Tc and resistivity variation induced by external bending strain in flexible film of strain-sensitive (La,Sr)2CuO4

Strain in materials changes their electronic structure, and the strain response realizes rich material properties and devices. Superconductivity under hydrostatic pressure and epitaxial strain suggests significant response to an external variable strain in a single sample, but this has not yet been demonstrated because the strain is usually a fixed parameter after sample fabrication. ${\left(\mathrm{La},\mathrm{Sr}\right)}_2\mathrm{Cu}{\mathrm{O}}_4$ films were fabricated on flexible metal substrates, and bending strain was applied to them to observe the critical temperature $\left(T_{\mathrm{c}}\right)$ and resistivity variation induced by strain. The compressive bending strain of −0.005 increased the $T_{\mathrm{c}}$ from 23.4 to 27.3 K. The magnitude of the $T_{\mathrm{c}}$ change by the bending strain is independent of the doping level and initial epitaxial strain. Furthermore, the irreversibility temperature was also improved by the compressive bending, and reasonable $T_{\mathrm{c}}$ variation with respect to the reversible strain was observed. $A$b initio density functional calculation for the mother compound $\mathrm{L}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_4$ clarified that the low-energy electronic structures are sensitive to the bending strain. While the carriers (holes) are preferentially injected into the in-plane orbitals of the $\mathrm{Cu}{\mathrm{O}}_2$ plane under the compressive strain, the tensile strain leads to the carrier injection into the perpendicular orbitals which is unfavorable to the superconductivity. The strain-sensitive high-$T_{\mathrm{c}}$ superconductor under the external strain highlights a new aspect for cuprate superconductors, which opens monitoring of the stress situation in the cryogenic systems such as superconducting magnet and liquid hydrogen container.