Excitations in Liquid Helium: Neutron Scattering Measurements

Yarnell, J. L.; Arnold, G.; Bendt, P.J.; Kerr, E.C.

doi:10.1103/physrev.113.1379

Cited by 167 publications

(64 citation statements)

References 15 publications

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“…Besides the well-known phonon mode, there is a clear parabolic minimum at finite quasimomentum in the excitation spectrum. Such a minimum is analogous to a roton excitation in the context of superfluid helium [118][119][120][121] and systems with long-range [122][123][124][125] (or finiterange [126,127]) interactions. However, in this regime the minimum at finite momentum originates from the double well structure of the linear spectrum.…”

Section: Bragg Spectroscopy Indicating Roton-like Excitationmentioning

confidence: 99%

Properties of spin–orbit-coupled Bose–Einstein condensates

et al. 2016

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The experimental and theoretical research of spin-orbit-coupled ultracold atomic gases has advanced and expanded rapidly in recent years. Here, we review some of the progress that either was pioneered by our own work, has helped to lay the foundation, or has developed new and relevant techniques. After examining the experimental accessibility of all relevant spin-orbit coupling parameters, we discuss the fundamental properties and general applications of spin-orbit-coupled Bose-Einstein condensates (BECs) over a wide range of physical situations. For the harmonically trapped case, we show that the ground state phase transition is a Dicke-type process and that spin-orbit-coupled BECs provide a unique platform to simulate and study the Dicke model and Dicke phase transitions. For a homogeneous BEC, we discuss the collective excitations, which have been observed experimentally using Bragg spectroscopy. They feature a roton-like minimum, the softening of which provides a potential mechanism to understand the ground state phase transition. On the other hand, if the collective dynamics are excited by a sudden quenching of the spin-orbit coupling parameters, we show that the resulting collective dynamics can be related to the famous Zitterbewegung in the relativistic realm. Finally, we discuss the case of a BEC loaded into a periodic optical potential. Here, the spin-orbit coupling generates isolated flat bands within the lowest Bloch bands whereas the nonlinearity of the system leads to dynamical instabilities of these Bloch waves. The experimental verification of this instability illustrates the lack of Galilean invariance in the system.

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Section: Bragg Spectroscopy Indicating Roton-like Excitationmentioning

confidence: 99%

Properties of spin–orbit-coupled Bose–Einstein condensates

et al. 2016

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“…For a weakly interacting homogeneous Bose-Einstein condensate (BEC), the excitation spectrum is given by the Bogoliubov dispersion relation [10], with low-q phonon excitations and highq particle-like excitations. Predictions of the Bogoliubov theory have been experimentally verified both in harmonically trapped gases, invoking the local density approximation [4,5], and in homogeneous atomic BECs [9].Much richer physics, including phenomena traditionally associated with superfluid liquid helium, such as the roton minimum in the excitation spectrum [11], is expected in strongly interacting atomic BECs (for a recent review see [12]). The strength of two-body interactions, characterised by the s-wave scattering length a, can be enhanced by exploiting magnetic Feshbach resonances [13].…”

mentioning

confidence: 99%

“…Much richer physics, including phenomena traditionally associated with superfluid liquid helium, such as the roton minimum in the excitation spectrum [11], is expected in strongly interacting atomic BECs (for a recent review see [12]). The strength of two-body interactions, characterised by the s-wave scattering length a, can be enhanced by exploiting magnetic Feshbach resonances [13].…”

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confidence: 99%

Quasiparticle Energy in a Strongly Interacting Homogeneous Bose-Einstein Condensate

et al. 2017

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Using two-photon Bragg spectroscopy, we study the energy of particle-like excitations in a strongly interacting homogeneous Bose-Einstein condensate, and observe dramatic deviations from Bogoliubov theory. In particular, at large scattering length a the shift of the excitation resonance from the free-particle energy changes sign from positive to negative. For an excitation with wavenumber q, this sign change occurs at a ≈ 4/(πq), in agreement with the Feynman energy relation and the static structure factor expressed in terms of the two-body contact. For a 3/q we also see a breakdown of this theory, and better agreement with calculations based on the Wilson operator product expansion. Neither theory explains our observations across all interaction regimes, inviting further theoretical efforts.PACS numbers: 03.75. Hh, 67.85.De, 67.85.Hj Spectroscopy of elementary excitations in a many-body system is one of the primary methods for probing the effects of interactions and correlations in the ground state of the system, which are at the heart of macroscopic phenomena such as superfluidity [1,2]. In ultracold atomic gases, two-photon Bragg spectroscopy provides a measurement of the excitation energy ω at a well defined wavenumber q [3-9]. For a weakly interacting homogeneous Bose-Einstein condensate (BEC), the excitation spectrum is given by the Bogoliubov dispersion relation [10], with low-q phonon excitations and highq particle-like excitations. Predictions of the Bogoliubov theory have been experimentally verified both in harmonically trapped gases, invoking the local density approximation [4,5], and in homogeneous atomic BECs [9].Much richer physics, including phenomena traditionally associated with superfluid liquid helium, such as the roton minimum in the excitation spectrum [11], is expected in strongly interacting atomic BECs (for a recent review see [12]). The strength of two-body interactions, characterised by the s-wave scattering length a, can be enhanced by exploiting magnetic Feshbach resonances [13]. However, this also enhances three-body inelastic collisions, making the experiments on strongly interacting bulk BECs [6,[14][15][16] challenging and still scarce [17]. A deviation from the Bogoliubov spectrum was observed in Bragg spectroscopy of large-q excitations in a harmonically trapped 85 Rb BEC [6], and has inspired various theoretical interpretations [6,12,[18][19][20][21][22], with no consensus or complete quantitative agreement with the experiments being reached so far.In this Letter, we use Bragg spectroscopy to study the large-q, particle-like excitations in a strongly interacting homogeneous 39 K BEC, produced in an optical box trap [24]. Our homogeneous system allows more direct comparisons with theory, and we also explore stronger interactions than in previous experiments. We show that at large a the excitation-energy shift from the free-particle dispersion relation strongly deviates from the Bogoliubov theory and even changes sign from positive to negative. For a 3/q our measurements are ...

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“…These equations were written in operational form and he never referred to Madelung's work. More surprisingly, he never went beyond this stage but used general features of what he considered some basic aspects of quantum hydrodynamics to obtain the qualitative form of the energy spectrum of a quantum liquid, applying the results to He 4 and from the well known form London (1964); Wilks (1966);Yarnell et al (1959) of such spectrum computed the temperature dependance of thermodynamic functions for this system. And further, without invoking any other quantum mechanical aspects of liquid helium he set forth, using only the form of such spectrum, his well known two-fluid theory of superfluid helium He 4 .…”

Section: Quantum Hydrodynamicsmentioning

confidence: 99%