A research group has succeeded in enhancement of spin lifetime one order longer by electrical control of a spin-orbit interaction (SOI), and micro-fabrication of narrow wires of semiconductor two-dimensional electron gas (2DEG). The group consists of Professor Junsaku Nitta, Assistant Professor Makoto Kohda, Yoji Kunihashi (a doctor course student and research fellowship for young scientists at Japan Society for the Promotion of Science (JSPS)) at Graduate School of Engineering, Tohoku University. The research results have been published in “Physical Review Letters” on June 5, 2009.

 

A spin field-effect transistor requires electrical control of spins and suppression of spin relaxation (losing spin information). SOI acts as an effective magnetic field on quantum spin, and therefore it is possible to rotate spin orientation by using SOI. The research group of Tohoku University had succeeded in controlling SOI and spin precession angle by gate electric field. However, SOI changes the direction of effective magnetic field after electron scattering events, which causes spin relaxation. There are two different SOIs, Rashba SOI and Dresselhaus SOI in semiconductors. It has been predicted that spin relaxation is suppressed by making two SOIs equal strengths (spin lifetime resonance), and this equal SOI strengths condition provides persistent spin helix state. A joint research group of University of California at Berkeley, Santa Barbara, and Stanford University has presented observation of persistent spin helix state by optical measurement in “Nature” vol. 458, pp. 610-614 (2009). However, the group needed different types of semiconductor samples because SOI was not controlled by gate electric field.

 

The group of Tohoku University controlled SOI by gate electric fields, and confirmed spin lifetime resonance state by precise magneto-conductance measurements of gate fitted wires. The group of Tohoku University uses semiconductor InGaAs 2DEG to make narrow wires. InGaAs has stronger SOI than GaAs that the U.S. research group used. Narrow wires less than 1 micro meter wide confine direction of electron motion, and suppress spin relaxation because effective magnetic field directions caused by SOI are more or less aligned. Furthermore, spin lifetime was successfully enhanced more than one order longer in gate fitted wires by making two SOIs equal strengths through gate electric field. The result shows that spin lifetime resonance state is tunable, and that gate voltage can control spin coherent and incoherent motions in semiconductor channels even including scatters.

 

These research results are expected to apply to spintronics. For example, application to a spin field-effect transistor or a spin lifetime resonance transistor is expected by combining a spin injection technology from magnetic electrodes into semiconductors

 

[Contact] Professor Junsaku Nitta

Graduate School of Engineering, Tohoku University

Tel: +81 22-795-7315

E-mail:　nitta@material.tohoku.ac.jp

Assistant Professor Makoto Kohda

Graduate School of Engineering, Tohoku University

Tel: +81 22-795-7316

E-mail:　makoto@material.tohoku.ac.jp