N. P. Mikhin

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.

Fast Diffusion Process in Quenched hcp Dilute Solid 3He–4He Mixture

The study of phase structure of dilute 3He–4He solid mixtures of differing quality is performed by the spin echo NMR technique. The diffusion coefficient is determined for each coexistent phase. Two diffusion processes are observed in rapidly quenched (non-equilibrium) hcp samples: the first process has a diffusion coefficient corresponding to hcp phase and the second one has huge diffusion coefficient corresponding to liquid phase, which is evidence of liquid-like inclusions formation during fast crystal growing. It is established that these inclusions disappear in equilibrium crystals after careful annealing.

Contribution of phonon and vacancion excitations to the thermodynamic properties of solid helium

Precision measurements of the temperature dependence of the pressure are made on high-quality crystals of He4 and He3–He4 solutions grown at a constant volume. The phonon and vacancion contributions to the pressure are separated on the basis of the Debye model for the phonons and the model of wide-band vacancion excitations. This approach is also used to analyze all the other available thermodynamic data for the solid pure isotopes of helium and their solutions. This yields information about the Debye temperature and vacancy activation energy, and a universal dependence of these parameters on the molar volume is found for He3, He4, and He3–He4 solutions. The values found for the corresponding Gruneisen parameters turn out to be independent of the molar volume.

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...

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 interpretat...

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.

Effect of Crystal Growth Rate on Liquid-like Droplets Formation in the hcp Solid Helium

The samples of hcp solid helium (1% 3He in 4He) are studied by NMR technique. The samples are grown by the blocking capillary method at different growth rates (about 8, 2, and 0.08 mK/s). The NMR technique is used for phase identification by measurements of the diffusion coefficient D and the spin-spin relaxation time T2 at temperatures of 1.3–2.0 K and pressures of 34–36 bar. Along with D and T2 for the hcp phase, we simultaneously observe the D and T2 typical for a liquid at growth rates 8 and 2 mK/s. It means that liquid-like inclusions are quenched from the melting curve during fast crystallization of the samples. It is also shown that the slower growth rate corresponds to a smaller size of liquid-like droplets. It results from lower spatially restricted values of D and, finally, absence of these inclusions at the longest crystallization times. The diffusion coefficient measured for liquid-like droplets is also decreasing during the NMR experiment at constant temperature which indicates the reduction of the size of these droplets. Liquid-like droplets are shown to disappear after sample annealing near the melting curve.

NMR Study of Disordered Inclusions in the Quenched Solid Helium

Phase structure of rapidly quenched solid helium samples is studied by the NMR technique on dilute 3He–4He mixtures. The pulse NMR method is used for measurements of spin–spin T2 relaxation time and spin diffusion coefficient D for all coexisting phases. It was found that quenched samples are two-phase systems consisting of the hcp matrix and some inclusions which are characterized by D and T2 values close to those in liquid phase. Such liquid-like inclusions undergo a spontaneous transition to a new state with anomalously short T2 times. It is found that inclusions observed in both the states disappear on careful annealing near the melting curve. It is assumed that the liquid-like inclusions transform into a new state—a glass or a crystal with a large number of dislocations. These disordered inclusions may be responsible for the anomalous phenomena observed in supersolid region.

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.