Type-II superconductivity in SrPd2Ge2
The mechanism responsible for superconductivity in iron pnictides is unclear to date and the roles of magnetic, chemical and structural properties of the compounds remain an issue. To unravel it, already several pnictide superconductors without magnetic elements were studied. These isostructural materials serve as a playground for investigating the role of the structure in the mechanism of superconductivity in the absence of a magnetic element. Regarding these intents, the recently discovered low-temperature superconducting compound SrPd2Ge2 isostructural with the “122” family is even pnictogen-free, in addition to the lack of a magnetic element, and therefore enables a different aspect of the investigation of the superconductivity in “122” iron pnictides. We have shown, by means of angle resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and band structure calculations, that SrPd2Ge2, unlike the “122” family of pnictides, has a strongly three-dimensional electronic structure. It is well described within local density approximation (LDA) and features a single isotropic superconducting energy gap with 2Δ/kTc close to the Bardeen– Cooper–Schrieffer (BCS) theory universal value (see Fig. 1), ruling out exotic electronic states. By examining the differential conductance spectra across a vortex (Fig. 2, Fig. 3) and estimating the upper and lower critical magnetic fields by tunneling spectroscopy and local magnetization measurements, we show that SrPd2Ge2 is a strong type-II superconductor. Also, we compare the differential conductance spectra in various magnetic fields to the pair breaking model of Maki – de Gennes for dirty limit type-II superconductor in the gapless region. By estimating the electron mean free path, we have been able to identify the Pippard coherence length and the London penetration depth valid in the clean limit with the values of the coherence length and penetration depth obtained from ARPES and STS data, which indicate type I superconductivity. In view of that, while the type-I superconductivity has been attributed to the clean limit, in the dirty limit SrPd2Ge2 behaves as a type-II superconductor.
Figure 1: Normalized differential conductance spectra acquired by STS, measured between the Au tip and the SrPd2Ge2 sample in zero magnetic field at different temperatures between 0.4 K and 2.6 K, increasing by 0.1 K. The BCS fit of the spectrum obtained at the lowest temperature (T = 0.4 K) is indicated by circles. The inset shows the temperature dependence of the superconducting gap of SrPd2Ge2 (circles), obtained by the BCS fit of individual curves, in comparison with the BCS theory (line).
Figure 2: Normalized differential conductance spectra along the green line in Figure 3 (a). Length of the line is 216 nm. The spectra demonstrate the variation of the SDOS across a vortex in the applied magnetic field of 50 mT. The vortex center is at d ∼ 125 nm, featuring a normal DOS. Far from the vortex center, a full superconducting gap is present. The red transparent stripe marks the distance 2ξ, roughly matching the vortex core size.
Figure 3: (a), (b) and (c): Conductance imaging tunneling spectroscopy (CITS) maps (500 × 500 nm2, 0.119 mV, 0.42 K) of the SrPd2Ge2 surface in magnetic fields of 50 mT (a), 100 mT (b) and 250 mT (c), illustrating the superficial variation of the sample DOS at 0.119 meV. Red transparent circles in (b) represent the Abrikosov lattice. The diameter of each circle is 2ξ and they are separated by a = 155 nm, the Abrikosov lattice constant for a magnetic field of 100 mT. (d) Top: STM topographic image (500 × 500 nm2, 1 nA, 1 mV, 0.42 K) of the area shown in (a), (b) and (c) without the applied magnetic field. Bottom: The line profile of the topography along the red line in the STM image above.