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BCS-BEC Crossover and Topological Phase Transition in 3D Spin-Orbit Coupled Degenerate Fermi Gases  [PDF]
Ming Gong,Sumanta Tewari,Chuanwei Zhang
Physics , 2011, DOI: 10.1103/PhysRevLett.107.195303
Abstract: We investigate the BCS-BEC crossover in three dimensional degenerate Fermi gases in the presence of spin-orbit coupling (SOC) and Zeeman field. We show that the superfluid order parameter destroyed by a large Zeeman field can be restored by the SOC. With increasing strengths of the Zeeman field, there is a series of topological quantum phase transitions from a non-topological superfluid state with fully gapped fermionic spectrum to a topological superfluid state with four topologically protected Fermi points (i.e., nodes in the quasiparticle excitation gap) and then to a second topological superfluid state with only two topologically protected Fermi points. The quasiparticle excitations near the Fermi points realize the long-sought low-temperature analog of Weyl fermions of particle physics. We show that the topological phase transitions can be probed using the experimentally realized momentum resolved photoemission spectroscopy.
BCS-BEC quantum phase transition and collective excitations in two-dimensional Fermi gases with p- and d-wave pairings  [PDF]
Gaoqing Cao,Lianyi He,Pengfei Zhuang
Physics , 2012, DOI: 10.1103/PhysRevA.87.013613
Abstract: It is generally believed that the BCS-BEC evolution in fermionic systems with s-wave pairing is a smooth crossover. However, for nonzero orbital-angular-momentum pairing such as p- or d-wave pairing, the system undergoes a quantum phase transition at the point where the chemical potential $\mu$ vanishes. In this paper, we study the BCS-BEC quantum phase transition and the collective excitations associated with the order-parameter fluctuations in two-dimensional fermionic systems with p- and d-wave pairings. We show that the quantum phase transition in such systems can be generically traced back to the infrared behavior of the fermionic excitation at $\mu=0$: $E_{\bf k}\sim k^l$, where $l=1,2$ is the quantum number of the orbital angular momentum. The nonanalyticity of the thermodynamic quantities is due to the infrared divergence caused by the fermionic excitation at $\mu=0$. As a result, the evolution of the Anderson-Bogoliubov mode is not smooth: Its velocity is nonanalytical across the quantum phase transition.
BCS-BEC Crossover in the Strongly Correlated Regime of ultra-cold Fermi gases  [PDF]
S. G. Bhongale,S. J. J. M. F. Kokkelmans
Physics , 2006,
Abstract: We study BCS-BEC crossover in the strongly correlated regime of two component rotating Fermi gases. We predict that the strong correlations induced by rotation will have the effect of modifying the crossover region relative to the non-rotating situation. We show via the two particle correlation function that the crossover smoothly connects the s-wave paired fermionic fractional quantum Hall state to the bosonic Laughlin state.
Virial relations for ultracold trapped Fermi gases with finite range interactions through the BCS-BEC crossover  [PDF]
L. E. C. Rosales-Zarate R. Jauregui
Physics , 2009,
Abstract: We study the virial relations for ultracold trapped two component Fermi gases in the case of short finite range interactions. Numerical verifications for such relations are reported through the BCS-BEC crossover. As an intermediate step, it is necessary to evaluate the partial derivatives of the many body energy with respect to the inverse of the scattering length and with respect to the interaction range. They are found to have extreme values at the unitary limit. The virial results are used to check the quality of the variational wave function involved in the calculations.
Vortex structures and zero energy states in the BCS-to-BEC evolution of p-wave resonant Fermi gases  [PDF]
T. Mizushima,K. Machida
Physics , 2010, DOI: 10.1103/PhysRevA.81.053605
Abstract: Multiply quantized vortices in the BCS-to-BEC evolution of p-wave resonant Fermi gases are investigated theoretically. The vortex structure and the low-energy quasiparticle states are discussed, based on the self-consistent calculations of the Bogoliubov-de Gennes and gap equations. We reveal the direct relation between the macroscopic structure of vortices, such as particle densities, and the low-lying quasiparticle state. In addition, the net angular momentum for multiply quantized vortices with a vorticity $\kappa$ is found to be expressed by a simple equation, which reflects the chirality of the Cooper pairing. Hence, the observation of the particle density depletion and the measurement of the angular momentum will provide the information on the core-bound state and $p$-wave superfluidity. Moreover, the details on the zero energy Majorana state are discussed in the vicinity of the BCS-to-BEC evolution. It is demonstrated numerically that the zero energy Majorana state appears in the weak coupling BCS limit only when the vortex winding number is odd. There exist the $\kappa$ branches of the core bound states for a vortex state with vorticity $\kappa$, whereas only one of them can be the zero energy. This zero energy state vanishes at the BCS-BEC topological phase transition, because of interference between the core-bound and edge-bound states.
Ultracold Fermi gases in the BEC-BCS crossover: a review from the Innsbruck perspective  [PDF]
Rudolf Grimm
Physics , 2007,
Abstract: A review of recent BEC-BCS crossover experiments in ultracold Fermi gases is given with particular emphasis on the work performed with lithium-6 at the University of Innsbruck.
Evolution from BCS to BEC superfluidity in p-wave Fermi gases  [PDF]
M. Iskin,C. A. R. Sá de Melo
Physics , 2005, DOI: 10.1103/PhysRevLett.96.040402
Abstract: We consider the evolution of superfluid properties of a three dimensional p-wave Fermi gas from weak (BCS) to strong (BEC) coupling as a function of scattering volume. We analyse the order parameter, quasi-particle excitation spectrum, chemical potential, average Cooper pair size and the momentum distribution in the ground state ($T = 0$). We also discuss the critical temperature $T_{\rm c}$, chemical potential and number of unbound, scattering and bound fermions in the normal state ($T = T_{\rm c}$). Lastly, we derive the time-dependent Ginzburg-Landau equation for $T \approx T_{\rm c}$ and extract the Ginzburg-Landau coherence length.
The quantum pressure correction to the excitation spectrum of the trapped superfluid Fermi gases in a BEC-BCS crossover

Dong Hang,Ma Yong-Li,

中国物理 B , 2009,
Abstract: Using quantum hydrodynamic approaches, we study the quantum pressure correction to the collective excitation spectrum of the interacting trapped superfluid Fermi gases in the BEC-BCS crossover. Based on a phenomenological equation of state, we derive hydrodynamic equations of the system in the whole BEC-BCS crossover regime. Beyond the Thomas--Fermi approximation, expressions of the frequency corrections of collective modes for both spherical and axial symmetric traps excited in the BEC-BCS crossover are given explicitly. The corrections of the eigenfrequencies due to the quantum pressure and their dependence on the inverse interaction strength, anisotropic parameter and particle numbers of the condensate are discussed in detail.
BCS to BEC Quantum Phase Transition in Spin Polarized Fermionic Gases  [PDF]
S. S. Botelho,C. A. R. Sá de Melo
Physics , 2004,
Abstract: We discuss the possibility of a quantum phase transition in ultracold spin polarized fermionic gases which exhibit a p-wave Feshbach resonace. We show that when fermionic atoms form a condensate that can be externally tuned between the BCS and BEC limits, the zero temperature compressibility and the spin susceptibility of the fermionic gas are non-analytic functions of the two-body bound state energy. This non-analyticity is due to a massive rearrangement of the momentum distribution in the ground state of the system. Furthermore, we show that the low temperature superfluid density is also non-analytic, and exibits a dramatic change in behavior when the critical value of the bound state energy is crossed.
Second sound in Fermi gases at the BCS-BEC crossover  [PDF]
H. Heiselberg
Physics , 2004,
Abstract: The thermodynamic potential is calculated for a uniform superfluid gas of fermi atoms from the mean field BCS equations including corrections from induced interactions, Hartree-Fock energies and quasiparticle selfenergies. The entropy, specific heat and sound modes are calculated as function of temperature, density and interaction strength from the BCS to the unitarity limit and around the BCS-BEC crossover. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature.
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