61 research outputs found
Probing correlations in the exciton landscape of a moir\'e heterostructure
Excitons are two-particle correlated bound states that are formed due to
Coulomb interaction between single-particle holes and electrons. In the
solid-state, cooperative interactions with surrounding quasiparticles can
strongly tailor the exciton properties and potentially even create new
correlated states of matter. It is thus highly desirable to access such
cooperative and correlated exciton behavior on a fundamental level. Here, we
find that the ultrafast transfer of an exciton's hole across a type-II
band-aligned moir\'e heterostructure leads to a surprising sub-200-fs upshift
of the single-particle energy of the electron being photoemitted from the
two-particle exciton state. While energy relaxation usually leads to an
energetic downshift of the spectroscopic signature, we show that this unusual
upshift is a clear fingerprint of the correlated interactions of the electron
and hole parts of the exciton quasiparticle. In this way, time-resolved
photoelectron spectroscopy is straightforwardly established as a powerful
method to access exciton correlations and cooperative behavior in
two-dimensional quantum materials. Our work highlights this new capability and
motivates the future study of optically inaccessible correlated excitonic and
electronic states in moir\'e heterostructures.Comment: 32 pages, 4 main figures, 5 supplemental figure
Formation of moir\ue9 interlayer excitons in space and time
Moir\ue9 superlattices in atomically thin van der Waals heterostructures hold great promise for extended control of electronic and valleytronic lifetimes1-7, the confinement of excitons in artificial moir\ue9 lattices8-13 and the formation of exotic quantum phases14-18. Such moir\ue9-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure19,20. To exploit the full potential of correlated moir\ue9 and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moir\ue9 interlayer excitons. First, we elucidate that interlayer excitons are dominantly formed through femtosecond exciton-phonon scattering and subsequent charge transfer\ua0at the interlayer-hybridized Ξ£ valleys. Second, we show that interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moir\ue9 modification. Third, we reconstruct the wavefunction distribution of the electronic part of the exciton and compare the size with the real-space moir\ue9 superlattice. Our work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moir\ue9 and exciton physics for the future realization of exotic quantum phases of matter
Ultrafast nano-imaging of dark excitons
The role and impact of spatial heterogeneity in two-dimensional quantum
materials represents one of the major research quests regarding the future
application of these materials in optoelectronics and quantum information
science. In the case of transition-metal dichalcogenide heterostructures, in
particular, direct access to heterogeneities in the dark-exciton landscape with
nanometer spatial and ultrafast time resolution is highly desired, but remains
largely elusive. Here, we introduce ultrafast dark field momentum microscopy to
spatio-temporally resolve dark exciton formation dynamics in a twisted
WSe/MoS heterostructure with 55 femtosecond time- and 500~nm spatial
resolution. This allows us to directly map spatial heterogeneity in the
electronic and excitonic structure, and to correlate these with the dark
exciton formation and relaxation dynamics. The benefits of simultaneous
ultrafast nanoscale dark-field momentum microscopy and spectroscopy is
groundbreaking for the present study, and opens the door to new types of
experiments with unprecedented spectroscopic and spatiotemporal capabilities.Comment: 39 pages, 4 main figures, 8 supplemental figure
Ultrafast dynamics of bright and dark excitons in monolayer WSe and heterobilayer WSe/MoS
The energy landscape of optical excitations in mono- and few-layer transition
metal dichalcogenides (TMDs) is dominated by optically bright and dark
excitons. These excitons can be fully localized within a single TMD layer, or
the electron- and the hole-component of the exciton can be charge-separated
over multiple TMD layers. Such intra- or interlayer excitons have been
characterized in detail using all-optical spectroscopies, and, more recently,
photoemission spectroscopy. In addition, there are so-called hybrid excitons
whose electron- and/or hole-component are delocalized over two or more TMD
layers, and therefore provide a promising pathway to mediate charge-transfer
processes across the TMD interface. Hence, an in-situ characterization of their
energy landscape and dynamics is of vital interest. In this work, using
femtosecond momentum microscopy combined with many-particle modeling, we
quantitatively compare the dynamics of momentum-indirect intralayer excitons in
monolayer WSe with the dynamics of momentum-indirect hybrid excitons in
heterobilayer WSe/MoS, and draw three key conclusions: First, we find
that the energy of hybrid excitons is reduced when compared to excitons with
pure intralayer character. Second, we show that the momentum-indirect
intralayer and hybrid excitons are formed via exciton-phonon scattering from
optically excited bright excitons. And third, we demonstrate that the
efficiency for phonon absorption and emission processes in the mono- and the
heterobilayer is strongly dependent on the energy alignment of the intralayer
and hybrid excitons with respect to the optically excited bright exciton.
Overall, our work provides microscopic insights into exciton dynamics in TMD
mono- and bilayers.Comment: 27 pages, 5 figure
Epithelial Tissues Have Varying Degrees of Susceptibility to KrasG12D-Initiated Tumorigenesis in a Mouse Model
Activating mutations in the Kras gene are commonly found in some but not all epithelial cancers. In order to understand the susceptibility of different epithelial tissues to Kras-induced tumorigenesis, we introduced one of the most common Kras mutations, KrasG12D, broadly in epithelial tissues. We used a mouse model in which the G12D mutation is placed in the endogenous Kras locus controlled by inducible, Cre-mediated recombination in tissues expressing cytokeratin 19 including the oral cavity, GI tract, lungs, and ducts of the liver, kidney, and the pancreas. Introduction of the KrasG12D mutation in adult mouse tissues led to neoplastic changes in some but not all of these tissues. Notably, many hyperplasias, metaplasias and adenomas were observed in the oral cavity, stomach, colon and lungs, suggesting that exposure to products of the outside environment promotes KrasG12D-initiated tumorigenesis. However, environmental exposure did not consistently correlate with tumor formation, such as in the small intestine, suggesting that there are also intrinsic differences in susceptibility to Kras activation. The pancreas developed small numbers of mucinous metaplasias with characteristics of early stage pancreatic intraepithelial neoplasms (PanINs), supporting the hypothesis that pancreatic ducts have the potential to give rise pancreatic cancer
gp130-mediated Stat3 activation in enterocytes regulates cell survival and cell-cycle progression during colitis-associated tumorigenesis
Although gastrointestinal cancers are frequently associated with chronic inflammation, the underlying molecular links have not been comprehensively deciphered. Using loss- and gain-of-function mice in a colitis-associated cancer model, we establish here a link comprising the gp130/Stat3 transcription factor signaling axis. Mutagen-induced tumor growth and multiplicity are reduced following intestinal epithelial cell (IEC)-specific Stat3 ablation, while its hyperactivation promotes tumor incidence and growth. Conversely, IEC-specific Stat3 deficiency enhances susceptibility to chemically induced epithelial damage and subsequent mucosal inflammation, while excessive Stat3 activation confers resistance to colitis. Stat3 has the capacity to mediate IL-6- and IL-11-dependent IEC survival and to promote proliferation through G1 and G2/M cell-cycle progression as the common tumor cell-autonomous mechanism that bridges chronic inflammation to tumor promotion
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