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We are developing a laser spectroscopic method to study the nuclear structure of radioactive isotopes utilizing superfluid helium (He II) as an efficient stopper for highly energetic ion beams and as an in-situ laser spectroscopic environment. Recently, we conducted an ion stopping experiment for the 84 Rb 37 + beams with the energy of approximately 350 A MeV at QST-HIMAC with a cryostat system used for 66 A MeV at RIKEN-RIPS experiment (Appl. Phys. Express 12, 016502 (2019)). The radioactive 84 Rb ions were produced via a projectile fragmentation reaction using accelerated 84 Kr 36 + beams and a 12-mm thick Be target. As a first step, we measured the stopping range distribution of the injected ion beams in liquid N 2 to estimate the stopping range distribution in He II and the spot size of the injected ion beams using a plastic scintillator. Then, a laser-induced fluorescence (LIF) detection experiment was performed using He II. We successfully observed the LIF from 84 Rb atoms. We estimated the longitudinal range straggling of 84 Rb ions in superfluid helium from the obtained results. The details of experiment at QST-HIMAC and results are given in this report. [ABSTRACT FROM AUTHOR]

Holmium orthophosphate (HoPO4) was prepared using the solid-state method. The XRD pattern analyses have proved that HoPO4 crystallises in the tetragonal structure (I41 / amd). The maximum magnetic entropy change (− Δ S M max) and the relative cooling power (RCP) reach 34.5 J / kg·K and 414.84 J kg−1 at 2.4 K under a magnetic field of 50 kOe, respectively. The lowest transition temperature and the large magnetocaloric effect (MCE) showed that HoPO4 is a potential candidate for superfluid helium magnetic refrigeration applications. [ABSTRACT FROM AUTHOR]

Holmium orthophosphate (HoPO4) was prepared using the solid-state method. The XRD pattern analyses have proved that HoPO4 crystallises in the tetragonal structure (I41 / amd). The maximum magnetic entropy change (− Δ S M max) and the relative cooling power (RCP) reach 34.5 J / kg·K and 414.84 J kg−1 at 2.4 K under a magnetic field of 50 kOe, respectively. The lowest transition temperature and the large magnetocaloric effect (MCE) showed that HoPO4 is a potential candidate for superfluid helium magnetic refrigeration applications. [ABSTRACT FROM AUTHOR]

Experimental data on the movement of the superfluid helium in a U-shaped cylindrical channel filled with and without a permeable porous backfill of metal monodisperse spheres are considered. The results of experiments are presented in the form of the time dependences of the position of the helium-II–vapor interface in the channel. A comparison of the amplitudes and frequencies of vibration of this interface, depending on the pressure in the channel, in the cases of free channel and where it is filled with porous backfill has been performed. It is shown that, in the channel with a monodisperse porous backfill, a stationary state of the interface is possible. [ABSTRACT FROM AUTHOR]

The oscillating quartz tuning fork method has been used to study resonance phenomena in experimental cells of different sizes filled with superfluid 4He and concentrated liquid mixtures of 3He–4He. An analysis of the temperature dependence of the resonance frequencies of the tuning forks showed that in a number of cases, the incompressible fluid model is not sufficient to interpret the experimental results and that acoustic processes in the cell should be taken into account. The frequencies of the resonances of the first sound in cylindrical geometry are estimated and their influence on the resonant frequencies of the tuning fork is shown, which can lead to a distortion of the shape of the resonant line. A comparison is made between experimental results for superfluid 4He and mixtures of 3He-4He with light isotope concentrations of 5% and 15%. It is shown that, in contrast to pure helium, the model of a viscous incompressible fluid cannot be applied to mixtures, even in the absence of first acoustic resonances. This can be explained by the fact that, when studying concentrated solutions, the excitation of the second sound along with the first can have a noticeable effect on the resonance characteristics of the tuning fork. [ABSTRACT FROM AUTHOR]

A high-density ultracold-neutron source based on superfluid helium is being developed at the Petersburg Nuclear Physics Institute (PNPI) of the National Research Center "Kurchatov Institute" for fundamental physics research. This ultracold-neutron source is intended for installation in the largest experimental channel of the PIK reactor complex: the horizontal experimental channel (HEC-4). Calculations indicate that the thermal-neutron flux density at the channel output is 3 × 1010 cm–2 s–1. The new ultracold-neutron source aims to achieve an ultracold neutron density of 3.5 × 103 cm–3 at the reactor-chamber output and 200 cm–3 in the spectrometer designated for measuring the neutron electric dipole moment. The neutron-guide system for ultracold neutrons is designed to support five experimental facilities alternately. Initially, the ultracold-neutron source will be equipped with existing experimental setups: a neutron electric-dipole-moment spectrometer and two setups for measuring the neutron lifetime (utilizing gravitational and magnetic traps). For this ultracold-neutron source, a unique technological cryogenic complex has been designed and implemented to work with superfluid helium under reactor-installation conditions. This complex includes equipment capable of achieving temperatures down to 1 K and removing heat from superfluid helium at a rate of up to 60 W. [ABSTRACT FROM AUTHOR]

Superfluid helium confined to nanofluidic systems is emerging as an important system for studies of two-dimensional turbulence and as the basis for novel quantum technologies. In fully enclosed nanofluidic geometries, only the fourth sound can propagate, which we show can be used for probing quantized vortices pinned in well-defined slab geometry. We show that similarly to well-established second sound attenuation, fourth sound attenuation can be used to infer the number of quantized vortices in a unit area. We experimentally verify fourth sound attenuation as a probe of vortex line density by injecting a known number of quantized vortices into a nanofluidic slab using a rotating cryostat. [ABSTRACT FROM AUTHOR]

We report the results of radioactivity assays and heat leak calculations for a range of common cryogenic materials, considered for use in the QUEST-DMC superfluid 3 He dark matter detector. The bolometer, instrumented with nanomechanical resonators, will be sensitive to energy deposits from dark matter interactions. Events from radioactive decays and cosmic rays constitute a significant background and must be precisely modelled, using a combination of material screening and Monte Carlo simulations. However, the results presented here are of wider interest for experiments and quantum devices sensitive to minute heat leaks and spurious events, thus we present heat leak per unit mass or surface area for every material studied. This can inform material choices for other experiments, especially if underground operation is considered – where the radiogenic backgrounds will dominate even at shallow depths. [ABSTRACT FROM AUTHOR]

We present a numerical study, using the vortex filament model, of vortex tangles in a flow of pure superfluid 4 He in the T = 0 limit through a channel of width D = 1 mm for various applied velocities V. The flat channel walls are assumed to be microscopically rough such that vortices terminating at the walls are permanently pinned; vortices are liberated from their pinned ends exclusively through self-reconnection with their images. Sustained tangles were observed, for a period of 80 s, above the critical velocity V c ∼ 0.20 cm s - 1 = 20 κ D . The coarse-grained velocity profile was akin to a classical parabolic profile of the laminar Poiseuille flow, albeit with a nonzero slip velocity ∼ 0.20 cm s - 1 at the walls. The friction force was found to be proportional to the applied velocity. The effective kinematic viscosity was ν ′ ∼ 0.1 κ , and effective Reynolds numbers within Re ′ < 200 . The fraction of the polarised vortex length varied between zero in the middle of the channel and ∼ 60% within the shear flow regions ∼ D / 4 from the walls. Therefore, we studied a state of statically polarised ultraquantum (Vinen) turbulence fuelled at short length scales by vortex reconnections, including those with vortex images due to the relative motion between the vortex tangle and the pinning rough surface. [ABSTRACT FROM AUTHOR]

High-intensity heavy-ion accelerator facility (HIAF) is an accelerator-driven high-current heavy ion facility, currently under construction in Guangdong, China. The proton linear accelerator of HIAF linac has 96 superconducting cavities and 17 cryomodules cooled by 2 K saturated superfluid helium. Cryogenic tests should be carried out in a test cryogenic system prior to the online operation of each cavity and cryomodule. The cryogenic distribution system (CDS) is the main part of the test cryogenic system, which is used for transporting cryogenic fluid between helium refrigerator and test terminals. A specific CDS is developed due to the long cryogenic test time. Each test terminal can be independently cooled and heated up, saving quite a bit of time without impacting the others. This article provides comprehensive explanations for the process design, process analysis, and special structure design which enhance the system's maintainability. Furthermore, particular technical designs, such as measuring system, helium guard, and elimination of thermoacoustic oscillations, are described in detail. [ABSTRACT FROM AUTHOR]

The paper presents the simulation of a complex of reserch with ultracold neutrons at the reactor PIK (Gatchina, Russia). The complex is being built on the basis of a high-intensity source of ultracold neutrons at the channel GEK-4. A Monte Carlo model has been developed, which includes a source, a neutron guide system and an experimental setup for search for the electric dipole moment of a neutron, taking into account their real location in the main hall of the reactor. Using the developed computer model the density of ultracold neutrons in the setup was obtained, which is 200 cm–3. It is 50 times higher than at the source at the Institut Laue-Langevin (Grenoble, France). This density will allow to achieve a sensitivity of measurements in the experiment of 1 × 10–27 electrons cm/year. [ABSTRACT FROM AUTHOR]

The PIK reactor at NRC "Kurchatov Institute"—PNPI is going to be equipped with a high-flux Ultra Cold Neutron (UCN) source for fundamental physics researches. The UCN source will use superfluid helium, which will make possible to achieve the density of UCN 2.2 × 103 cm–3, that has not yet been achieved anywhere in the world. The UCN source will be installed on the GEK-4 channel, which will make possible to obtain a low value of heat influx to cryogenic vessels from reactor radiation. The heat removal from the UCN source vessel will be implemented by using a heat exchanger. The calculated UCN density in the EDM spectrometer chamber at the PIK is going to be 200 cm–3, which is 20 times higher than the existing UCN densities in the world. For a new UCN source based on superfluid helium, an extensive research program has been developed in field of the physics of fundamental interactions, including the search for a nonzero neutron EDM, precision measurement of the neutron lifetime, and search for mirror dark matter. [ABSTRACT FROM AUTHOR]

A boron-doped diamond electrode with a network structure of PdNi alloy nanowires, which is deposited onto its surface by laser ablation in superfluid helium, is considered as a possible sensor for formaldehyde. It is shown that the electrode is highly sensitive to trace amounts of formaldehyde. [ABSTRACT FROM AUTHOR]

We report the use of a 60 μ m thick superconducting NbTi vibrating wire resonator as a local probe of quantum turbulence in superfluid 4 He (He II). Wire resonance is driven via magneto-motive force, exclusively in the laminar hydrodynamic regime. For the detection of quantized vortices, changes in the probe resonant frequency and peak amplitude are measured in reaction to the applied external counterflow. Calibration of the device response is obtained in thermal counterflow in the temperature range from 1.45 to 2.1 K against second sound attenuation data. The main motivation of this work is the development of local probes of quantum turbulence suitable for use in non-homogeneous systems such as flows with spherical or cylindrical symmetry. The frequency response of the devices is described with good accuracy at lower temperatures by considering the balance between viscosity and mutual friction and its effect on the boundary layer. Under the experimental conditions, the fluid–structure interaction cannot be modeled reliably by an effective turbulent viscosity and agrees better with a model of the boundary layer modified by mutual friction. The obtained results may be extended to the interaction of nanoscale devices with sufficiently dense vortex tangles. [ABSTRACT FROM AUTHOR]

The paper considers experimental data on the dynamics of superfluid helium in a U-shaped cylindrical channel. An experimental cell and a methodology for investigations, parameter measuring, and obtained data processing are described. When a heat flux is applied near the heater, vapor can appear, and the interfacial surface executes regular oscillations of macroscopic amplitude with a constant frequency. The dependences of the interfacial surface position and the corresponding parameters on time are given. The obtained experimental results are interpreted based on the earlier obtained analytical solution. [ABSTRACT FROM AUTHOR]

We propose a mathematical interpolation between several regimes of energy cascade in quantum turbulence in He II. On the basis of a physical interpretation of such mathematical expression we discuss in which conditions it is expected to appear an intermediate k 2 regime (equipartition regime) in the transition region between the hydrodynamic regime and the Kelvin wave regime (namely, between the k − 5 / 3 and k − 1 regions in coflow situations and between the k − 3 and k − 1 regions in counterflow situations). It is seen that if the energy rate transfer from the hydrodynamic region to the Kelvin wave region is sufficiently slow, such equipartition region will be present, but for higher values of such energy rate transfer it will disappear. For high rates of the energy rate transfer, the transition regime between the hydrodynamic and the Kelvin wave regimes will be monotonous, characterized by a negative exponent of k between − 5 / 3 and −1 (or between −3 and −1), instead of the positive 2 exponent of the equipartition regime. [ABSTRACT FROM AUTHOR]

The paper provides a theoretical consideration of the "hotline" phenomenon, a region with an increased phonon energy density, which occurs when two beams cross in helium. An analysis of the results of observations and a theoretical consideration of the formation of a "hotline" are carried out. The results of calculations are also compared with experimental data on the dependence of the "hotline" parameters on the power of the initial beams, and the angle between the directions of their movement. [ABSTRACT FROM AUTHOR]

• Nanowires (2 ÷ 5 nm) were obtained by laser ablation of a tungsten in superfluid helium. • It was found that the tungsten nanowires were formed in the β-phase. • The electrical resistance of the nanowire bundles weakly depends on temperature. Bundles of nanowires with diameters 2 ÷ 5 nm were obtained by laser ablation of a tungsten target immersed in superfluid helium. The morphology and structure of the nanowires were studied by means of transmission electron microscopy. It was found that the tungsten nanowires were formed in the β-phase. The thermal stability of the β-W nanowires was studied. The electrical resistance of the bundles manifested a very weak dependence on temperature with the range from 5 up to 290 К at helium gas pressure 1 bar. [ABSTRACT FROM AUTHOR]

By using a rotating disk electrode, it is shown for the first time that the network structures formed by long ultrathin (diameter ~4 nm) platinum nanowires deposited onto the surface of a glassy carbon electrode demonstrate the high specific catalytic activity in the oxygen reduction reaction. [ABSTRACT FROM AUTHOR]

We report the first observation of the microwave-induced intersubband (Rydberg) resonance of surface-bound electrons on superfluid helium confined in a single 20- μ m wide and 100- μ m long channel. The resonance signal from a few thousand surface electrons comprising a Wigner Solid (WS) is detected by observing WS melting due to the microwave absorption. The transition frequency of electrons, in the range of 0.4–0.5 THz determined by the microwave source used in our experiment, is tuned via the Stark shift of energy levels by a voltage applied to the channel electrode, and the resolution of the transition line requires the depth of the channel to be about 4 μ m. The observed large broadening of the resonance on the order of 10 GHz, which is due to the inhomogeneous distribution of the pressing electric field in the microchannel, is in reasonable agreement with our finite-element modeling calculations. This study of the confined electrons is motivated by their potential use for charge and spin qubits. [ABSTRACT FROM AUTHOR]

We reviewed systems formed in liquid and solid helium-4 at high concentrations (1016 cm−3 and higher) of impurity particles. A new explanation for the reversible coagulation of fine hydrogen particles into big flakes observed upon the transition of liquid helium into the superfluid phase is proposed. The importance of nanoclusters presence in helium crystals doped with impurity particle for the metastability of icebergs is discussed. [ABSTRACT FROM AUTHOR]

We report the first calorimetric detection of individual He2∗ excimers within a bath of superfluid 4He . The detector used in this work is a single superconducting titanium transition edge sensor (TES) with an energy resolution of ∼1eV , immersed directly in the helium bath. He2∗ excimers are produced in the surrounding bath using an external gamma-ray source. These excimers exist either as short-lived singlet or long-lived triplet states. We demonstrate detection (and discrimination) of both states: In the singlet case the calorimeter records the absorption of a prompt ≈15eV photon, and in the triplet case the calorimeter records a direct interaction of the molecule with the TES surface, which deposits a distinct fraction of the ≈15eV , released upon decay, into the surface. We also briefly discuss the detector fabrication and characterization.

This article reviews the development of various sources for ultracold neutrons (UCNs) at the Petersburg Nuclear Physics Institute (PNPI). For 45 years, PNPI has proposed and manufactured cryogenic devices for neutron conversion to low energies. Based on beryllium, hydrogen and deuterium, they can be operated in the intense radiation fields near the core of a nuclear reactor. A more recently launched UCN source development utilizes superfluid helium (He-II) as conversion medium. Initially proposed and designed for PNPI's old WWR-M reactor, the project has been reshaped to equip the institute's PIK reactor with a modern UCN source of this type. The projected UCN density in the closed source chamber is 2.2 x 10³ cm-3, which, as calculations of neutron transport show, will provide 200 cm-3 in the chambers of a neutron EDM spectrometer connected to the source by a UCN guide. Experiments at PNPI with a full-scale UCN source model have demonstrated that a heat load of 60 W can be removed from the He-II in the converter at a temperature of 1.37 K. This fact confirms the practical possibility to implement low-temperature converters under "in-pile" conditions with large heat inflows. The review concludes with a presentation of various proposed options for a He-II based UCN source at the European Spallation Source. [ABSTRACT FROM AUTHOR]

Low counting statistics is one of the most important challenges in modern experiments with ultracold neutrons (UCN). UCN densities in superthermal sources based on superfluid helium are normally much higher than those after UCN delivery to ex-situ volumes. Therefore, and due to the vanishing neutron absorption of 4He, storage-based experiments performed in-situ promise significant sensitivity gains. Scalable measurements offer a promising path to simultaneously address the inefficient use of cold neutron beams as precursors for UCN production in 4He, by recuperating the unused beam fraction, and confront the practical challenges of large-scale UCN infrastructure. We suggest strategies for the development of modular cryogenic cells, propose a novel approach for in-situ UCN detection, and discuss the ultimate statistical reach of such a multiplexed experiment for measuring the neutron's permanent electric dipole moment (EDM). While dedicated research and development are needed to evaluate the feasibility for many requirements, a neutron EDM measurement with sensitivity well beyond 10-28 e cm seems possible. Such an experiment could be pursued at any compatible cold neutron beamline, e.g., at the Institut Laue--Langevin, or later using the ANNI facility or large beam port (LBP) at the European Spallation Source. [ABSTRACT FROM AUTHOR]

A new source of ultracold neutrons (UCNs), developed at the Institut Laue-Langevin (ILL) and named SuperSUN, is currently being commissioned. Its operational principle is the conversion of cold neutrons, delivered by ILL's existing beam H523, to UCNs in a vessel filled with superfluid helium-4, wherein the neutron's energy and momentum are transferred by inelastic scattering to phonons in the superfluid. The inverse Boltzmann-suppressed process is negligible at temperatures below 0.6 K, enabling long storage times and high in-situ UCN densities as demonstrated at the ILL for two prototype sources. These two prototypes are installed at secondary beams behind crystal monochromators, whereas a primary beam with a white cold spectrum illuminates the SuperSUN conversion volume. This provides not only higher intensity around the wavelength 0.89 nm where the dominant single-phonon process for UCN production takes place, but also a contribution to UCN production by multi-phonon processes. In the first phase of the project, material walls will trap the UCNs, while in the second phase an octupole magnet will generate a 2.1 T magnetic field at the edge of the conversion volume. For low-field-seeking UCNs, this field increases the trapping potential and reduces wall losses so that the accumulated UCNs are spin-polarized as a result. SuperSUN aims to deliver the highest possible UCN densities to external storage experiments, the first of which will be the PanEDM experiment measuring the neutron's permanent electric dipole moment. [ABSTRACT FROM AUTHOR]

For twenty years of research, the processes of radiation and dissipation occurring during oscillations of quartz tuning forks in superfluid helium and its mixtures have turned from an object of research into a tool for studying the properties of helium. Quartz tuning forks are used to study various properties of helium - viscosity, thermal conductivity, radiation of the first and second sounds, and also as a precision temperature sensor. Experimental observations of these phenomena were carried out in a wide range of temperatures and pressures, but the results of observations have not yet been exhaustively described theoretically. The aim of this work is to study density and pressure oscillations to determine the conditions under which oscillations of a solid wall excite the first sound in superfluid helium and sound in supercritical helium, and to calculate the contributions of these processes to the formation of resonances during oscillations of closed tuning forks. In particular, the experimentally observed excitation of standing waves of pressure oscillations by an oscillating closed tuning fork, the appearance and properties of resonances depending on the temperature and pressure of helium are considered. As a result of the work, a model was built that described the physical features of the experimentally observed resonance phenomena.

From the experimental point of view, the works on the study of the so-called electrical activity in superfluid helium are analyzed. Based on the analysis of published data on the results of experiments on the appearance of such electrical activity under the propagation of second sound waves, during oscillations of a torsion oscillator filled with helium and microwave electromagnetic pumping of helium, one concludes that the experimental results published before 2016 do not give sufficient reasons to assume the presence of any special electrical activity of helium, and the appearance of an electric signal in a cell with superfluid helium at the propagation of acoustic waves in it can only be said after 2016. The nature of this signal needs further investigation. [ABSTRACT FROM AUTHOR]

Presented here is a brief selective survey of some of the experiments with superfluid 4 He that helped to move forward our understanding of the ability of helium to support persistent flow, especially for helium films. In addition, we discuss some of the experiments that helped to solidify an understanding of the two-dimensional 4 He phase transition. The selectivity is based on some of the areas of helium film work in which the work of John Reppy and his group had contact and parallels with the work of my group. [ABSTRACT FROM AUTHOR]

Superfluid helium has been used for cooling superconducting magnets in nuclear fusion and high energy physics. This paper presents a simplified final stage process of 1.8 K superfluid helium cryogenic system and its key component sub-atmospheric heat exchanger. Influence of variable thermal parameters of sub-atmospheric heat exchanger on the system performance is studied. The application of sub-atmospheric heat exchanger can increase refrigerating capacity by 61.34% under given conditions. The helically finned-tube heat exchanger (HFHE) is selected, and three-dimensional steady-state numerical simulations of flow and heat transfer performance have been carried out. Nu and f of tube side are 4.5-6 times higher and 95-97% lower than those of shell side, respectively. It is found that HFHE with low fin height and HFHE with filler (nylon rope) have better comprehensive heat transfer and flow performance, and PEC can reach 1.65 and 1.41 respectively under given conditions. [ABSTRACT FROM AUTHOR]

We have studied the resonance curves of a quartz tuning fork of the fundamental frequency 32 kHz immersed in superfluid 4He and mixtures 3He–4He with 3He concentration of 5 and 15% in the temperature range of 0.35–2.5 K at saturated vapor pressure. Two types of experiments have been carried out, with a tuning fork both not covered by a bulb and coated by a bulb, i.e., in the restricted geometry. In both cases, the velocity-force dependences for the tuning fork showed a linear damping force at low peak velocities and extra drag due to the appearance of vortex lines accompanying the transition to turbulence under increasing peak velocity. These dependencies are mainly determined by the density of the normal helium component. There is a pronounced difference between superfluid 4He and mixtures of 3He in 4He, where 3He impurity particles provide a constant temperature-independent contribution to the normal component of the mixture. The extra contribution to the damping force, so-called "turbulent drag force", decreases with concentration increase at the same peak velocity of the tuning fork that can be explained by the extension of the range of laminar flow with an increase in the concentration of 3He. We found that the drag coefficient in superfluid 4He and mixtures 3He–4He reaches a plateau at different peak velocities and different exciting forces and explained this fact by different conditions for vortex formation, depending on the different thickness of the near-wall viscous layer. The comparison between the data obtained in restricted and unrestricted geometries shows that there is an excessive dissipation of the tuning fork motion associated with the emission of the first sound wave in unrestricted geometry. [ABSTRACT FROM AUTHOR]

The quantum turbulence is a collection of the interacting quantum vortex loops and filaments. The main goal of this work is to systematize the information on the current state of the research on the reconnecting quantized vortex loops in the superfluid helium: the geometry, the dynamics, the properties of the vortex loops, the energy spectrum before and after the reconnections. The paper discusses the possible role of the reconnections in the formation of the turbulent spectrum. In addition, in this paper the main methods and approaches to the study of the reconnecting vortex loops and quantum turbulence are discussed. [ABSTRACT FROM AUTHOR]

• The values of the recovery heat flux at superfluid helium boiling is differ from immersion depth and liquid temperature. • There is no influence permeability on the recovery heat flux at backfill particles diameters greater than 240 µm. • The calculated results are in satisfactory agreement with the experiment. • Porous insert has a significant effect on the interface dynamics\ (amplitude, frequency, stationary vapor film). The paper presents experimental data on the recovery heat flux during the boiling of superfluid helium (He II) on a flat heater located inside a U-shaped channel. The lower part of the channel is filled with monodisperse porous backfill. The paper includes information on the experimental cell, the data processing and synchronization technique, and the results of experiments conducted at different heater immersion depths and liquid temperatures. The paper presents also a mathematical description of heat transfer processes in superfluid helium for confined conditions. The calculated values are compared with experimental data. The effect of the porous backfill permeability on the recovery heat flux, including the case of a free channel is discussed. It is demonstrated that the increase in the heat flux in the channel initiates oscillations of the phase interface. [ABSTRACT FROM AUTHOR]

The interaction between injected negative charges and vortex motion formed by standing waves with the frequency ω/2π = 49.88 Hz on the surface of normal and superfluid helium has been experimentally studied. It has been found that the vorticity generation results in the redistribution of the current at segments of a collector. A change in the trajectory of the motion of charges from a source to a receiving collector is caused by their interaction with the vortex flow of the normal component. [ABSTRACT FROM AUTHOR]

The interaction between injected negative charges and vortex motion formed by standing waves with the frequency ω/2π = 49.88 Hz on the surface of normal and superfluid helium has been experimentally studied. It has been found that the vorticity generation results in the redistribution of the current at segments of a collector. A change in the trajectory of the motion of charges from a source to a receiving collector is caused by their interaction with the vortex flow of the normal component. [ABSTRACT FROM AUTHOR]

The electric transport I–V characteristics of a tungsten filament immersed in superfluid helium are experimentally studied. The forward sweep I–V characteristics show an abrupt jump from the linear ohmic regime (C state) to the high-resistance non-ohmic regime (H state). In the H state, the filament is covered with a He gas bubble. In the C state, there is no gas bubble, that is, liquid He directly touches the filament surface. The transitions between these two states exhibit a well-developed hysteresis and bistability. The transition from the H state to the C state occurs at the equilibrium gas–liquid phase transition point, as reported by Date et al. (J Phys Soc Jpn 35(4):1190, 1973), whereas the C-to-H-state transition occurs in the superheat region. [ABSTRACT FROM AUTHOR]

Acoustic cavitation is a powerful technique to probe electron bubbles inside the liquid helium. The critical pressure to explode a bubble depends on the number and quantum state of electrons inside the bubble and if the bubble is trapped on a vortex. Here, we report cavitation events that occur at pressure magnitudes approximately 70% lower compared to single electron bubbles. We have considered various possibilities, e.g., single electron bubbles trapped on vortex lines or primary electrons depositing the energy at the acoustic focus and compared the results of our experiments with past measurements reported in the literature. We consider the possibility these new species of bubbles are multielectron bubbles with a small (< 20) number of electrons and discuss future experiments to confirm the same. [ABSTRACT FROM AUTHOR]

The dynamics of thermal radiation accompanying the condensation of tungsten nanoparticles in superfluid helium and vacuum was studied experimentally in the visible range. It was shown that during the first 100 μs the accompanying thermal energy of process in the case of superfluid helium is considerably higher than in vacuum at comparable temperatures after that it levels up. From a standpoint of the process' kinetics it is demonstrated that the reasons for this are, on one hand, an increased condensation rate in superfluid helium (due to the concentration of nanoparticles in quantized vortices), and on the other hand, higher efficiency of heating of the condensation products in superfluid helium (due to the prevalence of processes involving particles of similar sizes). [ABSTRACT FROM AUTHOR]

Single electrons are one of the simplest quantum objects in nature. In free space or conventional materials, single electrons hardly interact with optical (ultraviolet‐visible‐infrared) photons, so their quantum states cannot be easily manipulated by optics. Here, a notable exception is theoretically demonstrated: a single electron in condensed helium‐4 can be self‐confined in a nanometric bubble of 1 to 2 nm radius and resonantly interact with mid‐infrared photons of 3 to 12 µm wavelength. A femtosecond laser can drive the electron into Rabi oscillations and prepare it onto arbitrary superposition states between the ground and excited states. After the laser pulse, the electron bubble can exhibit remarkable nonequilibrium quantum dynamics and evolve toward various final states with various lifetimes. These processes are accompanied with intense phonon radiation. [ABSTRACT FROM AUTHOR]

The technique of simultaneous coagulation of two different materials in superfluid helium has been implemented, which allows one to create different ultrathin (d < 8 nm) heterostructures. A simultaneous ablation of two metal targets (indium and platinum) immersed in the volume of superfluid helium and targets carried out above the surface of the liquid has been performed. During the ablation of targets in the volume of helium, nanowires from individual metals are obtained. During the ablation of targets located above the surface, in addition to nanowires of individual metals, "combined" nanowires are formed when the wire from indium coaxially overlap the platinum wire. [ABSTRACT FROM AUTHOR]

The nonlinear stage in the evolution of anisotropic beams of high-energy and low-energy phonons is considered. The beams are interacting systems that are virtually one-dimensional in phase space. The observations and theoretical approaches to the space and time evolution of these systems are analyzed in the general case, when they are bounded in both the longitudinal and transverse directions. As a result, the system of equations for the beam parameters is reduced to a single nonlinear equation that describes both the longitudinal and transverse evolution of the beam. The numerical solution of this equation provides an explanation of the experimentally observed characteristic features of the beam motion and deformation; it also enables the determination of the beams' main parameters and stages of evolution. [ABSTRACT FROM AUTHOR]

The hydrodynamic forces acting on a quantized vortex in a superfluid have long been a highly controversial issue. A new approach, originally developed in the astrophysical context of compact stars, is presented to determine these forces by considering small perturbations of the asymptotically uniform flows in the region far from the vortex in the framework of Landau–Khalatnikov two-fluid model. Focusing on the irrotational part of the flows in the Helmholtz decomposition, the classical Kutta–Joukowski theorem from ordinary hydrodynamics is thus generalized to superfluid systems. The same method is applied to predict the hydrodynamic forces acting on vortices in cold atomic condensates and superfluid mixtures. [ABSTRACT FROM AUTHOR]