Superfluid 4He Research Articles

Competition of vortex core structures in superfluid He3−B

Among vortex structures identified so far in superfluid He3−B, the most common are the A-phase-core and double-core vortices. According to earlier numerical calculations, the double-core vortex is energetically favored nearly everywhere in the p−T phase diagram. Nevertheless, in experiments, the A-phase-core vortex has been observed down to temperatures of 0.6Tc at high pressures. We use the Ginzburg-Landau formalism to calculate the energies of the two vortex structures in the experimentally relevant magnetic field as well as the energy barrier for the transition between the two structures. Assigning vanishing barrier as the boundary of the metastability region of the A-phase-core vortex, we reproduce the experimentally measured vortex phase diagram and provide an explanation for the reappearance of the double-core vortex near the critical temperature Tc at low pressures: the difference in Zeeman energy between the two vortex structures becomes relatively more important close to Tc, and the A-phase-core vortex becomes unstable. In contrast to the equilibrium vortex structures, we suggest that the vortex nucleation process favors the A-phase-core vortex over the double-core vortex. Our approach can be used to analyze competition between different vortex structures in other unconventional superfluids and superconductors. Published by the American Physical Society 2024

Axion and dark fermion electromagnetic form factors in superfluid He4

Condensed matter materials have shown great potential in searching for light dark matter (DM) via detecting the phonon or magnon signals induced by the scattering of DM off the materials. In this paper, we study the possibility of detecting electromagnetic form factors of fermionic DM and axionlike particles (ALPs) using superfluid He4. The phonon induced by a sub-GeV fermionic DM scattering off the superfluid can be described using the effective field theory with the interaction between DM and the bulk He4. Signals arising from the electromagnetic form factors of light DM in the presence of an external electric field are calculated. Projected constraints on the charge radius, anapole moment, and magnetic moment of the DM are derived with 1 kg·yr exposure. The phonon signal induced by the scattering of ALPs off the superfluid is also calculated, which can put competitive and the first direct detection bounds on ALP-photon-dark photon couplings in the projected experiments. Published by the American Physical Society 2024

Quantum turbulence, superfluidity, non-Markovian dynamics, and wave function thermalization

While quantum turbulence has been addressed both experimentally (predominantly for superfluid He4 and He3) and theoretically, the dynamics of various ensembles of quantized vortices has been followed in time only until the vortices have decayed into phonons. How this “thermalization” is achieved is still an unaddressed and thus an unelucidated question. The unitary Fermi gas (UFG) is a unique quantum system, which has no classical counterpart and is of relevance to neutron stars, cold atoms, condensed-matter and nuclear many-body systems. The non-Markovian evolution of an isolated UFG is put in evidence and its entire nonequilibrium evolution can be studied theoretically within a unified theoretical framework. The initial lattice of quantum vortices and antivortices evolves through a couple of vortex tangles and excitation of Kelvin waves, where vortices cross and reconnect, until very slowly thermalization sets in. Published by the American Physical Society 2024

Influence of acoustic modes on resonance properties of a quartz tuning fork immersed in superfluid 4He and liquid mixtures 3He–4He

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.

Axion detection via superfluid 3He ferromagnetic phase and quantum measurement techniques

We propose to use the nuclear spin excitation in the ferromagnetic A1 phase of the superfluid 3He for the axion dark matter detection. This approach is striking in that it is sensitive to the axion-nucleon coupling, one of the most important features of the QCD axion introduced to solve the strong CP problem. We review a quantum mechanical description of the nuclear spin excitation and apply it to the estimation of the axion-induced spin excitation rate. We also describe a possible detection method of the spin excitation in detail and show that the combination of the squeezing of the final state with the Josephson parametric amplifier and the homodyne measurement can enhance the sensitivity. It turns out that this approach gives good sensitivity to the axion dark matter with the mass of O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(1) μeV depending on the size of the external magnetic field. We estimate the parameters of experimental setups, e.g., the detector volume and the amplitude of squeezing, required to reach the QCD axion parameter space.

Effect of temperature and heat input on helium isotope separation driven by an entropy filter

To understand the effect of temperature and heat input on helium isotope separation driven by an entropy filter, the thermomechanical flow of superfluid helium through an entropy filter for obtaining high purity 4He is experimentally investigated. For this method, there are two important indicators: separation flow (flow rate) and separation effect (3He concentration). The separation flow rate is examined at various temperatures ranging from 1.6 K to 1.9 K of feed helium. Different heat inputs (Q) are applied to the entropy filter outlet to drive superfluid 4He flowing through the porous element. The results demonstrate that the flow rate increases as the feed helium temperature decreases and heat input increases. Simultaneously, 3He diffusion is detected as the superfluid helium passes through the entropy filter. The concentration of 3He, filtered at different temperature ranging from 1.6 K to 1.9 K, are analyzed using HELIX SFT Static Vacuum Mass Spectrometer. The findings reveal that the 3He concentration decreases with an increase in the temperature of the feed helium bath. 3He concentration of feed helium is around 3.3×10-8. Specifically, the 3He concentration in the filtered helium at 1.6 K is approximately 3.2×10-10, while at 1.9 K, it reduced to 2.2×10-10. This suggests that 3He diffusion in He II is inversely proportional to the He II temperature from 1.6 K to 1.9 K, resulting in a lower 3He concentration at higher temperatures.

Aluminum nanosized beams as probes of superfluid 4He

Aluminum nanosized beams as probes of superfluid 4He

On the Mass Transfer in the 3D Pitaevskii Model

We examine a micro-scale model of superfluidity derived by Pitaevskii (Sov. Phys. JETP 8:282-287, 1959) which describes the interacting dynamics between superfluid He-4 and its normal fluid phase. This system consists of the nonlinear Schrödinger equation and the incompressible, inhomogeneous Navier-Stokes equations, coupled to each other via a bidirectional nonlinear relaxation mechanism. The coupling permits mass/momentum/energy transfer between the phases, and accounts for the conversion of superfluid into normal fluid. We prove the existence of global weak solutions in T3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\mathbb {T}}^3$$\\end{document} for a power-type nonlinearity, beginning from small initial data. The main challenge is to control the inter-phase mass transfer in order to ensure the strict positivity of the normal fluid density, while obtaining time-independent a priori estimates.

Time-resolved solvation of alkali ions in superfluid helium nanodroplets.

The sinking of alkali cations in superfluid 4He nanodroplets is investigated theoretically using liquid 4He time-dependent density functional theory at zero temperature. The simulations illustrate the dynamics of the buildup of the first solvation shell around the ions. The number of helium atoms in this shell is found to linearly increase with time during the first stages of the dynamics. This points to a Poissonian capture process, as concluded in the work of Albrechtsen et al. on the primary steps of Na+ solvation in helium droplets [Albrechtsen et al., Nature 623, 319 (2023)]. The energy dissipation rate by helium atom ejection is found to be quite similar between all alkalis, the main difference being a larger energy dissipated per atom for the lighter alkalis at the beginning of the dynamics. In addition, the number of helium atoms in the first solvation shell is found to be lower at the end of the dynamics than at equilibrium for both Li+ and Na+, pointing to a kinetic rather than thermodynamical control of the snowball size for small and strongly attractive ions.

Rotating curved spacetime signatures from a giant quantum vortex.

Gravity simulators1 are laboratory systems in which small excitations such as sound2 or surface waves3,4 behave as fields propagating on a curved spacetime geometry. The analogy between gravity and fluids requires vanishing viscosity2-4, a feature naturally realized in superfluids such as liquid helium or cold atomic clouds5-8. Such systems have been successful in verifying key predictions of quantum field theory in curved spacetime7-11. In particular, quantum simulations of rotating curved spacetimes indicative of astrophysical black holes require the realization of an extensive vortex flow12 in superfluid systems. Here we demonstrate that, despite the inherent instability of multiply quantized vortices13,14, a stationary giant quantum vortex can be stabilized in superfluid 4He. Its compact core carries thousands of circulation quanta, prevailing over current limitations in other physical systems such as magnons5, atomic clouds6,7 and polaritons15,16. We introduce a minimally invasive way to characterize the vortex flow17,18 by exploiting the interaction of micrometre-scale waves on the superfluid interface with the background velocity field. Intricate wave-vortex interactions, including the detection of bound states and distinctive analogue black hole ringdown signatures, have been observed. These results open new avenues to explore quantum-to-classical vortex transitions and use superfluid helium as a finite-temperature quantum field theory simulator for rotating curved spacetimes19.

Magnon Bose–Einstein condensates: From time crystals and quantum chromodynamics to vortex sensing and cosmology

Under suitable experimental conditions, collective spin-wave excitations, magnons, form a Bose–Einstein condensate (BEC), where the spins precess with a globally coherent phase. Bose–Einstein condensation of magnons has been reported in a few systems, including superfluid phases of 3He, solid state systems, such as yttrium-iron-garnet films, and cold atomic gases. The superfluid phases of 3He provide a nearly ideal test bench for coherent magnon physics owing to experimentally proven spin superfluidity, the long lifetime of the magnon condensate, and the versatility of the accessible phenomena. We first briefly recap the properties of the different magnon BEC systems, with focus on superfluid 3He. The main body of this review summarizes recent advances in the application of magnon BEC as a laboratory to study basic physical phenomena connecting to diverse areas from particle physics and cosmology to vortex dynamics and new phases of condensed matter. This line of research complements the ongoing efforts to utilize magnon BECs as probes and components for potentially room-temperature quantum devices. In conclusion, we provide a roadmap for future directions in the field of applications of magnon BEC to fundamental research.

Parylene-bonded micro-fluidic channels for cryogenic experiments at superfluid He-4 temperatures.

We present the manufacturing process of a (24.5 × 100) μm2-sized on-chipflow channel intended for flow experiments with normal and superfluid phases of 4He and showcase such a proof-of-concept experiment. This work proves the suitability of chip-to-chip bonding using a thin layer of Parylene-C for cryogenic temperatures as a simpler alternative to other techniques, such as anodic bonding. A monocrystalline silicon chip embeds the etched meander-shaped micro-fluidic channel and a deposited platinum heater and is bonded to a Pyrex glass top. We test the leak tightness of the proposed bonding method for superfluid 4He, reaching temperatures of ≈1.6K and evaluate its possible effects on flow experiments. We demonstrate that powering an on-chip platinum heater affects the superfluid flow rate by local overheating of a section of the micro-fluidic channel.

Fully gapped pairing state in spin-triplet superconductor UTe2.

The recently discovered superconductor UTe2 is a promising candidate for spin-triplet superconductors, but the symmetry of the superconducting order parameter remains highly controversial. Here, we determine the superconducting gap structure by the thermal conductivity of ultraclean UTe2 single crystals. We find that the a-axis thermal conductivity divided by temperature κ/T in zero-temperature limit is vanishingly small for both magnetic field H‖a and H‖c axes up to H/Hc2 ∼ 0.2, demonstrating the absence of nodes around the a axis contrary to the previous belief. The present results, combined with the reduction of nuclear magnetic resonance Knight shift, indicate that the superconducting order parameter belongs to the isotropic Au representation with a fully gapped pairing state, analogous to the B phase of superfluid 3He. These findings reveal that UTe2 is likely to be a long-sought three-dimensional strong topological superconductor, hosting helical Majorana surface states on any crystal plane.

Fermionic Quartet and Vestigial Gravity

We discuss the two-step transitions in superconductors, where the intermediate state between the Cooper pair state and the normal metal is the 4-fermion condensate, which is called the intertwined vestigial order. We discuss different types of the vestigial order, which are possible in the spin-triplet superfluid 3He, and the topological objects in the vestigial phases. Since in 3He the order parameter \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{A}_{{\\alpha i}}}$$\\end{document} represents the analog of gravitational tetrads, we suggest that the vestigial states are possible in quantum gravity. As in superconductors, the fermionic vacuum can experience two consequent phase transitions. At first transition the metric appears as the bilinear combination of tetrads \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{g}_{{\\mu \ u }}} = {{\\eta }_{{ab}}}\\langle \\hat {E}_{\\mu }^{a}\\hat {E}_{\ u }^{b}\\rangle $$\\end{document}, while the tetrad order parameter is still absent, \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e_{\\mu }^{a} = \\langle \\hat {E}_{\\mu }^{a}\\rangle = 0$$\\end{document}. This corresponds to the bosonic Einstein general relativity, which emerges in the fermionic vacuum. The nonzero tetrads \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e_{\\mu }^{a} = \\langle \\hat {E}_{\\mu }^{a}\\rangle \ e 0$$\\end{document} appear at the second transition, where a kind of the Einstein–Cartan–Sciama–Kibble tetrad gravity is formed. This suggests that on the levels of particles, gravity acts with different strength on fermions and bosons.

Cross-Component Energy Transfer in Superfluid Helium-4

The reciprocal energy and enstrophy transfers between normal fluid and superfluid components dictate the overall dynamics of superfluid 4He including the generation, evolution and coupling of coherent structures, the distribution of energy among lengthscales, and the decay of turbulence. To better understand the essential ingredients of this interaction, we employ a numerical two-way model which self-consistently accounts for the back-reaction of the superfluid vortex lines onto the normal fluid. Here we focus on a prototypical laminar (non-turbulent) vortex configuration which is simple enough to clearly relate the geometry of the vortex line to energy injection and dissipation to/from the normal fluid: a Kelvin wave excitation on two vortex anti-vortex pairs evolving in (a) an initially quiescent normal fluid, and (b) an imposed counterflow. In (a), the superfluid injects energy and vorticity in the normal fluid. In (b), the superfluid gains energy from the normal fluid via the Donnelly–Glaberson instability.

Thermomagnetoelectric convective effect in normal and superfluid systems

Thermomagnetoelectric effect for the dielectric hydrodynamic system is considered. It is established that in a liquid, which is placed in a magnetic field, the temperature gradient can cause a convective mass flow, which leads to the appearance of an electric field in the surrounding space. The distribution of electric potential for various geometric implementations is calculated. A comparison of the obtained effect for normal and superfluid 4He is performed.

Origins of anomalies in the temperature dependences of specific heat and superfluid density in doped high-Tc cuprates: Signatures of Bose-liquid superconductivity

We show that the temperature-dependent superconducting order parameter and related superconducting properties (in particular, the temperature dependences of specific heat, superfluid density and related London penetration depth) of high-[Formula: see text] cuprates are fundamentally different from those of conventional superconductors and cannot be understood within the existing theories based on the Bardeen–Cooper–Schrieffer (BCS)-type condensation of weakly bound Cooper pairs into a superfluid Fermi-liquid and on the usual Bose–Einstein condensation (BEC) of bosonic Cooper pairs. We examine the validity of an alternative approach to the unconventional superconductivity in high-[Formula: see text] cuprates and establish that these materials exhibiting a [Formula: see text]-like superconducting transition at the critical temperature [Formula: see text] are similar to the superfluid 4He and are also superfluid Bose systems. We argue that the doped high-[Formula: see text] cuprates, from underdoped to overdoped regime, are unconventional (bosonic) superconductors and the tightly bound (polaronic) Cooper pairs in these polar materials behave like composite bosons just like 4He atoms and condense into a Bose superfluid at [Formula: see text]. We identify the superconducting order parameter [Formula: see text] in underdoped and optimally doped cuprates as the coherence parameter [Formula: see text] of bosonic Cooper pairs, which appears just below [Formula: see text] and has a kink-like temperature dependence near the characteristic temperature [Formula: see text]. We find that the [Formula: see text]-like specific heat anomaly in high-[Formula: see text] cuprates near [Formula: see text] predicted by the theory of Bose-liquid superconductivity is similar to that observed both in superfluid 4He near [Formula: see text] and in Hg-based cuprate superconductor HgBa2Ca2Cu3O8 near [Formula: see text][Formula: see text]K. We demonstrate that in high-[Formula: see text] superconductor YBa2Cu3[Formula: see text] the superfluid density [Formula: see text] exhibits distinctly different temperature dependences in the temperature ranges [Formula: see text] and [Formula: see text]. In this superconductor, a pronounced anomaly in [Formula: see text] exists near [Formula: see text] and the temperature dependence of [Formula: see text] below [Formula: see text] deviates downwards from the high-temperature behavior. Our results for the normalized superfluid density [Formula: see text] are in good agreement with the experimental data on the temperature dependence of [Formula: see text] in YBa2Cu3[Formula: see text]. The anomalous temperature dependences of specific heat and superfluid density observed in Hg- and Y-based high-[Formula: see text] superconductors are clear signatures of Bose-liquid superconductivity.

Transport of bound quasiparticle states in a two-dimensional boundary superfluid

The B phase of superfluid 3He can be cooled into the pure superfluid regime, where the thermal quasiparticle density is negligible. The bulk superfluid is surrounded by a quantum well at the boundaries of the container, confining a sea of quasiparticles with energies below that of those in the bulk. We can create a non-equilibrium distribution of these states within the quantum well and observe the dynamics of their motion indirectly. Here we show that the induced quasiparticle currents flow diffusively in the two-dimensional system. Combining this with a direct measurement of energy conservation, we conclude that the bulk superfluid 3He is effectively surrounded by an independent two-dimensional superfluid, which is isolated from the bulk superfluid but which readily interacts with mechanical probes. Our work shows that this two-dimensional quantum condensate and the dynamics of the surface bound states are experimentally accessible, opening the possibility of engineering two-dimensional quantum condensates of arbitrary topology.

Excitation of Josephson Currents by Aerogel Vibrations in Superfluid 3He

The problem of mechanical vibrations of an aerogel attached to an elastic wire in superfluid 3He is solved for the case, where the aerogel is also in a superfluid state. The hydrodynamic boundary conditions at the aerogel surface formulated in this work allow one to explain the anomalously rapid increase in the frequency of mechanical vibrations of the system on cooling. The found relation between the phase jump at the aerogel boundary and the superfluid current flowing across the boundary suggests the Josephson character of such current.

Vortex wake patterns in superfluid 4He

Excitations in the form of quantized vortex rings are known to exist in superfluid 4He at energies and momenta exceeding those of the Landau phonon–roton spectrum. They form a vortex branch of elementary excitations spectrum which is disconnected from the Landau spectrum. Interference of vortex ring excitations determines wake patterns due to uniformly traveling sources in bulk superfluid at low speeds and pressures. The dispersion law of these excitations resembles that of gravity waves on deep water with infrared wave number cutoff. As a result, vortex wake patterns featuring elements of the Kelvin ship wake are predicted. Specifically, at lowest speeds the pattern with fully developed transverse and diverging wavefronts is present. At intermediate speeds transverse wavefronts are absent within a cone whose opening angle increases with the source velocity. At largest speeds only diverging wavefronts confined within a cone whose opening angle decreases with the source velocity are found. When experimentally observed, these changes in appearance of wake patterns serve as indicators of the beginning part of the vortex branch of elementary excitations.