Quench-condensed Films Research Articles

Heat Release of Quench-Condensed Hydrogen Films

Heat release experiments are reported on quench-condensed thin films of solid H 2 and HD, measured during the initial annealing. The measurements shed some light on the structural properties of quench-condensed films and particularly allow to estimate the initial grain size. For H 2 additional information about the ordering transition of ortho-molecules could be extracted from the data.

Ultrasonic Study of the Ordering of ortho–Hydrogen in Quench–Condensed Films

The ultrasonic attenuation and dispersion of quench–condensed H 2 films was measured for ortho–concentrations ranging from x = 75% to x = 32%. Orientational ordering was observed for all ortho–H 2 concentrations. The obtained results are compared to recent specific heat experiments by Birmingham et al.

Mass Transport of Molecular Hydrogen in Porous Vycor Glass

The dynamics of adsorbed hydrogen molecules on the pore surface of Vycor glass has been investigated with a torsional oscillator technique at temperatures between 7.5 K and 22 K. Whereas most experiments carried out previously on this system were performed under thermodynamic equilibrium, we concentrated on the dynamics of the mass transport. Since the free energy of hydrogen inside the Vycor is a function of both temperature and film thickness, a temperature change causes a gradient in the chemical potential between inside and outside of the Vycor which leads to mass transport of the hydrogen molecules as consequence of a wetting or a solidification transition. We find a thermally activated motion with an activation energy of 25 K corresponding to the value for surface self-diffusion of quench-condensed hydrogen films.

Specific heat of quench-condensed hydrogen films

The specific heatC(X, T) of quench-condensed films of H2 has been measured as a function of ortho concentration X with 0.28<X<0.75 for coverages between 24.3 and 92.3 A−2 at temperatures between 0.4 and 3.0 K. The films were condensed on evaporated gold substrates held at several temperaturesT. cond between 1.0 and 3.5 K. The observed specific heat is attributed to orientational ordering of the ortho-H2 molecules. For the films withX = 0.74 condensed atT cond>2.5 K, there is a peak which indicates a bulk-like ordering transition. At temperatures below the peak, there is a large contribution toC, which is not present in bulk H2, and which we attribute to short-range ordering size effects. AsT cond is decreased below 2.5 K, the shape of the specific heat curve changes, and the peak at 1.5 K is replaced by a gradual rise with a sharp drop above 2.6 K. Despite this strong dependence ofC onT cond, the entropy per molecule at 3 K is only weakly dependent onT cond and comparable to that for bulk H2. Film annealing at 3.4 K produces a change in the specific heat curve, and a study of this effect is presented. The ortho-para conversion rate of the films condensed at the various temperatures is found to be same as in bulk, well-annealed H2. As in bulk H2, the transition temperature inferred from the location of the specific heat peak or anomaly decreases withX. Unlike in bulk H2, there is no temperature hysteresis inC for any of the quench-condensed films. This implies that the ordering transitions are not accompanied by a martensitic transformation.

Structural relaxation and surface diffusion of quench-condensed hydrogen films

We have studied the structural relaxation of quench-condensed films of molecular hydrogen with ultrasonic surface acoustic waves (SAW). Upon annealing, frequency-dependent scattering effects occur indicating a growth of hydrogen crystallites of sizes around 1 μm. The dominant scattering mechanism appears to be a resonant coupling of the SAW to some elastic eigenmodes of the crystallites. During deposition at 1.4 K and subsequent annealing we measured the change of attenuation and sound velocity of the SAW and are able to extract activation energies for the underlying surface diffusion. A strong temperature dependence below 2.5 K of the resonance frequency is reported in the case of H 2- and D 2-crystallites.

Anomalous elastic properties of quench-condensed hydrogen films

Quench-condensed films of molecular hydrogen (H2, HD, and D2), prepared around 1 K, undergo a strong structural rearrangement upon annealing at 3–4 K. Caused by dewetting the film transforms into crystallites with sizes of the order of μm. Surface acoustic waves couple resonantly to the eigenmodes of these crystallites and are strongly scattered. The systematic variation with time of sound velocity and attenuation allows to trace the evolution of the crystallites. Moreover, at any stage of evolution a drastic temperature dependence of sound velocity and attenuation is observed between 0.1 K and 3 K. This has to be attributed to a strong change of the effective eigenfrequencies of the crystallites which cannot be readily explained by the bulk properties of hydrogen.

Wetting behavior of solid hydrogen films

We have studied the dewetting of quench-condensed solid hydrogen films on bare and Ne plated Ag substrates. Information about the morphology of the films during the dewetting process is obtained from measurements of photoelectron tunneling through the films and complementary surface plasmon resonance data. Plating the Ag substrate with Ne films of various thickness allows to tune the strength of the van der Waals interaction with the hydrogen and thus to change the H2 films from a state of incomplete to complete wetting.

Dimensionality Crossover of the Nonlinear Resistance in Quench-Condensed Indium Films

Thickness dependence of the nonlinear resistance ( I - V characteristics) has been measured in quench-condensed films of indium with thicknesses of 14 nm, 26 nm and 65 nm. The I - V curves for the 14 nm-thick film are qualitatively similar to the previous result for the much thinner film (5 nm) whose superconducting properties were well described by the two-dimensional (2D) Kosterlitz-Thouless (KT) theory. In contrast, those for the thicker films are markedly different. As I is raised from zero at low temperatures, there appears the nonlinear I - V curve (resistance) which exhibits negative curvature.

Superconductivity in ultrathin quench-condensed Pb/Sb and Pb/Ge multilayers.

We present the results of in situ transport and tunneling measurements on quench-condensed Pb/Sb and Pb/Ge multilayers. The superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) and the density of states increase concurrently with the number of Pb layers (n) and eventually saturate. Both the variation of ${\mathit{T}}_{\mathit{c}}$ with n and the saturated value of ${\mathit{T}}_{\mathit{c}}$ can be accounted for by electron transfer between Pb layers through localized states in the Sb(Ge). We find that the basic mechanism for superconductivity in these multilayers is not fundamentally altered from that in homogeneous amorphous superconducting films. Our results clarify the role the Ge or Sb underlayer and overlayer plays in the transport and superconductivity of uniform quench-condensed films.

Insulating phase of mercury in thin quench-condensed films.

We present experimental evidence that mercury forms an insulating phase in films condensed on glass substrates at liquid-helium temperatures. This insulating phase is metastable and it exists in films only up to some critical thickness ${\mathit{d}}_{\mathit{c}}$\ensuremath{\approxeq}60 \AA{}. Our results cannot be explained by the assumption that the films consist of isolated islands. We believe that similar insulating phases exist also in quench-condensed lead films and maybe in some other metals. As far as we know the possibility that a disordered system of metallic atoms may form an insulator has never been considered.

Electron tunneling studies of ultrathin films near the superconductor-to-insulator transition

Electron tunneling measurements on ultrathin quench-condensed films near the superconductor-to-insulator (SI) transition reveal that the superconducting state degrades with increasing normal state sheet resistance, R □, in a manner that depends strongly on film morphology. In homogeneously disordered films, the superconducting energy gap Δ o decreases continuously and appears to go to zero at the SI transition. In granular films the transport properties degrade while Δ o remains constant. Measurements in the normal state reveal disorder enhanced e - - e - interaction corrections to the density of states. These effects are strong and depend on morphology in a manner that is consistent with their playing an important role in driving the SI transition.

Atomic dynamics in quench-condensed films measured by universal conductance fluctuations.

Atomic dynamics in quench-condensed films measured by universal conductance fluctuations.

Mass density and refractive index of quench-condensed argon-krypton films

The refractive index and the mass density of ArKr films condensed on a silicon substrate at liquid helium temperature are determined by ellipsometric measurements at a wavelength Λ= 632.8 nm. The densities of the crystalline ArKr films are typically 3.5% less than the corresponding bulk values. This deviation is much less than in the case of recently investigated ArXe mixtures. No annealing effect has been observed except when the Ar atoms sublime from the layer. Like in the case of ArXe mixtures, ArKr films with a high initial Ar concentration remain in a highly porous sponge structure after the Ar atoms have left the layer.

Non-Ohmic Normal State Behavior and the Onset of Global Superconductivity in Partially Annealed Quench–Condensed Films of Tin

ABSTRACTFilms of tin condensed onto 4K pyrex substrates can be driven from “insulating” (i.e. dR/dT &lt; 0) to superconducting behavior by low temperature annealing. Global superconductivity occurs when the normul state sheet resistance, R□, shrinks to a value very near R□≡ h/4ea, as in experiments where R□is changed by increasing the film thickness. When R□&gt; R□, conduction is decidedly non-ohmic, with R□(T,V) = R□(T) + R1(T)exp(−m(T)|V|).

Superconductivity and disorder-driven metal-insulator transition in quench-condensed tin films

The authors have studied the effects of disorder in a series of Sn films quench-condensed onto cryogenically cooled substrates. They find a simple scaling of the conductivity with film thickness sigma approx. (d-d/sub c/)/sup ..nu../, with ..nu.. /approx equal/ 0.85, and a metal-insulator transition near d/sub c/ /approx equal/ 40-50 /angstrom/. The superconducting transition temperatures depend linearly on 1/d, provided d is not too small, and the films exhibit strong annealing effects. These features are explained using a model in which grain boundary scattering dominates intragrain scattering.

Absence of superconductivity in metallic granular aluminum.

Granular aluminum near the metal-insulator transition has been found to have a range of metal concentrations over which it is metallic (M) but not superconducting (S). For higher concentrations it is superconducting and at lower concentrations insulating (I). There is a striking resemblance of the temperature dependence of resistivity in the S,M, and I ranges of granular aluminum to the temperature dependence of sheet resistance in very thin quench-condensed films. However there is as yet no theory that gives a satisfactory explanation of this behavior in both two- and three-dimensional systems.

Destruction of superconductivity in quench-condensed two-dimensional films.

We have performed systematic tunneling measurements on two-dimensional quench-condensed films of Sn and Pb to investigate the destruction of superconductivity by localization effects. With increasing sheet resistance of the films, the energy gaps and ${T}_{c}$'s decrease only slightly, but the gap edges broaden until the width becomes comparable to the gap. Associated with this broadening are low-temperature finite-resistance tails in the resistive transitions.

Magnetic properties of quench-condensed CuFe films

Random substitutional alloys of CuFe with Fe concentrations 0.8-8 at.% Fe are produced by quenching from the vapour phase onto a cold substrate. The paramagnetic susceptibility chi is measured in situ using AC magnetic fields of 2.2-11.5 mT at frequencies between 33 and 5500 Hz and temperatures from 2.5 to 80K. The paramagnetic Curie temperature theta , the effective magnetic moment peff and the temperature Tf of the susceptibility maximum depend on Fe concentration and annealing. Alloys annealed at 80K do not show a frequency dependence of Tf. A reduction of chi near Tf in small DC fields is found only after annealing at 300K. A comparison with the properties of bulk CuFe alloys shows that the Tf values are nearly the same whereas the theta and peff values of the alloy films are much smaller than for the corresponding bulk alloys.

Insulator-conductor transition in very thin metal films

The insulator-conductor transition in very thin metal films is investigated by acoustoelectric measurements of the relation between the charge carrier density and the sheet resistance at various substrate temperatures from room temperature down to about 90 K. Near room temperature a rather sharp mobility edge is observed but at lower temperatures, in quench-condensed films, percolation effects mask the manifestation of the Anderson transition. This difference in behaviour is attributed to the change in surface mobility of the metal islands and its effect on the microscopic structure of the film.

Correction to the two-dimensional density of states.

We report tunneling measurements on quench-condensed films of tin in order to determine the correction to the two-dimensional electronic density of states due to the effects of electron-electron interactions. Comparison with recent theory yields good agreement in the regime that the theories are valid. These measurements illustrate the nature of the corrections over a wider range of parameters.