Superconductivity and charge carrier localization in ultrathin $\mathbf{{La_{1.85}Sr_{0.15}CuO_4}/{La_2CuO_4}}$ bilayers

摘要

$\mathrm{La_{1.85}Sr_{0.15}CuO_4}$/$\mathrm{La_2CuO_4}$ (LSCO15/LCO) bilayerswith a precisely controlled thickness of N unit cells (UCs) of the former and MUCs of the latter ([LSCO15\_N/LCO\_M]) were grown on (001)-oriented {\slao}(SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction andreciprocal space map (RSM) studies confirmed the epitaxial growth of thebilayers and showed that a [LSCO15\_2/LCO\_2] bilayer is fully strained,whereas a [LSCO15\_2/LCO\_7] bilayer is already partially relaxed. The\textit{in situ} monitoring of the growth with reflection high energy electrondiffraction (RHEED) revealed that the gas environment during deposition has asurprisingly strong effect on the growth mode and thus on the amount ofdisorder in the first UC of LSCO15 (or the first two monolayers of LSCO15containing one $\mathrm{CuO_2}$ plane each). For samples grown in pure$\mathrm{N_2O}$ gas (growth type-B), the first LSCO15 UC next to the SLAOsubstrate is strongly disordered. This disorder is strongly reduced if thegrowth is performed in a mixture of $\mathrm{N_2O}$ and $\mathrm{O_2}$ gas(growth type-A). Electric transport measurements confirmed that the first UC ofLSCO15 next to the SLAO substrate is highly resistive and shows no sign ofsuperconductivity for growth type-B, whereas it is superconducting for growthtype-A. Furthermore, we found, rather surprisingly, that the conductivity ofthe LSCO15 UC next to the LCO capping layer strongly depends on the thicknessof the latter. A LCO capping layer with 7~UCs leads to a strong localization ofthe charge carriers in the adjacent LSCO15 UC and suppresses superconductivity.The magneto-transport data suggest a similarity with the case of weakly holedoped LSCO single crystals that are in a so-called {"{cluster-spin-glassstate}"}