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1. Temperature-dependent optical properties of monocrystalline CaF ₂ , BaF ₂ , and MgF ₂

   https://doi.org/10.1364/OME.496246  

   Zheng, Qifan; Wang, Xinchao; Thompson, Dakotah (July 2023, Optical Materials Express)

   CaF₂, BaF₂, and MgF₂are low-index, infrared-transparent materials that are extensively used in optical systems. Despite their technological importance, a systematic investigation into the temperature dependence of their optical properties is lacking. In this study, spectroscopic ellipsometry was used to obtain the refractive index of monocrystalline CaF₂, BaF₂, and MgF₂for wavelengths between 220 nm and 1700 nm, and for temperatures between 21 °C and 368 °C. The raw ellipsometric data was fit to a Sellmeier model with temperature-dependent oscillator terms to extract the real part of the refractive index of each material. The refractive index of CaF₂and BaF₂was observed to decrease linearly with increasing temperature, which can be largely attributed to a reduction in the mass density due to thermal expansion. In contrast, the refractive index of MgF₂was found to vary nonlinearly with temperature, which suggests competing effects from the material’s electronic polarizability. The temperature-dependent refractive index data reported here provide a finely-resolved mapping of the thermo-optic coefficient for these three materials, which could inform the development of optical devices operating at elevated or unsteady temperatures. 

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2. Axis-Dependent Conduction Polarity in WSi ₂ Single Crystals

   https://doi.org/10.1021/acs.chemmater.3c00183  

   Koster, Karl G.; Deng, Ziling; Moore, Curtis E.; Heremans, Joseph P.; Windl, Wolfgang; Goldberger, Joshua E. (June 2023, Chemistry of Materials)
3. Composition‐Dependent Ferroelectricity of LuFeO ₃ Orthoferrite Thin Films

   https://doi.org/10.1002/aelm.202300059  

   Cho, Eunsoo; Klyukin, Konstantin; Su, Tingyu; Kaczmarek, Allison; Ross, Caroline_A (May 2023, Advanced Electronic Materials)

   Abstract This work characterizes the structural, magnetic, and ferroelectric properties of epitaxial LuFeO₃orthoferrite thin films with different Lu/Fe ratios. LuFeO₃thin films are grown by pulsed laser deposition on SrTiO₃substrates with Lu/Fe ratio ranging from 0.6 to 1.5. LuFeO₃is antiferromagnetic with a weak canted moment perpendicular to the film plane. Piezoresponse force microscopy imaging and switching spectroscopy reveal room temperature ferroelectricity in Lu‐rich and Fe‐rich films, whereas the stoichiometric film shows little polarization. Ferroelectricity in Lu‐rich films is present for a range of deposition conditions and crystallographic orientations. Positive‐up‐negative‐down ferroelectric measurements on a Lu‐rich film yield ≈13 µC cm⁻²of switchable polarization, although the film also shows electrical leakage. The ferroelectric response is attributed to antisite defects analogous to that of Y‐rich YFeO₃, yielding multiferroicity via defect engineering in a rare earth orthoferrite. 

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4. Layer and material-type dependent photoresponse in WSe ₂ /WS ₂ vertical heterostructures

   https://doi.org/10.1088/2053-1583/ac3c9c  

   Fu, ZhuangEn; Hill, Josh W; Parkinson, Bruce; Hill, Caleb M; Tian, Jifa (December 2021, 2D Materials)

   Abstract Transition metal dichalcogenide (TMD) heterostructures are promising for a variety of applications in photovoltaics and photosensing. Successfully exploiting these heterostructures will require an understanding of their layer-dependent electronic structures. However, there is no experimental data demonstrating the layer-number dependence of photovoltaic effects (PVEs) in vertical TMD heterojunctions. Here, by combining scanning electrochemical cell microscopy (SECCM) with optical probes, we report the first layer-dependence of photocurrents in WSe 2 /WS 2 vertical heterostructures as well as in pristine WS 2 and WSe 2 layers. For WS 2 , we find that photocurrents increase with increasing layer thickness, whereas for WSe 2 the layer dependence is more complex and depends on both the layer number and applied bias ( V b ). We further find that photocurrents in the WSe 2 /WS 2 heterostructures exhibit anomalous layer and material-type dependent behaviors. Our results advance the understanding of photoresponse in atomically thin WSe 2 /WS 2 heterostructures and pave the way to novel nanoelectronic and optoelectronic devices. 

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5. Temperature dependent performance of ITO Schottky contacts on β-Ga ₂ O ₃

   https://doi.org/10.1116/6.0001211  

   Xia, Xinyi; Xian, Minghan; Fares, Chaker; Ren, Fan; Tadjer, Marko; Pearton, Stephen J. (September 2021, Journal of Vacuum Science & Technology A)