Doubling of the critical temperature of FeSe observed in point contacts

A rise in the superconducting critical temperature $T_c$ of more than two times (exceeding 20 K) is discovered in point contacts created between iron chalcogenide FeSe single crystals and Cu. The possible reasons for such a $T_c$ increase in point contacts are discussed. The most likely cause for this may be the interfacial carrier doping and/or interfacial enhanced electron-phonon interaction.

Figure 1

Evolution of $dV/dI$ curves with temperature for a PC created by touching an edge of a FeSe crystal with a Cu wire. The upper two curves marked by the circle are measured at 18 and 22 K (top curve). The right inset shows two $dV/dI$ curves measured at 4 and 18 K for a larger bias. The left inset shows the dependence of the PC resistance at zero bias versus temperature. Dotted lines are given to guide the eye.

Figure 2

Evolution of $dV/dI$ curves with temperature for PC created by touching the $ab$ plane of an FeSe flake with a Cu wire. The upper two curves marked by the circle are measured at 20 and 22 K (top curve). The right inset shows $dV/dI$ measured at 3, 12, and 22 K for a larger bias. The left inset shows the dependence of the PC resistance at zero bias versus temperature. Red circles show the corrected PC resistance after subtracting additional PC resistance increase above 12 K (as seen from the shifting of the $dV/dI$ “background” at larger bias) coming, most likely, from the bulk. The dotted line is given to guide the eye.

Figure 3

Statistic data for $T_c^{\mathrm{onset}}$ variation for PCs with different resistances. The horizontal stripe marks $T_c^{\mathrm{onset}}$ for a bulk FeSe taken from the resistance data shown in the inset. The inset shows the resistance versus temperature for a FeSe sample from Ref. [9].