Solving mystery with the Meissner state in La3Ni2O7-δ

Recently, zero resistance state in highly compressed La3Ni2O7-δ has been observed. However, all attempts of many research groups to detect the Meissner state in the La3Ni2O7-δ have been failed. To explain this puzzle, an exotic superconducting state (for instance, filamentary superconductivity) in the La3Ni2O7-δ has been supposed. Here, I extracted temperature dependent self-field critical current, Ic(sf,T), dataset from current-voltage curves and performed the Ic(sf,T) analysis. As a result, I found that highly compressed La3Ni2O7-δ to exhibits d-wave superconductivity with the gap-to-transition temperature ratio 2Δ(0)/(kBTc) = =4.0 ± 0.3, a very large ground state London penetration depth, λ(0, P = 16.6 GPa) = 6.0 μm, and a very high Ginzburg-Landau parameter k(0, P = 16.6 GPa) = 1500. This implies that the ground state lower critical field Bc1(0, P = 16.6 GPa) = 34 μT is of the same order as the Earth’s magnetic field. Based on this, to detect the Meissner state in the La3Ni2O7-δ becomes a very challenging task. I can hypothesize that the magnetic flux trap effect recently proposed to eliminate the diamond anvil cell (DAC) background in experiments on magnetic properties of the superconducting hydrides can also apply in studies of magnetic properties in the La3Ni2O7-δ superconductor.