2016/06/29

A University of Tokyo research group found anomalous behavior, which is opposite to the current understanding in solid-state physics. When a semiconductor becomes ferromagnetic by doping magnetic atoms, the order of the electron waves is suddenly restored in the semiconductor. This cannot be explained by the conventional solid-state physics theory. This research may indicate a route to developing high-speed quantum spintronics devices.

In semiconductors and many other solid-state materials, an electrical current is carried by mobile electrons or holes (that are, essentially positively charged “holes” in filled electron seas). Such mobile electrons and holes are called carriers and behave as waves, so called wavefunctions. In order to improve the performance of semiconductor devices, it is highly important to suppress the disorder (or to enhance the order, so called coherence) of the propagation waves (wavefunctions) of electrons and holes. The coherence of the wavefunction is often expressed by the term “carrier mobility”. It has been a major issue to increase the carrier mobility to realize high-performance semiconductor devices. Doping impurity atoms in semiconductors is a commonly-used and indispensable technique to create mobile electron or hole carriers, and thus to reduce the resistance and flow an electrical current in semiconductors. However, as the impurity concentration increases, the wavefunction of electrons and holes is more strongly disturbed, the coherence and mobility decrease, and then the device performance degrades. This has been a fundamental problem based on the understanding in conventional solid-state physics and semiconductor physics, for a long time.

The research group of Project Researcher Iriya Muneta, Associate Professor Shinobu Ohya and Professor Masaaki Tanaka at the University of Tokyo Graduate School of Engineering doped magnetic impurity manganese (Mn) atoms in a semiconductor gallium arsenide (GaAs). Using a unique technique, they systematically investigated how strongly the wavefunction of holes is disturbed in the Mn-doped GaAs samples with various Mn concentrations. When the Mn concentration is less than 0.9%, the disturbance of the wavefunction becomes stronger and the coherence of the holes disappears as the Mn concentration increases, as is expected. However, when the Mn concentration exceeds 0.9% and the semiconductor becomes ferromagnetic, the disturbance of the wavefunction is suddenly suppressed and the coherence of holes is enhanced. This is the behavior opposite to the prediction of conventional solid-state physics that the coherence decreases as the impurity concentration increases.

The origin of this phenomenon is not fully understood as yet. However, it is thought to be induced by the alignment of the direction of spins in Mn-doped GaAs. In the future, this finding is expected to lead to high-speed quantum spintronics devices utilizing the long coherence of electrons and holes in semiconductors.

Restoring the disorder of the wavefunction of electrons (holes) by doping magnetic impurities
When the Mn concentration is less than 0.9%, the wavefunction of holes, which carry an electrical current, is disturbed by the Mn atoms (top). As the Mn content increases, the disturbance of the wavefunction of holes becomes stronger (middle). However, when the Mn concentration becomes more than 0.9%, the disturbance is strongly suppressed and the coherence is enhanced (bottom). This is the behavior opposite to the understanding in the conventional solid state physics. © 2016 Tanaka-Ohya Laboratory

“In the present research, we found that by doping magnetic impurities in a semiconductor, the scattering of carriers (holes) are dramatically suppressed and the order of holes (so-called coherence) is suddenly increased at the onset of ferromagnetism, that is a totally unexpected discovery,” says Tanaka. He continues, “This is because the order of the valence band, where holes are running, is restored when the semiconductor becomes ferromagnetic. Despite the long history of semiconductor physics research, this phenomenon cannot be understood by conventional theories, indicating there are still unexplored phenomena. Although practical applications remain to be realized, this result may lead to applications to quantum spintronics devices using ferromagnetic semiconductors.”

Muneta, who has carried out most of the experiments, said “I have been interested in spintronics and studying ferromagnetic semiconductors since I was a bachelor and graduate student, but this is the first time that I have found such an intriguing and unexpected phenomenon.” He continues, “Because it contradicts common sense, at first I felt that the experiment was a failure. But, at the same time, I sensed that there may be something meaningful because this phenomenon occurs at the onset of the ferromagnetic transition. Then, I prepared many samples and carried out many experiments very carefully, so it took a few years to complete this research and to finish writing a paper. Now I am confident in the results and am pleased to publish.”

Paper

Iriya Muneta, Shinobu Ohya, Hiroshi Terada, and Masaaki Tanaka, "Sudden restoration of the band ordering associated with the ferromagnetic phase transition in a semiconductor", Nature Communications: 2016/06/28 (Japan time), doi: 10.1038/ncomms12013.
Article link (Publication, UTokyo Repository)