High magnetic field ultrasound study of spin freezing in La1.88Sr0.12CuO4

Lanthanum-based high-temperature superconductors have long been known for featuring intriguing elastic anomalies, especially close to the notorious $p$ = 0.12 doping. Here, using ultrasound, very high magnetic fields, and nuclear magnetic resonance, the authors show that these anomalies originate from the coupling of the crystal lattice to slow spin fluctuations. Moreover, a symmetry analysis reveals the predominant ${B}_1g$ nematic character of the spin fluctuations, which are reinforced up to unexpectedly high magnetic fields.