Quench-condensed Films Research Articles

1D AND 2D QUANTUM INTERFERENCE EFFECTS IN ELECTRON TRANSPORT IN Au FILM

We present transport properties of quench-condensed Au film with nominal thickness ≈3.56 nm and R□ ≈ 5 k Ω for T > 10 K. This film has weak nonmetallic temperature dependence of resistance with logarithmic behavior above 10 K and somewhat stronger dependence at low temperatures. Above 3 K only two-dimensional (2D) quantum interference effects in electron transport have been found; whereas, below 3 K both one-dimensional (1D) and 2D effects of weak localization (WL) and electron–electron interaction (EEI) can be distinguished. This reflects inhomogeneous structure of the film near the thickness-controlled metal-insulator transition (MIT).

Magnetic-Field-Tuned Quantum Phase Transition in the Insulating Regime of Ultrathin Amorphous Bi Films

A surprisingly strong variation of resistance with a perpendicular magnetic field, and a peak in the resistance versus field, R(B) has been found in insulating films of a sequence of homogeneous, quench-condensed films of amorphous Bi undergoing a thickness-tuned superconductor-insulator transition. Isotherms of magnetoresistance, rather than resistance, versus field were found to cross at a well-defined magnetic field higher than the field corresponding to the peak in R(B). For all values of B, R(T) was found to obey an Arrhenius form. At the crossover magnetic field the prefactor became equal to the quantum resistance of electron pairs h/4e², and the activation energy returned to its zero-field value. These observations suggest that the crossover is the signature of a quantum phase transition between two distinct insulating ground states, tuned by magnetic field.

Superconductor-insulator transitions of quench-condensed films

The superconductor-insulator transitions of quench-condensed ultrathin films of metals are simple examples of continuous quantum phase transitions. Quantum phase transitions differ from thermal phase transitions in that they occur at zero temperature when the ground state of a system is changed in response to a variation of an external parameter of the Hamiltonian. In superconductor-insulator transitions, this control parameter is usually a parallel or perpendicular magnetic field, disorder, or charge density. Quantum phase transitions are studied through measurements, at nonzero temperature, of physical behavior influenced by the quantum fluctuations associated with the transition. Here we focus on the results of transport and magnetotransport measurements of disordered quench-condensed films of metals that are effectively two-dimensional. Open questions relating to the nature of the very puzzling insulating regime and whether there are several different types of superconductor-insulator transitions determined by material properties will be discussed.

Sputtering of cryogenic films of hydrogen by keV ions: Thickness dependence and surface morphology

The sputtering yield induced by keV hydrogen ions measured at CERN and at Risø National Laboratory for solid H 2 and D 2 at temperatures below 4.2 K decreases with increasing film thickness from about 100 × 10 15 molecules/cm 2. For a film thickness comparable to or larger than the ion range the data from Risø show a slight increase, whereas the yield from CERN continues to decrease up to very large film thicknesses, i.e. one order of magnitude larger than the ion range. The different behavior of the yield is discussed in terms of the probable growth modes of the films. The films produced at the Risø setup are quench-condensed films, while those produced at CERN are supposed to grow with large hydrogen aggregates on top of a thin bottom layer.

Electrostatic tuning of a quantum phase transition

Quantum phase transitions are transitions between the ground states of physical systems traversed by varying a parameter of the Hamiltonian. We have been examining the properties of ultrathin quench-condensed films of amorphous Bi, which exhibit a number of quantum phase transitions tuned by disorder, perpendicular and parallel magnetic fields and charge density. We have developed a procedure in which the substrate for film growth serves as the gate insulator in a field effect transistor configuration. This permits electrostatic tuning of a transition between insulating and superconducting behaviour. Finite size scaling of the conductivity has been carried out yielding critical exponents that suggest that the insulator-superconductor transition belongs to the universality class of the 3D XY model. This electric field-induced superconductivity can in turn be destroyed by magnetic fields and the data can also be scaled in with essentially the same results. These findings are quite different from those obtained in the study of substantially thicker films of indium oxide and titanium nitride.

Early Stage Morphology of Quench Condensed Ag, Pb and Pb/Ag Hybrid Films

In situ Scanning Tunneling Microscopy (STM) has been used to study the morphology of Ag, Pb and Pb/Ag bilayer films fabricated by quench condensation of the elements onto cold (T = 77 K) inert and atomically flat Highly Oriented Pyrolytic Graphite (HOPG) substrates. All films are thinner than 10 nm and show a granular structure that is consistent with earlier studies of Quench condensed (QC) films. The average lateral diameter, & of the Ag grains, however, depends on whether the Ag is deposited directly on HOPG (&=13 nm) or on a Pb film consisting of a single layer of Pb grains (&=26.8 nm). In addition, the critical thickness for electrical conduction (d G ) of Pb/Ag films on inert glass substrates is substantially larger than for pure Ag films. These results are evidence that the structure of the underlying substrate exerts an influence on the size of the grains in QC films. We propose a qualitative explanation for this phenomenon.

Electronic transition of vanadium impurities in different alkali hosts

Abstract Thin quench-condensed films of Na, K, Rb and Cs are covered with 1 100 of a monolayer of V. Then the impurities are covered with several atomic layers of the host. Measured by means of the anomalous Hall effect, V impurities show a magnetic moment of 6– 7 μ B in Na and 4 μ B in Cs. This contradicts the favored atomic model (predicting 3 5 μ B ). Furthermore, the sign of the anomalous Hall resistance changes from negative in Na to positive in Cs. This indicates a transition of the electronic structure of the impurity and the host environment.

Field‐induced space charge limited current flow in disordered ultrathin films

Slow, nonexponential relaxation of electrical transport accompanied by memory effects can be induced in quench-condensed ultrathin amorphous Bi films by the application of a parallel magnetic field. This behavior, which appears to be a form of space charge limited current flow, is found in extremely thin films on the insulating side of the superconductor–insulator transition. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Critical currents of strongly disordered ultra-thin superconducting films

Quench-condensed films of various metals are well known systems for studying the superconducting state in presence of strong disorder. Critical currents of superconducting films of Bi and Sn with sheet resistance <500 Ω were found to vary with temperature according to the relation I c( T)= I c(0)(1−( T/ T c) 4). The result was found to be independent of the substrate used. The observed dependence implies a variation of the superconducting gap Δ( T)∼(1−( T/ T c) 4) 1/2, near T c. This is different from the behaviour of a BCS type gap near T c but consistent with some existing measurements on fabricated 2-D Josephson junction arrays. The I– V characteristics of these films also show hysteresis, indicating the presence of underdamped intergrain Josephson coupling. The values of the intergrain resistances and capacitances are estimated from the ratios of the observed retrapping and critical currents.

Strongly enhanced magnetic moments of vanadium impurities in thin films of sodium and potassium.

Thin quench-condensed films of Na and K are covered with 1/100 of a monolayer of V. Then the impurities are covered with several atomic layers of the host. The magnetization of the films is measured by means of the anomalous Hall effect. For V impurities on the surface of Na and K, a magnetic moment of 7 Bohr magnetons is observed. After coverage with the host, the V moment became 6.5 mu(B) for the Na host. These results contradict the favored atomic model (predicting 3/5 mu(B)) and the resonance model. A polarization of the alkali host appears to be the only resolution.

Strong coupled superconductor in proximity with a quench-condensed ferromagnetic Ni film: A search for oscillatingTc

We present results of in situ transport and tunneling measurements on a quench-condensed (QC) Pb film covered by a QC ferromagnetic Ni layer. This experiment provides new evidence in the controversy over the existence of inhomogeneous superconductivity in a superconducting/ferromagnet $(S/F)$ system. The critical temperature of the superconducting Pb/Ni bilayer has been measured for numerous thicknesses of Pb ranging from 5 to 80 nm. No oscillatory behavior as a function of Ni thickness has been observed in the variation of ${T}_{c},$ implying no $\ensuremath{\pi}$ state in those bilayers. These results are interpreted in light of recent theoretical developments. Tunneling measurements of $(dI/dV)$ on Pb/Ni also show a monotonic depairing effect in the density of states of Pb versus the Ni thickness, which further confirms the absence of nonconventional superconductivity in this Pb/Ni system.

Nonlinear Current-Voltage Characteristics of Locally Superconducting Elemental Films

Nonlinear I-V characteristics have been observed in quench-condensed films which are locally superconducting. We suggest an interpretation of an apparent threshold phenomenon in terms of the enhancement of conduction by the depinning of a charge structure that may characterize this insulating regime. We propose that this is a more likely description than Coulomb blockade or charge-anticharge unbinding phenomena.

Giant moments of Fe and Co on and in rubidium aaand potassium films

Thin quench-condensed films of Rb and K are covered with 1/100 of a mono-layer of Fe or Co. Then the impurities are covered with several atomic layers of the host. The magnetization of the films is measured by means of the anomalous Hall effect (AHE). The magnetization follows a Brillouin function with a magnetic moment of more than 10 Bohr magnetons for bulk Co and Fe impurities. These moments are much larger than the moments of the atomic configurations of Fe and Co and suggest enhanced magnetic moments of the impurities.

Evidence of collective charge behavior in the insulating state of ultrathin films of superconducting metals.

Nonlinear I-V characteristics have been observed in insulating quench-condensed films which are locally superconducting. We suggest an interpretation in terms of the enhancement of conduction by the depinning of a Cooper pair charge density wave, Cooper pair crystal, or Cooper pair glass that may characterize this insulating regime. We propose that this is a more likely description than the Coulomb blockade or charge-anticharge unbinding phenomena.

Electron glass in ultrathin granular Al films at low temperatures.

Quench-condensed granular Al films of sheet resistance approximately 10 k Omega/square box display hysteresis and ultraslow, nonexponential relaxation in the resistance when the temperature is varied below 300 mK. The hysteresis is nonlinear and the relaxation time does not obey the Arrhenius form. Furthermore, large resistance fluctuations having a 1/f-type power spectrum are observed at low temperatures, with a low-frequency cutoff which shifts to lower frequencies with decreasing temperature. These observations combine to provide a coherent picture that there exists a new glassy electron state in ultrathin granular Al films, with a growing correlation length at low temperatures.

Annealing of quench-condensed Argon films

When prepared by quench-condensation below 3K, Argon films of several hundred nanometers in thickness are found to be considerably softer than bulk Argon. The shear stiffness increases when the films are subsequently annealed at higher temperatures, with a time dependence described by stretched exponential functions. The magnitude of the cumulative annealing has an exponential, non-Arrhenius temperature dependence, and thinner films exhibit more stiffening than thicker films.

Anomalous hopping exponents of ultrathin metal films

The temperature dependence of the resistance R(T) of ultrathin quench-condensed films of Ag, Bi, Pb and Pd has been investigated. In the most resistive films, R(T)=Roexp(To/T)^x, where x=0.75. Surprisingly, the exponent x was found to be constant for a wide range of Ro and To in all four materials, possibly implying a consistent underlying conduction mechanism. The results are discussed in terms of several different models of hopping conduction.

Sound velocity measurements of quench-condensed noble-gas films

Abstract We have developed a new technique to study the sound velocity in quench-condensed noble-gas films. In this method, we excite the phonons with a laser pulse and derive the speed of sound by tracing the phonon thickness resonance emission out of the film with a super-conducting tunneling junction as phonon detector. Measurements were performed on Ne, Ar, Kr, and Xe, condensed onto a 1.3 K Si-crystal under UHV-conditions. We found that all quench-condensed films have a longitudinal speed of sound comparable to that of the corresponding noble-gas crystal. However, the values of the transverse speed of sound are reduced by a factor of two in comparison with the bulk. The implications of these results on the structure of the condensates will be discussed.

Anomalous Coulomb blockade in nanoconstricted quench-condensed Bi films

Nanobridges in thin 1–3 nm granular bismuth quench-condensed films with lengths and widths of 10 nm have been formed on top of an electrostatic gate made of oxidized aluminum. Periodic gate modulation of the Coulomb blockade-type I( V) curves of these nanoconstrictions has been observed in the temperature range 4.2–11 K, indicating that the charge of a single grain controlled the transport in the constriction. A reversible transition between two states with the same charging energy of 35 meV occurred at a temperature around 8 K, with up to 5 times enhanced conductivity at low temperatures.

Annealing effects in quench-condensed neon and argon films

Films of argon and neon were prepared on a cold substrate by quench-condensation from the gas phase onto a surface acoustic wave device operated at 630 MHz. On annealing above the condensation temperature strong changes of the sound velocity occur indicating structural rearrangement of the film. The annealing procedure has also significant influence on the temperature dependence of the sound velocity and attenuation which is governed by the presence of two-level tunneling states.