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Spin Density Functional Theory Simulations of Quantum Point Contacts: An Investigation of Spin Filtering Effects

R. Akis and D.K. Ferry
Arizona State University, US

Keywords:
quantum wire, quantum point contact, spin, spintronics

Abstract:
We present simulations of quantum wires and quantum point contacts (QPCs) formed in semiconductor heterostructures. Conductance measurements for such structures is quantized with plateaus at integer multiples of G_0= (2e^2/h) as function of gate voltage, a result readily explained by a single electron quantum mechanical theory. More recent experiments however have found additional non-integer plateaus in QPCs and wires, in particular a ~0.7 G_0 conductance anomaly. These require electron-electron interactions to be included to account for them theoretically. Applying spin-density-functional theory (SDFT) to these systems, we obtain ~0.7 G_0 anomalies similar to experiment and find that these features can be correlated with the formation of spin-dependent energy barrier structure which can allow two modes of spin-down electrons to be almost fully transmitted through the channel before spin-up electrons start being allowed through. These barriers, which are largely the result of the exchange potential, rise and fall as function of the local density and one can exploit this to tune the spin-filtering effects. We can also account for additional features such as anomalies at ~0.25 G_0 and “missing” plateaus at higher conductances, features which have also been recently observed.

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