A. Ya. Parshin

Growth of 4He-Crystals at mK-Temperatures

Liquid-solid interface of4He has been investigated down to mK-temperatures using an optical interferometer in combination with a sensitive pressure gauge. Thec-facets with 5–100 screw dislocations/cm2grew with spiral growth which can be understood by including inertial terms and localization of steps to the standard theory. Crystals without screw dislocations revealed two novel growth mechanisms. At growth rates > 1 nm/s, these high-quality crystals grew in a burst-like manner, creating abruptly 200–2000 new atomic-layers. At rates below 0.5 nm/s, thec-facet revealed slow, continuous growth. Studies ofa-facets yielded a velocity vs. pressure dependence which can be explained by spiral growth. The shape of thec-facet was monitored down to 2 mK without any evidence of the freezing of kinks. Pressure measurements down to our minimum temperature did not show any anomalies connected with the supersolid transition. Indications of new faceting transitions were not observed down to 2 mK.

Structure and dynamics of the He2*(a3Σu+ molecular complex in condensed phases of helium

An investigation is made of the absorption spectra of triplet metastable helium molecules in the a3Σu+ state in liquid 4He and 3He at various pressures and in dense 3He gas. An analysis of the spectrum corresponding to the a3Σu+ → c3Σg+ transition confirms the conclusion that there is a microscopic bubble surrounding the molecule in liquid helium. A simple approximation is proposed for the wave function of the valence electron of the molecule and the parameters of the bubble are determined for various experimental conditions. The coefficient of molecular recombination in liquid 3He and 4He was determined experimentally at various pressures and in dense cold 3He gas. The results show good agreement with the theory of mutual recombination limited by molecular diffusion under conditions of strong van der Waals interaction. It is shown that in the condensed phases of helium the polarization of the molecules under the action of the magnetic field does not lead to suppression of their mutual recombination, and this is confirmed experimentally.

Spectroscopic study of excess electrons in liquid helium

Abstract The IR absorption spectra of electron bubbles in helium have been measured directly over the wavelength interval 5–8.5 μm using semiconductor lasers installed inside the cryostat. The characteristics of the 1s-1p absorption line have been studied as a function of pressure (over the range 2–40 atm) and temperature (2–4 K). The observed changes in the position of the center of the absorption line with temperature and pressure can be described in a simple model by introducing the effective surface tension of the bubble boundary as an adjustable parameter. The experimental absorption line turns out to be much wider than predicted by the theory. Data on the position of the line agree well with photomobility experiments.

Vibrating Wire Measurements in Superfluid 3He at the Melting Curve Down to 0.53 mK

Measurements of the vibrating wire spectrum have been carried out in superfluid 3He along the melting curve down to 0.53mK. We have observed that at temperatures below 0.3 Tc the width of the mechanical resonance of the wire decreases exponentially with 1/T, indicating the ballistic regime of collisions with quasiparticles. The value of the superfluid energy gap was found to be (1.99±0.05)Tc, in good agreement with the values obtained from heat capacity measurements. The vibrating wire was thereby calibrated for further experiments at temperatures below 0.5mK, where the sensitivity of the melting curve thermometry becomes rather poor.

Spectroscopic Studies of the Triplet-state 4He 2 * Molecules in Superfluid Helium Up to the Solidification Pressure

We report detailed investigations of the absorption spectra of the triplet excimer helium molecules in superfluid helium in a wide pressure range. The width of the (0–0) a → c absorption band shows peculiar dependence on helium pressure with maximum near 24 bar. Studies of dynamic properties of molecules in the controlled geometry allow for the first time to perform independent measurements of their mutual decay coefficient, light absorption cross section and probability of molecules formation in the ions recombination process.

On Mssbauer thermometry in the Millikelvin region

Resonant absorption of γ-rays by57Fe and119Sn in iron was investigated to study the possibility of ultralow-temperature measurements by the Mössbauer effect. A dilution refrigerator which could provide temperatures down to 6 mK was used. The Mössbauer temperature of119Sn coincided with the temperature in the mixing chamber down to 6 mK if the heat input to the absorber was low enough (≲10−5 erg/sec). But the Mössbauer temperatures of57Fe stayed almost invariable below 15 mK. In the same iron sample the nuclei of57Fe and119Sn had different Mössbauer temperatures below 15 mK. The reasons for this phenomenon are not yet clear.

Direct Observation of (110), (100) and (211) Facets on 3He Crystals

We present the results of our recent observations on 3He crystals grown from the superfluid phase at 0.55 mK. The crystal images were obtained with a low-temperature multiple-beam interferometer. The angles between the crystal facets were measured by employing a phase-shift technique and true 3D shapes of the crystals were reconstructed on the basis of the obtained information. Three different types of facets (110), (100) and (211) were clearly visible in these experiments.

Crystallization waves on an atomically smooth surface

For the atomically smooth surfaces (faces) of a quantum crystal, the possibility of the presence of traveling waves similar to crystallization waves observed on an atomically rough surface is demonstrated. However, unlike the latter wave type, the waves on atomically smooth surfaces are nonlinear even when their amplitudes are small. The propagation velocity of such waves is determined as a function of their amplitude and wavelength.

Kinematic multiplication of elementary steps on a crystal surface

The dynamics of elementary steps on an atomically smooth crystal-liquid interface and, in particular, the process of collisions of steps differing in sign are considered. It is shown that, along with the conventional annihilation of steps in such collisions, both the overthrow of steps to the neighboring row with the formation of a new atomic layer (passage) and the reflection of steps from each other can take place under certain conditions. The overthrow of steps gives a qualitatively new mechanism of the growth of facets in the absence of renewable sources such as grown-in dislocations. Under these conditions, the growth kinetics of a crystal with atomically smooth facets changes substantially. In particular, the processes considered above may form a basis for physical mechanisms of unconventional growth regimes observed for helium crystals at low temperatures.

Observations on Faceting of 3He Crystals at T=0.55 mK

We report on our recent observations on growing 3He crystals in which altogether eleven different types of facets were found. The crystals were imaged with a novel low-temperature Fabry-Pérot interferometer. Intensity based analysis methods combined with a phase shift technique were used to identify the observed facets.