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arXiv:1407.7157 · Physical Review Letters

Roton-Maxon Excitation Spectrum of Bose Condensates in a Shaken Optical Lattice

Li-Chung Ha, Logan W. Clark, Colin Parker, Brandon Anderson, Cheng Chin

Published Feb 3, 2015• 111 citations
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Abstract

We present experimental evidence showing that an interacting Bose condensate in a shaken optical lattice develops a roton-maxon excitation spectrum, a feature normally associated with superfluid helium. The roton-maxon feature originates from the double-well dispersion in the shaken lattice, and can be controlled by both the atomic interaction and the lattice modulation amplitude. We determine the excitation spectrum using Bragg spectroscopy and measure the critical velocity by dragging a weak speckle potential through the condensate-both techniques are based on a digital micromirror device. Our dispersion measurements are in good agreement with a modified Bogoliubov model.

DOI
10.1103/physrevlett.114.055301

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We present experimental evidence showing that an interacting Bose condensate in a shaken optical lattice develops a roton-maxon excitation spectrum, a feature normally associated with superfluid helium. The roton-maxon feature originates from the double-well dispersion in the shaken lattice, and can be controlled by both the atomic interaction and the lattice modulation amplitude. We determine the excitation spectrum using Bragg spectroscopy and measure the critical velocity by dragging a weak speckle potential through the condensate-both techniques are based on a digital micromirror device. Our dispersion measurements are in good agreement with a modified Bogoliubov model.
Dilute quantum gases offer a controllable platform for investigating quantum liquids, such as superfluid helium, in which correlations and fluctuations are encoded in the form of a quasiparticle energy (i.e., roton-maxon) dispersion. An approach to extracting the roton-maxon spectrum of a Bose-Einstein condensate is theoretically developed: the method relies on using an external one-dimensional optical lattice that effectively controls the response of the system, giving access to its elementary excitations.
Roton-type excitations usually emerge from strong correlations or long-range interactions, as in superfluid helium or dipolar ultracold atoms. However, in a weakly short-range interacting quantum gas, the recently synthesized spin-orbit (SO) coupling can lead to various unconventional phases of superfluidity and give rise to an excitation spectrum of roton-maxon character. Using Bragg spectroscopy, we study a SO-coupled Bose-Einstein condensate of ^{87}Rb atoms and show that the excitation spectrum in a "magnetized" phase clearly possesses a two-branch and roton-maxon structure. As Raman coupling strength Ω is decreased, a roton-mode softening is observed, as a precursor of the phase transition to a stripe phase that spontaneously breaks spatially translational symmetry. The measured roton gaps agree well with theoretical calculations. Furthermore, we determine sound velocities both in the magnetized and in the nonmagnetized phases, and a phonon-mode softening is observed around the phase transition in between. The validity of the f-sum rule is examined.
We present a scheme to generate maxon and roton solitons via the maxon and roton excitations of a Rydberg-dressed Bose-Einstein condensate (BEC). We show that the strong, long-range Rydberg-Rydberg interaction between Rydberg atoms can drastically alter the linear excitation spectrum of the quasiparticles (phonons) on the homogeneous background of the BEC, which displays obvious maxon-roton character and can be tuned actively by adjusting the strength of the Rydberg-Rydberg interaction and Rydberg blockade radius. We also show that the system supports both bright maxon and dark roton solitons if the center wave numbers of excited wave packets are chosen respectively to be those of the maxon and roton. Moreover, we corroborate that both the maxon and roton solitons are dynamical stable during the evolution if system parameters are kept in the domain where the homogeneous background of the BEC is modulationally stable. The results reported are beneficial not only for understanding the physical property of Rydberg-dressed BECs, but also for creating and actively manipulating new types of nonlinear matter-wave excitations in cold atomic gases.
We examine the stability of quasi-two-dimensional dipolar Bose-Einstein condensates in the presence of weak optical lattices of various geometries. We find that when the condensate possesses a roton-maxon quasiparticle dispersion, the conditions for stability exhibit a strong dependence both on the lattice geometry and the polarization tilt. This results in rich structures in the system's stability diagram akin to spectroscopic signatures. We show how these structures originate from the mode matching of rotons to the perturbing lattice. In the case of a one-dimensional lattice, some of the features emerge only when the polarization axis is tilted into the plane of the condensate. Our results suggest that the stability diagram may be used as a novel means to spectroscopically measure rotons in dipolar condensates.
We study the stability of a quasi-one-dimensional dipolar Bose-Einstein condensate that is perturbed by a weak lattice potential along its axis. Our numerical simulations demonstrate that systems exhibiting a roton-maxon structure destabilize readily when the lattice wavelength equals either half the roton wavelength or a low roton subharmonic. We apply perturbation theory to the Gross-Pitaevskii and Bogoliubov--de Gennes equations to illustrate the mechanisms behind the instability threshold. The features of our stability diagram may be used as a direct measurement of the roton wavelength for quasi-one-dimensional geometries.