  It is shown that many anomalies observed in underdoped cuprates, including
anomalous spectral weight transfer and a large pseudogap, appear to have a
common nature due to both the cluster structure of the underdoped phase and the
specific mechanism of superconducting pairing. The combined action of these
factors leads to the fact that at a temperature T lying in a certain
temperature range Tc<T<T*, the crystal contains small isolated clusters that
can exist both in superconducting and normal states, randomly switching between
them. In this case, below Tc with a very high probability the cluster is in a
superconducting state, and above T* it is in a normal state, and the interval
Tc<T<T* is the region of existence of the so-called pseudogap phase. The
temperatures Tc and T* for YBa2Cu3O6+d were calculated depending on the doping
level d. The calculation results are in good agreement with experiment without
the use of fitting parameters. At a given T in the same temperature range, the
time sequence of randomly arising superfluid density pulses from each cluster
can be represented as a random process. The effective width of the spectrum of
such a random process will be determined by a correlation time, i.e. the
characteristic time between successive on/off superconductivity in two
different clusters. This time, according to the estimate, is ~10^(-15) sec,
which corresponds to the effective width of the spectrum ~1 eV and explains the
effect of spectral weight transfer to the high-frequency region. This approach
also makes it possible to explain other anomalies observed in the vicinity of
Tc: the reversibility of magnetization curves in a certain temperature range
below Tc, the anomalous Nernst effect and anomalous diamagnetism above Tc.
