Step 1: Understanding the Question:
The question asks to identify the type of line defect (dislocation) where the Burgers vector ($\mathbf{b}$) is perpendicular to the dislocation line.
Step 2: Detailed Explanation:
Burgers Vector: A vector that represents the magnitude and direction of the lattice distortion resulting from a dislocation in a crystal.
Edge Dislocation: This defect is formed by inserting an extra half-plane of atoms into the crystal. The dislocation line is at the bottom edge of this extra half-plane.
In an edge dislocation, the Burgers vector is perpendicular to the dislocation line ($\mathbf{b} \perp L$).
Screw Dislocation: Formed by a shear stress that shifts one part of the crystal with respect to another in a ramp-like fashion.
In a screw dislocation, the Burgers vector is parallel to the dislocation line ($\mathbf{b} \parallel L$).
Mixed Dislocation: The angle between $\mathbf{b}$ and the line is between $0^{\circ}$ and $90^{\circ}$.
Step 3: Final Answer:
The Burgers vector is perpendicular to the line in an edge dislocation.