Spin-Stripe Order Tied to the Pseudogap Phase in La1.8−xEu0.2SrxCuO4

Abstract

Although spin and charge stripes in high-Tc cuprates have been extensively studied, the exact range of carrier concentration over which they form a static order remains uncertain, complicating efforts to understand their significance. The problem is challenging due to the combined effects of quenched disorder and competition with superconductivity—both significant in cuprates—which add to the inherent difficulty of determining phase boundaries. In La2−xSrxCuO4 (LSCO) and in zero external magnetic field, static spin stripes are confined to a doping range well below p*, the pseudogap boundary at zero temperature. However, when high fields suppress the competing effect of superconductivity, spin-stripe order is found to extend up to p*. Here, we investigate La1.8−xEu0.2SrxCuO4 (Eu-LSCO) using La139 nuclear magnetic resonance and observe field-dependent spin fluctuations suggesting a similar competition between superconductivity and spin order as in LSCO. Nevertheless, we find that static spin stripes are present practically up to p* irrespective of field strength: The stronger stripe order in Eu-LSCO prevents superconductivity from enforcing a nonmagnetic ground state, except very close to p*. Thus, spin-stripe order is consistently bounded by p* in both LSCO and Eu-LSCO, despite their differing balances between stripe order and superconductivity. This indicates that the canonical stripe order, where spins and charges are intertwined in a static pattern, is fundamentally tied to the pseudogap phase, though the exact nature of this connection has yet to be elucidated. Any stripe order beyond the pseudogap endpoint must then be of a different nature: Either spin and charge orders remain intertwined, but both fluctuating, or only spin order fluctuates while charge order remains static. The presence of spin-stripe order up to p* and the pervasive, slow, and field-dependent spin-stripe fluctuations, as well as the electronic inhomogeneity documented in this work, must all be carefully considered in discussions of Fermi surface transformations, putative quantum criticality, and strange metal behavior. Published by the American Physical Society 2025