Node-like excitations in superconducting $\mathrm{Pb}{\mathrm{Mo}}_6{\mathrm{S}}_8$ probed by scanning tunneling spectroscopy

We present a scanning tunneling spectroscopy study on the Chevrel phase $\mathrm{Pb}{\mathrm{Mo}}_6{\mathrm{S}}_8$, an extreme type-II superconductor with a coherence length only slightly larger than in high-$T_c$ cuprates. Tunneling spectra measured on atomically flat terraces are spatially homogeneous and show well-defined coherence peaks. The low-energy spectral weight, the zero bias conductance, and the temperature dependence of the gap are incompatible with a conventional isotropic $s$-wave interpretation, revealing the presence of low-energy excitations in the superconducting state. We show that our data are consistent with the presence of nodes in the superconducting gap.

Figure 1

(Color online) Topographic characterization. (a) Large-scale topography ($T=1.8\mathrm{K}$, $R_t=50\mathrm{M}\Omega$). (b) Corresponding height histogram showing a unit step of height of $5.9\pm 1.1\mathrm{\AA }$.

Figure 2

(Color online) (a) Spectroscopic trace along a $1100\mathrm{\AA }$ path with one spectrum every $8.8\mathrm{\AA }$. The spectra are offset for clarity ($T=1.8\mathrm{K}$, $R_t=25\mathrm{M}\Omega$). (b) Normalized conductance spectra obtained at various $R_t$ from $20\text{to}50\mathrm{M}\Omega$. (c) Histogram of the zero-bias conductance in $\sim 7500$ spectra normalized to the conductance at $8\mathrm{mV}$. The standard deviation is 0.04.

Figure 3

(Color online) (a) Experimental $dI∕dV$ spectrum at $1.8\mathrm{K}$ (thick line) compared to the $s$- and $d$-wave fits described in the text (dashed and solid lines, respectively). (b) $dI∕dV$ spectra obtained at various temperatures (thick lines) and $d$-wave fits (thin lines). Each experimental spectrum is normalized so as to ensure state conservation within the measurement energy range and is offset by 0.45 with respect to the previous one for clarity.

Figure 4

(Color online) (a) Temperature dependence of the superconducting gap from the spectra in Fig. 3b (circles) compared to the $s$- and $d$-wave cases from solving the BCS gap equation for ${\Delta }_0\left(0\right)=2.95\mathrm{meV}$ (dashed and solid curves, respectively). (b) The same data with gaps rescaled by ${\Delta }_0\left(0\right)$ and temperatures rescaled by the respective $T_c$ values.