Strain-induced superconductor-insulator transition on a Lieb lattice

Abstract

We report the numerical investigation of strain-induced superconductor-insulator quantum phase transition on a Lieb lattice. Based on a nonperturbative Monte Carlo technique, which retains the spatial fluctuations of the superconducting pairing field at all orders but neglects the temporal fluctuations, we show that in two dimensions, an s-wave superconductor undergoes transition to a highly correlated bosonic insulator under the influence of strain, applied as staggered hopping amplitudes. We further demonstrate a strain-induced BCS-BEC like crossover in the superconducting state, such that the superconductor-insulator transition takes place between a bosonic superconductor and a bosonic insulator. Our results suggest that it is the contribution of the dispersive bands towards the superconducting order that dictates this crossover. To the best of our knowledge, this is the first work to report a theoretical investigation of "disorder free" superconductor-insulator phase transition in systems with Lieb lattice structure. With the recent experimental realization of the Lieb lattice in ultracold atomic gases, photonic lattices as well as in solid state systems, we believe that the results presented in this paper would be of importance to initiate experimental investigation of such novel quantum phase transitions. We further discuss the fate of such systems at finite temperature, highlighting the effect of fluctuations on the superconducting pair formations, thermal scales, and quasiparticle behavior. Our nonperturbative numerical approach to the problem enables us to capture the thermal scales of the system accurately and provides us with mean-field estimates of the ground state properties. The high-temperature quasiparticle signatures discussed in this paper are expected to serve as benchmarks for experiments such as radio frequency and momentum resolved radio frequency spectroscopy measurements carried out on systems such as ultracold atomic gases.