Dr. Martin Kliesch

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This work is concerned with thermal quantum states of Hamiltonians on
spin and fermionic lattice systems with short range interactions. We
provide results leading to a local definition of temperature, thereby
extending the notion of "intensivity of temperature'' to interacting
quantum models. More precisely, we derive a perturbation formula for
thermal states. The influence of the perturbation is exactly given in
terms of a generalized covariance. For this covariance we prove
exponential clustering of correlations above a universal critical
temperature, which upper bounds physical critical temperatures such as
the Curie temperature. As a corollary, we obtain that above a the
critical temperature, thermal states are stable against distant
Hamiltonian perturbations. Moreover, our results imply that above the
critical temperature local expectation values can be approximated with a
computational cost independent of the system size.

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FU Berlin
Seminar room 0.03, ETP
Contact: David Gross