Critical role of elemental hydrogen for superconductivity in nickel oxide superconductors
H is the lightest element in the universe,and detecting its presence and distribution in materials is challenging because most of the advanced characterization techniques are not sensitive enough to detect it.However,H plays an important role in materials science,as it can act as an"invisible key"that unlocks unusual physical properties in different materials.For instance,H can induce metal-insulator transition,ferromagnetism,superconductivity,and other phenomena in various oxides,hydrides,and intermetallic compounds.In 2019,the discovery of nickel oxide superconducting materials(nickel-based superconductors)based on the infinite-layer structure has encouraged researchers to find more similar high-temperature superconducting materials.Currently,nickel oxide superconductors are mainly synthesized via the topotactic reduction of metal hydrides,which is a process that removes oxygen atoms from the metal oxide lattice and replaces them with H atoms.The challenge now is to confirm whether the CaH2-assisted topotactic reduction in nickel-based superconductors will introduce"extra"H elements in the films and affect their superconducting state.This is an important question because H can remarkably affect the charge carrier density,band structure,electron-phonon coupling,and disorder levels of the materials,which are all related to superconductivity.Thus,this article focuses on reviewing the influence rules of H on the physical properties of materials.Moreover,recent traces of the existence of H in nickel oxides detected by various techniques,such as X-ray absorption spectroscopy and resonant inelastic X-ray scattering,are highlighted,and the atomic occupation positions of H inside the materials are revealed by first-principles calculations and neutron diffraction experiments.Also,the influence mechanism of H on the superconductivity of nickel-based materials is analyzed in detail by considering both intrinsic and extrinsic factors.This paper further discusses future research directions in this field,such as optimization approaches of synthesis conditions and film quality,more accurate and reliable measurements of the H-related physical parameters,and design of novel materials with enhanced superconductivity by using H as a tuning knob.
H elementstopological reductionnickel oxidesuperconductivity
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