Point-contact spectroscopy of Cu${}_{0.2}$Bi${}_2$Se${}_3$ single crystals

We report point-contact measurements in high-quality single crystals of Cu${}_{0.2}$Bi${}_2$Se${}_3$. We observe three different kinds of spectra: (1) Andreev-reflection spectra, from which we infer a superconducting gap size of 0.6 mV; (2) spectra with a large gap which closes above $T$${}_c$ at about 10 K; and (3) tunnelinglike spectra with zero-bias conductance peaks. These tunneling spectra show a very large gap of $\sim$2 meV ($2\Delta /K_b{T}_c\sim$14).

Figure 1

(a) Resistivity data showing a zero resistance at $T_c=3.3$ K. (b) SQUID magnetization data showing $T_c=3.3$ K and a SC volume fraction of 17$\%$ at 1.8 K. (c), (d) ARPES data taken along the $\Gamma$-$K$ direction at 25 K with photon energy of 22 eV. The data was measured in the PGM beamline at the SRC Madison, WI. Panel (c) shows the data for a pristine Bi${}_2$Se${}_3$ sample and panel (d) for the same Cu${}_{0.2}$Bi${}_2$Se${}_3$ sample used in the point-contact experiments. A large change in the chemical potential can be seen.

Figure 2

Differential conductance versus the voltage bias for the Au point contact at a temperature of 1.8 K at different magnetic fields.

Figure 3

(a) Differential conductance versus the voltage bias for the Au point contact at a temperature of 1.8 K at different magnetic fields. Inset: Differential conductance of a point contact on top of a nonsuperconducting spot on the sample. (b) Differential conductance of a point contact with a subtracted background and a fit to an $s$-wave BTK model with the following parameters: $\Delta =0.5$ mV and $Z=0$ with a quasiparticle broadening parameter of $\Gamma =0.23$ mV. Inset: The size of the superconducting gap, extracted from the BTK fit as a function of temperature and a fit to the data using a BCS model with a gap size of 0.625 mV and $T_c$ of 2.65 K.

Figure 4

(a) Differential conductance of a 20 $\Omega$ point contact at different magnetic fields. (b) Temperature dependence of the differential conductance.

Figure 5

Differential conductance measurements made at high temperatures, showing a large gap.