Ground State Instability in Non-relativistic QFT and Euler-Heisenberg Lagrangian via Holography
We study the ground state instability of a strongly coupled QFT with the $z=2$ Schr\"odinger symmetry in a constant electric field using probe branes holography. The system is $N_f$ $\mathcal{N}=2$ hypermultiplet fermions at zero charge density in the supergravity Schr\"odinger background. We show that the instability occurs due to Schwinger-like effect and an insulator state will undergo a transition to a conductor state. We calculate the decay rate of instability and pair production probability by using the $gauge/gravity$ duality. At zero temperature for massive fermions, we suggest that the instability occurs if the critical electric field is larger than the confining force between fermions, which is proportional to an effective mass. We demonstrate that, at zero temperature, the Schr\"odinger background simulates the role of a crystal lattice for massive particles. We also show that at finite 't Hooft coupling for particles with a mass higher than $\frac{\sqrt{\lambda}}{\pi \b}$, in this background, instability does not occur, no matter how large the external electric field is, meaning that we have a \textit{perfect insulator}. Moreover, we derive Euler-Heisenberg effective Lagrangian for the non-relativistic strongly correlated quantum theory from probe branes holography in Schr\"odinger spacetime.