Revisit of the Growth of Co on Cu(111) Introduction  Cobalt thin films on Cu(111) are model systems for magnetic investigations and their structures and electronical properties are widely studied[1-6].  Bilayer triangular Co islands with different orientations are observed by room-temperature scanning tunneling spectroscope(STM)[2]. People assume islands with different orientations have faulted and unfaulted fcc structures, but it needs to be identified. Low-temperature(LT) scanning tunneling spectroscopy[3-4] of Co/Cu(111) show that the surface states of faulted and unfaulted islands are different and vary with their size[5].  We want to use LT-STM to identify structure of Co islands and find that the islands can be in either fcc or hcp structures where the fcc islands are dominant. The fcc and hcp islands can be distinguished by spectroscopy and the comparison of atomic rows of both islands and substrate. This is in contrast to previous findings where only faulted and unfaulted fcc islands are found. Nanjing National Laboratory of Microstructures and Department of Physics, Nanjing University 22 Hankou Road, Nanjing, China (210093) 1.J. de la Figuera, et al., Phys. Rev. B 47, (1993) 2.A. L. Vázquez de Parga, et al., Phys. Rev. Lett. 85, 4365 (2000) 3. L. Diekhoner, et al., Phys. Rev. Lett. 90, (2003) 4. O. Pietzsch, et al., Phys. Rev. Lett. 92, (2004) 5.M. V. Rastei, et al., Phys. Rev. Lett. 99, (2007) 6.N. N. Negulyaev, et al., Phys. Rev. B 77, (2008) edge  Co island Island A hcp Hard Sphere Model  Hard sphere model gives eight possible stacking sequences of the Co layers on Cu(111) as shown above. According to Negulyaev’s theoretical results[6], the first Co layer has {100}-microfaceted step (  step), so only ABCab- , ABCba- , ABCac-  β and ABCbc-  β (β step: {111}- microfaceted step) are possible stacking sequences.  If the first Co layer in fcc structure has  step, the distance between atomic row of topmost Co layer and nearest atomic row of substrate toward island edge equals L/3. And it’s 2L/3 if the first Co layer has β step. In hcp stacking sequence, the distance is zero. R. X. Cao, X. P. Zhang, L. Sun, B. You and H. F. Ding Experimental Results dI/dV map of Co/Cu(111) at U= -0.28V. Bilayer triangular islands with different orientations are observed. And ratio of up to down triangular islands is 30:19 which agrees with others’ results[1-6]. B A B C D STS spectrum on the bare Cu(111) surface. It proves that tip is in good condition. STS spectra on islands A-D as marked in dI/dV map. Spectrum on island A has a much larger peak intensity. Atomic resolved STM image on island D. The distance between atomic row of topmost Co layer and nearest atomic row of substrate toward island edge is about L/3, so Co bilayer has   step in fcc structure. STM image on island A. Atomic row of topmost layer and substrate are colinear and it proves that Co island has hcp stacking sequence. Conclusion In combination with low temperature atomic resolution scanning tunneling microscope image and spectroscopy measurements, we find the cobalt islands on Cu(111) can be in either fcc or hcp structures and they can be distinguished by spectroscopy and the comparison of atomic rows of both islands and substrate. Cu