C OMPU TATION FOR THE F U T URE
VALLEY POLARIZATION GETS A MAGNETIC BOOST Access to valley-polarized charge carriers by magnetic doping could transform electronics.
P S E The field of valleytronics is emerging as a way of exploiting specific energy extrema known as valleys. This field has the potential to revolutionize conventional electronics, which rely on the control and manipulation of charged particles, such as electrons and holes, and surpass the capacity of spintronics, which depends on the intrinsic spin property of particles. Valleytronics is based on the crucial trapping of charge carriers in energy valleys; however, device development is hindered by the complexity involved with the necessary valley polarization. Several researchers have generated valley polarization in semiconductors called transition-metal
From left to right: Professor Younis and PhD students Nizar and Saad examine a silicon chip before placing it into an environmental chamber for further electrical probing and testing.
GOOD VIBRATIONS FOR THE FUTURE OF COMPUTING A vibration-driven logic gate could form the basis for the next generation of efficient, low-power computers.
dichalcogenides but the effectiveness of this approach has been hampered because it demands sustained optical pumping, which requires light to pump electrons to a higher energy state. “This problem can be eliminated by creating a
P S E
are connected to much larger
Vibrating mechanical
complex operations.
switches that can be
networks to allow increasingly With each transistor
permanent valley polarization by means of magnetic
cascaded to perform
consuming electrical current
doping,” says Udo Schwingenschlögl, a professor of
complex computational
and generating heat even
material science and engineering at KAUST.
operations could take
when not being actively
computing significantly
switched, and with transistors
scientist Nirpendra Singh computationally investigated
further than today’s
approaching their physical
the effect of magnetic doping using chromium and
technologies. KAUST
limits of miniaturization and
vanadium on valley polarization in single layers of
researchers have
efficiency, the search is on for
To test this hypothesis, Schwingenschlögl and research
molybdenum disulfide (MoS2). This two-dimensional
demonstrated an alternative
alternative technology that
transition-metal dichalcogenide comprises molybdenum
technology based on
will eventually replace the
atoms connected to six sulfur atoms, forming a transition
mechanical vibrations.
electrical transistor and take
metal sheet sandwiched between two sulfur sheets. The researchers’ first option for a magnetic dopant
The microcomputer processors found inside
computing into the future. Saad Ilyas and Nizar
was chromium, which presents the same electronic
every computer, mobile
Jaber, doctoral researchers
configuration as molybdenum. They chose this dopant
phone and microwave
in the laboratory of
thinking it might induce spin polarization—a state in
comprise mind-bogglingly
Mohammad Younis, have
which all spins are oriented in the same direction—and
complex networks of millions
now demonstrated a scalable,
promote valley polarization. But, instead of the desired
or billions of microscopic
efficient alternative technology,
effect, the team observed a complex magnetic structure.
transistors—electrical
not based on electrical current,
According to Schwingenschlögl this surprising
switches that turn on when
but on mechanical vibrations
extended-moment formation counteracts the valley
a current flows across their
excited by multifrequency
polarization. The team experimented with vanadium-
terminals. These transistors
electrical inputs.
doped single-layer MoS2 to find that the resulting
are networked together to
“Electromechanical
material exhibited permanent valley polarization.
construct logic gates that
systems offer a major
perform operations, such as
advantage over existing
Singh, N. & Schwingenschlögl, U. A route to permanent
AND (when two inputs are on)
technology in that they
valley polarization in monolayer MoS2. Advanced
and OR (when either input is
are leakage free: that is,
Materials 29, 1600970 (2017).
on). In turn, these logic gates
unlike electrical transistors,
14
J a n u a r y 2 018