2D SUPERFULID HELIUM

Research Overview

LIST OF RESEARCHERS

Dr. Oleksiy Zadorozhko (12-16), Dr. Jaewon Choi (16-17)

TWO-DIMENSIONAL SUPERFLUID

Helium adsorbed on graphite provides an excellent realization of a number of nearly two-dimensional (2D) phenomena. The helium film grows layer by layer as the density of the ascorbate increases. The interaction potential between the helium atoms and graphite is much stronger than the He–He interaction. Due to this potential the first layer of helium on graphite is solid. The phase diagram of first layer is well known and has the variety of phases. Experiments revealed behavior corresponding to 2D gas, 2D liquid, commensurate and incommensurate phases.

TORSIONAL OSCILLATOR STUDY ON 2D SUPERFLUID

The phase diagram of second layer is not so clear. The interaction potential between the helium atoms and graphite is weaker comparing to the first layer, but the second layer remains solid. Experiments revealed that there is the drop of period of torsional oscillator at some densities of 2nd layer of solid helium film. It can be explained as an existence of zero point vacancy at commensurate phase or a superfluid. Main point in the studying helium adsorbed on graphite is the quality of substrate. For example, the surface homogeneities in Grafoil (size of crystallite ≈ 200A) substrates induces the broadening of the heat capacity near the density of the √3*√3. At the same time the heat capacity of helium films adsorbed on the ZYX (size of crystallite ≈ 1800A) substrate shows the much sharper transition. Therefore, in order to solve the problem of unusual drop of period of torsional oscillator in 2nd layer of helium film we use torsional oscillators with graphite substrates of different quality.

- Researcher: Dr. Oleisiy Zadorozhko