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Posted: Jul 20, 2012
Electrons get off the track - Scientists find new principle for spin transistor
(Nanowerk News) Performance of present day electronics is reaching its boundaries since faster transistor operation lead to high power consumption and heat generation. Several alternative schemes are being explored to possibly overcome these limitations, including the use of the electrons’ spin in electronics. Now a research team from the University of Regensburg around Dieter Weiss and Klaus Richter in Germany together with colleagues from the Polish Academy of Sciences in Warsaw has made a significant step in utilizing the electrons’ spin for transistor action (see paper in Science: "Spin-Transistor Action via Tunable Landau-Zener Transitions").
If spin-based electronics prevails the new switching concept might turn out to be useful as it allows for switching the spin-polarization of an electric current on and off, tuning it continuously or reading it off electrically by simple resistance measurements.
In conventional field effect transistors the current through the device can be switched on and off by an electric field. The Regensburg/Warsaw team has developed a new way to control electron current in a transistor-like structure by using the electrons’ spin, a property which causes electrons to act like tiny compass needles in a magnetic field. However, in contrast to a classical compass needle the quantum mechanical version can align parallel (spin-up) or anti-parallel (spin-down) to the externally applied magnetic field direction.
What is really new is that one can not only tune the electrical current in the device but also the spin-polarization of the electron current, i.e. the ratio of spin-up and spin-down electrons carrying the electrical current. To do so they use the rate of change of the electrons’ spin direction in a spatially varying magnetic field orientation. In the transistor 'on'-state, electrons travel through the device unhindered, their spin direction following a slowly rotating magnetic guiding field. In the 'off'-state the guiding field is twisted and changes direction rapidly which causes electrons to deflect into energetically forbidden tracks, suppressing current. An analogy of the process would be a car going around a sharp turn. If the car is sufficiently slow it stays on the road and makes it around the turn (‘on’-state). If the car is too fast it veers off the street (‘off’-state).
In the experiment the research team placed ferromagnetic stripes on top of a two-dimensional electron gas which usually serves as an electrically conducting channel in transistors. The material of choice was the semiconductor CdMnTe, known for the large splitting between energy levels for spin-up and spin-down electrons. The magnetic stray field around the ferromagnetic stripes forms in the plane of the electron gases a helical structure of the magnetic field vector. With an externally applied magnetic field B, generated by large coils, the stray field components in the direction of the external field get larger, the ones opposite to the B-field weaker and eventually vanish.
Without or with sufficiently small external B-field the electron spin is rotated continuously by the helical stray field as it traverses the device following the helical B-field pattern. This corresponds to the car moving slowly through the turn. If the external magnetic field is switched to a certain value the electron spins are no longer able to follow the changes of the magnetic field and need to jump to the energetically higher spin level, giving rise to a higher resistance. In the car picture this corresponds to getting off the track.
As the effect allows for tuning the resistance of a two-dimensional electron system and – under certain circumstances – to switch the current in the channel on and off, it constitutes transistor action. In contrast to other switching schemes the Regensburg team uses so-called Landau-Zener transitions between spin-down and spin-up energy levels. The simplicity of the concept might be transferable to other systems and could be straightforwardly implemented into a device which works at liquid helium temperatures and allows switching the spin-polarization of an electric current on and off.