Fig. 2: Temperature dependence of resistance (R) under various pressures for orthorhombic La3Ni2O7-δ.
a and b Temperature-dependent resistance curves for the orthorhombic La3Ni2O7-δ under different pressures. The inset of (b) shows the gradual suppression of Tc by pressure above 20 GPa. c The R(T) curves for the orthorhombic La3Ni2O7-δ under various magnetic field at 20.1 GPa. The superconducting transition is gradually suppressed to a low temperature with increasing magnetic field. The inset clearly shows that the temperature of zero resistance is as high as 45 K, which is the highest temperature of zero resistance reported in the La3Ni2O7-δ single crystals so far. d The upper critical field for the orthorhombic La3Ni2O7-δ sample is shown in (c). The upper critical fields were fitted with different criteria using a 3D GL model. In the 3D GL model, the Hc-T relation for a superconductor follows the equation \({\mu }_{0}{H}_{c2}\left(T\right)={\mu }_{0}{H}_{c2}\left(0\right)\frac{1-{(\frac{T}{{T}_{C}})}^{2}}{1+{(\frac{T}{{T}_{C}})}^{2}}\) where \({H}_{c2}\) and \({T}_{c}\) are the upper critical field and superconducting temperature. As there is a jump in the resistance curves recorded at 0 T and 0.1 T in (c), the fitted upper critical field will be influenced by the criterion of the Tc defined. We show the fitting results of the upper critical field with different criteria of 10%, 50% and 90% RN. There is a gap region between the red and cyan dotted lines, which indicates the thermally activated flux flow16. The positive curvature close to Tc in the low magnetic field region indicates the multi-band characteristic in nickelate superconductors under pressure.
