H. J. Maris

Thermal conductivity of GaAs/AlAs superlattices

Abstract We have used a novel picosecond optical technique to measure the temperature dependence of the thermal conductivity of two short-period GaAs/AlAs superlattices. We find that the thermal conductivity is reduced by about one order of magnitude relative to the conductivity of bulk GaAs.

Supercooling of liquid H2 and the possible production of superfluid H2

We discuss the extent to which it may be possible to supercool liquid H2 below its normal freezing temperature. We show that there is a significant prospect that H2 could be supercooled to arbitrarily low temperatures, and that in this way superfluid H2 might be produced. A brief discussion is given of the properties of the anticipated superfluid phase.

On the Fission of Elementary Particles and the Evidence for Fractional Electrons in Liquid Helium

We consider the possibility that as a result of interactions between an elementary particle and a suitably designed classical system, the particle may be divided into two or more pieces that act as though they are fractions of the original particle. We work out in detail the mechanics of this process for an electron interacting with liquid helium. It is known that when an electron is injected into liquid helium, the lowest energy configuration is with the electron localized in a 1s state inside a spherical cavity from which helium atoms are excluded. These electron bubbles have been studied in many experiments. We show that if the electron is optically excited from the 1s to the 1p state, the bubble wall will be set into motion, and that the inertia of the liquid surrounding the bubble can be sufficient to lead to the break-up of the bubble into two pieces. We call the electron fragments “electrinos.” We then show that there is a substantial amount of experimental data in the published literature that gives support to these theoretical ideas. The electrino bubble theory provides a natural explanation for the photoconductivity experiments of Northby, Zipfel, Sanders, Grimes and Adams, and possibly also the ionic mobility measurements of Ihas, Sanders, Eden and McClintock. Previously, these experimental results have not had a satisfactory explanation. In a final section, we describe some further experiments that could test our theory and consider the broader implications of these results on fractional particles.

Heat Flow in Glasses on a Picosecond Timescale

The transient thermal properties of amorphous materials have been the subject of several recent investigations [1–4]. These experiments have mainly been designed to study the time-dependence of the specific heat at low temperature (T≤2K) on time scales ≥0.1µsec. In this paper we describe a new technique which enables measurements of thermal properties to be made on a time scale into the picosecond range, and we present preliminary results of measurements of this type.

Magnetic Levitation of Liquid Helium

We report on the stable levitation of liquid helium drops of up to 2 cm diameter in a magnetic trap at temperatures down to 1.5 K in the earths gravitational field. The production and properties of a magnetic trap for diamagnetic materials is discussed. The behavior of liquids in such a trap is analyzed, including the deformation of a liquid drop by the trap forces. We frequently observe two drops in the magnetic trap which are held in apparent contact for up to 3 minutes without coalescing. This non-coalescence effect was only seen above the superfluid transition temperature. We explain this effect in terms of the existence of a vapor layer between the drops caused by evaporation of the drops, much like the suspension of a liquid drop above a hot surface known as the Leidenfrost effect.

Electrons and cavitation in liquid helium

We report preliminary results from a study of the effect of electrons on cav-itation in liquid3He. We have measured the negative pressure at which electron bubbles explode. The results are in good agreement with theoretical expectations.

Shape Oscillations in Levitated He II Drops

We describe experiments to study the shape oscillations of levitated He II drops. Drops of approximately 0.5 cm diameter are levitated magnetically with a superconducting solenoid, and shape oscillations are induced with an ac electric field. We have measured the damping of shape oscillations as a function of temperature. The damping rate is compared to that predicted by a two fluid, hydrodynamic model, which takes account the effect of the motion in the vapor. The effects of condensation and evaporation on the motion of the drop are also considered.

Simultaneous calorimetric detection of rotons and photons generated by particles in superfluid helium

Abstract A charged particle stopped in superfluid helium produces both excitations in the liquid (phonons and rotons) and photons by fluorescence. Using a calorimeter with a response time of 50 μs we are able to time-resolve in a single detector signals from the photons and from the excitations of the liquid at 25 mK. The implications of this observation for the detection of low energy solar neutrinos are discussed.

Thermodynamic Properties of Superfluid 4He at Negative Pressure

We calculate the thermodynamics of superfluid 4He at negative pressures. We use the Landau theory in which thermodynamic properties are expressed as sums over the thermal distribution of elementary excitations. The excitation dispersion curve is taken from the density functional theory developed by Dalfovo et al. To give a good description of the liquid near to the lambda line, we include the interaction between the elementary excitations using a modification of the roton liquid theory of Bedell, Pines, and Fomin. The calculated quantities include the location of the lambda line, the liquid-vapor spinodal, and lines of constant entropy in the pressure-temperature (P-T) plane. We have also calculated the line of minimum density (zero expansion coefficient) in theP-T plane. This joins the lambda line tangentially at about −5.3 bars. Using the critical properties near the lambda line, we find that the line of maximum density in He I also joins the lambda line tangentially at this pressure. For use in cavitation experiments, we have calculated the states on the lambda line reached by isentropic expansion from He I.

Laser levitation of superfluid helium

We describe experiments on the laser levitation of superfluid helium. Liquid drops of 10–20 μm diameter are produced by a submerged piezoelectric transducer and suspended in vapor at 2K for up to 3 minutes. The optical trap is formed by two counter-propagating, horizontal, focused laser beams. Calculations show that Brillouin and Raman scattering of the laser light in the liquid helium produces a negligible rate of evaporation of the drop. Evaporation caused by the enhanced vapor pressure of the curved drop surfaces appears to be a significant effect limiting the drop lifetimes. We calculate the characteristics of the optical trap and the deformation of the liquid drops by the light forces. The observed simultaneous trapping of multiple drops is also discussed.