QCM Measurements of Superfluid Response in 4He Films up to 180 MHz

At finite frequencies, a dynamic Kosterlitz–Thouless (KT) theory predicts a frequency dependence of the superfluid transition in ⁴He films on planar surfaces. We report results of QCM measurements to study the superfluid response on planar gold surfaces for very high frequencies up to 180 MHz in the temperature range of 0.6–1.0 K. As the frequency is increased, we observed the expected KT behavior that the superfluid transition shifts to a higher temperature from the static transition temperature T _KT and the transition temperature region broadens. The frequency dependence of the dissipation peak temperature at the transition agrees with a simple equation of the frequency dependence based on the dynamic KT theory. The microscopic parameter for the dynamic transition, the ratio of the diffusion constant to the square of the vortex core radius D/r ₀ ², is estimated to be on the order of 10¹⁰ s⁻¹.     

The solubility of spin-polarized3He in4He and the superfluidity of3He in3He-4He mixtures

We investigate the possibility of a large enhancement of the T = 0 finite solubility of³He in⁴He due to spin-polarization. The size of the effect depends on the fraction of³He atoms in the system. We present two different approaches for the limits of a small and a large number of³He atoms compared to the number of⁴He atoms. Since the possible³He superfluid phase transition depends on³He density, we calculate the consequences of this change in the solubility for its superfluid transition temperature. It is shown that for small fractions of³He, the transition temperature is enhanced mostly due to the enlargement of the up-spin Fermi sphere. In the opposite limit the transition temperature is enhanced as a result of the increased³He solubility.     

Wigner Solid Transition of Electrons Confined in Microchannel

We have investigated the finite size effect on the Wigner solid transition of the electrons trapped on the liquid ⁴He surface by the mobility measurement of the electrons confined in a 15 μm-wide channel. The resistance above the Wigner solid transition temperature is well reproduced by the theory including the scattering from ⁴He vapor atoms and ripplons. At the transition, the resistance shows the increase as similar to that on the bulk liquid, but the transition is broader. The transition temperature shows the similar density dependence to the two-dimensional system.      

EPR in a barium borosilicate glass containing titanium ions

EPR spectra were studied in an equimolar barium borosilicate glass containing 0 to 12 mol% titanium ions. The valence concentration ratio, defined as the ratio of the concentration of the lower valence state transition metal ion Ti³⁺ to the total concentration of the transition metal ion Ti³⁺ + Ti⁴⁺, was varied between 0.15 and 0.85 at each concentration. The spectra were studied as a function of temperature down to 15 K. A broad asymmetric absorption peak is observed with effectiveg ~ 1.938 at room temperature. This absorption is attributed to a tetragonally distorted octahedral ligand field around Ti³⁺ ion. Theg value decreases as the temperature decreases, with a distinct transition between 60 and 100 K. Accompanied by this decrease ing is a distinct broadening of the curves with a transition in the same transition range. These and other observations are explained in terms of a lengthening of the spin-lattice relaxation time in this transition range and this transition is analogous to the NMR motional narrowing phenomena in solids. Many observations at low temperatures suggest a non-random distribution of titanium ions in these glasses.     

Extraordinary behavior of4He on hydrogen and deuterium surfaces

Using third sound resonance below 1 K, for⁴He on solid H₂ and D₂ surfaces, we have found new phenomena. On H₂ we find surface superfluidity for⁴He coverages less than one layer, with the zero temperature Kosterlitz-Thouless (KT) transition extrapolated to a coverage of 0.35±0.05 layers. On both H₂ and D₂ we find a new line of transitions below the KT transitions. On H₂, at zero temperature, the new line of transitions starts at zero temperature at a coverage of 0.6±0.1 layers of⁴He and has a slope of 0.58 (K/layer) of the universal KT slope. We also find, on H₂, two modes of surface wave propagation between the new transition and the KT transition. Below the new transition we also find anomalous third sound absorption which increases exponentially with temperature by five orders of magnitude.     

The large modification of phase transition characteristics of VO2 films on SiO2/Si substrates

The vanadium oxide (VO₂) films have been prepared on SiO₂/Si substrates by using a modified Ion Beam Enhanced Deposition (IBED) method. During the film deposition, high doses of Ar⁺ and H⁺ ions have been implanted into the deposited films from the implanted beam. The resistance change of the VO₂ films with temperature has been measured and the phase transition process has been observed by using the X-ray Diffraction technique. The phase transition of the IBED VO₂ films starts at a low temperature of 48 °C and ends at a high temperature of 78 °C. It is found that the phase transition characteristics can be adjusted by changing the annealing temperature or the time and the phase transition characteristics of the IBED VO₂ films depend on the quantity and location of argon atoms in the film matrix.     

The compressive property and brittle-to-ductile transition of Ti3SiC2 ceramics

Compressive tests of polycrystalline Ti₃SiC₂ were performed from room temperature to 1423 K at strain rates of 1×10^–4 s^–1 and 2.5×10^–5 s^–1, respectively. The effect of strain rates on high-temperature compressive property was also investigated. Polycrystalline Ti₃SiC₂ exhibited positive temperature dependence of flow stress (flow stress anomaly) and showed a temperature peak at 1173 K. The brittle-to-ductile transition temperature (BDTT) for polycrystalline Ti₃SiC₂ was strain-rate sensitive, an approximately 100 K decrease in transition temperature was associated with four times of magnitude decrease in strain rate. In addition, the fracture morphology changed from predominately intergranular to mostly transgranular. The mechanism responsible for the brittle-to-ductile transition in Ti₃SiC₂ was involved in the onset of a thermally activated deformation process. Received: 6 July 1999 / Reviewed and accepted: 9 August 1999     

A–BTransition of Superfluid 3He with a Film Geometry

We have performedcwNMR experiments on superfluid ³ He confined to a parallel-plate geometry with a m scale spacing for a wide pressure range. A static field was applied parallel or perpendicular to the plate surface. The spectra of two absorption signals, a main and a satellite, have been observed below the superfluid transition temperature in a parallel field. As the temperature decreased, the main signal decreased with shifts to higher frequencies, and the satellite grew with shifts to much higher frequencies. From the temperature dependence of these signals and the result in the perpendicular field, it is confirmed that the main signal and the satellite correspond to the A phase signal (ABM state) and the B phase signal (BW state), respectively. The temperature dependence of the two signals indicates that a phase transition from the A phase to the B phase occurs with decreasing temperature. By analyzing these signals, we determine A–B transition temperatures experimentaly. TheA–Btransition temperature normalized by the superfluid transition temperature is 0.95 at 20 bar, and decreased further to 0.70 at 0 bar for a thickness of 0.88 m for pure ³ He. The values of T_AB/T_C were slightly elevated when covering the surface with 4.5 layers of ⁴ He film, which suggests that this transition is also influenced by the surface condition.     

Preparation and characterization of stoichiometric CaFeO3

Stoichiometric CaFeO₃ was prepared and was characterized by crystallographic, magnetic and electrical measurements. A slight tetragonal distortion from the ideal cubic perovskite structure was found. The tetramolecular unit cell has a = 5.325 (3) A and c = 7.579 (5) A. The susceptibility showed a maximum at about 115 K and the temperature dependence of electrical resistivity changed from metallic to semiconductive in the vicinity of the magnetic transition temperature. These indicate a phase transition from the metallic-paramagnetic (the high-temperature phase) to the semiconductive-antiferromagnetic phase. The Mössbauer spectra indicated that a charge disproportionation 2Fe⁴⁺ → Fe³⁺ + Fe⁵⁺ associates with the transition.     

Relationship between ultraviolet emission and electron concentration of ZnO thin films

ZnO thin films were deposited on (0001) Al₂O₃ substrates depending on oxygen partial pressure by pulsed laser deposition. Optical properties of ZnO were investigated by photoluminescence (PL). The relationship between PL and electron concentration has been investigated. Origin of the dominant ultraviolet (UV) emission in ZnO thin film measured at room temperature was identified as a free electron-neutral-acceptor transition (eA⁰) through temperature dependence of PL measurement. The UV emission intensity at room temperature is related to variation of electron concentration because a free-electron-neutral-acceptor transition (eA⁰) as origin of UV emission at room temperature is related to impurity concentration of ZnO.     

4He in Nano-porous Media: Superfluidity and Quantum Phase Transition

This paper reviews recent findings of novel phenomena in ⁴He confined to a nano-porous glass. We examined pressure–temperature (P-T) phase diagram of ⁴He confined in a porous Gelsil glass that had nanopores 2.5 nm in diameter, by torsional oscillator and pressure studies. The obtained phase diagram is fairly unprecedented the superfluid transition temperature approaches zero at 3.4 MPa, and a novel nonsuperfluid phase exists between the superfluid and solid phase. These observations indicate that the confined ⁴He undergoes a superfluid-nonsuperfluid-solid quantum phase transition at zero temperature. We propose that the nonsuperfluid phase may be a localized Bose-condensed state in which global phase coherence is destroyed by a strong correlation between the ⁴He atoms or by a random potential. ⁴He in nanospace is an excellent model system for studying a strongly correlated Bose liquid and solid in a confinement potential.     

Quartz Crystal Microbalance Study of Superfluid 4He Films on Gold and Porous Gold Surfaces

The superfluid transition in sub-rnonolayer ⁴He films was studied using quartz crystal microbalance (QCM). We recently improved the QCM technique by replacing the conventional gold electrodes with porous gold, thus increasing the surface area and therefore sensitivity by 40 times. Simugtaneous measurements were taken for QCMs with flat and porous gold electrodes contained within the same copper cell. For both QCMs the superfluid transition temperature was found to be proportional to the ⁴He film thickness. The superfluid transition temperature at each film coverage was greater for porous gold than for flat gold electrodes.     

On the Order–Disorder Surface Phase Transition and Critical Temperature of Pure Metals Originating from BCC, FCC, and HCP Crystal Structures

The excess surface Gibbs energy and surface tension of pure liquid metals (originating from bcc, fcc, and hcp solid metals) of ordered and disordered surface structures are compared in this paper. It is shown that at a special temperature T ^* an order–disorder surface phase transition is expected in all liquid metals from a low-temperature ordered surface state to a high-temperature disordered surface state. This surface phase transition is similar to the first-order bulk solid–liquid phase transition (melting). The values of T ^* appear in the temperature interval between the melting point and the critical point of metals. Critical temperatures of metals are estimated from the equation for high-temperature disordered surfaces.     

Inverse ductile–brittle transition in metallic glasses?

There is evidence that metallic glasses can show increased plasticity as the temperature is lowered. This behaviour is the opposite to what would be expected from phenomena such as the ductile–brittle transition in conventional alloys. Data collected for the plasticity of different metallic–glass compositions tested at room temperature and below, and at strain rates from rate 10^?5 to 10³ s^?1, are reviewed. The analogous effects of low temperature and high strain rate, as observed in conventional alloys, are examined for metallic glasses. The relevant plastic flow in metallic glasses is inhomogeneous, sharply localised in thin shear bands. The enhanced plasticity at lower temperature is attributed principally to a transition from shear on a single dominant band to shear on multiple bands. The origins of this transition and its links to shear bands operating ‘hot’ or ‘cold’ are explored. The stress drop on a shear band after initial yielding is found to be a useful parameter for analysing mechanical behaviour. Schematic failure mode maps are proposed for metallic glasses under compression and tension. Outstanding issues are identified, and design rules are considered for metallic glasses of improved plasticity.     

Conductivity in human tooth enamel

When human tooth enamel is heated either in vacuum or air it presents drastic changes in electrical susceptibility, conductivity and structural properties. In this paper we report an insulator-conductive transition which is observed in air around 350°C where enamel conductivity changes drastically and its electrical resistance decreases from 10¹⁵ to 10⁵ that is, it goes from an insulator to a super-ionic ceramic behavior. This transition, first evidenced in vacuum by electron microscopy observations, is now completely determined by a.c. impedance spectroscopy technique and its characterization was carried out as a function of the frequency and temperature. X-ray in situ heating diffraction experiments show that there is no structural phase transition during a wide range of temperature including the one where the conductivity transition occurs.     

A-B Phase Conversion and Coexistence of Superfluid 3He in Aerogel

We have studied phase transition of superfluid ³He at 2.4 MPa in cylindrical aerogel by NMR method. When the liquid is cooled down from the normal state, the A-like phase appears below superfluid transition temperature T _c ^a which is suppressed in comparison with the transition temperature of the bulk liquid. With further cooling below the certain temperature T _ab,c ^a , the A-like phase is converted into the B-like phase gradually. Both phases stably coexist within about 90 μK. When you keep the temperature constant in which both phases coexist, the A-B phase conversion stops. With furthermore cooling, the whole liquid becomes the B-like phase. The cwNMR spectra at the coexistence state suggest that the B-like phase is not uniformly distributed in the A-like phase like a large number of small bubbles in a liquid, but separated as a whole from the A-like phase. By applying a field gradient which changes as a function of square of radius, we found that the A-like phase is in the edge part with a cylindrical shape and the B-like phase is in the central part with a columnar shape.      

A Preliminary Heat Capacity Measurement of 3He in Aerogel near its Superfluid Transition

We report simultaneous heat capacity and torsional oscillator measurement of ³ He in aerogel near the superfluid transition. The heat capacity has a peak at the temperature T_c where the torsional oscillator shows the onset of superfluid decoupling. The coincidence of these signatures suggests that ³ He in aerogel does undergo a true thermodynamic transition.     

Search for superfluidity of3He in3He-4He solution

We have tried to cool a³He-⁴He solution down to the microkelvin temperature region to search for a superfluid transition of the³He component in the solution. The contact surface area between the solution and a sintered powder has been increased enormously by the use of a fine platinum powder, to reduce the effect of the residual heat leak directly entering into the solution. Although the heat leak was found to be time dependent, the ultimate ratio of the heat leak to the surface area is about 0.046 pW/m², improved very much from those by the other groups. But the lowest temperature of the solution is still around 0.2 mK, and no evidence of the transition has been observed yet.     

Effect of Pressure on the Superconducting Transition Temperature in Mercury-Based Cuprates

The pressure dependence of the transition temperature of the mercury-based cuprates is analyzed through a phenomenological model, based on the inverted parabolic relation between the critical temperature (T _c ) and the hole concentration per CuO₂ layer (n). It is found that another inverted parabolic relation between the pressure dependence of the superconducting transition temperature at the optimum hole concentration and the pressure fit the recent experimental results of the mercury-based superconductors. This relation leads to a universal relation that is obeyed not only by mercury-based cuprates but also by many other high T _c compounds. In contrast to earlier studies, the transition temperature at pressure p (T _c (p)) is always less than the transition temperature for the optimum hole concentration (T ^op _c (p)) in agreement with the experiment. The effect of the pressure-induced change in the hole concentration on the transition temperature is found to be small compared to the intrinsic effects.     

Molecular dynamics simulation of pressure effect on phase transition behaviour and dynamics in the isotropic and discotic nematic phases

Molecular dynamics simulation was performed at constant temperature and pressure to investigate the effect of pressure on molecular dynamics for disc-shaped molecules. The generic Gay-Berne model, GB(0.345, 5.0, 1, 3), was used to study the phase transition behaviour, and translational and rotational dynamics, under two different reduced pressures P^?, 10.0 and 20.0. Obvious shifts were detected in the transition temperatures. Both systems have the same phase sequence with different pressures: isotropic, discotic nematic and columnar phases. Translational motion is characterised by the parallel and perpendicular components of diffusion coefficients, with respect to the director in the orientational ordered phase. With regard to rotational dynamics, the correlation time of the first-rank orientational time autocorrelation function, which corresponds to end-over-end rotational motion of a molecule, has been investigated. A clear jump in the temperature dependence of the correlation time has been found at the isotropic-nematic phase transition point. The retardation factor g_|| as a function of the reduced temperature T^?/T_NI^? shows an apparent pressure effect on the rotational dynamics.