Conventional superconductivity in SrPd${}_2$Ge${}_2$

The electronic structure of SrPd${}_2$Ge${}_2$ single crystals is studied by angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and band structure calculations within the local-density approximation (LDA). The STS measurements show a single $s$-wave superconducting energy gap $\Delta \left(0\right)$ $=$ 0.5 meV. The photon-energy dependence of the observed Fermi surface reveals a strongly three-dimensional character of the corresponding electronic bands. By comparing the experimentally measured and calculated Fermi velocities a renormalization factor of 0.95 is obtained, which is much smaller than typical values reported in Fe-based superconductors. We ascribe such an unusually low band renormalization to the different orbital character of the conduction electrons and, using ARPES and STS data, argue that SrPd${}_2$Ge${}_2$ is likely to be a conventional superconductor, which makes it clearly distinct from isostructural iron pnictide superconductors of the “122” family.

Figure 1

Temperature dependence of magnetization of the SrPd${}_2$Ge${}_2$ single crystal with a field of 10 Oe, perpendicular to the $c$ axis, in both field-cooling (FC) and zero-field-cooling (ZFC) modes.

Figure 2

LDA band structure and density of states for SrPd${}_2$Ge${}_2$. Bands in different panels (from top to bottom) are marked with circles, whose radii are proportional to the weight of Sr $d$, Pd $d$, and Ge $p$ states in the corresponding Bloch wave functions.

Figure 3

(a) Brillouin zone of SrPd${}_2$Ge${}_2$ and (b, c, d) cuts of the Fermi surface from the calculated electronic band structure for different $k_z$ values.

Figure 4

Comparison of the experimental and simulated MDMs: Photoemission intensity measured with (a) $h\nu$ $=$ 80 eV at the FL and (c) 500 meV below the FL and (b, d) the corresponding density of states calculated with $k_z$ $=$ 0.75 (2$\pi /c$). Photoemission intensity measured with (e) $h\nu$ $=$ 60 eV at the FL and (g) 500 meV below the FL and (f, h) the corresponding density of states calculated with $k_z$ $=$ 0.5 (2$\pi /c$). Sample temperature is 1.3 K.

Figure 5

Comparison of the experimental and simulated energy distribution maps: Photoemission intensity measured with $h\nu$ $=$ 80 eV in the (a) $M$-$\Gamma$-$M$ and (d) $X$-$\Gamma$-$X$ directions, (b, 3) second derivatives, and (c, f) the corresponding density of states calculated with $k_z=0.75$ (2$\pi /c$). (g) Photoemission intensity plot along the $X$-$\Gamma$-$X$ direction together with extracted MDC peak positions (black circles) and linear fit (red line) to band dispersion; (h) the corresponding raw and fitted MDS. Sample temperature is 1.3 K.

Figure 6

STS conductance spectra of the superconductor-superconductor junction, Pb-SrPd${}_2$Ge${}_2$, measured in zero magnetic field at different temperatures between 0.45 K (lowest curve) and 2.7 K, increasing by 0.05 K (upper curves are shifted for clarity). The inset shows the temperature dependence of the superconducting gap of SrPd${}_2$Ge${}_2$ (circles, squares, and triangles) in comparison with the BCS theory (line).