Anomalous suppression of superfluidity in 4He confined in a nanoporous glass

We explore the superfluidity of 4He confined in a porous glass, which has nanopores of 2.5 nm in diameter, at pressures up to 5 MPa. With increasing pressure, the superfluidity is drastically suppressed, and the superfluid transition temperature approaches 0 K at some critical pressure, Pc approximately 3.4 MPa. The feature suggests that the extreme confinement of 4He into the nanopores induces a quantum phase transition from a superfluid to a nonsuperfluid at 0 K and at Pc.     

Study of the electrical properties of thin SmS films at high pressures

The pressure-induced shift of impurity levels under hydrostatic compression (?1.9 × 10^?2 meV/MPa) at T = 300 K has been derived from measurements of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films on glass substrates at different pressures. The difference between the pressure-induced shifts of impurity levels in thin films and single crystals has been attributed to the effect of elastic properties of the substrate material. It has been shown that the semiconductor-metal phase transition in SmS films does not occur at pressures of up to 1000 MPa, because the impurity levels triggering the mechanism of phase transition at such pressures are not in the conduction band.     

High-pressure,high-resolution NMR probe working at 400 MHZ

Abstract

A probe for obtaining high-resolution multinuclear NMR spectra at elevated pressures with a Bruker AM-400 spectrometer is described. The probe is designed for pressures up to 200 MPa and has been used between -40 to 150°C. We obtain routinely a resolution of about 1 Hz for proton (400 MHz) spectra using deuterium as an internal field lock. This probe is easily interchangeable with a commercial probe. We also describe a simple sample tube made of a 5 mm commercial NMR tube and a machinable glass cap with a total volume of about 1 cm³.     

Excitations of metastable superfluid 4He at pressures up to 40 bars

Neutron scattering measurements of the fundamental excitations of liquid 4He confined in 44 A pore diameter gelsil glass at pressures up to 40 bars in the wave vector range 0.41.6 A(-1), especially the rotons, are observed up to complete solidification of all the liquid at a pressure of approximately 40 bars where the roton vanishes. At and above a pressure of 35.1 bars, Bragg peaks are observed, indicating coexistence of liquid and solid in the pores at pressures 35 less than or approximately equal P less than or approximately equal 40 bars.     

On the thermodynamic equilibrium in the <Superscript>3</Superscript>He-aerogel system at low temperatures

A new method for studying the processes of the establishment of the thermodynamic equilibrium in the adsorbed ³He layers in highly porous media has been proposed. Using this method, the thermalization of adsorbed ³He on silica aerogel at a temperature of 1.5 K has been studied. The process of the establishment of the thermodynamic equilibrium has been controlled by measuring the pressure in an experimental cell, the amplitude of the NMR signal, and the nuclear spin-spin and spin-lattice relaxation times of adsorbed ³He. It has been shown that the establishment of the thermodynamic equilibrium in the adsorbed ³He-aerogel system is characterized by a time of 26 min.     

Heat capacity and glass transition of pure and doped cubic ices2

Heat capacities of pure and KOH (mole fraction 1.8 × 10⁻³)-doped cubic ices were measured at nearly atmospheric pressures in the temperature ranges of 12–160 K and of 80–160 K, respectively. They were prepared from water at 250 MPa through the high pressure ice III and IX. The preparation and the subsequent heat capacity measurement were performed within a cell of an adiabatic high pressure calorimeter. A new glass transition, where the heat capacity showed a jump of 0.1 J K⁻¹mol⁻¹, was found at 140 K in the pure ice. The KOH doping caused a decrease in the glass transition temperature by about 30 K. For the bulk samples which had little surface, the enthalpy change during the irreversible cubic-hexagonal transformation at about 200 K was determined to be −(37.0 ± 0.8) J mol⁻¹. With the heat capacity data of the cubic and hexagonal ices, this value was converted to the enthalpy difference at OK, −(35.6 ± 0.8) J mol⁻¹.     

High pressure viscosity and density measurements of the binary mixture tridecane + 2,2,4,4,6,8,8-heptamethylnonane

Abstract

The dynamic viscosity η of the binary mixture tridecane + 2,2,4,4,6,8-heptamethyl-nonane has been measured in the temperature range 293.15-353.15K (in progressive 10 K steps) at pressures 0.1,20,40,60,80 and 100 MPa. The system is described by 9 molar fractions (0 to 1 in 0.125 progressive steps). The density ρ has been measured at pressures from 0.1 to 65 MPa in progressive 5 MPa steps. The whole set of experimental data represents 378 points for η and 882 for ρ. The measurements of η allow to determine the excess viscosity η^E and the excess activation energy of viscous flow ΔG^E versus pressure, temperature and composition.     

Development of Very-Low-Temperature Millimeter-Wave Electron-Spin-Resonance Measurement System

We report the development of a millimeter-wave electron-spin-resonance (ESR) measurement system at the University of Fukui using a ³He/⁴He dilution refrigerator to reach temperatures below 1 K. The system operates in the frequency range of 125–130 GHz, with a homodyne detection. A nuclear-magnetic-resonance (NMR) measurement system was also developed in this system as the extension for millimeter-wave ESR/NMR double magnetic-resonance (DoMR) experiments. Several types of Fabry–Pérot-type resonators (FPR) have been developed: A piezo actuator attached to an FPR enables an electric tuning of cavity frequency. A flat mirror of an FPR has been fabricated using a gold thin film aiming for DoMR. ESR signal was measured down to 0.09 K. Results of ESR measurements of an organic radical crystal and phosphorous-doped silicon are presented. The NMR signal from ¹H contained in the resonator is also detected successfully as a test for DoMR.     

Depression of the 3He melting curve by adsorptive solidification on carbon substrates

The depression of the ³He melting curve by up to one bar lower than the bulk melting curve for temperatures varying from the mK range up to 0.5 K was observed. Adsorptive solidification isotherms at several different temperatures were studied.     

Nuclear magnetic resonance investigation of metallic sodium nanoparticles in porous glass

Sodium nanoparticles embedded in porous glass have been studied by NMR. The measurements have been carried out on pulse spectrometers in magnetic fields of 9.4 and 17.6 T in a wide temperature range. Changes in the magnitude and temperature dependence of the ²³Na Knight shift with respect to those in bulk sodium have been discovered. An additional component of the NMR line shifted to high frequencies has been observed in the temperature range from 240 to 100 K. Investigation of the specific heat has revealed a considerable decrease in the melting and crystallization temperatures of sodium under nanoconfinement, which were not accompanied by abrupt changes in the Knight shift.     

NMR investigation of defect properties in single crystal SrTiO3

Abstract

⁴⁹Ti and ⁸⁷Sr nuclear magnetic resonance measurements were performed in single crystalline SrTiO₃ between 90 K and 1800 K at various oxygen partical pressures. The NMR lines are found to be shifted with rising temperature due to oxygen vacancies acting as donors. Furthermore, three distinct motion-induced nuclear spin relaxation rates could be observed. The corresponding jump rates are attributed to the following processes: translational jumps of charged oxygen vacancies between oxygen sites, localized motion of oxygen vacancies in Fe-vacancy complexes, self diffusion of Sr²⁺.     

NMR on superfluid<Superscript>3</Superscript>He in aerogel

The transition to superfluidity of³He in high porosity (98.2%) acrogel has been observed by nuclear magnetic resonance techniques. The onset of the transition at all pressures above 13 bar is marked by a sharp increase in NMR frequency similar to that observed in bulk³He-A. This suggests that the aerogel/superfluid phase is highly homogeneous although both the transition temperature, T_c, and the amplitude of the order parameter are substantially suppressed. The acrogel strands are ≈ 50Å in diameter, much smaller than the superfluid coherence length. Consequently, we have attempted to interpret our observations as an impurity scattering problem. Based on our measurements of the magnetic field dependence of T_c it appears that both magnetic and potential scattering play important roles where the magnetic scattering can be associated with solid³He on the aerogel surface. This is determined by isotopic exchange with⁴He, a process which appears to stabilize a new superfluid state similar to the bulk B-phase.     

Molecular mobility in compressed fluid n-butane

Abstract

An autoclave machined from a high strength titanium alloy for high pressure high resolution NMR studies is described. The autoclave has been used at pressures up to 600 MPa and temperatures between 150 K and 450 K. The molecular mobility of fluid n-butane has been studied by proton- and deuteron spin lattice relaxation time measurements of selectively deuterated butanes. Additional information about the dynamic is obtained from isotopic dilution experiments. The p, T dependence of the overall mobility and of the rotation of the methyl groups is derived from the data.     

Research on glass cells for He neutron spin filters

Glass cells play an important role in polarized ³He neutron spin filters. To evaluate the scattering and absorption contribution from glass cells during neutron scattering experiments, we measured small-angle scattering and neutron transmission in GE180 and other glasses. The small-angle neutron scattering measurements revealed that the glasses used for ³He spin filters have acceptably lower scattering: dΣ(Q)/dΩ=4-7×10⁻⁴ cm⁻¹ at Q=0.03-0.12 Å⁻¹. The transmission measurement was performed at J-PARC. Neutron transmission of about 92% through empty GE180 cells was observed over a wide wavelength range 0.014-7.0 Å. To pursue the possibility of being a structural influence on ³He spin relaxation in GE180 glass cells, we performed precise X-ray diffraction measurement using synchrotron radiation at SPring-8. From these measurements, a structural difference was observed among GE180 glasses with different thermal treatments.     

Migration and agglomeration of heliumatoms in copper

The magration and agglomeration of Helium atoms in He irradiated Cu was studied between 10 K and 900 K by perturbed γγ angular correlation (PAC) measurements using the radioactive probe atom¹¹¹In. Trapping of interstitial and substitutional He atoms is observed already after irradiation at 10 K. Substitutional He (He-yacancy pair) is bound in the nearest neighbourhood to the¹¹¹In probe atom and seems to be stable up to 725 K (E _He1V1 ^b ≥2.1 eV). The onset of vacancy assisted He agglomeration in Cu is observed at 250 K.     

Generation of microcellular poly(methyl methacrylate) by compressed gases

Abstract

Generation of microcellular poly(methy1 methacrylate) (PMMA) was studied in CO₂ and N₂O at pressures from 2 to 15MPa at three temperatures, 293.2K, 308.2K, and 323.2 K. The average diameter d and average number density N of voids generated by a rapid expansion of compressed gases in PMMA were measured by use of an optical microscope. Effects of gases, temperature, and pressure on the d and N values were examined. Even at pressure below glass transition pressure of PMMA with both gases, voids of diameter being as small as those found at high pressure, 15MPa, were obtained at each temperature. However, the void density of PMMA at lower pressure by both gases was not so good as those obtained at high pressures.     

Nonlinear NMR in a superfluid B phase of <Superscript>3</Superscript>He in aerogel

The properties of liquid ³He in a low-density aerogel preliminarily covered with a few monolayers of ⁴He were studied by pulsed and nonlinear CW NMR techniques. It was found that an NMR frequency shift from the Larmor value exhibits a sharp increase at a magnetization tilting angle exceeding 104°. Nonlinear CW NMR signals related to the formation of a macroscopic region featuring homogeneous precession of the magnetization (homogeneous precession domain) were observed. The experimental results confirm that the low-temperature superfluid ³He phase in the aerogel is analogous to the B-phase in bulk ³He and indicate that the spin supercurrents play an important role in the spin dynamics of superfluid ³He in aerogel.     

Energy transfer from Tb3+ to Nd3+ in glycerol:D2O solution and frozen glass

The mechanism of energy transfer from Tb³⁺ to Nd³⁺ in glycerol:D₂O solution at 280 K, 300 K and in its frozen glass at 80 K, 120 K, 190 K and 250 K has been examined. Analysis of the intensity and lifetime measurements of ⁵D₄ level of Tb³⁺ shows that at 280 K and 300 K in solution the transfer rate varies as the square of the Nd³⁺ concentration. This suggests a dipole-dipole transfer in the glycerol:D₂O solution. However, at lower temperatures up to 190 K, where the solution freezes into a glass, the dependence observed is linear at lower Nd³⁺ concentrations and quadratic at higher concentrations. Moreover, the linear concentration dependence becomes more pronounced as the temperature of the glass is increased. This linear concentration dependence of the transfer rate and its temperature dependence have been ascribed to the migrational transfer accompanying the multipolar transfer. The coefficients of both the interactions at the above temperatures have been calculated.     

Novel MRI applications of laser-polarized noble gases

Gas-phase nuclear magnetic resonance (NMR) has great potential as a probe for a variety of interesting physical and biomedical problems that are not amenable to study by water or similar liquid. However, NMR of gases was largely neglected due to the low signal obtained from the thermally polarized gases with very low sample density. The advent of optical pumping techniques for enhancing the polarization of the noble gases³He and¹²⁹Xe has bought new life to this field, especially in medical imaging where³He lung inhalation imaging is approaching a clinical application. However, there are numerous applications in materials science that also benefit from the use of these gases. We review primarily nonmedical applications of laser-polarized noble gases for both NMR imaging and spectroscopy and highlight progress with examples from our laboratory including high-resolution imaging at millitesla applied field strength and velocity imaging of convective flow. Porous media microstucture has been probed with both thermal and laser-polarized xenon, as xenon is an ideal probe due to low surface interaction with the grains of the porous media.     

Observation of micropores in hard-carbon using Xe NMR porosimetry

The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using ¹²⁹Xe NMR. For high-pressure (4.0 MPa) ¹²⁹Xe NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.