Susumu Kurihara

A Microscopic Theory of the Pinning Effect in Peierls Systems with Dilute Impurities

Effect of dilute impurities on Frohlichs collective mode is investigated by solving the Dyson equation for the collective mode Greens function which includes self-consistently an infinite series of impurity scattering processes occuring through electronic bubbles. The Dyson equation is shown to reduce to a simple algebraic equation in the low frequency region of interest. The electrical conductivity σ(ω) is examined and related to the collective mode Greens function. An analytic solution to the reduced Dyson equation is obtained. The resulting expression for conductivity is very simple: \(\sigma(\omega){\propto}\omega^{-3}\sqrt{\omega^{2}-(1/2)\omega_{\text{T}}^{2}}\) where ω T is a constant. the analytic expression for σ(ω) describes the pinning effect fairly well: it gives a sharp asymmetric peak at \((\sqrt{3}/2)\omega_{\text{T}}\) with width ∼ω T , and it satisfies exactly the conductivity sum rule for the collective mode.

Solitons in superfluid 4He films

The Korteweg-de Vries (KdV) equation is derived from Landau two-fluid hydrodynamics applied to the thickness oscillation of the superfluid 4He film at low temperatures, where the main restoring force is van der Waals attraction from the substrate and the thermomechanical force due to phonons is a small correction. Since the usual third-sound generators and detectors are far wider than the individual solitons, the asymptotic solution of the KdV equation provided by the inverse scattering method is coarse-grained by regarding it as a continuous train of sharp pulses. The envelope so obtained still shows a singular front proportional to (t−t0)−1/2, where t0 is the arrival time of the fastest soliton, and should therefore be observable with the appropriate experimental arrangement.

Solitary waves in saturated films of superfluid 4He

It is shown that there should exist in saturated films of superfluid 4He at low temperature a nonlinear surface wave described by the Korteweg-de Vries equation. The effect of surface tension makes the solitary wave cold, in contrast to very thin films discussed in a preceding paper. An electrostatic method is proposed to generate cold solitary waves. The asymptotic behavior can be found theoretically by the inverse scattering method. Estimates of parameters imply the possibility of observing each solitary wave separately.

A Model for Elastic Anomalies in K2Pt(CN)4Br0.3·3H2O

A “spin-lattice” model is proposed for K 2 Pt(CN) 4 Br 0.3 ·3H 2 O (KCP) to solve the puzzling problem of the apparent complete softening of shear modulus C 66 without any structural phase transition at low temperatures. Our model emphasizes the importance of the site II water molecules lying between bromine ions and Pt(CN) 4 complexes. Each water molecule is assumed to have three configurations, which are represented by an S =1 Ising spin. An almost complete agreement is obtained between calculation and experimental data. A marked “hardening” of C 66 at low temperatures ( T <90 K) is predicted. A structural phase transition is possible if the spin-lattice coupling can be enhanced by e.g. external pressure. Ultrasonic attenuation coefficient α 66 , and also C 44 , α 44 , are calculated.

Collective-mode conductivity of TTF-TCNQ in the metallic phase

A non-linear interaction between phase mode and amplitude mode of a charge-density wave is derived from a microscopic model. This interaction offers a new mechanism for the electrical conductivity of TTF-TCNQ in the metallic phase. Temperature dependence, magnitude and pressure dependence of the static conductivity are successfully explained.

Nonlinear Amplitude-Phase Interaction in Charge-Density-Wave Systems

A nonlinear amplitude-phase interaction Hamiltonian is derived for charge-density-wave (CDW) systems from the microscopic Frohlich model. A perturbative formalism with the small parameter \(1/\sqrt{\mu}\) (µ is the dimensionless mass parameter of a CDW) is presented, and illustrated by calculating the amplitude-mode spectrum in one dimension. The calculation gives, without adjustable parameters, a quantitative explanation of the temperature dependence of the damping constant observed in KCP by Raman experiments. This result, together with the calculation of the metallic conductivity of TTF-TCNQ (S. Kurihara: J. Phys. Soc. Jpn. 44 (1978) 2011.), shows the importance of the nonlinear amplitude-phase interactions in CDW systems. It is suggested that some other dynamical degrees of freedom (most probably the motion of water molecules) should be taken into account to understand the behavior of the amplitude-mode frequency in KCP.

Dynamics of phase-separated 3He-4He films

A Frohlich-type Hamiltonian is derived for third sound and 3 He quasi particles in phase-separated double layer of superfluid 4 He and normal 3 He liquid. It is stressed that our system is unique and valuable in that characteristic parameters can be varied in a wide range, simply by adjusting the film thickness. The effect of fermion-boson coupling on the velocity and damping of the third sound is examined. It is predicted that a rather drastic change in the third sound spectrum will occur when the Fermi velocity of 3 He system and the third sound velocity are nearly the same. It is pointed out that the system under consideration may show a variety of interesting phenomena, in addition to the softening of the third sound.

Singlet Superfluidity in ^3He Film on ^4He Film

It is predicted that spin-singlet superfluidity can be realized in a submonolayer 3 He film on the surface of thin film of superfluid 4 He. The basic mechanism of the superfluidity is the attraction between 3 He quasi particles arising from exchange of virtual third sound quanta. It is argued that the superfluid transition of 3 He film is of Kosterlitz-Thouless nature if the 4 He film is thick, and changes its nature as the 4 He thickness is reduced, with a crossover thickness of ∼2 atomic layers. Several experiments are suggested to detect the new phase transition, to test whether the proposed mechanism is really working, and to exanaine the nature of the transition. The transition temperature is estimated to be in a temperature range attainable with a dilution refrigerator.

Quasi-solitons and their collisions in superfluid films

Abstract A Schrodinger equation is examined which describes the motion of superfluid condenste in thin 4 He films. The equation includes full nonlinearity arising from the van der Waals potential due to substrate. In contrast to the ordinary nonlinear Schrodinger equation (|ψ| 2 ψ type) which has been solved by inverse-scattering method, it is extremely difficult to apply such analytic method to our equation. Numerical study shows, however, that there indeed exist, in our system, solitons which have asymmetric shape, and are quite stable through collisions with each other and with fixed boundary walls. Thus the present work generalizes KdV-soliton theories for 4 He films to the fully nonlinear case.