Low Temperature Physics Laboratory

Outline

We study prominent quantum phenomena in condensed matter over a wide temperature range, including the micro-Kelvin region. Work we have carried out inched an experiment on surface phenomena of superfluid 3He, study of nonlinear transport of ion pool trapped under the surface of superfluid 4He under rotation, torsional oscillator measurement in the context of a supersolid, under rotation, and millimeter-wave absorption-induced resistance of surface state electrons on liquid He under strong excitation limits, in which distortion of resonance curves and hysteretic behavior were observed. The zero resistance state, as discussed below, will also be intensively studied. We have fabricated a vertical double quantum dot with a different g-factor based on a GaAs semiconductor and studied transport properties under high magnetic fields. We are also studying interactions between electron spin, nuclear spin and nuclear spin polarization in the double quantum dot and in quantum Hall effect breakdown regime. We are preparing to measure such phenomena at ultraslow temperatures of such phenomena and investigating new nano-structures, employing capillary condensed He.

Recent Research Topic

Novel magnetoresistance oscillation and zero resistance state of 2D electrons on a He surface

Fig. 1 Energy spectrum of two-dimensional surface state electrons on a liquid helium surface

Relaxation takes place at the rate 1/τ, due to scattering after the electron is excited by millimeter waves at the rate r. Px and Py are electron momenta in the direction parallel to the liquid surface.

Fig. 2

Two-dimensional electrons on a liquid He surface under a magnetic field perpendicular to the electron sheet show magnetoresistance oscillation. Eventually the resistance reaches zero in certain regions.

When an electron approaches a free surface of liquid helium, the electron is trapped in a surface state in which the motion perpendicular to the surface is quantized, whereas the motion parallel to the surface remains free. This brings about the formation of a two-dimensional electron system with a subband energy spectrum structure. Since the electron exists mostly in a vacuum, its effective mass is very close to the bare electron mass regardless of the subband index. Fig. 1 schematically shows the dynamics of millimeter wave absorption and relaxation of surface state electrons. The millimeter wave is absorbed by surface state electrons with a rate r, to excite electrons from the ground to the first excited subband, after which electrons are scattered by He atoms or surface capillary waves, or ripplons, of liquid He with a rate of 1/τ back into the ground subband, preserving electron energy. When the excited electron returns to the ground subband, it obtains large kinetic energy parallel to the surface. Because of a strong electron-electron interaction, this kinetic energy is quickly redistributed in the whole electron system. In comparison with the rate of energy absorption from millimeter waves, energy relaxation from the electron system to the environment is small, and a hot-electron state can be achieved. The transport properties under this condition are strongly influenced by intersubband scattering, thus the resistance of 2D electrons gives a good measure of electron temperature, effectively providing an electron thermometer. We then observed a shift of resonance frequency as a function of the electron population of excited states. This shift, ascribed to electron-electron interaction, results in a strong nonlinear behavior, that is the resistance shows hysteretic behavior with respect to the direction of frequency sweep. This is a typical optical bistability. This method of electron thermometry, by measuring the resistance, will be applied to study quantum dynamics of strongly interacting electron excitation, by combining the method with pulse irradiation of millimeter waves.

Application of magnetic fields perpendicular to the electron sheet results in further quantization of electronic motion parallel to the surface to form Landau levels so that strong dependence of relaxation process on the magnetic field emerges. This may produce oscillations in resistance as a function of the magnetic field, which is something we have observed for the first time. By increasing millimeter wave power at low temperatures where mobility becomes high, we observed “zero-resistance” states, as shown in Fig. 2. The mechanism responsible for the creation of this zero-resistance state will hopefully be elucidated in future investigations. The effect is probably closely associated with a strongly non-equilibrium condition, thus a unique investigation carried out employing an extraordinary clean system, like electrons on liquid helium, may open a new door into the field.