Scanning tunneling microscopy in the superconductor LaSb${}_2$

We present very low temperature (0.15 K) scanning tunneling microscopy and spectroscopy experiments in the layered superconductor LaSb${}_2$. We obtain topographic microscopy images with surfaces showing hexagonal and square atomic size patterns, and observe in the tunneling conductance a superconducting gap. We find well defined quasiparticle peaks located at a bias voltage comparable to the weak coupling $s$-wave BCS expected gap value (0.17 meV). The amount of states at the Fermi level is however large and the curves are significantly broadened. We find $T_c$ of 1.2 K by following the tunneling conductance with temperature.

Figure 1

Lattice structure of LaSb${}_2$. A top view of one La/Sb bilayer and one sheet of Sb is shown. Lattice parameters are $a=6.319$ Å, $b=6.1739$ Å, and $c=18.57$ Å.

Figure 2

In the left panels we show atomic resolution topography images at 0.15 K on two different LaSb${}_2$ surfaces, with (a) hexagonal and (b) square atomic sized shapes. These images are unfiltered. The corresponding Fourier transforms are given in the right panels. Middle panels are Fourier filtered images, made by filtering out everything except green circles, which show the position of the Bragg peaks due to the atomic modulation.[27]

Figure 3

(a) Temperature dependence of the experimental tunneling conductance (black curves) and calculated conductance (red line). The data are, from bottom to top, taken at 0.15, 0.27, 0.36, 0.48, 0.55, 0.65, 0.77, 0.82, 0.92, 1.1, and 1.2 K. (b) Temperature dependence of the position of the quasiparticle peak in the density of states used to calculate the red lines in (a). The black solid line is the temperature dependence of the superconducting gap in BCS theory. Inset shows again the lowest temperature curve in (a). Blue dashed lines give quasiparticle peak positions for a weak coupling BCS superconductor with $T_c$ of 1.2 K.

Figure 4

Magnetic field dependence of tunneling conductance curves at 0.15 K for magnetic field applied perpendicular (a) and parallel (b) to the $ab$ plane. Insets show the magnetic field dependence of the zero bias conductance $\sigma (V=0\text{mV})$.