No abstract

We consider a layered system of fermionic molecules with permanent dipole moments aligned perpendicular to the layers by an external field. The dipole interactions between fermions in adjacent layers are attractive and induce interlayer pairing. Because of the competition for pairing among adjacent layers, the mean-field ground state of the layered system is a dimerized superfluid, with pairing only between every other layer. We construct an effective Ising-XY lattice model that describes the interplay between dimerization and superfluid phase fluctuations. In addition to the dimerized superfluid ground state, and high-temperature normal state, at intermediate temperature, we find an unusual dimerized "pseudogap" state with only short-range phase coherence. We propose light-scattering experiments to detect dimerization.

We analyze the effects of a random magnetic potential in a microfabricated waveguide for ultracold atoms. We find that the shape and position fluctuations of a current carrying wire induce a strong Gaussian correlated random potential with a length scale set by the atom-wire separation. The theory is used to explain quantitatively the observed fragmentation of the Bose-Einstein condensates in atomic waveguides. Furthermore, we show that nonlinear dynamics can be used to provide important insights into the nature of the strongly fragmented condensates. We argue that a quantum phase transition from the superfluid to the insulating Bose glass phase may be reached and detected under the realistic experimental conditions.

We reexamine dipolar motion of condensate atoms in one-dimensional optical lattices and harmonic magnetic traps including quantum fluctuations within the truncated Wigner approximation. In the strong tunneling limit we reproduce the mean field results with a sharp dynamical transition at the critical displacement. When the tunneling is reduced, on the contrary, strong quantum fluctuations lead to finite damping of condensate oscillations even at infinitesimal displacement. We argue that there is a smooth crossover between the chaotic classical transition at finite displacement and the superfluid-to-insulator phase transition at zero displacement. We further analyze the time dependence of the density fluctuations and of the coherence of the condensate and find several nontrivial dynamical effects, which can be observed in the present experimental conditions.

We explain quantitatively why resonant Raman scattering spectroscopy, an extensively used experimental tool in studying elementary electronic excitations in doped low-dimensional semiconductor nanostructures, always produces an observable peak at the so-called "single particle" excitation although the standard theory predicts that there should be no such single particle peak in the Raman spectra. We have thus resolved an experimental puzzle which dates back more than 25 years. PACS numbers: 73.20.Mf, 78.30.Fs Resonant Raman scattering (RRS) involving inelastic scattering of light by electrons has long been a powerful and versatile spectroscopic tool for studying [1][2][3][4][5][6][7][8][9][10] elementary excitations in doped low-dimensional semiconductor structures such as quantum wells, superlattices, and, more recently, one-dimensional quantum wire systems. RRS has been extensively used in experimentally studying the collective charge density excitation (CDE) dispersion in semiconductor quantum wells, quantum wires, and superlattices for both intrasubband and intersubband transitions. In the standard theory [11,12], which ignores the role of the valence band and simplistically assumes the photon to be interacting entirely with conduction band electrons, RRS intensity is proportional to the dynamical structure factor [13] of the conduction band electron system and therefore has peaks at the collective mode frequencies at the appropriate wave vectors defined by the experimental geometry. Restricting to the polarized RRS geometry [12], where the incident and scattered photons have the same polarization vectors indicating the absence of spin flips in the electronic excitations, the dynamical structure factor peaks should correspond to the poles of the reducible density response function, which are given by the collective CDEs of the system. In particular, the single particle electron-hole excitations (SPE), which are at the poles of the corresponding irreducible response function, carry no long wavelength spectral weight in the density response function and should not, as a matter of principle, show up in the polarized RRS spectra. The remarkable experimental fact, however, is that there is always a relatively weak (but quite distinct) low energy SPE peak in the observed RRS spectra (near resonance) in addition to the expected strong CDE peak at higher energy. This observed SPE peak in the polarized RRS spectra is a factor of 10 3 10 4 times stronger than that given by the calculated dynamical structure factor in the standard theory. This puzzling feature of an ubiquitous anomalous SPE peak (in addition to the expected CDE peak) in the observed RRS spectra occurs in one-dimensional GaAs-AlGaAs quantum wires, twodimensional GaAs quantum wells, and even in the doped three-dimensional bulk GaAs systems [1]. It exists in the low-dimensional structures both for intrasubband and intersubband (i.e., transitions along and perpendicular to the confining quantization direction) excitations, and in zero and finite magnetic...