Department of Physics of Quantum Fluids and Crystals B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, Kharkov, Ukraine
13/06/2018/ at 15:00h
IF - predavaonica u zgradi Mladen Paić
Solid helium, as representative of atomic crystals, is the unique modeling object for the investigation of various solid state phase transformations, as well as a wide range of kinetic properties of stable and metastable condensed phases. The advantage of helium is that the condensed phases of helium can be obtained in a controlled manner in a very pure form without impurities, and their properties can easily be changed by the external factors – temperature, pressure, electric or magnetic field. Moreover, the condensed phases of helium are representatives of quantum systems in which the quantum effects such as superfluidity and quantum diffusion are observed.
New data on the properties and conditions of the formation of the metastable disordered phases of helium, crystals growth and annealing and on the processes of diffusion and magnetic relaxation in condensed phases of helium may be used in scientific researches in physics of multicomponent (dispersed) systems and colloidal chemistry. Our research is devoted to the experimental studies of the kinetic properties of the metastable disordered phases of helium and the conditions of their formation.
By precision pressure measurement technique the kinetics of bcc-hcp phase transition in solid 4He and solid mixture of 3He in 4He was studied resulting in the phase diagram. The new kinetic phenomenon was observed – hysteresis of the phase diagram of the weak solid 3He-4He mixture. It was shown that the hysteresis is a result of the influence of 3He impurities on the nature of the bcc-hcp phase transition.
The kinetics of nucleation under bcc-hcp structural phase transition in solid helium was also studied. It was observed experimentally that in pure solid 4He both homogeneous and heterogeneous nucleation mechanisms are realized, whereas in weak solid 3He – 4He mixture the heterogeneous nucleation regime dominates. For the first time the quantitative description of the nucleation mechanisms in the bcc-hcp phase transition in solid helium was given. The activation energy for homogeneous and heterogeneous nucleation was obtained and it was shown that its frequency is sensitive to the helium crystals quality.
By pulsed NMR technique we detected the presence of metastable liquid inclusions in the quenched hcp 1% 3Не-4Не crystals which were grown by blocking capillary technique. The most intriguing results are the new original NMR data about the possibility of evolution of the metastable liquid inclusions in the hcp matrix of the solid mixture 1% 3Не-4Не in the disordered solid state, which is accompanied by a significant decrease of the speed of spin-spin relaxation and the diffusion coefficient. The diffusion coefficient of 3He in the liquid inclusions, as they evolve, was measured by a spin echo technique with two probe pulses. It was found that during the evolution, 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.
The temperature dependence of nuclear magnetic susceptibility of 3He adsorbed in 2.5 nm 1D channels (300 nm length) of the MCM-41 (Mobile Crystalline Material-41) powder has been studied by pulse NMR method (9.15 MHz). Using the difference of both spin-lattice T1 and spin-spin T2 relaxation times in both “solid” and “fluid” “phases” of adsorbed 3He, we separated the contribution of each phase to the amplitude of the NMR echo signal. It was shown that magnetic susceptibility of any 3He phase follows the Curie law, so the temperature in 1.5 monolayers of the condensed helium-3 in MCM-41 corresponds to the measured temperature of the cell.