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| 2. |
- Baym, Gordon, et al.
(författare)
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Damping of gravitational waves by matter
- 2017
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Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 96:8
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Tidskriftsartikel (refereegranskat)abstract
- We develop a unified description, via the Boltzmann equation, of damping of gravitational waves by matter, incorporating collisions. We identify two physically distinct damping mechanisms-collisional and Landau damping. We first consider damping in flat spacetime, and then generalize the results to allow for cosmological expansion. In the first regime, maximal collisional damping of a gravitational wave, independent of the details of the collisions in the matter is, as we show, significant only when its wavelength is comparable to the size of the horizon. Thus damping by intergalactic or interstellar matter for all but primordial gravitational radiation can be neglected. Although collisions in matter lead to a shear viscosity, they also act to erase anisotropic stresses, thus suppressing the damping of gravitational waves. Damping of primordial gravitational waves remains possible. We generalize Weinberg's calculation of gravitational wave damping, now including collisions and particles of finite mass, and interpret the collisionless limit in terms of Landau damping. While Landau damping of gravitational waves cannot occur in flat spacetime, the expansion of the universe allows such damping by spreading the frequency of a gravitational wave of given wave vector.
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| 3. |
- Baym, Gordon, et al.
(författare)
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Landau critical velocity in weakly interacting Bose gases
- 2012
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 86:2, s. 023602-
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Tidskriftsartikel (refereegranskat)abstract
- The flow of a uniform Bose gas at speeds greater than the Landau critical velocity nu(c) does not necessarily destroy superfluidity, but rather needs only to lead to a decrease of the superfluid mass density rho(s). Analyzing a weakly interacting Bose gas with a finite-range interparticle interaction that leads to a Landau critical velocity at nonzero quasiparticle momentum, we explicitly construct the (nonuniform) condensate for fluid flow faster than nu(c) and calculate the accompanying decrease in rho s. We briefly comment on the relation of the physics to other problems in superfluids, e.g., solitons and vortices in Bose-Einstein condensates, and critical currents in superconductors.
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| 4. |
- Baym, Gordon, et al.
(författare)
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Low-Temperature Transport Properties of Very Dilute Classical Solutions of He in Superfluid He
- 2015
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Ingår i: Journal of Low Temperature Physics. - : Springer Science and Business Media LLC. - 0022-2291 .- 1573-7357. ; 178:3-4, s. 200-228
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Tidskriftsartikel (refereegranskat)abstract
- We report microscopic calculations of the thermal conductivity, diffusion constant, and thermal diffusion constant for classical solutions of He in superfluid He at temperatures K, where phonons are the dominant excitations of the He. We focus on solutions with He concentrations , for which the main scattering mechanisms are phonon-phonon scattering via 3-phonon Landau and Beliaev processes, which maintain the phonons in a drifting equilibrium distribution, and the slower process of He-phonon scattering, which is crucial for determining the He distribution function in transport. We use the fact that the relative changes in the energy and momentum of a He atom in a collision with a phonon are small to derive a Fokker-Planck equation for the He distribution function, which we show has an analytical solution in terms of Sonine polynomials. We also calculate the corrections to the Fokker-Planck results for the transport coefficients.
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| 5. |
- Baym, Gordon, et al.
(författare)
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Normal mass density of a superfluid Fermi gas at unitarity
- 2013
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 88:4, s. 043631-
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Tidskriftsartikel (refereegranskat)abstract
- We calculate the normal mass density of a paired Fermi gas at unitarity. The dominant contribution near the superfluid transition is from fermionic quasiparticle excitations and is, thus, sensitive to the pairing gap. A comparison with the recent experiment of Sidorenkov et al. [Nature (London) 498, 78 (2013)] suggests that the superfluid gap near the transition temperature is larger than the BCS value, but the data do not permit a quantitative inference of the gap. Calculations of the quenched moment of inertia of a BCS superfluid in a harmonic trap are in reasonable agreement with the earlier experiment of Riedl et al. [New J. Phys. 13, 035003 (2011)].
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| 6. |
- Baym, Gordon, et al.
(författare)
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Searching for low mass dark matter via phonon creation in superfluid He-4
- 2020
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Ingår i: Physical Review D. - : American Physical Society (APS). - 1550-7998 .- 1550-2368. ; 102:3
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Tidskriftsartikel (refereegranskat)abstract
- We consider the scattering of dark matter particles from superfluid liquid He-4, which has been proposed as a target for their direct detection. Focusing on dark matter masses below similar to 1 MeV, we demonstrate from sum-rule arguments the importance of the production of single phonons with energies. omega less than or similar to 1 meV. We show further that the anomalous dispersion of phonons in liquid He-4 at low pressures [i.e., d(2)omega(q)/dq(2) > 0, where q and omega(q) are the phonon momentum and energy] has the important consequence that a single phonon will decay over a relatively short distance into a shower of lower-energy phonons centered on the direction of the original phonon. Thus, the experimental challenge in this regime is to detect a shower of low-energy phonons, not just a single phonon. Additional information from the distribution of phonons in such a shower could enhance the determination of the dark matter mass.
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| 7. |
- Baym, Gordon, et al.
(författare)
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Transport in ultradilute solutions of He-3 in superfluid He-4
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:2
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Tidskriftsartikel (refereegranskat)abstract
- We calculate the effect of a heat current on transporting He-3 dissolved in superfluid He-4 at ultralow concentration, as will be utilized in a proposed experimental search for the electric dipole moment of the neutron (nEDM). In this experiment, a phonon wind will be generated to drive (partly depolarized) 3He down a long pipe. In the regime of 3He concentrations less than or similar to 10(-9) and temperatures similar to 0.5 K, the phonons comprising the heat current are kept in a flowing local equilibrium by small angle phonon-phonon scattering, while they transfer momentum to the walls via the He-4 first viscosity. On the other hand, the phonon wind drives the 3He out of local equilibrium via phonon-He-3 scattering. For temperatures below 0.5 K, both the phonon and He-3 mean free paths can reach the centimeter scale, and we calculate the effects on the transport coefficients. We derive the relevant transport coefficients, the phonon thermal conductivity, and the He-3 diffusion constants from the Boltzmann equation. We calculate the effect of scattering from the walls of the pipe and show that it may be characterized by the average distance from points inside the pipe to the walls. The temporal evolution of the spatial distribution of the He-3 atoms is determined by the time dependent He-3 diffusion equation, which describes the competition between advection by the phonon wind and He-3 diffusion. As a consequence of the thermal diffusivity being small compared with the He-3 diffusivity, the scale height of the final He-3 distribution is much smaller than that of the temperature gradient. We present exact solutions of the time dependent temperature and He-3 distributions in terms of a complete set of normal modes.
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| 8. |
- Baym, Gordon, et al.
(författare)
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Transport in very dilute solutions of He-3 in superfluid He-4
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:1, s. 014512-
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by a proposed experimental search for the electric dipole moment of the neutron (nEDM) utilizing neutron-He-3 capture in a dilute solution of He-3 in superfluid He-4, we derive the transport properties of dilute solutions in the regime where the He-3 are classically distributed and rapid He-3-He-3 scatterings keep the He-3 in equilibrium. Our microscopic framework takes into account phonon-phonon, phonon-He-3, and He-3-He-3 scatterings. We then apply these calculations to measurements by Rosenbaum et al. [J. Low Temp. Phys. 16, 131 (1974)] and by Lamoreaux et al. [Europhys. Lett. 58, 718 (2002)] of dilute solutions in the presence of a heat flow. We find satisfactory agreement of theory with the data, serving to confirm our understanding of the microscopics of the helium in the future nEDM experiment.
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