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result(s) for
"helium"
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Quantum liquid droplets in a mixture of Bose-Einstein condensates
In recent years, quantum fluids have been studied largely in gaseous form, such as the Bose-Einstein condensates (BECs) of alkali atoms and related species. Quantum liquids, other than liquid helium, have been comparatively more difficult to come by. Cabrera et al. combined two BECs and manipulated the atomic interactions to create droplets of a quantum liquid (see the Perspective by Ferrier-Barbut and Pfau). Because the interactions were not directional, the droplets had a roughly round shape. The simplicity of this dilute system makes it amenable to theoretical modeling, enabling a better understanding of quantum fluids. Science , this issue p. 301 ; see also p. 274 Tuning interatomic interactions in two ultracold gases of potassium atoms creates quantum liquid droplets. Quantum droplets are small clusters of atoms self-bound by the balance of attractive and repulsive forces. Here, we report on the observation of droplets solely stabilized by contact interactions in a mixture of two Bose-Einstein condensates. We demonstrate that they are several orders of magnitude more dilute than liquid helium by directly measuring their size and density via in situ imaging. We show that the droplets are stablized against collapse by quantum fluctuations and that they require a minimum atom number to be stable. Below that number, quantum pressure drives a liquid-to-gas transition that we map out as a function of interaction strength. These ultradilute isotropic liquids remain weakly interacting and constitute an ideal platform to benchmark quantum many-body theories.
Journal Article
A review of defect engineering, ion implantation, and nanofabrication using the helium ion microscope
The helium ion microscope has emerged as a multifaceted instrument enabling a broad range of applications beyond imaging in which the finely focused helium ion beam is used for a variety of defect engineering, ion implantation, and nanofabrication tasks. Operation of the ion source with neon has extended the reach of this technology even further. This paper reviews the materials modification research that has been enabled by the helium ion microscope since its commercialization in 2007, ranging from fundamental studies of beam–sample effects, to the prototyping of new devices with features in the sub-10 nm domain.
Journal Article
Signatures of exciton condensation in a transition metal dichalcogenide
Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. Using the recently developed technique of momentum-resolved electron energy-loss spectroscopy (M-EELS), we studied electronic collective modes in the transition metal dichalcogenide semimetal 1T-TiSe₂. Near the phase-transition temperature (190 kelvin), the energy of the electronic mode fell to zero at nonzero momentum, indicating dynamical slowing of plasma fluctuations and crystallization of the valence electrons into an exciton condensate. Our study provides compelling evidence for exciton condensation in a three-dimensional solid and establishes M-EELS as a versatile technique sensitive to valence band excitations in quantum materials.
Journal Article
Visualization of Oscillatory Electron Dynamics on the Surface of Liquid Helium
We investigate time traces of the currents generated by the motion of electrons on the surface of liquid helium that are placed in a perpendicular magnetic field and exposed to microwave radiation. Nonlinear dynamics methods are utilized to explore the characteristic features of the current oscillations from five electrodes at different electron densities and pressing voltages. The wavelet phase coherence and phase shift are calculated to obtain the coherence relationships between the currents in the five electrodes, and the direction of motion of electrons inside the cell, as functions of the pressing voltage. These classical methods reveal that the electron motion is oscillatory with varying frequency and with a constant frequency modulation. Higher harmonics due to nonlinearity arise at higher frequencies where the the resonance condition for inter-subband transition is satisfied at a pressing voltage of 4.20 V for low electron density. Our approach provides a platform for investigating these phenomena analytically. We show that slow helium gravity waves modulate the electronic oscillatory behaviour and illustrate that the model in fact produces 3D dynamics. Motion of electrons on the surface of liquid helium is shown to be a paradigmatic example of a chronotaxic system, i.e. a system that undergoes continuous perturbation and is nonetheless capable of maintaining its stability.
Dissertation
Kosterlitz-Thouless melting of magnetic order in the triangular quantum Ising material TmMgGaO4
Frustrated magnets hold the promise of material realizations of exotic phases of quantum matter, but direct comparisons of unbiased model calculations with experimental measurements remain very challenging. Here we design and implement a protocol of employing many-body computation methodologies for accurate model calculations—of both equilibrium and dynamical properties—for a frustrated rare-earth magnet TmMgGaO
4
(TMGO), which explains the corresponding experimental findings. Our results confirm TMGO is an ideal realization of triangular-lattice Ising model with an intrinsic transverse field. The magnetic order of TMGO is predicted to melt through two successive Kosterlitz–Thouless (KT) phase transitions, with a floating KT phase in between. The dynamical spectra calculated suggest remnant images of a vanishing magnetic stripe order that represent vortex–antivortex pairs, resembling rotons in a superfluid helium film. TMGO therefore constitutes a rare quantum magnet for realizing KT physics, and we further propose experimental detection of its intriguing properties.
TmMgGaO
4
is one of a number of recently-synthesized quantum magnets that are proposed to realize important theoretical models. Here the authors demonstrate the agreement between detailed experimental measurements and state-of-the-art predictions based on the 2D transverse-field triangular lattice Ising model.
Journal Article
Distributions and accumulation mechanisms of helium in petroliferous basins
Helium is an irreplaceable strategic mineral resource, and commercial helium-rich gas fields (He>0.1%) worldwide are typically discovered serendipitously during hydrocarbon exploration efforts. According to an analysis of 75 helium-rich gas fields and 1048 natural gas samples worldwide, helium in natural gas generally exhibits “scarce”, “accompanying”, and “complex” properties, and helium-rich gas fields often occur at depths <4500 m. Helium concentrations in He-CH
4
and He-CO
2
gas fields are notably lower than those in He-N
2
gas fields (He>1%). However, geological reserves in the former two types of gas fields are mainly in the range of 10
7
–10
11
m
3
, whereas in the latter, they are only in the range of 10
5
–10
7
m
3
. There are nevertheless notable disparities in the genesis and migration patterns between helium and gaseous hydrocarbons. Helium necessitates carriers (such as formation water, hydrocarbon fluids, N
2
, mantle-derived fluids, etc.) during both accumulation and long-distance migration processes, where migration conduits are not confined to sedimentary strata, and may extend to the basin’s basement, lower crust, and even lithospheric mantle. However, the accumulation conditions of both helium and gaseous hydrocarbons are generally considered equivalent. The presence of gaseous hydrocarbons facilitates both the rapid exsolution of helium within helium-containing fluids and subsequent efficient aggregation in gaseous hydrocarbons, while both reduce helium diffusion and diminish escape flux. In terms of caprock, gypsum, salt, and thick shale as sealing layers contribute to the long-term preservation of helium over geological timescales. Large helium-rich gas fields, predominantly crust-derived gas fields, are primarily concentrated in uplifted zones of ancient cratonic basins and their peripheries. Based on a diagram of the He concentration versus He/N
2
ratio, crust-derived helium fields can be categorized as basement, combined basement-sedimentary rock, and sedimentary rock helium supply types. Comprehensively given China’s helium grade, helium resource endowment, natural gas industrialization process, and current helium purification processes, the foremost deployment zones for the commercial production of helium should be the helium-rich gas fields located in the Ordos, Tarim, Sichuan, and Qaidam Basins in western and central China. In addition, certain (extra) large helium-containing gas fields serve as important replacement zones.
Journal Article
Observation of first and second sound in a BKT superfluid
Superfluidity in its various forms has been of interest since the observation of frictionless flow in liquid helium II
1
,
2
. In three spatial dimensions it is conceptually associated with the emergence of long-range order at a critical temperature. One of the hallmarks of superfluidity, as predicted by the two-fluid model
3
,
4
and observed in both liquid helium
5
and in ultracold atomic gases
6
,
7
, is the existence of two kinds of sound excitation—the first and second sound. In two-dimensional systems, thermal fluctuations preclude long-range order
8
,
9
; however, superfluidity nevertheless emerges at a non-zero critical temperature through the infinite-order Berezinskii–Kosterlitz–Thouless (BKT) transition
10
,
11
, which is associated with a universal jump
12
in the superfluid density without any discontinuities in the thermodynamic properties of the fluid. BKT superfluids are also predicted to support two sounds, but so far this has not been observed experimentally. Here we observe first and second sound in a homogeneous two-dimensional atomic Bose gas, and use the two temperature-dependent sound speeds to determine the superfluid density of the gas
13
–
16
. Our results agree with the predictions of BKT theory, including the prediction of a universal jump in the superfluid density at the critical temperature.
First and second sound are experimentally observed in a two-dimensional superfluid, and the temperature-dependent sound speeds reveal the predicted jump in the superfluid density at the infinite-order Berezinskii–Kosterlitz–Thouless transition.
Journal Article