Feb+2014

flat = = = Feb 24-Feb 28, Bo Liu =

= = =Feb 28=

1.[|arXiv:1402.6939] [[|pdf], [|other]] [|M. O. J. Heikkinen], [|D.-H. Kim], [|M. Troyer], [|P. Törmä] We study fermionic superfluidity in strongly anisotropic optical lattices with attractive interactions utilizing cluster DMFT and real-space DMFT methods and focusing in particular on the role of non-local quantum fluctuations. We show that non-local quantum fluctuations impact the BCS superfluid transition dramatically. Moreover, we show that exotic superfluid states with delicate order parameter structure, such as the Fulde-Ferrell-Larkin-Ovchinnikov phase driven by spin population imbalance, can emerge even in the presence of such strong fluctuations.
 * Non-local quantum fluctuations and fermionic superfluidity in the attractive Hubbard model **

2. [|arXiv:1402.6704] [[|pdf], [|other]] [|Lei Wang], [|Hsiang-Hsuan Hung], [|Matthias Troyer] We study the interplay between topological and conventional long range order of attractive fermions in a time reversal symmetric Hofstadter lattice using quantum Monte Carlo simulations, focussing on the case of one-third flux quantum per plaquette. At half-filling, the system is unstable towards s-wave pairing and charge-density-wave order at infinitesimally small interactions. At one-third-filling, the noninteracting system is a topological insulator, and a nonzero critical interaction strength is needed to drive a transition from the quantum spin Hall insulator to a superfluid. We probe the topological signature of the phase transition by threading a magnetic flux through a cylinder and observe quantized topological charge pumping.
 * Topological Phase Transition in the Hofstadter-Hubbard Model **

=Feb 27=

1.[|arXiv:1402.6532] [[|pdf], [|ps], [|other]] [|Balázs Dóra], [|Igor F. Herbut], [|Roderich Moessner] A (single flavor) quadratic band crossing in two dimensions is known to have a generic instability towards a quantum anomalous Hall (QAH) ground state for infinitesimal repulsive interactions. Here we introduce a generalization of a quadratic band crossing which is protected only by rotational symmetry. By focusing on the representative case of a parabolic and flat band touching, which also allows for a straightforward lattice realization, the interaction induced nematic phase is found generally to compete successfully with the QAH insulator, and to become the dominant instability in certain parts of the phase diagram already at weak coupling. The full phase diagram of the model, together with its topological properties, is mapped out using a perturbative renormalization group, strong coupling analysis, the mean-field theory. Interestingly, the Berry flux varies continuously in the single flavour limit with various control parameters.
 * Nematic, topological and Berry phases when a flat and a parabolic band touch **

=Feb 26=

1.[|arXiv:1402.6086] [[|pdf], [|ps], [|other]] [|J. Smyrnakis], [|M. Magiropoulos], [|Nikolaos K. Efremidis], [|G. M. Kavoulakis] We present variational and numerical solutions for the problem of stability of persistent currents in a two-component Bose-Einstein condensate of distinguishable atoms which rotate in a ring potential. We consider the general class of solutions of constant density in the two components separately, thus generalizing the results of Zhigang Wu and Eugene Zaremba (e-print [|arXiv:1309.1734] ). Our approach provides a physically transparent solution of this delicate problem.
 * Persistent currents in a two-component Bose-Einstein condensate confined in a ring potential **

=Feb 25= = = 1. [|arXiv:1402.5799] [[|pdf], [|ps], [|other]] [|Nicolas Rougerie], [|Jakob Yngvason] This paper has its motivation in the study of the Fractional Quantum Hall Effect. We consider 2D quantum particles submitted to a strong perpendicular magnetic field, reducing admissible wave functions to those of the Lowest Landau Level. When repulsive interactions are strong enough in this model, highly correlated states emerge, built on Laughlin's famous wave function. We investigate a model for the response of such strongly correlated ground states to variations of an external potential. This leads to a family of variational problems of a new type. Our main results are rigorous energy estimates demonstrating a strong rigidity of the response of strongly correlated states to the external potential. In particular we obtain estimates indicating that there is a universal bound on the maximum local density of these states in the limit of large particle number. We refer to these as incompressibility estimates.
 * Incompressibility estimates for the Laughlin phase **

=Feb 3-Feb 7, Li-jun Lang= = = =Feb 7= 1. [|arXiv:1402.1204] [ [|pdf], [|other] ] Probe of Three-Dimensional Chiral Topological Insulators in an Optical Lattice [|Sheng-Tao Wang], [|Dong-Ling Deng] , [|Lu-Ming Duan] Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph) We propose an experimental scheme to realize a three-dimensional chiral topological insulator with cold fermionic atoms in an optical lattice, which is characterized by an integer topological invariant and a zero-energy flat band. The topological state is protected by the chiral symmetry instead of the time-reversal symmetry. To probe its property, we show that its characteristic edge states --- the Dirac cones --- can be probed through the time-of-flight imaging or the Bragg spectroscopy and the flat band can be detected via measurements of the atomic density profile in a weak global trap. The realization of this novel topological phase with a flat band in an optical lattice will provide an excellent experimental platform to study the interplay between interaction and topology and open new avenues for application of the topological states.

2. [|arXiv:1402.1196] [ [|pdf], [|ps] , [|other] ] Lifetime of Single-Particle Excitations in a Dilute Bose-Einstein Condensate at Zero Temperature [|Kazumasa Tsutsui], [|Takafumi Kita]   Subjects: Quantum Gases (cond-mat.quant-gas) ; Superconductivity (cond-mat.supr-con) We study the lifetime of single-particle excitations in a dilute homogeneous Bose-Einstein condensate at zero temperature based on a self-consistent perturbation expansion of satisfying Goldstone's theorem and conservation laws simultaneously.It is shown that every excitation for each momentum p  should have a finite lifetime proportional to the inverse a − 1  <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> of the <span class="MathJax" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"><span class="mi" style="font-family: MathJax_Math; font-size: 19px; vertical-align: 0px;">s  <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">-wave scattering length <span class="MathJax" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"><span class="mi" style="font-family: MathJax_Math; font-size: 19px; vertical-align: 0px;">a  <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, instead of <span class="MathJax" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"><span class="mi" style="font-family: MathJax_Math; font-size: 19px; vertical-align: 0px;">a <span class="mo" style="font-family: MathJax_Main; font-size: 13px; vertical-align: 0px;">− <span class="mn" style="font-family: MathJax_Main; font-size: 13px; vertical-align: 0px;">2  <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> for the normal state, due to a new class of Feynman diagrams for the self-energy that emerges upon condensation. We calculate the lifetime as a function of <span class="MathJax" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"><span class="mo" style="font-family: MathJax_Main; font-size: 19px; vertical-align: 0px;">| <span class="mi" style="font-family: MathJax_Main; font-size: 19px; vertical-align: 0px;">p <span class="mo" style="font-family: MathJax_Main; font-size: 19px; vertical-align: 0px;">| <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> approximately. =Feb 6= 1. [|arXiv:1402.1020] [ [|pdf], [|other] ] Vortex lattices in dipolar two-component Bose-Einstein condensates [|N. Ghazanfari], [|A. Keleş] , [|M. Ö. Oktel] Subjects: Quantum Gases (cond-mat.quant-gas) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We consider a rapidly rotating two-component Bose-Einstein condensate with short-range s-wave interactions as well as dipolar coupling. We calculate the phase diagram of vortex lattice structures as a function of the intercomponent s-wave interaction and the strength of the dipolar interaction. We find that the long-range interactions cause new vortex lattice structures to be stable and lead to a richer phase diagram. Our results reduce to the previously found lattice structures for short-range interactions and single-component dipolar gases in the corresponding limits.

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> 2. [|arXiv:1402.0873] [ [|pdf], [|ps] , [|other] ] Detecting quadrupole interactions in ultracold Fermi gases [|M. Lahrz], [|Mikhail Lemeshko] , [|Klaus Sengstock] , [|Christoph Becker] , [|L. Mathey]   Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We propose to detect quadrupole interactions of neutral ultra-cold atoms via their induced mean-field shift. We consider a Mott insulator state of spin-polarized atoms in a two-dimensional optical square lattice. The quadrupole moments of the atoms are aligned by an external magnetic field. As the alignment angle is varied, the mean-field shift shows a characteristic angular dependence, which constitutes the defining signature of the quadrupole interaction. For the <span class="MathJax" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"><span class="mn" style="font-family: MathJax_Main; font-size: 13px; vertical-align: 0px;">3 <span class="mi" style="font-family: MathJax_Math; font-size: 19px; vertical-align: 0px;">P <span class="mn" style="font-family: MathJax_Main; font-size: 13px; vertical-align: 0px;">2 <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> states of Yb and Sr atoms, we find a frequency shift of the order of tens of Hertz, which can be realistically detected in experiment with current technology. We compare our results to the mean-field shift of a spin-polarized quasi-2D Fermi gas in continuum. =Feb 5= 1. [|arXiv:1402.0819] [ [|pdf], [|other] ] Observation of the Meissner effect with ultracold atoms in bosonic ladders [|M. Atala], [|M. Aidelsburger] , [|M. Lohse] , [|J. T. Barreiro] , [|B. Paredes] , [|I. Bloch] Subjects: Quantum Gases (cond-mat.quant-gas) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We report on the observation of the Meissner effect in bosonic flux ladders of ultracold atoms. Using artificial gauge fields induced by laser-assisted tunneling, we realize arrays of decoupled ladder systems that are exposed to a uniform magnetic field. By suddenly decoupling the ladders and projecting into isolated double wells, we are able to measure the currents on each side of the ladder. For large coupling strengths along the rungs of the ladder, we find a saturated maximum chiral current corresponding to a full screening of the artificial magnetic field. For lower coupling strengths, the chiral current decreases in good agreement with expectations of a vortex lattice phase. Our work marks the first realization of a low-dimensional Meissner effect and, furthermore, it opens the path to exploring interacting particles in low dimensions exposed to a uniform magnetic field. =Feb 4= 1. [|arXiv:1402.0011] [ [|pdf], [|ps] , [|other] ] Dissipation through localised loss in bosonic systems with long-range interactions [|Ivana Vidanovic], [|Daniel Cocks] , [|Walter Hofstetter] Subjects: Quantum Gases (cond-mat.quant-gas) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">In recent years, controlled dissipation has proven to be a useful tool for probing of a quantum system in the ultracold setup. In this paper we consider dynamics of bosons induced by a dissipative local defect. We address superfluid and supersolid phases close to half-filling that are ground states of an extended Bose-Hubbard Hamiltonian. To this end, we solve the master equation using the Gutzwiller approximation and find that in the superfluid phase repulsive nearest neighbour interactions can lead to enhanced dissipation processes. On the other hand, our mean-field approach indicates that the effective loss rates are significantly suppressed deep in the supersolid phase where repulsive nearest neighbour interactions play a dominant role. Our numerical results are explained by analytical arguments and in particular, in the limit of strong dissipation we recover the quantum Zeno effect.

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> 2. [|arXiv:1402.0231] [ [|pdf], [|other] ] Low-energy excitations of a one-dimensional Bose gas with weak contact repulsion [|M. Pustilnik], [|K. A. Matveev]   Subjects: Quantum Gases (cond-mat.quant-gas) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We study elementary excitations of a system of one-dimensional bosons with weak contact repulsion. We show that the Gross-Pitaevskii regime, in which the excitations are the well-known Bogoliubov quasiparticles and dark solitons, does not extend to the low energy limit. Instead, the spectra of both excitations have finite curvatures at zero momentum, in agreement with the phenomenological picture of fermionic quasiparticles. We describe analytically the crossover between the Gross-Pitaevskii and the low-energy regimes, and discuss implications of our results for the behavior of the dynamic structure factor. =Feb 3= 1. [|arXiv:1401.8240] [ [|pdf], [|other] ] Tomography of band-insulators from quench dynamics [|Philipp Hauke], [|Maciej Lewenstein] , [|André Eckardt] Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We propose a simple scheme for a complete tomography of band-insulating states in one- and two-dimensional optical lattices with two sublattice states. In particular, the proposed scheme allows for mapping out the Berry curvature in the entire Brillouin zone and to extract topological invariants such as the Chern number. The measurement simply relies on observing---via time-of-flight imaging---the dynamics following a sudden quench in the band structure. We illustrate the performance of the scheme in noisy settings with limited resolution by considering two examples of experimental relevance: the Harper model with <span class="MathJax" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"><span class="mi" style="font-family: MathJax_Math; font-size: 19px; vertical-align: 0px;">π <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">-flux and the Haldane model on a honeycomb lattice.

2. [|arXiv:1401.8267] [ [|pdf], [|other] ] Phase Imprinting in Equilibrating Fermi Gases: The Transience of Vortex Rings and Other Defects [|Peter Scherpelz], [|Karmela Padavić] , [|Adam Rançon] , [|Andreas Glatz] , [|Igor S. Aranson] , [|K. Levin] Subjects: Quantum Gases (cond-mat.quant-gas) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We present numerical simulations of phase imprinting experiments in ultracold trapped Fermi gases. The dynamics we consider is associated with a time-dependent Ginzburg-Landau equation, which contains dissipation (arising from fermionic dissociation of the pairs) as well as a noise contribution. In contrast to other simulations we find that vortex rings in confined geometries are highly transient. Small noise-induced asymmetries in their position within the trap lead to their migration to the trap surface and subsequent decay into vortex lines. In the simplest terms this follows because asymmetrically positioned rings are attracted to ``quasi-image" rings which leads to their reconnection, leaving more stable vortex filaments behind.

=Feb 10-Feb 14, Zhifang Xu,=

=Feb 10= 1. [|arXiv:1402.1539] [[|pdf], [|ps], [|other]] Collective modes of a one-dimensional trapped atomic Bose gas at finite temperatures [|Hui Hu], [|Gao Xianlong], [|Xia-Ji Liu] Comments: 8 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We theoretically investigate collective modes of a one-dimensional (1D) interacting Bose gas in harmonic traps at finite temperatures, by using a variational approach and local density approximation. We find that the temperature dependence of collective mode frequencies is notably different in the weakly and strongly interacting regimes. Therefore, the experimental measurement of collective modes could provide a sensitive probe for different quantum phases of a 1D trapped Bose gas, realized by tuning the interatomic interaction strength and temperature. Our prediction on the temperature dependence of the breathing mode frequency is in good qualitative agreement with an earlier experimental measurement for a weakly interacting 1D Bose gas of rubidium-87 atoms in harmonic traps [Moritz et al., Phys. Rev. Lett. 91, 250402 (2003)].

=Feb 11= 1. [|arXiv:1402.1808] [[|pdf], [|other]] Current reversals in rapidly rotating ultra-cold Fermi gases [|K. Bencheikh], [|S. Medjedel], [|G. Vignale] Comments: 5 pages, 2 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We study the equilibrium current density profiles of harmonically trapped ultra-cold Fermi gases in quantum Hall-like states that appear when the quasi-two-dimensional trap is set in fast rotation. The density profile of the gas (in the rotating reference frame) consists of incompressible strips of constant quantized density separated by compressible regions in which the density varies. Remarkably, we find that the atomic currents flow in opposite directions in the compressible and incompressible regions -- a prediction that should be amenable to experimental verification. = = =Feb 12= 1. [|arXiv:1402.2434] [[|pdf], [|ps], [|other]] Measuring Z2 topological invariants in optical lattices using interferometry [|Fabian Grusdt], [|Dmitry Abanin], [|Eugene Demler] Comments: 17 pages, 13 figures, 6 pages appendix Subjects: Quantum Gases (cond-mat.quant-gas)

We propose an interferometric method to measure Z2 topological invariants of time-reversal invariant topological insulators realized with optical lattices in two and three dimensions. We suggest two schemes which both rely on a combination of Bloch oscillations with Ramsey interferometry and can be implemented using standard tools of atomic physics. In contrast to topological Zak phase and Chern number, defined for individual 1D and 2D Bloch bands, the formulation of the Z2 invariant involves at least two Bloch bands related by time- reversal symmetry which one has keep track of in measurements. In one of our schemes this can be achieved by the measurement of Wilson loops, which are non-Abelian generalizations of Zak phases. The winding of their eigenvalues is related to the Z2 invariant. We thereby demonstrate that Wilson loops are not just theoretical concepts but can be measured experimentally. For the second scheme we introduce a generalization of time-reversal polarization which is continuous throughout the Brillouin zone. We show that its winding over half the Brillouin zone yields the Z2 invariant. To measure this winding, our protocol only requires Bloch oscillations within a single band, supplemented by coherent transitions to a second band which can be realized by lattice-shaking.

2. [|arXiv:1402.2354] [[|pdf], [|other]] Many-body dynamics of dipolar molecules in an optical lattice [|Kaden R. A. Hazzard], [|Bryce Gadway], [|Michael Foss-Feig], [|Bo Yan], [|Steven A. Moses], [|Jacob P. Covey], [|Norman Y. Yao], [|Mikhail D. Lukin], [|Jun Ye], [|Deborah S. Jin], [|Ana Maria Rey] Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

Understanding the many-body dynamics of isolated quantum systems is one of the central challenges in modern physics. To this end, the direct experimental realization of strongly correlated quantum systems allows one to gain insights into the emergence of complex phenomena. Such insights enable the development of theoretical tools that broaden our understanding. Here, we theoretically model and experimentally probe with Ramsey spectroscopy the quantum dynamics of disordered, dipolar-interacting, ultracold molecules in a partially filled optical lattice. We report the capability to control the dipolar interaction strength, and we demonstrate that the many-body dynamics extends well beyond a nearest-neighbor or mean-field picture, and cannot be quantitatively described using previously available theoretical tools. We develop a novel cluster expansion technique and demonstrate that our theoretical method accurately captures the measured dependence of the spin dynamics on molecule number and on the dipolar interaction strength. In the spirit of quantum simulation, this agreement simultaneously benchmarks the new theoretical method and verifies our microscopic understanding of the experiment. Our findings pave the way for numerous applications in quantum information science, metrology, and condensed matter physics.

=Feb 13= 1. [|arXiv:1402.2958] [[|pdf], [|other]] Quantized hysteresis in a superfluid atomtronic circuit [|Stephen Eckel], [|Jeffrey G. Lee], [|Fred Jendrzejewski], [|Noel Murray], [|Charles W. Clark], [|Christopher J. Lobb], [|William D. Phillips], [|Mark Edwards], [|Gretchen K. Campbell] Comments: 19 pages, 4 figures Journal-ref: Nature 506, 200-203 (13 February 2014) Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph)

Atomtronics is an emerging interdisciplinary field that seeks new functionality by creating devices and circuits where ultra-cold atoms play a role analogous to the electrons in electronics. Hysteresis in atomtronic circuits may prove to be a crucial feature for the development of practical devices, just as it has in electronic circuits like memory, digital noise filters (e.g., Schmitt triggers), and magnetometers (e.g., superconducting quantum interference devices [SQUIDs]). Here we demonstrate quantized hysteresis in an atomtronic circuit: a ring of superfluid, dilute-gas, Bose-Einstein condenstate (BEC) obstructed by a rotating weak link. Hysteresis is as fundamental to superfluidity (and superconductivity) as quantized persistent currents, critical velocity, and Josephson effects, but has not been previously observed in any atomic-gas, superfluid BEC despite multiple theoretical predictions. By contrast, hysteresis is routinely observed in superconducting circuits, and it is essential in rf-SQUIDs. Superfluid liquid helium experiments observed hysteresis directly in systems where the quantization of flow could not be observed and only indirectly in systems that showed quantized flow. We directly detect quantized hysteresis, using techniques that allow us to tune the size of the hysteresis loop and study the fundamental excitations that accompany hysteresis.

2. [|arXiv:1402.2911] (cross-list from quant-ph) [[|pdf], [|ps], [|other]] Three-body Physics in Strongly Correlated Spinor Condensates [|V. E. Colussi], [|Chris H. Greene], [|J. P. D'Incao] Comments: 5+ pages, 1 figure, and supplementary material (4 pages) Subjects: Quantum Physics (quant-ph) ; Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)

Spinor condensates have proven to be a rich area for probing many-body phenomena richer than that of an ultracold gas consisting of atoms restricted to a single spin state. In the strongly correlated regime, the physics controlling the possible novel phases of the condensate remains largely unexplored, and few-body aspects can play a central role in the properties and dynamics of the system through manifestations of Efimov physics. The present study solves the three-body problem for bosonic spinors using the hyperspherical adiabatic representation and characterizes the multiple families of Efimov states in spinor systems as well as their signatures in the scattering observables relevant for spinor condensates. These solutions exhibit a rich array of possible phenomena originating in universal few-body physics, which can strongly affect the spin dynamics and three-body mean-field contributions for spinor condensates. The collisional aspects of atom-dimer spinor condensates are also analyzed and effects are predicted that derive from Efimov physics.

=Feb 14= 1. [|arXiv:1402.3098] [[|pdf], [|other]] Higgs amplitude mode in the vicinity of a $(2+1)$-dimensional quantum critical point [|A. Rancon], [|N. Dupuis] Comments: 5+3 pages Subjects: Quantum Gases (cond-mat.quant-gas)

We study the "Higgs" amplitude mode in the relativistic quantum O(<span class="MathJax_Preview">N ) model in two space dimensions. Using the nonperturbative renormalization group we compute the O(<span class="MathJax_Preview">N )-invariant scalar susceptibility in the vicinity of the zero-temperature quantum critical point. In the zero-temperature ordered phase, we find a well defined Higgs resonance for <span class="MathJax_Preview">N=2 with universal properties in agreement with quantum Monte Carlo simulations. The resonance persists at finite temperature below the Berezinskii-Kosterlitz-Thouless transition temperature. In the zero-temperature disordered phase, we find a maximum in the spectral function which is however not related to a putative Higgs resonance. Furthermore we show that the resonance is strongly suppressed for <span class="MathJax_Preview">N\geq 3.