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Journal ArticleDOI

High-Energy Neutron Scattering from Liquid He 4

01 Jan 1966-Physical Review (American Physical Society)-Vol. 152, Iss: 1, pp 198-200
TL;DR: In this article, high-energy neutrons are used to probe the momentum distribution of helium atoms in liquid helium, and detect the presence of a zero-momentum condensate below a certain value.
Abstract: An experiment is proposed using high-energy neutrons to probe the momentum distribution of helium atoms in liquid helium, and detect the presence of a zero-momentum condensate below ${T}_{\ensuremath{\lambda}}$. It is suggested that for momentum transfers to the neutron much larger than a roton momentum, the energy transfer should be equal to the recoil energy of a single helium atom, Doppler-shifted by its initial motion in the helium bath. Thus, if a finite fraction of atoms are initially in the zero-momentum state, they will contribute a peak to the spectrum of scattered neutrons. Corrections due to final-state interactions are discussed briefly and estimated.
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TL;DR: The theory of critical phenomena in systems at equilibrium is reviewed at an introductory level with special emphasis on the values of the critical point exponents α, β, γ,..., and their interrelations as mentioned in this paper.
Abstract: The theory of critical phenomena in systems at equilibrium is reviewed at an introductory level with special emphasis on the values of the critical point exponents α, β, γ,..., and their interrelations. The experimental observations are surveyed and the analogies between different physical systems - fluids, magnets, superfluids, binary alloys, etc. - are developed phenomenologically. An exact theoretical basis for the analogies follows from the equivalence between classical and quantal `lattice gases' and the Ising and Heisenberg-Ising magnetic models. General rigorous inequalities for critical exponents at and below Tc are derived. The nature and validity of the `classical' (phenomenological and mean field) theories are discussed, their predictions being contrasted with the exact results for plane Ising models, which are summarized concisely. Pade approximant and ratio techniques applied to appropriate series expansions lead to precise critical-point estimates for the three-dimensional Heisenberg and Ising models (tables of data are presented). With this background a critique is presented of recent theoretical ideas: namely, the `droplet' picture of the critical point and the `homogeneity' and `scaling' hypotheses. These lead to a `law of corresponding states' near a critical point and to relations between the various exponents which suggest that perhaps only two or three exponents might be algebraically independent for any system.

1,792 citations

Journal ArticleDOI
TL;DR: In this article, a review of single-particle momentum distributions in light atoms and molecules is presented with specific emphasis on experimental measurements using the deep inelastic neutron scattering technique at eV energies.
Abstract: Studies of single-particle momentum distributions in light atoms and molecules are reviewed with specific emphasis on experimental measurements using the deep inelastic neutron scattering technique at eV energies. The technique has undergone a remarkable development since the mid-1980s, when intense fluxes of epithermal neutrons were made available from pulsed neutron sources. These types of measurements provide a probe of the short-time dynamics of the recoiling atoms or molecules as well as information on the local structure of the materials. The paper introduces both the theoretical framework for the interpretation of deep inelastic neutron scattering experiments and thoroughly illustrates the physical principles underlying the impulse approximation from light atoms and molecules. The most relevant experimental studies performed on a variety of condensed matter systems in the last 20 years are reviewed. The experimental technique is critically presented in the context of a full list of published work. ...

215 citations

Journal ArticleDOI
TL;DR: In this paper, the theoretical background for the interpretation of neutron Compton scattering, with emphasis on studies of solids, is reviewed, with a focus on solids and quantum liquids and solids.
Abstract: Neutron Compton scattering measurements have the potential to provide direct information about atomic momentum distributions and adiabatic energy surfaces in condensed matter. First applied to measuring the condensate fraction in superfluid helium, the technique has recently been extended to study a variety of classical and quantum liquids and solids. This article reviews the theoretical background for the interpretation of neutron Compton scattering, with emphasis on studies of solids.

157 citations

Journal ArticleDOI
TL;DR: These spectra probe the dynamic and static structure factors of the gas and provide a direct link to two-body correlations and are characterized and measured their density dependence across the broad Feshbach resonance at 834 G.
Abstract: We present a comprehensive study of the Bose-Einstein condensate to Bardeen-Cooper-Schrieffer (BEC-BCS) crossover in fermionic 6Li using Bragg spectroscopy. A smooth transition from molecular to atomic spectra is observed with a clear signature of pairing at and above unitarity. These spectra probe the dynamic and static structure factors of the gas and provide a direct link to two-body correlations. We have characterized these correlations and measured their density dependence across the broad Feshbach resonance at 834 G.

153 citations

Journal ArticleDOI
TL;DR: There has been considerable progress in the study of the structure and elementary excitations of liquid helium as discussed by the authors, which has largely arisen from the development and refinement of neutron, X-ray and light scattering techniques and also from more detailed ultrasonic measurements.
Abstract: During the past decade there has been considerable progress in the study of the structure and elementary excitations of liquid helium. This has largely arisen from the development and refinement of neutron, X-ray and light scattering techniques and also from more detailed ultrasonic measurements. A large part of this review is concerned with describing the results of these experiments and their implications for theories of liquid helium. During the last few years there has also been considerable progress made in the theory of the excitations in liquid helium. These developments are briefly reviewed and the most promising lines of development indicated.

151 citations