Below a critical filling factor (v c =0.2&) and critical temperature (Tc^XA K at 26 T) an additional line has been observed in the luminescence spectrum of a GaAs-Al x Gaix As heterojunction; it grows in intensity with decreasing v, dominating the spectrum at v < 17 • Its appearance is accompanied by a strong reduction in overall integrated intensity, while its relative intensity decreases sharply at v = K j, and f. The lack of correlation between v c and the disorder-related properties of the system indicates the intrinsic nature of the line which we propose signals the formation of a pinned Wigner solid.

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