Possible resonance effect of axionic dark matter in Josephson junctions
  We provide theoretical arguments that dark matter axions from the galactic
halo that pass through the earth may generate a small observable signal in
resonant S/N/S Josephson junctions. The corresponding interaction process is
based on uniqueness of the gauge-invariant axion Josephson phase angle modulo 2
pi and is predicted to produce a small Shapiro step-like feature without
externally applied microwave radiation when the Josephson frequency resonates
with the axion mass. A resonance signal of so far unknown origin observed in
[C. Hoffmann et al. PRB 70, 180503(R) (2004)] is consistent with our theory and
can be interpreted in terms of an axion mass of 0.11 meV and a local galactic
axionic dark matter density of 0.05 GeV/cm^3. We discuss future experimental
checks to confirm the dark-matter nature of the observed signal.
