Wetting of Superfluid 4He by Liquid 3He

We have investigated optically the spreading of He3 on top of the He4-rich solution in phase-separated helium-mixture films, 20-50 μm thick. In equilibrium, the He3 layer wets the He4-rich phase completely, but nearly circular or stripelike pools of He3-rich phase are stabilized instead when He4 atoms are condensed to the liquid sample at the rate 10 exp 15-5 × 10 exp 15 atoms/cm2s. For the contact angle we obtain about 10 mrad, which suggests a fractional change of the He3 surface tension by a factor of 10 exp −5 from the equilibrium value.Peer reviewe

Observation of a New Surface State on 4He Crystal Interfaces

The equilibrium shape of hcp 4He crystals has been studied at temperatures 0.05≤T≤0.7 K by means of a high-precision optical interferometer. We find that the profile of the interfacial boundary, close to an almost horizontal c-facet, has a well-defined slope discontinuity separating two angular regions with different behavior of the surface stiffness α̃. For surfaces tilted by an angle φ≲100 μrad with respect to the c-facet, we obtain α̃=κ/φ, contrary to the linear angular dependence predicted by current theories. Our results on four large crystals yield κ=(11±3)×10 exp −4T erg/ cm exp 2 K.Peer reviewe

Evidence of 4He Crystallization via Quantum Tunneling at mK Temperatures

We have investigated creation of 4He crystals from the superfluid phase at the temperature range 2 mK–1.0 K. Statistical nucleation-event distributions in overpressure were found to be broad, asymmetric, and temperature independent below 100 mK. Our statistical analysis agrees with a theoretical model suggesting that solid formation is driven by macroscopical quantum-mechanical fluctuations from a seed preexisting in a cavity on the wall.Peer reviewe

Cooled video camera for optical investigations below 1 mK

An optical imaging system for milliKelvin temperatures has been developed based on a regular B/W surveillance camera (25 frames/s), with its CCD sensor inside the 4‐K vacuum can of our nuclear demagnetization cryostat. The heat leak to the nuclear stage, caused by the operation of the video camera, was reduced below 1 nW by careful rf shielding. The construction of the system and its limits of operation are discussed.Peer reviewe

Observations on Superfluid Meniscus in Rotating 3He-B

The parabolic superfluid meniscus has been seen for states in which only (1) the normal or (2) the superfluid component of 3He-B rotates. A reduced, temperature-dependent meniscus (1) was formed at small speeds Ω≲0.21 rad /s. A deeper-than-expected meniscus (2) was observed after a rapid halt of the cryostat, when the normal fluid stops during a short relaxation time, followed by slow decay of the superfluid circulation. The depth of the meniscus (2) was explained by a reactive radial force between the rotating superfluid and the stationary normal liquid.Peer reviewe

Facet Growth of 4He Crystals at mK Temperatures

We have investigated growth of c facets in good quality helium crystals with screw dislocation densities 0–20 cm exp −2 along the c axis. Three distinct regimes of growth were observed. One of them can be explained by spiral growth provided that kinetic energy of moving steps and their tendency to localization at large driving forces are taken into account. In the absence of screw dislocations we find burstlike growth unless the speed is less than 0.5 nm/s, in which case anomalous, intrinsic growth of facets is detected.Peer reviewe

Management of space surveillance radar temporal resource on fuzzy set theory

This paper addresses the problem of optimizing the use of temporal resources of a radar station (RS) under limited time resources. Special attention is given to the necessity of considering a multitude of compensatory optimality criteria when allocating the RS operating time. The proposed approach is based on the use of fuzzy set theory which represents an innovative solution in the context of this task. The task of managing the RS temporal resources is formulated as the search for an optimal work schedule among all potentially possible options. This schedule should minimize the values of all partial optimality criteria. Fuzzy set theory is applied to solve this problem, allowing for the consideration of uncertainty and variability in task execution conditions. An algorithm for managing the RS temporal resources was presented. The review results confirm the probable increase in efficiency, especially in conditions of acute shortage of temporal resources, ensuring their optimal distribution among current tasks. Furthermore, the algorithm enables decisions to be made about the possibility of performing special or additional tasks without compromising the main monitoring functions. The review of the proposed algorithm provides a basis for hypothesizing its advantages over traditional methods of managing the RS temporal resources. In particular, the use of fuzzy set theory allows for more flexible responses to changes in task execution conditions and enhances the overall adaptability of the system. In the future, this approach could be adapted and applied in other areas where there is a need for resource optimization under conditions of limitation and uncertainty of external factors

Observation of Dispersion in the J=2+ Collective Modes of He-3-B by Nonlinear Acoustic Spectroscopy

Parametric excitation yields new results, which cannot be obtained by conventional linear spectroscopy, on the collective modes in superfluid He-3-B. In our ultrasonic experiments, performed in a chamber with four quartz crystals, we have excited the real squashing (J = 2+) modes by two simultaneous sound waves which are mutually either parallel, orthogonal, or antiparallel. Qualitatively these results are in agreement with the theoretical dispersion relation of the J = 2+ modes, and quantitatively we have been able to extract the collective-mode velocities from the line splittings in zero magnetic field