-2 -1 0 1 2 x (mm) 1 0 -1 1 0 -1 1 0 -1 1 0 -1 (d) d = 31.9 mm, $^{2}$ = 5.0 (c) d = 25.5 mm, $^{2}$ = 8.9 (b) d = 23.2 mm, $^{2}$ = 11.0 (a) d = 20.8 mm, $^{2}$ = 13.6 y (mm) y (mm) y (mm) y (mm) Figure 2: Measured particle configurations vs increasing trap width d , or equivalently, decreasing well anisotropy α $^{2}$. (a) Straight line configuration, (b) and (c) zigzag configurations and (d) elliptical configuration. Values of the anisotropy parameter α 2 were determined from measured c.m. frequencies. Here x$_{rms}$ represents the longitudinal cluster size (i.e., its length), while y$_{rms}$ is the trans- verse cluster width. In particular, y$_{rms}$ can be used as an order parameter for the zigzag transition since y$_{rms}$ = 0 in the 1D straight line configuration and y$_{rms}$ > 0 for the 2D zigzag configuration [16]. The measured c.m. frequencies ω$_{x}$ and ω$_{y}$ are plotted vs d in Fig. 3(a). Here ω$_{x}$ (the frequency associated with the fixed 50.8 mm separation) shows a weak linear increase with 6