As an essential ingredient in forming cer-
tain topological phases, spin-orbit coupling
(SOC) provides an additional experimental
knob to engineer the topological properties
of moiré structures ( 17 – 19 ). It was recently
proposed that SOC endows the moiré energy
band with nonzero Berry curvature, making
ferromagnetic order at half moiré filling a pos-
sibility without alignment with the hexagonal
boron nitride (hBN) substrate ( 16 , 20 ). Unlike
bulk materials, where tuning the chemical
composition is required to produce spin-orbit
locking, an alternative route through the prox-
imity effect exists in vdW structures. Close
proximity between graphene and transition
metal dichalcogenide crystals, such as tungsten
diselenide (WSe 2 ), allows the electron wave
functions from both crystals to overlap and
hybridize, endowing graphene with strong SOC
( 21 – 28 ). In this work, we use transport mea-
surement to examine the effect of proximity-
induced SOC on properties of the moiré band
and its associated quantum phases.
The geometry of the twisted bilayer graphene
(tBLG)/WSe 2 heterostructure is shown in
Fig. 1A. An atomic interface is created by stack-
ing a few-layer WSe 2 crystal on top of magic-
angle tBLG, which is further encapsulated
with dual hBN and graphite crystals above and
below to achieve optimal sample quality ( 29 ).
Transport measurement indicated excellent
sample quality with low charge fluctuation
dn~0.08×10^12 cm–^2 (fig. S10). Longitudinal
resistanceRxx(measured from tBLG) exhib-
ited a series of well-defined resistance peaks
emerging at partial filling of the moiré band,
n=–2, +1, +2, and +3, which are associated
with the correlated insulator states. The posi-
tions of these peaks were consistent with a
twist angle ofq≈0.98°. The tBLG and the hBN
substrate were maximally misaligned, accord-
ing to the optical image of the heterostructure
(fig. S2) ( 16 ), which is consistent with the gap-
less appearance of the sample at the charge
neutrality point (CNP) [see ( 16 ) for more dis-
cussion of the coupling between tBLG and
hBN] ( 8 , 9 ). Transverse resistance measure-
ments revealed large Hall resistanceRxyat
n= +1 and +2; the Hall resistance exhibited
hysteretic switching behavior as the field
effect–induced doping in tBLG,ntBLG, was
swept back and forth (Fig. 1C). Hysteresis in
magnetization reversal was also observed while
sweeping an external magnetic field alignedperpendicular to the 2D interface,B⊥(Fig. 1,
D and E). We note that the resistance peak at
n= +2 vanishes at large in-planeBfield (fig.
S9) ( 16 ), indicating a spin-unpolarized isospin
configuration. As such, the ground state is
likely valley-polarized and the ferromagnetic
order is orbital. This is further illustrated by a
schematic representation of the band struc-
ture (Fig. 1, D and E, right panels), where the
two lowest conduction bands feature the same
valley index with nonzero Chern numbers
C=–3 and +1.
A valley-polarized state atn= 2 is unfavor-
able in the absence of SOC because of the in-
fluence of intervalley Hund’scoupling( 13 – 16 ).
As a result, observations of orbital ferromag-
netism at the half-filled moiré band have re-
mained elusive ( 8 , 9 , 30 ). To this end, the AHE
atn= +2 in our sample provides strong evi-
dence that the moiré band structure is trans-
formed by proximity-induced SOC, which is
more dominant than intervalley Hund’s cou-
pling ( 20 ). Although the presence of SOC
endows the moiré flatband with a nonzero
Chern number (Fig. 1, D and E), the observed
Hall resistance is much smaller than the ex-
pected value of the quantum AHE. We ascribe438 28 JANUARY 2022•VOL 375 ISSUE 6579 science.orgSCIENCE
Fig. 1. Emerging ferromagnetic
order from the tBLG/WSe 2
interface.(A) Schematic of the
heterostructure, consisting of
a tBLG/WSe 2 interface that
is doubly encapsulated with hBN
and graphite. (B) Calculated
dispersion of moiré bands for a
single valley.lIandlRindicate
the strength of Ising and Rashba
SOC, respectively, in units of
meV. Red and blue colors denote
the out-of-plane component of the
spin moment of each band. The
Chern number is expected to
be zero for all energy bands in the
absence oflR(left and center
panels). The combination of strong
Ising and Rashba SOC gives rise
to a nonzero valley Chern number.
Values oflIandlRare taken
from previous measurements
of proximity-induced SOC ( 27 , 44 ).
(C) Longitudinal and transverse
resistance,RxxandRxy, as carrier
densityntBLGis swept back and
forth. Carrier densityntBLGand
moiré fillingnare denoted as the
bottom and top axis, respectively.
The measurement is performed
atB= 10 mT,T= 20 mK, and
D= 252 mV/nm. (DandE)Rxymeasured atntBLG= 0.55 × 10^12 cm–^2 nearn= 1 (D) andntBLG= 1.22 × 10^12 cm–^2 nearn= 2 (E) asB⊥is swept back and forth. These
measurements are performed atD= 0. The hysteresis loop disappears at high temperature. Right: Schematic band structure atn= +1 (D) andn= +2 (E). In (E),
the two lowest bands feature the same valley index and have nonzero Chern numbers ofC=–3 and +1. As a result, the ground state atn= 2 is valley-polarized
with net Chern numberCnet=–2.
ABDEgraphitegraphitehBNhBN-1.0-0.50.00.51.0T (K)1.8
3.04.0
5.0
R 7.0(kΩ)xyν = 2R(kΩ)xy-0.6-0.30.30.00.6 ν = 18.0 K
6.0 K
4.0 K
2.0 KT (K)B (mT)T-100 -50 0 50 100-3
B = 10 mT
T = 20 mK
D = 252 mV/nm-2 -1 0 1 2 3ν01020R(kΩ)xx-1 0 1
n (10 cm )tBLG
12 -2-202R(kΩ)xyB-2 2Cτ = K C = -3C net = -3τ = K C = -3τ = K C = 1C net = -2Γ K’ K’’ K Γ Γ K’ K’’ K Γ Γ K’ K’’ K Γ-505E(meV)Sz-1^01λ =0, I λ R =0 λ =3, I λ R =0 λ =3, I λ R =2RESEARCH | REPORTS