Inelastic Light Scattering to probe Strongly Correlated Bosons in Optical Lattices

In condensed matter physics, inelastic scattering of waves or particles provided a very powerful method to characterize the systems under study. Neutron scattering allowed the first experimental observation of a condensate in superfluid He-4 [1]. As soon as experiments on ultracold quantum gases in optical lattices started to simulate many-body systems [2], theoretical papers appeared proposing to measure their dynamical structure factor through inelastic light scattering [3]. In general, inelastic light scattering allows to measure correlation functions, which could identify different many-body states [4]. The response of the system to an excitation with frequency ? and momentum p is probed thanks to a two photon transition coupling two states with the same internal degrees of freedom (Bragg spectroscopy) [5]. We will report on the experimental investigation of inelastic light scattering from an array of 1D ultracold samples of Rb-87 atoms in an optical lattice across the superfluid to Mott Insulator transition [6]. We performed the investigation in the linear regime [7] where the excitation in the lowest band is proportional to the dynamical structure factor. Besides the frequency regions attributed to the superfluid and insulator phases, the system can be excited suggesting the presence of temperature effects and the appearance of a gapped mode in the strongly correlated superfluid regime [8]. We also investigated transitions toward excited bands coupling the many-body insulator state to free particle states and giving information on single-particle spectral functions. -- References: [1] D.G. Henshaw and A.D.B. Woods Phys. Rev. 121, 1266 (1961). [2] M. Greiner O. Mandel, T. Esslinger, T.W. Haensch and I. Bloch, Nature 415, 39 (2002); I. Bloch, J. Dalibard and W. Zwerger, Rev. Mod. Phys. 80, 885 (2008). [3] D. Van Oosten, D.B.M. Dickerscheid, B. Farid, P. van der Straten, and H.T.C. Stoof, Phys. Rev. A 71, 021601R (2005); A.M. Rey, P.B. Blakie, G. Pupillo, C.J.Williams, and C.W. Clark, Phys. Rev. A 72, 023407 (2005). [4] J.S. Caux and P. Calabrese, Phys. Rev. A 74, 031605 (2006); J. Ye, J.M. Zhang, W.M. Liu, K. Zhang, Y. Li and W. Zhang, arXiv:1001.3230v2 (2010). [5] J. Stenger, S. Inouye, A.P. Chikkatur, D.M. Stamper-Kurn, D.E. Pritchard, and W. Ketterle Phys. Rev. Lett. 82, 4569 (1999); M. Kozuma L. Deng, E.W. Hagley, J.Wen, R. Lutwak, K. Helmerson, S.L. Rolston, and W.D. Phillips, Phys. Rev. Lett. 82, 871 (1999); J. Steinhauer R. Ozeri, N. Katz, and N. Davidson, Phys. Rev. Lett. 88, 120407 (2002); X. Du, S. Wan, E. Yesilada, C. Ryu, and D.J. Heinzen, arXiv:0704.2623v2 (2007); S.B. Papp, J.M. Pino, R.J. Wild, S. Ronen, C.E. Wieman, D.S. Jin, E.A. Cornell Phys. Rev. Lett.101, 135301 (2008); G. Veeravalli, E. Kuhnle, P. Dyke and C.J. Vale Phys. Rev. Lett. 101, 250403 (2008); P.T. Ernst, S. Goetze, J.S. Krauser, K. Pyka, D. L¨uhmann, D. Pfannkuche and K. Sengstock, Nature Physics 5, 1 (2009). [6] D. Clément N. Fabbri, L. Fallani, C. Fort, and M. Inguscio, Phys. Rev. Lett. 102, 155301 (2009). [7] D. Clément, N. Fabbri, L. Fallani, C. Fort and M. Inguscio, New J. Phys. 11, 103030 (2009). [8] S.D. Huber, E. Altman, H.P. Buechler and G. Blatter Phys. Rev. B 75, 085106 (2007); C. Menotti and N. Trivedi, Phys. Rev. B 77, 235120 (2008).