Much recent attention has focused on the voltage-driven reversible topotactic transformation between the ferromagnetic metallic perovskite (P) SrCoO 3−δ and oxygen-vacancy-ordered antiferromagnetic insulating brownmillerite (BM) SrCoO 2.5 . This is emerging as a paradigmatic example of the power of electrochemical gating (using, e.g., ionic liquids/gels), the wide modulation of electronic, magnetic, and optical properties generating clear application potential. SrCoO 3 films are challenging with respect to stability, however, and there has been little exploration of alternate compositions. Here, we present the first study of ion-gel-gating-induced P → BM transformations across almost the entire La 1−x Sr x CoO 3 phase diagram (0 ≤ x ≤ 0.70), under both tensile and compressive epitaxial strain. Electronic transport, magnetometry, and operando synchrotron X-ray diffraction establish that voltage-induced P → BM transformations are possible at essentially all x, including x ≤ 0.50, where both P and BM phases are highly stable. Under small compressive strain, the transformation threshold voltage decreases from approximately +2.7 V at x = 0 to negligible at x = 0.70. Both larger compressive strain and tensile strain induce further threshold voltage lowering, particularly at low x. The P → BM threshold voltage is thus tunable, via both composition and strain. At x = 0.50, voltage-controlled ferromagnetism, transport, and optical transmittance are then demonstrated, achieving Curie temperature and resistivity modulations of ∼220 K and at least 5 orders of magnitude, respectively, and enabling estimation of the voltage-dependent Co valence. The results are analyzed in the context of doping-and strain-dependent oxygen vacancy formation energies and diffusion coefficients, establishing that it is thermodynamic factors, not kinetics, that underpin the decrease in the threshold voltage with x, that is, with increasing formal Co valence. These findings substantially advance the practical and mechanistic understanding of this voltage-driven transformation, with fundamental and technological implications.

The properties of quantum materials are commonly tuned using experimental variables such as pressure, magnetic field and doping. Here we explore a different approach: irreversible, plastic deformation of single crystals. We show for the archetypal unconventional superconductor SrTiO3 that compressive plastic deformation induces lowdimensional superconductivity significantly above the superconducting transition temperature (Tc) of undeformed samples. We furthermore present evidence for unusual normal-state transport behaviour that suggests superconducting correlations at temperatures two orders of magnitude above the bulk Tc. The superconductivity enhancement is correlated with the appearance of structural features related to selforganized dislocation structures, as revealed by diffuse neutron and X-ray scattering.These results suggest that deformed SrTiO3 is a potential high-temperature superconductor, and push the limits of superconductivity in this low-density electronic system. More broadly, we demonstrate the promise of plastic deformation and dislocation engineering as tools to manipulate electronic properties of quantum materials.

We study the properties of the eigenvalues of real random matrices and their products. It is known that when the matrix elements are Gaussian-distributed independent random variables, the fraction of real eigenvalues tends to unity as the number of matrices in the product increases. Here we present numerical evidence that this phenomenon is robust with respect to the probability distribution of matrix elements, and is therefore a general property that merits detailed investigation. Since the elements of the product matrix are no longer distributed as those of the single matrix nor they remain independent random variables, we study the role of these two factors in detail. We study numerically the properties of the Hadamard (or Schur) product of matrices and also the product of matrices whose entries are independent but have the same marginal distribution as that of normal products of matrices, and find that under repeated multiplication, the probability of all eigenvalues to be real increases in both cases, but saturates to a constant below unity showing that the correlations amongst the matrix elements are responsible for the approach to one. To investigate the role of the non-normal nature of the probability distributions, we present a thorough analytical treatment of the 2 × 2 single matrix for several standard distributions. Within the class of smooth distributions with zero mean and finite variance, our results indicate that the Gaussian distribution has the maximum probability of real eigenvalues, but the Cauchy distribution characterised by infinite variance is found to have a larger probability of real eigenvalues than the normal. We also find that for the two-dimensional single matrices, the probability of real eigenvalues lies in the range [5/8, 7/8].a

Electrolyte-based transistors utilizing ionic liquids/gels have been highly successful in the study of charge-density-controlled phenomena, particularly in oxides. Experimental probes beyond transport have played a significant role, despite challenges to their application in electric double-layer transistors. Here, we demonstrate application of synchrotron soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) as operando probes of the charge state and magnetism in ion-gel-gated ferromagnetic perovskite films. Electrochemical response via oxygen vacancies at positive gate bias in LaAlO3(001)/La0.5Sr0.5CoO3-δ is used as a test case. XAS/XMCD measurements of 4-25 unit-cell-thick films first probe the evolution of hole doping (from the O K-edge pre-peak) and ferromagnetism (at the Co L-edges), to establish a baseline. Operando soft XAS/XMCD of electrolyte-gated films is then demonstrated, using optimized spin-coated gels with thickness ~1 µm, and specific composition. Application of gate voltages up to +4 V is shown to dramatically suppress the O K-edge XAS pre-peak intensity and Co L-edge XMCD, thus enabling the Co valence and ferromagnetism to be tracked upon gateinduced reduction. Soft XAS and XMCD, with appropriate electrolyte design, are thus established as viable for the operando characterization of electrolyte-gated oxides.

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