Dresden 2026 – wissenschaftliches Programm
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MA: Fachverband Magnetismus
MA 31: Frustrated Magnets II (joint session MA/TT)
MA 31.1: Vortrag
Mittwoch, 11. März 2026, 09:30–09:45, POT/0361
Finite-size spectral signatures of order by quantum disorder: A perspective from Anderson's tower of states — •Subhankar Khatua1, Griffin C. Howson2, Michel J. P. Gingras2, and Jeffrey G. Rau3 — 1IFW Dresden, Germany — 2University of Waterloo, Canada — 3University of Windsor, Canada
In frustrated magnetic systems with a subextensive number of classical ground states, quantum zero-point fluctuations can select a unique long-range ordered state, a celebrated phenomenon referred to as order by quantum disorder (ObQD). While ObQD is well understood in the semiclassical, large spin length limit, its behavior in quantum spin-1/2 systems is less clear. As exact analytical solutions are scarce for frustrated systems, numerical approaches are essential. We show that ObQD can be identified from exact diagonalization (ED) calculations through an analysis akin to the Anderson tower of states associated with spontaneous symmetry breaking. By defining an effective quantum rotor model, we describe the competition between ObQD-induced localization of the rotor and its tunneling between symmetry-related ground states, identifying the crossover lengthscale from the finite-size regime where the rotor is delocalized, to the infinite system-size limit where it becomes localized. This rotor model relates the characteristic splittings in the ED energy spectrum to the ObQD selection energy scale, providing an estimate that can be compared to spin wave calculations. We demonstrate the general applicability of this approach in one-, two- and three-dimensional frustrated spin models that exhibit ObQD.
Keywords: Order by quantum disorder; Finite-size spin-1/2 systems; Exact diagonalization; Anderson tower of states; Quantum rotor