Person:

Silvera, Isaac

The traditional pathway that researchers have used in the goal of producing atomic metallic hydrogen is to compress samples with megabar pressures at low temperature. A number of phases have been observed in solid hydrogen and its isotopes, but all are in the insulating phase. The results of experiment and theory for this pathway are reviewed. In recent years a new pathway has become the focus of this challenge of producing metallic hydrogen, namely a path along the melting line. It has been predicted that the hydrogen melt line will have a peak and with increasing pressure the melt line may descend to zero Kelvin so that high pressure metallic hydrogen may be a quantum liquid. Even at lower pressures hydrogen may melt from a molecular solid to an atomic liquid. Earlier attempts to observe the peak in the melting line were thwarted by diffusion of hydrogen into the pressure cell components and other problems. In the second part of this paper we present a detailed description of our recent successful demonstration of a peak in the melting line of hydrogen

We show that a strategy for increased accuracy in temperature determination by optical pyrometry when the wavelength dependence of the emissivity is unknown is to measure the spectral irradiance at short wavelengths. We then introduce an improved method of determining the temperature in laser heated diamond anvil cells. In general a blackbody source is used to determine the optical transfer function required for determining the blackbody curve. By using the thermal radiation of a heated absorber at ambient pressure and known temperature, uncertainties in the temperature determination caused by the wavelength dependence of the emissivity of the heated absorber can be eliminated. Temperature determination reduces to a one-parameter fit to the blackbody curve rather than the usual two parameters (emissivity and temperature), leading to increased precision and accuracy.

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.

The insulator to metal transition in solid hydrogen was predicted over 70 years ago but the demonstration of this transition remains a scientific challenge. In this regard, a peak in the temperature vs. pressure melting line of hydrogen may be a possible precursor for metallization. However, previous measurements of the fusion curve of hydrogen have been limited in pressure by diffusion of hydrogen into the gasket or diamonds. To overcome this limitation we have used an innovative technique of pulsed laser heating of the sample and final peak in the melting line at (P=64.7 \pm 4) GPa and (T=1055 \pm 20) K.

Tungsten filaments used as sources of electrons in a low temperature liquid or gaseous helium environment have remarkable properties of operating at thousands of degrees Kelvin in surroundings at temperatures of order 1 K. We provide an explanation of this performance in terms of important changes in the thermal transport mechanisms. The behavior can be cast as a first-order phase transition.

The free surface of thin films of liquid helium adsorbed on a solid substrate has been studied using x-ray reflectivity. The film thickness and interfacial profile are extracted from the angular dependence of measured interference between signals reflected from the liquid-vapor and liquid-substrate interfaces. Polished silicon wafers, chemically cleaned and passivated, were used as substrates. Results are reported for measurements for 4He films 35 to 130 Å thick in the temperature range 0.44 to 1.3 K. The 10%/90% interfacial width for temperature T=0.45 K varies from 5.3±0.5 Å for 36±1.5 Å thick films to 6.5±0.5 Å for 125±1.5 Å thick films. The profile width at zero temperature should not differ significantly from that measured at T=0.45 K. For T=1.22 K, the width is 7.8±1.0 Å.

Films of isotopic mixtures of helium have been studied using x-ray specular reflectivity techniques. In contrast with superfluid 4He films, x-ray exposure causes a reduction in the thickness of 4He films above the superfluid transition as well as films of pure 3He and 3He/4He mixtures. One proposed model that could account for this effect is a charging model, in which thinning is caused by electrostatic pressure of free charges that accumulate on the helium surface. Unfortunately, this model is not fully consistent with all of the experimental observations. A localized heating model, in which indirect heating of the film causes it to thin would explain the data if there were dissipative film flow in the 3He/4He mixtures at temperatures where the bulk is superfluid. We argue that various published experimental results suggest such an effect. In this model, film thinning data for dilute 3He/4He films indicates dissipation that is linear in 3He content of the film over two orders of magnitude.