Pure 4He Research Articles

The geyser effect in the vacuum expansion of solid 3He0.544He0.46 and the determination of the Poisson ratio

Periodic bursts, also called the geyser effect, previously observed in the vacuum expansion of pure and <1% 3He-doped solid 4He, are shown to occur also in solid 3He–4He alloys. Results are reported for 3He0.544He0.46 for the temperature range of 1.73–2.13 K and pressures up to 92 bar, and then compared with the results for pure solid 4He. The geyser period is found to increase with pressure much faster in the mixed solid than in pure solid 4He. A direct measurement of the internal pressure exerted on the lateral walls of the source chamber provides the first information on the Poisson ratio of polycrystalline pure and mixed solids.

Modification of theHe3Phase Diagram by Anisotropic Disorder

Motivated by the recent prediction that uniaxially compressed aerogel can stabilize the anisotropic A phase over the isotropic B phase, we measure the pressure dependent superfluid fraction of (3)He entrained in 10% axially compressed, 98% porous aerogel. We observe that a broad region of the temperature-pressure phase diagram is occupied by the metastable A phase. The reappearance of the A phase on warming from the B phase, before superfluidity is extinguished at T(c), is in contrast to its absence in uncompressed aerogel. The phase diagram is modified from that of pure (3)He, with the disappearance of the polycritical point (PCP) and the appearance of a region of A phase extending below the PCP of bulk (3)He, even in zero applied magnetic field. The expected alignment of the A phase texture by compression is not observed.

Acoustic Resonances and Non-linearity in Solid 4He

Direct measurements of the low frequency shear modulus of solid 4He show a remarkable increase below 150 mK, a regime where torsional oscillator (TO) experiments show evidence of mass decoupling. Acoustic resonance measurements at higher frequencies confirm the unusual elastic behavior. A striking feature of both TO and shear modulus measurements is their amplitude dependence—the low temperature anomalies are reduced as the drive amplitudes increase. We have studied the amplitude dependence of acoustic resonances in both standard and isotopically pure 4He crystals. The resonance peaks shift to lower frequencies as the drive amplitude increases. At high amplitudes the peaks are asymmetric and exhibit both bistability and hysteresis—classic features of nonlinear oscillators. At the lowest amplitudes the peak frequency and shape are independent of drive amplitude and are non-hysteretic. The threshold for nonlinear behavior is lower for the isotopically pure crystal. We also studied the effects of annealing on the acoustic resonances. The nonlinear behavior was qualitatively unchanged but annealing often affected the shape of the resonance peaks, which often had a complicated structure at low temperatures.

Studies on Helium Liquids by Vibrating Wires and Quartz Tuning Forks

We present results of low-temperature experiments on dilute mixtures of 3He in 4He and on pure 3He, obtained by means of two kinds of mechanical oscillators immersed in the liquid sample: vibrating wires and quartz tuning forks. The helium sample was cooled either by adiabatic demagnetization of an immersed copper nuclear stage or by adiabatic melting of 4He in superfluid 3He. The measured effect of the surrounding fluid on the mechanical resonance of the oscillators is compared with existing theories. We also discuss resonances of second sound and the state of supersaturation, both observed by a tuning fork in helium mixtures.

An accurate optical technique for measuring the nuclear polarisation of 3He gas

In the metastability exchange optical pumping cells of our on-site production unit and of our other experimental set-ups, we use a light absorption technique to measure the 3He nuclear polarisation. It involves weak probe beams at 1083 nm, that are either perpendicular or parallel to the magnetic field and cell axis, with suitable light polarisations. When metastability exchange collisions control the populations of the sublevels in the 23S state, absolute values of the 3He ground state nuclear polarisation are directly inferred from the ratio of the absorption rates measured for these probe beams. Our report focuses on the transverse detection scheme for which this ratio, measured at low magnetic field for σ and π light polarisations, hardly depends on gas pressure or the presence of an intense pump beam. This technique has been systematically tested both in pure 3He and isotopic mixtures and it is routinely used for accurate control of the optical pumping efficiency as well as for calibration of the NMR system.

Ab initiolow-energy dynamics of superfluid and solidH4e

We have extracted information about real time dynamics of 4He systems from noisy imaginary time correlation functions f(tau) computed via Quantum Monte Carlo (QMC): production and falsification of model spectral functions s(omega) are obtained via a survival-to-compatibility with f(tau) evolutionary process, based on Genetic Algorithms. Statistical uncertainty in f(tau) is promoted to be an asset via a sampling of equivalent f(tau) within the noise, which give rise to independent evolutionary processes. In the case of pure superfluid 4He we have recovered from exact QMC simulations sharp quasi-particle excitations with spectral functions displaying also the multiphonon branch. As further applications, we have studied the impuriton branch of one 3He atom in liquid 4He and the vacancy-wave excitations in hcp solid 4He finding a novel roton like feature.

Low-temperature instability of uniform spin precession in the B phase of pure 3He and 3He in an aerogel

The magnetic-field dependences of the threshold temperature of the low-temperature instability of uniform spin precession in pure 3He-B and 3He-B in an aerogel have been determined for the bulk mechanism. These dependences appear to be different. The theoretical dependence of the threshold temperature for the pure case has been compared with the experimental dependence. The threshold temperature of the instability for 3He in the aerogel has been estimated for typical experimental conditions.

Thermodynamic Properties of Liquid 3He-4He Mixtures Between 0–10 bar below 1.5 K

The thermodynamic properties of liquid 3He-4He mixtures at pressures of up to 10 bar and temperatures below 1.5 K are determined. The calculations are based on previously determined thermodynamic properties of 3He-4He mixtures at saturated (zero) pressure, and available experimental measurements of the molar volume, which are used to determine an expression for the molar volume. Since available experimental data for mixtures at higher pressures are restricted to low temperatures (below about 0.7 K), the calculated molar volumes at high pressure and high temperature are largely based on pure 3He and pure 4He data.

Hysteresis of the bcc–hcp transition in a solid mixture of He3 in He4

Hysteresis of the bcc–hcp transition is observed by precision barometry in samples of a solid mixture of 1% He3 in He4 grown by the capillary blocking method. It is shown that the lines of equilibrium bcc–hcp and hcp–bcc transitions on the P–T diagram on cooling and heating do not coincide (the cooling line corresponds to higher pressure). In the process of thermocycling in the two-phase region the system executes a closed thermodynamic cycle, two branches of which correspond to the “normal” bcc–hcp transformation, with a slope of dP∕dT=6–12bar∕K, and the other two, to an “anomalous” transformation with a slope of dP∕dT=2.5–7bar∕K (the slope increases with decreasing molar volume). This effect is not observed in crystals of pure He4, and it can therefore be attributed to properties of the He3 impurity subsystem. A hierarchy of relaxation times for the pressure in the system is established which indicates that the mechanisms of transformation on different branches of the cycle are different. An interpretation of the observed effects is proposed in the framework of a model of a heterophase polydomain superstructure consisting of second-phase domains localized at He3 impurities in the He4 host matrix.

Superfluid Transition of a 4He Thin Film Pressurized by Bulk Liquid 3He

We measured transverse acoustic impedance Z of normal fluid 3He at 46.6 MHz on a surface coated with a thin 4He film. The real component of the impedance, Z′, in the coated samples deviates from Z′ in the pure 3He in the low temperature region. Z′ on the coated samples is almost identical with Z′ in the pure sample at high temperature and gradually deviates below a particular temperature T onset . T onset is possibly the superfluid onset temperature of the 4He film pressurized by the bulk liquid 3He. The gradual decrease in Z′ means that the superfluid component in 4He film increases gradually, which is expected from the dynamic KT transition at high frequency. The thicker is the film, the higher is the T onset . The range of T onset we observed was between 40 and 160 mK. This is much lower than that at the saturated vapor pressure. Suppression of T onset achieved by the applied pressure from bulk liquid 3He was presumably caused by the dissolved 3He in the film, thickening of the inert layers and/or by the strong correlation effect. The result shows that the specularity of 3He quasiparticle scattering is strongly affected by superfluidity of the 4He film.

Pulsed Nuclear Magnetic Resonance on 3He Adsorbed on Bare and 4He Preplated MCM-41 Using DC SQUID Detection

We report low field DC SQUID NMR measurements down to 1.5 K of 3He adsorbed in the pores of the mesoporous molecular sieve MCM-41. In the first experiment measurements were made on 3He adsorbed onto the bare pore walls of MCM-41 with coverages ranging from $n_{^{3}\mathrm{He}}=0.86n_{1}$ to full pores at $n_{^{3}\mathrm{He}}=1.79n_{1}$ , where n 1 is the coverage for monolayer completion. A second experiment was performed with low 3He coverages ( $n_{^{3}\mathrm{He}}\sim0.01n_{1}$ ) on 4He preplated pores, where a crossover to a quasi-1D state is expected to occur at temperatures sufficiently below 700 mK. In both experiments relaxation times T 1 and T 2 * were measured as a function of temperature and coverage at frequencies from 80 to 240 kHz. The frequency dependence of the linewidth in the pure 3He experiment is extremely weak therefore T 2 * ≈T 2. The 1.5 K isotherm shows a small minimum in T 2 * at a coverage corresponding to monolayer completion. In the experiment with 4He preplating there was no significant change in T 1 or T 2 * when the 3He coverage was doubled from $n_{^{3}\mathrm{He}}=0.01n_{1}$ to 0.02n 1 at a 4He preplating of $n_{^{4}\mathrm{He}}=1.05n_{1}$ . This suggests that the relaxation times are dominated by single particle effects in the low density regime.

Transition to Turbulence and Critical Velocity in Superfluid Solution of 3He in 4He

By the method of oscillating tuning fork, we carried out researches of the transition to turbulence in superfluid solution of 5% 3He in 4He at temperatures of 100 mK–300 mK. The critical velocity υc of the turbulence appearance is determined through measuring the volt-ampere characteristics. It is established that in the mixture the temperature dependence of the critical velocity is non-monotonous and differs strongly from that in pure 4He. Unlike 4He, the step-like anomalies on resonance curves were observed which, presumably, is connected with instability of the vortex system under the conditions where the core of the vortex is filled by the atoms of 3He. It is shown that such anomalies appear at the temperatures below 0.9 K, at the same time at temperatures below ∼0.5 K they appear even at υ<υc .

Decoupling of Confined Normal 3He

Anodic bonding was used to fabricate a 10 mm diameter × 640 nm tall annular geometry suitable for torsion pendulum studies of confined 3He. For pure 3He at saturated vapor pressure the inertia of the confined fluid was seen to be only partially coupled to the pendulum at 160 mK. Below 100 mK the liquid’s inertial contribution was negligible, indicating a complete decoupling of the 3He from the pendulum.

Development of MRI Microscope

We have been developing an ultra high spatial resolution MRI, “MRI Microscope”, especially for 3He physics at ultra low temperature. The ultimate goal of our MRI Microscope is to achieve 1 μm×1 μm two dimensional spatial resolution comparable to optical microscopes. We constructed the MRI Microscope using a magnetic field of 7.2 T, with tri-axial magnetic field gradients of 2.0 T/m. We visualized the pure liquid 3He in a 230 μm diameter tube to study the effect of nonlinearity on the MRI Microscope at low temperature and in high magnetic fields. An MRI image was obtained at 0.22 MPa, 1 K with 1.8 μm×1.8 μm pixel size. At 65 mK, the MRI image became more blurred. We speculate that it was caused by large spin diffusion and nonlinearity.

Improved capacitive melting curve measurements

Sensitivity of the capacitive method for determining the melting pressure of helium can be enhanced by loading the empty side of the capacitor with helium at a pressure nearly equal to that desired to be measured and by using a relatively thin and flexible membrane in between. This way one can achieve a nanobar resolution at the level of 30 bar, which is two orders of magnitude better than that of the best gauges with vacuum reference. This extends the applicability of melting curve thermometry to lower temperatures and would allow detecting tiny anomalies in the melting pressure, which must be associated with any phenomena contributing to the entropy of the liquid or solid phases. We demonstrated this principle in measurements of the crystallization pressure of isotopic helium mixtures at millikelvin temperatures by using partly solid pure 4He as the reference substance providing the best possible universal reference pressure. The achieved sensitivity was good enough for melting curve thermometry on mixtures down to 100 μK. Similar system can be used on pure isotopes by virtue of a blocked capillary giving a stable reference condition with liquid slightly below the melting pressure in the reference volume. This was tested with pure 4He at temperatures 0.08–0.3 K. To avoid spurious heating effects, one must carefully choose and arrange any dielectric materials close to the active capacitor. We observed some 100 pW loading at moderate excitation voltages.

Heat capacity of kagome 3He monolayer film on graphite

The heat capacities of second layer 3He-4He mixture solid films are measured. The total areal densities are adjusted to be the 4/7 phase commensurate to the underlying 4He first layer, and the fractions of 3He in the second layer are adjusted to be almost three-fourths of the amount of second-layer helium atoms. According to the recent theoretical prediction of the adsorption structure of the 4/7 phase by Takagi, 3He atoms in such a 3He-4He mixture film are expected to be a nuclear spin system of the pure kagome lattice. The heat capacity measurements of such 3He-4He films between 0.1 and 80 mK reveal double peak structures. The higher-temperature peaks appear at around 30 mK; these are thought to arise from the replacement of 3He and 4He atoms. The magnitude of the higher-temperature peak strongly supports the adsorption structure proposed by Takagi, and not the one previously proposed by Elser, and gives strong supporting evidence of the realization of a pure kagome 3He lattice. The lower-temperature peaks must arise from the short-range spin ordering of 3He nuclear spins on the kagome lattice. As compared to the heat capacity of the pure 3He in the 4/7 phase, i.e., in the triangular phase, slower short-range ordering is observed in the kagome lattice, which agrees with the theoretical predictions.

Motion of 3He quasiparticles in aerogel driven by fourth sound

While the shear viscous motion of the normal fluid component causes the energy loss of the fourth sound resonance in superfluid 3He confined in narrow pores, it is not yet clear what causes the energy loss of the fourth sound in the aerogel system. We prepared two aerogel samples of 99.0% and 97.5% porosities, which were directly grown in sintered silver sponges and performed the fourth sound resonance experiments. We found that the energy loss of the fourth sound resonance in aerogel is much smaller than that in pure liquid 3He, showing that the mean free path of 3He quasiparticles is strongly suppressed by aerogel strands. Such behavior is explained with the frictional flow. The temperature dependence of the frictional relaxation time will be discussed.

1D Crossover, universality and finite-size scaling of the specific heat

We report measurements of the specific heat of 3He-4He mixtures near the superfluid transition when confined to channels of 1 /tm square cross section. These data test the universality of finite-size scaling as function of 3He concentration for 1D crossover. The analysis of these data requires that data measured at fixed concentration be converted to a specific heat at constant chemical potential difference ϕ = μ3 - μ4. This is carried out according to a procedure performed for planar mixtures by Kimball and Gasparini. We find that, in the most self-consistent analysis of the data, the mixtures define a separate scaling locus from that of pure 4He, both above and below Tλ. An analysis whereby the exponent a is forced to have the same universal value—as opposed to the best-fit value—yields a good collapse of all the data. This is achieved, however, at a cost of self-consistency. These results mirror very closely those obtained for finite-size scaling of confined planar mixtures, i.e. for 2D crossover.

Unusual behavior of 3He-4He mixtures in connected OD boxes

We report on measurements of the specific heat of 3He-4He mixtures confined to boxes connected with fill channels 1 μm wide and 19 nm high. These data were meant as a study of mixtures to test universality of finite-size scaling along the lambda-line. We find instead that the mixtures have very unusual behavior relative to mixtures studied in geometries for 2D and 1D crossover. These latter data can be scaled successfully as long as one does not include data for pure 4He. In contrast, the 0D crossover data do not scale on either side of the bulk transition temperature Tλ and have rather different overall behavior: They show a dramatically larger temperature shift of the specific heat maximum Cmax relative to data for 2D and ID, the value of the specific heat at Tλ is substantially smaller than that observed for the other dimensionalities, and the value of Cmax exceeds the magnitude of the bulk specific heat at the same temperature. This overall behavior is so unusual that it suggests there is a role being played by the connecting channels even though the liquid within them remains in the normal state over the temperature range in which the specific heat is measured.

Impact of ICRH on the measurement of fusion alphas by collective Thomson scattering in ITER

Collective Thomson scattering (CTS) has been proposed for measuring the phase space distributions of confined fast ion populations in ITER plasmas. This study determines the impact of fast ions accelerated by ion cyclotron resonance heating (ICRH) on the ability of CTS to diagnose fusion alphas in ITER. Fast ions with large perpendicular velocities, such as the populations investigated here, can be detected with the ‘enabled’ part of the proposed ITER CTS diagnostic. The investigated ICRH scenarios include pure second harmonic tritium heating and 3He minority heating at a frequency of 50 MHz, corresponding to an off-axis resonance. The sensitivities of the results to the 3He concentration (0.1–4%) and the heating power (20–40 MW) are considered. Fusion born alphas dominate the total CTS signal for large Doppler shifts of the scattered radiation. The tritons generate a negligible fraction of the total fast ion CTS signal in any of these heating scenarios. The minority species 3He, however, contributes more than 10% of the fast ion CTS signal at locations close to the resonance layer for 3He concentrations larger than ∼1%. In this particular region in space for resolution of near perpendicular velocities, it may be difficult to draw conclusions about the physics of alpha particles alone by CTS. With this exception, the CTS diagnostic can reveal the physics of the fusion alphas in ITER even under the presence of fast ions due to ICRH.