Static and Dynamic Magnetism in Underdoped Superconductor ${\mathrm{BaFe}}_{1.92}{\mathrm{Co}}_{0.08}{\mathrm{As}}_2$

We report neutron scattering measurements on single crystals of ${\mathrm{BaFe}}_{1.92}{\mathrm{Co}}_{0.08}{\mathrm{As}}_2$. The magnetic Bragg peak intensity is reduced by 6% upon cooling through $T_C$. The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional interactions. Below $T_C$ additional intensity appears at an energy of $\sim 4.5(0.5)\mathrm{meV}$, similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below $T_C$ for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance, demonstrating the direct competition between static magnetic order and superconductivity.

Figure 1

(a) Peak intensity of the $(\frac{1}{2}\frac{1}{2}3)$ magnetic Bragg peak. The solid line is the result of a power law fit as described in the text. (b) Peak intensity of the $(\frac{1}{2}\frac{1}{2}1)$ and $(\frac{1}{2}\frac{1}{2}3)$ magnetic peaks through $T_C$. The FWHM (c) and peak position (d) extracted from transverse and longitudinal scans in terms of $\theta$ (squares) and $2\theta$ (circles) for the $(\frac{1}{2}\frac{1}{2}3)$ peak through $T_C$. The solid lines in (c) and (d) represent the weighted average.

Figure 2

(a) Constant-$Q$ scans, $Q=(\frac{1}{2}\frac{1}{2}\overline{1})$, for temperatures above and below $T_C$. For comparison the background scattering determined as described in the text is also shown. (b) The temperature dependence of the inelastic intensity at $(\frac{1}{2}\frac{1}{2}\overline{1})$ and $E=5\mathrm{meV}$. The solid line is a power law fit yielding $T_C=11(1)\mathrm{K}$. $L$ dependence (c) and $H$ dependence (d) of the inelastic intensity near $Q=(\frac{1}{2}\frac{1}{2}\overline{1})$ and $E=5\mathrm{meV}$ at 1.6 and 20 K. The solid lines are guides to the eye, and the horizontal bars represent instrumental resolution. The unit r.l.u. denotes reciprocal lattice units. In (d) a scan around $(\frac{3}{2}\frac{3}{2}\overline{1})$ is included to emphasize the magnetic origin of the scattering.

Figure 3

(a) ${\chi }^{\prime \prime }(\mathbf{Q},\omega )$ for $\mathbf{Q}=(\frac{1}{2}\frac{1}{2}\overline{1})$ at 1.6 and 20 K. The line is a fit to ${\chi }^{\prime \prime }$ at 20 K as described in the text. (b) The difference between ${\chi }^{\prime \prime }(\mathbf{Q},\omega )$ at 1.6 and 20 K showing an additional component to the spin dynamics which appears below $T_C$. The line is a guide to the eye.

Figure 4

(a) Constant-$E$ scans along $(HH\overline{1})$. (b) Constant-$E$ scans along $(\frac{1}{2}\frac{1}{2}L)$. All data are measured at 1.6 K except $E=5\mathrm{meV}$, measured at 20 K to prevent the introduction of spin resonance scattering. The solid lines represent fits to the model with a single amplitude as described in the text. The scans are offset by 50 counts for clarity.