Phonon anomalies in pure and underdoped $R_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ ($R=\text{Ba}$, Sr) investigated by Raman light scattering

We present a detailed temperature-dependent Raman light scattering study of optical phonons in ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ ($x\sim 0.28$, superconducting $T_c\sim 29\text{K}$), ${\text{Sr}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ ($x\sim 0.15$, $T_c\sim 29\text{K}$), and nonsuperconducting ${\text{BaFe}}_2{\text{As}}_2$ single crystals. In all samples we observe a strong continuous narrowing of the Raman-active Fe and As vibrations upon cooling below the spin-density wave transition $T_s$. We attribute this effect to the opening of the spin-density wave gap. The electron-phonon linewidths inferred from these data greatly exceed the predictions of ab initio density-functional calculations without spin polarization, which may imply that local magnetic moments survive well above $T_s$. A first-order structural transition accompanying the spin-density wave transition induces discontinuous jumps in the phonon frequencies. These anomalies are increasingly suppressed for higher potassium concentrations. We also observe subtle phonon anomalies at the superconducting transition temperature $T_c$ with a behavior qualitatively similar to that in the cuprate superconductors.

Figure 1

In-plane resistivity of BFA and BKFA single crystals determined by a standard four-probe method.

Figure 2

Field-cooled (FC) and zero-field-cooled (ZFC) magnetization of a SKFA crystal in a magnetic field of 10 Oe applied along the crystallographic $c$ axis.

Figure 3

(Color online) Raman spectra of underdoped BKFA at 300 K in y(xu)y and y(zu)y polarization configurations. The additional peaks below $\sim 150{\text{cm}}^{-1}$ are artifacts of the optical setup. The insets show a comparison of the Raman spectra at 5 and 300 K. The displacement patterns show the Raman-active $A_{1g}$ and $B_{1g}$ modes as obtained from shell-model calculations (Ref. 24).

Figure 4

(Color online) Temperature dependence of the frequency and FWHM of the $A_{1g}$ and $B_{1g}$ modes in underdoped BKFA. The solid line is the result of a fit to the normal-state data according to anharmonic phonon decay processes (see text for details). The dashed lines indicate the superconducting $T_c\sim 29\text{K}$ and the SDW transition temperature $T_s\sim 75\text{K}$ as obtained from Ref. 22.

Figure 5

(Color online) Temperature dependence of the frequency and FWHM of the $A_{1g}$ and $B_{1g}$ modes in underdoped SKFA. The dashed lines indicate the superconducting $T_c\sim 29\text{K}$ and the SDW transition temperature $T_s\sim 178\text{K}$.

Figure 6

Temperature dependence of the frequency and FWHM of the $A_{1g}$ and $B_{1g}$ modes in undoped BFA (grown in self-flux). The dashed lines indicate the structural-SDW transition temperature $T_s\sim 138\text{K}$ as obtained from Fig. 1.