Quench-condensed Films Research Articles

Erratum: Localized excited states observed in superconducting quench-condensed and annealed manganese-doped zinc films

Erratum: Localized excited states observed in superconducting quench-condensed and annealed manganese-doped zinc films

Quench-condensed indium-hydrogen films. II. Changes in electric and superconducting characteristics near the metal-insulator transition

Electrical and superconducting properties of indium films condensed in a H2 atmosphere (pressurep H 2=6×10−6 to 1.4×10−4 Torr) onto a substrate cooled with liquid helium are investigated. As hydrogen content is increased, a continuous increase in residual resistivity ρ* is observed, permitting systematic study of the resistance vs. temperature dependenceR(T) and the superconducting transition temperatureT c on approaching the metal-insulator transition (MIT). With regard to ρ*, four regimes of conductivity can be observed: (1) conductivity with a positive temperature resistance coefficient (TRC), (2) conductivity with a small, constant, negative TRC, (3) conductivity under weak localization with ΔR (T) ∼ln T or $$ \sim \sqrt T $$ type corrections, (4) hopping conductivity.T c rises continuously with ρ* and reaches its peak (∼5.2K) in the second regime. A further increase of ρ* leads to a decrease ofT c and complete suppression of superconductivity. The experimental dependenceR(T) is compared with theory. TheT c variation on approaching the MIT and the relation between Mooij's rule and the superconducting properties are discussed.

Localized excited states observed in superconducting quench-condensed and annealed manganese-doped zinc films

We have measured the electron-tunneling characteristics of a dilute, superconducting zinc-manganese alloy, both for a quench-condensed thin film and for one annealed at room temperature. The ac-conductance curve of the annealed film showed a well-defined structure in the energy gap corresponding to localized states predicted by the theory of Shiba and Rusinov or that of M\uller-Hartmann and Zittartz. The observed band was broader than predicted. The ac conductance of the quench-condensed film showed an even broader structure. The annealed film's density of states at zero energy agreed reasonably well with that calculated from critical-field measurements of Smith on bulk Zn-Mn.

Quench-condensed indium-hydrogen films. I. Superconductivity enhancement and metastable state formation

The electrical and superconducting properties of indium films are studied. The films were condensed in H2 atmosphere (\({\text{p}}_{{\text{H}}_{\text{2}} } \) ranged from 4×10−7 to 6×10−5 Torr) onto a substrate cooled with liquid helium down to about 3 K. As the hydrogen concentration increased, the normal resistivity ρ n of the films continuously increased up to a point where the metal-insulator transition occurred. It was found for the first time that saturation of In films with hydrogen lead to a significant increase (up to ∼5.15 K) inT c and to zero-dimensional paraconductivity aboveT c . Possible mechanisms for an effect of dissolved hydrogen on ρ n andT c are considered along with the influence of structural defects onT c . At\({\text{p}}_{{\text{H}}_{\text{2}} } \)>2×10−6 Torr the prepared films appear to be a metastable phase In(H) with high nonequilibrium hydrogen concentration, which decayed at ∼ 5 K. The destruction of In(H) was accompanied by a release of H2 from the film, a sharp drop of resistance (by three and more orders of magnitude), and a rise inT c . The properties and the formation and stabilization conditions of the In(H) phase are considered in detail and this phase is compared with the a-Bi(H) phase observed earlier.

Electron-tunneling search for bound states in superconducting manganese-doped aluminum

It has been predicted that the virtual bound states localized at Mn atoms in bulk samples of Al-Mn alloys in the normal state will yield an observable band of bound states below the gap edge of the alloy when it is cooled and becomes superconducting. We present reasons why we believe that such bound states would be too broad to be seen in such samples but might be present in quench-condensed films. We have used electron-tunneling measurements to look for these bound states in superconducting, quench-condensed, Al-Mn alloy films, but have not found them. We believe our results indicate that the manganese atoms are nonmagnetic and that any bound states that exist must be too broad to be seen in this tunneling experiment.

Localized modes of oscillation in hydrogen and deuterium doped lead

We have measured the tunneling characteristics of quench-condensed films of pure Pb, hydrogen-doped Pb, and deuterium- doped Pb. We have determined phonon spectra to fit the electronic densities of states calculated from our data. The phonon spectra for Pb/H and Pb/D exhibit additional phonon peaks which we associate with a localized mode (or modes) of the impurity. The magnitudes of the peaks indicate that this mode is strongly coupled to the electronic system.

Quantum oscillations of the superconducting transition temperature in Al-SiO systems

We report measurements on the auperconducting transition temperature T/sub c/ and the normal resistance R/sub n/ of quench-condensed pure and granular aluminum films. The changes of the superconducting transition temperature are characteristically dependent on the coating film thickness and are inversely dependent on the Al film thickness. The observed oscillations of the transition temperature are interpreted in the framework of the theory of quantum oscillations developed by Kagan and Dubovskii.

Effect of elastic strain and stress on the superconducting transition temperature of quench-condensed films

The volume dependence of the superconducting transition temperatureT c of amorphous and highly disordered films is investigated by bending the film substrate. The volume coefficientd InT c /d InV of annealed Sn and Pb films agrees with the volume coefficient known from hydrostatic pressure experiments. This bending technique is applied to superconducting modifications which can be produced only by quench condensation onto a substrate at liquid helium temperature. Amorphous films such as Bi, Ga, Sn90Cu10 and PbxBi1−x, as well as highly disordered Sn and Pb films, are investigated. The relations found between the volume coefficient, the electron-phonon coupling constant λ and the Gruneisen parameter γ G are in reasonable agreement with the volume dependence ofT c for non-transition metals, obtained on the basis of the McMillan equation. In particular, the results represent an expansion of this kind of investigation to very strong-coupling superconductors.

Apparatus for the measurement of the initial susceptibility of quench-condensed spin glass films

An apparatus for the measurement of the absolute value of the paramagnetic susceptibility is described. With this apparatus the susceptibility of quench-condensed films is measured in situ in the temperature range 2.5-300K. The applied AC magnetic field varies between 0.5 and 11.5 mT at frequencies of 11-5500 Hz. DC fields up to 11.5 mT can be applied parallel to the AC field. Magnetic moments of mu >or=1012 mu B are detected at 2.2 mT (AC) and 1100 Hz. This sensitivity allows the study of films about 1 mu m thick with various magnetic additions, e.g. Mn, Fe and Co in non-magnetic host metals. As an example the initial susceptibility chi of amorphous Au75Si19Fe6 and of polycrystalline Au92Fe8 is measured.

Specific heat of superconducting films of indium and of indium alloyed with magnetic impurities

Measurements were made of the dicontinuity ..delta..c in the specific heat at the superconducting transition for quench-condensed films of indium--manganese, and indium--chromium. Manganese and chromium are 3-d transition elements, and retain their magnetism when dissolved in indium. This is the first investigation of this sort where 3-d magnetic impurities have been studied in a simple metal superconductor. Magnetic impurities depress the superconducting transition temperature T/sub c/, and predictions based on the Shiba theory for classical spins in superconductors indicate that ..delta..c is also depressed by the presence of magnetic impurities. It is useful to express the results in terms of ..delta..c/sub 0/ and T/sub c0/, the values for the pure superconductor. The Shiba theory indicates that ..delta..c/..delta..c/sub 0/ is depressed more rapidly than T/sub c//T/sub c0/ as magnetic impurities are introduced into a superconductor. The results of the present work confirm this prediction, but show an even greater depression of ..delta..c/..delta..c/sub 0/ relative to T/sub c//T/sub c0/ than the Shiba theory predicts. Similar results have been obtained by previous workers who dissolved 3-d magnetic impurities in transition metal superconductors. This work also shows that ..delta..c for pure indium increases with increasing disorder, where the degree of disorder is indicated bymore » the residual resistivity of the sample. This result is to be expected qualitatively from Bergmann's studies of the transition temperature and energy gap width for disordered samples of indium. The present results, however, indicate that ..delta..c increases roughly ten times more rapidly with the degree of disorder than would be estimated from the results of Bergmann.« less

Magnetic and superconducting properties of dilute Cd alloys with Cr, Mn and Ni impurities

Measurements of the superconducting transition temperature T c , the electrical resistance and the temperature and field dependence of the magnetic susceptibility were performed on dilute alloys of CdCr, CdMn and CdNi. For CdMn, they were supplemented by tunneling studies on quenchcondensed films. The sample preparation was crucial. A small amount (about 9 at.ppm) of magnetic Cr or Mn content was lost even in our best preparation procedure. Well-developed magnetic moments exist in CdCr and CdMn with spin values of 1.5 and 2.0, respectively, while Ni is non-magnetic. The residual resistance ratio R(4 K) R(295 K) -R(4 K) varied as 0.78 (at.% Mn) −1, 1 (at.% Cr) −1 and 0.1 (at.% Ni) −1. Extremely strong depression rates of T c with concentration c were registered for CdCr and CdMn, 140 and 220 K (at.%) −1, but only about 0.1 K (at.%) −1 for CdNi. Comparisons with theoretical expressions for dT c dc gave an s-d exchange interaction J of about 0.6 eV for both Cr and Mn in Cd. T c versus c followed an Abrikosov-Gorkov relation for CdMn, but small deviations due to a temperature-dependent scattering could not be excluded. Traces of re-entrant transitions were detected in two CdMn samples, and signs of a low temperature tail were seen at high concentrations. These features might be connected with a spin-glass ordering. Quench-condensed films of CdMn displayed the same degree of pair breaking as bulk samples. A weak tunneling structure indicated the presence of an impurity band within the superconducting energy gap of CdMn films backed by Pb.

Characteristic temperatures in the spin-glassAuFe

The binary-alloy system $\mathrm{Au}\mathrm{Fe}$ with varying concentration, $c$, of Fe is shown to have three distinct regimes with quite different physical properties. These are the Kondo regime at low con\ifmmode \acute{c}\else \'{c}\fi{}entrations, the spin-glass regime at intermediate concentrations, and the dilute-ferromagnet regime at high concentrations. This paper contains a comprehensive compilation of experimental data expressed in terms of characteristic temperatures, like, for example, the spin-glass freezing temperature and the temperature of the resistance maximum. A coincidence of the characteristic temperatures is evident in the spin-glass regime $0.05%\ensuremath{\lesssim}c\ensuremath{\lesssim}15%$. Here a recent theoretical calculation based on the Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect-exchange interaction between the Fe spins accounts quantitatively for the observed dependence on concentration and electronic mean free path in a variety of different experiments. This includes, for example, recent experiments in quench-condensed films showing a large mean-free-path effect on the resistance maximum. The agreement extends to remarkably high concentrations and demonstrates substantial self-damping of the RKKY interaction. At the transition into the dilute ferromagnet regime $c\ensuremath{\gtrsim}15%$, near the threshold for nearest-neighbor percolation, there is a sharp departure of the observed freezing temperatures away from the predicted concentration dependence. This indicates a change-over between the spin-glass state dominated by the RKKY interactions and the dilute ferromagnet state dominated by nearest-neighbor direct-exchange interactions and is interpreted qualitatively in terms of recent percolation theories. Entering the Kondo regime $c\ensuremath{\lesssim}0.05%$ leads to a dispersion of the characteristic temperatures, whereby the complete agreement with the theoretical calculation is lost. Only one of the characteristic temperatures, the noise temperature derived from the resistance maximum by a method which incorporates the Kondo effect, remains in agreement with the RKKY calculation.

Al-type superconducting quench-condensed Cs, Ba, La and Ce films

Al-type superconducting quench-condensed Cs, Ba, La and Ce films

Electronic thermal conductivity of a superconducting indium-chromium alloy film

Measurements have been made of the electronic thermal conductivity of a quench-condensed film of an indium-chromium alloy. The results are compared with calculations based on Shiba's theory of superconductors containing magnetic impurities by using ${\ensuremath{\epsilon}}_{0}$ (the normalized position in the energy gap of the excited state associated with a single impurity atom) as an adjustable parameter. The data are in good agreement with the theory for a value ${\ensuremath{\epsilon}}_{0}=0.7$.

Anomalous Conductivity of Quench-Condensed Sodium-Ammonia Films

Films of sodium and ammonia have been formed on substrates held at 5 K. The electrical resistivity of the films has been measured and a metal-nonmetal transition has been observed, but the transition was found to be anomalous in that the resistivity was not a monotonic function of the sodium concentration. No evidence was found for the anomalously high electrical conductivity reported for bulk sodium-ammonia solids which were quick-frozen from liquid solutions.

A1-type superconducting quench-condensed Cs, Ba, La and Ce films

A1-type superconducting overlays of Cs, Ba, La and Ce have been obtained by quench-condensation of these elements on suitable fcc base films vapour-quenched onto glass substrates. A similar technique has already been used for producing A1-type overlays of Si, Ge, Sn, Se and Te. On annealing cyclically at increasing temperatures, a critical temperature is reached at which the overlay breaks because of the transformation of the A1-type modification to the usual crystal structure. Both the density and the superconducting transition temperature of the overlay depend on the base-film material and increase, for each element, with decreasing base-film lattice constant. The density is larger than the ordinary, but smaller than would be expected for a perfect pseudomorph or an ideal close-packed atom arrangement. The superconducting transition temperature is higher the thinner the overlay and the lower the annealing temperature. A comparison with previous results shows that the superconducting transition temperature of the A1-type overlays is higher than both that of films with normal crystal structure quench-condensed directly on glass substrates and that of samples subjected to high pressures.

Influence of reduced mean free path on the electrical resistivity of Fe spin glass alloys

Abstract The resistivity of quench-condensed films of the Au Fe spin glass system has been investigate with different concentrations pf structural defects above 0.3 K. The temperature of the resistance maximum is considerabley lowered by a reduced mean free path. The temperature dependence of the resistivity below Tmax is also discussed.

Electronic thermal conductivity of superconducting lead-manganese and indium-manganese alloy films

Measurements have been made of the electronic thermal conductivity of quench-condensed films of lead-manganese and indium-manganese alloys. The results are compared with calculations based on Shiba's theory of superconductors containing magnetic impurities. For Pb-Mn, the value of ${\ensuremath{\epsilon}}_{0}$ (the normalized position in the energy gap of the excited state associated with a single impurity) which was used earlier to explain tunneling measurements, is 0.55; this value is consistent with our data. For In-Mn, the value ${\ensuremath{\epsilon}}_{0}=0.85$ gives excellent agreement with our data.

A new metallic superconducting Sn modification observed in quench-condensed films

A fcc Sn phase was obtained by vapour-quenching fine-grained Sn overlays on microcrystalline Rh, Pd or Ag base films. This metastable modification is metallic and becomes superconducting at a temperature higher than that of white Sn.

Resistivity minima in quench-condensed films of palladium : S. Schmid-Marcic and J. A. Mydosh. Solid State Communications17, 795 (1975)

Resistivity minima in quench-condensed films of palladium : S. Schmid-Marcic and J. A. Mydosh. Solid State Communications17, 795 (1975)