Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems 1,2 , and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr 2 RuO 4 for example 3-5 , leading to a pronounced peak in T c vs. strain whose origin is still under active debate 4,6,7 . Here, we develop a simple and compact method to apply large uniaxial pressures passively in restricted sample environments, and utilize this to study the evolution of the electronic structure of Sr 2 RuO 4 using angle-resolved photoemission. We directly visualize how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in T c 7 . Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr 2 RuO 4 . More generally, our novel experimental approach opens the door to future studies of straintuned phase transitions not only using photoemission, but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.The layered perovskite Sr 2 RuO 4 has been extensively studied both because of its celebrated unconventional superconductivity 5,8-11 and the accuracy with which its normal state properties can be measured 12-15 and analysed [16][17][18][19] . In spite of a quarter of a century of work, there is still no consensus on the symmetry of its superconducting order parameter, or the mechanism by which the superconductivity condenses 5 . This is a major unsolved problem because its electronic structure, which is relatively simple compared to that of many other unconventional superconductors, is now known in considerable detail and its metallic state is firmly established to be a Fermi liquid below approximately 30 K 13 . A full understanding of the Sr 2 RuO 4 problem is therefore a benchmark for the progress of the fields of strongly interacting systems and unconventional superconductivity.
We present a combined study from angle-resolved photoemission and density-functional theory calculations of the temperature-dependent electronic structure in the excitonic insulator candidate Ta2NiSe5. Our experimental measurements unambiguously establish the normal state as a semimetal with a significant band overlap of >100 meV. Our temperature-dependent measurements indicate how these low-energy states hybridise when cooling through the well-known 327 K phase transition in this system. From our calculations and polarisationdependent photoemission measurements, we demonstrate the importance of a loss of mirror symmetry in enabling the band hybridisation, driven by a shear-like structural distortion which reduces the crystal symmetry from orthorhombic to monoclinic. Our results thus point to the key role of the lattice distortion in enabling the phase transition of Ta2NiSe5.
Self-powered photodetectors operating in the UV–visible–NIR window made of environmentally friendly, earth abundant, and cheap materials are appealing systems to exploit natural solar radiation without external power sources. In this study, we propose a new p–n junction nanostructure, based on a ZnO–Co3O4 core–shell nanowire (NW) system, with a suitable electronic band structure and improved light absorption, charge transport, and charge collection, to build an efficient UV–visible–NIR p–n heterojunction photodetector. Ultrathin Co3O4 films (in the range 1–15 nm) were sputter-deposited on hydrothermally grown ZnO NW arrays. The effect of a thin layer of the Al2O3 buffer layer between ZnO and Co3O4 was investigated, which may inhibit charge recombination, boosting device performance. The photoresponse of the ZnO–Al2O3–Co3O4 system at zero bias is 6 times higher compared to that of ZnO–Co3O4. The responsivity (R) and specific detectivity (D*) of the best device were 21.80 mA W–1 and 4.12 × 1012 Jones, respectively. These results suggest a novel p–n junction structure to develop all-oxide UV–vis photodetectors based on stable, nontoxic, low-cost materials.
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