arXiv subjects

A. P. Sebedash

Publications and source records attributed to A. P. Sebedash.

4 recordsLinked to original sources

arXiv2019

Performance of adiabatic melting as a method to pursue the lowest possible temperature in $^3$He and $^3$He-$^4$He mixture at the $^4$He crystallization pressure

We studied a novel cooling method, in which $^3$He and $^4$He are mixed at the $^4$He crystallization pressure at temperatures below $0.5\,\mathrm{mK}$. We describe the experimental setup in detail, and present an analysis of its performance under varying isotope contents, temperatures, and operational modes. Further, we developed a computational model of the system, which was required to determine the lowest temperatures obtained, since our mechanical oscillator thermometers already became insensitive at the low end of the temperature range, extending down to $\left(90\pm20\right)\,\mathrm{μK\approx}\frac{T_{c}}{\left(29\pm5\right)}$ ($T_{c}$ of pure $^3$He). We did not observe any indication of superfluidity of the $^3$He component in the isotope mixture. The performance of the setup was limited by the background heat leak of the order of $30\,\mathrm{pW}$ at low melting rates, and by the heat leak caused by the flow of $^4$He in the superleak line at high melting rates up to $500\,\mathrm{μmol/s}$. The optimal mixing rate between $^3$He and $^4$He, with the heat leak taken into account, was found to be about $100..150\,\mathrm{μmol/s}$. We suggest improvements to the experimental design to reduce the ultimate achievable temperature further.

cond-mat.other↗

arXiv2019

Thermal conductivity of superfluid $^3$He-B in a tubular channel down to 0.1$T_c$ at the $^4$He crystallization pressure

We studied the thermal conductivity of superfluid $^3$He in a 2.5 mm effective diameter and 0.15 m long channel connecting the two volumes of our experimental assembly. The main volume contained pure solid $^4$He, pure liquid $^3$He and saturated liquid $^3$He-$^4$He mixture at varying proportions, while the separate heat-exchanger volume housed sinter and was filled by liquid $^3$He. The system was cooled externally by a copper nuclear demagnetization stage, and, as an option, internally by the adiabatic melting of solid $^4$He in the main volume. The counterflow effect of superfluid just below the transition temperature $T_c$ resulted in the highest observed conductivity about five times larger than that of the normal fluid at the $T_c$. Once the hydrodynamic contribution had practically vanished below $0.5T_c$, we first observed almost constant conductivity nearly equal to the normal fluid value at the $T_c$. Finally, below about $0.3T_c$, the conductivity rapidly falls off towards lower temperatures.

cond-mat.other↗

arXiv2018

Thermodynamics of adiabatic melting of solid $^4 \mathrm{He}$ in liquid $^3 \mathrm{He}$

In the cooling concept by adiabatic melting, solid $^{4}$He is converted to liquid and mixed with $^{3}$He to produce cooling power directly in the liquid phase. This method overcomes the thermal boundary resistance that conventionally limits the lowest available temperatures in the helium fluids, and hence makes it possible to reach for the temperatures significantly below 100 $μ$K. In this paper we focus on the thermodynamics of the melting process, and examine the factors affecting the lowest temperatures achievable. We show that the amount of $^{3}$He-$^{4}$He mixture in the initial state, before the melting, can substantially lift the final temperature, as its normal Fermi fluid entropy will remain relatively large compared to the entropy of superfluid $^{3}$He. We present the collection of formulas and parameters in order to work out the thermodynamics of the process at very low temperatures, study the heat capacity and entropy of the system with different liquid $^{3}$He, mixture, and solid $^{4}$He contents, and use them to estimate the lowest temperatures achievable by the melting process, as well as compare our calculations to the experimental saturated $^{3}$He-$^{4}$He mixture crystallization pressure data. Realistic expectations in the execution of the actual experiment are considered. Further, we study the cooling power of the process, and find the coefficient connecting the melting rate of solid $^{4}$He to the dilution rate of $^{3}He$.

cond-mat.other↗

arXiv2018

Effects of $^4$He film on quartz tuning forks in $^3$He at ultra-low temperatures

In pure superfluid $^3$He-B at ultra-low temperatures, quartz tuning fork oscillator response is expected to saturate when the dissipation caused by the superfluid medium becomes substantially smaller than the internal dissipation of the oscillator. However, even with small amount of $^4$He covering the surfaces, we have observed saturation already at significantly higher temperatures than anticipated, where we have other indicators to prove that the $^3$He liquid is still cooling. We found that this anomalous behavior has a rather strong pressure dependence, and it practically disappears above the crystallization pressure of $^4$He. We also observed a maximum in the fork resonance frequency at temperatures where the transition in quasiparticle flow from the hydrodynamic to the ballistic regime is expected. We suggest that such anomalous features derive from the superfluid $^4$He film on the oscillator surface.

cond-mat.other↗