  The power spectrum of finite-temperature quantum electromagnetic fluctuations
produced by elementary charge carriers under the influence of external electric
field is investigated. It is found that under the combined action of the photon
heat bath and the external field, the low-frequency asymptotic of the power
spectrum is modified both qualitatively and quantitatively. The new term in the
power spectrum is inversely proportional to, but is odd in frequency. It comes
from the connected part of the correlation function, and is related to the
temperature and external field corrections to the photon and charge carrier
propagators. In application to the case of a biased conducting sample, this
term gives rise to a contribution to the voltage power spectrum which is
proportional to the absolute system temperature, the charge carrier mobility,
the bias voltage squared, and a factor describing dependence of the noise
intensity on the sample geometry. It is verified that the derived expression is
in agreement with the experimental data on 1/f-noise measurements in metal
films.
