Feng-Chuan Liu

Finite-size scaling and universality of the thermal resistivity of liquid 4He near Tlambda.

We present measurements of the thermal resistivity rho(t,P,L) near the superfluid transition of 4He at saturated vapor pressure and confined in cylindrical geometries with radii L=0.5 and 1.0 microm [t identical with T/T(lambda)(P)-1]. For L=1.0 microm measurements at six pressures P are presented. At and above T(lambda) the data are consistent with a universal scaling function F(X)=(L/xi(0))(x/nu)(rho/rho(0)), X=(L/xi(0))(1/nu)t valid for all P (rho(0) and x are the pressure-dependent amplitude and effective exponent of the bulk resistivity rho, and xi=xi(0)t(-nu) is the correlation length). Indications of breakdown of scaling and universality are observed below T(lambda).

The λ-transition of4He in a heat current and under gravity

Abstract We searched experimentally for hysteresis at the λ-transition of 4 He in a heat current and in the gravitational field, and found none. With the temperature controlled by a thermometer with 3 nK (rms) resolution, the transition was crossed from below and above at ramp rates from 1.14 to 10.42nK/s in the presence of heat currents from 13.76 to 37.00μ W/cm 2 . In all cases, the hysteresis was less than 0.1μK. This is an order of magnitude less than the theoretical estimates of 1.6 or 3.1μK.

4He very nearT λ heated from above

We discuss the feasibility and likely benefits of studying the superfluid transition of4He very nearTλ in the presence of gravity, and of a finite heat currentQ applied from the top of the sample and extracted from below. Part of or the entire sample can exist in a homogeneous critical state represented by a uniform reduced temperaturet=[T(z)−Tλ(z)]/Tλ (z is the vertical coordinate) without inducing convection in the HeI layer. Precision measurements of the thermal conductivity and boundary resistance very nearTλ, including inside the nonlinear region, should be possible; but only along a single path in theQ−t plane.