Decrausaz, B.Pikulski, M.Ivashko, OlehChristensen, N. B.Choi, JaewonUdby, LindaNiedermayer, Ch.Lefmann, K.Rønnow, H. M.Mesot, J.Ollivier, JacquesKurosawa, T.Momono, N.Oda, M.Chang, J.Mazzone, D. G.2025-05-132025-05-132025-05-122025-05-0910.1103/physrevresearch.7.023131https://infoscience.epfl.ch/handle/20.500.14299/250084Quantum matter phases may coexist microscopically even when they display competing tendencies. A fundamental question is whether such a competition can be avoided through the elimination of one phase while the other one condenses into the ground state. Here, we present a high-resolution neutron spectroscopy study of the low-energy spin excitations in the high-temperature superconductor La 1.855 Sr 0.145 CuO 4 . In the normal state, we find low-energy magnetic fluctuations at incommensurate reciprocal lattice positions where spin-density-wave order emerges at lower Sr concentration or at high magnetic fields. While these spin excitations are largely suppressed by the emergence of the superconducting spin gap, some low-energy magnetic fluctuations persist deep inside the superconducting state. We interpret this result in terms of a dynamic competition between superconductivity and magnetism, where superconductivity impedes the condensation of low-energy magnetic fluctuations through the formation of magnetically mediated Cooper pairs.entext::journal::journal article::research article