19 research outputs found
Electrically Tunable Valley Dynamics in Twisted WSeâ/WSeâ Bilayers
The twist degree of freedom provides a powerful new tool for engineering the electrical and optical properties of van der Waals heterostructures. Here, we show that the twist angle can be used to control the spin-valley properties of transition metal dichalcogenide bilayers by changing the momentum alignment of the valleys in the two layers. Specifically, we observe that the interlayer excitons in twisted WSeâ/WSeâ bilayers exhibit a high (>60%) degree of circular polarization (DOCP) and long valley lifetimes (>40ââns) at zero electric and magnetic fields. The valley lifetime can be tuned by more than 3 orders of magnitude via electrostatic doping, enabling switching of the DOCP from âŒ80% in the n-doped regime to <5% in the p-doped regime. These results open up new avenues for tunable chiral light-matter interactions, enabling novel device schemes that exploit the valley degree of freedom
Electrically Tunable Valley Dynamics in Twisted WSeâ/WSeâ Bilayers
The twist degree of freedom provides a powerful new tool for engineering the electrical and optical properties of van der Waals heterostructures. Here, we show that the twist angle can be used to control the spin-valley properties of transition metal dichalcogenide bilayers by changing the momentum alignment of the valleys in the two layers. Specifically, we observe that the interlayer excitons in twisted WSeâ/WSeâ bilayers exhibit a high (>60%) degree of circular polarization (DOCP) and long valley lifetimes (>40ââns) at zero electric and magnetic fields. The valley lifetime can be tuned by more than 3 orders of magnitude via electrostatic doping, enabling switching of the DOCP from âŒ80% in the n-doped regime to <5% in the p-doped regime. These results open up new avenues for tunable chiral light-matter interactions, enabling novel device schemes that exploit the valley degree of freedom
Excitons in a reconstructed moirĂ© potential in twisted WSeâ/WSeâ homobilayers
MoirĂ© superlattices in twisted van der Waals materials have recently emerged as a promising platform for engineering electronic and optical properties. A major obstacle to fully understanding these systems and harnessing their potential is the limited ability to correlate direct imaging of the moirĂ© structure with optical and electronic properties. Here we develop a secondary electron microscope technique to directly image stacking domains in fully functional van der Waals heterostructure devices. After demonstrating the imaging of AB/BA and ABA/ABC domains in multilayer graphene, we employ this technique to investigate reconstructed moirĂ© patterns in twisted WSeâ/WSeâ bilayers and directly correlate the increasing moirĂ© periodicity with the emergence of two distinct exciton species in photoluminescence measurements. These states can be tuned individually through electrostatic gating and feature different valley coherence properties. We attribute our observations to the formation of an array of two intralayer exciton species that reside in alternating locations in the superlattice, and open up new avenues to realize tunable exciton arrays in twisted van der Waals heterostructures, with applications in quantum optoelectronics and explorations of novel many-body systems
Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe/MoSe bilayers
Structural engineering of van der Waals heterostructures via stacking and
twisting has recently been used to create moir\'e superlattices, enabling the
realization of new optical and electronic properties in solid-state systems. In
particular, moir\'e lattices in twisted bilayers of transition metal
dichalcogenides (TMDs) have been shown to lead to exciton trapping, host Mott
insulating and superconducting states, and act as unique Hubbard systems whose
correlated electronic states can be detected and manipulated optically.
Structurally, these twisted heterostructures also feature atomic reconstruction
and domain formation. Unfortunately, due to the nanoscale sizes (~10 nm) of
typical moir\'e domains, the effects of atomic reconstruction on the electronic
and excitonic properties of these heterostructures could not be investigated
systematically and have often been ignored. Here, we use near-0 twist angle
MoSe/MoSe bilayers with large rhombohedral AB/BA domains to directly
probe excitonic properties of individual domains with far-field optics. We show
that this system features broken mirror/inversion symmetry, with the AB and BA
domains supporting interlayer excitons with out-of-plane (z) electric dipole
moments in opposite directions. The dipole orientation of ground-state
-K interlayer excitons (X) can be flipped with electric fields,
while higher-energy K-K interlayer excitons (X) undergo
field-asymmetric hybridization with intralayer K-K excitons (X). Our study
reveals the profound impacts of crystal symmetry on TMD excitons and points to
new avenues for realizing topologically nontrivial systems, exotic
metasurfaces, collective excitonic phases, and quantum emitter arrays via
domain-pattern engineering.Comment: 29 pages, 4 figures in main text, 6 figures in supplementary
informatio
Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSeâ/MoSeâ bilayers
Van der Waals heterostructures obtained via stacking and twisting have been used to create moirĂ© superlattices, enabling new optical and electronic properties in solid-state systems. MoirĂ© lattices in twisted bilayers of transition metal dichalcogenides (TMDs) result in exciton trapping, host Mott insulating and superconducting states6 and act as unique Hubbard systems whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures feature atomic reconstruction and domain formation. However, due to the nanoscale size of moirĂ© domains, the effects of atomic reconstruction on the electronic and excitonic properties have not been systematically investigated. Here we use near-0°-twist-angle MoSeâ/MoSeâ bilayers with large rhombohedral AB/BA domains to directly probe the excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane electric dipole moments in opposite directions. The dipole orientation of ground-state ÎâK interlayer excitons can be flipped with electric fields, while higher-energy KâK interlayer excitons undergo field-asymmetric hybridization with intralayer KâK excitons. Our study reveals the impact of crystal symmetry on TMD excitons and points to new avenues for realizing topologically non-trivial systems, exotic metasurfaces, collective excitonic phases and quantum emitter arrays via domain-pattern engineering
Nonlinear optical diode effect in a magnetic Weyl semimetal
Weyl semimetals have emerged as a promising quantum material system to
discover novel electrical and optical phenomena, due to their combination of
nontrivial quantum geometry and strong symmetry breaking. One crucial class of
such novel transport phenomena is the diode effect, which is of great interest
for both fundamental physics and modern technologies. In the electrical regime,
giant electrical diode effect (the nonreciprocal transport) has been observed
in Weyl systems. In the optical regime, novel optical diode effects have been
theoretically considered but never probed experimentally. Here, we report the
observation of the nonlinear optical diode effect (NODE) in the magnetic Weyl
semimetal CeAlSi, where the magnetic state of CeAlSi introduces a pronounced
directionality in the nonlinear optical second-harmonic generation (SHG). By
physically reversing the beam path, we show that the measured SHG intensity can
change by at least a factor of six between forward and backward propagation
over a wide bandwidth exceeding 250 meV. Supported by density-functional theory
calculations, we establish the linearly dispersive bands emerging from Weyl
nodes as the origin of the extreme bandwidth. Intriguingly, the NODE
directionality is directly controlled by the direction of magnetization. By
utilizing the electronically conductive semimetallic nature of CeAlSi, we
demonstrate current-induced magnetization switching and thus electrical control
of the NODE in a mesoscopic spintronic device structure with current densities
as small as 5 kA/cm. Our results advance ongoing research to identify novel
nonlinear optical/transport phenomena in magnetic topological materials. The
NODE also provides a way to measure the phase of nonlinear optical
susceptibilities and further opens new pathways for the unidirectional
manipulation of light such as electrically controlled optical isolators.Comment: 28 pages, 12 figure
Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure
Quantum geometry - the geometry of electron Bloch wavefunctions - is central
to modern condensed matter physics. Due to the quantum nature, quantum geometry
has two parts, the real part quantum metric and the imaginary part Berry
curvature. The studies of Berry curvature have led to countless breakthroughs,
ranging from the quantum Hall effect in 2DEGs to the anomalous Hall effect
(AHE) in ferromagnets. However, in contrast to Berry curvature, the quantum
metric has rarely been explored. Here, we report a new nonlinear Hall effect
induced by quantum metric by interfacing even-layered MnBi2Te4 (a PT-symmetric
antiferromagnet (AFM)) with black phosphorus. This novel nonlinear Hall effect
switches direction upon reversing the AFM spins and exhibits distinct scaling
that suggests a non-dissipative nature. Like the AHE brought Berry curvature
under the spotlight, our results open the door to discovering quantum metric
responses. Moreover, we demonstrate that the AFM can harvest wireless
electromagnetic energy via the new nonlinear Hall effect, therefore enabling
intriguing applications that bridges nonlinear electronics with AFM
spintronics.Comment: 19 pages, 4 figures and a Supplementary Materials with 66 pages, 4
figures and 3 tables. Originally submitted to Science on Oct. 5, 202