A continuous confinement-deconfinement transition in a triangular quantum magnet.

A continuous transition between phases hosting distinct excitations-bosonic magnons versus fermionic spinons-is a long-sought phenomenon in quantum magnetism, analogous to the confinement-deconfinement transition in quantum chromodynamics. We report evidence for such a transition in the triangular-lattice antiferromagnet thallium ytterbium disulfide. Antiferromagnetic order develops below TN ≈ 0.53 kelvin. A c-axis field suppresses this order, driving the system into a gapless quantum spin liquid with a spinon Fermi surface above [Formula: see text], evidenced by a finite residual linear term in thermal conductivity, a Pauli-like susceptibility, and a temperature-independent nuclear magnetic resonance Knight shift. Approaching [Formula: see text] from above, the scattering rate of itinerant excitations is strongly enhanced, while their density of states shows no critical enhancement, atypical of conventional magnetic quantum criticality. These results point to a continuous confinement-deconfinement transition governed by fractionalized excitations beyond the Ginzburg-Landau paradigm.