Properties of two level systems in current carrying superconductors

At low temperatures, the only low energy excitations in disordered S-wave superconductors are two level systems (TLS). We show that at a sufficiently low frequency, ω ,the strength of the interaction between the ac electric field and the TLS is strongly enhanced by the presence of a dc supercurrent. As a result, the real part of conductivity, σ(ω), and consequently the equilibrium current correlation function are parametrically enhanced. If the distribution of TLS relaxation times is broad, then σ(ω) ∼ 1/ω, and the equilibrium supercurrent fluctuations have a form of 1/f noise.