40 research outputs found
Enhanced photoenergy harvesting and extreme Thomson effect in hydrodynamic electronic systems
The thermoelectric (TE) properties of a material are dramatically altered
when electron-electron interactions become the dominant scattering mechanism.
In the degenerate hydrodynamic regime, the thermal conductivity is reduced and
becomes a {\it decreasing} function of the electronic temperature, due to a
violation of the Wiedemann-Franz (WF) law. We here show how this peculiar
temperature dependence gives rise to new striking TE phenomena. These include
an 80-fold increase in TE efficiency compared to the WF regime, dramatic
qualitative changes in the steady state temperature profile, and an anomalously
large Thomson effect. In graphene, which we pay special attention to here,
these effects are further amplified due to a doubling of the thermopower.Comment: 6 pages, 3 figure
Observation of interlayer phonon modes in van der Waals heterostructures
We have investigated the vibrational properties of van der Waals
heterostructures of monolayer transition metal dichalcogenides (TMDs),
specifically MoS2/WSe2 and MoSe2/MoS2 heterobilayers as well as twisted MoS2
bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered
Raman features (at 30 ~ 40 cm-1) that arise from the layer-breathing mode (LBM)
vibrations between the two incommensurate TMD monolayers in these structures.
The LBM Raman intensity correlates strongly with the suppression of
photoluminescence that arises from interlayer charge transfer. The LBM is
generated only in bilayer areas with direct layer-layer contact and atomically
clean interface. Its frequency also evolves systematically with the relative
orientation between of the two layers. Our research demonstrates that LBM can
serve as a sensitive probe to the interface environment and interlayer
interactions in van der Waals materials
Observation of Interlayer Phonon Modes in van der Waals Heterostructures
We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically MoS2/WSe2 and MoSe2/MoS2 heterobilayers and twisted MoS2 bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at 30â40 cmâ1) that arise from the layer-breathing mode (LBM) vibration between the two incommensurate TMD monolayers in these structures. The LBM Raman intensity correlates strongly with the suppression of photoluminescence that arises from interlayer charge transfer. The LBM is generated only in bilayer areas with direct layer-layer contact and an atomically clean interface. Its frequency also evolves systematically with the relative orientation between the two layers. Our research demonstrates that the LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials
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
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
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