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 T N ≈ 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 μ 0 H c ≈ 3 T , evidenced by a finite residual linear term in thermal conductivity, a Pauli-like susceptibility, and a temperature-independent nuclear magnetic resonance Knight shift. Approaching H c 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.