There are few Luka/Carter fics out there (this is a doom ship, doom, doom) but the best of them is this:
Incarnate by Destina.
Marvellous, and hot as hell (ah, the Luka/Carter smut, life cannot get better than that). Don´t forget to give feedback to the author.
For the daring out there you can also try my own L/C fic. It´s NC-17 kiddos.