We use variational approach to analyze the evolution of the dynamics of a neutral s-wave superconductor between BCS and BEC regimes. We consider 2D case, when BCS-BEC crossover occurs already at weak coupling and is governed by the ratio of the two scales - the Fermi energy EF and the bound state energy for two fermions in a vacuum, E0. BCS and BEC regimes correspond to EF ≥ E0 and EF ≤ E0, respectively. We compute the spectrum of low-energy bosonic excitations and show that the velocity of phase fluctuations remains v F / 2 through BCS-BEC crossover. We also discuss the topological A φ term in the effective action.
Bibliographical noteFunding Information:
We thank I. Aleiner, J.J. Dziarmaga, I. Martin, M. Stone, J. Sauls, D. Solenov, and G.E. Volovik for valuable discussions. The work at Los Alamos was performed under the auspices of the United States Department of Energy under Contract DE5AC52-06NA25396 and supported by LDRD and BES Grant No’s. 20170460ER and E3B7, respectively. The work by AVC was supported by the Office of Basic Energy Sciences U.S. Department of Energy under award DE-SC0014402. AVC is thankful to KITP at UCSB, where part of the work has been done. KITP is supported by NSF grant PHY-1125915. ________
© 2018 J.I. Vestgarden, T.H. Johansen, and Y.M. Galperin.
- Berry phase