A. V. Polev

Observation of anomalously fast diffusion in 3He–4He solid solutions near the BCC-HCP transition

The pulsed NMR technique was used to investigate diffusion on the BCC-HCP phase-equilibrium and melting curves of a dilute solution of 3He in 4He. The contributions from all coexisting phases were identified using the spin-echo method. It is established that, along with the contributions from the equilibrium BCC and HCP phases or from bulk liquid (in the melting curve measurements), there is an additional diffusional process that is characterized by an anomalously high diffusion coefficient. It is found that the latter is close to the diffusion coefficient in liquid helium, while the diffusion itself is spatially restricted. The observed effect may be caused by the formation of liquid droplets in the course of the BCC-HCP transition.

Kinetics of the bcc–hcp transition in He4 off the melting curve

The kinetics of the bcc–hcp structural phase transition in He4 is investigated by the method of precision barometry in the pressure range 25–31 bar and temperature range 1.25–1.90K. Under constant-volume conditions the kinetics of the pressure and temperature variations is recorded in the range of molar volumes Vm=20.85–21.10cm3∕mole. It is found that the process of cooling is accompanied by an unusual two-stage relaxation of the pressure: initially an exponential decrease of the pressure occurs due to thermal contraction of the supercooled bcc phase, and then the structural transition itself occurs very rapidly. The latter is accompanied by a pronounced thermal effect due to the release of the heat of the phase transition. It is shown that the inverse, hcp–bcc phase transition occurs in one stage (without a delay) and is accompanied by absorption of the heat of transition. Experimental data on the variation of the pressure are obtained in the bcc and hcp single-phase regions and along the bcc–hcp phase e...

Spin–lattice relaxation in phase-separated 3He–4He solid solution

The spin–lattice relaxation time in a 3He–4He solid solution with the initial concentration 3.18%3He is measured during phase separation by using the pulsed NMR technique. The relaxation time in a concentrated bbc phase formed as a result of phase separation is found to be independent of temperature over the entire range of its existence and is determined by the Zeeman-exchange interaction mechanism. In the dilute hcp daughter phase, the spin–lattice relaxation time increases on cooling according to the law T1∼x−n, where n=0.88±0.12, and x is the 3He concentration. The values of T1 in this phase coincide with the values corresponding to a homogeneous (nonseparated) solution of the same concentration.

Giant asymmetry of the processes of separation and homogenization of 3He–4He solid mixtures

A comparison of the kinetics of the separation processes and homogenization of 3He–4He solid mixtures is made with the use of precision barometry for samples of three types—dilute mixtures of 3He in 4He and of 4He in 3He and concentrated 3He–4He mixtures. It is found that in all types of mixtures studied the rate of the initial stage of homogenization can exceed the rate of separation by more than 500 times. An appreciable rate of phase separation in the concentrated mixtures, where, according to existing ideas, the impurity atoms in quantum crystals should be localized, attests to a new, unknown mechanism of mass transfer under those conditions, while the fast homogenization indicates that this process is nondiffusional in nature.

Spin Diffusion Processes in Solid 3He-4He Mixtures Near the BCC-HCP Phase Transition at the Melting Curve

The spin diffusion coefficient of 1% 3He in solid of 4He has been measured in the vicinity of the BCC-HCP phase transition at the melting curve by pulsed NMR. The applied spin echo technique does allow to distinguish the contributions from all of coexisting phases. In addition to well-known diffusion in BCC, HCP, and bulk liquid phases the new fast diffusion process is observed and the diffusion coefficient of this process is shown to be close to that in liquid mixture being dependent on the time between the NMR pulses (bounded diffusion). The possible reason for the effect may be connected with formation of liquid droplets during the BCC-HCP transition.

Thermomagnetic relaxation in two-phase solid 3He–4He mixtures at ultralow temperatures

NMR investigations of restoration of longitudinal equilibrium magnetization in phase-separated solid 3He–4He mixtures are carried out in the temperature range 1–200 mK. It is found that below 100 mK, the results depend on the energy of tipping NMR pulses, while at the lowest temperatures the restoration of magnetization becomes nonmonotonic. The obtained results are explained on the basis of a proposed model in which both magnetic (spin-lattice) and thermal relaxation are assumed to take place between the Zeeman system and the lattice.

Spin–lattice relaxation in the bcc phase of phase-separated 3He–4He solid mixtures

The spin–lattice relaxation time in two samples of 3He–4He solid mixtures with initial concentrations of 0.5% 3He in 4He and 0.5% 4He in 3He is measured by the pulsed NMR method. As a result of phase separation, in both cases two-phase crystals form, having the same helium concentration in the concentrated bcc phase. However, in the first sample the bcc phase forms as small inclusions in an hcp matrix, while in the second sample the bcc phase is the matrix. It is established that in the second case the spin–lattice relaxation occurs in the same way as in pure bulk 3He, while in the first case one observes anomalous behavior of the spin–lattice relaxation time at low temperatures. Experiments have shown that this anomaly is due not to the possible influence of the small 4He impurity but to the small dimensions of the inclusions of the of the bcc phase. In this case the main contribution to the relaxation is apparently due to defects formed at the boundaries of the bcc inclusions and the hcp matrix.

Nuclear spin–spin relaxation in 3He–4He two-phase solid solutions at ultralow temperatures

The spin–spin relaxation time in a 3He–4He solid solution is measured before and after phase separation in the temperature range 1–250 mK. The spin echo technique is used, which permits separating the contributions of the two separated phases to the magnetic relaxation. It is found that in the concentrated phase the spin–spin relaxation time is practically independent of temperature above 50 mK and is described by the same exchange mechanisms as in pure 3He. In the dilute phase the relaxation time is inversely proportional to the concentration and agrees with the corresponding values for homogeneous solutions. The dominant contribution to the spin–spin relaxation process is from 3He–4He tunneling exchange. At the lowest temperatures the spin echo exhibits anomalous behavior, which may be a manifestation of quasi-one-dimensional diffusion.

Kinetics of Phase Transition at the BCC‐HCP‐Liquid Triple Points of 4He

The kinetics of BCC‐HCP phase transition along the 4He melting curve is studied by precise pressure measurements in the temperature range of 1.25–2.0 K in the vicinity of the triple points (BCC‐HCP‐He II and BCC‐HCP‐He I). It is found that the time dependence of the pressure change along the melting curve far from the triple point can be fitted to an exponential. During the crossing of the triple points, an extra contribution to the kinetics of pressure is found in addition to the exponential one. These anomalies can be explained by assumption a re‐melting of the crystal during the structural phase transition.

Crystallization thermometer for ultralow temperatures with a cooled FET oscillator

The existing methods of measuring pressure in a crystallization thermometer used for measuring ultralow temperatures are analyzed. It is shown that a method based on measuring the resonance frequency of a resonance circuit, which includes a capacitive pressure gauge, can be used to increase measurement sensitivity and accuracy. A low-temperature FET oscillator is described. This oscillator makes it possible to increase the sensitivity and accuracy of temperature measurements in the range 0.9 mK–1 K by more than an order of magnitude.