Producing and imaging a thin line of He*2 molecular tracers in helium-4.

Cryogenic helium-4 has long been recognized as a useful material in fluids research. The unique properties of helium-4 in the gaseous phase and the normal liquid phase allow for the generation of turbulent flows with exceptionally high Reynolds and Rayleigh numbers. In the superfluid phase, helium-4 exhibits two-fluid hydrodynamics and possesses fascinating properties due to its quantum nature. However, studying the flows in helium-4 has been very challenging largely due to the lack of effective visualization and velocimetry techniques. In this article, we discuss the development of novel instrumentation for flow-visualization in helium based on the generation and imaging of thin lines of metastable He*2 tracer molecules. These molecular tracers are created via femtosecond-laser field-ionization of helium atoms and can be imaged using a laser-induced fluorescence technique. By observing the displacement and distortion of the tracer lines in helium, quantitative information about the flow field can be extracted. We present experimental results in the study of thermal counterflow in superfluid helium that validate the concept of this technique. We also discuss anticipated future developments of this powerful visualization technique.

Excimers He2* as tracers of quantum turbulence in 4He in the t = 0 limit.

We have studied the interaction of metastable 4He2* excimer molecules with quantized vortices in superfluid 4He in the zero temperature limit. The vortices were generated by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96±6 nm at a pressure of 0.1 bar and 27±5 nm at 5.0 bar. We have also demonstrated that a moving tangle of vortices can carry the molecules through the superfluid helium.

Magnetoresistance of TVO temperature sensor at T<1 K.

A magnetoresistance (MR) of the well known TVO resistor temperature sensors has been studied at ultralow temperatures from approximately 0.8 K down to approximately 0.1 K under strong magnetic fields up to 8 T. A crossover from positive to negative MR with lowering temperature is found at weak magnetic fields. A zero MR-value at the magnetic field of 4 T, for example, is reached at T approximately 0.2 K. At sufficiently strong magnetic field the negative MR is suppressed and MR returns to positive values. A constant negative MR, which does not depend on the magnetic field from 2 to 8 T, is revealed at the lowest temperature T approximately 0.1 K. The observed behavior of the TVO sensor is explained basing on the model of hopping conduction via localized states in the weak localization regime.