The charge compensation mechanisms that occur when Li+ substitutes a 2+ element in superionic conductor (MgCoNiCuZn)O high‐entropy oxide have been studied using a combination of thermogravimetric analysis and X‐ray photoemission spectroscopy. Depending on the concentration of Li+ in the compound, the charge compensation involves first partial oxidation of Co2+ into Co3+ for low fraction of Li+, and then a combination of both the oxidation of cobalt and the formation of oxygen vacancies for large fraction of Li+.

The phase diagram of high-temperature superconductors is still to be understood 1 . In the low-carrier-doping regime, a loss of spectral weight in the electronic excitation spectrum-the so-called pseudogap-is observed above the superconducting temperature T c , and below a characteristic temperature T * (ref. 2). First observed in the spin channel by NMR measurements, the pseudogap has also been observed in the charge channel by scanning probe microscopy and photoemission experiments, for instance 2 . An important issue to address is whether this phenomenon is related to superconductivity or to a competing 'hidden' order. In the superconductivity case, it has been suggested that superconducting pairing fluctuations may be responsible, but this view remains to be tested experimentally. Here, we have designed a Josephson-like experiment to probe directly the fluctuating pairs in the normal state. We show that fluctuations survive only in a restricted range of temperature above T c , well below T * , and therefore cannot explain the opening of the pseudogap at higher temperature.Angle-resolved photoemission spectroscopy 3,4 and scanning tunnelling spectroscopy 5 showed a characteristic energy of the pseudogap that merges with the superconducting gap when the temperature is lowered below T c . This reveals a smooth crossover rather than a sharp transition line between the pseudogap regime and the superconducting state, and has led to the superconducting precursor scenario. As opposed to the conventional Bardeen-Cooper-Schrieffer (BCS) transition, where pairing and condensation occur simultaneously at T c , in underdoped cuprates fluctuating pairs may form at T * , with no long-range coherence, and condense in the superconducting state at T c (refs 6,7). Difficulties in confirming (or invalidating) this scenario arise from the fact that most of the experimental techniques used to investigate the pseudogap are sensitive only to the one-particle excitations, and therefore cannot provide a test of pairing above T c . Owing to its ability to probe the properties of the superconducting wavefunction, the Josephson effect is a natural way to address the fluctuation issue.In a second-order phase transition, the susceptibility is given by the linear response of the order parameter to a suitable external field. In the case of the superconducting phase transition, the role of the external field can be played by the rigid pair field of a second superconductor below its own T c (refs 8,9). In a Josephson junction in which one side of the junction is the fluctuating superconductor of interest above its T c , whereas the other side is a superconductor below its T c , the coupling between the pairing fluctuations and the well-established pair field gives rise to an excess current I ex proportional to the imaginary part of the frequency-and wavenumber-dependent pair susceptibility χ(ω, q). For a conventional superconductor above its T c (ref. 9)where Γ 0 = (16k B /h)(T − T c ) is the relaxation rate of the fluctuations, ξ(T ) is the ...