Superconducting correlations above $T_c$ in the pseudogap state of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ cuprates revealed by angular-dependent magnetotunneling

We present an angular-dependent magnetotunneling technique, which facilitates unambiguous separation of superconducting (supporting circulating screening currents) and nonsuperconducting (not supporting screening currents) contributions to the pseudogap phenomenon in layered ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ cuprates. Our data indicate persistence of superconducting correlations at temperatures up to $1.5T_c$ in a form of both phase and amplitude fluctuations of the superconducting order parameter. However, despite a profound fluctuations region, only a small fraction of the pseudogap spectrum is caused by superconducting correlations, while the dominating part comes from a competing nonsuperconducting order, which does not support circulating orbital currents.

Figure 1

(a) Sketch of a sample with contact configurations for measurements of $c$-axis (top) and in-plane (bottom) characteristics. (b) Resistive transitions $R\left(T\right)$ of the base crystal in the $ab$ plane (blue) and of a mesa in the $c$-axis direction (magenta). (c) Current-voltage characteristics of the same mesa in zero field (black) and at 10 T parallel to the planes (red). (d) $dI/dV$ vs voltage per junction at $T=2$ K in zero field and at 10 T parallel and perpendicular to the planes. (e) $dI/dV$ characteristics in parallel and perpendicular fields at different $T$. (f) Ratios of $dI/dV$ characteristics at parallel to that at perpendicular field of 10 T. Such angular-dependent magnetotunneling is selective to the anisotropic superconducting contribution and allows discrimination between superconducting and nonsuperconducting parts to the spectra.

Figure 2

(a) and (b) Angular-dependent magnetotunneling curves above $T_c$ at $H=10$ T for mesas at (a) Bi-2212 No. 1 and (b) Bi-2212 No. 2 crystals. (c) Low-bias parts of the curves for the Bi-2212 No. 1 mesa. The zero-bias maximum is due to an interlayer Josephson current. (d) Angular dependencies of in-plane resistances at 10 T for the Bi-2212 No. 1 crystal. The 2D-superconducting cusp at $\mathrm{\Theta }=0$ is distinguishable up to $\sim 100$ K. (e) Temperature dependencies of the superconducting (SC) peak and the pseudogap (PG) hump voltages for Bi-2212 No. 1. (f) Amplitudes of the PG hump, the SC peak, the 2D cusp in the in-plane angular magnetoresistance, and the Josephson $c$-axis supercurrent.