ELFT HÍRADÓ

[Janos Asboth <asboth.janos AT ttk.bme.hu>]

Meghívó

BME Elméleti Fizika Szeminárium,

febr. 16. péntek 10h15,

1111 Budapest, Budafoki út 8., BME F III. magasföldszint 01.,
Elméleti Fizika Tanszék szemináriumi szoba

Felix Werner (Laboratoire Kastler Brossel, ENS & Collège de France):
Large-order summation of connected Feynman diagrams for strongly correlated
fermions

A major long-standing goal is to develop generic unbiased methods for
many-fermion problems. Conventional Quantum Monte Carlo methods (e.g.
auxiliary-field QMC or lattice QCD) generically suffer from the "fermion
sign problem": The computational time increases exponentially with the
number of fermions. In contrast, expansions of intensive quantities in
series of connected Feynman diagrams can be computed directly in the
thermodynamic limit. Over the last decade, diagrammatic Monte Carlo
algorithms made it possible to reach large expansion orders, and to obtain
state-of-the-art results for various models of interacting fermions in 2
and 3 dimensions. The "Connected Determinant" algorithm allows to sum a
factorial number of connected diagrams in an exponential time [1], leading
to a computational time that increases only polynomially with the precision
[2]. The range of applicability of the approach can be considerably
enlarged by two ingredients: divergent-series summation methods, and
modification of the expansion by changing the starting point and/or using
dressed propagators/vertices. An important aspect is to understand the
singularities of the function that stands behind the series. I will present
illustrative results for the unitary Fermi gas in the normal phase [3,4]
and the superconducting phase of the polarized attractive Hubbard model
[5].

[1] Rossi, PRL 119, 045701 (2017)
[2] Rossi, Prokof’ev, Svistunov, Van Houcke, FW, EPL 118, 10004 (2017)
[3] Rossi, Ohgoe, Van Houcke, FW, PRL 121, 130405 (2018)
[4] Rossi, Ohgoe, Kozik, Prokof’ev, Svistunov, Van Houcke, FW, PRL 121,
130406 (2018)
[5] Spada, Rossi, Simkovic, Garioud, Ferrero, Van Houcke, FW,
arXiv:2103.12038

Minden érdeklődőt szeretettel várunk.
Asbóth János,
 szemináriumi koordinátor