The oxygen isotope (${}^{16}\mathrm{O}$/${}^{18}\mathrm{O}$) effect (OIE) on the pseudogap (charge-stripe ordering) temperature $T^{*}$ is investigated for the cuprate superconductor ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ as a function of doping $x$ by means of x-ray absorption near edge structure studies. A strong $x$ dependent and sign reversed OIE on $T^{*}$ is observed. The OIE exponent ${\alpha }_{T^{*}}$ systematically decreases from ${\alpha }_{T^{*}}=-0.6\left(1.3\right)$ for $x=0.15$ to ${\alpha }_{T^{*}}=-4.4\left(1.1\right)$ for $x=0.06$, corresponding to increasing $T^{*}$ and decreasing superconducting transition temperature $T_c$. Both $T^{*}\left({}^{16}\mathrm{O}\right)$ and $T^{*}\left({}^{18}\mathrm{O}\right)$ exhibit a linear doping dependence with different slopes and critical end points [where $T^{*}\left({}^{16}\mathrm{O}\right)$ and $T^{*}\left({}^{18}\mathrm{O}\right)$ fall to zero] at $x_c\left({}^{16}\mathrm{O}\right)=0.201\left(4\right)$ and $x_c\left({}^{18}\mathrm{O}\right)=0.182\left(3\right)$, indicating a large positive OIE of $x_c$ with an exponent of ${\alpha }_{x_c}=0.84\left(22\right)$. The remarkably large and strongly doping dependent OIE on $T^{*}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.

While it is known that strongly correlated transition metal oxides described by a multi-band Hubbard model show microscopic multiscale phase separation, little is known about the possibility to manipulate them with vacuum ultraviolet (VUV), 27 eV lighting. We have investigated the photo-induced effects of VUV light illumination of a super-oxygenated La₂NiO_4+y single crystal by means of scanning photoelectron microscopy. VUV light exposure induces the increase of the density of states (DOS) in the binding energy range around E_b = 1.4 eV below E_F. The photo-induced states in this energy region have been predicted due to clustering of oxygen interstitials by band structure calculations for large supercell of La₂CuO_4.125. We finally show that it is possible to generate and manipulate oxygen rich domains by VUV illumination as it was reported for X-ray illumination of La₂CuO_4+y. This phenomenology is assigned to oxygen-interstitials ordering and clustering by photo-illumination forming segregated domains in the La₂NiO_4+y surface.

The oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) on the pseudogap (charge-stripe ordering) temperature $T^{\ast}$ is investigated for the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ as a function of doping $x$ by means of x-ray absorption near edge structure (XANES) studies. A strong $x$ dependent and sign reversed OIE on $T^{\ast}$ is observed. The OIE exponent $\alpha_{T^{\ast}}$ systematically decreases from $\alpha_{T^{\ast}} = - 0.6(1.3)$ for $x = 0.15$ to $\alpha_{T^{\ast}} = - 4.4(1.1)$ for $x = 0.06$, corresponding to increasing $T^{\ast}$ and decreasing superconducting transition temperature $T_{c}$. Both $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ exhibit a linear doping dependence with different slopes and critical end points (where $T^{\ast}(^{16}{\rm O})$ and $T^{\ast}(^{18}{\rm O})$ fall to zero) at $x_{c}(^{16}{\rm O}) = 0.201(4)$ and $x_{c}(^{18}{\rm O}) = 0.182(3)$, indicating a large positive OIE of $x_{c}$ with an exponent of $\alpha_{x_{c}} = 0.84(22)$. The remarkably large and strongly doping dependent OIE on $T^{\ast}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.