Filippo Tramonto - Università degli Studi di Milano #
Quantum Monte Carlo study of dynamic properties of ultracold gases #

We obtain ab-initio estimations of the dynamic structure factor, S(q,w),
of ultracold Bose gases at zero temperature. More precisely, we use the Genetic
Inversion via Falsification of Theories (GIFT) algorithm [1], which uses genetic
algorithms to perform accurate analytic continuations of imaginary time
correlation functions under ill-posed conditions. The imaginary time correlation 
functions have been computed with an exact projector method, the path integral ground
 state (PIGS) method [2]. Using the hard-sphere potential to model the two-body
interactions between the atoms, we compute S(q,w) changing the gas parameter
from the diluteregime up to densities above the freezing point of the hard-spheres
system. With increasing density, we observe the emergence of a broad multiphonon
contribution accompanying the quasi-particle peak and a crossover of the dispersion
of elementary excitations from a Bogoliubov-like spectrum to a phonon-maxon-roton 
curve.
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In collaboration with: R. Rota, D. E. Galli, S. Giorgini

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References:
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[1] E. Vitali, M. Rossi, L. Reatto and D.E. Galli, Phys. Rev. B 82, 174510,(2010).
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[2] A. Sarsa, K. E. Schmidt, and W. R. Magro, J. Chem. Phys. 113, 1366 (2000).