4 research outputs found
Electron Pumping in Graphene Mechanical Resonators
The combination of high-frequency vibrations and metallic
transport
in graphene makes it a unique material for nanoelectromechanical devices.
In this Letter, we show that graphene-based nanoelectromechanical
devices are extremely well suited for charge pumping due to the sensitivity
of its transport coefficients to perturbations in electrostatic potential
and mechanical deformations, with the potential for novel small scale
devices with useful applications
Tunable Phonon-Induced Transparency in Bilayer Graphene Nanoribbons
In the phenomenon of plasmon-induced
transparency, which is a classical
analogue of electromagnetically induced transparency (EIT) in atomic
gases, the coherent interference between two plasmon modes results
in an optical transparency window in a broad absorption spectrum.
With the requirement of contrasting lifetimes, typically one of the
plasmon modes involved is a dark mode that has limited coupling to
the electromagnetic radiation and possesses relatively longer lifetime.
Plasmon-induced transparency not only leads to light transmission
at otherwise opaque frequency regions but also results in the slowing
of light group velocity and enhanced optical nonlinearity. In this
article, we report an analogous behavior, denoted as phonon-induced
transparency (PIT), in AB-stacked bilayer graphene nanoribbons. Here,
light absorption due to the plasmon excitation is suppressed in a
narrow window due to the coupling with the infrared active Γ-point
optical phonon, whose function here is similar to that of the dark
plasmon mode in the plasmon-induced transparency. We further show
that PIT in bilayer graphene is actively tunable by electrostatic
gating and estimate a maximum slow light factor of around 500 at the
phonon frequency of 1580 cm<sup>–1</sup>, based on the measured
spectra. Our demonstration opens an avenue for the exploration of
few-photon nonlinear optics and slow light in this novel two-dimensional
material
Local Strain Engineering in Atomically Thin MoS<sub>2</sub>
Controlling the bandstructure through
local-strain engineering
is an exciting avenue for tailoring optoelectronic properties of materials
at the nanoscale. Atomically thin materials are particularly well-suited
for this purpose because they can withstand extreme nonhomogeneous
deformations before rupture. Here, we study the effect of large localized
strain in the electronic bandstructure of atomically thin MoS<sub>2</sub>. Using photoluminescence imaging, we observe a strain-induced
reduction of the direct bandgap and funneling of photogenerated excitons
toward regions of higher strain. To understand these results, we develop
a nonuniform tight-binding model to calculate the electronic properties
of MoS<sub>2</sub> nanolayers with complex and realistic local strain
geometries, finding good agreement with our experimental results
High-Mobility Compensated Semimetals, Orbital Magnetization, and Umklapp Scattering in Bilayer Graphene Moiré Superlattices
Twist-controlled moiré superlattices (MSs) have
emerged
as a versatile platform for realizing artificial systems with complex
electronic spectra. The combination of Bernal-stacked bilayer graphene
(BLG) and hexagonal boron nitride (hBN) can give rise to an interesting
MS, which has recently featured a set of unexpected behaviors, such
as unconventional ferroelectricity and the electronic ratchet effect.
Yet, the understanding of the electronic properties of BLG/hBN MS
has, at present, remained fairly limited. Here, we combine magneto-transport
and low-energy sub-THz excitation to gain insights into the properties
of this MS. We demonstrate that the alignment between BLG and hBN
crystal lattices results in the emergence of compensated semimetals
at some integer fillings of the moiré bands, separated by van
Hove singularities where the Lifshitz transition occurs. A particularly
pronounced semimetal develops when eight holes reside in the moiré
unit cell, where coexisting high-mobility electron and hole systems
feature strong magnetoresistance reaching 2350% already at B = 0.25 T. Next, by measuring the THz-driven Nernst effect
in remote bands, we observe valley splitting, indicating an orbital
magnetization characterized by a strongly enhanced effective gv-factor of 340. Finally, using THz photoresistance
measurements, we show that the high-temperature conductivity of the
BLG/hBN MS is limited by electron–electron umklapp processes.
Our multifaceted analysis introduces THz-driven magnetotransport as
a convenient tool to probe the band structure and interaction effects
in van der Waals materials and provides a comprehensive understanding
of the BLG/hBN MS