Gapless quantum spin liquid behavior in spin-1/2 triangular-lattice antiferromagnet Ba$_3$CuNb$_2$O$_9$ with in-plane anisotropic exchange interactions

Quantum spin liquids (QSL) are prototypical examples of ground states with massive many-body entanglement. While an unambiguous realization of this long-sought-after state remains elusive, a growing number of materials candidates keep emerging with varieties of novel and exotic properties. Here, we report a combined experimental and theoretical study of a new antiferromagnet Ba$_3$CuNb$_2$O$_9$ which has an in-plane anisotropic exchange interaction arising from distorted triangular arrangements of magnetic Cu$^{2+}$ (spin-1/2) ions. We show an unconventional gapless quantum spin liquid (QSL) like ground state in this material which does not exhibit any signature of magnetic ordering down to 0.3 K, while a finite Curie-Weiss temperature $\sim$ - 41 K is evident. The gapless QSL state is supported by a high spin fluctuation, a power-law dependence of magnetic specific heat ($C_m$ $\propto$ $T^{\gamma}$) and a unique scaling of magnetic susceptibility and specific heat with temperature and magnetic field respectively. Our ab-initio simulation also confirms the anisotropic exchange interaction which can only be explained via superexchange mechanism. While the combined results strongly suggest a random singlet state driven QSL behavior in present material, we strongly believe that the anisotropic exchange interaction has a possible role for this exotic ground state behavior.