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Quantum Dots and Spin Pumps

Spin pumped quantum dotIt is not so long ago, that the first thing that sprang to mind when one read the phrase ‘quantum dot’ was the idea of some rather esoteric and complicated aspect of avant garde physics. This is still partly true, there is some rather complex experimental work underway underpinned by even more complex theoretical work investigating the bizarre properties of tiny devices that can trap a single electron in zero-dimensions.

Practical applications of quantum dots have emerged recently in sensor science but US and Brazilian researchers hope to exploit them in a new kind of electronics, known as spintronics where electron charge and quantum spin add an extra dimension to electronic operations and computation. Spin currents might also be used to allow quantum communications take place “in-chip” in devices so small that light propagation is not practical. Such developments will open up quantum dots that can increase processing speed, storage capacity, and functionality of conventional electronics, communication, and computations and technologies.

Eduardo Mucciolo of the Department of Physics at the University of Central Florida, Orlando and Caio Lewenkopf of the Department of Theoretical Physics at State University of Rio de Janeiro, Brazil, are investigating lateral semiconductor quantum dots. They believe that such devices could be used as pumps to produce spin polarised currents, by exploring quantum phase coherence phenomena. The effect, called pure spin pumping, is analogous to charging a battery in conventional electronics. Such a spin pump might provide the much-needed circuit element for spin-based electronics.

Writing in the International Journal of Nanotechnology (2007, 4, 482-495), Mucciolo and Lewenkopf describe a lateral semiconductor quantum dot. In these systems, electrons within a two-dimensional gas are trapped within small puddles by the application of a voltage; applied voltages control the shape and size of these puddles. Electrodes can be used to vary the width of the point contacts between the electron puddle and the 2D gas. Controlling these point contacts allows quantum dots to be “opened” and “closed”.

Controlling these point contacts allows them to “open” and “close” the quantum dots. This effect dates back to the early 1990s, points out Mucciolo. “Closing and opening the propagation through a constriction, the point contact, can be used to detect spin-polarized currents,” he explains, “This is how Susan Watson and colleagues at Middlebury College managed to see spin currents coming out of their quantum dot pump in 2003.”

“Recently, our spin pump proposal passed its first experimental test,” say the researchers, who now hope that other teams will take up the challenge and investigate the potential of spin pump quantum dots.

“The main idea behind the spin pumping mechanism was actually published for the first time in Physical Review Letters in a paper I co-authored with Claudio Chamon (Boston University) and Charles Marcus (Harvard University),” adds Mucciolo. The main development since that earlier work presented in the current paper with Lewenkopf is that now they have carried out a much more detailed analysis to demonstrate the precise details, this was entirely missing from the PRL paper, Mucciolo told us. “In the J Nanotech paper we also develop a general formalism that could serve as a basis for the theoretical investigation of several aspects of spin pumps which, albeit important, have not yet been considered in the literature,” Mucciolo adds.

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