  The mechanism of high temperature superconductivity is not resolved for so
long because the normal state of cuprates is not yet understood. Here we show
that the normal state pseudo-gap exhibits an unexpected non-monotonic
temperature dependence, which rules out the possibility to describe it by a
single mechanism such as superconducting phase fluctuations. Moreover, this
behaviour, being remarkably similar to the behaviour of the charge ordering gap
in the transition-metal dichalcogenides, completes the correspondence between
these two classes of compounds: the cuprates in the PG state and the
dichalcogenides in the incommensurate charge ordering state reveal virtually
identical spectra of one-particle excitations as function of energy, momentum
and temperature. These results suggest that the normal state pseudo-gap, which
was considered to be very peculiar to cuprates, seems to be a general complex
phenomenon for 2D metals. This may not only help to clarify the normal state
electronic structure of 2D metals but also provide new insight into electronic
properties of 2D solids where the metal-insulator and metal-superconductor
transitions are considered on similar basis as instabilities of particle-hole
and particle-particle interaction, respectively.
