Simple vertex correction improves GW band energies of bulk and two-dimensional crystals

The GW self-energy method has long been recognized as the gold standard for quasiparticle (QP) calculations of solids in spite of the fact that the neglect of vertex corrections and the use of a DFT starting point lacks rigorous justification. In this work we remedy this situation by including a simple vertex correction that is consistent with an LDA starting point. We analyse the effect of the self-energy by splitting it into a short-range and long-range term which are shown to govern respectively the center and size of the band gap. The vertex mainly improves the short-range correlations and therefore has a small effect on the band gap, while it shifts the band gap center up in energy by around 0.5 eV in good agreement with experiments. Our analysis also explains how the relative importance of short- and long-range interactions in structures of different dimensionality is reflected in their QP energies. Inclusion of the vertex comes at practically no extra computational cost and even improves the basis set convergence compared to GW. The method thus provides an efficient and rigorous improvement over the GW approximation and sets a new standard for quasiparticle calculations of solids

Auxiliary Field Diffusion Monte Carlo calculation of nuclei with A<40 with tensor interactions

We calculate the ground-state energy of 4He, 8He, 16O, and 40Ca using the auxiliary field diffusion Monte Carlo method in the fixed phase approximation and the Argonne v6' interaction which includes a tensor force. Comparison of our light nuclei results to those of Green's function Monte Carlo calculations shows the accuracy of our method for both open and closed shell nuclei. We also apply it to 16O and 40Ca to show that quantum Monte Carlo methods are now applicable to larger nuclei.Comment: 4 pages, no figure

Contact interaction in an unitary ultracold Fermi gas

An ultracold Fermi atomic gas at unitarity presents universal properties that in the diluted limit can be well described by a contact interaction. By employing a guide function with correct boundary conditions and making simple modifications to the sampling procedure we are able to handle for the first time a true contact interaction in a quantum Monte Carlo calculation. The results are obtained with small variances. Our calculations for the Bertsch and contact parameters are in excellent agreement with published experiments. The possibility of using a more faithfully description of ultracold atomic gases can help uncover features yet unknown of the ultracold atomic gases. In addition, this work paves the way to perform quantum Monte Carlo calculations for systems interacting with contact interactions, where in many cases the description using potentials with finite effective range might not be accurate

Nonequilibrium gas-liquid transition in the driven-dissipative photonic lattice

We study the nonequilibrium steady state of the driven-dissipative Bose-Hubbard model with Kerr nonlinearity. Employing a mean-field decoupling for the intercavity hopping $J$, we find that the steep crossover between low and high photon-density states inherited from the single cavity transforms into a gas$-$liquid bistability at large cavity-coupling $J$. We formulate a van der Waals like gas$-$liquid phenomenology for this nonequilibrium situation and determine the relevant phase diagrams, including a new type of diagram where a lobe-shaped boundary separates smooth crossovers from sharp, hysteretic transitions. Calculating quantum trajectories for a one-dimensional system, we provide insights into the microscopic origin of the bistability.Comment: 5 pages, 4 figures + Supplemental Material (2 pages, 2 figures