Low-Frequency Spin Dynamics in a Canted Antiferromagnet

Resistively detected nuclear spin relaxation measurements in closely separated two-dimensional electron systems reveal strong low-frequency electron-spin fluctuations in the quantum Hall regime. As the temperature is decreased, the spin fluctuations, manifested by a sharp enhancement of the nuclear spin-lattice relaxation rate$1/T_1$, continue to grow down to the lowest temperature of 66 millikelvin. The observed divergent behavior of$1/T_1$signals a gapless spin excitation mode and is a hallmark of canted antiferromagnetic order. Our data demonstrate the realization of a twodimensional system with planar broken symmetry, in which fluctuations do not freeze out when approaching the zero temperature limit.