Alexa W. Harter

Criticality and superfluidity in liquid ^4He under nonequilibrium conditions

We review a striking array of recent experiments and their theoretical interpretations on the superfluid transition in ^4He in the presence of a heat flux Q. We define and evaluate a new set of critical point exponents. The statics and dynamics of the superfluid-normal interface are discussed, with special attention to the role of gravity. If Q is in the same direction as gravity, a self-organized state can arise, in which the entire sample has a uniform reduced temperature, on either the normal or superfluid side of the transition. Finally, we review recent theory and experiment regarding the heat capacity at constant Q. The excitement that surrounds this field arises from the fact that advanced thermometry and the future availability of a microgravity experimental platform aboard the International Space Station will soon open to experimental exploration decades of reduced temperature that were previously inaccessible.

The fluctuation-imposed limit for temperature measurement

AbstractIn experimental sciences, random processes often place a fundamental limit on the achievable measurement resolution. A well known example is the Johnson noise voltage across a resistor. In this paper, we describe observations of the spontaneous transfer of heat in two equilibrium systems: one consisting of a thermometer linked to a reservoir, the other consisting of two thermometers connected to each other and linked to a reservoir for the purpose of temperature stabilization. In the second system, we find anti-correlations between the temperature fluctuations of the two thermometers at intermediate frequencies. We also find that the low frequency temperature noise density of the thermometers, in units of

Heat capacity of superfluid ^4He in the presence of a heat current near T_λ

K/\sqrt {Hz}

Quest to observe the bulk superfluid breakdown in a heat flux

, is given by

Enhanced heat capacity and a new temperature instability in superfluid He-4 in the presence of a constant heat flux near T-lambda

\sqrt {4Rk_B T^2 }

Comment on “Heat-Flow Induced Anomalies in Superfluid 4He near Tλ”

, whereR is the thermal resistance of the link between the thermometer and the reservoir. This implies that for noise reduction purposes,R is the only available engineering parameter to adjust. In a recent thermometer design, we have reduced R to achieve a low frequency temperature noise density of 5×10−11