Superconducting density of states and vortex lattice of LaRu$_2$P$_2$ observed by Scanning Tunneling Spectroscopy

Abstract

We provide the superconducting density of states of the iron based superconductor LaRu2P2 (Tc= 4.1 K), measured using millikelvin Scanning Tunneling Microscopy. From the tunneling conductance, we extract a density of states which shows the opening of a s-wave single superconducting gap. The temperature dependence of the gap also follows BCS theory. Under magnetic fields, vortices present Caroli de Gennes Matricon states, although these are strongly broadened by defect scattering. From the vortex core size we obtain a superconducting coherence length of ξ = 50 nm, compatible with the value extracted from macroscopic Hc2 measurements. We discuss the comparison between s-wave LaRu2P2 and pnictide unconventional multiple gap and strongly correlated Fe based superconductors.

Figures (3)

• Figure 1: a) Scanning Electron Microscope (SEM) image of a LaRu$_2$P$_2$ sample after cryogenic cleaving and after removing the sample from the STM. b) STM image of the topography of the sample, taken with a bias voltage V=2.5 mV, a constant tunneling current I$_t$=2 nA and at T= 80 mK. In the inset we show the profile normalized to the c-axis lattice parameter of LaRu$_2$P$_2$, taken along the blue line. White scale bar is 40 nm long. c) Normalized conductance as a function of bias voltage measured at 80 mK (black) and a fit to the BCS theory (dashed line), with $\Delta$ = 0.61 meV.
• Figure 2: a) The normalized conductance as a function of bias voltage measured from 84 mK to 3 K is shown as black data points. The solid colored lines represent the convoluted density of states (DOS) with their respective temperatures. b) The DOS obtained from the curves in a) is plotted as a function of energy from 84 mK to 3 K. c) The temperature dependence of the superconducting gap is determined by the quasiparticle peak position of the DOS.
• Figure 3: a),b) Zero bias tunneling conductance maps made at magnetic fields 0.05 T and 0.07 T respectively. Images are taken at different locations at 90 mK. White scale bar: 50 nm. The bar at the left provides the color scale in units of the normalized conductance. Vortex positions are Delaunay triangulated. c) Normalized conductance along a line from inside to outside of a vortex. The conductance curves follow the same color code as the conductance map in a) and b). d) Normalized conductance vs distance from vortex centre normalized as Physrevb.97.134501, at different magnetic fields. e) Vortex core size vs the magnetic field in LaRu$_2$P$_2$ compared to the core size in other superconductors ($\beta$-Bi$_2$Pd, 2H-NbS$_2$, 2H-NbSe$_{1.8}$S$_{0.2}$Physrevb.97.134501 and CaKFe$_4$As$_4$Physrevb.97.134501). Dashed line shows the fit $C = \eta\sqrt{\Phi_o/\pi H}$, described in the text Physrevb.94.014517.