Fractional Quantum Hall Effect

Discovery (Tsui, Stormer & Gossard, 1982) that in a very-high-purity 2D electron gas at strong magnetic field and low temperature, the Hall conductance is quantized in exact fractional (not just integer) multiples of e^2/h -- an unexpected result implying collective, strongly-correlated electron behavior rather than the single-particle physics behind the 1980 integer effect. Tsui and Stormer shared the 1998 Nobel Prize with Laughlin.