Unconventional Superconductivity in the Layered Iron Germanide ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$

The iron-based intermetallic ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ stands out among transition metal compounds for its high Sommerfeld coefficient of the order of $100\mathrm{mJ}/(\mathrm{mol}{\mathrm{K}}^2)$, which signals strong electronic correlations. A new generation of high quality samples of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ show superconducting transition anomalies below 1.8 K in thermodynamic, magnetic, and transport measurements, establishing that superconductivity is intrinsic in this layered iron compound outside the known superconducting iron pnictide or chalcogenide families. The Fermi surface geometry of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ resembles that of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ in the high pressure collapsed tetragonal phase, in which superconductivity at temperatures as high as 10 K has recently been reported, suggesting an underlying connection between the two systems.

Figure 1

Electrical resistivity of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ versus temperature, displaying a sharp superconducting drop of the resistivity with midpoint at 1.83 K below a $T^{3/2}$ normal state temperature dependence. (inset) Temperature dependence of the resistivity up to room temperature.

Figure 2

Correlation between superconducting transition temperature $T_c$ and residual resistance ratio $\mathrm{RRR}=\rho (300\mathrm{K})/\rho (2\mathrm{K})$. Data points and error bars show resistive transition midpoint temperature and transition width, determined by an $80\%/20\%$ criterion. Samples annealed at $800°\mathrm{C}$ (filled red circles) can reach a higher RRR and higher $T_c$ than samples which have not been annealed at $800°\mathrm{C}$ (unfilled blue circles). The $T$-dependent heat capacity $C(T)$ was measured for a subset of the samples. Large red circles mark those cases in which a clear $C(T)$ anomaly was observed, whereas the absence of such an anomaly is marked with black crosses. The $C(T)$ anomalies occur at about 1 K, as is shown in Fig. 3 for the sample with $\mathrm{RRR}=185$. Shaded areas illustrate the step in $T_c$ discussed in the text.

Figure 3

$C/T$ and magnetization of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ versus temperature show anomalies characteristic of bulk superconductivity. Low temperature extrapolation using linear (red line), quadratic (orange line), and single-gap BCS (blue line) forms obeys entropy matching at $T_c$ (see text). The background-subtracted zero-field-cooled low temperature dc magnetization $\mathrm{\Delta }M$ divided by the applied field ${\mu }_{0}H=0.5\mathrm{mT}$ of the same sample displays a step with midpoint at $\simeq 0.95\mathrm{K}$, which coincides with the steepest descent in $C/T$. The size of the step corresponds to $(95\pm 5)\%$ diamagnetic screening. (inset) Temperature dependence of the upper critical field determined from the midpoint of resistive transitions (labeled $\rho$) and from the peak in $C/T$ (labeled $C$).

Figure 4

Fermi surface calculated within DFT for ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ and for ${\mathrm{KFe}}_2{\mathrm{As}}_2$ in the uncollapsed and collapsed tetragonal structure. The Fermi surfaces of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ and uncollapsed-tetragonal ${\mathrm{KFe}}_2{\mathrm{As}}_2$ are fundamentally different. Cylindrical hole sheets characterize the Fermi surface structure in uncollapsed-tetragonal ${\mathrm{KFe}}_2{\mathrm{As}}_2$, whereas ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ features nested 3D hole pockets on the face of the Brillouin zone ($Z$) and an electron pocket in the corner of the zone ($X$). Conversely, the Fermi surface of collapsed-tetragonal ${\mathrm{KFe}}_2{\mathrm{As}}_2$ is strikingly similar to that of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ apart from the vicinity of the $\mathrm{\Gamma }$ point, which is enclosed by an outgrowth of the largest hole pocket in collapsed-tetragonal ${\mathrm{KFe}}_2{\mathrm{As}}_2$, whereas it is avoided by the corresponding hole pocket in ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$.