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arXiv:1410.8431 · Physical Review Letters

Topological Superradiant States in a Degenerate Fermi Gas

Jian-Song Pan, Xiong-Jun Liu, Wei Zhang, Wei Yi, Guang‐Can Guo

Published Jul 22, 2015• 87 citations
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Abstract

We predict the existence of a topological superradiant state in a two-component degenerate Fermi gas in a cavity. The superradiant light generation in the transversely driven cavity mode induces a cavity-assisted spin-orbit coupling and opens a bulk gap at half filling. This mechanism can simultaneously drive a topological phase transition in the system, yielding a topological superradiant state. We map out the steady-state phase diagram in the presence of an effective Zeeman field, and identify a critical tetracritical point beyond which the topological and the conventional superraidiant phase boundaries separate. The topological phase transition can be detected from its signatures in either the momentum distribution of the atoms or the variation of the cavity photon occupation due to the nontrivial feedback of the atoms on the cavity field.

DOI
10.1103/physrevlett.115.045303

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We predict the existence of a topological superradiant state in a two-component degenerate Fermi gas in a cavity. The superradiant light generation in the transversely driven cavity mode induces a cavity-assisted spin-orbit coupling and opens a bulk gap at half filling. This mechanism can simultaneously drive a topological phase transition in the system, yielding a topological superradiant state. We map out the steady-state phase diagram in the presence of an effective Zeeman field, and identify a critical tetracritical point beyond which the topological and the conventional superraidiant phase boundaries separate. The topological phase transition can be detected from its signatures in either the momentum distribution of the atoms or the variation of the cavity photon occupation due to the nontrivial feedback of the atoms on the cavity field.
Coherently driven atomic gases inside optical cavities hold great promise for generating rich dynamics and exotic states of matter. It was shown recently that an exotic topological superradiant state exists in a two-component degenerate Fermi gas coupled to a cavity, where local order parameters coexist with global topological invariants. In this work, we characterize in detail various properties of this exotic state, focusing on the feedback interactions between the atoms and the cavity field. In particular, we demonstrate that cavity-induced interband coupling plays a crucial role in inducing the topological phase transition between the conventional and topological superradiant states. We analyze the interesting signatures in the cavity field left by the closing and reopening of the atomic bulk gap across the topological phase boundary and discuss the robustness of the topological superradiant state by investigating the steady-state phase diagram under various conditions. Furthermore, we consider the interaction effect and discuss the interplay between the pairing order in atomic ensembles and the superradiance of the cavity mode. Our work provides many valuable insights into the unique cavity–atom hybrid system under study and is helpful for future experimental exploration of the topological superradiant state.
Coherently driven atomic gases inside optical cavities hold great promises in generating rich dynamics and exotic states of matter. Recently, it has been shown that a novel topological superradiant state exists in a two-component degenerate Fermi gas coupled to a cavity, where local order parameters coexist with global topological invariants. In this work, we characterise in detail various properties of this exotic state, focusing on the feedback interactions between the atoms and the cavity field. In particular, we demonstrate that the cavity-induced inter-band couplings play a crucial role in inducing the topological phase transition between the conventional and the topological superradiant state. Furthermore, we analyse how the closing and reopening of the atomic bulk gap across the topological phase boundary leaves interesting signatures in the cavity field. We also discuss the robustness of the topological superradiant state by investigating the steady-state phase diagram under various circumstances. Our work provides many valuable insights into the unique atom-cavity hybrid system under study, and is helpful for future experimental exploration of the topological superradiant state.
In the present paper, we investigate a one-dimensional spinful degenerate Fermi gas coupled to an optical cavity, thereby realizing a cavity-assisted hourglass Creutz-like ladder model characterized by a Peierls-like instability between identical spins and a modulated spin-orbit coupling. Furthermore, we demonstrate that flat-band states induced by the optical cavity emerge through appropriate detuning of the parameters within our system. We perform a comprehensive analysis for the effects of cavity-assisted Raman processes that involve either distinct or identical spins on the topological superradiant properties. Notably, our findings reveal that our system exhibits rich nontrivial topological superradiant characteristics by varying different coupling strengths. Specifically, we identify a topological superradiant phase with winding numbers two and four corresponding to degenerate edge states. Additionally, we discuss various quantum phases and provide the corresponding steady-state phase diagram. Finally, we briefly outline parameter estimation techniques and address potential experimental observations related to these predicted quantum phases and their associated phase diagrams.
In the present paper, we consider the steady state of Fermi atoms coupled to a shaken cavity field at half filling. This shaking generates a time-dependent dynamical optical lattice with backaction on the atomic degrees of freedom. In the high-frequency limit, the shaken dynamical optical lattice produces interesting cavity-field-dependent long-range hoppings, including the nearest-, next-nearest-, and next-next-nearest-neighbor hoppings. They can change the band structures of the system and thus induce nontrivial topological superradiant phases, especially with a large winding number and four degenerate edge states. We also briefly address possible experimental observations of the predicted topological superradiant phases. Our results provide an alternative way to explore novel many-body physics in cavity quantum electrodynamics systems.
We study the superradiance transition of a two-component three-dimensional Fermi gas interacting with a single-mode light field of Dicke-type coupling. We find that for a noninteracting gas, due to the Fermi blocking, a unique superradiant state with a superradiant outer shell surrounding an inner Fermi sea may appear, and the critical atom-light coupling strength gc{g}_{c}gc​ to trigger the superradiance approaches $\sqrt{{\ensuremath{\omega}}_{c}{E}_{F}/3}$ even for a vanishing transition frequency between a two-spin state (${\ensuremath{\omega}}_{a}\ensuremath{\rightarrow}0$), in contrast to ${g}_{c}\ensuremath{\sim}\sqrt{{\ensuremath{\omega}}_{c}{\ensuremath{\omega}}_{a}}\ensuremath{\rightarrow}0$ for a bosonic or spin system. When the atom-atom attraction is included, we find that the atomic superfluid would compete with the superradiance directly and both orders cannot coexist, giving rise to an interesting ground-state phase diagram with a tricritical point. The resultant phases and phase transitions are characterized by the unique fluctuation spectrum beyond the mean-field level. We further analyze the effects caused by the decay of the light field, which is inevitable for a possible realization in a cavity with a cold atom system. Our results would be beneficial for the understanding of the interplay between Fermi superfluid and superradiance.
We consider a spinless ultracold Fermi gas tightly trapped along the axis of an optical resonator and transversely illuminated by a laser closely tuned to a resonator mode. At a certain threshold pump intensity, the homogeneous gas density breaks a Z_{2} symmetry towards a spatially periodic order, which collectively scatters pump photons into the cavity. We show that this known self-ordering transition also occurs for low field seeking fermionic particles when the laser light is blue detuned to an atomic transition. The emergent superradiant optical lattice in this case is homopolar and possesses two distinct dimerizations. Depending on the spontaneously chosen dimerization, the resulting Bloch bands can have a nontrivial topological structure characterized by a nonvanishing Zak phase. In the case where the Fermi momentum is close to half of the cavity-mode wave number, a Peierls-like instability here creates a topological insulator with a gap at the Fermi surface, which hosts a pair of edge states. The topological features of the system can be nondestructively observed via the cavity output: the Zak phase of the bulk coincides with the relative phase between laser and cavity field, while the fingerprint of edge states can be observed as additional broadening in a well-defined frequency window of the cavity spectrum.