Possible coexistence of short-range resonating valence bond and long-range stripe correlations in the spatially anisotropic triangular-lattice quantum magnet Cu2 (OH) 3NO3

Abstract

We show that short-range resonating valence bond correlations and long-range order can coexist in the ground state (GS) of a frustrated spin system. Our study comprises a comprehensive investigation of the quantum magnetism on the structurally disorder-free single crystal of ${\mathrm{Cu}}_{2}{(\mathrm{OH})}_{3}{\mathrm{NO}}_{3}$, which realizes the $s$ = 1/2 Heisenberg model on a spatially anisotropic triangular lattice. Competing exchange interactions determined by fitting the magnetization measured up to 55 T give rise to an exotic GS wave function with the coexistence of the dominant short-range resonating valence bond correlations and weak long-range stripe order (ordered moment ${M}_{0}=|\ensuremath{\langle}{s}_{i}^{z}\ensuremath{\rangle}|\ensuremath{\sim}0.02$). At low temperatures, a first-order spin-flop transition is visible at $\ensuremath{\sim}$1--3 T. As the applied field further increases, another two magnetic field induced quantum phase transitions are observed at $\ensuremath{\sim}$14--19 and $\ensuremath{\sim}$46--52 T, respectively. Simulations of the Heisenberg exchange model show semiquantitative agreement with the magnetic-field modulation of these unconventional phases, as well as the absence of visible magnetic reflections in neutron diffraction, thus supporting the GS of the spin system of ${\mathrm{Cu}}_{2}{(\mathrm{OH})}_{3}{\mathrm{NO}}_{3}$ may be approximate to a quantum spin liquid. Our study establishes structurally disorder-free magnetic materials with spatially anisotropic exchange interactions as a possible arena for spin liquids.