Abstract-Light-induced electron localization in a quantum Hall system

• T. Arikawa,
• K. Hyodo,
• Y. Kadoya
• & K. Tanaka

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4078.html

An insulating bulk state is a prerequisite for the protection of topological edge states¹. In quantum Hall systems, the thermal excitation of delocalized electrons is the main route to breaking bulk insulation². In equilibrium, the only way to achieve a clear bulk gap is to use a high-quality crystal under high magnetic field at low temperature. However, bulk conduction could also be suppressed in a system driven out of equilibrium such that localized states in the Landau levels are selectively occupied. Here we report a transient suppression of bulk conduction induced by terahertz wave excitation between the Landau levels in a GaAs quantum Hall system. Strikingly, the Hall resistivity almost reaches the quantized value at a temperature where the exact quantization is normally disrupted by thermal fluctuations. The electron localization is realized by the long-range potential fluctuations, which are a unique and inherent feature of quantum Hall systems. Our results demonstrate a new means of effecting dynamical control of topology by manipulating bulk conduction using light.

Terahertz pulse increases electron density 1,000-fold

http://www.eurekalert.org/pub_releases/2011-12/ific-tpi121811.php

Findings point to advances in transistor and solar cell development

		IMAGE: A picosecond terahertz pulse causes an avalanche of excitons to burst forth from semiconductor GaAs. Click here for more information.

Kyoto, Japan -- Researchers at Kyoto University have announced a breakthrough with broad implications for semiconductor-based devices. The findings, announced in the December 20 issue of the journal Nature Communications, may lead to the development of ultra-high-speed transistors and high-efficiency photovoltaic cells.
Working with standard semiconductor material (gallium arsenide, GaAs), the team observed that exposing the sample to a terahertz (1,000 gigahertz) range electric field pulse caused an avalanche of electron-hole pairs (excitons) to burst forth. This single-cycle pulse, lasting merely a picosecond (10^-12 s), resulted in a 1,000-fold increase in exciton density compared with the initial state of the sample.
"The terahertz pulse exposes the sample to an intense 1 MV/cm^2 electric field," explains Hideki Hirori, team leader and Assistant Professor at Kyoto University's Institute for Integrated Cell-Material Sciences (iCeMS). "The resulting exciton avalanche can be confirmed by a bright, near-infrared luminescence, demonstrating a three-order of magnitude increase in the number of carriers."
Research in Kyoto using terahertz waves is led by Professor Koichiro Tanaka, whose lab at the iCeMS pursues numerous applications including the development of new biological imaging technologies.
"Since terahertz waves are sensitive to water, our goal is to create a microscope that will allow us to look inside living cells in real time," says Prof. Tanaka. "These just-released results using semiconductors are an entirely different field of science, but they demonstrate the rich potential that lies in the study of terahertz waves."

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The article, "Extraordinary carrier multiplication gated by a picosecond electric field pulse" by H. Hirori, K. Shinokita, M. Shirai, S. Tani, Y. Kadoya, and K. Tanaka was published online in the December 20, 2011 issue ofNature Communications.
Acknowledgements: This work was supported by Grant-in-Aid for Young Scientists (B) (Grant No. 21760038) of the Japan Society for the Promotion of Science, and also Grant-in-Aid for Scientific Research on Innovative Area "Optical science of dynamically correlated electrons (DYCE)" (Grant No. 20104007) and Grant-in-Aid for Creative Scientific Research (Grant No. 18GS0208) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
About the iCeMSThe Institute for Integrated Cell-Material Sciences (iCeMS) at Kyoto University in Japan aims to advance the integration of cell and material sciences -- both traditionally strong fields for the university -- in a uniquely innovative global research environment. The iCeMS combines the biosciences, chemistry, materials science, and physics to capture the potential power of stem cells (e.g., ES/iPS cells) and of mesoscopic sciences (e.g., porous coordination polymers). Such developments hold the promise of significant advances in medicine, pharmaceutical studies, the environment, and industry.