Gapless spin liquid behavior in two-dimensional solid 3He

Direct demagnetization has been made for two-dimensional solid 3He in both the paramagnetic and the antiferromagnetic phases. The lowest temperature is about 10 microK, judging from the observed magnetization for the paramagnetic solid 3He. The magnetization of the antiferromagnetic solid 3He shows a gradual increase to about 10 microK for the 4/7 phase adsorbed on both one layer of 4He and two layers of HD preplated graphite. This strongly suggests that the triangular antiferromagnet with the higher order multiple exchange has a quantum spin liquid ground state with nearly zero or extremely small spin gap less than 10 microK.     

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.     

Direct measurement of spreading pressures of films adsorbed on crystalline substrates

A direct reading strain gauge has been developed for the measurement of spreading pressures of films adsorbed on solid substrates. In contrast to previous instruments employing heterogeneous adsorbents, the present device, using Grafoil, has linear response to spreading pressure and displays no hysteresis. Sensitivity is adequate for studies of low coverages, down to small fractions of a monolayer. Surveys of benzene, butanol, and propanol monolayers were made with the device at room temperature. The films were found to behave as two-dimensional imperfect gases, and their second and third order virial coefficients were measured. A general theory of the dilational effect is presented, in which the strains produced by a film adsorbed on a face of a crystalline substrate are related to the elastic constants and dimensions of the crystal. The theory is applied to the case of graphite such as in Grafoil. Quantitative results consistent with the behavior of the actual device are obtained.     

Hyperpolarized xenon nuclear spins detected by optical atomic magnetometry

We report the use of an atomic magnetometer based on nonlinear magneto-optical rotation with frequency-modulated light to detect nuclear magnetization of xenon gas. The magnetization of a spin-exchange-polarized xenon sample (1.7 c m(3) at a pressure of 5 bars, natural isotopic abundance, polarization 1% ), prepared remotely to the detection apparatus, is measured with an atomic sensor. An average magnetic field of approximately 10 nG induced by the xenon sample on the 10 cm diameter atomic sensor is detected with signal-to-noise ratio approximately 10 , limited by residual noise in the magnetic environment. The possibility of using modern atomic magnetometers as detectors of nuclear magnetic resonance and in magnetic resonance imaging is discussed. Atomic magnetometers appear to be ideally suited for emerging low-field and remote-detection magnetic resonance applications.     

Acoustic crystallization and heterogeneous nucleation.

By focusing a high-intensity acoustic wave in liquid helium, we have observed the nucleation of solid helium inside the wave above a certain threshold in amplitude. The nucleation is a stochastic phenomenon. Its probability increases continuously from 0 to 1 in a narrow pressure interval around P(m) + 4.7 bars ( P(m) = 25.3 bars is the melting pressure where liquid and solid helium are in equilibrium). This overpressure is larger by 2 to 3 orders of magnitude than what had been previously observed. Our result strongly supports the recent suggestion by Balibar, Mizusaki, and Sasaki that, in all previous experiments, solid helium nucleated on impurities.     

Evidence for a Mott-Hubbard transition in a two-dimensional 3He fluid monolayer

The heat capacity and magnetization of a fluid 3He monolayer adsorbed on graphite plated with a bilayer of HD have been measured in the temperature range 1-60 mK. Approaching the density at which the monolayer solidifies into a sqrt[7]xsqrt[7] commensurate solid, we observe an apparent divergence of the effective mass and magnetization corresponding to a T=0 Mott-Hubbard transition between a 2D Fermi liquid and a magnetically disordered solid. The observations are consistent with the Brinkman-Rice-Anderson-Vollhardt scenario for a metal-insulator transition. We observe a leading order T2 correction to the linear term in heat capacity.     

3He/graphite commensurate bilayer films in the antiferromagnetic regime

We have measured the nuclear magnetic susceptibility of second layer³He films adsorbed on graphite in the antiferromagnetic regime. The nuclear exchange parameter J is found to be constant over a coverage range centered around that expected for a 4/7 commensurate phase. The ratio of the magnetization of the first and second layers provides a direct test of the bilayer commensuration.     

Chemisorption and surface ferromagnetism: Theoretical analysis of spin-resolved photoemission data from Ni(1 1 0)(2×1)−O

To extract information from recent experimental spin-resolved photoemission data from Ni(1 1 0)(2×1)?O, one-step theory calculations were performed for several geometrical and magnetic model structures. The observed strong influence of oxygen chemisorption on the normal photoemission spectra at room temperature is quantitatively reproduced assuming a (2×1) saw-tooth reconstruction with the third and fourth Ni layers magnetically dead, while the magnetization of the topmost two Ni layers is almost irrelevant for the spectra. It is concluded that bulk-like deeper layers are strongly quenched at least with regard to the near-X₂ exchange splitting.     

Liquid 3He thermomechanical effect in regular cylinders

We have measured the normal liquid 3He thermomechanical effect with the liquid confined in the regular 210 nm diameter cylinders of Anopore. The pressure difference resulting from the temperature difference was deltaP/deltaT=25 Pa/mK, independent of temperature, from 0.7 to 12 mK. This value was 100x the theoretical value and 3x the result with a packed powder constriction. Two monolayers of 4He did not change the value, but four monolayers reduced it by 2 orders of magnitude; the high value was therefore attributed to boundary scattering from magnetic 3He surface layers.     

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.     

Thermal conductivity of liquid 3He in aerogel: a gapless superfluid

We have measured the thermal conductivity of liquid 3He in 98% aerogel at ultralow temperatures. Aerogel introduces disorder on a scale comparable to the superfluid coherence length. At low pressures the liquid in the aerogel shows normal-state behavior with conductivity linear in temperature. At pressures above approximately 6 bars the onset of superfluidity suppresses the conductivity and the thermal conductivity again tends towards linear behavior in the very low temperature limit, providing strong evidence that here the liquid 3He in the aerogel is behaving as a gapless superfluid.     

Magnetization studies of the nuclear spin ordered phases of solid <Superscript>3</Superscript>He in silver sinters

Solid ³He, in the bcc lattice between 34 and 100 bar, exhibits two nuclear magnetic ordered phases in the sub-mK temperature range, the so called U2D2 low (magnetic) field phase and the “high field phase” above 0.4 T. To determine the exact spin structure of these phases we started a project of neutron scattering from the ordered solid in collaboration with the Hahn-Meitner Institute, Berlin, and other European and US groups. For this experiment it is crucial to grow a single crystal within the sinter needed for cooling the solid to temperatures of the order of 500 μK (or even twenty times lower in the case of the hcp lattice which is formed above 100 bar) and to keep it there long enough to measure a magnetic neutron reflection. We studied the growth of crystals in Ag sinters of different pore size and with different growth speeds to find an optimal way to obtain single crystalline samples. As a first diagnostic step we performed pulsed NMR measurements in the ordered phases of solid ³He in a sinter of 2700 Å particle size down to temperatures of 450 μK at various molar volumes. We could keep the samples in the ordered state for as long as 140 h. The second method we used was SQUID magnetometry. For the low field phase T_N was indicated by a drop of the intensity, both in the NMR signal and in the dc magnetization, whereas in the high field phase an increase of about 30% was observed below the ordering temperature. For the fabrication of the sinters a packing fraction of 50% and subsequent annealing proved to be very favorable to obtain cold ordered solid. Furthermore, we find that a paramagnetic surface contribution from a few monolayers of ³He exists down to 500 μK in addition to the bulk magnetization.     

3He spin diffusion measurements in 3He- 4He mixture films

3He spin diffusion measurements for 3He-4He mixture films on Nuclepore are reported as a function of 3He coverage for 0. 030相似文献   

Effect of phase transitions of helium-3 in pores of wood carbonizate on the spin kinetics of 3He nuclei

The time of the nuclear magnetic relaxation of ³He gas in pores of Astronium carbonizate powder is studied as a function of gas pressure. Using the results of the measurements of the spin-echo amplitude, the dependences of the rates of spin-lattice and spin-spin relaxations on the number of helium atoms on the surface, in pores, and in the interparticle space of carbonizate powder have been determined. Analysis of the relaxation rates allows the identification of three possible basic phases of ³He in the system under consideration: solid film, gas phase, and liquid. Moreover, the character of spin kinetics in transitions between these phases has been determined.     

NMR magnetization transfer as a tool for characterization of nanoporous materials

The application of nuclear magnetic resonance magnetization transfer experiments to probe the surface-to-volume ratio and pore morphology of porous materials with characteristic pore sizes of 1-100 nm is described. The method is based on the phenomenon of incomplete freezing of liquids in small pores where a few monolayers adjacent to the pore walls remain liquid. Sufficient difference between the transverse relaxation times in the solid frozen core and liquid surface layer allows the initial preparation and subsequent re-equilibration of a solid-liquid magnetization grating. The method is demonstrated using model nanoporous materials with known characteristics. The ensuing problems of the mechanism of the magnetization transfer through the interface and within the frozen core are discussed and elucidated by pulsed-field-gradient NMR experiments.     

Interpretation of measured spectral line shapes using a square-well potential: The absorption lines of cesium broadened by rare gases

A simple square-well potential is used as an aid in understanding the complicated evolution of the absorption lines and their associated satellites for cesium perturbed by the rare gases. New experimental halfwidth data for cesium perturved by Xe, Ar, and He are presented. In the case of Xe, data for the second, third and fourth members are presented to 30, 7, 1 rd (relative density), respectively. For Ar, data are presented for the third through the ninth member. The pressure range for the third and fourth members is from 0 to 20 rd. For the other members, it ranges to 8 rd for the fifth and to 2 rd for the ninth. Third and fourth member helium data are presented to about 20 and 8 rd, respectively. Several theoretical computer profiles are used to illustrate the evolution of the line and satellites with foreign gas pressure and these are compared with the experimental data.     

Polarization decreases the specific heat of liquid 3He

We report on the first measurements of the polarization dependence of the specific heat of liquid 3He. Transient polarizations m of up to 70% were reached by using the rapid melting technique. The specific heat at 60-100 mK and 27 bars is found to decrease approximately as m(2), the reduction reaching at least 30% for m = 70%. These results contradict the nearly localized picture of 3He, and are in agreement with the idea that a large part of the specific heat is due to spin fluctuations.     

Ground state of a spin system with two- and four-spin exchange interactions on the triangular lattice

We study a spin system with both two- and four-spin exchange interactions on the triangular lattice as a possible model for the nuclear magnetism of solid ³He layers adsorbed on grafoil. The ground state is analyzed by the use of the mean-field approximation. It is shown that the four-sublattice state is favored by introduction of the fourspin exchange interaction. A possible phase transition at a finite temperature into a phase with the scalar chirality is predicted. Application of a magnetic field is shown to cause a variety of phase transitions.     

NMR of Liquid 3He in Pores of a Clay Sample

In the present work a new method for studying porous media by nuclear magnetic resonance of liquid ³He has been proposed. This method has been demonstrated by an example of a clay mineral sample. For the first time the integral porosity of the clay sample has been measured. For investigated samples the value of the integral porosity is in the range of 10–30%. The inverse Laplace transform of the ³He longitudinal magnetization recovery curve has been carried out and the distribution of the relaxation times T ₁ has been obtained.