Kimitoshi Kono

Restitution coefficient in a collision between two spheres

On the basis of elastic theory the restitution coefficient e was derived as a function of the elastic constants, radii, masses and colliding velocities of two spheres by taking the visco-elastic property into consideration. For a coefficient close to one, the value of (1-e) was found to be proportional to (velocity)1/5. This result was compared with a treatment based on the plastic property of solids.

Two-dimensional Coulomb liquids and solids

1 Two-Dimensional Interface Electron Systems.- 2 Strongly Correlated Coulomb Liquid.- 3 Quantum Transport Framework for Highly Correlated Electrons.- 4 Unconventional Hall Effect.- 5 Quantum Cyclotron Resonance.- 6 Interface Polarons.- 7 Wigner Solid. I. Dynamics on Rigid and Soft Interfaces.- 8 Wigner Solid. II. Transport Properties.- References.

Surface study of liquid3He using surface state electrons

We have measured the mobility of surface state electrons (SSE) on liquid3He, μ3, aiming to study the elementary surface excitations of the Fermi liquid. A gradual increase of μ3 below 300 mK is attributed to the scattering of electrons by ripplons. Ripplons do exist in3He down to 100 mK. We observe an abrupt decrease of μ3, due to the transition to the Wigner solid (WS). The dependences of the WS conductivity and mobility on temperature and magnetic field differ from the SSE behavior on liquid4He.

Sliding Wigner solid on liquid 4He

We report a systematic experimental study of the anomalous nonlinear magnetotransport in the Wigner solid (WS) trapped on a liquid4He surface. The ac Corbino conductivity σxx exhibits an abrupt jump at a certain driving voltage. The threshold input voltage Vth for the σxx jump varies as Vth ∝ B−0.8 ω−1 n1.5sE⊥, where B, ω, ns, and E⊥ are magnetic field, frequency, electron density and pressing electric field, respectively. We interpret the σxx jump as the transition between the WS accompanied with the periodic 4He surface deformation and the WS which decouples from the surface, due to the large driving force. A simple model is presented: The rigid-potential sliding model qualitatively explains the above mentioned behaviors of Vth. We have found that the dependences of σxx on B and E⊥ are different from those of the liquid phase. The anomalous behaviors of σxx play a crucial role on the sliding model.

Novel radiation-induced magnetoresistance oscillations in a nondegenerate two-dimensional electron system on liquid helium.

We report the observation of novel magnetoresistance oscillations induced by the resonant intersubband absorption in nondegenerate 2D electrons bound to the surface of liquid 3He. The oscillations are periodic in B-1 and originate from the scattering-mediated transitions of the excited electrons into the Landau levels of the first subband. The structure of the oscillations is affected by the collision broadening of the Landau levels and by many-electron effects.

Evidence of Supersolidity in Rotating Solid Helium

Supersolidity in a Spin Observing superfluid flow in a solid is a counterintuitive finding that has been accomplished by freezing 4He inside a torsional oscillator and monitoring the oscillating period as the temperature is lowered: A reduction in the oscillating period will be observed at the supersolid transition when the mass of the superfluid decouples from the oscillator and the remaining normal component of the solid. However, extraneous classical effects can also cause this reduction, and so, to confirm supersolid formation, Choi et al. (p. 1512, published online 18 November) performed a slightly different measurement. Rotation was superimposed onto the oscillating motion, and the period and the shear modulus of the system were measured simultaneously. These two quantities exhibited very different responses to the rotation speed, suggesting that supersolidity (rather than classical effects that would also affect the shear modulus) is indeed at the root of the previously observed change in the oscillating period. Measurements on rotating frozen helium support the formation of a quantum, or supersolid, phase. Supersolidity, the appearance of zero-viscosity flow in solids, was first indicated in helium-4 torsional oscillator (TO) experiments. In this apparatus, the irrotationality of the superfluid component causes it to decouple from the underlying normal solid, leading to a reduction in the resonant period of the TO. However, the resonant period may be altered for reasons other than supersolidity, such as the temperature dependence of the elastic modulus of solid helium. Superimposing rotation onto oscillatory measurements may distinguish between supersolidity and classical effects. We performed such simultaneous measurements of the TO and the shear modulus, and observed substantial change in the resonant period with rotational speed where the modulus remained unchanged. This contrasting behavior suggests that the decrease in the TO period is a result of supersolidity.

Photon-induced vanishing of magnetoconductance in 2D electrons on liquid helium.

We report on a novel transport phenomenon realized by optical pumping in surface state electrons on helium subjected to perpendicular magnetic fields. The electron dynamics is governed by the photon-induced excitation and scattering-mediated transitions between electric subbands. In a range of magnetic fields, we observe vanishing longitudinal conductivity σ(xx)→0. Our result suggests the existence of radiation-induced zero-resistance states in the nondegenerate 2D electron system.

Transmission Spectra of Third Sound in a Fibonacci Lattice

Transmission spectra were experimentally studied in third sound that was propagated through a Fibonacci lattice. Thin aluminum films were used to configure the Fibonacci lattice on a glass substrat...

Dynamic Properties of the Two-Dimensional Wigner Solid on the Surface of Normal and Superfluid 3He.

We present the theory of transport properties and vibration modes of the two-dimensional Wigner solid (WS) coupled to surface dimples on normal and superfluid 3 He. Compared to the case of liquid 4 He, Fermi liquid properties of the substrate crucially affect the low-frequency dynamics of the electron solid. At ultralow temperatures, WS transport is shown to be determined mostly by the reflection of ballistic quasiparticles of the bulk liquid at the dimple sublattice. The long mean free path regime of normal 3 He is characterized by the temperature-independent conductivity. Below the superfluid transition, the appearance of the quasiparticle energy gap sharply reduces the momentum adsorbed by moving dimples, which leads to the rapid increase of the WS conductivity. The analysis presented shows that the WS can serve as a new powerful experimental probe for superfluid phases of 3 He.

Chiral Symmetry Breaking in Superfluid 3He-A

Quantum Handedness When a rotating object is placed in circulating fluid, an imbalance of pressures on either side of it causes a deflecting force called the Magnus force. The quantum analog of this effect has been predicted to appear in the low-temperature A phase of 3He, where the Cooper pairs forming the superfluid have a specific handedness. An impurity traveling through such a superfluid would experience a deflecting force in the direction determined by the chirality of the pairs. Ikegami et al. (p. 59) trapped impurities beneath the free surface of 3He, set them in motion, and demonstrated the existence of this deflecting force by measuring the differential transverse current. The sign of the deflection varied over different cooling runs, indicating that the system was choosing one or the other chirality upon entering the superfluid phase—a signature of spontaneous symmetry breaking. The asymmetric deflection of impurities beneath the surface of helium-3 reveals the handedness of the superfluid A phase. Spontaneous symmetry breaking is an important concept in many branches of physics. In helium-3 (3He), the breaking of symmetry leads to the orbital chirality in the superfluid phase known as 3He-A. Chirality is a fundamental property of 3He-A, but its direct detection has been challenging. We report direct detection of chirality by transport measurements of electrons trapped below a free surface of 3He-A. In particular, we observed the so-called intrinsic Magnus force experienced by a moving electron; the direction of the force directly reflected the chirality. We further showed that, at the superfluid transition, the system selected either right- or left-handed chirality. The observation of such selection directly demonstrates chiral symmetry breaking.