We have performed scanning tunneling spectroscopy imaging of vortices in ${\mathrm{MgB}}_2$ with both the tunneling current and the applied field perpendicular to the crystalline c axis. This revealed a well ordered vortex lattice with a relatively low anisotropy of $1.19\pm \mathrm{0.04.}$ This is significantly below the upper-critical-field anisotropy, but in good agreement with theoretical predictions for the penetration depth anisotropy, which at low fields is expected to be dominated by the $\pi$ band. The value of the anisotropy is smaller than estimated from small-angle neutron-scattering measurement.

• Received 19 May 2003

DOI:https://doi.org/10.1103/PhysRevB.68.100508