van Hove singularities play a key role in the physics of correlated electron systems. They are points in the band structure where the density of states is singular. When the Fermi level is tuned towards a van Hove singularity, new and exotic physics is expected, particularly in low dimensional systems. In twisted bilayer graphene, for example, van Hove singularities occuring at « magic angles » are believed to produce unconventional superconducting and insulating states.
In two-dimensional d-wave superconductors, the presence of a van Hove singularity produces unconventional vortex lattice phases. This issue has been studied theoretically and an exotic vortex lattice transition, occuring close to the upper critical field, has been predicted by Nakai et al., (Phys. Rev. Lett. 89 237004 (2002)). However, after two decades this prediction had still been lacking experimental confirmation.
Using ultrasound measurements in high magnetic field, researchers from LNCMI discovered a new transition in the vortex lattice of the high-Tc cuprate superconductor LaSrCuO (LSCO). Combining the ultrasound observations with theory of the vortex lattice of LSCO, this work shows that the transitions observed in ultrasound correspond to the vortex lattice transition predicted 20 years ago.
Publication – Evidence for a Square-Square Vortex Lattice Transition in a High-Tc Cuprate Superconductor
D. J. Campbell, M. Frachet, S. Benhabib, I. Gilmutdinov, C. Proust, T. Kurosawa, N. Momono, M. Oda, M. Horio, K. Kramer, J. Chang, M. Ichioka, and D. LeBoeuf
Phys. Rev. Lett. 129, 067001 (2022)