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Research On Going Projects High resolution scanning tunneling spectroscopy of quantized spectrum: evolution from quantum well to quantum wire to quantum dot. Bistable transport properties in STM - passivated semiconductor tunnel junctions. Coherent transport measurements with dual probe STM. Imaging spatial distribution of quantum liquids in a high magnetic fields. Recent Discoveries Making Fizeau fringes with a quantum wedge and internal electron waves. We have introduced a new and fascinating quantum object which we named "quantum wedge". A quantum wedge is a nano-scale wedge with its thickness varying monotonically by discrete atomic planes. In a quantum wedge, electron waves are strongly quantized normal to its surface, and electrons occupy a set of two-dimensional energy bands. An increase of the thickness by each atomic plane introduces new sub-bands in that particular slab, causing the electron energy spectrum to shift, and the energy separation to narrow. Thus, a quantum wedge can be viewed as an assembly of artificial atoms across a "periodic table". It offers us a rather unique new structure for studying quantum confinement at atomic precision. To realize such a geometry experimentally, we use epitaxial growth of Pb on a stepped Si substrate. The need for this hetero-epitaxial system to reduce its internal strain and its surface energy drives the Pb to form a flat-topped island; thus the desired wedge geometry is obtained. One of the most striking results on these Pb wedges is the observation of the electron interference fringes spontaneously formed on the surface of the wedge. This experiment demonstrates a quantum mechanical analog of the classical optical Fizeau fringe experiment. For a detailed description see Altfeder, I.B.; Matveev, K.A.; Chen, D.M. "Electron fringes on a quantum wedge", Phys. Rev. Lett. 78, 2815-2818 (1997). The Princess and the Pea: Imaging buried interfacial lattices with quantized electrons. We demonstrate that the well-known Si(111)-(7 X 7) superlattice buried under as much as 100 Å of crystalline Pb can be directly imaged with a scanning tunneling microscope at 77 K. The unexpected transparency of the metal and the high lateral resolution are the result of a nondiffractive scattering of the electrons at the interface. We attribute this phenomenon to a large anisotropy of the effective masses associated with the free in-plane and the quantized transverse motion of the electrons. For a detailed description see Altfeder, I.B.; Chen, D.M.; Matveev, K.A. "Imaging Buried Interfacial Lattices with Quantized Electrons" Phys. Rev. Lett. 80, 4895-4898 (1998). See also "Dowsing for Silicon" in Physical Review Focus 1, story 17, (1998) and Science News, 153, 358 (1998). Pinhole formation in a solid phase epitaxial film of CoSi2. The long-standing pinhole problem in solid phase epitaxial growth of a CoSi2 film on Si(111) has been revisited with in situ scanning tunneling microscopy. While the as-deposited film with 5 Å of CO at room temperature shows a smooth granular texture with original substrate terraces remaining intact, annealing at 580o C produces an epitaxail CoSi2 film with large pinholes enclosed by a thin ring of CoSi2, exhibiting a volcano feature. Quantitative analysis shows that the formation of pinholes is a result of rapid Si outward diffusion from bulk to surface, and of the subsequent Si reaction with Co on the outer surface. Evidence suggests that inhibiting the Si diffusion channels during the thermal annealing process is the key to solving the pinhole problem. For a detailed description see Ruan, L.; Chen, D.M. "Pinhole formation in a solid phase epitaxial film of CoSi2 on Si(111)" Appl. Phys. Lett. 72, 3464-3466 (1998) and the online poster "Pinhole formation in a solid phase epitaxial film of CoSi2 on Si(111)". Bistability in Scanning Tunneling Spectroscopy of Ga-Terminate Si (111) Bistable electron transport, a phenomenon usualyl associate with double-barrier sturctures has been observed with a conventional STM junction formed between a metal tip and a Ga-terminate Si(111) surface at 77K. Large hysteresis loops appear in teh current-voltage characteristics when electrons are injected from the tip to the surgace. The turn-on bias varies from -3.1 to -4.0 V and shows an inverse dependence on the tip-sample distance, indicating a stong field effect. The turn-off bisas, however, is essentially pinned at a conductance threshold of -2.7V. For information about previous discoveries, see the Past Findings section.