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1. Observation of orbital selective charge transfer in a Fe/BaTiO3 interfacial two-dimensional electron gas

   https://doi.org/10.1103/PhysRevB.109.155106  

   Sohn, Byungmin; Marcaud, Guillaume; Shin, Yeongjae; Lee, Sangjae; Werner, Tyler; Yilmaz, Turgut; Yang, Jinming; Wei, Wenzheng; Fedorov, Alexei; Vescovo, Elio; et al (April 2024, Physical Review B)

   Two-dimensional electron gas (2DEG) states at oxide interfaces between two ferroic materials have been fertile ground to realize controllable multiferroicity. Here, we investigate the 2DEG states at the interface of ferroelectric BaTi⁢O3 and a magnetic layer of iron using angle-resolved photoemission spectroscopy. Orbital-selective charge transfer occurs on the surprisingly robust 2DEG. Based on first-principles calculations, we show how the interfacial hybridization can give rise to the unexpected charge transfer in the magnetic 2DEG. Our study reveals a close interplay on a 2DEG between magnetic and ferroelectric interfaces, which sheds light on future design principles of multiferroic 2DEG states. 

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2. Theory of Coulomb driven nematicity in a multivalley two-dimensional electron gas

   https://doi.org/10.1103/PhysRevB.111.155135  

   Calvera, Vladimir; Valenti, Agnes; Huber, Sebastian D; Berg, Erez; Kivelson, Steven A (April 2025, Physical Review B)

   The properties of a two-dimensional electron gas (2DEG) in a semiconductor host with two valleys related by an underlying C4 rotational symmetry are studied using Hartree-Fock and various other many-body approaches. A familiar artifact of the HF approach is a degeneracy between the valley-polarized-"Ising nematic"-and spin-polarized-ferromagnetic-phases, which is inconsistent with recent variational Monte Carlo (VMC) results. Correlation effects, computed either within the random phase approximation (RPA) or the T-matrix approximation, enhance the valley susceptibility relative to the spin susceptibility. Extrapolating the results to finite interaction strength, we find a direct first-order transition from a symmetry-unbroken state to a spinunpolarized Ising nematic fluid with full valley polarization, in qualitative agreement with VMC. The RPA results are also reminiscent of experiments on the corresponding 2DEG in AlAs heterostructures. 

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3. Interfacial charge transfer and its impact on transport properties of LaNiO ₃ /LaFeO ₃ superlattices

   https://doi.org/10.1126/sciadv.adq6687  

   Wang, Le; Yang, Zhifei; Koirala, Krishna Prasad; Bowden, Mark E; Freeland, John W; Sushko, Peter V; Kuo, Cheng-Tai; Chambers, Scott A; Wang, Chongmin; Jalan, Bharat; et al (December 2024, Science Advances)

   Charge transfer or redistribution at oxide heterointerfaces is a critical phenomenon, often leading to remarkable properties such as two-dimensional electron gas and interfacial ferromagnetism. Despite studies on LaNiO₃/LaFeO₃superlattices and heterostructures, the direction and magnitude of the charge transfer remain debated, with some suggesting no charge transfer due to the high stability of Fe³⁺(3d⁵). Here, we synthesized a series of epitaxial LaNiO₃/LaFeO₃superlattices and demonstrated partial (up to ~0.5 e⁻/interface unit cell) charge transfer from Fe to Ni near the interface, supported by density functional theory simulations and spectroscopic evidence of changes in Ni and Fe oxidation states. The electron transfer from LaFeO₃to LaNiO₃and the subsequent rearrangement of the Fe 3d band create an unexpected metallic ground state within the LaFeO₃layer, strongly influencing the in-plane transport properties across the superlattice. Moreover, we establish a direct correlation between interfacial charge transfer and in-plane electrical transport properties, providing insights for designing functional oxide heterostructures with emerging properties. 

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4. MOCVD-grown β-Ga2O3 as a Gate Dielectric on AlGaN/GaN-Based Heterojunction Field Effect Transistor

   https://doi.org/10.3390/cryst13020231  

   Hasan, Samiul; Jewel, Mohi Uddin; Crittenden, Scott R.; Lee, Dongkyu; Avrutin, Vitaliy; Özgür, Ümit; Morkoç, Hadis; Ahmad, Iftikhar (February 2023, Crystals)

   We report the electrical properties of Al0.3Ga0.7N/GaN heterojunction field effect transistor (HFET) structures with a Ga2O3 passivation layer grown by metal–organic chemical vapor deposition (MOCVD). In this study, three different thicknesses of β-Ga2O3 dielectric layers were grown on Al0.3Ga0.7N/GaN structures leading to metal-oxide-semiconductor-HFET or MOSHFET structures. X-ray diffraction (XRD) showed the (2¯01) orientation peaks of β-Ga2O3 in the device structure. The van der Pauw and Hall measurements yield the electron density of ~ 4 × 1018 cm−3 and mobility of ~770 cm2V−1s−1 in the 2-dimensional electron gas (2DEG) channel at room temperature. Capacitance–voltage (C-V) measurement for the on-state 2DEG density for the MOSHFET structure was found to be of the order of ~1.5 × 1013 cm−2. The thickness of the Ga2O3 layer was inversely related to the threshold voltage and the on-state capacitance. The interface charge density between the oxide and Al0.3Ga0.7N barrier layer was found to be of the order of ~1012 cm2eV−1. A significant reduction in leakage current from ~10−4 A/cm2 for HFET to ~10−6 A/cm2 for MOSHFET was observed well beyond pinch-off in the off-stage at -20 V applied gate voltage. The annealing at 900° C of the MOSHFET structures revealed that the Ga2O3 layer was thermally stable at high temperatures resulting in insignificant threshold voltage shifts for annealed samples with respect to as-deposited (unannealed) structures. Our results show that the MOCVD-gown Ga2O3 dielectric layers can be a strong candidate for stable high-power devices. 

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5. Large bi-axial tensile strain effect in epitaxial BiFeO3 film grown on single crystal PrScO3

   https://doi.org/10.1038/s41598-023-45980-w  

   Bae, In-Tae; Lingley, Zachary R.; Foran, Brendan J.; Adams, Paul M.; Paik, Hanjong (December 2023, Scientific Reports)

   Abstract A BiFeO₃film is grown epitaxially on a PrScO₃single crystal substrate which imparts ~ 1.45% of biaxial tensile strain to BiFeO₃resulting from lattice misfit. The biaxial tensile strain effect on BiFeO₃is investigated in terms of crystal structure, Poisson ratio, and ferroelectric domain structure. Lattice resolution scanning transmission electron microscopy, precession electron diffraction, and X-ray diffraction results clearly show that in-plane interplanar distance of BiFeO₃is the same as that of PrScO₃with no sign of misfit dislocations, indicating that the biaxial tensile strain caused by lattice mismatch between BiFeO₃and PrScO₃are stored as elastic energy within BiFeO₃film. Nano-beam electron diffraction patterns compared with structure factor calculation found that the BiFeO₃maintains rhombohedral symmetry, i.e., space group ofR3c. The pattern analysis also revealed two crystallographically distinguishable domains. Their relations with ferroelectric domain structures in terms of size and spontaneous polarization orientations within the domains are further understood using four-dimensional scanning transmission electron microscopy technique. 

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