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  1. Boron arsenide (BAs) is a covalent semiconductor with a theoretical intrinsic thermal conductivity approaching 1300 W/m K. The existence of defects not only limits the thermal conductivity of BAs significantly but also changes its pressure-dependent thermal transport behavior. Using both picosecond transient thermoreflectance and femtosecond time-domain thermoreflectance techniques, we observed a non-monotonic dependence of thermal conductivity on pressure. This trend is not caused by the pressure-modulated phonon–phonon scattering, which was predicted to only change the thermal conductivity by 10%–20%, but a result of several competing effects, including defect–phonon scattering and modification of structural defects under high pressure. Our findings reveal the complexity of the defect-modulated thermal behavior under pressure.

     
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  4. Abstract

    Two distinct stacking orders in ReS2are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one‐unit cell displacement along theaaxis. First‐principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm−1for AA stacking, and 20 cm−1for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump–probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular tobaxis. The findings underscore the stacking‐order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

     
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  5. Abstract

    The intensity‐scan (I‐scan) technique to study the polarization‐dependent, nonlinear processes in exfoliated bulk ReS2is utilized. The polarization‐dependent reflection and transmission of ReS2, from which the absorption coefficients are extracted using the transfer matrix method, are measured. Absorption coefficients under high laser peak power show a transition from saturable absorption (SA) to reverse saturable absorption when rotating the laser polarization with respect to theb‐axis. It is found that SA and excited‐state absorption (ESA) contribute to the nonlinear optical processes. Both the SA and ESA show strong dependence on the polarization angle, which is attributed to the anisotropic optical transition probability and electronic band structure in ReS2. The anisotropic nonlinear optical properties of ReS2may find applications as saturable absorbers in lasers and optical modulators.

     
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