| In quantum spin liquid (QSL), zero-point fluctuations prevent a transition to an ordered state down to the lowest temperatures. There have now been observations of such states in some two-dimensional organic Mott insulators with triangular lattice, yet the ground state of QSLs remains elusive. Here we report very high sensitivity magnetic torque measurements of EtMe3Sb[Pd(dmit)2]2 and its deuterated compound, both of which are likely to have a QSL ground state. In both compounds the magnetic susceptibility remains finite down to low temperatures (30 mK) and magnetization increases nearly linearly up to high field (35 T). These provide direct evidence of the prevailing gapless magnetic excitations, revealing that a ground state of this system is an algebraic spin liquid, where there is no spin-gap and spin-spin correlations decay as a power law, rather than a topological liquid with a spin-gap and a topological order. Moreover, the fact that similar behavior is observed in both compounds with different degree of frustration implies the presence of an extended quantum critical phase where the algebraic liquid state is stabilized. These findings of interacting critical phase in two-dimensional quantum magnets point towards important implications for the presence of a novel quantum state near the Mott transition. |
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