Nonlinear near-field microwave probing of Andreev bound states in ultrathin YBa2Cu3O7−x films

Boundary surfaces in epitaxial films of high-temperature oxides can host Andreev bound states. At low temperatures, these states should contribute to nonlinear microwave response and strongly enhance it. Using the nonlinear near-field microwave technique, the temperature dependence of the third harmonic power $P_{3\omega }\left(T\right)$ and its anisotropy in the $\left(ab\right)$ plane have been studied for ultrathin ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_{7-x}$ films. We observe a kink on the dependence $P_{3\omega }\left(T\right)$ at temperature $T_{\mathrm{kink}}\sim 20-30K$. For $T<T_{\mathrm{kink}}$, the third harmonic power monotonously increases as the temperature decreases. Additionally, the nonlinear microwave response demonstrates a larger contribution when the supercurrent flows along the $a$ and $b$ axes of the films. Our experimental results indicate that these low temperature features can be associated with the presence of Andreev bound states at the twin boundaries and/or the film edges.