High-pressure material syntheses using shear strain for development of novel superconducting materials

Material synthesis and material manipulation using shear stress under compression have been conducted to enhance electronic functionalities of inorganic magnets and superconductors. Through the shear stress under compression, dislocation-like defects are inserted, which result in grain structure changes. During subsequent pressure release, lattice strains occur at the unit-cell level, such as anisotropic change in lattice parameters and/or lattice expansion, resulting in the creation of novel functionalities in electronic materials. Here, we demonstrate the possibility of stabilizing novel metastable materials using shear stress under high pressure through high-pressure torsion (HPT) processing. Metastable states created via HPT processing, which may be regarded as excited states, can be modified by simultaneous and/or successive heating. Regarding successful properties in the metastable states, high- T c superconductivity appears in single-element metals such as Re with lattice expansion accompanied by an increase in density-of-states at Fermi energy and Ba with residual high-pressure phases accompanied by strained unit cell, Al-Ti oxides characterized by Magnéli phases with unique valence states, and hyperoxia cuprates of La 2 CuO 4 with carrier control via oxygen defects. These features are different from those in high-quality materials obtained via the high-pressure synthesis using isotropic compression at high temperature.