Associate Professor, Department of Physics, Keio University
Superfluid helium is the purest substance in nature. The free surface of superfluid helium has been of great interest as it is recognized as a surface (boundary) of topological super fluids. My research work concerning the Sir Martin Wood Prize was the first experimental study of the free surface of liquid helium 3, a fermion isotope of helium, in particular in the superfluid states.
One can trap many electrons on a free surface of liquid helium. The electrons form a two-dimensional system and undergo a phase transition to the Wigner crystal with a triangular lattice. Since electrons on helium surface was the only system that realizes the Wigner crystal state, the interest of community concentrated to understand its properties. With Dr. Kimitoshi Kono at ISSP The University of Tokyo, I explored the possibility of utilizing the crystal for the study of helium surface. This idea was successfully proven by a series of experiments. Our early transport measurement of crystal on a liquid helium 3 surface showed extremely high electron mobility (μ ~ 108cm2/Vsec), which is still the record of electron mobility in condensed matters. This result not only showed the ultra-clean nature of helium surface, but also opened the possibility of utilizing surface electrons for quantum computing.
Fig.1) An illustration of the Wigner solid on a liquid helium surface. Electrons (shown as yellow balls) form a triangular lattice and press the helium surface. The "dimples" formed beneath each electron produce unique dynamical phenomena.
We succeeded to measure the mobility of the Wigner crystal on the surface of superfluid states of helium 3 down to 200 μK. It was unexpectedly found that the mobility is dominated by scattering of helium quasiparticles to the periodically deformed surface. Since the quasiparticle spectrum is determined by superfluid energy gap, we can study various gap structures of the superfluid A, B, A1 and possible new phases and change in gap structures near the surface. The first order transition between the A and B phases and a nonlinear transport caused by deformation of the surface were also clearly observed.