Two-dimensional 3He Research Articles

Intertwined superfluid and density wave order in two-dimensional 4He

A detailed analysis of low-temperature torsional oscillation measurements on two-dimensional 4He reveals evidence for intertwined superfluid and density wave order in this system. Superfluidity is a manifestation of the operation of the laws of quantum mechanics on a macroscopic scale. The conditions under which superfluidity becomes manifest have been extensively explored experimentally in both quantum liquids (liquid 4He being the canonical example) and ultracold atomic gases1,2, including as a function of dimensionality3,4. Of particular interest is the hitherto unresolved question of whether a solid can be superfluid5,6. Here we report the identification of a new state of quantum matter with intertwined superfluid and density wave order in a system of two-dimensional bosons subject to a triangular lattice potential. Using a torsional oscillator we have measured the superfluid response of the second atomic layer of 4He adsorbed on the surface of graphite, over a wide temperature range down to 2 mK. Superfluidity is observed over a narrow range of film densities, emerging suddenly and subsequently collapsing towards a quantum critical point. The unusual temperature dependence of the superfluid density in the limit of zero temperature and the absence of a clear superfluid onset temperature are explained, self-consistently, by an ansatz for the excitation spectrum, reflecting density wave order, and a quasi-condensate wavefunction breaking both gauge and translational symmetry.

Whole lung morphometry with 3D multiple b-value hyperpolarized gas MRI and compressed sensing.

PurposeTo demonstrate three‐dimensional (3D) multiple b‐value diffusion‐weighted (DW) MRI of hyperpolarized 3He gas for whole lung morphometry with compressed sensing (CS).MethodsA fully‐sampled, two b‐value, 3D hyperpolarized 3He DW‐MRI dataset was acquired from the lungs of a healthy volunteer and retrospectively undersampled in the k y and k z phase‐encoding directions for CS simulations. Optimal k‐space undersampling patterns were determined by minimizing the mean absolute error between reconstructed and fully‐sampled 3He apparent diffusion coefficient (ADC) maps. Prospective three‐fold, undersampled, 3D multiple b‐value 3He DW‐MRI datasets were acquired from five healthy volunteers and one chronic obstructive pulmonary disease (COPD) patient, and the mean values of maps of ADC and mean alveolar dimension (Lm D) were validated against two‐dimensional (2D) and 3D fully‐sampled 3He DW‐MRI experiments.ResultsReconstructed undersampled datasets showed no visual artifacts and good preservation of the main image features and quantitative information. A good agreement between fully‐sampled and prospective undersampled datasets was found, with a mean difference of +3.4% and +5.1% observed in mean global ADC and Lm D values, respectively. These differences were within the standard deviation range and consistent with values reported from healthy and COPD lungs.ConclusionsAccelerated CS acquisition has facilitated 3D multiple b‐value 3He DW‐MRI scans in a single breath‐hold, enabling whole lung morphometry mapping. Magn Reson Med 77:1916–1925, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Application of low frequency SQUID NMR to the ultra-low temperature study of atomically layered 3He films adsorbed on graphite

Low frequency pulsed NMR is a powerful technique for the investigation of the nuclear susceptibility and spin-dynamics of strongly correlated systems down to ultra-low temperatures. We describe a versatile broadband pulsed NMR spectrometer using a two- stage Superconducting Quantum Interference Device (SQUID). This instrument has enabled the investigation of the spin-dynamics of 3He films adsorbed on exfoliated graphite into the microkelvin temperature range. In prior work we reported a SQUID NMR study of twodimensional ferromagnetism on a triangular lattice, where 3He films adsorbed on graphite provide an ideal model system. Here we describe the detection of the much weaker signals from strongly correlated fluid films in the second layer of 3He on graphite, using a spectrometer with improved sensitivity. We have measured the low frequency spin-spin relaxation and spin-lattice relaxation of two-dimensional 3He into the microkelvin range. These show an unusual time and frequency dependence. We also describe the use of the 13C-signal from the exfoliated graphite for thermometry, and the unusual properties of the spin-lattice relaxation of that system.

Density fluctuation spectrum of two-dimensional correlated fermion systems

Density fluctuation spectrum of two-dimensional fermions that interact with short-range repulsive interaction is calculated with the self-consistent perturbation theory. The spectrum extends beyond the particle-hole continuum band in the noninteracting case because of the multiparticle excitations. At a large wave vector, a peak develops in the spectrum near the lower threshold of the particle-hole continuum. These results are compared with the recent inelastic neutron scattering experiment on two-dimensional 3He adsorbed on graphite.

Multi-Pair and Exchange Effects in the Dynamic Structure of Two-Dimensional 3He

We examine the effect of fermionic exchange interactions on the dynamic structure function of two-dimensional 3He within a manifestly microscopic theory of excitations. These exchanges have, at different wave lengths and densities, different consequences: At low densities, exchanges are decisive to determine whether the phonon is Landau-damped or not. In the intermediate wave number regime, exchanges are relatively unimportant but they become important again at short wave length corresponding to about four times the Fermi wave number.

Multiple-Spin Exchange in Strongly Correlated Fermion Systems

The concept of Multiple-Spin Exchange is reviewed from the pioneering theoretical paper of Dirac (1926), to recent experimental observations in two-dimensional highly-correlated fermion systems. Special emphasis is put on two-dimensional 3He: a model system with tunable magnetic frustration, exhibiting exotic magnetic states (like spin-liquid) and Mott-Hubbard transition from liquid to commensurate solid.

Compressed sensing in hyperpolarized3He Lung MRI

In this work, the application of compressed sensing techniques to the acquisition and reconstruction of hyperpolarized (3)He lung MR images was investigated. The sparsity of (3)He lung images in the wavelet domain was investigated through simulations based on fully sampled Cartesian two-dimensional and three-dimensional (3)He lung ventilation images, and the k-spaces of 2D and 3D images were undersampled randomly and reconstructed by minimizing the L1 norm. The simulation results show that temporal resolution can be readily improved by a factor of 2 for two-dimensional and 4 to 5 for three-dimensional ventilation imaging with (3)He with the levels of signal to noise ratio (SNR) (approximately 19) typically obtained. The feasibility of producing accurate functional apparent diffusion coefficient (ADC) maps from undersampled data acquired with fewer radiofrequency pulses was also demonstrated, with the preservation of quantitative information (mean ADC(cs) approximately mean ADC(full) approximately 0.16 cm(2) sec(-1)). Prospective acquisition of 2-fold undersampled two-dimensional (3)He images with a compressed sensing k-space pattern was then demonstrated in a healthy volunteer, and the results were compared to the equivalent fully sampled images (SNR(cs) = 34, SNR(full) = 19).

Evaluation of residual stress assessment methods using a repair weld benchmark

A simple design of a letterbox-type repair weld on a plate is evaluated as a benchmark problem. Residual stresses are recorded using the non-destructive neutron diffraction technique and compared with predictions by full three-dimensional finite element modelling. The comparison is performed over a number of path lines in an attempt to evaluate both methods as potential residual stress assessment tools, for weld repairs. The proposed finite element methodology is based on uncoupled quasi-static thermoelasticity and incorporates the element activation–deactivation technique for simulating weld deposition. Sensitivity analysis indicates that the effect of heat loss due to radiation on predicted residual stresses is negligible, whereas incorporation of full annealing into the model is recommended. The diffractometer used, is STRESS-SPEC, located at the FRMII research reactor (Munich, Germany) and comprises a highly flexible monochromator arrangement and a two-dimensional position-sensitive 3He detector. The success of the repair weld benchmark design is illustrated by not only the satisfactory correlation between experimental and computational results in the form of residual stress but also temperature history data.

Magnetic susceptibility and saturation magnetic field in the t-J2-J3-K model: 3He on graphite

We consider dynamics of zero-point vacancies introduced in the two-dimensional (2D) solid 3He adsorbed on a graphite surface. In our previous paper, we calculated the temperature dependence of the heat capacity, single-particle spectra and effective mass, as well as the spin and density excitation spectra of the t-J-K model, and discussed their experimental implications. As an extension of this model, we here propose an improved model, i.e., the t-J2-J3-K model, which takes into account the proper cut of the ring-exchange paths by the zero-point vacancies, and we discuss doping dependence of the magnetic susceptibility and saturation magnetic field in the 2D 3He. We thus show that the improved model is essential in particular for considering the magnetic field dependence of the system.

Zero-point vacancies in the two-dimensional [formula omitted] model: 3He on graphite surface

We consider dynamics of zero-point vacancies introduced in the two-dimensional (2D) solid 3He adsorbed on a graphite surface. We thereby propose an improved model, i.e., t–J2–J3–J4 model, which takes into account the proper truncation of the ring-exchange paths in the presence of the vacancies. We find that, in contrast to the t–J–K model, the doping of vacancies makes the system less ferromagnetic in consistent with experiment and thus the improved model is essential for understanding the magnetic-field dependence of the system.

Torsional oscillator studies for possible supersolidity in two-dimensional 4He solid

The origin of "supersolidity" of solid 4He, which was reported by Kim and Chan, has not yet been clarified. Although Bose-Einstein condensation of zero-point vacancies (ZPVs) was initially considered to be a possible origin, recent studies imply that crystal imperfections in the solid samples are strongly associated with the observed supersolid behaviors. In two-dimensional (2D) 3He system on graphite, on the other hand, a novel phase due to quantum hopping of ZPVs has been found. This suggests that ZPVs form also in 2D 4He solid systems and their quantum hopping results in superfluidity of ZPVs, namely, supersolidity. In order to search for the possible 2D supersolid state, we perform torsional oscillator studies for 4He films adsorbed on graphite. At the 4He coverage of 18.7 nm-2 a positive frequency shift was observed below 300 mK. The size of the shift decreases with increasing the oscillation velocity. This behavior closely resembles to the supersolid behavior of bulk solid 4He.

A high-resolution two-dimensional 3He neutron MSGC with pads for neutron scattering experiments

High-resolution neutron imaging detectors are required for future high-intensity neutron sources such as the J-PARC project in Japan. We are developing a high-resolution gaseous neutron detector by using the microstrip gas chamber (MSGC) with floating-pad readout method. In this paper, we describe the neutron detector performance obtained with this MSGC and the readout method. For a 3 cm×3 cm MSGC filled with 5 bar CF 4 and 0.5 bar 3He, we could achieve a measured position resolution of 0.6 mm (FWHM) including the beam size and the parallax error.

Heat capacities of the anomalous fluid phase in two-dimensional 3He

We measured heat capacities of two dimensional 3He adsorbed on graphite preplated with a 4He monolayer (3He/4He/gr) in a wide temperature (0.3 ≤ T ≤ 80 mK) and density range (1.3 ≤ ρ ≤ 7.3 nm−2). We found that the system behaves as a normal Fermi fluid at low densities between 1.3 and 5.6 nm−2 where the quasiparticle effective mass seems to diverge at a density for the 4/7 phase (ρ4/7 = 6.8 nm−2). At higher densities but below ρ4/7, we observed anomalous temperature dependencies of heat capacity with two round maxima near 1 and 30 mk. With increasing density in this region, a high temperature weight of heat capacity decreases selectively leaving the 30 mK peak, while the 1 mK peak develops. This unexpected behavior can not be explained by the conventional two-phase co-existence model. Instead, we propose here that the 2D normal Fermi fluid is continuously transformed to the 4/7 Mott localized phase through a new quantum phase where a hopping of the zero-point vacancy plays an important role.

Gapless Spin Liquid Behavior in Two-Dimensional SolidHe3

Direct demagnetization has been made for two-dimensional solid 3He in both the paramagnetic and the antiferromagnetic phases. The lowest temperature is about 10 microK, judging from the observed magnetization for the paramagnetic solid 3He. The magnetization of the antiferromagnetic solid 3He shows a gradual increase to about 10 microK for the 4/7 phase adsorbed on both one layer of 4He and two layers of HD preplated graphite. This strongly suggests that the triangular antiferromagnet with the higher order multiple exchange has a quantum spin liquid ground state with nearly zero or extremely small spin gap less than 10 microK.

Direct Nuclear Demagnetization of Two Dimensional Solid3He Adsorbed on Graphite

Direct nuclear demagnetization has been made for two-dimensional solid 3He adsorbed on graphite. The lowest temperature is estimated to be about 10 µK from the observed magnetization of the paramagnetic solid 3He. The deviation from, ideal behavior is explained by a simple heat flow model.

Effective mass of two-dimensional 3He.

We use structural information from diffusion Monte Carlo calculations for two-dimensional 3He to calculate the effective mass. Static effective interactions are constructed from the density and spin-structure functions using sum rules. We find that both spin and density fluctuations contribute about equally to the effective mass. Our results show, in agreement with recent experiments, a flattening of the single-particle self-energy with increasing density, which eventually leads to a divergent effective mass.

Quantum frustration in the "spin liquid" phase of two-dimensional 3He.

We have measured the ultralow temperature and low field magnetic susceptibility of the 4/7 phase of two-dimensional 3He adsorbed on graphite preplated by one layer of 4He. The experiments are performed by progressively adding 4He to the system, thus suppressing in a controlled way the 3He atoms trapped in substrate heterogeneities. This procedure enables us to determine the intrinsic properties of this spin 1/2 model magnet in the zero field limit. The results show quantitatively that the system is strongly frustrated by multiple spin exchange interactions. A characteristic gapped spin liquid behavior is observed at ultralow temperature.

Dimers in two-dimensional 3He- 4He mixtures

Ground-state properties of two-dimensional 3He- 4He mixtures are studied at zero temperature. A general argument based on the long-ranged attraction of the phonon exchange is given for the existence of 3He dimers in low-concentration mixtures with 4He. The binding energy of dimers ranges from milli- to microkelvins with increasing 4He density. By comparing the 3He impurity chemical potential in 4He with the one in pure 3He we conclude that at finite pressures 3He dimers form a mixture with 4He with a maximum solubility of approximately 3%.

3He dimers in two-dimensional 3He– 4He mixture films

Ground state properties of the two-dimensional 3He– 4He mixture are studied at zero temperature. We find that at low concentrations 3He atoms dimerize and form bosonic mixture with 4He. The binding energy of the dimer ranges from 10 −3 K near the saturation density to 10 −6 K at the solidification density. The radius of such a weakly bound dimer is tens of Ångströms. A stable atomic mixture is found at concentration larger than ≈5% varying with density. At high 3He concentrations the mixture phase separates and critical concentrations are obtained by calculating the second derivative of the enthalpy.

Low-temperature magnetization of two-dimensional 3He adsorbed on HD preplated graphite

NMR studies of submonolayer solid 3He adsorbed on graphite preplated by a bilayer of HD have been made down to the order of 0.1 mK at the areal densities before solidification. In the submillikelvin region we found a substantial increase of the magnetization with decreasing temperature, which is unexpected from a degenerate Fermi liquid. This temperature dependence is understood as the existence of a small amount of glassy solid 3He trapped on the substrate heterogeneities.