Excitons
form across layers in a 3D superlattice of stacked 2D semiconductors. Credit:
Olivia Kong
Ultra-low-energy
electronics ‘straight out of the fridge’?
Could a
stack of 2D materials allow for supercurrents at ground-breakingly warm
temperatures, easily achievable in the household kitchen?
An
international study published in August opens a new route to high-temperature
supercurrents at temperatures as ‘warm’ as inside a kitchen fridge.
The ultimate aim is to achieve superconductivity (ie, electrical current without any energy loss to resistance) at a reasonable temperature.
Towards
room-temperature superconductivity
Previously,
superconductivity has only been possible at impractically low temperatures,
less than -170°C below zero – even the Antarctic would be far too warm!
For this
reason, the cooling costs of superconductors have been high, requiring
expensive and energy-intensive cooling systems.
Superconductivity
at everyday temperatures is the ultimate goal of researchers in the field.
This new
semiconductor superlattice device could form the basis of a radically new class
of ultra-low energy electronics with
vastly lower energy consumption per computation than conventional,
silicon-based (CMOS) electronics.
Such
electronics, based on new types of conduction in which solid-state transistors
switch between zero and one (ie, binary switching) without resistance at room
temperature, is the aim of the FLEET Centre of Excellence.
Exciton
supercurrents in energy-efficient electronics
Because
oppositely-charged electrons and holes in semiconductors are strongly attracted
to each other electrically, they can form tightly-bound pairs. These composite
particles are called excitons, and they open up new paths towards conduction
without resistance at room temperature.
Excitons
can in principle form a quantum, ‘superfluid’ state, in which they move
together without resistance. With such
tightly bound excitons, the superfluidity should exist at high
temperatures—even as high as room temperature.
But
unfortunately, because the electron and hole are so close together, in practice
excitons have extremely short lifetimes—just a few nanoseconds, not enough time
to form a superfluid.
As a
workaround, the electron and hole can be kept completely apart in two,
separated atomically-thin conducting layers, creating so-called ‘spatially
indirect’ excitons. The electrons and
holes move along separate but very close conducting layers. This makes the
excitons long-lived, and indeed superfluidity has recently been observed in
such systems.
Counterflow
in the exciton superfluid, in which the oppositely charged electrons and holes
move together in their separate layers, allows so-called ‘supercurrents’
(dissipationless electrical currents) to flow with zero resistance and zero
wasted energy. As such, it is clearly an
exciting prospect for future, ultra-low-energy electronics.
Stacked
layers overcome 2D limitations
Sara Conti
who is a co-author on the study, notes another problem however: atomically-thin
conducting layers are two-dimensional, and in 2D systems there are rigid
topological quantum restrictions discovered by David Thouless and Michael
Kosterlitz (2016 Nobel prize), that eliminate the superfluidity at very low
temperatures, above about –170°C.
The key
difference with the new proposed system of stacked atomically-thin layers of
transition metal dichalcogenide (TMD) semiconducting materials, is that it is
three dimensional.
The
topological limitations of 2D are overcome by using this 3D `superlattice’ of
thin layers. Alternate layers are doped
with excess electrons (n-doped) and excess holes (p-doped) and these form the
3D excitons.
The study
predicts exciton supercurrents will flow in this system at temperatures as warm
as –3°C.
David
Neilson, who has worked for many years on exciton superfluidity and 2D systems,
says “The proposed 3D superlattice breaks out from the topological limitations
of 2D systems, allowing for supercurrents at –3°C. Because the electrons and
holes are so strongly coupled, further design improvements should carry this
right up to room temperature.â€
“Amazingly,
it is becoming routine today to produce stacks of these atomically-thin layers,
lining them up atomically, and holding them together with the weak van der
Waals atomic attraction,†explains Prof Neilson. “And while our new study is a
theoretical proposal, it is carefully designed to be feasible with present
technology.â€
The study
The study
looked at superfluidity in a stack made of alternating layers of two different
monolayer materials (n- and p-doped TMDC transition metal dichalcogenides WS2
and WSe2).