APS Practice Talks for New Orleans March Meeting

Irreversible Matrix Remodeling Between Cells and Cellular Interactions with Collagen Bundle
Jihan Kim
When cells are surrounded by complex environment, they continuously probe and interact with it by applying
cellular traction forces. As cells apply traction forces, they can sense rigidity of their local environment and remodel
the matrix microstructure simultaneously. Previous study shows that single human carcinoma cell (MDA-MB-231)
remodeled its surrounding extracellular matrix (ECM) and the remodeled matrix was reversible. However, ECM
microstructural change between a pair of cells is significantly different from a single cell case. Highly densified
collagen bundle was formed between a pair of cells. In this study, we examined microstructure of collagen bundle
and cellular interactions along the bundle using quantitative confocal microscopy. The result shows that
irreversibility of collagen bundle is correlated with stress dwelling time. The bundle becomes more permanent as
cells apply traction force longer. Highly aligned and densified bundle become more rigid than surrounding ECM.
As a result, cell forms more actin fibers, which is a key element for cell traction force, towards the bundle direction
than other directions. Due to the actin dynamics along the bundle, cell pairs tend to migrate toward each other.
Altogether this study provides insights of cell-cell interactions within ECM and their responses as a direction of
migration.

Binding Energy of Interlayer Excitons in Twisted Bilayer Graphene
Hiral Patel

When two sheets of graphene are stacked at off axis angle, 2p orbitals hybridize, giving angletunable absorption resonances. By comparing the ultrafast intra-band and multi-photon transient absorption spectra, our results agree best with a recent theoretical simulations which predict that overlapping interlayer 2p orbitals interfere deconstructively to produce symmetrizedbound exciton states that are decoupled from lower lying continuum states. We further map out the excited state manifold of exciton fine states using 2-photon photoluminescence microscopy. Our spectral analysis suggests that the observed photoluminescence emission from a bright exciton state is thermally populated by a lower-lying, long lived dark-exciton state. For this dark state, both our ultrafast transient absorption and two-photon photoluminescence studiessupport a large binding energy of the order of ~0.6 eV. We believe twisted bilayer graphene isthe first 2D metallic material that can form stable, strongly bound excitons upon resonant excitation. Such strong excitonic effects coupled with enhanced hot carrier lifetimes suggest possible new routes for hot carrier extraction from an otherwise predictable 2D metallic system.

Ultrafast Resolution of Photocurrent Generation Bottlenecks in Stacked van der Waals Materials
Kyle Vogt
Combining E-Field dependent ultrafast photocurrent and transient absorption microscopy, we determine the fundamental electron extraction rates that determine photocurrent efficiency in stacked WSe2 devices. We find that both measurement techniques yield the same rate limited ultrafast time constant of 87 ps, associated with exciton dissociation and electron escape. Using the corresponding the recombination rates, we can calculate the upper bound of IQE in our device to be ∼51% which agrees with our directly measured ‘on-chip’ photoefficiency.