Alexander Sebedash

Superconductivity in lithium below 0.4 millikelvin at ambient pressure.

Elements in the alkali metal series are regarded as unlikely superconductors because of their monovalent character. A superconducting transition temperature as high as 20 K, recently found in compressed lithium (the lightest alkali element), probably arises from pressure-induced changes in the conduction-electron band structure. Superconductivity at ambient pressure in lithium has hitherto remained unresolved, both theoretically and experimentally. Here we demonstrate that lithium is a superconductor at ambient pressure with a transition temperature of 0.4 mK. As lithium has a particularly simple conduction electron system, it represents an important case for any attempts to classify superconductors and transition temperatures, especially to determine if any non-magnetic configuration can exclude superconductivity down to zero temperature. Furthermore, the combination of extremely weak superconductivity and relatively strong nuclear magnetism in lithium would clearly lead to mutual competition between these two ordering phenomena under suitably prepared conditions.

Towards Superfluidity of 3He Diluted by 4He

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.

Melting Pressure Thermometry for Dilute 3He‐4He Mixtures

At very low temperatures, the melting pressure of a dilute 3He‐4He mixture depends quadratically on temperature. This can be used for thermometry much in a similar fashion that has become customary for pure 3He. We have measured the difference between the melting curves of pure 4He and a mixture with about 7% 3He down to 1 mK. At the T = 0 limit, such mixture solidifies at a pressure 34.1 kPa higher than pure 4He. The melting curve followed pm(T) = p0 + 1.1 Pa (T/mK)2 with good precision at least up to 50 mK. The noise level of our system corresponded to 6 mPa variation in pressure.

Acoustic resonator providing fixed points of temperature between 0.1 and 2 K

Below 2 K the speed of second sound in mixtures of liquid 3He and 4He first increases to a maximum of 30?40 m/s at about 1 K and then decreases again at lower temperatures to values below 15 m/s. The exact values depend on the concentration and pressure of the mixture. This can be exploited to provide fixed points in temperature by utilizing a resonator with appropriate dimensions and frequency to excite standing waves in the resonator cavity filled with helium mixture. We demonstrate that commercially mass produced quartz tuning forks can be used for this purpose. They are meant for frequency standards operating at 32 kHz. Their dimensions are typically of order 1 mm matching the wavelength of the second sound in helium mixtures at certain values of temperature. Due to the complicated geometry, we observe some 20 sharp acoustic resonances in the range 0.1? 2 K having temperature resolution of order 1 ?K. The quartz resonators are cheap, compact, simple to implement, easy to measure with great accuracy, and, above all, they are not sensitive to magnetic field, which is a great advantage compared to fixed point devices based on superconductivity transitions. The reproducibility of the resonance pattern upon thermal cycling remains to be verified.

Melting pressure of saturated helium mixture at temperatures between 10 mK and 0.5 K

The melting curves of helium mixtures are, in general, equilibrium states between one or two liquid phases and one solid phase. In the case of a solubility saturated system, three phases coexist and the melting pressure depends uniquely on temperature. Thus, it forms a univariant system and can be used as a thermometric standard. The melting pressure of the saturated helium mixture has been studied experimentally, but an independent calculation of this quantity supports its use in thermometry. We have calculated the melting pressure of the saturated helium mixture as a function of temperature between 10 mK and 0.5 K by using thermodynamic considerations and recently determined effective interaction potential between 3He quasiparticles in the liquid mixture.

Improved capacitive melting curve measurements

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 ∞exible 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 4 He 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 „K. 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 4 He at temperatures 0.08{0.3 K. To avoid spurious heating efiects, one must carefully choose and arrange any dielectric materials close to the active capacitor. We observed some 100 pW loading at moderate excitation voltages.