Finite Spinon Density-of-States in Triangular-Lattice Delafossite TlYbSe₂

We introduce the rare-earth delafossite compound TlYbSe₂ - extending the search for quantum spin liquids in frustrated triangular lattice magnets. While the DC magnetisation suggests magnetic exchange interactions in the order of several Kelvin, the zero-field AC magnetisation and heat capacity measurements reveal no signs of long-range magnetic order down to 20 mK, indicating a quantum-disordered ground state. We observe a spin glass transition around ∼ 30 mK at zero field, arguably originating from a small fraction of free spins–with an associated entropy of < 3% of the total R ln 2, which is suppressed by an applied field of ∼ 0.02 T. A broad anomaly in the heat capacity measurements between 2 − 5 K is indicative of short-range spin correlations. Below 350 mK, we observe a robust linear temperature dependence of the heat capacity, accompanied by the complete absence of long-range order at low fields. We propose that a phenomenological theory, based on the interplay between spinons and thermally excited gauge flux excitations, can account for the linear temperature dependence of the heat capacity, and could be widely applicable to similar critical quantum spin liquid candidate materials. The results establish the low-temperature, low-field regime of TlYbSe₂ as a prime candidate for field-tunable triangular quantum spin liquid behavior and highlight the importance of thermally excited gauge field excitations.