107 research outputs found
Exciton Diamagnetic Shifts and Valley Zeeman Effects in Monolayer WS and MoS to 65 Tesla
We report circularly-polarized optical reflection spectroscopy of monolayer
WS and MoS at low temperatures (4~K) and in high magnetic fields to
65~T. Both the A and the B exciton transitions exhibit a clear and very similar
Zeeman splitting of approximately 230~eV/T (), providing
the first measurements of the valley Zeeman effect and associated -factors
in monolayer transition-metal disulphides. These results complement and are
compared with recent low-field photoluminescence measurements of valley
degeneracy breaking in the monolayer diselenides MoSe and WSe. Further,
the very large magnetic fields used in our studies allows us to observe the
small quadratic diamagnetic shifts of the A and B excitons in monolayer WS
(0.32 and 0.11~eV/T, respectively), from which we calculate exciton
radii of 1.53~nm and 1.16~nm. When analyzed within a model of non-local
dielectric screening in monolayer semiconductors, these diamagnetic shifts also
constrain and provide estimates of the exciton binding energies (410~meV and
470~meV for the A and B excitons, respectively), further highlighting the
utility of high magnetic fields for understanding new 2D materials.Comment: 9 pages, 5 figure
Magneto-reflection spectroscopy of monolayer transition-metal dichalcogenide semiconductors in pulsed magnetic fields
We describe recent experimental efforts to perform polarization-resolved
optical spectroscopy of monolayer transition-metal dichalcogenide
semiconductors in very large pulsed magnetic fields to 65 tesla. The
experimental setup and technical challenges are discussed in detail, and
temperature-dependent magneto-reflection spectra from atomically thin tungsten
disulphide (WS) are presented. The data clearly reveal not only the valley
Zeeman effect in these 2D semiconductors, but also the small quadratic exciton
diamagnetic shift from which the very small exciton size can be directly
inferred. Finally, we present model calculations that demonstrate how the
measured diamagnetic shifts can be used to constrain estimates of the exciton
binding energy in this new family of monolayer semiconductors.Comment: PCSI-43 conference (Jan. 2016; Palm Springs, CA
Magneto-Optics of Exciton Rydberg States in a Monolayer Semiconductor
We report 65 tesla magneto-absorption spectroscopy of exciton Rydberg states
in the archetypal monolayer semiconductor WSe. The strongly field-dependent
and distinct energy shifts of the 2s, 3s, and 4s excited neutral excitons
permits their unambiguous identification and allows for quantitative comparison
with leading theoretical models. Both the sizes (via low-field diamagnetic
shifts) and the energies of the exciton states agree remarkably well with
detailed numerical simulations using the non-hydrogenic screened Keldysh
potential for 2D semiconductors. Moreover, at the highest magnetic fields the
nearly-linear diamagnetic shifts of the weakly-bound 3s and 4s excitons provide
a direct experimental measure of the exciton's reduced mass, .Comment: To appear in Phys. Rev. Lett. Updated version (25 jan 2018) now
includes detailed supplemental discussion of Landau levels, Rydberg exciton
energies, exciton mass, Dirac Hamiltonian, nonparabolicity, and dielectric
effect
System-Optimal Routing of Traffic Flows with User Constraints in Networks with Congestion
The design of route-guidance systems faces a well-known dilemma. The approach that theoretically yields the
system-optimal traffic pattern may discriminate against some users, for the sake of favoring others. Proposed
alternate models, however, do not directly address the system perspective and may result in inferior performance.
We propose a novel model and corresponding algorithms to resolve this dilemma. We present computational
results on real-world instances and compare the new approach with the well-established traffic assignment model.
The quintessence is that system-optimal routing of traffic flow with explicit integration of user constraints leads to a
better performance than the user equilibrium while simultaneously guaranteeing a superior fairness compared to the
pure system optimum
Probing the Dark Exciton in Monolayer MoS by Quantum Interference in Second Harmonic Generation Spectroscopy
We report resonant second harmonic generation (SHG) spectroscopy of an
hBN-encapsulated monolayer of MoS. By tuning the energy of the excitation
laser, we identify a dark state transition (D) that is blue detuned by +25 meV
from the neutral exciton X. We observe a splitting of the SHG spectrum into
two distinct peaks and a clear anticrossing between them as the SHG resonance
is tuned through the energy of the dark exciton D. This observation is
indicative of quantum interference arising from the strong two-photon
light-matter interaction. We further probe the incoherent relaxation from the
dark state to the bright excitons, including X and localized excitons LX,
by the resonant enhancement of their intensities at the SHG-D resonance. The
relaxation of D to bright excitons is strongly suppressed on the bare substrate
whilst enabled when the hBN/MoS/hBN heterostructure is integrated in a
nanobeam cavity. The relaxation enabled by the cavity is explained by the
phonon scattering enhanced by the cavity phononic effects. Our work reveals the
two-photon quantum interference with long-lived dark states and enables the
control through nanostructuring of the substrate. These results indicate the
great potential of dark excitons in 2D-material based nonlinear quantum
devices
Selective Exciton-Phonon-Phonon Coupling and Anharmonicity with Cavity Vibrational Phonons and MoS Lattice Phonons in Hybrid Nanobeam Cavities
We report selective coupling between neutral excitons X, vibrational
phonon modes of a freestanding nanobeam cavity and lattice phonons of a MoS
monolayer fully encapsulated by hBN. Our experimental findings demonstrate that
the cavity vibrational phonons selectively couple to neutral excitons (X),
and the coupling to negatively charged trion (X) being significantly
weaker. We establish this result by studying the lattice temperature induced
broadening of exciton linewidths, where the contribution from the X-cavity
phonon coupling is clearly observed while the X-cavity phonon coupling is
not. Furthermore, when the Raman modes of MoS lattice phonons A and
2LA are tuned into an outgoing resonance with exciton emissions, we observe the
X-cavity phonon-lattice phonon coupling which inherits the characteristics
rule the of X-cavity phonon coupling. As a result, X-induced Raman
scatterings are enhanced, while X-induced scatterings are suppressed,
revealed by the detuning-dependent Raman intensities and the ratio of
X/X emission intensities. The phonon anharmonicity from the coupling
between cavity vibrational phonons and MoS lattice phonons is further
demonstrated by the observed Raman linewidth. Such hybrid couplings between
materials and nanostructures enable the control of phonon-induced processes in
nanophotonic and nanomechanical systems incorporating 2D semiconductors
Charged exciton kinetics in monolayer MoSe near ferroelectric domain walls in periodically poled LiNbO
Monolayers of semiconducting transition metal dichalcogenides are a strongly
emergent platform for exploring quantum phenomena in condensed matter, building
novel opto-electronic devices with enhanced functionalities. Due to their
atomic thickness, their excitonic optical response is highly sensitive to their
dielectric environment. In this work, we explore the optical properties of
monolayer thick MoSe straddling domain wall boundaries in periodically
poled LiNbO. Spatially-resolved photoluminescence experiments reveal
spatial sorting of charge and photo-generated neutral and charged excitons
across the boundary. Our results reveal evidence for extremely large in-plane
electric fields of 3000\,kV/cm at the domain wall whose effect is manifested in
exciton dissociation and routing of free charges and trions toward oppositely
poled domains and a non-intuitive spatial intensity dependence. By modeling our
result using drift-diffusion and continuity equations, we obtain excellent
qualitative agreement with our observations and have explained the observed
spatial luminescence modulation using realistic material parameters.Comment: 29 pages, 6 figures, submetted materia
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