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Quantum materials (QMs) with strong correlation and nontrivial topology are indispensable to next-generation information and computing technologies. Exploitation of topological band structure is an ideal starting point to realize correlated topological QMs. Here, we report that strain-induced symmetry modification in correlated oxide SrNbO 3 thin films creates an emerging topological band structure. Dirac electrons in strained SrNbO 3 films reveal ultrahigh mobility (μ max ≈ 100,000 cm 2 /Vs), exceptionally small effective mass ( m * ~ 0.04 m e ), and nonzero Berry phase. Strained SrNbO 3 films reach the extreme quantum limit, exhibiting a sign of fractional occupation of Landau levels and giant mass enhancement. Our results suggest that symmetry-modified SrNbO 3 is a rare example of correlated oxide Dirac semimetals, in which strong correlation of Dirac electrons leads to the realization of a novel correlated topological QM. 

Above‐bandgap femtosecond optical excitation of a ferroelectric/dielectric BaTiO₃/CaTiO₃superlattice leads to structural responses that are a consequence of the screening of the strong electrostatic coupling between the component layers. Time‐resolved X‐ray free‐electron laser diffraction shows that the structural response to optical excitation includes a net lattice expansion of the superlattice consistent with depolarization‐field screening driven by the photoexcited charge carriers. The depolarization‐field‐screening‐driven expansion is separate from a photoacoustic pulse launched from the bottom electrode on which the superlattice is epitaxially grown. The distribution of diffracted intensity of superlattice X‐ray reflections indicates that the depolarization‐field‐screening‐induced strain includes a photoinduced expansion in the ferroelectric BaTiO₃and a contraction in CaTiO₃. The magnitude of expansion in BaTiO₃layers is larger than the contraction in CaTiO₃. The difference in the magnitude of depolarization‐field‐screening‐driven strain in the BaTiO₃and CaTiO₃components can arise from the contribution of the oxygen octahedral rotation patterns at the BaTiO₃/CaTiO₃interfaces to the polarization of CaTiO₃. The depolarization‐field‐screening‐driven polarization reduction in the CaTiO₃layers points to a new direction for the manipulation of polarization in the component layers of a strongly coupled ferroelectric/dielectric superlattice.

 

Distinct properties of multiple phases of vanadium oxide (VO_x) render this material family attractive for advanced electronic devices, catalysis, and energy storage. In this work, phase boundaries of VO_xare crossed and distinct electronic properties are obtained by electrochemically tuning the oxygen content of VO_xthin films under a wide range of temperatures. Reversible phase transitions between two adjacent VO_xphases, VO₂and V₂O₅, are obtained. Cathodic biases trigger the phase transition from V₂O₅to VO₂, accompanied by disappearance of the wide band gap. The transformed phase is stable upon removal of the bias while reversible upon reversal of the electrochemical bias. The kinetics of the phase transition is monitored by tracking the time‐dependent response of the X‐ray absorption peaks upon the application of a sinusoidal electrical bias. The electrochemically controllable phase transition between VO₂and V₂O₅demonstrates the ability to induce major changes in the electronic properties of VO_xby spanning multiple structural phases. This concept is transferable to other multiphase oxides for electronic, magnetic, or electrochemical applications.