Tunneling characteristics and low-frequency noise of high-T_c superconductor/noble-metal junctions

We report extensive measurements of transport characteristics and low-frequency resistance noise of c-axis yttrium-barium-copper-oxide (YBCO)/Au junctions. The dominant conduction mechanism is tunneling at low temperatures. The conductance characteristic is asymmetric, and the conductance minimum occurs at a nonzero voltage. These features can be qualitatively explained by modeling the YBCO/Au interface with a Schottky barrier. The model shows that the YBCO surface behaves like a p-type degenerate semiconductor, with a Fermi degeneracy of about 0.1 eV. The barrier height is approximately 1.0 eV. We present evidence that interface states and disorder play an important role in determining the conductance characteristics. Low-frequency noise measurements of these junctions reveal that junction noise is dominated by resistance fluctuations with a 1/f-like power spectrum over a wide range of temperature and bias voltage. For temperatures between 4.2 and 77 K, the junction noise can be parameterized in terms of a normalized resistance fluctuation: δR/R≃6.3×10^-4/sqrt[f], in units of Hz^-1/2, where f is the center frequency of the measurement bandwidth. At f=10 Hz, for example, it is 2×10^-4 Hz^-1/2. This noise figure should prove to be useful for engineering design of high-T_c electronics. A more detailed analysis shows that at low temperatures the noise spectrum is characterized by random telegraph signals withe a Lorentzian power spectrum, which can have a distribution of corner frequencies that mimics a 1/f dependence. The random telegraph signals provide evidence for the existence of localized states.