In this paper we investigate the properties of Bose gases with Raman-induced spin-orbit(SO) coupling. It is found that the SO coupling can greatly modify the single particle density-of-state, and thus lead to non-monotonic behavior of the condensate depletion, the Lee-Huang-Yang correction of groundstate energy and the transition temperature of a non-interacting Bose-Einstein condensate. The presence of the SO coupling also breaks the Galileaan invariance, and this gives two different critical velocities, corresponding to the movement of the condensate and the impurity respectively. Finally, we show that with SO coupling, the interactions modify the BEC transition temperature even at Hartree-Fock level, in contrast to the ordinary Bose gas without SO coupling. All results presented here can be directly verified in the current cold atom experiments using Raman laser-induced gauge field. arXiv:1212.6832v1 [cond-mat.quant-gas]
We construct a variational wave function to study whether a fully polarized Fermi sea of ultracold atoms is energetically stable against a single spin flip. Our variational wave function contains short-range correlations at least to the same level as Gutzwiller's projected wave function. For the Hubbard lattice model and the continuum model with pure repulsive interaction, we show that a fully polarized Fermi sea is generally unstable even for infinite repulsive strength. By contrast, for a resonance model, the ferromagnetic state is possible if the s-wave scattering length is positive and sufficiently large and the system is prepared to be orthogonal to the molecular bound state. However, we cannot rule out the possibility that more exotic correlations can destabilize the ferromagnetic state. Whether a fermion system with repulsive interaction will become ferromagnetic is a long-standing problem in condensed matter physics. In the early 1930s, Stoner used a simple mean-field theory to predict that ferromagnetism will always take place with a sufficiently large repulsive interaction [1]. However, this conclusion is later challenged by Gutzwiller, who took the short-range correlation into account [2]. So far, except for a few specific cases [3,4], there is no conclusive result on itinerant ferromagnetism. Recently, the MIT group reports an experiment on itinerant fermions in an ultracold Fermi gas with large positive scattering length close to a Feshbach resonance [5], and they attribute their observations to Stoner ferromagnetism by comparing their results to theories [6,7]. However, these theories are basically mean-field theory or a second-order perturbation, neither of which includes the Gutzwiller-type short-range correlation nor do they consider the unitary limited interaction near the Feshbach resonance. Moreover, many of the experimental signatures can be reproduced qualitatively by a nonmagnetic correlated state [8]. Thus, a serious study including the effects of both correlation and unitarity in this problem is called for.In this Rapid Communication we address the question of whether a fully magnetized state is stable against a single spin flip. We compare the energy of N + 1 spin-up particles with that of one spin-down particle and N spin-up particles. A fully magnetized ferromagnetic state is definitely unstable if we can find a variational state of the latter whose energy is lower. Similar idea has been used previously in studying the stability of Nagaoka ferromagnetism in the Hubbard model [3,[9][10][11] and attractively interacting Fermi gases with large population imbalance [12,13]. In this work, we will explore different realizations of "repulsive interactions" in ultracold Fermi gases: I. A single-band Hubbard model in a two-dimensional (2D) square or three-dimensional (3D) cubic lattice. The Hamiltonian isĤ =Ĥ t +Ĥ int , withĤ t = −t ij ,σ c † iσ c jσ + H.c., where ij are nearest-neighbor sites, and H int = U i n i↑ n i↓ (U > 0). II. A continuum model with a finite-range interaction potential in...
We present a scheme for generating a synthetic magnetic field and spin-orbit coupling via Raman coupling in highly magnetic lanthanide atoms such as dysprosium. Employing these atoms offer several advantages for realizing strongly correlated states and exotic spinor phases. The large spin and narrow optical transitions of these atoms allow the generation of synthetic magnetic fields an order of magnitude larger than those in the alkalis, but with considerable reduction of the heating rate for equal Raman coupling. The effective hamiltonian of these systems differs from that of the alkalis' by an additional nematic coupling term, which leads to a phase transition in the dressed states as detuning varies. For high-spin condensates, spin-orbit coupling leads to a spatially periodic structure, which is described in Majorana representation by a set of points moving periodically on a unit sphere. We name this a "Majorana spinor helix" in analogy to the persistent spin-1 2 helix observed in electronic systems.In the past few years, several groups have realized a synthetic magnetic field either in traps or in optical lattices [1][2][3][4], and spin-orbit (SO) coupling [5][6][7][8][9][10][11] with alkali atoms. These developments have highlighted intriguing physics in the ultracold atomic gas context. Vortices and the classical Hall effect have been observed with a Bose condensate exposed to a synthetic magnetic field [1,2]. SO-coupling in a Bose gas can lead to superfluid phases with stripe order [12,13] and a rich phase diagram [5,13,14], as well as modify the effective interaction between dressed-state atoms [6]. In addition, SO-coupling also leads to a divergent spin susceptibility, and the magnetic transition it implies has recently been observed [8,15]. Recently, SO-coupled Fermi gases are found to display interesting spin dynamics, topological transitions of Fermi surfaces, and spin-dependent band structure [9,10].However, there are serious challenges with creating exotic quantum matter using the current scheme of generating synthetic gauge fields. The small fine-structure splitting of the excited level used in the Raman-coupling process for alkalis leads to significant heating through spontaneous emission [16]. While lowering the laser intensity will reduce heating, it will also reduce the strength of the synthetic gauge field, pushing the high-field regime of novel correlated physics beyond reach. As of now, the number of vortices generated by synthetic magnetic field is far below that generated by rotations [1].We suggest the use of lanthanide atoms such as Dy [17,18] and Er [19] to overcome these challenges. The particular atomic structure of these atoms-narrow linewidth transitions, large ground-state orbital and spin angular momenta, and large fine-structure splittingoffer many advantages over the alkalis. As we explain later, the narrow-line transitions (2-kHz wide in Dy versus 6 MHz in Rb) and the L > 0 nature of the Dy ground and excited states provide a significant increase in Raman coupling without addit...
The Bose polaron is a quasi-particle of an impurity dressed by surrounding bosons. In fewbody physics, it is known that two identical bosons and a third distinguishable particle can form a sequence of Efimov bound states in the vicinity of inter-species scattering resonance. On the other hand, in the Bose polaron system with an impurity atom embedded in many bosons, no signature of Efimov physics has been reported in the existing spectroscopy measurements up to date. In this work, we propose that a large mass imbalance between a light impurity and heavy bosons can help produce visible signatures of Efimov physics in such a spectroscopy measurement. Using the diagrammatic approach in the Virial expansion to include three-body effects from pairwise interactions, we determine the impurity self-energy and its spectral function. Taking 6 Li-133 Cs system as a concrete example, we find two visible Efimov branches in the polaron spectrum, as well as their hybridizations with the attractive polaron branch. We also discuss the general scenarios for observing the signature of Efimov physics in polaron systems. This work paves the way for experimentally exploring intriguing few-body correlations in a many-body system in the near future.Top-down and bottom-up are two major approaches to studying correlations in a quantum many-body system. The cold atom system has intrinsic advantage for the bottom-up approach since it is a dilute system and the few-body problems therein are well understood. In this approach, one would like to understand how manybody physics is built up from few-body correlations. In cold atom system, one of the most intriguing three-body correlations lies in Efimov physics, which is characterized by an infinite number of trimer states nearby a two-body resonance and following a universal scaling law [1,2]. Efimov physics has been observed in a number of cold atoms experiments, while all of them are at the few-body level [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The manifestation of Efimov physics in the many-body system has yet to be observed.In this context, a convenient and non-trivial testbed is the highly-polarized ultracold gases, which consist of minority impurity atoms interacting with the majority of fermionic or bosonic atoms, respectively called the Fermi or the Bose polarons. Lots of theoretical efforts have been paid to study the Fermi polaron [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] and the Bose polaron [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Nearby a Feshbach resonance, a Fermi polaron displays an attractive branch [20][21][22][23][24][25]29] and a repulsive branch [26][27][28], which directly manifests two-body correlations in this system. In the past few years, the Fermi polaron has been studied by a number of experiments [52][53][54][55][56][57], while the Bose polaron has only recently been explored [58][59][60]. Most of these experiments are the injection radio-frequency spectroscopy measurements, with which both the r...
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