562 research outputs found
Impact of electrode density of states on transport through pyridine-linked single molecule junctions
We study the impact of electrode band structure on transport through
single-molecule junctions by measuring the conductance of pyridine-based
molecules using Ag and Au electrodes. Our experiments are carried out using the
scanning tunneling microscope based break-junction technique and are supported
by density functional theory based calculations. We find from both experiments
and calculations that the coupling of the dominant transport orbital to the
metal is stronger for Au-based junctions when compared with Ag-based junctions.
We attribute this difference to relativistic effects, which results in an
enhanced density of d-states at the Fermi energy for Au compared with Ag. We
further show that the alignment of the conducting orbital relative to the Fermi
level does not follow the work function difference between two metals and is
different for conjugated and saturated systems. We thus demonstrate that the
details of the molecular level alignment and electronic coupling in
metal-organic interfaces do not follow simple rules, but are rather the
consequence of subtle local interactions
Transport Study of Charge Carrier Scattering in Monolayer WSe
Employing flux-grown single crystal WSe, we report charge carrier
scattering behaviors measured in -BN encapsulated monolayer field effect
transistors. We perform quantum transport measurements across various hole
densities and temperatures and observe a non-monotonic change of transport
mobility as a function of hole density in the degenerately doped sample.
This unusual behavior can be explained by energy dependent scattering amplitude
of strong defects calculated using the T-matrix approximation. Utilizing long
mean-free path (500 nm), we demonstrate the high quality of our electronic
devices by showing quantized conductance steps from an
electrostatically-defined quantum point contact. Our results show the potential
for creating ultra-high quality quantum optoelectronic devices based on
atomically thin semiconductors.Comment: 6 pages, 4 figure
Thermodynamic behavior of correlated electron-hole fluids in van der Waals heterostructures
Coupled two-dimensional electron-hole bilayers provide a unique platform to
study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons
and holes in spatially separated layers can bind to form interlayer excitons,
composite Bosons expected to support high-temperature exciton superfluids. The
interlayer excitons can also interact strongly with excess charge carriers when
electron and hole densities are unequal. Here, we use optical spectroscopy to
quantitatively probe the local thermodynamic properties of strongly correlated
electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe a
discontinuity in the electron and hole chemical potentials at matched electron
and hole densities, a definitive signature of an excitonic insulator ground
state. The excitonic insulator is stable up to a Mott density of ~ and has a thermal ionization temperature of ~70 K.
The density dependence of the electron, hole, and exciton chemical potentials
reveals strong correlation effects across the phase diagram. Compared with a
non-interacting uniform charge distribution, the correlation effects lead to
significant attractive exciton-exciton and exciton-charge interactions in the
electron-hole fluid. Our work highlights the unique quantum behavior that can
emerge in strongly correlated electron-hole systems
Controlled Interlayer Exciton Ionization in an Electrostatic Trap in Atomically Thin Heterostructures
Atomically thin semiconductor heterostructures provide a two-dimensional (2D)
device platform for creating high densities of cold, controllable excitons.
Interlayer excitons (IEs), bound electrons and holes localized to separate 2D
quantum well layers, have permanent out-of-plane dipole moments and long
lifetimes, allowing their spatial distribution to be tuned on demand. Here, we
employ electrostatic gates to trap IEs and control their density. By
electrically modulating the IE Stark shift, electron-hole pair concentrations
above cm can be achieved. At this high IE density, we
observe an exponentially increasing linewidth broadening indicative of an IE
ionization transition, independent of the trap depth. This runaway threshold
remains constant at low temperatures, but increases above 20 K, consistent with
the quantum dissociation of a degenerate IE gas. Our demonstration of the IE
ionization in a tunable electrostatic trap represents an important step towards
the realization of dipolar exciton condensates in solid-state optoelectronic
devices.Comment: 14 pages, 4 main figures, 1 extended data figur
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
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