16 research outputs found
Electrical switching of magnetic order in an orbital Chern insulator
Magnetism typically arises from the joint effect of Fermi statistics and
repulsive Coulomb interactions, which favors ground states with non-zero
electron spin. As a result, controlling spin magnetism with electric fields---a
longstanding technological goal in spintronics and multiferroics---can be
achieved only indirectly. Here, we experimentally demonstrate direct electric
field control of magnetic states in an orbital Chern insulator, a magnetic
system in which non-trivial band topology favors long range order of orbital
angular momentum but the spins are thought to remain disordered. We use van der
Waals heterostructures consisting of a graphene monolayer rotationally faulted
with respect to a Bernal-stacked bilayer to realize narrow and topologically
nontrivial valley-projected moir\'e minibands. At fillings of one and three
electrons per moir\'e unit cell within these bands, we observe quantized
anomalous Hall effects with transverse resistance approximately equal to
, which is indicative of spontaneous polarization of the system into a
single-valley-projected band with a Chern number equal to two. At a filling of
three electrons per moir\'e unit cell, we find that the sign of the quantum
anomalous Hall effect can be reversed via field-effect control of the chemical
potential; moreover, this transition is hysteretic, which we use to demonstrate
nonvolatile electric field induced reversal of the magnetic state. A
theoretical analysis indicates that the effect arises from the topological edge
states, which drive a change in sign of the magnetization and thus a reversal
in the favored magnetic state. Voltage control of magnetic states can be used
to electrically pattern nonvolatile magnetic domain structures hosting chiral
edge states, with applications ranging from reconfigurable microwave circuit
elements to ultralow power magnetic memory.Comment: 21 pages, 17 figure
The Future of the Correlated Electron Problem
The understanding of material systems with strong electron-electron
interactions is the central problem in modern condensed matter physics. Despite
this, the essential physics of many of these materials is still not understood
and we have no overall perspective on their properties. Moreover, we have very
little ability to make predictions in this class of systems. In this manuscript
we share our personal views of what the major open problems are in correlated
electron systems and we discuss some possible routes to make progress in this
rich and fascinating field. This manuscript is the result of the vigorous
discussions and deliberations that took place at Johns Hopkins University
during a three-day workshop January 27, 28, and 29, 2020 that brought together
six senior scientists and 46 more junior scientists. Our hope, is that the
topics we have presented will provide inspiration for others working in this
field and motivation for the idea that significant progress can be made on very
hard problems if we focus our collective energies.Comment: 55 pages, 19 figure
Quantitative Transport Measurements of Fractional Quantum Hall Energy Gaps in Edgeless Graphene Devices
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Electrical switching of magnetic order in an orbital Chern insulator
Magnetism typically arises from the joint effect of Fermi statistics and repulsive Coulomb interactions, which favors ground states with non-zero electron spin. As a result, controlling spin magnetism with electric fields---a longstanding technological goal in spintronics and multiferroics---can be achieved only indirectly. Here, we experimentally demonstrate direct electric field control of magnetic states in an orbital Chern insulator, a magnetic system in which non-trivial band topology favors long range order of orbital angular momentum but the spins are thought to remain disordered. We use van der Waals heterostructures consisting of a graphene monolayer rotationally faulted with respect to a Bernal-stacked bilayer to realize narrow and topologically nontrivial valley-projected moiré minibands. At fillings of one and three electrons per moiré unit cell within these bands, we observe quantized anomalous Hall effects with transverse resistance approximately equal to h/2e2, which is indicative of spontaneous polarization of the system into a single-valley-projected band with a Chern number equal to two. At a filling of three electrons per moiré unit cell, we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential; moreover, this transition is hysteretic, which we use to demonstrate nonvolatile electric field induced reversal of the magnetic state. A theoretical analysis indicates that the effect arises from the topological edge states, which drive a change in sign of the magnetization and thus a reversal in the favored magnetic state. Voltage control of magnetic states can be used to electrically pattern nonvolatile magnetic domain structures hosting chiral edge states, with applications ranging from reconfigurable microwave circuit elements to ultralow power magnetic memory.Work at UCSB was
primarily supported by the ARO under MURI W911NF-
16-1-0361. Measurements of twisted bilayer graphene
(Extended Data Fig. E8) and measurements at elevated
temperatures (Extended Data Fig. E3) were supported
by a SEED grant and made use of shared facilities of the
UCSB MRSEC (NSF DMR 1720256), a member of the
Materials Research Facilities Network (www.mrfn.org).
AFY acknowledges the support of the David and Lu-
cille Packard Foundation under award 2016-65145. AHM
and JZ were supported by the National Science Founda-
tion through the Center for Dynamics and Control of
8
Materials, an NSF MRSEC under Cooperative Agree-
ment No. DMR-1720595, and by the Welch Founda-
tion under grant TBF1473. CLT acknowledges support
from the Hertz Foundation and from the National Sci-
ence Foundation Graduate Research Fellowship Program
under grant 1650114. KW and TT acknowledge sup-
port from the Elemental Strategy Initiative conducted by
the MEXT, Japan, Grant Number JPMXP0112101001,
JSPS KAKENHI Grant Numbers JP20H00354 and the
CREST(JPMJCR15F3), JST.Center for Dynamics and Control of Material
Cascade of phase transitions and Dirac revivals in magic-angle graphene
© 2020, The Author(s), under exclusive licence to Springer Nature Limited. Twisted bilayer graphene near the magic angle1–4 exhibits rich electron-correlation physics, displaying insulating3–6, magnetic7,8 and superconducting phases4–6. The electronic bands of this system were predicted1,2 to narrow markedly9,10 near the magic angle, leading to a variety of possible symmetry-breaking ground states11–17. Here, using measurements of the local electronic compressibility, we show that these correlated phases originate from a high-energy state with an unusual sequence of band population. As carriers are added to the system, the four electronic ‘flavours’, which correspond to the spin and valley degrees of freedom, are not filled equally. Rather, they are populated through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moiré lattice. At each transition, a single spin/valley flavour takes all the carriers from its partially filled peers, ‘resetting’ them to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breaking. The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state that we report here, with its strongly broken electronic flavour symmetry and revived Dirac-like electronic character, is important in the physics of magic-angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge