Eur. Phys. J. B (2018) 91: 205
https://doi.org/10.1140/epjb/e2018-90350-y

Regular Article

Study of supersolidity in the two-dimensional Hubbard–Holstein model^★

¹ CMP Division, Saha Institute of Nuclear Physics, HBNI, Kolkata, India
² Theoretical Physics Department, Indian Association for the Cultivation of Science, Kolkata, India

^a e-mail: y.sudhakar@saha.ac.in

Received: 30 May 2018
Published online: 17 September 2018

Abstract

We derive an effective Hamiltonian for the two-dimensional Hubbard–Holstein model in the regimes of strong electron–electron and strong electron–phonon interactions by using a nonperturbative approach. In the parameter region where the system manifests the existence of a correlated singlet phase, the effective Hamiltonian transforms to a t₁ − V ₁ − V ₂ − V ₃ Hamiltonian for hard-core-bosons on a checkerboard lattice. We employ quantum Monte Carlo simulations, involving stochastic-series-expansion technique, to obtain the ground state phase diagram. At filling 1∕8, as the strength of off-site repulsion increases, the system undergoes a first-order transition from a superfluid to a diagonal striped solid with ordering wavevector = (π∕4, 3π∕4) or (π∕4, 5π∕4). Unlike the one-dimensional situation, our results in the two-dimensional case reveal a supersolid phase (corresponding to the diagonal striped solid) around filling 1∕8 and at large off-site repulsions. Furthermore, for small off-site repulsions, we witness a valence bond solid at one-fourth filling and tiny phase-separated regions at slightly higher fillings.