Continuous wave NMR of submonolayer 3He adsorbed on grafoil

NMR linewidths and magnetic susceptibilities of ³He adsorbed on Grafoil have been measured by a continuous wave technique between 0.40 K to 4.21 K for five fractional monolayer coverages, x, between 0.20 and 0.96. Interpretations in terms of the phases of the submonolayers are presented.     

Pulsed NMR of submonolayer 3He adsorbed on grafoil

Relative susceptibility and free induction decay, (FID), time measurement were made on ³He submonolayers adsorbed on Grafoil in the temperature range 0.07 K to 4.2 K. The susceptability measurements for coverages of 0.5 monolayers up, showed Curie-law behavior.     

Growth of a third ferromagnetic solid 3He layer on graphite

We have measured the nuclear susceptibility of 3He in Grafoil filled with pure liquid 3He over the pressure region between 0.6 and 31.38 bars and at temperatures down to 0.5 mK with a cw NMR method. The nuclear magnetization corresponding to the adsorbed 3He layers on the Grafoil surface shows a strong ferromagnetic tendency with a periodic behavior as a function of liquid pressure. This observation is attributable to the growth of third and fourth solid 3He layers with the liquid pressure increase. The pressure dependence of the Weiss temperature indicates the third layer is completed at 19 bars and the fourth probably at 28 bars. The number of localized spins estimated from the solid magnetization is almost doubled from 0 to 28 bars, being consistent with this scenario.     

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.     

NMR studies on polarized12B implanted in highly oriented pyrolytic graphite

NMR measurements on¹²B nuclei implanted in a thin sheet of highly oriented pyrolytic graphite (Grafoil) have been made to study the magnitude of the polarization of¹²B and its electric quadrupole coupling. The magnitude of the residual polarization of¹²B held in carbon material is of crucial importance to the experiment measuring the average polarization of¹²B produced in the muon capture by¹²C. It was found that the polarization of¹²B can be maintained at 101.0±2.2% in Grafoil under a magnetic field of 3 kG. The electric quadrupole coupling was determined to bee ² qQ/h=+1207.3±8.2 kHz. From the magnitude and the sign of the electric field gradienteq, the location of¹²B was found to be a substitutional site in the carbon lattice. The anomalous B-C bond involving thesp ₂ hybrid orbitals with partially filledp _z electrons is also discussed.     

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.     

Heat capacity study of the first monolayer of adsorbed methane on grafoil

The heat capacity of CH₄ adsorbed on Grafoil below monolayer completion has been determined experimentally for the first time. The specific heat at constant density shows two characteristic features, very broad, weak anomalies at 47.6 K (around one third of a monolayer) and (for occupation numbers below two thirds of a monolayer) very narrow, strong peaks at 56.35 K. The anomalies at 47.6 K are interpreted as being due to a registry — out-of-registry transition of the two dimensional (2D) solid CH₄ adsorbate into an incommensurate expanded structure. The anomalies at 56.35 K define the 2D triple point of adsorbed CH₄. For higher occupation numbers the liquid-solid phase boundary shifts to higher temperatures. The present data make partial re-definition of the 2D-CH₄ phase diagram necessary.     

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.     

Topological Insulators and Topological Superconductors,by B. Andrei Bernevig with Taylor L. Hughes

After introductory remarks the recent discovery of the superfluid phases of ³He is described. This is followed by a discussion of the P, T-diagram below 3 mK. NMR and susceptibility measurements on liquid ³He are then explained. Next, heat capacity and heat flow data are briefly mentioned and experiments involving the zero and fourth sound are described. The viscosity and the density of the normal component of liquid ³He are then discussed. The paper is concluded by a theoretical section.     

Solidification of 3He in a porous glass

NMR relaxation measurements of ³He in porous high silica glass within a temperature range of 0.5 to 4.2 K at various applied pressures up to 14.6 MPa are reported. The onset of solidification of ³He in the pores was observed at excess pressures ≈ 2.3?4.7 MPa above the bulk melting pressure between 1.3 and 2.1 K. The specific heat of ³He in the porous glass was measured to supplement the NMR measurements and the result confirmed the solidification.     

Intravascular delivery of hyperpolarized129Xenon forin vivo MRI

The nuclear polarization of¹²⁹Xe and³He can be enhanced by several orders of magnitude by using optical pumping techniques, thus allowing NMR detection of xenon and helium in very low concentrations. The benefits of optically enhanced magnetic resonance (MR) are already exploited in MR imaging of the lungs using optically polarized³He. The high solubility of xenon in blood and lipids suggests a variety ofin vivo MR applications, for instance perfusion measurements or functional MR studies. This article reviews some current work directed towards delivery of optically polarized xenon forin vivo MR applications.     

The Calcium Carbonate Geological Samples Study by <Superscript>3</Superscript>He NMR

Geological samples of calcium carbonates (CaCO₃) were investigated by ³He NMR, nitrogen porosimetry, X-ray diffraction and electron microscopy methods. The integral porosity of the samples was obtained by ³He nuclear magnetic resonance method and compared with nitrogen adsorption isotherms data. The advantages of ³He porometry method are discussed.     

Size effect in the (PrF3 nanoparticles-3He) system

Spin kinetics of adsorbed and liquid ³He in contact with crystalline nanopowders of the Van Vleck paramagnet PrF₃ at a temperature of 1.5 K has been studied by nuclear magnetic resonance. The correlation between the parameters of the nuclear magnetic relaxation of ³He and the sizes of the sample particles has been found. A qualitative model of the magnetic relaxation of ³He describing the experimental results has been proposed.     

Nuclear magnetic relaxation of <Superscript>3</Superscript>He in contact with an aerogel above the Fermi temperature

The spin kinetics of ³He in an aerogel has been studied above the Fermi temperature. The magnetic relaxation times T ₁ and T ₂ of adsorbed, gaseous, and liquid ³He in a 95% silica aerogel at a temperature of 1.5 K have been determined as functions of frequency by means of pulse nuclear magnetic resonance. It has been found that the time T ₁ is linear in frequency in all three cases, whereas T ₂ is independent of frequency. To explain the observed behavior of the longitudinal relaxation rate, a theoretical model of relaxation in the adsorbed layer of ³He taking into account the filamentary structure of the aerogel is proposed.     

Superfluid 3He in Squeezed Nematic Aerogel

We present results of nuclear magnetic resonance (NMR) experiments in superfluid ³He in two samples of nematic aerogel consisting of nearly parallel mullite strands. The samples were cut from the same piece of the aerogel, but one of them was squeezed by 30% in the direction transverse to the strands. In both samples, the superfluid transition of ³He occurred into the polar phase, where no qualitative difference between NMR properties of ³He in these samples was found. The difference, however, has appeared on further cooling, after the transition to the polar-distorted A phase (PdA phase) with the orbital part of the order parameter in the 2D Larkin–Imry–Ma (LIM) state. In the squeezed sample, the 2D LIM state is anisotropic, which results in changes in the NMR, which can be used as an additional marker of the PdA phase and have allowed us to measure the value of the anisotropy.

     

Superfluidity of 3He in dilute solutions of 3He in liquid 4He

The possibility of superfluidity of ³He in dilute solutions of ³He in liquid ⁴He is investigated, and the corresponding critical temperature is estimated. The results indicate S-state pairing, i.e., for small concentrations of ³He, the highest transition temperature is obtained for L = 0, but the obtained critical temperatures are very low.     

NMR on the new phases of liquid 3He in contact with platinum powder

Liquid ³He has been cooled by adiabatic demagnetization of copper nuclei to 1.6 mK, well into the region of the superfluid phases. NMR measurements on ³He in a coil partly filled with platinum powder give results different from those expected for bulk liquid.     

Recent results on hyperpolarized<Superscript>3</Superscript>He−<Superscript>4</Superscript>He liquid mixtures

Laser optical pumping in low magnetic field provides very high nuclear polarizations in gaseous helium mixtures, and is used to prepare polarized liquid. Wall relaxation in glass cells is effiently reduced using cesium coatings, and bulk longitudinal relaxation times are measured. In highly magnetized samples, dipolar fields control the spin dynamics in anisotropic volumes and weak external magnetic field inhomogeneities. Long lived magnetostatic modes are observed by pulsed NMR. Detailed analysis of their frequency and damping gives information on magnetization density and spin diffusion coefficient in polarized mixtures. Experiments are performed above 0.2 K on mixtures with³He concentrations of order a few percents or larger. When phase separation occurs, the³He-rich phase retains a high polarization.     

Inelastic electron tunneling spectroscopy of carboxylic acids on alumina at low coverage

The difficulties in assigning certain modes of carboxylic acids adsorbed on alumina are discussed. The study of the spectra of marked acetic acid CH₃C¹⁸O¹⁸OH adsorbed on alumina has allowed us to clarify the assignments of the acetate ion formed. Then an examination of the spectra at very low coverages has enabled us to propose a model for the adsorption, which occurs on (at least) two different sites.