Quarter-filled supersolid and solid phases in the extended Bose-Hubbard model

First-Principles Study of the Optical Properties of TMDC/Graphene Heterostructures

The transition-metal dichalcogenide (TMDC) in the family of MX2 (M=Mo,W; X=S,Se) and the graphene (Gr) monolayer are an atomically thin semiconductor and a semimetal, respectively. The monolayer MX2 has been discovered as a new class of semiconductors for electronics and optoelectronics applications. Because of the hexagonal lattice structure of both materials, MX2 and Gr are often combined with each other to generate van der Waals heterostructures. Here, the MX2/Gr heterostructures are investigated theoretically based on density functional theory (DFT). The electronic structure and the optical properties of four different MX2/Gr heterostructures are computed. We systematically compare these MX2/Gr heterostructures for their complex permittivity, absorption coefficient, reflectivity and refractive index.

Twisted complex superfluids in optical lattices

We show that correlated pair tunneling drives a phase transition to a twisted superfluid with a complex order parameter. This unconventional superfluid phase spontaneously breaks the time-reversal symmetry and is characterized by a twisting of the complex phase angle between adjacent lattice sites. We discuss the entire phase diagram of the extended Bose-Hubbard model for a honeycomb optical lattice showing a multitude of quantum phases including twisted superfluids, pair superfluids, supersolids and twisted supersolids. Furthermore, we show that the nearest-neighbor interactions lead to a spontaneous breaking of the inversion symmetry of the lattice and give rise to dimerized density-wave insulators, where particles are delocalized on dimers. For two components, we find twisted superfluid phases with strong correlations between the species already for surprisingly small pair-tunneling amplitudes. Interestingly, this ground state shows an infinite degeneracy ranging continuously from a supersolid to a twisted superfluid.