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Say 'cheese' To understand the quantum spin Hall state, it's
key to first understand the related quantum Hall state. Imagining a
cheese sandwich will help. Swap semiconductor sheets for the bread,
and turn the cheese into an electron gas. Instead of sticking your
cheese sandwich in the fridge, place your semiconductor-electron gas
concoction in an environment where it's way colder (below 1 degree
Kelvin). Apply an intense magnetic field of several Teslas-more than
10,000 times greater than Earth's magnetic field.
''In such a state, the electrical current does not flow through the
two-dimensional sheet, but is confined at the edges,'' Zhang explains.
''The current at a given edge flows without dissipation, and only in
one direction; it cannot be scattered backward by impurities.''
In essence, current flows only around the bread crusts. ''This
property gives rise to the remarkable observation of quantized Hall
voltage measured in the direction perpendicular to the current flow.''
In contrast, in conventional electronics, currents flow in the same
direction as applied voltage, and the resistance can take arbitrary or
nonquantized values. That means greater energy dissipation.
So that's the recipe for creating the quantum Hall effect, which
Zhang calls ''one of the most profound phenomena in physics.'' The
stuff of dreams, the quantum Hall effect was the basis of Nobel Prizes
in 1985 and 1998.
A new state
Physicists often use math to convert complex physics concepts into
terms of shape, or topology. It makes it easier to describe the
extraordinary properties of different states of matter. ''If one
performs smooth distortions of the donut, one can never get rid of the
hole in its center and transform it into a sphere,'' Zhang says. ''Similarly,
the electronic state of the quantum Hall effect is topologically
distinct from that of any conventional semiconductor states.''
As cool and exotic as the quantum Hall state is, it has a serious
drawback, Zhang notes. ''Unfortunately, the quantum Hall effect can
only be realized under high magnetic field and low temperature, and
cannot, therefore, be used for semiconductor devices operating under
ambient conditions.''
In their report in Science, the three Stanford researchers proposed
that a new state, called the quantum spin Hall effect, could be
realized without applying an external magnetic field. They stacked and
skewed alternating layers of mercury telluride and cadmium telluride.
Just as in a slightly skewed stack of checkerboards, where red squares
are bordered by black squares and vice versa, the material made a
crystal lattice structure similar to that of the silicon or gallium
arsenide of semiconductors. The researchers say that by controlling
the thickness of wells in the mercury telluride, the result will be a
quantum phase transition into a new state that is distinct from that
of conventional semiconductor states.
Conventional semiconductors are insulators at low temperatures.
That means the resistance of the material is so high that no current
can flow. But insulators can be turned into conductors-materials with
some resistance, but not enough to stop current from flowing-using
n-type doping, which adds electrons to the material, or p-type doping,
which removes electrons to leave behind holes.
But matter in the quantum spin Hall state can carry electric
currents without any doping, Zhang says. Just like with the quantum
Hall effect, electrical current flows only at the edges of the sample.
What's more, the quantum spin Hall state would display an ''extraordinary''
property, Zhang says. On any given edge, electrons oriented with their
spins aligned pointing up would flow in one direction, while the
electrons oriented with their spins aligned pointing down would flow
in the opposite direction. Because impurities usually do not flip the
spin orientation, they cannot easily scatter the electrons into the
backward direction, thus leading to far less energy dissipation or
heat generation compared to conventional semiconductors. Basically,
the quantum spin Hall effect has most of the desirable features of the
quantum Hall effect, but without the cost of applying a huge magnetic
field to a device, Zhang says.
''Similar to the quantum Hall effect, the quantum spin Hall effect
is also topologically distinct from any conventional semiconductors,''
says Zhang. ''In this precise mathematical sense, the quantum spin
Hall effect is a topologically distinct new state of matter.''
Putting theory to the test
Since quantum wells in mercury telluride/cadmium telluride sheets
can be readily fabricated, it is possible to experimentally test the
theoretical predictions of Zhang, Bernevig and Hughes. A research
group at the University of Würzburg in Germany, under the direction of
Professor Laurens Molenkamp, is currently doing this.
If the theory pans out, the quantum spin Hall effect may eventually
inspire room-temperature devices with new capabilities. Zhang notes
the potential for getting around a well-known roadblock of the
electronics industry, the dictum saying the number of transistors
fitting on a computer chip will double every 18 months: ''Transistors
built based on the quantum spin Hall effect are expected to dissipate
far less heat compared to conventional transistors, thus paving the
way for extending Moore's law.''
In fact, hoping to turn Zhang's vision into a commercial reality,
the Microelectronics Advanced Research Corporation, a consortium of
leading U.S. semiconductor companies, has started to fund his research
on the quantum spin Hall effect. |
