Collège de France

About: Collège de France is a education organization based out in Paris, France. It is known for research contribution in the topics: Population & Receptor. The organization has 6541 authors who have published 11983 publications receiving 648742 citations. The organization is also known as: College de France.

Direct imaging of topological edge states in cold-atom systems

Abstract: Detecting topological order in cold-atom experiments is an ongoing challenge, the resolution of which offers novel perspectives on topological matter. In material systems, unambiguous signatures of topological order exist for topological insulators and quantum Hall devices. In quantum Hall systems, the quantized conductivity and the associated robust propagating edge modes—guaranteed by the existence of nontrivial topological invariants—have been observed through transport and spectroscopy measurements. Here, we show that optical-lattice-based experiments can be tailored to directly visualize the propagation of topological edge modes. Our method is rooted in the unique capability for initially shaping the atomic gas and imaging its time evolution after suddenly removing the shaping potentials. Our scheme, applicable to an assortment of atomic topological phases, provides a method for imaging the dynamics of topological edge modes, directly revealing their angular velocity and spin structure.

H.E.S.S. Observations of PKS 2155-304

Abstract: The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been detected with high significance (~45 sigma) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of 3.32 +/- 0.06. An improved chi-square per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2 sigma). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.

Theoretical Implications from the Spectral Evolution of Markarian 501 Observed with BeppoSAX

Abstract: We present new BeppoSAX observations of the TeV-emitting blazar Mrk 501 performed in 1999 June and a homogeneous reanalysis of all the BeppoSAX observations during the period from 1997 April to 1999 June. Recently published TeV spectra by the Cerenko Array at Themis, which are quasi-simultaneous with the 1997 BeppoSAX pointings, are used to constrain further the physical parameters of the jet in 1997 April with detailed models of the X-ray and TeV components. During the 1997-1999 period, the peak frequency of the synchrotron emission moved from about 100 keV in 1997 April to 0.5 keV in 1999 June. The shift in the peak frequency appears to be correlated with the long-term luminosity decrease. We interpret the results in the framework of a homogeneous synchrotron self-Compton model, discussing the evolution of the physical parameters of the emitting region under simple assumptions for the variability model. We also compute the expected variations of the TeV flux under the same assumptions.

Exponential mixing for the Teichmüller flow

Abstract: We study the dynamics of the Teichmuller flow in the moduli space of Abelian differentials (and more generally, its restriction to any connected component of a stratum). We show that the (Masur-Veech) absolutely continuous invariant probability measure is exponentially mixing for the class of Holder observables. A geometric consequence is that the SL(2, R) action in the moduli space has a spectral gap.