Twofold symmetry of proximity-induced superconductivity in ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ heterostructures revealed by scanning tunneling microscopy

We observe proximity-induced superconductivity in in situ prepared heterostructures constructed by topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films and high-temperature cuprate superconductors ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. The superconducting gap maximum is about 7.6 meV on the surface of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the principal crystalline axes of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. This gap form is consistent with the ${\mathrm{\Delta }}_{4y}$ notation of the topological superconductivity proposed in such systems. Our results provide fruitful information on the possible topological superconductivity induced by the proximity effect in high-temperature superconducting cuprates.

Figure 1

Typical atomically resolved topographic image of (a) the Bi2212 single crystal and (b) 2 QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin film grown on the top surface of Bi2212. The inset in (b) shows the FT image of (b). (c) Topography of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ film with different thicknesses. Set-point conditions: (a) $V_{\text{set}}=100$ mV, $I_{\text{set}}=50$ pA; (b) $V_{\text{set}}=50$ mV, $I_{\text{set}}=50$ pA; (c) $V_{\text{set}}=250$ mV, $I_{\text{set}}=50$ pA. (d) Spatial distribution of height measured along the arrowed line in (c). (e) A series of tunneling spectra measured in the heterostructures with different thicknesses of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ layer. The arrows point out the kinks probably arising from the Dirac points of the surface states.

Figure 2

A series of typical tunneling spectra (open circles) measured on (a) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$/Bi2212 heterostructures and (b) the Bi2212 single crystal. The solid curves in (a) and (b) are the fitting results by using a one-gap Dynes model with (a) an anisotropic $s$-wave gap or (b) a $d$-wave gap. The short dashes near zero bias denote the zero differential conductance for the corresponding spectrum with the same color. (b) Semilog plot of the film-thickness-dependent superconducting gap maximum obtained from fittings. The error bars are determined in the fitting procedure by changing other fitting parameters.

Figure 3

(a)–(f) The FT-QPI patterns measured at different energies from 0 to $+12$ meV on the 2 QL ${\mathrm{Bi}}_2{\mathrm{Te}}_3$/Bi2212 heterostructure ($T=1.5$ K). (g) Schematic figure of the surface state in the normal state and (h) that when the DOS is partially gapped in the presence of a twofold symmetric gap. The angular dependence of the anisotropic superconducting gap is also shown schematically by the orange solid line in (h). (i) Simulated FT-QPI pattern by doing self-correlation to (h) and considering the spin selection rules.

Figure 4

Angle dependence of averaged intensity of the scattering spots measured in the heterostructures with different thicknesses of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ but all at $E=+2$ meV. The averaged background intensities are also plotted as dotted lines by using the same color as the corresponding averaged curve in this figure. Both the experimental integrated curves and the background dotted lines are shifted for clarification.