Molecular blockade

Artificial ‘molecules’ with an asymmetric structure can control the
flow of electrons in semiconductor materials

Nanoscale devices confine electrons and enable manipulation of electron
spin—an inherent property akin to the direction in which the particle is rotating.
An unexpected mechanism for this control in asymmetric structures has now
been reported by Keiji Ono at the RIKEN Advanced Science Institute, Wako,
in collaboration with a team of researchers from Japan and Taiwan.
Artificial systems that trap electrons in a tiny volume can display many
of the properties of atoms because they create an analogous series of
discrete electron energy levels. “One example is the Zeeman Effect in
which an applied magnetic field splits a single electron energy level
into two, depending on its spin,” explains Ono.
Taking this analogy further, two closely spaced ‘artificial atoms’ can
behave like an artificial molecule. In principle, it is possible to transfer an
electron between these atoms by tuning the energy level of an electron
in one atom to that of the second by, for example, applying an electric
field. Indeed, this phenomenon, known as resonant tunneling, occurs in
artificial molecules consisting of two identical atoms. Ono and his team
showed, however, that the situation is not so simple in artificial
molecules comprising two different atoms.

They investigated a structure that was a stack of alternating layers of
semiconductor. Electrons become trapped in the semiconductor with
the smaller bandgap by the surrounding layers of wide-bandgap
material. The top ‘atom’ was 7.5 nanometers thick and made of indium
gallium arsenide. A 6.5-nanometer barrier separated this from the
second atom: 10 nanometers of gallium arsenide. Etched pillars
with a diameter of less than one micrometer confined the electrons
in the transverse direction.
The difference in size and composition meant that the Zeeman Effect
was stronger in the top atom than the bottom one. This made it impossible to
align both of the Zeeman-split levels in the two atoms at the same time. Ono
and colleagues demonstrated that because of this, when an energy state
from one atom is aligned with one in the second, the electron
flow through the molecule reduces, an effect they call spin
blockade. The flow increased when they tuned the two Zeeman
levels in one atom to the midpoint of those in the other atom.
“This finding can be used as a basic tool for selecting, filtering, or initializing
an individual electron spin,” comments Ono. “I hope this can be applied to
quantum information technology.”