Spintronics sounds like a Harlem Globetrotter trick, but it is actually a branch of physics in which a UCSB professor has received a nationally recognized prize and published a landmark paper.
David Awschalom, professor of physics and computer and electrical engineering, was awarded the 2005 Oliver E. Buckley prize Nov. 9 for his work in quantum mechanics and spintronics. Awschalom’s contribution to the field has centered on experimental studies that may lead to enormous increases in computing power and data storage. The prize is awarded annually by the American Physical Society to recognize significant research, as only 70 scientists share this honor.
In addition to this, Science magazine published a breakthrough paper Nov. 11 written by Awschalom, Art Gossard, Roberto Myers and Yuichiro Kato titled “Observation of the Spin Hall Effect in Semiconductors.” This research provides the first definitive proof – ending a 33-year search – for a physical phenomenon known as the spin Hall effect.
Spintronics is the field of study that is based on a quantum property called “spin.” Every electron has a spin. It is either spin-up or spin-down, but quantum mechanics allows an electron to be up some of the time and down the rest of the time. Awschalom has studied electron spin and created devices in his lab to control and measure this property. The Buckley prize was awarded in large part for this work.
“I’ve done other things, but this particular award is based on the fundamental research that led to spintronics and quantum computation in semiconductors,” Awschalom said. “I think the idea is to bring attention to new scientific discoveries that would have an impact on physics.”
One application of spintronics already being investigated at UCSB is quantum computing: the field in which subatomic properties of matter are used to store and process information. Currently, computers use electronic impulses to represent data. This system is binary, and encodes data in combinations of only ones or zeros. A one means there is an electrical charge present; a zero means there is no charge. There is no intermediate value.
In quantum computing, spin is used to represent data. Since spin can be sometimes up and sometimes down, it allows for many more possibilities than just a static one or zero. This increase in possibilities means that quantum computers have the potential for being tremendously more powerful than current conventional computers.
“I really do believe the most exciting thing about all of these developments is that it is difficult to predict where they will lead,” Awschalom said. “It offers a pathway to an enormous infrastructure of technology that can manipulate these states of information. This is one of the reasons I think this is so intriguing.”
Before quantum computers become common technology, scientists will need to control the spin of individual or small groups of electrons. Awschalom’s publication addresses this by providing proof for the spin Hall effect. This effect is named after the conventional Hall effect, which describes how flowing electrons move in a magnetic field.
In the conventional Hall effect, electrons that are moving through a conductor are pushed to one side of the conductor by an external magnetic field. This is analogous to water being pushed toward one bank of a flowing river. By placing a Hall effect sensor in a magnetic field, the strength of the field can be measured by how much the electrons are pushed. This effect is utilized in many industrial and commercial devices, such as proximity switches and electric motors.
Awschalom’s research group has found evidence that there is also a spin Hall effect. This has been theorized but not observed until very recently at UCSB.
“About 35 years ago there were theory predictions that you could have spin currents without charge currents in matter,” Awschalom said. “It’s kind of a weird concept that you could have electron spins moving, but no net charge flow.”
This means that quantum computers may be one step closer to reality. By controlling the flow of spin instead of the flow of electrical current, computers may be able to operate on less power without overheating.
“Obviously technology people were pretty interested in this because there would be a way to manipulate spins on a chip and maybe with very little heating,” Awschalom said. “It’s incredibly cool. You send electrons into a wire and spin-up goes one way and spin-down goes the other way. So the net charge current is zero, but there is spin current flowing.”
Another development in the study of spin current is Awschalom’s discovery that spin can be controlled by electric fields. In the past, magnetic fields have been used to control spin which are not as easy to produce in small areas on a microchip.
“What’s come out of these research projects at Santa Barbara is that you can also use electric fields to manipulate spins, and the advantage of that is you can do it on a chip,” Awschalom said. “That’s a very important development both in science but also for impacting technology.”
The main reason that the spin Hall effect has not been seen already is because it required extremely specialized equipment.
“[The effect] is all electrical, but we observed it optically,” Awschalom said. “The reason you cannot observe it electrically is because if you have spin-up on one side of the wire and spin-down on the other side, there is no spin-to-charge converter, so when you put two probes to measure it electrically, you can’t distinguish up and down.”
To get around this problem, Awschalom’s group used a specialized optical system to directly observe the spin of the electrons.
“The trick to observing it was developing high quality instrumentation,” Awschalom said. “We bounced light on and off the wire and if electrons are polarized when the photon hits the wire, it then becomes polarized.”
The polarized photons can be detected, showing that there was indeed a separation of spins in the wire.
“I’m pleased our group will be known for the first observation of this,” Awschalom said. “We think this is going to create an enormous theoretical activity now.”
Awschalom has had help from post-doctoral students visiting from abroad. Florian Meier and Oliver Gywat, both postdoctoral students from the University of Basel in Switzerland are visiting UCSB to help with Awschalom’s work.
“There is a strong collaboration between Basel and Santa Barbara. In Basel, there is a theory group, and the experiments are here,” Gywat said. “The theory helps you to explore the properties of these systems and to formulate expectations you would have [of them].”
Meier said that he came to UCSB about a year ago for the specific purpose to work with Awschalom.
“He is really doing outstanding research,” Meier said. “The scope of the work and of the group is very broad.”