Study of the Quantum Phase Transition in the Two-dimensional Heisenberg Antiferromagnet Cr(dien)(O$_2$)$_2$$\cdot$H$_2$O by means of Proton NMR

We have investigated Cr(dien)(O$_2$)$_2$$\cdot$H$_2$O, a Cr(IV) based compound, by means of proton solid state NMR. The compound is a two-dimensional square-lattice antiferromagnet, experiencing a phase transition at 2.55 K in zero field. We present data from spectrum and spin-lattice relaxation time measurements in the vicinity of the critical field, $H_{c}\approx$12.4 T, and in the temperature region $0.3\--1.5$ K, along with high temperature single crystal angular variation spectra. The spin-lattice relaxation time goes through a minimum as the phase boundary is crossed, emphasizing the change in the spin dynamics of Cr(dien)(O$_2$)$_2$$\cdot$H$_2$O due to fluctuations near the quantum critical point.