Transition-metal dichalcogenide monolayers and heterostructures are highly tunable material systems that provide excellent models for physical phenomena at the two-dimensional (2D) limit. While most studies to date have focused on electrons and electron–hole pairs, phonons also play essential roles. Here, we apply ultrafast electron diffraction and diffuse scattering to directly quantify, with time and momentum resolution, electron–phonon coupling (EPC) in monolayer molybdenum disulfide and phonon transport from the monolayer to a silicon nitride substrate. Optically generated hot carriers result in a profoundly anisotropic distribution of phonons in the monolayer within ∼5 ps. A quantitative comparison with ab initio ultrafast dynamics simulations reveals the essential role of dielectric screening in weakening EPC. Thermal transport from the monolayer to the substrate occurs with the phonon system far from equilibrium. While screening in 2D is known to strongly affect equilibrium properties, our findings extend this understanding to the dynamic regime.

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Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity of Humboldt Bay, California. During these surveys, a band of enhanced acoustic backscatter was observed oVshore from the bay entrance, approximately parallel to the isobaths, in water depths ranging from 16± 24 m. In order to assess the cause of the increase in backscatter levels, a more comprehensive study was conducted in August and September 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwater video recording. New observations indicated that a dense population of sand dollars (Dendraster excentricus) coincided with the enhanced backscatter band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the band shifted eastward by 160 m, possibly resulting from a change in the biological or physical factors which inXuence the location and breadth of sand dollars. 135We wish to thank our colleagues, P. Ratta, M. Forguson, and J. T. Walsh, who helped to develop and carry out this project. We are very grateful to Dr. J. E. Pequegnat for his guidance at every stage, Dr. M. A. de Angelis for the invaluable use of the video equipment, and the captain and crew of the R/V Coral Sea for their assistance during the cruises. Lastly, we are indebted to Humboldt State University for providing the facilities and support necessary to complete this project.The relationship between high sand dollar abundance and enhanced acoustic backscatter was further veriWed in the nearshore region oV Samoa Beach California, where a dense, banded population of sand dollars was previously observed. Video footage conWrmed the presence of a band of sand dollars, also nominally parallel to the isobaths, in water depths of 8± 15 m. A band of enhanced backscatter coincided with the dense sand dollar population. The identiWcation of dense aggregations of sand dollars through enhanced acoustic backscatter could lead to the use of acoustic techniques to study sand dollar distributions and abundance .

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