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We have studied Single donor transistors with designed nanochannels in which the tunnel barrier height is enhanced due to dielectric confinement. For single electron tunneling effects the charging energy is significantly large. Which is a first limiting factor for shallow dopants? At high temperatures, thermally activated transport became quickly dominant. We have found that at high temperature operation of electron tunneling occurred via a single donor. We have found that the single dopant devices operating at room temperature paving the way for practical applications based on individual dopant atom in silicon nanodevices. We have also found that in nanopatterned channel transistors, single electron tunneling via individual positive donors was observed at elevated temperatures. The obtained results were found in good agreement with previously obtained results. [ABSTRACT FROM AUTHOR]

We have studied and found coupled cluster wave functions as important functions for a projection quantum Monte Carlo algorithm within the configuration interaction scheme. The ground state wave function of a configuration interaction Hamiltonian is filtered out by propagating the amplitudes of an initial arbitrary wave function via a random walk in the many body Hilbert space spanned by a basis of Slater determinants. The coupled cluster wave functions are used to guide this random walk via importance sampling in order to circumvent the sign problem. This approach has provided upper bounds to the ground state energy whose tightness can be systematically improved by including higher order excitations in the coupled cluster wave function. We have applied three dimensional homogenous electron gas in momentum space. The electron gas was studied for large single particle basis sizes. We found that coupled cluster wave functions are very accurate and good approximation for ground state wave function for many physical systems. The obtained results were in good agreement with previously obtained results. [ABSTRACT FROM AUTHOR]