Deconfined Fermi liquid to Fermi liquid transition

We propose the possibility of a deconfined quantum phase transition between two symmetric Fermi liquids with a jump of Fermi surface volume by 1/2 of the Brillouin Zone per flavor. We consider a bilayer t-J model with inter-layer spin-spin coupling $J_perp$ but no inter-layer hopping. The Oshikawa-Luttinger theorem allows two distinct Fermi liquids for this particular model. At fixed filling $ u=frac{1-x}{2}$ per flavor, we argue that there is a transition between the conventional Fermi liquid (FL) with large Fermi surface and a second Fermi liquid (sFL) with small Fermi surface through tuning the $J_perp$ coupling. We formulate a deconfined critical theory with the FL and the sFL phase as higgs or confined phases of a $U(1) imes U(1)$ gauge theory. The critical theory has a pairing instability below a finite but low temperature scale. Above the small pairing scale, the system is governed by a deconfined Fermi liquid to Fermi liquid transition (DFFT). In the critical regime the elementary particles are fractionalized with the physical electron as a composite object. This is similar to the well-known Neel to VBS DQCP. Across the transition, the quasi particle residue $Z$ first vanishes and then revives. The DFFT critical point here offers a simple and clean example of small to large Fermi surface transition, without the controversy in the similar Fermi surface transitions in the heavy fermion systems and in the hole doped cuprates. We comment on possible realizations in the recently found nickelate superconductor La3Ni2O7.