sub-Kelvin Temperatures Research Articles

Novel technique for producing ultracold 4He beams.

We have developed a technique for producing ultracold beams of ground state [sup 4]He atoms by evaporation from a film at subkelvin temperature in a dilution refrigerator. Our analysis shows that we have cooled atoms to below 700 [mu]K and that the cooling process can be continued to below 100 [mu]K. This new technique has great promise for determining the He-He interaction potential to high precision and searching for [sup 4]He dimer formation. It might also be used to study quantum reflection off surfaces and to search for Bose-Einstein condensation in a dilute gas of [sup 4]He.

Heat transport via evaporation of superfluid helium films: Giant effective Kapitza resistance.

We report measurements of heat transport from a solid substrate into a thick superfluid $^{4}\mathrm{He}$ film at sub-kelvin temperatures. We believe this transport is bottlenecked by the insufficient generation of excitations in the film capable of causing evaporation of atoms. An effective film temperature and an effective Kapitza resistance are defined. The effective Kapitza resistance is more than 3 orders of magnitude larger than for bulk helium, with an exponential temperature dependence.

Diffuse phase transition and low temperature dielectric constant in PbTiO3

Abstract The dielectric constant minimum in the sub-Kelvin temperature region has been found both for amorphous and crystalline PbTiO3. The glass-like behavior gradually vanishes as the transition changes from the diffuse to the sharp transition.

The anomalous thermal conductivity of La2-xSrxCuO4-yat very low temperatures

The authors report measurements of thermal conductivity ( kappa ) extending to 100 mK on a series of La2-xSrxCuO4-y specimens with 0.10<or=x<or=0.25. For two superconducting samples (x=0.20, x=0.25) the temperature dependence of kappa weakens below about 0.5 K, approaching a variation linear in T near 0.1 K. A non-superconducting sample (x=0.10) exhibits a kappa varies as T3 dependence below 1 K, characteristic of phonon boundary scattering. After annealing in a vacuum, the x=0.20 specimen became insulating and had a thermal conductivity similar in magnitude and temperature dependence to that of the x=0.10 sample. Thus superconducting and non-superconducting samples are clearly distinguished by the behaviour of their thermal conductivities at sub-kelvin temperatures. These results are shown to be consistent with the presence of a small number of free carriers well below Tc in the superconductors. They suggest that a non-superconducting band of electrons is present in the highly doped materials.

Laser Cooling of Magnesium Ions Confined in a Penning Trap

The construction of a Penning ion trap is described. The trap has been used to confine clouds of magnesium ions for periods of many hours and the ions have been cooled to sub-kelvin temperatures by the method of laser cooling. Preliminary experiments with the cooled ions are reported.

High-pressure valence instability andTcmaximum in superconductingCeCu2Si2

The high-pressure electrical resistivity of the Kondo compound ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ was investigated down to the subkelvin temperature range. The main result is the occurrence of a valence transition associated with a steep increase of the superconducting critical temperature ${T}_{c}$ to about 2 K near 25 kbar. In the intermediate-valence phase, $\frac{\ensuremath{\partial}{T}_{c}}{\ensuremath{\partial}P}$ is negative, and no evidence for superconductivity is found above 1.2 K for $P&gt;100$ kbar. At pressure of the order of 200 kbar, the $\ensuremath{\rho}(T)$ curves between 0 and 300 K are similar to those for ${\mathrm{LaCu}}_{2}$${\mathrm{Si}}_{2}$ at ambient pressure. The preceding results are discussed in terms of a two-component ($sd+f$) picture.

Secondary amines: A low-temperature dielectric relaxation study

Polyethylene matrices containing dibutylamine and dioctylamine were studied at temperatures down to 4.4 mK, and the mean inversion splitting of each amine was estimated to be 425 ± 25 MHz. This result prompted a new study of some thiophenol derivatives from which it is concluded that internal-rotation barriers and inversion barriers both tend to be a little higher (on the order of 100 cm−1) in hydrocarbon matrices than in the vapours. An unexplained non-linearity of the dielectric response at sub-Kelvin temperatures was investigated by superposing alternating voltages of different frequencies. Thirty other secondary amines and similar compounds were also examined to evaluate the effects on nitrogen inversion of bulky alkyl groups, neighbouring π-bonds and ring strain. Derivatives of allylamine, benzylamine and pyrrolidine apparently had alternative configurations distinguished by markedly different inversion rates. The proportions depended on the matrix, polyethylene or polypropylene, as well as on the derivative.