The growth of atomically rough4He crystals

We have studied the growth of atomically rough bcc and hcp⁴He crystals from the superfluid phase for temperaturesT>0.9 K. The growth coefficient displays a temperature dependence which can be represented bym ₄ K . The parameter E is found to be in close agreement with the energy gap of rotons, suggesting that these thermal excitations dominate the growth kinetics. Besides, the absolute value of the growth coefficient depends on crystal orientation, with an anisotropy for the hcp phase of about a factor of 2.5 between the and {0001} planes.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

Quantum kinks on the null plane

We consider the quantization of a 1+1 dimensional field theory with kink solutions on a null plane. We present a field expansion which diagonalizes the operatorM ²=2P ⁺ P ^? including first order quantum corrections, reobtaining thereby the well known result for the kink mass. The quantization scheme treats classical solutions of different rapidity on an equal footing and the translation mode cancels completely, at least in the order considered here.     

Dynamics of kinks in modulated crystals

The dynamics of kinks (domain walls) in a linear chain system with frustration, in which incommensurate phases occur, is studied. In this both numerical and analytical techniques are used. The kinks, which can be interpreted as discommensurations in incommensurate phases, are found to be unstable, in a continuum approximation, but their lifetime may be long near the lock-in transition due to pinning effects.     

Charge mobility anisotropy in hcp 4He crystals

A strong anisotropy of the positive-charge mobility is observed in hcp ⁴He crystals. The mobility activation energies in the principal directions are determined. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 90–95 (10 January 1997)     

Kosterlitz-Thouless transition for 4He films adsorbed to rough surfaces

We report the study of adsorption isotherms of 4He on several well characterized rough CaF2 surfaces using a quartz crystal microbalance technique at 1.672 K. The signature of decoupled mass observed on crossing the Kosterlitz-Thouless transition as a function of 4He film thickness decreases and becomes increasingly difficult to identify as the surface roughness is increased. A peak in the dissipation, indicative of the onset of superfluidity, changes little with roughness.     

Quantum Monte Carlo simulation of overpressurized liquid 4He

A diffusion Monte Carlo simulation of superfluid 4He at zero temperature and pressures up to 275 bar is presented. Increasing the pressure beyond freezing (approximately 25 bar), the liquid enters the overpressurized phase in a metastable state. In this regime, we report results of the equation of state and the pressure dependence of the static structure factor, the condensate fraction, and the excited-state energy corresponding to the roton. Along this large pressure range, both the condensate fraction and the roton energy decrease but do not become zero. The roton energies obtained are compared with recent experimental data in the overpressurized regime.     

Quantum momentum distribution and kinetic energy in solid 4He

We present measurements of neutron scattering from solid 4He at high momentum transfer. The solid is held close to the melting line at molar volume 20.87 cm3/mol and temperature T=1.6 K. From the data, we determine the shape of the momentum distribution, n(k), of atoms in the solid and the leading final state contribution to the scattering. We show that n(k) in this highly anharmonic, quantum solid differs significantly from a Gaussian. The n(k) is more sharply peaked with larger occupation of low momentum states than in a Maxwell-Boltzmann distribution, as found in liquid 4He and predicted qualitatively by path integral Monte Carlo calculations. The atomic kinetic energy is =(24.25+/-0.30) K. If n(k) is assumed to be Gaussian, as is usually the practice, a 10% smaller is obtained.     

Dynamics of kinks on dislocations in SiGe single crystals

The method of intermittent pulse loading is used for obtaining the dependences of the mean free path of individual dislocations in SiGe single crystals with various concentration of Ge (0–5.5 at. %) on the duration of loading pulses and time intervals between them. It is found that these dependences change qualitatively upon an increase in the Ge concentration. It is shown that the motion of dislocations in SiGe crystals under small shear stresses is characterized by a nonlinear drift of kinks and the formation of superkinks. A theory of the motion of dislocations under the action of intermittent pulse loading under the conditions of heterogeneous kink dynamics is developed. Extended quasi-one-dimensional defects repeating the shape of a part of a segment of a moving dislocation are discovered in SiGe crystals containing 0.96 at. % Ge. The mechanism of formation of such defects as the result of the shedding of a part of the impurity atmosphere by a dislocation segment during overcoming of a local obstacle is proposed.     

Wetting of He on rough cesium substrates

We discuss the behaviour of ⁴He meniscus on various disordered Cs substrates. We have first studied the dynamics of the contact line on Cs substrates evaporated at low temperature. The activated motion of the line is consistent with a substrate disorder of mesoscopic length scale. We have performed further studies of the contact line behaviour on substrates with roughness of macroscopic length scale. Close to the wetting transition, we find that a film invades the substrate leading to marked changes in the value of the contact angle.     

Scattering of He atoms from rough surfaces

Three diverse rough surfaces have been studied using inelastic He scattering. The first is an ion bombarded Ag(110) surface where a broadened specular peak remains visible but scattering remains entirely elastic as viewed by the incoherent component. The second is a cleaved Te(10 ) surface composed of helical chains where the roughness originates from thermal oscillations and scattering is almost entirely inelastic. Thirdly, a glass (7740) disc is shown to be physically rough but the surface is sufficiently rigid to yield largely elastic incoherent scattering.     

Smoothing of an atomically rough vicinal surface – STM observation and MC simulation

Smoothing of an atomically rough vicinal surface of SrTiO₃ is studied by scanning tunneling microscope (STM) observation and by Monte Carlo (MC) simulation. A complex step pattern that resembles a two-dimensional phase separation pattern is observed on the surface. Analysis of the step pattern during annealing obtained by the STM in comparison with the MC simulation reveals an asymmetry of the relaxation pattern between islands and holes. The asymmetry is attributed to the difference of the mobility of an adatom and an atomic hole, and the asymmetry is enhanced by the step edge diffusion barrier. Values of an effective bond energy and an effective diffusion barrier as well as the surface diffusion coefficient are deduced from the relaxation pattern.