Antiferromagnetic Excitations and Van Hove Singularities in YBa${}_2$Cu${}_3$O${}_{6+x}$

G. Blumberg, Branko P. Stojkovic, M. V. Klein

We show that in quasi-two-dimensional $d$-wave superconductors Van Hove singularities close to the Fermi surface lead to novel magnetic quasi-particle excitations. We calculate the…