  High-temperature copper oxide superconductors (cuprates) display
unconventional physics when they are lightly doped whereas the standard theory
of metals prevails in the opposite regime. For example, the thermoelectric
power, that is the voltage that develops across a sample in response to a
temperature gradient, changes sign abruptly near optimal doping in a wide class
of cuprates, a stark departure from the standard theory of metals in which the
thermopower vanishes only when one electron exists per site. We show that this
effect arises from proximity to a state in which particle-hole symmetry is
dynamically generated. The operative mechanism is dynamical spectral weight
transfer from states that lie at least 2eV away from the chemical potential. We
show that the sign change is reproduced quantitatively within the Hubbard model
for moderate values of the on-site repulsion, $U$. For sufficiently large
values of on-site repulsion, for example, $U=20t$, ($t$ the hopping matrix
element), dynamical spectral weight transfer attenuates and our calculated
results for the thermopower are in prefect agreement with exact atomic limit.
The emergent particle-hole symmetry close to optimal doping points to pairing
in the cuprates being driven by high-energy electronic states.
