Superfluid Liquid Helium Research Articles

Electronic rotons and Wigner crystallites in a two-dimensional dipole liquid.

A key concept proposed by Landau to explain superfluid liquid helium is the elementary excitation of quantum particles called rotons1-8. The irregular arrangement of atoms in a liquid leads to the aperiodic dispersion of rotons, which played a pivotal role in understanding fractional quantum Hall liquids (magneto-rotons)9,10 and the supersolidity of Bose-Einstein condensates11-13. Even for a two-dimensional electron or dipole liquid, in the absence of a magnetic field, the repulsive interactions have been predicted to form a roton minimum14-19, which can be used to trace the transition to Wigner crystals20-24 and superconductivity25-27, although this has not yet been observed. Here, we report the observation of such electronic rotons in a two-dimensional dipole liquid of alkali-metal ions donating electrons to surface layers of black phosphorus. Our data reveal the striking aperiodic dispersion of rotons, which is characterized by a local minimum of energy at finite momentum. As the density of dipoles decreases so that interactions dominate over the kinetic energy, the roton gap reduces to 0, as in a crystal, signalling Wigner crystallization. Our model shows the importance of short-range order arising from repulsion between dipoles, which can be viewed as the formation of Wigner crystallites (bubbles or stripes) floating in the sea of a Fermi liquid. Our results reveal that the primary origin of electronic rotons (and the pseudogap) is strong correlations.

Paul Harry Roberts. 13 September 1929—17 November 2022

Paul Roberts was a physicist and applied mathematician. He made important and often pioneering contributions in diverse research areas, but mainly with a fluid dynamic theme, which include rotating fluids, geophysical and astrophysical flows and superfluids. The cornerstone was magnetohydrodynamics with particular application to the geodynamo. His most notable achievement was the first three-dimensional self-consistent numerical geodynamo model, which built on the equations he had derived previously governing the Earth's core dynamics. His considerations included the role of thermal and compositional convection, and the inner core boundary layer (a mixed phase region). In addition to his remarkable illustration of magnetic field reversals, he applied his model to core–mantle coupling and variations in the length of the day. Paul investigated superfluid liquid helium on two fronts. He had a lifelong interest in Bose–Einstein condensates on the microscale governed by the nonlinear Schrodinger equation. His early work on the robust derivation of the mean-field Hall–Vinen–Bekharevic–Khalatnikov equations was far ahead of its time. Their importance and relevance was not appreciated until much later.

Superfluid liquid helium control for the primordial inflation polarization explorer balloon payload.

The Primordial Inflation Polarization Explorer (PIPER) is a stratospheric balloon payload to measure the polarization of the cosmic microwave background. Twin telescopes mounted within an open-aperture bucket dewar couple the sky to bolometric detector arrays. We reduce detector loading and photon noise by cooling the entire optical chain to 1.7K or colder. A set of fountain-effect pumps sprays superfluid liquid helium onto each optical surface, producing helium flows of 50-100 cm3 s-1 at heights up to 200cm above the liquid level. We describe the fountain-effect pumps and the cryogenic performance of the PIPER payload during two flights in 2017 and 2019.

Revealing new processes with superfluid liquid helium detectors: the coherent elastic neutrino atom scattering

Revealing new processes with superfluid liquid helium detectors: the coherent elastic neutrino atom scattering

A superfluid liquid helium target for low-momentum electron scattering experiments at the S-DALINAC

The superconducting electron accelerator S-DALINAC enables electron scattering experiments with low momentum transfer and high energy resolution. In order to perform experiments on helium with high precision and high luminosity, a superfluid liquid helium target with good temperature stability was developed. The functionality of this target could be confirmed and its properties were characterized in a commissioning experiment.

Quantum Optomechanics in a Liquid.

We measure the quantum fluctuations of a single acoustic mode in a volume of superfluid He that is coupled to an optical cavity. Specifically, we monitor the Stokes and anti-Stokes light scattered by a standing acoustic wave that is confined by the cavity mirrors. The intensity of these signals (and their cross-correlation) exhibits the characteristic features of the acoustic wave's zero-point motion and the quantum backaction of the intracavity light. While these features are also observed in the vibrations of solid objects and ultracold atomic gases, their observation in superfluid He opens the possibility of exploiting the remarkable properties of this material to access new regimes of quantum optomechanics.

The hydrogen migration in cryogenic tubes of superconductive accelerator with gas source

In superconductive linear accelerator, the performance and stability can be impacted by gas adsorbed on cryogenic surfaces adversely. The cryogenic devices usually work at 4.2 K or 1.9 K and are refrigerated by normal or super fluid liquid helium, respectively. The purpose of this paper is to study the character of gas migration in cryogenic tubes. Adsorption coefficient for hydrogen at 4.2 K is measured by experimental study. Then, a gas migration model is established based on the experimental results to depict the hydrogen migration process in cryogenic tubes. The experimental results and model analysis indicated that at cryogenic temperature (4.2 K), adsorption coefficient for hydrogen is very close to 1, which is several orders higher than the adsorption coefficient at room temperature, resulting in a unique pressure distribution pattern in cryogenic tubes when compared with the pressure distribution in room temperature tubes. At 4.2 K or 1.9 K, the gas migration process is obviously different from the process at room temperature and is significantly affected by the gas adsorption on the cryogenic surfaces. The model established in this article can be applied not only to hydrogen migration in cryogenic tubes but also to other gas migration in tubes with high adsorption coefficient.

Self-consistent one-dimensional electron system on liquid helium suspended over a nanoscale dielectric substrate

For electrons above a superfluid helium film suspended on a specially designed dielectric substrate, z = h(y), we obtain that both the transverse, along z, and the lateral, along y, quantizations are strongly enhanced due to a strong mutual coupling. The self-consistent quantum wires (QWs) with non-degenerated one-dimensional electron systems (1DESs) are obtained over a superfluid liquid helium (LH) suspended self-consistently on different dielectric substrates with a nanoscale modulation. A gap ≳10 meV (≳1 meV) is obtained between the lowest two electron levels due to mainly the transverse (lateral) quantization. Our analytical model takes into account a strong interplay between the transverse and the lateral quantizations of an electron. It uses that the characteristic length (energy) along the former direction is essentially smaller (larger) than the one along the latter, in a close analogy with the adiabatic approximation.

In-orbit performance of a helium dewar for the soft X-ray spectrometer onboard ASTRO-H

ASTRO-H was an X-ray astronomy satellite that the Japan Aerospace Exploration Agency (JAXA) developed to study the evolution of the universe and physical phenomena yet to be discovered. The primary scientific instrument of ASTRO-H was the Soft X-ray Spectrometer (SXS). Its detectors were to be cooled to 50 m K using a complex cryogenic system with a multistage adiabatic demagnetization refrigerator (ADR) developed by the National Aeronautics and Space Administration (NASA), and a cryogenic system developed by Sumitomo Heavy Industries, Ltd. (SHI). SHI’s cryogenic system was required to cool the ADR’s heatsink to 1.3 K or less in orbit for three years or longer. To meet these requirements, SHI developed a hybrid cryogenic system consisting of a liquid helium tank, a 4 K Joule-Thomson cooler, and two two-stage Stirling coolers.ASTRO-H was launched from Tanegashima Space Center on February 17, 2016. The initial operation of the SXS cryogenic system in orbit was completed successfully. The cooling performance was as expected and could have exceeded the lifetime requirement of three years.This paper describes results of ground tests, results of top-off filling of superfluid liquid helium just before launch, and cooling performance in orbit.

Performance of the helium dewar and the cryocoolers of the Hitomi soft x-ray spectrometer

The soft x-ray spectrometer (SXS) was a cryogenic high-resolution x-ray spectrometer onboard the Hitomi (ASTRO-H) satellite that achieved energy resolution of 5 eV at 6 keV, by operating the detector array at 50 mK using an adiabatic demagnetization refrigerator (ADR). The cooling chain from room temperature to the ADR heat sink was composed of two-stage Stirling cryocoolers, a He4 Joule–Thomson cryocooler, and superfluid liquid helium and was installed in a dewar. It was designed to achieve a helium lifetime of more than 3 years with a minimum of 30 L. The satellite was launched on February 17, 2016, and the SXS worked perfectly in orbit, until March 26 when the satellite lost its function. It was demonstrated that the heat load on the helium tank was about 0.7 mW, which would have satisfied the lifetime requirement. This paper describes the design, results of ground performance tests, prelaunch operations, and initial operation and performance in orbit of the flight dewar and the cryocoolers.

Temperature-dependent energy levels of electrons on liquid helium

We present measurements of the resonant microwave absorption by the Rydberg energy levels of surface state electrons on the surface of superfluid liquid helium, in the frequency range 165 - 220 GHz. The resonant frequency was strongly temperature dependent from 0.1 to 2 K. The experiments are in agreement with recent theoretical calculations of the renormalisation of the electron energy levels due to zero-point and thermal ripplons, analogous to a condensed matter Lamb shift. The temperature-dependent contribution to the linewidth for excitation to the first excited state at 189.6 GHz is compared with other measurements and theoretical predictions.

Coagulation of Metals in Superfluid and Normal Liquid Helium.

The thermal emission study in this work has shown that coagulation of metals in liquid helium is accompanied by enormous local overheating of several thousand degrees. Direct experiments demonstrated, for the first time, that condensation of metals in superfluid helium occurs via the specific mechanism which is substantially faster than that in normal liquid helium. It has been stated that coagulation of metals in superfluid helium indeed occurs in two stages, a "hot" one of nanoparticles coalescence with the formation of molten nanospheres and the subsequent stage of their sticking together into nanowires. It turned out that if a laser ablation of metal targets immersed in superfluid helium was used for introducing a metal into liquid, the formation of nanowires occurs at distances of only about 1 mm from the laser focus. This leads to the presence of a considerable number of spherical inclusions in nanowires grown in such a way.

Superfluidity of the confined water

Superfluidity means non-sticky and frictionless when two bodies are set contacting motion. It seems that a substance can be transported in a channel with high velocity and negligible resistance. As a typical example, helium-4 below 2.17 K will become a superfluid. In this case, the flow rate of helium is independent of the driving pressure, and it is impossible to calculate the significant viscosity coefficient from the experimental data. The research on superfluid and supersolid has attracted considerable attention, which mainly focused on extreme conditions (ultra-low temperature, high pressure, etc.). One may ask that can certain substances show superfluid and supersolid behaviors under room temperature. We already know that the flow rate of water in nanometer channel is several orders of magnitude higher than that predicted by the classical theory of fluid mechanics. The molecular dynamics simulation predicts that the water molecules spontaneously form a two-dimensional solid-like square structure in graphene nanocapillaries. This is considered to be the reason for the rapid passage of water molecules through such nano-channels. Water transport in carbon nanotubes exibit similar behaviours. The friction coefficient of water transport in carbon nanotubes is positively correlated with the curvature radius of the nanotubes. That is, the smaller the radius of carbon nanotubes, the smaller the friction coefficient, the greater the flow enhancement. However, for boron nitride nanotubes, although with a similar structure with the carbon nanotubes, it does not show significant flow enhancement because of the different electrical properties of walls. Based on the recent experimental and theoretical simulations, we discussed the superfluid of liquid water exhibits superfluidity in nanochannels, and pointed out that superfluidity of the confined water under room temperature originates from the hydrogen bond relaxation between the low coordination water molecules and the high elasticity and polarization at the interface. We also revealed that the superfluidity of water and liquid helium has both commonality and individuality. The commonality is the quantum pinning and polarization of the local strain induced by the low coordination of atoms or molecules; the individuality is that the superfluid of liquid helium at ultra-low temperature is driven by the spin-resolved polarization, while the superfluidity of the water in the nanochannels may still need some external forces. There are wide applications for the superfluidity of the confined water under room temperature. In recent years, the experimental facilities and the computer capacity have been improved continuously. It has a significant impact on the understanding and cognition of the water transport mechanism at the nanometer scale. Moreover, it can also provide important reference significance for the study of superfluidity of common substances under conventional conditions. Through this article, we hope to stimulate more scholars’ interest in the basic scientific problem of superfluidity, further enhance the understanding of this problem and combine it with macroscopic superfluidity, super-lubrication, super-hydrophobic and super-solidity.

Superfluid Brillouin optomechanics

Optomechanical systems couple an electromagnetic cavity to a mechanical resonator which is typically formed from a solid object. The range of phenomena accessible to these systems depends on the properties of the mechanical resonator and on the manner in which it couples to the cavity fields. In both respects, a mechanical resonator formed from superfluid liquid helium offers several appealing features: low electromagnetic absorption, high thermal conductivity, vanishing viscosity, well-understood mechanical loss, and in situ alignment with cryogenic cavities. In addition, it offers degrees of freedom that differ qualitatively from those of a solid. Here, we describe an optomechanical system consisting of a miniature optical cavity filled with superfluid helium. The cavity mirrors define optical and mechanical modes with near-perfect overlap, resulting in an optomechanical coupling rate ~3 kHz. This coupling is used to drive the superfluid; it is also used to observe the superfluid's thermal motion, resolving a mean phonon number as low as 11.

Flight model performance test results of a helium dewar for the soft X-ray spectrometer onboard ASTRO-H

ASTRO-H is a Japanese X-ray astronomy satellite, scheduled to be launched in fiscal year 2015. The mission includes a soft X-ray spectrometer instrument (SXS), which contains an X-ray micro calorimeter operating at 50mK by using an adiabatic demagnetization refrigerator (ADR). The heat sink of the ADR is superfluid liquid helium below 1.3K. The required lifetime of the superfluid helium is 3years or more. In order to realize this lifetime, we have improved the thermal performance from the engineering model (EM) while maintaining the mechanical performance. Then, we have performed a thermal test of the flight model (FM). The results were that the heat load to the helium tank was reduced to below 0.8mW in the FM from 1.2mW in the EM. Therefore, the lifetime of the superfluid helium is more than 3years with 30L of liquid helium.In this paper, the thermal design and thermal test results are described.

Achievement of 1020 MHz NMR

We have successfully developed a 1020MHz (24.0T) NMR magnet, establishing the world’s highest magnetic field in high resolution NMR superconducting magnets. The magnet is a series connection of LTS (low-Tc superconductors NbTi and Nb3Sn) outer coils and an HTS (high-Tc superconductor, Bi-2223) innermost coil, being operated at superfluid liquid helium temperature such as around 1.8K and in a driven-mode by an external DC power supply. The drift of the magnetic field was initially ±0.8ppm/10h without the 2H lock operation; it was then stabilized to be less than 1ppb/10h by using an NMR internal lock operation. The full-width at half maximum of a 1H spectrum taken for 1% CHCl3 in acetone-d6 was as low as 0.7Hz (0.7ppb), which was sufficient for solution NMR. On the contrary, the temporal field stability under the external lock operation for solid-state NMR was 170ppb/10h, sufficient for NMR measurements for quadrupolar nuclei such as 17O; a 17O NMR measurement for labeled tri-peptide clearly demonstrated the effect of high magnetic field on solid-state NMR spectra.

The Road to Precision: Extraction of the Specific Shear Viscosity of the Quark-Gluon Plasma

For a few fleeting moments after the Big Bang the universe was filled with an astonishingly hot and dense soup known as the Quark-Gluon Plasma (QGP), a precursor to the matter we observe today that consisted of elementary particles. Although this Quark-Gluon Plasma also contained leptons and weak gauge bosons, its transport properties were dominated by the strong interaction between quarks and gluons. Utilizing the most powerful particle accelerators, physicists now conduct head-on collisions between heavy ions, such as gold or lead nuclei, to recreate conditions that existed at the birth of the universe. The most striking discovery in relativistic heavy-ion collisions is that the hot and dense matter created during the collisions behaves like an almost perfect (inviscid) liquid, meaning that it can be characterized by a very small shear viscosity (η) to entropy density (s) ratio (or, “specific shear viscosity”) [1–4]. It turns out that the η/s for the QGP is smaller than that of any known substance, including that of superfluid liquid helium. In Figure 1, we illustrate schematically the specific shear viscosity η/s normalized by , the minimum bound in a large class of theories with infinitely strong coupling [5], for four different types of fluids. The QGP at high temperature exhibits the smallest value of η/s of any fluids occurring in nature.

Development status of the mechanical cryocoolers for the Soft X-ray Spectrometer on board Astro-H

Astro-H is the Japanese X-ray astronomy satellite to be launched in 2015. The Soft X-ray Spectrometer (SXS) on board Astro-H is a high energy resolution spectrometer utilizing an X-ray micro-calorimeter array, which is operated at 50mK by the ADR with the 30liter superfluid liquid helium. The mechanical cryocoolers, 4K-class Joule Thomson (JT) cooler and 20K-class double-staged Stirling (2ST) cooler, are key components of the SXS cooling system to extend the lifetime of LHe cryogen beyond 3years as required. Higher reliability was therefore investigated with higher cooling capability based on the heritage of existing cryocoolers. As the task of assessing further reliability dealt with the pipe-choking phenomena by contaminant solidification of the on-orbit SMILES JT cryocooler, outgassing from materials and component parts used in the cryocoolers was measured quantitatively to verify the suppression of carbon dioxide gas by their storage process and predict the total accumulated carbon dioxide for long-term operation. A continuous running test to verify lifetime using the engineering model (EM) of the 4K-JT cooler is underway, having operated for a total of 720days as of June 2013 and showing no remarkable change in cooling performance. During the current development phase, prototype models (PM) of the cryocoolers were installed to the test SXS dewar (EM) to verify the overall cooling performance from room temperature to 50mK. During the EM dewar test, the requirement to reduce the transmitted vibration from the 2ST cooler compressor was recognized as mitigating the thermal instability of the SXS microcalorimeter at 50mK.

Hysteresis in a quantized superfluid ‘atomtronic’ circuit

Atomtronics is an emerging interdisciplinary field that seeks to develop new functional methods by creating devices and circuits where ultracold atoms, often superfluids, have a role analogous to that of electrons in electronics. Hysteresis is widely used in electronic circuits-it is routinely observed in superconducting circuits and is essential in radio-frequency superconducting quantum interference devices. Furthermore, it is as fundamental to superfluidity (and superconductivity) as quantized persistent currents, critical velocity and Josephson effects. Nevertheless, despite multiple theoretical predictions, hysteresis has not been previously observed in any superfluid, atomic-gas Bose-Einstein condensate. Here we directly detect hysteresis between quantized circulation states in an atomtronic circuit formed from a ring of superfluid Bose-Einstein condensate obstructed by a rotating weak link (a region of low atomic density). This contrasts with previous experiments on superfluid liquid helium where hysteresis was observed directly in systems in which the quantization of flow could not be observed, and indirectly in systems that showed quantized flow. Our techniques allow us to tune the size of the hysteresis loop and to consider the fundamental excitations that accompany hysteresis. The results suggest that the relevant excitations involved in hysteresis are vortices, and indicate that dissipation has an important role in the dynamics. Controlled hysteresis in atomtronic circuits may prove to be a crucial feature for the development of practical devices, just as it has in electronic circuits such as memories, digital noise filters (for example Schmitt triggers) and magnetometers (for example superconducting quantum interference devices).

Universal dynamics of a degenerate unitary Bose gas

Understanding the rich behavior that emerges from systems of interacting quantum particles, such as electrons in materials, nucleons in nuclei or neutron stars, the quark-gluon plasma, and superfluid liquid helium, requires investigation of systems that are clean, accessible, and have tunable parameters. Ultracold quantum gases offer tremendous promise for this application largely due to an unprecedented control over interactions. Specifically, $a$, the two-body scattering length that characterizes the interaction strength, can be tuned to any value. This offers prospects for experimental access to regimes where the behavior is not well understood because interactions are strong, atom-atom correlations are important, mean-field theory is inadequate, and equilibrium may not be reached or perhaps does not even exist. Of particular interest is the unitary gas, where $a$ is infinite, and where many aspects of the system are universal in that they depend only on the particle density and quantum statistics. While the unitary Fermi gas has been the subject of intense experimental and theoretical investigation, the degenerate unitary Bose gas has generally been deemed experimentally inaccessible because of three-body loss rates that increase dramatically with increasing $a$. Here, we investigate dynamics of a unitary Bose gas for timescales that are short compared to the loss. We find that the momentum distribution of the unitary Bose gas evolves on timescales fast compared to losses, and that both the timescale for this evolution and the limiting shape of the momentum distribution are consistent with universal scaling with density. This work demonstrates that a unitary Bose gas can be created and probed dynamically, and thus opens the door for further exploration of this novel strongly interacting quantum liquid.