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Event Number 2877
Thursday, March 28th, 2013
 R. G. Herb Condensed Matter Seminar
 Control and possible applications of valley degree of freedom: Valleytronics?
 Time: 10:00 am
 Place: 5310 Chamberlin
 Speaker: Belita Koiller, Federal University of Rio de Janeiro
 Abstract: The conduction electrons in Si are not in a welldefined single Bloch state. Instead, the Si conduction band is sixfold degenerate, with minima (valleys) along the x, y and z crystallographic directions. This imposes limitations to the spin manipulation and coherence. It was recently proposed to encode quantum information directly into the valley degree of freedom, converting the spurious valley Hilbert subspace into a useful ingredient for a quantum computer. In this talk, valley degrees of freedom in Si are addressed in 3 different contexts.
1) Based on an atomistic pseudopotential theory, we demonstrate that ordered GeSi layered barriers confining a Si slab can be optimized to enhance the VS in the active Si region by up to one order of magnitude compared to the random alloy barriers adopted so far. We identify Ge/Si layer sequences leading to a VS as large as ~9 meV. The splitting is "protected" even if some mixing occurs at the interfaces.
2) Interface states form spontaneously at some semiconductorbarrier interfaces and they may improve or hinder the electron control and coherence for semiconductorbased qubits. From a simple 1D Tightbinding model, new insights emerge regarding the interface state's energy, as well as the exponential longer (shorter) localization lengths into the Si (barrier) material. The interface state may be probed experimentally by an external electric field, which modulates the capacitance of the system and the lowest level spacing (valley splitting).
3) We analyze the valley composition of one electron bound to a shallow donor close to a Si/barrier interface. A full sixvalley effective mass model Hamiltonian is adopted. For low fields, the electron ground state is essentially confined at the donor. At high fields the ground state is such that the electron is drawn to the interface, leaving the donor practically ionized. Valley splitting at the interface occurs due to the valleyorbit coupling, taken here as a complex parameter. A sequence of two anticrossings takes place and the complex phase affects the symmetries of the eigenstates and level anticrossing gaps.
References:
1) L. Zhang, JWi Luo, A. L. Saraiva, B. Koiller, A. Zunger, arXiv:1303.4932.
2) A. L. Saraiva, B.Koiller, M. Friesen, Phys. Rev. B 82, 245314 (2010).
3) A. Baena, A. L. Saraiva, B.Koiller, M. J. Calderón Phys. Rev. B 86, 035317 (2012).  Host: Coppersmith
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