Uncollapsed ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ phase: Compensated, highly doped, electron-phonon-coupled, iron-based superconductor

The recently discovered ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ superconducting compound, member of the 122 family of iron pnictide superconductors, becomes superconducting below $T_c\approx 13\mathrm{K}$, yet its nominal doping apparently places it in the extreme overdoped limit, where superconductivity should be suppressed. In this work, we investigate the normal state of magneto- and thermoelectric transport and specific heat of this compound. The experimental data are consistent with the presence of highly compensated electron and hole bands, with ∼0.42 electrons per unit cell just above $T_{\mathrm{c}}$, and high effective masses $\sim 3m_0$. The temperature dependence of transport properties strongly resembles that of conventional superconductors, pointing to a key role of electron-phonon coupling. From this evidence, ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ can be regarded as the connecting compound between unconventional and conventional superconductors.

Figure 1

Resistivity of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$. The continuous red line represents the fit with the generalized Bloch-Grüneisen law, Eq. (1). Upper left inset: resistivity curves in different magnetic fields up to 9 T. Lower right inset: $H_{\mathrm{c}2}$ and $H_{\mathrm{irr}}$ extracted from the resistive transitions with the criteria of 90 and 10% of the normal state resistivity, respectively.

Figure 2

Isothermal magnetoresistivity of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ at different temperatures. The curve at 25 K is shown in the inset magnifying the low-field regime.

Figure 3

Hall resistance of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ at different temperatures.

Figure 4

Measured Seebeck coefficient of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ (filled symbols). The diffusive Seebeck coefficient calculated from the two-band parameters is also shown in the main panel (open symbols). Inset: S/T vs $T^2$ plot, with a linear regime identified in correspondence of the temperature range 40–85 K, as described by Eq. (2), with fitting parameters $A\approx -0.186\mu \mathrm{V}/{\mathrm{K}}^2$ and $B\approx 5.13\times {10}^{-6}\mu \mathrm{V}/{\mathrm{K}}^4$.

Figure 5

Specific heat $c$ of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ in zero field and in a 7-T field, plotted as $c$/T vs $T^2$. The continuous red line represents a linear fit to the 7-T data in the temperature range between $T_{\mathrm{c}}$ and 20 K, with best-fit coefficients $c/T\left[\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}\right]=\left(35+0.73T^2\right)\left[\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}\right]$ . In the upper left inset, the electronic superconducting contribution to the specific heat $c_{\mathrm{es}}/T=c/T-(\gamma +\beta T^2)$, which better evidences the superconducting transition, is plotted vs T. In the lower right inset, $c$ in units of R is plotted vs T and the horizontal dashed line represents the value $3\times N_{\mathrm{at}}\times R=15R$, expected from the Dulong-Petit law for the lattice contribution.

Figure 6

Carrier concentrations and mobilities extracted from data analysis in a single-band framework (upper panel) and in a two-band framework (lower panel).