Iolanda Di Bernardo
In a semimetal or narrow-gap semiconductor the Coulomb interaction can drive a unique many-body ground state in which excitons condense. The exciton condensate is a macroscopic quantum-coherent state, analogous to a superconductor, with an order parameter [1]. However, because excitons are charge-neutral, the condensate allows no supercurrent and is an example of an exciton insulator. The existence and behaviour of the exciton condensate is highly conditional on the strength of the Coulomb interaction (U0), and so is expected to depend strongly on doping, electric fields, and screening from the dielectric environment. Monolayer WTe2 is theoretically predicted to host an exciton condensate, with the order parameter at low U0 to exhibit a spin density wave (SDW) of order qc and a charge density wave (CDW) of order 2qc [2], where qc is defined by the separation of the electron and hole pockets in momentum space. At high values of U0, however, the condensate should exhibit a spin spiral phase, which is absent of both charge and spin density modulations, but these can be weakly revealed upon breaking time-reversal symmetry [2] – for example, by applying a magnetic field.
Here we investigate the electronic structure of monolayer WTe2 grown on highly oriented pyrolytic graphite (HOPG) using quasiparticle interference (QPI), and analyse the behaviour of QPI features and but also additional modulations of the local density of states (LDOS) as a function of an external magnetic field perpendicular to the van der Waals stack.References[1] Phys Rev 158, 461 (1967)[2] arXiv, pp. 1–12, 2020.