I've got a uniform quadrangular mesh and for each node there's a vector quantity $u$ defined. I also have a non-aligned material interface across the mesh. Now I need that vector quantity to have a zero jump across the interface $[u\cdot n]=0$ where $n$ is the interface normal. I want to project $u$ into another space that satisfies this condition. Could I use Raviart-Thomas finite element for this case? I have never worked with RT elements and I was wondering if they could be built using an arbitrary direction instead of the element's normal (recall that the interface normal is not aligned with the elements and also that I am only interested in having $[u\cdot n]=0$, whereas the other components can have non-zero jumps) Thanks.
In the interior of cells, the Raviart-Thomas functions are continuous. As a consequence, the normal component is of course also continuous and the jump is zero. That may not be the case at places where the interface intersects cell boundaries, though. The set of these intersection points, however, consists of a finite number of points along a curve in 2d, or similar in 3d, i.e., a lower-dimensional manifold.
That said, I'm surprised that you need the condition $[u \cdot n]=0$. Typically, one would require that the flux across an interface separating two materials is continuous. The flux is usually a material constant times the vector-valued solution.