Electrons and Cavitation in Liquid Helium-3

We report preliminary results from a study of the effect of electrons on cav-itation in liquid ³ He. We have measured the negative pressure at which electron bubbles explode. The results are in good agreement with theoretical expectations.     

NMR in Superfluid Helium-3 in the Non-Hydrodynamic Regime

This article is dedicated to the memory of Olli Lounasmaa, the physicist, who directed my scientific life along the path of superfluid ³He! In the article I review the results of experimental and theoretical studies of superfluid ³He spin dynamics at ultralow temperatures, where the density of the normal component is virtually zero. I describe our current understanding of the phenomena of catastrophic relaxation, NMR in the Landau field and the surface instability of homogeneous precession. PACS numbers: 67.57.Lm.     

Spin-Orbital Dynamics in the B-Phase of Superfluid Helium-3

We report numerical calculations on the spin–orbital dynamics of ³He-B within the formalism of Poisson brackets with the two-fluid model by Leggett and Takagi. We incorporate an additional orbital term in the equations for the spin–orbit dynamics which plays an important role at very low temperatures, when the damping of orbit dynamics is small. We also find that, under the relevant experimental conditions, the Brinkman–Smith mode is strongly modified by the orbital dynamics. The orbital momentum does not relax completely to the direction of magnetic field, but remains significantly deflected, particularly at very low temperatures, and precesses at the dipole–dipole frequency and nutate at the NMR frequency. We report numerical calculations of the spin–orbit dynamics in the spatially inhomogeneous case. We solved for the mode of precession near the walls of the experimental cell, which significantly deviates from that, obtained in theoretical calculations in previous publications. We also identified the mechanism of instability of homogeneous precession at very low temperatures, the exponential growth of textural-spin waves of the longitudinal mode of NMR.     

Application of Surface Acoustic Wave Sensors for Liquid Helium-4 and Helium-3

A surface acoustic wave (SAW) sensor can be used to determine the acoustic properties of liquid He. We applied this sensor for liquid ³He and liquid ⁴He. Two kinds of SAW were used in this experiment, Rayleigh-SAW and shear horizontal SAW. The damping of Rayleigh-SAW was measured in liquid ⁴He. The results agreed well with the reported values of the acoustic impedance, and the superfluid transition was clearly seen. The damping and the phase shift of shear horizontal SAW were measured in the normal liquid ³He. Temperature dependence of the damping and the phase shift was analyzed by the viscoelastic model, and the results were compared with the transverse acoustic impedance in the normal ³He.     

Quantum Cavitation: A Comparison Between Superfluid Helium-4 and Normal Liquid Helium-3

Cavitation has now been studied in superfluid helium-4 and in normal liquid helium-3, both theoretically and experimentally. We compare the two cases and discuss the existence of a crossover from quantum cavitation, where bubbles are nucleated by tunneling, to classical cavitation where their nu-deation is thermally activated. In helium-3, where evidence for quantum nucleation is lacking, the interpretation of the experimental results leads to two related questions. The first one concerns the extrapolation of the properties of this Fermi liquid at negative pressure. The second one concerns the validity of present theories of quantum cavitation in a Fermi liquid.     

Polarized Dipolar Bose Gas with Strong Interactions

We present calculations of properties of a polarized dipolar Bose gas, trapped in the polarization direction by a harmonic potential while translationally invariant in the perpendicular direction. This system is of particular interest because the dipolar interaction is not only anisotropic, but also long-ranged and it already showed an interesting behaviour in the weakly interacting limit (Santos et al. in Phys. Rev. Lett. 90:250403, 2003), where a roton-maxon like excitation spectrum was found. Here we stabilize the dipolar Bose gas by a repulsive core of the form (σ/r)¹² to avoid a collapse of the system. For our calculation we use the hypernetted-chain Euler-Lagrange method which is not limited to weakly interacting or dilute systems, but is also valid for strongly interacting systems. We find strong evidence that under certain conditions a quantum phase transition occurs to a state where pairs of dipoles bind to form dimers. Close to this phase transition a roton-maxon like excitation spectrum is observed.     

A Slab of Liquid Helium-4 with Two Free Surfaces

We describe a method of creating a freely suspended ‘slab’ of superfluid helium-II by using a dense array of 51 µm diameter parallel cylindrical holes in a glass disc of 190 µm thickness. By adjusting the chemical potential in the cell, the holes could be made to fill with liquid, and the surface-curvature controlled. We have measured the transmission of atom beams, generated by a thin-film heater and detected with a sensitive bolometer, through this slab at low temperature. The resugts show that R ⁺ rotons can undergo total internal reflection at the free liquid surfaces and confirm that the dominant transmission channel is atom-R ⁺ roton-atom with a maximum probability p～0.15 for 5 K atoms.     

Nucleation of Bubbles by Electrons in Liquid Helium-4

We report on experiments in which we study cavitation resulting from electrons in liquid helium. Electrons are introduced into the liquid by a radioactive source. After an electron comes to rest in the liquid, it forces open a small cavity referred to as an electron bubble. To study cavitation, a sound pulse is generated by means of a hemispherical piezoelectric transducer producing a large-amplitude pressure oscillation at the acoustic focus. If an electron is in the vicinity of the focus and the negative-going pressure swing exceeds a critical value, a cavitation bubble is produced which can be detected by light scattering. Two distinct critical pressures \( P_{\text{el}} \) and \( P_{\text{rare}} \) have been measured. The first corresponds to cavitation resulting from the application of a reduced pressure to liquid containing an electron which has already formed an electron bubble. The second is the critical pressure needed to lead to cavitation when an electron enters the liquid at a time and place where there is already a reduced pressure. We have measured these two pressures as a function of temperature and consider possible explanations for the difference between them. In addition to these clearly seen cavitation thresholds, there are some cavitation events that have been detected with a threshold that is at an even smaller negative pressure than \( P_{\text{el}} \) and \( P_{\text{rare}} \).     

Equation of State for Normal Liquid Helium-3 from 0.1 to 3.3157 K

An equation of state for normal liquid ³He has been constructed in the form of Helmholtz free energy as a function of independent parameters—temperature, and density. The equation was fitted simultaneously to the collected experimental p-ρ-T, specific heat, sound velocity, isobaric expansion coefficient and isothermal compressibility coefficient from the world’s literature to accuracies comparable with reasonable experimental errors in the measured quantities. Extensive comparisons between the equation of state and experimental data have been made by a set of deviation plots. The state equation is valid in the region for temperatures from 0.1 K to T _c = 3.3157 K, and for pressures from vapor pressures to melting pressures.     

A Study of the Motion of Particles in Superfluid Helium-4 and Interactions with Vortices

We describe experiments in which a sound pulse is used to inject small particles into liquid helium at temperatures between 1 and 3 K. The motion of these particles is then observed. We are able to observe the effect of applying an impulse to a particle that is attached to a vortex. We also report on a number of surprising motions of particles and discuss possible explanations.     

Generation of Effective Field Gradient and Spin Current by a Flow of Liquid Helium-3

Journal of Low Temperature Physics - The coupling of spin angular momentum and mechanical rotation is utilized for generating a spin current of nuclear spins of the liquid helium-3. A hydrodynamic...     

Probing Silica Aerogel Structure with Spin-Polarised Helium-3 Gas

We report on systematic low-field NMR measurements on room temperature ³ He gas imbibed into a 98% porous silica aerogel. Laser polarised helium is used to obtain large NMR signals at 0.1 T even for small amounts of gas. We use a multi-echo technique in a controlled magnetic field gradient to characterize spin diffusion. Measurements are carried out for pressures P ranging from 10 mbar to 1 bar (mean free path of free ³ He ranging from 19 to 0.19 m respectively). In a free gas, the damping rate 1/T ₂ of the amplitude of the echoes is checked to be proportional to the known diffusion coefficient (T ₂ P). Damping rates 2–5 times smaller than those in a free gas are measured within the aerogel. This reduction is quite significant at 1 bar, suggesting that ³ He- ³ He and ³ He-silica collisions occur with comparable frequencies at this density. However, a stong dependence of T ₂ on P is still observed at low pressure, revealing a significant reduction of diffusion due to binary atomic collisions even for the lowest gas density. Measurements over such a large range of gas densities indirectely probe collisional effects over a wide range of sizes up to tens of m. In our sample, results are not consistent with a single geometrical mean free path for ³ He-silica collisions.     

Ultra-Low Temperature Magnetic Properties of Liquid 3He Films

We report measurements of the nuclear magnetization of submonolayer liquid ³ He films adsorbed on a graphite substrate (Papyex) preplated by a monolayer of ⁴ He. In the submilliKelvin temperature range we observe a substantial enhancement of the nuclear magnetization with respect to the degenerate Fermi Liquid value. The unusual temperature dependence of this new contribution to the liquid ³ He film magnetization agrees well with that expected from the theory of weak disorder in two-dimensional (2D) correlated Fermion systems. The effects of disorder and reduced dimensionality suppress the superfluid transition at least to below 180 K.     

Low Temperature Heat-Capacities of Liquid 3He Thin Films

We have measured the heat capacity of ³ He thin films adsorbed on graphite at an areal density of 15.0 nm ^–2 down to as low as 100 K. The second-layer ³ He behaves as a degenerate 2D Fermi fluid in the whole temperature range we studied. We observed no anomalous behavior in the heat capacity near 3 mK in contradiction to the recent report by other workers. This indicates that possible superfluid transitions would be below 100 K. Instead, a small and temperature-independent contribution to the heat capacity was observed, which we attribute to nuclear-spin degrees of freedom in glassy solid ³ He trapped in substrate heterogeneities.     

Sound Attenuation in U2D2 Solid Helium-3

We calculated the sound attenuation in the U2D2 phase of solid ³He using methods similar to those developed by Khalatnikov et al. ¹ Anisotropy of the crystal is also taken into consideration and the kinetic equation and the Bogtzmann equation are expanded with the strain tensor. We have considered the collision integrals for the scattering of magnons by magnons and the sound attenuation resugting from this process is evaluated. The resugts are compared with the measurements performed by Yamaguchi et al. ²     

Correlating the Dipolar Interactions Induced Magneto-Viscoelasticity and Thermal Conductivity Enhancements in Nanomagnetic Fluids

The effective thermal management of electronic system holds the key to maximize their performance. The recent miniaturization trends require a cooling system with high heat flux capacity, localized cooling, and active control. Nanomagnetic fluids (NMFs) based cooling systems have the ability to meet the current demand of the cooling system for the miniaturized electronic system. However, the thermal characteristics of NMFs have a long way to go before the internal mechanisms are well understood. This review mainly focuses on the three aspects to establish a correlation between the thermal and rheological properties of the NMFs. First, the background, stability, and factors affecting the properties of the NMFs are discussed. Second, the ferrohydrodynamic equations are introduced for the NMFs to explain the rheological behavior and relaxation mechanism. Finally, different theoretical and experimental models are summarized that explain the thermal characteristics of the NMFs. Thermal characteristics of the NMFs are significantly affected by the morphology and composition of the magnetic nanoparticles (MNPs) in NMFs as well as the type of carrier liquids and surface functionalization that also influences the rheological properties. Thus, understanding the correlation between the thermal characteristics of the NMFs and rheological properties helps develop cooling systems with improved performance.     

Hydrodynamics in Falling Liquid Films on Surfaces with Complex Geometry

The paper present the results of experimental studies on dynamics of the film flow of cryogenic liquid over the complex surfaces: vertical plane and ribbed aluminum plates with horizontal wave microtexture. Experimental results were obtained at the flow of liquid nitrogen. According to high-speed video recording, in case of horizontal microstructure the wave dynamics influences considerably liquid redistribution in transverse direction. This is caused by the fact that liquid flows intensively at propagation of secondary waves along the channels of horizontal microstructure. According to results of experiments on the ribbed structured perforated aluminum surfaces with the horizontal microstructure with the use of highly wetting low-viscous liquids, the film flow character depends significantly on the irrigation rate. The direction and parameters of microstructure, position and diameter of holes effect significantly the zones of liquid spreading and its distribution over the surface of the ribbed structured plates, especially, within the range of low liquid flow rates.     

Dynamical Response of Liquid 3He Films Adsorbed on Solid Substrates

Within the general framework of linear response theory, we investigate the excitation spectrum of ³He films adsorbed on substrates. Starting from the quasiparticle spectrum and the effective interaction derived in a finite range density functional scheme, we analyze the properties of the particle-hole (ph) excitations as well as the collective modes in the Random Phase Approximation. On the one hand, the most important features of the ph spectrum, originated in the band structure of the single particle spectrum together with the fermionic character of the quasiparticles, are clearly exhibited when the free response to a probe parallel or transverse to the film is considered. On the other hand, for any film, the collective spectrum presents various branches which, in many situations, can be associated with predominantly intraband longitudinal transitions or with almost pure interband transitions. Analyzing the RPA response of a monolayer on substrates of very different adsorbing power (cesium and graphite) we can interpret the nature of the existing collective excitations and determine the influence of the geometrical features of the film on the effective quasiparticle interaction responsible of these oscillations.     

Constant-Volume Gas Thermometry with Different Helium-3 Gas Densities at NMIJ/AIST

Constant-volume gas thermometer (CVGT) measurements are conducted using ³He of three different densities as the working gas to obtain the thermodynamic temperature T _CVGT and the second virial coefficient of ³He, B, at temperatures down to 3 K, using the triple point of Ne as a reference temperature. Densities of 127 mol ?? m^?3 and 278 mol ?? m^?3 are used in addition to the density of 168 mol ?? m^?3 used in the measurement reported previously, where T _CVGT was obtained using the virial coefficient adopted by the International Temperature Scale of 1990 (ITS-90), B _ITS-90. T _CVGT is obtained by two methods, by the single- and multi-isotherm fitting of B to the three densities and by the method used in the previous work using one of the three densities and B _ITS-90. B obtained from the isotherm fitting agrees with B _ITS-90 within the uncertainty of the data used to derive B _ITS-90. Moreover, B obtained from a multi-isotherm fit agrees with that of recent theoretical ab initio calculations within 0.05?cm³ ?? mol^?1 at 5 K and above, and within 0.3?cm³ ?? mol^?1 down to 3 K. The values of T _CVGT obtained from the multi-isotherm fits assuming different forms for the temperature dependence of B agree with each other within 0.1?mK. T _CVGT obtained from the multi-isotherm fitting agrees with that obtained from the method in the previous report within 0.22?mK. The tendency of the difference between T _CVGT and the ITS-90 temperature reported in the previous work is confirmed in the present work.