Portland, Ore. - Researchers at the Weizmann Institute of Science have experimentally verified a long-postulated possibility for one-dimensional quantum devices. The team fabricated wires so small that they were able to witness electron spin separating from its charge. The observation holds out more hope for the development of spin-based circuits.
"If you were to naively scale down the width of the wires in your device, they would cease to conduct at some point, even if you were able to make them perfect, which you won't," said Ophir Auslaender. The Weizmann Institute postdoctoral researcher was the one who first observed spin separating from charge. "We were able to observe some of these effects because our experiment circumvents some of the experimental barriers," Auslaender said.
In the new breed of spintronic devices, digital logic is represented by electron spin rather than charge. In some research centers, experiments have shown that spin can flow and be modified by spintronic logic gates. Without the independent propagation of spin, spintronic devices would be limited-unable to add much functionality to the array of options available to today's circuit designers.
Researchers Auslaender and Hadar Steinberg performed the work in the laboratory of professor Amir Yacoby at the Weizmann Institute of Science's Condensed Matter Physics Department.
The team used cleaved-edge overgrowth on GaAs/AlGaAs heterostructures to craft side-by-side quantum wires. The 10-micron-long wires were only 10 nanometers in diameter and were separated from each other by an ultrathin insulating barrier of aluminum gallium arsenide. The quantum wires also operate in a ballistic mode where the electrons move down the wire under their own momentum, rather than being carried along by a voltage potential.
Quantum tunneling allows occasional jumps of electrons from quantum wire to quantum wire. The researchers were able to observe the separation of spin from charge because spin caused a domino effect that propagated at a speed that's different from the electron's own speed. The separation effect became prominent and observable as the density of electrons in the wire decreased. The system was cooled to 0.25 Kelvin to eliminate nonquantum effects.
The theory of spin states that could be separated from charge, enabling them to travel independently, has been around since the 1950s, according to Auslaender. One problem has been the difficulty of observing the subtle effect of spin decoupled from electrons in three-dimensional conductors.
The separation between spin and charge was a part of a 1950 model made by Shin-Ichiro Tomonaga, who described a hypothetical one-dimensional electron liquid. Tomonaga was a prominent Japanese theoretical physicist who won the Nobel Prize, along with Richard Feynman and Julian Schwinger, for fundamental work in electrodynamics. The model was refined by Joaquin Mazdak Luttinger in 1963, then in 1980 by F. Duncan M. Haldane, who made the mathematics more realistic, according to Auslaender. But Haldane's work was still based on the Tomonaga-Luttinger concept of a one-dimensional electron liquid.
In the experiment conducted by Yacoby, Auslaender and Steinberg, when an electron tunnels from one of the quantum wires to the other, it necessarily violates the first rule of one-dimensional quantum behavior of electrons-namely, the electron's first quantum effect is that it arranges its spin to be the opposite of its nearest neighbor. Thus, when electrons travel along quantum wires they do so in alternating spin-up/spin-down pairs.
What the researchers observed was that, because of tunneling, the newly arrived electron will clash with either the electron in front of or behind it. When it does, the clashing electron spontaneously switches its spin, thus causing a chain reaction up that side of the the wire. The domino-like chain reaction was what the researchers called the spin state separating from the charge state and propagating separately.
"We have been concentrating on the behavior of interacting electrons, said Auslaender. "When electrons are allowed to move in more than one dimension, they are able to screen out most of the interaction effects. In one dimension they cannot do this, so interactions have a very profound effect, giving rise to several interesting phenomena, of which spin-charge separation is the most spectacular," said Auslaender.
"Our devices are fabricated from GaAs/AlGaAs heterostructures. After growing a crystal with high-quality quantum wells, metallic gates are deposited across the surface of the sample and it is reintroduced into the molecular-beam epitaxy chamber. In this pristine environment, the crystal is cleaved and a second growth sequence is initiated," Auslaender explained. An electric field is created along the whole cleaved edge. The field pulls electrons from the bulk of the sample and traps them along the one-dimensional edge.
"We have been using samples with two parallel wires because they allow us to map the dispersions of the one-dimensional, many-body elementary excitations. We energize the top gates in a unique configuration that allows us to contact each individual wire. Once the experiment is set up, we perform ultrasensitive transport measurements as a function of magnetic field," he said.
Now that the researchers have successfully separated spin from charge, they want to try it in a different material using holes instead of electrons as the charge carriers. Holes-the absence of an electron-have even stronger spin interactions with neighboring charge carriers than do electrons.
The group also plans to perform the experiment in even cooler circumstances. In the current experiment, the wires are cooled to ensure that the electrons' motion is truly one-dimensional and not masked by phonon effects. Cooling the wires further "would unmask more interesting phenomena," said Auslaender.
As designers plumb the nanoscale with ever-shrinking sizes, eventually devices will become so small that quantum effects will predominate. Quantum dots, for instance, can function using a single electron at a time. Likewise, quantum wires have an aspect ratio so high that electrons must propagate through them in single file. The zero-dimensional behavior of quantum dots, and the one-dimensional behavior of quantum wires, will become the norm when design rules shrink to the limits of the nanoscale.
"It is important to understand many-body quantum effects for several practical reasons, said Auslaender. The first is that big leaps of technology frequently arise from basic research. The second is that some day devices are going to get so small that quantum effects will have to be taken into account in design."
One of the surprises of this experiment was that the highest spin separation came when the electron flow was low. High-density electron flow tended to tolerate exceptions to the strict lockstep of opposite spins, whereas low-density electron flow tended to tolerate no exceptions, thereby easily propagating spin down the wire's length separately from charge movement.
Yacoby's research group was funded by the Rosa and Emilio Segre Fund, and the Joseph H. and Belle R. Braun Center for Submicron Research.