Emergent Quasicrystalline Symmetry in Light-Induced Quantum Phase Transitions

The discovery of quasicrystals with crystallographically forbidden rotational symmetries has changed the notion of the ordering in materials, yet little is known about the dynamical emergence of such exotic forms of order. Here we theoretically study a nonequilibrium cavity-QED setup realizing a zero-temperature quantum phase transition from a homogeneous Bose-Einstein condensate to a quasicrystalline phase via collective superradiant light scattering. Across the superradiant phase transition, collective light scattering creates a dynamical, quasicrystalline optical potential for the atoms. Remarkably, the quasicrystalline potential is " emergent" as its eightfold rotational symmetry is not present in the Hamiltonian of the system, rather appears solely in the low-energy states. For sufficiently strong two-body contact interactions between atoms, a quasicrystalline order is stabilized in the system, while for weakly interacting atoms the condensate is localized in one or few of the deepest minima of the quasicrystalline potential

Hofstadter butterfly in a cavity-induced dynamic synthetic magnetic field

Energy bands of electrons in a square lattice potential threaded by a uniform magnetic field exhibit a fractal structure known as the Hofstadter butterfly. Here we study a Fermi gas in a 2D optical lattice within a linear cavity with a tilt along the cavity axis. The hopping along the cavity axis is only induced by resonant Raman scattering of transverse pump light into a standing-wave-cavity mode. Choosing a suitable pump geometry allows us to realize the Hofstadter-Harper model with a cavity-induced dynamical synthetic magnetic field, which appears at the onset of the superradiant phase transition. The dynamical nature of this cavity-induced synthetic magnetic field arises from the delicate interplay between collective superradiant scattering and the underlying fractal band structure. Using a sixth-order expansion of the free energy as a function of the order parameter and by numerical simulations, we show that at low magnetic fluxes the superradiant ordering phase transition is first order, while it becomes second order for higher flux. The dynamic nature of the magnetic field induces a nontrivial deformation of the Hofstadter butterfly in the superradiant phase. At strong pump far above the self-ordering threshold, we recover the Hofstadter butterfly one would obtain in a static magnetic field

Driven-Dissipative Supersolid in a Ring Cavity

Supersolids are characterized by the counterintuitive coexistence of superfluid and crystalline order. Here we study a supersolid phase emerging in the steady state of a driven-dissipative system. We consider a transversely pumped Bose-Einstein condensate trapped along the axis of a ring cavity and coherently coupled to a pair of degenerate counterpropagating cavity modes. Above a threshold pump strength the interference of photons scattered into the two cavity modes results in an emergent superradiant lattice, which spontaneously breaks the continuous translational symmetry towards a periodic atomic pattern. The crystalline steady state inherits the superfluidity of the Bose-Einstein condensate, thus exhibiting genuine properties of a supersolid. A gapless collective Goldstone mode correspondingly appears in the superradiant phase, which can be nondestructively monitored via the relative phase of the two cavity modes on the cavity output. Despite cavity-photon losses the Goldstone mode remains undamped, indicating the robustness of the supersolid phase

INFRARED SPECTRA OF He--CS2_2, Ne--CS2_2, AND Ar--CS2_2

Author Institution: Department of Physics and Astronomy, University of Calgary, Calgary, AB T2N; 1N4, CanadaInfrared spectra of weakly bound Rg--CS$_2$ (Rg = He, Ne, and Ar) clusters formed in a pulsed supersonic slit-jet expansion have been recorded by exciting the CS$_2$ $\nu_3$ fundamental band ($\sim$ 1535 cm$^{-1}$) using a tuneable diode laser. Spectra were well fitted to a conventional semi-rigid asymmetric rotor Hamiltonian. The He--CS$_2$ spectrum was assigned to an a-type band, while spectra of Ne--CS$_2$ and Ar--CS$_2$ were well described by b-type bands, indicating a/b axis switching in transition from the He--CS$_2$ complex to the Ne--CS$_2$ and Ar--CS$_2$ complexes. The results show that the complexes have vibrationally averged T-shaped structures. The determined structural parameters along with the observed vibrational shifts are $R=3.81, 3.57$ and $3.71$ {\AA}, $\theta = 80.0, 86.9$ and 86.4^irc} and $\Delta\nu = 0.171, 0.181$ and $0.067$ cm$^{-1}$ for He--CS$_2$, Ne--CS$_2$ and Ar--CS$_2$, respectively. Here, $R$ is the distance between the rare gas and the carbon atom, $\theta$ is the the angle between $R$ and and the CS$_2$ axis and $\Delta\nu$ is the vibrational shift with respect to the free CS$_2$ monomer

Cavity-induced emergent topological spin textures in a Bose-Einstein condensate

The coupled nonlinear dynamics of ultracold quantum matter and electromagnetic field modes in an optical resonator exhibits a wealth of intriguing collective phenomena. Here we study a Lambda-type, three-component Bose-Einstein condensate coupled to four dynamical running-wave modes of a ring cavity, where only two of the modes are externally pumped. However, the unpumped modes play a crucial role in the dynamics of the system due to coherent backscattering of photons. On a mean- field level we identify three fundamentally different steady-state phases with distinct characteristics in the density and spatial spin textures: a combined density and spin-wave, a continuous spin spiral with a homogeneous density, and a spin spiral with a modulated density. The spin-spiral states, which are topological, are intimately related to cavity-induced spin-orbit coupling emerging beyond a critical pump power. The topologically trivial density-wave-spin-wave state has the characteristics of a supersolid with two broken continuous symmetries. The transitions between different phases are either simultaneously topological and first-order, or second-order. The proposed setup allows the simulation of intriguing many-body quantum phenomena by solely tuning the pump amplitudes and frequencies, with the cavity output fields serving as a built-in nondestructive observation tool

Infrared spectra of rare gas–carbon disulfide complexes: He–CS2, Ne–CS2, and Ar–CS2

Spectra of the weakly-bound van der Waals complexes Ar-CS2, Ne-CS2, and He-CS2 are studied in the region of the CS2 ν3 fundamental band (≈1535 cm-1) using a tunable diode laser to probe a pulsed supersonic expansion from a slit jet nozzle. He-CS2 is also observed in the ν1 + ν3 region (≈2185 cm-1). These are the first reported spectra for rare gas-CS2 complexes, which are shown to have T-shaped structures similar to their CO2 analogs but with intermolecular separations which are 0.2-0.3 Å greater. © 2012 Elsevier Inc. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

STRUCTURES OF TWO ISOMERS OF NITROUS OXIDE TETRAMER FROM THEIR INFRARED SPECTRA

Author Institution: Department of Physics and Astronomy, University of Calgary, Calgary, AB T2N; 1N4, Canada; Steacie Institute for Molecular Sciences, National Research Council of; Canada, Ottawa, ON K1A 0R6, CanadaWeakly bound complexes provide a convenient starting point for a detailed understanding of different pathways that can be taken between the gas and condensed phases of matter. In this regard, it is of considerable interest to determine the number of isomers for a cluster size and if and how geometrical choices made in the early stages of condensation influence the growth of larger clusters. Although it is expected that the number of isomers grows rapidly with cluster size, in many cases only a single isomer is observed experimentally. High resolution spectroscopy has provided information on structural and vibrational dynamics of a large number of binary and ternary complexes formed from related linear triatomic molecules such as CO$_{2}$, N$_{2}$O, OCS and CS$_{2}$. But, there are much fewer detailed experimental studies which provide structures for the important cluster sizes in the range of $$ monomers. Here we report the structural determination of two isomers of nitrous oxide tetramer from their infrared spectra in the $\nu _{1}$ fundamental region ($$ cm$^{-1}$). Two bands are observed and analyzed, corresponding to two distinct isomers of the complex. A perpendicular band centered at $$ cm$^{-1}$ has been assigned to an isomer with S$_{4}$ symmetry (a subgroup of D$_{2d}$ group). This is a rare symmetry group, but provides all the necessary ingredients. It allows for a tilt of the monomers from the symmetry axis of the complex as well as a twist. The experimentally determined structure has the monomers more or less perpendicular to the symmetry axis. A second band centered at 2237.424 cm$^{-1}$ was assigned to a structure close to a perfect barrel-shaped tetramer with D$_{2d}$ symmetry. This is a prolate symmetric top which gives parallel bands for the pure isotopomers and a c-type band for the mixed isotopomer containing three $^{15}$N$_{2}$O monomers. This isomer is the same species as that observed by R.E. Miller and L. Pederson

INFRARED SPECTRA OF (CO2_{2})2_{2}-OCS COMPLEX: INFRARED OBSERVATION OF TWO DISTINCT BARREL-SHAPED ISOMERS

Author Institution: Department of Physics and Astronomy, University of Calgary, Calgary, AB T2N; 1N4, Canada; Steacie Institute for Molecular Sciences, National Research Council of; Canada, Ottawa, ON K1A 0R6, CanadaSpectra of (CO$_{2}$)$_{2}$-OCS complex in the region of the OCS $\nu _{1}$ fundamental ( $\sim$2062 cm$^{-1}$) are observed using a tunable diode laser to probe a pulsed supersonic slit jet expansion. A previous microwave study of the complex by Peebles and Kuczkowskia gave a distorted triangular cylinder. The geometerical disposition of the three dimer faces of this trimer are quite similar to the slipped CO$_{2}$ dimer, the lowest energy form of OCS-CO$_{2}$ (isomer a), also observed and analyzed in the microwave region, and the higher energy form of OCS-CO$_{2}$ (isomer b), first observed by our group in the infrared region. Here we report the observation and analysis of two infrared bands, corresponding to two distinct isomers of the (CO$_{2}$)$_{2}$-OCS complex. A band around 2058.8 cm$^{-1}$ was assigned to isomer I, which is the same as that studied previously by microwave spectroscopy. A second band around 2051.7 cm$^{-1}$ was assigned to a higher energy isomer of the complex, isomer II, has not been observed previously, but expected on the basis of \textit{ab initio} calculations. Approximate structural parameters for this new isomer were obtained by means of isotopic substitution. In contrast to isomer I, the geometerical disposition of the faces containing OCS and CO$_{2}$ in isomer II are similar to isomer b of the OCS-CO$_{2}$ complex