D. A. Sergatskov

PdMn and PdFe : New materials for temperature measurement near 2 K

Interest in the critical dynamics of superfluid4He in microgravity conditions has motivated the development of new high resolution thermometry technology for use in space experiments near 2 K. We have developed a magnetic thermometer using dilute magnetic alloys of Mn or Fe dissolved in a pure Pd matrix, similar to previous thermometers used at ultra-low temperatures. These metallic thermometers are easy to fabricate, chemically inert, and can have a low thermal resistance to the stage to be measured. Also, the Curie temperature can be varied by changing the concentration of Fe or Mn, making them available for use in a wide temperature range. The derivative of the magnetic susceptibility was measured for PdMn and PdFe between 1.5 K and 4 K using a SQUID magnetometer. These measurements, as well as preliminary noise and drift measurements, show them to have sub-nK resolution with a drift of less than 10−13K/s.

New Paramagnetic Susceptibility Thermometers for Fundamental Physics Measurements

New paramagnetic susceptibility thermometers have been developed for use in fundamental physics missions in earth orbit. These devices use a SQUID magnetometer to measure the variation in the dc magnetization of a thermometric element that consists of a dilute concentration of manganese in a palladium matrix. Near 2.2 K these new PdMn thermometers have demonstrated a temperature resolution of better than 100 pK/√Hz and a time constant of 50 ms when operated with a 50 K/W thermal resistance to the liquid helium sample. These thermometers have been observed to be remarkably stable, with a drift of less than 10 fK/s. The observed power spectral density of the noise from these thermometers is consistent with separate measurements of the device’s time constant and thermal standoff from the bath. Recently these PdMn materials have been made into thin films and microstructures for use in future studies of quantum liquids, and for possible use in a new class of bolometers and radiometers. These thermometers have been integrated into an experimental cell and thermal isolation network that are adequate to keep stray heats stable to within a few picowatts, with no systematic temperature errors greater than 60 pK, over the course of a planned fundamental physics experiment on Earth orbit.

Decoherence Under a Heat Flux Near the Superfluid Transition in 4He

Measurements of heat transport at the transition from perfect thermal superconductivity to nonlinear heat diffusion in pure 4He provide a very sensitive probe of matter wave coherence. Superfluid heat transport is proportional to the product of the superfluid density and the superfluid velocity, which are both directly related to the superfluid order parameter. From dynamic scaling theory, the correlation length near the superfluid transition provides a measure of the length over which phase fluctuations of the order parameter persist. Our measurements suggest that both the hydrostatic pressure variation within the liquid helium column, together with the heat flux Q, limit the otherwise divergent correlation length near the superfluid transition. Future measurements planned for the microgravity laboratory will provide the fast extensive experimental test of a renormalized, field theoretic description of heat transport near the superfluid transition. It will also provide a conclusive experimental study of the influence of hydrostatic pressure effects and dynamical effects on the correlation length. A new class of microgravity experiments is proposed that will permit measurements to within 10 pK of the superfluid transition temperature, allowing an entirely new class of ultra-accurate scientific investigations to be performed.

Gravitational Effects on Nonlinear Heat Transport Near the Superfluid Transition in 4He

We have recently observed nonlinear heat transport within 30 nK of the superfluid transition temperature using heat flux, Q, in the range 0.1 < Q < 2 erg/(s cm2). While Haussmann and Dohm (HD) accurately predict the initial departure of the thermal conductivity, κ, from the linear response region, κ is greater than expected very close to Tλ. We anticipate that the nature of the thermal conductivitys nonlinearity may depend upon Earths gravity in the low heat flux limit (Q < 0.5 erg/(s cm2)). Comparison of our data to similar data to be taken in a microgravity laboratory will provide an experimental determination of the effect of gravity on nonlinear heat transport near the superfluid transition. The microgravity measurements will also permit the first experimental test of theories that do not consider gravitational effects, such as those by HD.

The Magnetic Properties of Sputtered Pd1−xMnx Films for Thermometry and Bolometry

The magnetic susceptibility of thin films and microstructures consisting of a Pd1−xMnx alloy have been measured as a function of temperature and magnetic field. Sputtering from a 0.68% manganese target produced films with a concentration of approximately 0.90%, as judged by comparison with results from bulk PdMn sensitivity measurements. The thinnest films (thickness ≤1.0μm) show significant domain scale noise below the Curie Temperature, Tc, while thicker films (thickness ≥10μm) show reliable non-hysteretic behavior throughout the temperature range of interest. The thin films show the effects of demagnetization with the field perpendicular to the surface, but a fine screen in this orientation shows no evidence of saturation and a predictable decrease in sensitivity due to demagnetization. These films will serve as the thermometric element in a new class of bolometers and thermometers for fundamental physics applications.

CW Measurement of the Upward‐Going Temperature Wave in the Helium‐4 Self‐Organized Critical State

We describe the first continuous‐wave (CW) measurements of the upward‐going temperature wave in the self‐organized‐critical (SOC) state which forms in 4He under conditions of downward heat flow near Tλ under gravity. The CW technique permits measurements with extremely low (<1 nK) excitation amplitudes, allows continuous measurement of the wave velocity as the SOC state grows, and has yielded the first quantitative measurements of the attenuation. The CW measurements appear to support predictions for the velocity but disagree with predictions for the attenuation. This new technique may help us understand the underlying mechanism of the SOC state.

New propagating mode near the superfluid transition in 4He

Abstract We have observed a new temperature-entropy wave that propagates opposite to the direction of a steady heat flux Q when the helium column is heated from above. Counter-intuitively this new mode, which resembles second sound, propagates on the normal fluid side of the transition, but it exists only when the column of helium is heated from above. Such a new mode had been predicted to exist on the self-organized heat transport state for Q less than about 100 nW / cm 2 . We confirm that this mode exists in this regime, however we also observe that it propagates even when the helium is held away from the self-organized heat transport state.

Sputtered Films of PdMn for Thermometry and Bolometry

Measurements of the magnetic sensitivity of thin sputtered films of PdMn alloy demonstrate the viability of this material for high resolution thermometry. The thinnest films (thickness ≤1.0 μm) show significant domain scale noise below the Curie Temperature, Tc, while thicker films (thickness ≥10 μm) show reliable non-hysteretic behavior throughout the temperature range of interest. The thin films show the effects of demagnetization with the field perpendicular to the surface, but a fine screen in this orientation shows good response with no evidence of saturation and a manageable degree of demagnetization.

Self-Organized Heat Transport Near the Superfluid Transition in 4He

Heat transport through a column of superfluid 4He has been observed experimentally to self-organize, resulting in a thermal gradient that exactly matches the gradient in the superfluid transition temperature across the column, leaving the entire sample at a constant distance from the superfluid critical point1. We describe a new experiment that is designed to accomplish three objectives: 1) search for the upper critical heat flux above which self-organized heat transport can no longer occur, 2) measure the heat capacity of the self-organized heat transport state, and 3) test recent theoretical predictions2,3 of the microscopic mechanism that is responsible for this self-organization.

Onset of superfluidity far from equilibrium: dynamical effects on the correlation length

Abstract Nonlinear heat conduction has recently been measured near the superfluid transition in pure 4He at very low heat flux Q. Since both dynamic effects and gravity limit the divergence of the superfluid correlation length near the transition at low-Q, these measurements must be repeated in the microgravity environment in order to observe the dynamic effects in isolation. Comparison of the microgravity data to similar data obtained on Earth will provide experimental insight into the effect of gravity on this nonlinear conduction region at low heat flux where theoretical predictions are lacking. While some measurement advantages exist in the microgravity laboratory, it is the study of the direct effect of gravity on the nonlinear conduction measurements that motivate the microgravity need.