Scanning tunneling spectroscopic studies of the iron-arsenic superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals

Scanning tunneling spectra of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.06, 0.08, 0.12) single crystals are studied as a function of temperature ($T)$ and magnetic field ($H)$. For $H$ = 0, direct evidence for two-gap superconductivity at energies $\omega$ = $\Delta _{\beta}$ and $\Delta _{\alpha ,\gamma /\delta}$ ($\approx $ 2$\Delta _{\beta })$ and for magnetic resonance modes at $\Omega \quad \approx \quad \Delta _{\beta }+\Delta _{\alpha ,\gamma /\delta}$ are found for all samples at $T \quad < \quad T_{c}$. Fourier transformation of the tunneling spectra reveals $x$- and $\omega$-dependent quasiparticle interference (QPI) wave-vectors \textbf{q}$_{2}$ near ($\pm \pi $,0)/(0,$\pm \pi )$ and \textbf{q}$_{1}$ near ($\pm $2$\pi $,0)/(0,$\pm $2$\pi )$. The spectral intensity of \textbf{q}$_{2}$ exhibits strong $\omega$-dependence, peaking sharply at $\omega$ = $\Delta _{\beta }$, $\Delta _{\alpha ,\gamma /\delta}$ and $\Omega$. This is in stark contrast to the Bragg diffraction peaks that are independent of $\omega$, $T$ and $x$. For $H \quad >$ 0, additional QPI wave-vector \textbf{q}$_{3}$ appears near ($\pm \pi $,$\pm \pi )$. These findings are consistent with the sign-changing $s$-wave pairing symmetry. Additionally, for the optimally doped sample, a pseudogap at $\omega$ $\sim \quad \Delta _{\gamma /\delta }$ is found inside the vortex core, possibly due to coexisting superconductivity and spin density waves. This result is in contrast to the zero-bias conductance peaks observed inside the vortex core of (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$, implying asymmetry in the hole and electron-doping of the iron arsenides. This work was supported by NSF DMR-0907251.