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1. The anomalous temperature dependent low energy electron diffraction intensity at epitaxial Sr ₃ Ir ₂ O ₇ thin film surfaces

   https://doi.org/10.1088/1361-648X/adbaa8  

   Adegbite, Peace Ikeoluwa; Subedi, Arjun; Zhang, Yuanyuan; Hong, Xia; Komesu, Takashi; Dowben, P A (March 2025, Journal of Physics: Condensed Matter)

   Abstract We report on the temperature dependent low energy electron diffraction (LEED) studies of 12 nm epitaxial Sr₃Ir₂O₇(001) thin films. The Debye temperature has been extracted from the temperature-dependence of LEED intensity at elevated temperatures and different electron kinetic energies. For the most surface sensitive LEED, obtained at the lowest electron kinetic energies, the extracted surface Debye temperature is 270 ± 22 K, which is much lower than the 488 ± 40 K Debye temperature obtained using higher electron kinetic energies. Surprisingly, the LEED diffraction intensity, at the lowest electron kinetic energies, increases rather than decreases, with increasing sample temperatures up to about 440 K. This anomalous behavior has been attributed to the reduction of the lattice vibrational amplitudes along the surface normal. This damping of the normal mode vibrations with increasing temperature results from the enhanced electronic screening via thermally activated carriers. This scenario is corroborated by the transport measurement, showing that Sr₃Ir₂O₇is a narrow band Mott insulator with a band gap of about 32 meV. We have identified criteria for finding anomalous scattering behavior in other transition metal oxide systems. 

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2. An interpretation for the components of 2p _3/2 core level x-ray photoelectron spectra of the cations in some inverse spinel oxides

   https://doi.org/10.1088/1361-648X/ad3271  

   Subedi, Arjun; Yang, Detian; Chin, Wai Kiat; Tamang, Binny; Sahoo, Sushrisangita; Yancey, Paul; Mahbub, Rifat; Shield, Jeffrey; Lai, Rebecca Y; Xu, Xiaoshan; et al (April 2024, Journal of Physics: Condensed Matter)

   Abstract In an effort to reconcile the various interpretations for the cation components of the 2p_3/2observed in x-ray photoelectron spectroscopy (XPS) of several spinel oxide materials, the XPS spectra of both spinel alloy nanoparticles and crystalline thin films are compared. We observed that different components of the 2p_3/2core level XPS spectra, of these inverse spinel thin films, are distinctly surface and bulk weighted, indicating surface-to-bulk core level shifts in the binding energies. Surface-to-bulk core level shifts in binding energies of Ni and Fe 2p_3/2core levels of NiFe₂O₄thin film are observed in angle-resolved XPS. The ratio between surface-weighted components and bulk-weighted components of the Ni and Fe core levels shows appreciable dependency on photoemission angle, with respect to surface normal. XPS showed that the ferrite nanoparticles Ni_xCo_1−xFe₂O₄(x= 0.2, 0.5, 0.8, 1) resemble the surface of the NiFe₂O₄thin film. Surface-to-bulk core level shifts are also observed in CoFe₂O₄and NiCo₂O₄thin films but not as significantly as in NiFe₂O₄thin film. Estimates of surface stoichiometry of some spinel oxide nanoparticles and thin films suggested that the apportionment between cationic species present could be farther from expectations for thin films as compared to what is seen with nanoparticles. 

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3. Extraordinary Thermoelectric Properties of Topological Surface States in Quantum‐Confined Cd ₃ As ₂ Thin Films

   https://doi.org/10.1002/adma.202311644  

   Ouyang, Wenkai; Lygo, Alexander C; Chen, Yubi; Zheng, Huiyuan; Vu, Dung; Wooten, Brandi L; Liang, Xichen; Heremans, Joseph P; Stemmer, Susanne; Liao, Bolin (July 2024, Advanced Materials)

   Abstract Topological insulators and semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport associated with the Fermi arc topological surface states on topological Dirac semimetals remains less explored. This work systematically examines thermoelectric transport in a series of topological Dirac semimetal Cd₃As₂thin films grown by molecular beam epitaxy. Surprisingly, significantly enhanced Seebeck effect and anomalous Nernst effect are found at cryogenic temperatures when the Cd₃As₂layer is thin. In particular, a peak Seebeck coefficient of nearly 500 µV K⁻¹and a corresponding thermoelectric power factor over 30 mW K⁻² m⁻¹are observed at 5 K in a 25‐nm‐thick sample. Combining angle‐dependent quantum oscillation analysis, magnetothermoelectric measurement, transport modeling, and first‐principles simulation, the contributions from bulk and surface conducting channels are isolated and the unusual thermoelectric properties are attributed to the topological surface states. The analysis showcases the rich thermoelectric transport physics in quantum‐confined topological Dirac semimetal thin films and suggests new routes to achieving high thermoelectric performance at cryogenic temperatures. 

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4. Probing electronic dead layers in homoepitaxial n -SrTiO ₃ (001) films

   https://doi.org/10.1063/5.0098500  

   Chambers, S. A.; Lee, D.; Yang, Z.; Huang, Y.; Samarakoon, W.; Zhou, H.; Sushko, P. V.; Truttmann, T. K.; Wangoh, L. W.; Lee, T.-L.; et al (July 2022, APL Materials)

   We combine state-of-the-art oxide epitaxial growth by hybrid molecular beam epitaxy with transport, x-ray photoemission, and surface diffraction, along with classical and first-principles quantum mechanical modeling to investigate the nuances of insulating layer formation in otherwise high-mobility homoepitaxial n-SrTiO 3 (001) films. Our analysis points to charge immobilization at the buried n-SrTiO 3 /undoped SrTiO 3 (001) interface as well as within the surface contamination layer resulting from air exposure as the drivers of electronic dead-layer formation. As Fermi level equilibration occurs at the surface and the buried interface, charge trapping reduces the sheet carrier density ( n 2 D ) and renders the n-STO film insulating if n 2 D falls below the critical value for the metal-to-insulator transition. 

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5. Gate‐Tunable Spin‐To‐Charge Conversion in Topological Insulator‐Magnetic Insulator Heterostructures at Room Temperature

   https://doi.org/10.1002/adfm.202501880  

   Sun, Wenxuan; Chen, Yequan; Xu, Ruijie; Zhuang, Wenzhuo; Wang, Di; Liu, Long; Song, Anke; Xing, Guozhong; Xu, Yongbing; Zhang, Rong; et al (March 2025, Advanced Functional Materials)

   Abstract Over the past decade, topological insulators have received enormous attention for their potential in energy‐efficient spin‐to‐charge conversion, enabled by strong spin‐orbit coupling and spin‐momentum locked surface states. Despite extensive research, the spin‐to‐charge conversion efficiency, usually characterized by the spin Hall angle (θ_SH), remains relatively low at room temperature. In this work, pulsed laser deposition is employed to fabricate high‐quality ternary topological insulator (Bi_0.1Sb_0.9)₂Te₃thin films on magnetic insulator Y₃Fe₅O₁₂. It is found that the value ofθ_SHreaches ≈0.76 at room temperature and increases to ≈0.9 as the Fermi level is tuned to cross topological surface states via electrical gating. These findings provide an innovative approach to tailoring the spin‐to‐charge conversion in topological insulators and pave the way for their applications in energy‐efficient spintronic devices. 

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