EVIDENCE FOR A SQUARE-SQUARE VORTEX-LATTICE TRANSITION IN A HIGH-T_c CUPRATE SUPERCONDUCTOR

J. Chang, Zürich University, M. Ichioka, Okayama University, D. Campbell, D. Le Boeuf, LNCMI Grenoble.

In the physics of correlated electron systems, van Hove singularities play a key role. These 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 two-dimensional d-wave superconductors, the presence of a van Hove singularity produces unconventional vortex-lattice phases and vortex transitions. These issues have been studied theoretically and an exotic vortex-lattice transition, occurring close to the upper critical field, has been predicted by Nakai and collaborators two decades ago (Phys. Rev. Lett. 89 237004 (2002)). Indeed, using Eilenberger’s theory of the vortex lattice, they predicted that the competition between the four-fold symmetry of the d-wave superconducting order parameter and that of the Fermi velocity produced by the van Hove singularity would result in a rotation of the square vortex lattice. However, after two decades their prediction was still lacking an experimental confirmation. The best current prospect material to observe such a transition is the high-T_c cuprate superconductor La_2-xSr_xCuO₄ (LSCO) close to its doping-induced Lifshitz transition. However, this is a challenging task experimentally, as techniques traditionally used to study the vortex lattice are not appropriate in that case. The magnetic fields required to observe the vortex transition are far beyond the capabilities of small-angle neutron scattering and scanning tunneling microscopy is not  applicable to LSCO as high-quality surfaces are not realized in this compound. In this work, researchers at LNCMI, in collaboration with Zürich University (Switzerland), Hokkaido University (Japan), and Okayama University (Japan), use a new approach to test this prediction with ultrasound measurements up to 60 T. Ultrasound is sensitive to the elasticity and pinning of the vortex lattice, both of which are affected by such kind of transition. They found a new transition in the compression modulus of the vortex lattice of LSCO, observed in magnetic fields exceeding 35 T, deep within the mixed state. The transition is only observed within the pinned vortex-solid phase indicating it is related to a structural transformation of the vortex-solid. Theoretical analysis, based on Eilenberger’s theory of the vortex lattice using tight-binding parameters specific to LSCO with x = 0.17 support the interpretation of the discovered ultrasound anomaly in terms of the square-square vortex lattice transformation predicted by Nakai and co-workers 20 years ago.

Figure: (a) The change in sound velocity of the in-plane longitudinal mode (c₁₁) of an LSCO crystal with x = 0.17 at different temperatures, as a function of magnetic field. The arrow marks the field of the anomaly, B_anom. (b) Theoretical phase diagram showing the different vortex-lattice configurations as a function of T and B. The hexagonal phase is stable in the green-colored area. In regions A and D, two different square vortex lattices are stabilized with a range of metastability emerging in rapidly varying magnetic field shown in regions B and C.

Evidence for a Square-Square Vortex Lattice Transition in a High-T_c 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).

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.067001

Contact: david.leboeuf@lncmi.cnrs.fr