Mott transition and high-temperature crossovers at half-filling

The interaction-driven Mott transition in the half-filled Hubbard model is a first-order phase transition that terminates at a critical point $(T_mathrm{c},U_mathrm{c})$ in the temperature-interaction plane $T-U$. A number of crossovers occur along lines that extend for some range above $(T_mathrm{c},U_mathrm{c})$. Asymptotically close to $(T_mathrm{c},U_mathrm{c})$, these lines coalesce into the so-called Widom line. The existence of $(T_mathrm{c},U_mathrm{c})$ and of the associated crossovers becomes unclear when long-wavelength fluctuations or long-range order occur above $(T_mathrm{c},U_mathrm{c})$. We study this problem using continuous-time quantum Monte Carlo methods as impurity solvers for both Dynamical Mean-Field Theory (DMFT) and Cellular Dynamical Mean-Field Theory (CDMFT). We contrast the cases of the square lattice, where antiferromagnetic fluctuations dominate in the vicinity of the Mott transition, and the triangular lattice where they do not. The inflexion points and maxima found near the Widom line for the square lattice can serve as proxy for the triangular lattice case. But the only crossover observable in all cases at sufficiently high temperature is that associated with the opening of the Mott gap. The same physics also controls an analog crossover in the resistivity called the Quantum Widom line.

Surface background rejection technique for liquid argon dark matter detectors using a thin scintillating layer

Future large liquid argon direct dark matter detectors can benefit greatly from an efficient surface background rejection technique. To aid the development of these large scale detectors a test stand, Argon-1, has been constructed at Carleton University, Ottawa, Canada, in the noble liquid detector development lab. It aims to test a novel surface background rejection technique using a thin layer of slow scintillating material at the surface of the vessel. Through pulse-shape discrimination of the slow light from the scintillating layer, events from the surface of the detector can be discriminated from liquid argon events. The detector will be implemented with high-granularity SiPMs for light detection which will be used to accurately identify surface events to characterize the proposed technique. An overview of the technique and the status of the experiment are discussed here.