. The results are compared with band-structure calculations for the crystalline materials. A splitting, $\ensuremath{\Delta}E$, of the $s$-like peak for the crystalline phase disappears in the amorphous phase and the $p$-like peak is shifted towards higher energies. This similarity to tetrahedrally coordinated covalent semiconductors is explained by describing the semimetals as layers of distorted covalently bonded hexagonal rings. This generalizes the even-odd ring effect previously observed in group-IV semiconductors to the $A7$ lattice of As, Sb, and Bi. The relation of $\ensuremath{\Delta}E$ to the interatomic distance is described by a universal curve, which applies to the group-V semimetals as well as group-IV semiconductors.

Superconductivity in amorphous Bi and Ga

The electron-phonon α2(ω)F(ω) has been measured by Chenet al. in films of amorphous Bi and Ga. This was accomplished by inversion of low-temperature quasiparticle tunneling data. From α2(ω)F(ω), the phonon renormalization of the electronic effective mass was calculated and was found to be considerably greater than it is in the case of the well-known strong-coupling superconductors Pb and Hg. These amorphous materials should, therefore, be ideal for a study of strong-coupling effects, i.e., deviations from BCS laws. In this paper, we present results of a theoretical study of select thermodynamic properties of these materials based on numerical solutions, at finite temperatures, of the Eliashberg equations.

The hall effect of amorphous-non transition metals

The amorphous metals Bi, Ga, Sn and Pb are obtained by quenched condensation. Their Hall effect is measured and compared with the model of free electrons. For Ga and Sn we find a good agreement with the free electron model. However, the Hall coefficient of Bi and Pb is only half as large as the theoretical value. This is particularly surprising because the corresponding liquids agree well with the free electron model and the amorphous state closely resembles the liquid. A trace effect of band structure may be responsible for the deviation.

Effect of Surface Charge on the Superconducting Transition Temperature and Normal-State Conductivity of Disordered Metal Films

Measurements were made of the shifts with added electrostatic charge of the normal-state residual conductivity and superconducting transition temperature of films of amorphous Bi and Ga, metastable crystalline Ga, and disordered crystalline Pb. For the amorphous Bi samples adding electrons increased both the superconducting transition temperature Tc and the residual contivity σ0. For the other materials adding electrons lowered both Tc and σ0. In each case removing electrons produced a shift of equal magnitude but of opposite sign to that observed for negative charging. For a given applied charge the shifts in Tc and σ0 were proportional to the reciprocal of the film thickness within the accuracy of the measurements. Results were reproducible from sample to sample. The correlation between the signs of the shifts Tc and σ0 for the metals investigated was the reverse of that observed previously for Sn, In, and Tl. Measurements were also made of the effect of condensed oxygen on the superconducting transition temperature of the films. Physically adsorbed oxygen lowered Tc for all of the materials. Increasing the temperature to the point that a reaction between the metal and the condensed oxygen began produced a further reduction in Tc for Pb and metastable crystalline Ga.

Electron tunneling study of amorphous Pb-Bi superconducting alloys

Electron tunneling studies carried out on amorphous Bi and Ga strong-coupling superconductors have been extended to a series of amorphous Pb-Bi superconducting alloys, ranging from 100% Pb to 100% Bi. The energy gap and transition temperature have been measured for each alloy composition, both in the amorphous state and after annealing to obtain the crystalline state. Phonon spectra have been derived for the amorphous state of the alloys by using the McMillan inversion program on our derivative tunneling data. Strong-coupling parameters derived from our phonon spectra have been used to test recent theories of the transition temperature of amorphous superconductors. The results are in reasonable agreement with the theories of Benda and of Garland et al.

Phonon spectra of amorphous superconductors

The techniques electron tunneling were applied to the study of the amorphous superconductors Ga, Bi(+Pb impurity), and Pb(+Bi impurity). The phonon spectra were computed using McMillan's inversion program. The characteristics of this kind of superconductor are the large Coulomb pseudopotentials and electron-phonon coupling constant. Calculated values of the T c for a disordered superconductor using the theory of Garland et al. were in good agreement with the measured values.

Temperature dependence of the superconducting energy gap of amorphous Bi

The temperature variation of the energy gap in amorphous Bi has been calculated from the Eliashberg Kernels which have recently been inferred from low temperature quasi particle tunneling data. The gap is found to exhibit important departures from a B.C.S. temperature variation. These deviations are a result of strong coupling effects and are considerably larger than those expected in Pb. They should be easily measurable.

Conduction in amorphous MgBi and MgSb alloys

The electrical properties of amorphous MgBi and MgSb alloys are discussed in terms of a model of a pseudogap containing localized states with the electrons moving by thermally activated hopping. However, in the MgSb alloys on the Sb rich side of the intermetallic compound it appears that the density of states in the pseudogap is too low for hopping between localized states to be of importance, and the alloy behaves more like an extrinsic semiconductor with electrons being excited into acceptor states a few tenths of an eV above the valence band.

Solid Amorphous Bi, Ga, and Fe as Examples of Liquid-Like Amorphous Substances

Experiments show that two structures simultaneously occur in liquid-like solid amorphous and in molten Bi and Ga, i.e., the spherical close packing in the form of a straight atomic chain and the layer lattice structure. In Bi the layer lattices are derived from the respective lattice and in Ga they are remainders of the lattice. In solid amorphous Fe the spherical close packing shows up as a zigzag chain in the atomic distribution curve as in molten Na. Differing from Bi and Ga, in amorphous Fe the layer lattices are parts of the lattice.

Solid Amorphous Bi, Fe, and Ga as Examples of Liquid-Like Amorphous Substances

Solid amorphous Bi, Fe, and Ga are liquid-like, i.e., they have the same structure as the respective melt. However, the degree of order is higher than in the liquid state. A detailed analysis of the structure of liquid-like solid amorphous Bi and Fe (8-coordination) is given. Their atomic distribution curves are compared with those of molten Bi and Na (8-coordination). By discussion of the electron diffraction patterns of solid amorphous and of molten Bi one finds a layer lattice structure (IInd structure) which is derived from the appertaining lattice. The Fourier analysis of the intensity curves, however, demands the spherical close packing in the form of a straight atomic chain (Ist structure). Fujime’s electron diffraction patterns of solid amorphous Fe also show that there exist two structures simultaneously: a layer lattice structure of the appertaining lattice (IInd structure) and the spherical close packing, this time as a plane zigzag chain (Ist structure) as in molten Na. This double structure is also obtained in solid amorphous and in molten Ga. The noncrystalline Ga has an especially simple structure as found immediately by discussion of the intensity curve. In solid amorphous and in molten Bi, Fe, and Ga the simultaneous occurrence of two structures stabilizes the noncrystalline state.

Fluctuation Rounding of the Resistive Superconducting Transition in Thin Lead Films

The effect of fluctuations on the resistive transition to superconductivity has been studied in thin (100-200 \AA{}) Pb films. Although the temperature dependence of the fluctuation contribution is given by ${(T\ensuremath{-}{T}_{c})}^{\ensuremath{-}1}$, and the width of the transition increases with increasing film resistance as predicted by Aslamazov and Larkin, this width is found to be smaller than the theoretical value by factors of 2-14. The transition from two-to three-dimensional behavior, expected theoretically in one of the samples studied, was not observed. A similar failure is found on analyzing published data for Al and (possibly for) amorphous Bi films. The various approximations used to derive the theory, and their effect on the analysis of the data (which has been carried out with no fitted parameters), are discussed in detail. The quantitative applicability of the Aslamazov-Larkin theory is considered in the light of its discrepancies with experiment. However, no explanation is found consistent with all published data. For one sample, measurements were performed in a magnetic field; a discussion of additional broadening mechanisms in a magnetic field is presented.

Phonon Spectrum of Superconducting Amorphous Bismuth and Gallium by Electron Tunneling

The phonon spectra for amorphous Bi and Ga are derived by using McMillan's inversion program on our tunneling data. The results indicate an extreme "softening of the phonon spectrum" from what is found for crystalline superconductors. Parameters derived from our phonon spectra are used to test recent theories of the transition temperature of amorphous and strong-coupling superconductors, and also to test the McMillan equation for the maximum possible transition temperature in the limit of extremely strong coupling.

Size dependence of the superconducting transition temperature

A linear dependence of the superconducting transition temperature on the reciprocal of sample thickness is reported for amorphous Bi and Ga films having thicknesses from 150 up to several thousand Angstroms. Decreasing the thickness lower the transition temperature.

Tunnelexperimente an amorphen Supraleitern

Superconducting disordered and amorphous films of Bi, Ga, Al are studied by the tunneling technique. The values of the reduced energy gap 2δ/kT c (4.56 in amorphous Bi, 4.52 in amorphous Ga, 4.15 in amorphous Sn, and 3.72 in amorphous Al) indicate a very strong coupling in some amorphous superconductors. An increase of the tunneling resistance by crystallization of the amorphous Ga-films is interpreted as a decrease in the concentration of free electrons. The influence of the high film resistance on the measured slope of the tunneling curves is observed as long as both films or one of them are normal.

The absolute quantum yield of the internal photoeffect in high-resistivity semiconductors : S.G. Greshinin and Yu.A. Tcherkasov (Vol. 6, No. 9, pp. 2831–2836)

The development and characterization of an amorphous SeBiI alloy system whose electrophotographic sensitivity is substantially greater than either selenium or arsenic triselenide at all visible wavelengths and whose sensitivity extends beyond 800 nm are described. Amorphous films were prepared by flash evaporation in vacuum of thermally blended alloys. A composition matrix of bismuth and iodine in selenium was surveyed to identify the optimum alloy. Alloys containing approximately equal amounts of bismuth and iodine were found to be more structurally stable and have more desirable electrophotographic properties than either alloys varying from these compositions or the iodine-free selenium-bismuth binary system. The optimum alloy contained about 3 at% bismuth, 3 at% iodine and 94 at% selenium. The alloy system would appear to have utility wherever panchromaticity or near infrared sensitivity is required and thin photogeneration layers can be tolerated. Potential applications lie in devices with multilayer configurations, in image orthicons and laser coupled devices.