Features of the nonlinear microwave response of ultrathin YBaCuO films

Using the method of nonlinear near-field microwave microscopy, the temperature dependences of the third harmonic power were studied for different orientations of the $a$–$b$ axes of ultrathin YBaCuO films with a thickness of 4–5 nm. The experimental results obtained indicate that the nonlinear response at low temperatures may be associated with the presence of Andreev bound states.