An artificial atom^{1} can be characterized by the
discrete quantum states (QS) of the electrons confined by its
boundaries. The number of occupied QS and their energy
spectrum depends on the size of the artificial atom. With
the advanced nano fabrication technology, it is possible to
tailor the size of an artificial atom so that the number of
occupied QS can be adjusted by unity, hence create a periodic
table of "artificial elements", in resemblance to
Mendeleev's Periodic Table of The Elements.

In this paper, we will demonstrate the validity of this
concept with a recent experiment^{2} performed with
a low temperature scanning tunneling microscope housed together
with a UHV fabrication chamber. We show that an array of
artificial atoms with incremental QS can be realized through the
fabrication of a quantum wedge, i.e. a nanoscale wedge
whose thickness changes monotonically by discrete atomic
planes. Each atomic layer increase in the
thickness adds a new QS into the system as a result of the
quantum confinement. Thus a quantum wedge is an assembly of
artificial atoms with incremental sizes, or "artificial
elements".

Our quantum wedge is fabricated by the epitaxial growth of Pb
on a stepped surface of Si(111). Tunneling spectroscopy
reveals that each slab of equal thickness is associated with a
set of QS, and the shift of the energy level of the highest
occupied QS (HOQS) between two neighboring slabs is nearly one
half of a energy quantization step. Thus slabs with the
even number layers have their HOQS aligned at one level
while those of odd number layers aligned at another. This
two fold repetition of the HOQS gives rise to a binary electron
interference fringes when imaged in a conventional constant
current mode, showing for the first time a quantum mechanical
analog of the classical Fizeau fringes on an optical wedge.

References
[1] M. Kastner, Physics Today, 46(1), 24 (1993)
[2] I. B. Altfeder, K. A. Matveev, and D. M. Chen, Phys. Rev.
Lett. 78, 2815 (1997).

*Corresponding Address:
Dongmin Chen, The Rowland Institute for Science,100 Edwin H. Land
Blvd. Cambridge, MA 02142, ph: (617)-497-4620, fax:
(617)-497-4627, email: chen@risvax.rowland.org