## The Mermin-Wagner theorem and quantum criticality in physical quasi-1d systems

Peter Richard Crompton Hamburg University

Abstract:
We identify the leading corrections to Finite-Size Scaling relations for the Correlation length and Twist Order parameter of three mixed-spin chain systems via QMC analysis, arguing that our results imply these systems can be described by the same underlying conformal picture in correspondence with recent pictures of deconfined quantum criticality at the vacuum angle, $\theta=\pi$, both in zero and finite temperature quantum spin chains. We propose a new effective theory for the critical region of quantum fluctuation driven transitions and derive a renormalization group equation, suitable for numerical evaluation via Quantum Monte Carlo analysis, that rigorously defines the mapping between critical features in zero temperature and finite temperature chains. We comment on the Mermin-Wagner theorem, and its rotational symmetry breaking extensions, in this context.