A. P. Birchenko

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.

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...

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.

The spin-lattice relaxation in superfluid inclusions formed in the solid helium matrix

Processes of spin-lattice relaxation in superfluid inclusions, formed during growth and rapid cooling of helium crystals, are studied by the pulse nuclear magnetic resonance method. Measurements of spin-lattice relaxation times were carried out using the two spin-echoes method. This allows to obtain reliable data for each of the coexisting phases of a two-phase system HCP matrix—superfluid inclusions. The samples of the solid solution 1% 3He in 4He with molar volume 20.2 cm3/mol are investigated in the temperature range 1.4–1.9 K. It is found that the time of spin-lattice relaxation in superfluid drops differs significantly from that in bulk superfluid. It is shown that in both cases the dominant relaxation mechanism is the wall relaxation, which is a few orders of magnitude faster than the bulk relaxation. However, in the bulk fluid, the velocity of wall relaxation is determined by the time of diffusion of atoms to the walls, while the spin relaxation processes directly on the wall play a major role in t...

Effect of an intermediate bcc phase on the evolution of superfluid inclusions in an hcp 3He–4He matrix

Pulsed NMR is used to study the evolution of liquid inclusions formed in an hcp matrix during rapid cooling of a 3He-4He solution containing 1.05% 3He. The diffusion coefficient of 3He in the liquid inclusions as they evolve is measured by a spin echo technique with two probe pulses. The measurements were made at 1.67 K, which corresponds to the region of the bcc phase in the phase diagram, and at 1.38 K, where the bcc phase is absent. It is found that during the evolution in both cases, the liquid inclusions are smaller than the diffusion length and diffusion is restricted. The measured coefficient of restricted diffusion made it possible to determine the characteristic size of the inclusions. In the first case, during the evolution of the liquid inclusions an intermediate bcc phase in the form of dendrites develops and separates the liquid inclusions into a mass of fine droplets. Because of the rapid growth of the bcc phase, the size of the droplets decreases rapidly and the process ends with the disapp...

Spin Diffusion in 3He, Adsorbed Nanostructured Material MCM-41

The pulsed nuclear magnetic resonance method was used for the first time to study diffusion processes in 3He, adsorbed nanostructured material MCM-41. The material contained bundles of nanotubes with internal cylindrical channels whose diameter was 2.5 nm. The measurements were made using the Hahn–Carr–Purcell spin echo method with a sequence of 90°–τ–180° probing pulses, where τ is the time interval between pulses. Two different diffusion processes were detected in the system, with diffusion coefficients differing by an order of magnitude. This result was confirmed by an independent multi-echo method with a pair of 90°–τ–120° probing pulses, and with a noticeable increase in the duration of the pulses themselves. It was also found that both diffusion coefficients depend on the value of τ, which is a characteristic feature of restricted diffusion. The results obtained set new directions for the further study of this system.

Nuclear magnetization of 3He adsorbed by the nanostructured material MCM-41

The nuclear magnetization of 3He adsorbed by the nanostructured material MCM-41 with a channel size of 2.5 nm was investigated using the pulse nuclear magnetic resonance method. The amplitude of the spin-echo signal was measured in the experiment after the application of two probe pulses to the system. The resulting temperature dependence of the signal magnitude, for various 3He coatings, is described by common dependence corresponding to the Curie law. It is demonstrated that the values of the echo signal amplitude determined in the experiment coincide with the calculated value of the corresponding amplitude due to nuclear magnetization that arises in the system of 3He nuclear spins in an external magnetic field.The nuclear magnetization of 3He adsorbed by the nanostructured material MCM-41 with a channel size of 2.5 nm was investigated using the pulse nuclear magnetic resonance method. The amplitude of the spin-echo signal was measured in the experiment after the application of two probe pulses to the system. The resulting temperature dependence of the signal magnitude, for various 3He coatings, is described by common dependence corresponding to the Curie law. It is demonstrated that the values of the echo signal amplitude determined in the experiment coincide with the calculated value of the corresponding amplitude due to nuclear magnetization that arises in the system of 3He nuclear spins in an external magnetic field.

Pressure relaxation and diffusion of vacancies in rapidly grown helium crystals

An experimental study of the features of pressure relaxation in rapidly grown crystals of a diluted solid solution 3He–4He, at temperatures above 1.3 K, was performed. A cylindrical cell with capacitive pressure sensors at the ends was used for measurements. It was found that, when the helium crystals were grown at cooling rates ≳4 mK/s, the difference in pressure ΔP registered by the sensors at 1.3 K reached 2.4 bars. The ΔP value decreased with subsequent stepwise increase in temperature, but reached zero only after thorough annealing at the premelting temperatures. The kinetics of pressure changes at the sample ends at different temperatures was recorded. The results obtained were interpreted within the framework of the structural relaxation model based on the monovacancy diffusion mechanism. The proposed model made it possible to explain the dependence of ΔP on the time and temperature recorded in the experiment, as well as to determine the activation energy of the structural relaxation process and th...