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arXiv:2011.04620 · Classical and Quantum Gravity

Rotational superradiance with Bogoliubov dispersion

Sam Patrick

Published Mar 25, 2021• 11 citations
arXivHTMLPDF

Abstract

Abstract Rotational superradiance affects the dynamics of many rotating systems in nature, through either stimulated or spontaneous extraction of energy and angular momentum. By now, this process is well-studied in the relativistic setting, where systems are intrinsically dispersion-free. In many condensed matter systems, however, dispersion is an unavoidable aspect of the description for the short wavelength modes. For these systems, how might one expect superradiance to be modified? In this work, an answer to this question is provided using an illustrative example. The scattering of linear excitations of a Bose–Einstein condensate (BEC) are studied in the presence of a rotating, draining vortex flow using the full Bogoliubov dispersion relation. It is shown that dispersion suppresses the extraction of energy and angular momentum, firstly, by decreasing the superradiant bandwidth, and secondly, by preventing high-angular momentum modes from superradiating.

DOI
10.1088/1361-6382/abf1fc

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Abstract Rotational superradiance affects the dynamics of many rotating systems in nature, through either stimulated or spontaneous extraction of energy and angular momentum. By now, this process is well-studied in the relativistic setting, where systems are intrinsically dispersion-free. In many condensed matter systems, however, dispersion is an unavoidable aspect of the description for the short wavelength modes. For these systems, how might one expect superradiance to be modified? In this work, an answer to this question is provided using an illustrative example. The scattering of linear excitations of a Bose–Einstein condensate (BEC) are studied in the presence of a rotating, draining vortex flow using the full Bogoliubov dispersion relation. It is shown that dispersion suppresses the extraction of energy and angular momentum, firstly, by decreasing the superradiant bandwidth, and secondly, by preventing high-angular momentum modes from superradiating.
The creation of a topological vortex by a superradiant scattering of a Laguerre-Gaussian (LG) beam off an atomic Bose-Einstein condensate (BEC) is theoretically investigated. It is shown that scattered superradiant radiation can be either in a Gaussian mode without angular momentum or in a LG mode with angular momentum. The conditions leading to these two qualitatively distinct regimes of superradiance are determined in terms of the width for the pump laser and the condensate size for the limiting cases where the recoil energy is both much smaller and larger than the atomic interaction energy.
The scattering of sound-wave perturbations from vortex excitations in hydrodynamic systems with typical Bose-Einstein-condensate (BEC) parameters is investigated by numerical integration of the associated Klein-Gordon equation. The simulations indicate that at sufficiently high angular speeds, in the perturbative limit where back-reaction effects can be neglected, sound wave packets can extract a sizable fraction of the vortex energy through a mechanism of superradiant scattering. It is conjectured that this superradiant regime may be detectable in BEC experiments.
In a rotating two-component Bose-Einstein condensate (BEC), the traditional triangular vortex lattice can be replaced by a rectangular vortex lattice or even a structure characterized in terms of vortex sheets, depending on the interspecies interactions. We study the dynamics of this system by analyzing the Bogoliubov excitation spectrum. Excitations familiar to BEC vortex systems are found such as Tkachenko modes, hydrodynamic modes, and surface waves, however, the complex two-component morphology also gives rise to phenomena including shear flow between vortex sheets.
We examine a scheme for the optical creation of a superfluid vortex in a trapped Bose-Einstein condensate (BEC), using the stimulated Raman adiabatic passage (STIRAP) technique. By exposing an oblate, axis-symmetric condensate to two co-propagating laser pulses, one can transfer external angular momentum from the light field to the matter wave, if one of the beams is the fundamental Gaussian mode and the other is a Gauss-Laguerre mode of angular momentum 1ℏ1\hbar1ℏ. We demonstrate the complete transfer efficiency by numerical integration of the multi-component Gross-Pitaevskii equation and explain the results with an intuitive and accurate approximation within the Thomas-Fermi limit. In addition, we discuss residual excitations (breathing modes) which occur in the two-dimensional regime and present the Bogoliubov excitation spectrum.
An analysis of superradiant scattering of light by ultracold atoms is presented in which the pump and scattered light consist of hollow counterrotating Laguerre-Gaussian beams possessing orbital angular momentum. It is shown that the interaction between the Laguerre-Gaussian beams and the atoms can give rise to a superradiant instability in which orbital angular momentum is transferred to the atoms and the probe field is amplified. In the case where the atoms constitute a dilute Bose-Einstein condensate (BEC) that is trapped in a one-dimensional ring geometry, the transfer of orbital angular momentum to the BEC during the instability means that the BEC spontaneously develops rotation in the ring.
We investigate low-energy collective excitations in Bose-Einstein condensates (BECs) with a single-vortex line or many vortices by using the microscopic theory based on the Bogoliubov-Popov formalism. The paper is divided into the following two section: (i) We first study the damping process of the quadrupole modes in a three-dimensional cigar-shaped BEC. The selective excitation of such the modes occurs during the rotation of the condensate in the opposite or same direction relative to a vortex velocity field. The dramatic change of the damping process of these modes is explained by the linear response theory. It is proposed that Kelvin modes play a leading part in the decay process of the quadrupole excitation. These results are quantitatively compared with the recent experiment on 87 Rb by Bretin et al. [Phys. Rev. Lett. 90, 100403 (2003)]. (ii) In addition, the complete spectrum of the collective excitations in vortex-lattice states is obtained by solving the Bogoliubov-de Gennes equation in a two-dimensional disk-shaped BEC. We also discuss the frequency shift of totally symmetric and twofold symmetric Tkachenko modes.