Spin City

Researchers are investigating a new kind of microelectronics called spintronics. These devices will rely on the spindependent behavior of electrons in addition to (or even instead of) conventional charge-based circuitry. Researchers in physics and engineering anticipate that these devices will process data faster, use less power than today's conventional semiconductor devices, and work well in nanotechnologies, where quantum effects predominate. Spin-FETs (field effect transistors), spin-LEDs (light-emitting diodes), spin-RTDs (resonant tunneling devices), terahertz optical switches, and quantum computers are some of the multifunctional spintronic devices being envisioned. The realization of this vision, however, is predicated on a much deeper understanding of fundamental spin interactions in semiconductors.

That's where researchers in Fellow Steve Cundiff's lab come in. Postdoc Sam Carter and graduate student Zhigang Chen are collaborating on laser studies of two electron spin properties that are essential for the development of spin-based electronics: (1) spin transport and (2) spin coherence. Spin transport depends on spin mobility and spin diffusion. Spin mobility is a measure of how fast a spin packet moves in response to an electric field, and spin diffusion describes how rapidly the spin packet spreads out. The coherence time is how long the phase of spin-synchronized electrons remains predictable or, in other words, how long such electrons can carry information. The group studies spin properties in quantum wells in semiconductors because confinement in the wells increases the strength of the light-electron interaction and allows the electrons to be isolated.

In the process of probing spin transport and coherence time, the researchers are also gaining new insights into optical spin control. Although the Cundiff group currently has no plans to actually build a spintronic device, it seeks a better understanding of the fundamental physics that will make such devices possible.

Transport

Sam Carter's studies of spin transport in semiconductor quantum wells have shown that the optical pulses he uses to manipulate and probe electron spin also affect the transport of spin packets. The figure above displays the optical excitation of a spin packet and how the spins move and spread out in an electric field. Carter believes that, with further study, it may even be possible to use lasers to control spin transport. Optical control could well play a role in the design of spin-based quantum computers. However, other spintronic devices, such as the spin transistor, will likely not rely on lasers.

Carter recently performed systematic studies of spin diffusion using transient spin gratings. In this technique, two laser pulses interfere on the semiconductor to generate a spin grating - alternating regions of spin up and spin down electrons, separated by a few microns. Spin diffusion causes this grating to wash out, so measuring how long the grating lasts determines how fast diffusion occurs.

Carter has shown that increasing the power of the lasers used to excite the electrons increases spin diffusion. Stronger laser excitation can free electrons from their local environment, allowing them to move more freely about the quantum well. Raising the temperature of the semiconductor had a similar effect. The difference between diffusion of electron spins was also compared to diffusion of bound electron-hole pairs called excitons, which diffuse much more rapidly.

The Cundiff group's research is helping physicists better understand spin transport experiments and aiding in the development of a better theory to explain them.

Law &Order

Zhigang Chen investigates how disorder in quantum wells affects spin coherence. His goal is figuring out what parameters influence spin coherence and why. He's discovered that weak electron localization (due to disorder) plays a key role in lengthening spin coherence times. One form of disorder is defects in the interface between two different materials that constitute a semiconductor quantum well. These defects lead to fluctuations in the quantum confinement energy, which trap electrons and prevent them from moving around freely in the quantum well. The figure on the right shows localized and delocalized electrons in a quantum well.

When an electron is delocalized and moving freely in the quantum well, it rapidly loses its spin coherence because of decreased momentum scattering. A strongly localized electron also loses coherence, but for different reasons. For example, the influence of stronger nuclear interactions will relax spin coherence in strongly localized electrons. Also, when electrons are localized at different defect sites, these different environments will cause a relaxation of the ensemble spin.

The longest spin coherence times occur when electrons are weakly localized, i.e., near the crossover point between the localized and delocalized states. Spintronic device designers will likely take these findings into account as they look for a compromise between increasing the spin coherence time and improving transport properties.

In this research, Chen and his colleagues optically adjusted the environment inside a quantum well to favor either localization or delocalization. By using a specially designed sample, they were able to continuously vary the electron density in particular quantum wells. Chen also worked out a new technique for characterizing the localization of electrons. These discoveries will guide future device design.   - Julie Phillips