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It has been known that noble metal surfaces bear an isotropic and nearly-free-electron-like (parabolic) energy dispersion to form an ideal two-dimensional (2D) electronic system. Since the surface states are localized in a few atomic layers on surfaces, defects such as steps and adsorbates act as a scattering center for surface electrons. Such a scattering phenomenon has been studied through standing wave observations by scanning tunneling microscopy (STM). Previous studies were focused on the intrinsic 2D surface states on the metals [1,2]. However, it would be interesting if one could easily modify the electron density of the 2D electronic system on metals, as studied in the low dimensional systems localized at semiconductor interfaces. Actually, such a modification has been studied in alloy formation or alkali metal adsorption by photoemission electron spectroscopy [3]. In this work, we performed a direct and real-space study of the electronic structure of the (111) surface of Cu-9%Al alloy, using low-temperature STM, LEED and AES. Two kinds of structures, (1~1) and (3~3)R30, appear on this surface, depending on the amount of Al segregation, which is controlled by annealing temperature [4]. We observed by differential conductance mapping that the standing waves were formed on the terraces of both surfaces. However, the magnitudes of the standing waves on these structures were weaker than that on the pristine Cu(111) surface. This is probably due to the limited mean free path of the surface electrons affected by surface alloying. Furthermore, we measured the energy dispersion of the surface states on both structures. The binding energies were observed to shift, depending on the amount of surface Al segregation. References: [1] D. Fujita et al., Phys. Rev. Lett. 78, 3904 (1997). [2] M. F. Crommie et al., Nature 524, 363 (1993). [3] T. C. Hsieh et al., Phys. Rev. Lett. 55, 2483 (1985); Surf. Sci. 166, 544 (1986). [4] H. Asonen et al., Phys. Rev. Lett. 46, 1696 (1981). |