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

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.PACS numbers: 03.75. Kk, 05.30.Jp, 37.10.Jk, In his seminal papers in the 1940s [1,2], L. D. Landau formulated the theory of superfluid helium-4 (He II) and showed that the energy-momentum relation (dispersion) of He II supports two types of elementary excitations: acoustic phonons and gapped rotons. This dispersion underpins our understanding of superfluidity in helium, and explains many experiments on heat capacity and superfluid critical velocity. What is now called the "rotonmaxon" dispersion in He II has been precisely measured in neutron scattering experiments [3,4] and is generally considered a hallmark of Bose superfluids in the strong interaction regime.The roton-maxon dispersion carries a number of intriguing features that distinguish excitations in different regimes. The low-lying excitations are acoustic phonons with energy E = pv s , where p is the momentum and v s is the sound speed. At higher momenta, the dispersion exhibits both a local maximum at p = p m with energy E = ∆ m and a minimum at p = p r with energy E = ∆ r . The elementary excitations associated with this maximum and minimum are known as maxons and rotons, respectively. The roton excitations, in particular, are known to reduce the superfluid critical velocity below the sound speed. This is best understood based on the Landau criterion for superfluidity in which the critical velocity set by the roton minimum v c ≈ ∆ r p r is lower than the sound speed v s . The roton minimum also suggests the emergence of density wave order [5] and dynamical instability [6].To explore the properties of these unconventional excitations, many theoretical works have proposed schemes for producing the roton-maxon dispersion outside of the He II system. Many proposals have been devoted to atomic systems with long-range or enhanced interactions, e.g. dipolar gases [6][7][8], Rydberg-excited condensates [9], or resonantly interacting gases [10]. Other candidates are 2D Bose gases [11,12], spinor condensates [13,14], and spin-orbit coupled condensates [15,16]. Experimentally, mode softening resulting from cavity-induced interaction has recently been reported [17], which provides strong evidence for an underlying rotonlike excitation spectrum.In this Letter, we generate and characterize an asymmetric roton-maxon excitation spectrum based on a Bose-Einstein condensate (BEC) in a one dimensio...

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Bose-Einstein condensate in an optical lattice with Raman-assisted two-dimensional spin-orbit coupling

“…Importantly, the Raman-assisted SOC couple atoms to high-lying bands, which have significant impact on the system [17][18][19]. In particular, the high-band effects stabilize the plane-wave (PW) phase at low Zeeman fields, which lead to the opening of "roton" gaps in the excitation spectrum that are absent in a simple two-band model [20,22,23]. We also * Electronic address: wzhangl@ruc.edu.cn † Electronic address: wyiz@ustc.edu.cn identify a high-band induced topological phase boundary for the lowest band of the single-particle spectrum, which is carried over to the bands in the Bogoliubov excitation spectrum.…”

Atom-optics approach to studying transport phenomena

“…Moreover, the general scheme of developing link-resolved control of tunneling by use of an inhomogeneous potential and global field addressing may be transportable to strongly-correlated studies. In an optical lattice simulator, for example, tunable inhomogeneous potentials may be created by projective methods [82][83][84], and global addressing via laser-assisted tunneling [85,86] may be used to reintroduce site-to-site coupling in a link-dependent fashion, allowing local control over tunneling amplitudes and phases.…”