Person:

Tempere, Jacques

We study two techniques to create electrons in a liquid helium environment. One is thermionic emission of tungsten filaments in a low temperature cell in the vapor phase with a superfluid helium film covering all surfaces; the other is operating a glowing filament immersed in bulk liquid helium. We present both the steady state and rapid sweep I-V curves and the electron current yield. These curves, having a negative dynamic resistance region, differ remarkably from those of a vacuum tube filament. A novel low temperature vapor-phase electron collector for which the insulating helium film on the collector surface can be removed is used to measure emission current. We also discuss our achievement of producing multielectron bubbles in liquid helium by a new method.

We show that on superconducting spherical nanoshells, the coexistence of the Meissner state with a variety of vortex patterns drives the phase transition to higher magnetic fields. The spherical geometry leads to a Magnus-Lorentz force pushing the nucleating vortices and antivortices toward the poles, overcoming local pinning centers, preventing vortex-antivortex recombination, and leading to the appearance of a Meissner belt around the sphere equator. In sufficiently small and thin spherical shells paramagnetic vortex states can be stable, enabling spatial separation of freely moving shells with different radii and vorticity in an inhomogeneous external magnetic field.

An equilibrium multielectron bubble (MEB) in liquid helium is a fascinating object with a spherical two-dimensional electron gas on its surface. We discuss two ways in which they have been created. For MEBs that have been observed in the dome of a cylindrical cell with an unexpectedly short lifetime, we show analytically why these MEBs can discharge by tunneling. Using a novel method, MEBs have been extracted from a vapor sheath around a hot filament in superfluid helium by applying electric fields up to 15 kV/cm, and photographed with high-speed video. Charges as high as (1.6 × 10^{−9} C) ((∼10^{10} electrons)) have been measured. The latter method provides a means of capture in an electromagnetic trap to allow the study of the extensive exciting properties of these elusive objects.

In this article, we present calculations that indicate that nanoshells can be used as a high-pressure gauge in diamond anvil cells (DACs). Nanoparticles have important advantages in comparison with the currently used ruby fluorescence gauge. Because of their small dimensions, they can be spread uniformly over a diamond surface without bridging between the two diamond anvils. Furthermore, their properties are measured by broad-band optical transmission spectroscopy leading to a very large signal-to-noise ratio even in the multi-megabar pressure regime where ruby measurements become challenging. Finally, their resonant frequencies can be tuned to lie in a convenient part of the visible spectrum accessible to CCD detectors. Theoretical calculations for a nanoshell with a SiO2 core and a golden shell, using both the hybridization model and Mie theory, are presented here. The calculations for the nanoshell in vacuum predict that nanoshells can indeed have a measurable pressure-dependent optical response desirable for gauges. However, when the nanoshells are placed in commonly used DAC pressure media, resonance peak positions as a function of pressure are no longer single valued and depend on the pressure media, rendering them impractical as a pressure gauge. To overcome these problems, an alternative nanoparticle is studied: coating the nanoshell with an extra dielectric layer (SiO2) provides an easy way to shield the pressure gauge from the influence of the medium, leaving the compression of the particle as a result of the pressure as the main effect on the spectrum. We have analyzed the response to pressure up to 200 GPa. We conclude that a coated nanoshell could provide a new gauge for high-pressure measurements that has advantages over current methods.

The highest quality pressures on samples in a diamond anvil cell (DAC) at high pressures are produced using quasi-hydrostatic pressurization media such as helium or hydrogen. In this paper we carry out a finite element analysis of pressure distributions in a DAC using helium and non-hydrostatic argon pressurization media. We find that samples and ruby chips are at substantially higher pressures than the pressurization media, although this is sharply reduced by using helium, which has a low yield strength for the shear modulus. The deviations in pressure of the different samples (and ruby) from the pressurization media differ and depend on their elastic constants. Our observations may account for the distribution of pressures in metallic markers found in a recent calibration of the ruby scale to high pressures.

Multielectron bubbles (MEBs) in liquid helium were first observed in the late 1970s, but their properties have never been explored experimentally due to their short lifetimes and the difficulty to localize them. We report the observation of long- lived MEBs in a novel cell filled with superfluid helium at static negative pressures. MEBs were extracted from the electron filled vapor sheath of a heated filament loop embedded in the superfluid helium and observed by high-speed photography. MEBs are 2D electron gases on the 3D surface of hollow helium bubbles. Diameters can range from nanometers to millimeters, depending on the number of enclosed electrons. Electrons move in angular momentum states; deformations of the surface are called spherical ripplons. The attractive electron-ripplon interaction leads to an unusual form of superconductivity. If they can be compressed, Wigner crystallization and quantum melting can be observed, as well as a new phase for localization called the ripplo- polaron lattice. MEBs are unstable to tunneling discharge when pressed against a surface. Just as Bose gases are captured in a trap for study, MEBs must also be localized away from walls. We shall discuss methods of capturing them in an electromagnetic trap embedded in the liquid helium.