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THU: Thursday Contributed Sessions

THU 8: Frustrated Quantum Systems: Contributed Session to Symposium

THU 8.6: Talk

Thursday, September 11, 2025, 15:30–15:45, ZHG009

Diamond-decorated quantum antiferromagnets in two dimensions — •Andreas Honecker¹, Katarína Karl’ová¹, Malo Rouxel¹, Jozef Strečka², Taras Verkholyak³, Stefan Wessel⁴, and Nils Çaçi⁵ — ¹Laboratoire de Physique Théorique et Modélisation, CNRS, CY Cergy Paris Université, France — ²Department of Theoretical Physics and Astrophysics, P.J. Šafárik University, Košice, Slovakia — ³Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine, L’viv — ⁴Institute for Theoretical Solid State Physics, RWTH Aachen University, Germany — ⁵Laboratoire Kastler Brossel, Collège de France, France

The spin-1/2 Heisenberg antiferromagnet on the diamond-decorated square and honeycomb lattices is a highly frustrated quantum spin system that in the presence of a magnetic field displays a rich phase diagram, including the Lieb-Mattis ferrimagnetic, dimer-tetramer, monomer-dimer, and spin-canted phases, in addition to the fully saturated state. We investigate the thermodynamic properties of this model using exact diagonalization, an effective monomer-dimer description, and sign-problem-free quantum Monte Carlo simulations. In the parameter region favoring the dimer-tetramer phase, the ground-state problem can be represented by a classical hard-dimer model and retains a macroscopic degeneracy even under a magnetic field. We detect an enhanced magnetocaloric effect on the square lattice. The ground-state degeneracy in the zero-field dimer-tetramer phase can be lifted by a small distortion. In a particular case on the honeycomb lattice, this gives rise to a Kastelyn-type phase transition.

Keywords: Frustrated magnetism; Ground-state degeneracy; Exact diagonalization; Quantum Monte Carlo; Monomer-dimer model