Charge density wave transport in NbSe<Subscript>3</Subscript> at low temperatures under high magnetic field

Charge density wave (CDW) depinning and sliding regimes have been studied in NbSe₃ at low temperatures down to 1.5 K under magnetic field of 19 T oriented along the c-axis. We found that the threshold field for CDW depinning becomes temperature independent below T ₀ ≈ 15 K. Also CDW current to frequency ratio characterizing CDW sliding regime increases by factor 1.7 below this temperature. The results are discussed as a crossover from thermal fluctuation to tunneling CDW depinning at T < T ₀. Besides, we found that CDW sliding strongly suppresses the amplitude of Shubnikov-de Haas oscillations of magnetoresistance.     

Phosphoric acid and pH sensors based on polyaniline films

Polyaniline (PANI)-chemically coated the electrode of quartz-crystal microbalance (QCM) has been developed for the determination of phosphoric acid (H₃PO₄) in the liquid phase. The sensing mechanism is based on the fact that the QCM devices produce a change in the oscillating frequency when the PANI coating the electrode of QCM interacts with different concentration of the acid. This was made during the subsequent redoping–dedoping processes of the PANI film in acid and ammonia solutions, respectively. Also, this was made during the successive redoping in different acid solutions. The QCM sensor demonstrated a rapid response to the acid with an excellent reversibility. A linear response of the sensor for different concentrations of H₃PO₄ was found in region of 1 M. The conductivity at different concentration of the acid was also determined. Finally, pH dependence on the electronic absorption of PANI phosphate film was studied which indicates that the film can be used as a sensor over a wider pH range of 3–12. This is in comparison to a PANI-sulphate film which shows limited pH absorption dependence (5–8).     

Mesure des forces d'oscillateur par absorption dans un jet atomique—I. Methode de mesure

Absolute oscillator strengths of the ground state transitions of metallic elements (mercury, zinc) can be determined by measuring the optical absorption from resonance lines in an atomic beam. The apparatus makes use of a Fabry-Perot analyzer for the optical signal in the vacuum ultraviolet region (1800–2200 Å) and of a quartz-crystal microbalance for the determination of the beam density. A computer program is used for folding the apparatus functions with the source and absorption functions. This program compares experimental profiles with calculated values.     

Ultrathin films of lead oxide on gold: Dependence of stoichiometry, stability and thickness on O2 pressure and annealing temperature

Ultrathin oxide films grown in vacuum are important in many industrial areas, including microelectronics and heterogeneous catalysis. In this paper, the dependence of oxide stoichiometry, growth kinetics, thickness and stability on O₂ pressure and annealing temperature are explored using a high-stability quartz-crystal microbalance and Auger spectroscopy, for the oxidation of lead on gold as a model system. The oxide thickness increases abruptly at specific values of the O₂ pressure, as explained previously using Gibbs free energies. A qualitative difference is found between lead-oxide films which are 1 monolayer thick and those which are 2 or more monolayers thick; the former apparently involve exclusively chemisorbed oxygen and can be oxidized and reduced reversibly using thermal oxidation/annealing cycles, whereas the latter involve an extended lead oxide, are more thermally stable, and have a smaller electron inelastic mean free path. Accurate values of the O₂ sticking probability are obtained.     

Ultrasonic detection of the pinning of dislocations by point defects in lead-I determination of migration energies

The ultrasonic attenuation of a γ-irradiated, deformed, single-crystal of lead has been measured between 77 and 270 K. Constant heating rate and isothermal anneals after low temperature irradiation yielded two dislocation pinning stages centered at 140 and 200 K and two dislocation depinning stages centered about 170 and 250 K. For an unirradiated sample one pinning stage centered at 170 and one depinning stage centered at 250 K were observed. Activation energies were calculated from isothermal anneals using an eigenfunction expansion model to be 0·16±0·04 and 0·36±0·05 eV for the pinning stages in the irradiated crystal and 0·28±0·05 eV for the pinning stage in the unirradiated crystal. The activation energy calculated from the same isothermal anneals using a Cottrell-Bilby analysis was 0·23±0·03 eV for the low temperature pinning stage in the irradiated crystal. The other activation energies were not changed. The discrepancy is discussed in Part II.     

Wireless-electrodeless quartz-crystal-microbalance biosensors for studying interactions among biomolecules: A review

The mass sensitivity of quartz-crystal microbalance (QCM) was drastically improved by removing electrodes and wires attached on the quartz surfaces. Instead of wire connections, intended vibrations of quartz oscillators were excited and detected by antennas through electromagnetic waves. This noncontacting measurement is the key for ultrahigh-sensitive detection of proteins in liquids as well as quantitative measurements. This review shows the principle of wireless QCMs, their applications to studying interactions among biomolecules and aggregation reactions of amyloid β peptides, and the next-generation MEMS QCM, the resonance acoustic microbalance with naked embedded quartz (RAMNE-Q).     

Depinning line of granular YBa2Cu3O7?δ measured with vibrating-reed method

The depinning temperatures in YBa₂Cu₃O_7−δ ceramics and grains have been studied at magnetic field up to 4 T and various amplitudes of vibration. For ceramics sample, large amplitude dependence of depinning temperature T_D was observed, which indicate the decoupling of grains. In the case of a suspension sample (grain), T_D was much lower than that of ceramics.     

Transmission electron microscopy study of epitaxial La0.8MnO3 thin films on SrTiO3

The structure and microstructure of La_0.8MnO₃ thin films on SrTiO₃ substrates, fabricated by pulsed laser deposition at substrate temperatures of 873?K and 1073?K, have been studied by transmission electron microscopy. In both films, columnar growth morphology has been observed. The columnar grain size is found to increase with increasing substrate temperature. In the film deposited at a substrate temperature of 1073?K, there is only one rhombohedral phase. However, two phases, a rhombohedral one and an orthorhombic one, have been observed in the film deposited at 873?K.     

Nernst effect in Tl-Ba-Ca-Cu-O superconducting films due to infrared laser heating

A Nernst effect has been observed in a high temperature superconductor for the first time. Irradiating superconducting Tl–Ba–Ca–Cu–O thin films by short pulses of a TEA-CO₂ laser, a photovoltaic signal is detected perpendicular to a magnetic field applied parallel to the film surface. The signal is attributed to magnetic flux line depinning and flux line transport driven by the laser induced temperature gradient. The results are described by thermal flux line activation leading to a calculated distribution of pinning energies from 100 K to 4000 K.     

The behavior of the critical current density below and above the first matching field in superconductors with periodic square arrays of pinning sites

We have studied the effect of the applied magnetic field on critical depinning force at zero and finite temperatures and for several values of pinning strength. This was achieved by conducting extensive series of molecular dynamic simulations on driven vortex lattices interacting with periodic square arrays of pinning sites. We have found that the critical depinning force decreases as the applied magnetic field is increased. We have also observed two distinct behaviors of dependence of the critical depinning force on the applied magnetic field below and above the first matching filed.     

Thermal expansion of nanostructured PbS films and anharmonicity of atomic vibrations

The dependences of the coherent scattering region size and thermal expansion coefficient α of a PbS nanofilm on the annealing temperature in the range of 293–473 K and on the duration of annealing at a constant temperature of 423 K have been measured. It has been found that the thermal expansion coefficient α of the PbS nanofilm is almost twice as much as the coefficient α of coarse-grained lead sulfide. It has been shown that the large difference in the coefficients α is associated with the small size of particles in the film, which leads to an increase in the anharmonicity of atomic vibrations. The contribution from the small size of particles to the thermal expansion coefficient of the PbS nanofilm has been evaluated theoretically.     

Localized superconductivity in the quantum-critical region of the disorder-driven superconductor-insulator transition in TiN thin films

We investigate low-temperature transport properties of thin TiN superconducting films in the vicinity of the disorder-driven superconductor-insulator transition. In a zero magnetic field, we find an extremely sharp separation between superconducting and insulating phases, evidencing a direct superconductor-insulator transition without an intermediate metallic phase. At moderate temperatures, in the insulating films we reveal thermally activated conductivity with the magnetic field-dependent activation energy. At very low temperatures, we observe a zero-conductivity state, which is destroyed at some depinning threshold voltage V{T}. These findings indicate the formation of a distinct collective state of the localized Cooper pairs in the critical region at both sides of the transition.     

Driven depinning of strongly disordered media and anisotropic mean-field limits

Extended systems driven through strong disorder are modeled generically using coarse-grained degrees of freedom that interact elastically in the directions parallel to the drive and slip along at least one of the directions transverse to the motion. In the limit of infinite-range elastic and viscous coupling this model has a tricritical point separating a region where the depinning is continuous, in the universality class of elastic depinning, from a region where depinning is hysteretic. Many of the collective transport models discussed in the literature are special cases of the generic model.     

The influence of substrate temperature on the structure and morphology of sexiphenyl thin films on Au(111)

The layer growth of ultra-thin films of sexiphenyl on Au(111) has been studied as a function of substrate temperature under well-defined ultra-high-vacuum conditions. In situ X-ray photoelectron spectroscopy in combination with thermal desorption spectroscopy was applied to reveal the kinetics of layer growth and recrystallisation. Ex situ atomic force microscopy and X-ray diffraction were used to determine the film morphology and film structure. A continuous small-grain film is obtained at 93 K. The crystalline structure of the grains has been confirmed by X-ray diffraction. However, the grains are not preferentially oriented with respect to the substrate. Around room temperature, recrystallisation takes place and the molecules within the crystallites orient parallel to the substrate surface. Further small substrate temperature changes during growth lead to further changes of the film morphology and to reorientation of the crystallites. However, the molecules in the crystallites stay aligned parallel to the surface. At temperatures greater than 350 K, terraced islands of standing molecules are observed, in addition to needle-like islands with flat-lying molecules. PACS 68.55.-a; 61.66.Hq; 81.10.Bk     

Nanotribology: Microscopic mechanisms of friction

Friction is one of the oldest problems in physics with a huge practical significance. However, during the last decade this problem gets strong acceleration due to the development of new experimental techniques (surface-force apparatus, quartz-crystal microbalance technique, friction-force microscopy) and essentially due to the great progress in molecular dynamics (MD) simulation of tribological systems. In the present review we describe the modern state of the problem from the point of view of surface science physicists. The main accent is devoted to recent MD results in their connection with experiments.     

Stark Broadening of Ne II and Ne III Spectral Lines

Using a semiclassical approach, the widths and shifts of the spectral lines of three Ne II and six Ne III multiplets caused by collisions with electrons, protons, and helium ions at a density of perturbing particles of 10¹⁷ cm^–3 and different temperatures have been calculated. The results obtained have been compared with the known experimental and theoretical data.     

Dynamic critical phenomena in two-dimensional fully frustrated Coulomb gas model with disorder

The dynamic critical phenomena near depinning transition in two-dimensional fully frustrated square lattice Coulomb gas model with disorders was studied using Monte Carlo technique. The ground state of the model system with disorder σ=0.3 is a disordered state. The dependence of charge current density J on electric field E was investigated at low temperatures. The nonlinear J-E behavior near critical depinning field can be described by a scaling function proposed for three-dimensional flux line system [M.B. Luo, X. Hu, Phys. Rev. Lett. 98 (2007) 267002]. We evaluated critical exponents and found an Arrhenius creep motion for field region Ec/2<E<Ec. The scaling law of the depinning transition is also obtained from the scaling function.     

Molecular dynamics simulation about porous thin-film growth in secondary deposition

The thin film growth has been confirmed to be assembled by an enormous number of clusters in experiments of CVD. Sequence of clusters’ depositions proceeds to form the thin film at short time as gas fluids through surface of substrate. In order to grow condensed thin film using series of cluster deposition, the effect of initial velocity, substrate temperature and density of clusters on property of deposited thin film, especially appearance of nanoscale pores inside thin film must be investigated. In this simulation, three different cluster sizes of 203, 653, 1563 atoms with different velocities (0, 10, 100, 1000 and 3000 m/s) were deposited on a Cu(0 0 1) substrate whose temperatures were set between 300 and 1000 K. Four clusters and one cluster were used in primary deposition and secondary deposition, respectively. We have clarified that adhesion between clusters and substrate is greatly influenced by initial velocity. As a result, the exfoliation pattern of deposited thin film is dependent on initial velocity and different between them. One borderline dividing whole region into porous region and nonporous region are obtained to show the effect of growth conditions on appearance of nanoscale pores inside thin film. Moreover, we have also shown that the likelihood of porous thin film is dependent on the point of impact of a cluster relative to previously deposited clusters.     

Vortices trapped in the damaged surroundings of antidots in Nb films – Depinning transition

The depinning transition of Vortex Matter in the presence of antidots in superconducting Nb films has been investigated. The antidots were fabricated using two different techniques, resulting in samples with arrays of diverse pinning efficiency. At low temperatures and fields, the spatial arrangement of Vortex Matter is governed by the presence of the antidots. Keeping the temperature fixed, an increase of the field induces a depinning transition. As the temperature approaches T_c, the depinning frontier exhibits a characteristic kink at the temperature T_k, above which the phase boundary exhibits a different regime. The lower-temperature regime is adequately described by a power-law expression, whose exponent n was observed to be inversely proportional to the pinning capability of the antidot, a feature that qualifies this parameter as a figure of merit to quantify the pinning strength of the defect.