Linear response theory is a general formalism applicable to any physical system slightly departing from equilibrium. It provides a microscopical description of small perturbations on a system in equilibrium by relating properties of the out-of-equilibrium system, such as transport coefficients, to correlations functions of the equilibrium system. Taking the Fourier transform of these quantities easily allows us to compare the results to scattering experiments.
The Stochastic Series Expansion (SSE) method is a powerful scheme for performing Monte Carlo (MC) simulations of quantum lattice systems. Through an approximation-free high-temperature expansion of the canonical partition function, it is possible to divide the sampling space in states and operator strings. With the directed loop algorithm, it is possible to efficiently sample the sampling space through nonlocal operator updates.
Getting estimations of transport coefficients in any MC method has proved a challenge. Approximation-free routines rely on the extrapolation of such quantities to the DC regime through Matsubara frequencies, limiting the results to low temperatures. In the beginning of the 21st century, much progress was made towards an efficient sampling routine of such quantities. This approach, however, suffers from scaling problems for larger system at low temperatures.
Here we present a novel and efficient sampling routine based on an imaginary time discretisation of the operator string to sample transport coefficients in the linear response regime. Results for the non-integrable spin-S XXZ-model are also computed.
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As you might know, some of our master's students are about to defend their thesis soon. Until then, we are arranging a series of 8 weekly open sessions to practice for their presentation and share their research with the rest of the department.
The presentations shall have the typical 30-minute exposition + a round of questions of around 15 minutes. It would be significant to have a good number of PhDs and individuals interested in the field so that the question round can be engaging and serve as good practice for the student. Make sure to attend if the topic seems interesting.
We are serving pizza. Welcome!