Interest
in
atomically-thin
transition
metal
dichalcogenide
(TMD)
semiconductors
such
as
MoS2
and
WSe2
has
exploded
in
the
last
few
years,
driven
by
the
new
physics
of
coupled
spin/valley
degrees
of
freedom
and
their
potential
for
new
spintronic
and
‘valleytronic’
devices.
Although
robust
spin
and
valley
degrees
of
freedom
have
been
inferred
from
polarized
photoluminescence
(PL)
studies
of
excitons,
PL
timescales
are
necessarily
constrained
by
short-lived
(1–30
ps)
recombination
timescales
of
excitons.
Direct
probes
of
spin
and
valley
dynamics
of
the
resident
electrons
and
holes
in
n-type
or
p-
type
doped
TMD
monolayers,
which
may
persist
long
after
recombination
ceases,
are
still
at
a
relatively
early
stage.
In
this
work,
we
directly
measure
the
coupled
spin-valley
dynamics
of
resident
electrons
and
resident
holes
in
n-type
and
p-type
monolayer
TMD
semiconductors
using
time-resolved
Kerr
rotation.
Very
long
relaxation
timescales
in
the
nanosecond
to
microsecond
range
are
observed
at
low
temperatures
–
orders
of
magnitude
longer
than
typical
exciton
lifetimes.
In
contrast
with
III-V
or
II-VI
semiconductors,
electron
spin
relaxation
in
monolayer
MoS2
is
found
to
accelerate
rapidly
in
small
transverse
magnetic
fields
(By).
This
indicates
a
novel
mechanism
of
electron
spin
dephasing
in
monolayer
TMDs
that
is
driven
by
rapidly-fluctuating
internal
spin-
orbit
fields
that,
in
turn,
are
due
to
fast
electron
scattering
between
the
K
and
K’
conduction
bands
[1].
More
recent
studies
of
gated
TMD
monolayers
also
allow
observation
of
very
long
spin/valley
relaxation
of
resident
holes,
a
consequence
of
spin-valley
locking
[2].
These
studies
provide
direct
insight
into
the
physics
underpinning
the
spin
and
valley
dynamics
of
resident
electrons
and
holes
in
2D
TMD
semiconductors.
[1]
L.
Yang
et
al.,
Nature
Physics
11,
830
(2015).
[2]
P.
Dey
et
al.,
Phys.
Rev.
Lett.
119,
137401
(2017).
Luyi
Yang