Observation of lock-in phenomena in heavy-ion-irradiated single crystal ofBa(Fe0.93Co0.07)2As2

The angular dependence of magnetization was measured in an optimally Co-doped BaFe${}_{2}$As${}_{2}$ single crystal, which is irradiated by 2.6 GeV uranium to introduce columnar defects (CDs). At low fields ($\mathbit{H}$), we observe the irreversible magnetization ${M}_{\mathrm{irr}}$ insensitive to the angle $\ensuremath{\theta}$ around $\mathbit{H}\ensuremath{\parallel}\mathrm{CDs}$. The field dependence of the angular range $2{\ensuremath{\theta}}_{L}$ is proportional to $1/H$, quite consistent with the expectation of the lock-in transition. This manifests the first observation of the fingerprint of the Bose glass phase in iron-based superconductors.