We have studied dilute liquid mixtures of H3e in H4e at millikelvin temperatures to find the maximum solubility in the zero-temperature limit, covering for the first time the whole pressure range of the liquid phase. Injecting pure H4e... more
We have studied dilute liquid mixtures of H3e in H4e at millikelvin temperatures to find the maximum solubility in the zero-temperature limit, covering for the first time the whole pressure range of the liquid phase. Injecting pure H4e into the sample cell through a superleak made it possible to pressurize the mixture up to the melting curve at low temperatures, unattainable through ordinary capillaries due to the minimum of the melting pressure—which would block any customary filling lines at around 1 K. The possibility to selectively drain H4e out of the cell through the superleak enabled us to reversibly cover the full span of pressures with a given amount of H3e in the system and to add H3e into the sample volume through an ordinary filling line without the necessity to warm up in between, thus probing the concentration range in small steps in one continuous run in an unprecedented way. Here we report the results on the pressure and temperature dependence of the saturation concentration of the mixture, based on the response of an oscillating quartz tuning fork immersed in the helium mixture. Our data generally agree with earlier results, but suggest that above the solubility maximum around 10 bar the decrease in the saturation concentration as a function of pressure is not as steep as observed in earlier capacitive concentration measurements.
The osmotic pressures of dilute 3He-4He solutions were determined at 25.3 bar from measurements of crystallization curves at temperatures from 5 mK to 60 mK, when the 3He component of the solution obeying the Fermi-Dirac statistics was... more
The osmotic pressures of dilute 3He-4He solutions were determined at 25.3 bar from measurements of crystallization curves at temperatures from 5 mK to 60 mK, when the 3He component of the solution obeying the Fermi-Dirac statistics was deep in the degenerate state. We determine the shift of the crystallization pressure of the solution of interest relative to pure 4He when both these substances are present in the cell in two separate volumes at the same temperature. We used our novel ultra-sensitive capacitive gauge for measurements of small pressure differences between the two substances. We used a quartz resonator for determination of solution’s concentration in situ. The difference between the crystallization pressure of the saturated solution and pure 4He, both extrapolated to zero temperature, is (339±2) mbar.
Sensitivity of the capacitive method for determining the melting pressure of helium can be enhanced by loading the empty side of the capacitor with helium at a pressure nearly equal to that desired to be measured and by using a relatively... more
Sensitivity of the capacitive method for determining the melting pressure of helium can be enhanced by loading the empty side of the capacitor with helium at a pressure nearly equal to that desired to be measured and by using a relatively thin and flexible membrane in between. This way one can achieve a nanobar resolution at the level of 30 bar, which is two orders of magnitude better than that of the best gauges with vacuum reference. This extends the applicability of melting curve thermometry to lower temperatures and would allow detecting tiny anomalies in the melting pressure, which must be associated with any phenomena contributing to the entropy of the liquid or solid phases. We demonstrated this principle in measurements of the crystallization pressure of isotopic helium mixtures at millikelvin temperatures by using partly solid pure 4He as the reference substance providing the best possible universal reference pressure. The achieved sensitivity was good enough for melting curve thermometry on mixtures down to 100 μK. Similar system can be used on pure isotopes by virtue of a blocked capillary giving a stable reference condition with liquid slightly below the melting pressure in the reference volume. This was tested with pure 4He at temperatures 0.08-0.3 K. To avoid spurious heating effects, one must carefully choose and arrange any dielectric materials close to the active capacitor. We observed some 100 pW loading at moderate excitation voltages.
Adiabatic melting of 4He crystal to phase separated 3He 4He solution (at T< 2 mK) is probably the most promising method to cool the dilute phase down to temperatures substantially below 0.1 mK. When started well below the superfluid... more
Adiabatic melting of 4He crystal to phase separated 3He 4He solution (at T< 2 mK) is probably the most promising method to cool the dilute phase down to temperatures substantially below 0.1 mK. When started well below the superfluid transition temperature T c of pure 3He, this process allows, in principle, to get the final temperature (T f ) several orders of magnitude less than the initial one (T i ). This work is the first practical implementation of the method below the T c of 3He. The observed cooling factor was T i /T f =1.4 at 0.9 mK, being mainly limited by the bad performance of the superleak filling line, by incomplete solidification of 4He in the cell, and by the improper thermal contact between the cell wall and the liquid.
Use of a quartz tuning fork for precise measurements of density has been studied in normal 3He liquid and in 4He liquid and vapor, spanning a reasonably wide range of fluid densities. It is evident that the compressibility of the fluid... more
Use of a quartz tuning fork for precise measurements of density has been studied in normal 3He liquid and in 4He liquid and vapor, spanning a reasonably wide range of fluid densities. It is evident that the compressibility of the fluid must be accounted for in order to properly interpret the resonator response.
The difference between the equilibrium concentration of He3 in the solid and liquid phases alters the crystallization pressure of helium mixtures due to osmotic pressure. This effect was determined with high precision at several... more
The difference between the equilibrium concentration of He3 in the solid and liquid phases alters the crystallization pressure of helium mixtures due to osmotic pressure. This effect was determined with high precision at several concentrations from 0.6% to the zero temperature solubility limit of 8.1% by a specific capacitive pressure gauge. Thereby, the osmotic pressure was deduced at 2.53 MPa—the crystallization pressure of pure He4—and at millikelvin temperatures. The experimental results are compared with numerical calculations for an interacting Fermi liquid using a quasiparticle potential and an effective mass fitted to experimental data.
Search for the superfluid state of dilute 3He dissolved to 4He is one of the major remaining problems of low temperature physics. We describe our two experiments designed to pursue the lowest achieved temperature in such mixtures... more
Search for the superfluid state of dilute 3He dissolved to 4He is one of the major remaining problems of low temperature physics. We describe our two experiments designed to pursue the lowest achieved temperature in such mixtures essentially below the values reported before.
We describe the improvements made to our earlier experiment, aiming to cool saturated helium mixtures at the melting pressure to ultra-low temperatures in the microkelvin regime. Cooling is produced by dissolving pure 3He in the... more
We describe the improvements made to our earlier experiment, aiming to cool saturated helium mixtures at the melting pressure to ultra-low temperatures in the microkelvin regime. Cooling is produced by dissolving pure 3He in the superfluid state to pure 4He being released from the solid phase within the mixture of isotopes at the melting pressure. The limiting factor for the performance was considered to be the inevitable coupling of the liquid mixture with the surroundings at higher temperatures, such as through the filling line and the sintered surfaces needed for the pre-cooling phase. These issues could be largely eliminated by the new design of the experiment. Results of testing the new components at low temperatures are presented and discussed.
532 J. Tuoriniemi et al. atures below 10 /Ж for extended periods of time. Both the extremely low temperature and its holding time are of importance as the time constants of experiments at such low temperatures may grow up to tens of hours... more
532 J. Tuoriniemi et al. atures below 10 /Ж for extended periods of time. Both the extremely low temperature and its holding time are of importance as the time constants of experiments at such low temperatures may grow up to tens of hours or even days. We explore two alternative ...
