Infrared Measurement of the Pseudogap of P-Doped and Co-Doped High-Temperature ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ Superconductors

We report on infrared studies of charge dynamics in a prototypical pnictide system: the ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ family. Our experiments have identified hallmarks of the pseudogap state in the ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.

Figure 1

(a)–(d) $T$-dependent ${\sigma }_1(\omega )$ of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, OPD $\mathrm{Ba}({\mathrm{Fe}}_{0.92}{\mathrm{Co}}_{0.08}{)}_2{\mathrm{As}}_2$, ${\mathrm{BaFe}}_2({\mathrm{As}}_{0.67}{\mathrm{P}}_{0.33}{)}_2$, and OD $\mathrm{Ba}({\mathrm{Fe}}_{0.75}{\mathrm{Co}}_{0.25}{)}_2{\mathrm{As}}_2$ from top to bottom. Inset of (d): spectral weight obtained by integrating ${\sigma }_1(\omega )$ at 295 K up to $1500{\mathrm{cm}}^{-1}$. The gray bar indicates the SDW gap in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. The arrows in (b) and (c) denote the frequency above which ${\sigma }_1(\omega )$ displays slight increase due to the pseudogap. (e)–(h) Scattering rate $1/\tau (\omega )$. The gray bars represent the frequency range below which $1/\tau (\omega )$ exhibits suppression in the magnitude. Note that this frequency region correlates with the SDW gap in ${\sigma }_1(\omega )$ of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. Inset of (e): the hatched area denotes the contribution from interband transitions. (i)–(l) Calculated $1/\tau (\omega )$ spectra. The dashed line in (k) shows the calculation result for $1/\tau (\omega )$ at 32 K assuming vanishing pseudogap (${\Delta }_{\mathrm{PG}}=0$). (m)–(p) ${\alpha }^{2}F(\omega )$ spectra and DOS (insets) obtained from the $1/\tau (\omega )$ analysis. The dashed lines in (o) and its inset represent ${\alpha }^{2}F(\omega )$ and DOS for $1/\tau (\omega )$ result with ${\Delta }_{\mathrm{PG}}=0$, respectively.

Figure 2

${\sigma }_1(\omega )$ in the far-infrared region. (a) ${\mathrm{BaFe}}_2({\mathrm{As}}_{0.67}{\mathrm{P}}_{0.33}{)}_2$. Vertical bars on top of ${\sigma }_1(\omega )$ represent error bars. (b) OPD $\mathrm{Ba}({\mathrm{Fe}}_{0.92}{\mathrm{Co}}_{0.08}{)}_2{\mathrm{As}}_2$.