ALBERT

Notes: We have investigated the influence of electron-beam writing on the creation of charge trapping centers which cause 1/f noise in single electron transistors (SET). Two Al/AlOx/Al devices were compared: one where the SET is on a {100} silicon wafer covered by a 120-nm-thick layer of Si3N4, and another one in which the Si was etched away from below the nitride membrane before patterning the SET. The background charge noise was found to be 1×10−3e/Hz at 10 Hz in both devices, independent of the substrate thickness. © 1999 American Institute of Physics.

Notes: An optical imaging system for milliKelvin temperatures has been developed based on a regular B/W surveillance camera (25 frames/s), with its CCD sensor inside the 4-K vacuum can of our nuclear demagnetization cryostat. The heat leak to the nuclear stage, caused by the operation of the video camera, was reduced below 1 nW by careful rf shielding. The construction of the system and its limits of operation are discussed.

Notes: Abstract On vicinal planes the surface stiffness is quite anisotropic and a crystal defect, terminating at the interface, is predicted to produce a 10...50 nm deep dimple with macroscopic lateral extent (up to a few mm). We have searched for such depressions using high resolution interferometry. Sometimes our measured interferograms of the super fluid/solid interface display unexpectedly large dimples. The volume and shape of the observed objects suggest that these dimples are caused by bundles of about 10 dislocations.

Notes: Abstract We have investigated the spreading of superfluid4He on top of a polished fused silica plate which is coated by a thin layer of MgF2. Our interferometric experiments indicate that a superfluid4He film of about 100 nm thickness is not able to spread uniformly over a nearly horizontal substrate, and hence these films cannot be governed by the Van der Waals forces only. Even in the stationary state, we observe nonzero (1 – 20 mrad) contact angles which display hysteresis, spatial variation, and strong history dependence.

Notes: Abstract Liquid-solid interface of 4 He has been investigated down to mK-temperatures using an optical interferometer in combination with a sensitive pressure gauge. The c- facets with 5–100 screw dislocations/cm 2 grew 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, the c- facet revealed slow, continuous growth. Studies of a- facets yielded a velocity vs. pressure dependence which can be explained by spiral growth. The shape of the c- 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.

Notes: Abstract We have performed NMR measurements on the flare-out texture of superfluid3He-B in a cylindrical container of 5 mm diameter in axial magnetic fields of 28.4 and 56.9 mT. The transverse cw NMR spectra have been analyzed both with respect to their overall shape and the spin-wave absorption peaks close to the Larmor frequency. Our analysis of the stationary state spectra, based on texture computations, yields the longitudinal resonance frequency v L (T), the magnetic healing length ξ H (T), and the dipolar length ξ D (T), which we report for pressures below 29 bar. A lattice of quantized vortex lines appears in the rotating state, and two additional textural free energy terms have to be included in the analysis. One of the terms is linear in the applied magnetic field and arises from the spontaneous magnetization of the vortex cores. The second term is quadratic in magnetic field; it is generated both by the superflow field v s (r) about the vortex core and the difference in the induced magnetizations of the vortex-core and the bulk superfluids. The rotational orienting effects have been studied for rotation speedsβ up to 2red/sec.

Notes: Abstract We have investigated theoretically the profile of electrically charged helium interfaces in the presence of a superfluid vortex line perpendicular to the interface. The free surface of superfluid3He-B as well as the interface between phase-separated3He and4He mixtures have been considered. The superflow-induced dimple profile around the vortex line was calculated within the linear theory assuming a weak curvature of the interface. Our results show that the depth of the dimple increases strongly with an external electric field as the critical value of the electrohydrodynamic surface instability is approached. The chances of observing these charge-enhanced dimples are also discussed.

Notes: The structure and the energy of vortices in rotating 3He-A are considered in the presence of a weak magnetic field. It is shown, using the logarithmic approximation for the free energy, that there will be a sequence of textural transitions when the magnetic field is increased. The transition from a nonsingular texture to a singular texture is studied in detail. It is shown that the l vector is almost uniform outside the cores of vortices. To verify these results, measurements by NMR and by ultrasound are suggested.

Notes: Abstract Observation of enhanced magnetic relaxation due to counterflow and quantized vortices is reported in rotating superfluid3He-B. These phenomena were studied using a novel method based on the homogeneously precessing domain (HPD) mode, characteristic of the B-phase spin dynamics. The additional HPD absorption can be related to the equilibrium vortex structures and, therefore, this method yields information complementary to textural CW-NMR measurements, which are sensitive to the static magnetic properties of vortex cores. Primarily, the HPD method provides a simple and effective means for studying transient processes, e.g., vortex creation in3He-B; a clear difference between axisymmetric and non-axisymmetric vortices was observed.

Notes: Abstract We have constructed a rotating nuclear demagnetization cryostat and used it for continuous-wave NMR experiments on superfluid3He-A and3He-B. The measurements were performed in a long cylindrical geometry of 5 mm diameter, with the cylinder axis parallel to the axis of rotation and with the external magnetic field H0=284 or 142 Oe in the same direction. The angular velocity of rotation Ω was varied between 0.2 and 1.5 rad/sec, and the experiments were done under 29.3 bar pressure at temperatures between Tc=2.72 and about 1.4 mK. As a guide to the new and esoteric field of superfluid3He in rotation, we first review the general theory at some length in relatively simple terms. Pictorial explanations are often given. In3He-A, a rotation-dependent NMR satellite was found; its intensity a rotation-dependent NMR satellite peak was discovered; its relative intensity increases linearly with Ω. The position of the satellite is independent of Ω and H, and does not depend on whether the sample was cooled from the Fermi-liquid region to the A phase while rotating or at rest. At temperatures 0.1〈1−T/Tc〈0.3, the frequency shift of the satellite can be described by the parameter Rt=0.86−1.1(1−T/Tc). Cooldown under rotation produced systematically larger satellite intensities than cooldown at rest. A second, metastable satellite, best seen at rest and disappearing in less than 30 min, was also discovered. Furthermore, the main NMR peak broadens during rotation, while the total NMR absorption remains the same. The behavior of the rotation-dependent satellite strongly supports the existence of vortices in3He-A, their number being proportional to Ω; the satellite is caused by localized spin wave modes trapped by vortex cores. Theoretical calculations agree quite well with our experimental data if continuous vortices, without a singularity in the order parameter, are assumed. Their presence is also responsible for the additional broadening of the main peak, due either to increased spin diffusion or to scattering of spin waves. The metastable satellite is caused by textural boundaries, probably by twist solitons in the superfluid, created by the rapid cooldown of the sample. In3He-B, a series of nearly equally spaced NMR satellites was found on the high-frequency side of the main peak with the cryostat at rest. Under rotation the separation between the satellites increases linearly with Ω. The spacing displays a jump, proportional to Ω, at 1−T/Tc=0.40. The discontinuity occurred only during start/stop experiments, not if the cryostat was continuously rotated while warming over the transition region. Immediately after rotation had been started the whole NMR spectrum shifted toward higher frequencies for about 30 sec; these transients were seen only at Ω〉0.25 rad/sec. In3He-B, the order parameter is strongly influenced by the wall of the container, producing the so-called flareout texture, with the angle β between the $$\hat n$$ vector andH equal to 63° at the walls. The satellites can be explained as spin wave modes arising from an almost harmonic potential well formed by the $$\hat n$$ texture. The creation of vortices changes the texture and increases the steepness of the potential and therefore increases the satellite spacing during rotation. The vortices themselves perturb the $$\hat n$$ texture due to the long-range orientating effect of their cores on the order parameter. The discontinuity in the satellite splitting at 1−T/Tc=0.40 is explained as being due to a first-order phase change in the vortex core at this temperature. The transient shift in the NMR spectrum, immediately after the start of rotation when vortices are not yet present, is caused by the large superfluid vs. normal liquid counterflow; this phenomenon thus gives an estimate for the time needed to create vortices in3He-B.