  The low-energy excitation spectrum of HTS cuprates is examined in the light
of thermodynamic, transport, quasiparticle and spin properties. Changes in the
thermodynamic spectrum associated with the normal-state pseudogap disappear
abruptly at a critical doping state, $p_{crit}$ = 0.19 holes per Cu. Moreover,
ARPES data at 100K show that heavily damped quasiparticles (QP) near ($\pi$,0)
suddenly recover long lifetimes at $p_{crit}$, reflecting an abrupt loss of
scattering from AF spin fluctuations. This picture is confirmed by $\mu$SR
zero-field relaxation measurements which indicate the presence of a novel
quantum glass transition at $p_{crit}$. Consistent with this picture
resistivity studies on thin films of
Y$_{0.7}$Ca$_{0.3}$Ba$_2$Cu$_3$O$_{7-\delta}$ reveal linear behavior confined
to a V-shaped domain focussed on $p_{crit}$ at $T$=0. The generic phase
behavior of the cuprates may be governed by quantum critical fluctuations above
$p_{crit}$ and the pseudogap appears to be caused by short-range AF
correlations.
