David Leboeuf and Marc-Henri Julien, LNCMI-Grenoble.
Extensive studies of cuprate superconductors have shown that, after three-dimensional Néel order disappears upon hole doping (p), there are still remnants of spin order at low temperature in the form of a glass-like freezing of incommensurate antiferromagnetic correlations. However, the importance of the coexistence of incommensurate spin order with superconductivity has been unclear. The glass-like characteristics and material-dependent phase boundaries of this ‘antiferromagnetic glass‘ suggest that it is favored by disorder and that it is not unequivocally connected to either charge order, the pseudogap phase, or superconductivity. We have used local (nuclear magnetic resonance, NMR) and bulk (ultrasound) measurements to show that, once competing effects from superconductivity are removed by high magnetic fields, the spin-glass phase of La2-xSrxCuO4 survives up to a hole-doping level much higher than hitherto believed: Actually, up to the critical doping level p* that defines the end of the pseudogap phase at zero temperature. This means that, in the non-superconducting ground state, this end of the pseudogap phase coincides with a quantum phase transition from glassy antiferromagnetic order to a correlated metal with only short-lived antiferromagnetism. This discovery has implications for the interpretation of high-field experiments performed in La-based cuprates: The frozen striped antiferromagnetic pattern should influence low-lying electronic states up to p*, which could affect transport or thermodynamic measurements at low temperatures and high fields. A fascinating issue is whether this is related to the quantum critical behavior observed at p*. More generally, our results exemplify that the properties of the field-induced normal state may not necessarily serve as a proxy for the whole pseudogap state. Here, in La2-xSrxCuO4, the field-induced normal state at zero temperature is an ordered antiferromagnet (albeit freezing like a glass) but the temperature-induced normal state above Tc shows only short-lived antiferromagnetic correlations. This work further shows that the spin-glass phase spans from the weakly doped insulator at p = 0.02 all the way up to p* ≈ 0.19. This suggests that the same local-moment antiferromagnetism as that found in the doped Mott insulator survives throughout the pseudogap state, which provides a connection between the pseudogap and the physics of the Mott insulator.
Figure: Temperature – doping phase diagram representing Tmin, the temperature of the minimum in the sound velocity, at different fields. Because superconductivity precludes the observation of Tmin in zero-field, the dashed line (brown area) represents the extrapolated zero-field Tmin, which is close to the freezing temperature at the NMR and ultrasound time scales of about 10-8s.
Hidden magnetism at the pseudogap critical point of a cuprate superconductor, M. Frachet, I. Vinograd, R. Zhou, S. Benhabib, S. Wu, H. Mayaffre, S. Krämer, S. K. Ramakrishna, A. P. Reyes, J. Debray, T. Kurosawa, N. Momono, M. Oda, S. Komiya, S. Ono, M. Horio, J. Chang, C. Proust, D. LeBoeuf, and M.-H. Julien, Nat. Phys. 16, 1064 (2020).
https://www.nature.com/articles/s41567-020-0950-5
Contact: david.leboeuf@lncmi.cnrs.fr marc-henri.julien@lncmi.cnrs.fr
