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.
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.
Spin-Transistor Action via Tunable Landau-Zener Transitions
Source: Universität Regensburg
