Abstract:
The generic doping dependence of the thermodynamic, electrodynamic and
transport properties of high-temperature superconductors remains a puzzle
despite many years of study. We are still awaiting a rigorous scientific theory
that explains the resistance-free flow of electric current in these novel
materials. In conventional superconductors, observations of the predicted
dependence of the superconducting transition temperature on isotopic mass
played a key role in identifying a phononic pairing mechanism.
In order to elucidate the role of phonons in the high-Tc superconductors,
the oxygen isotope effect in the separate components of the penetration depth
tensor of the high-temperature superconductor YBa2Cu4O8 was determined
from AC susceptibility measurements, performed on biaxially-aligned powders
set in epoxy. The results, extracted after assuming values for the upper
cut-off radii in the particle size distributions, show that the isotope effect in
the bc-plane is negligible compared to those of the ab- and ac-planes. This
suggests that the electrons prefer to couple to phonon modes in which the
motion of the atoms is perpendicular to the plane of transport.
The electronic entropy, superfluid density, Raman response, spin susceptibility
and thermoelectric power were calculated from energy-momentum dispersions
determined by angle-resolved photoemission spectroscopy (ARPES). An excellent match with experimental data was obtained. This is a highly
significant result because it provides the first comprehensive link between
these bulk properties and the ARPES measurements which are dominated
by the outermost CuO2 layer. Thus, in most respects surface effects do
not appear to seriously modify or obscure the band structure which governs
bulk properties. The calculations reveal the presence of a van Hove singularity
(vHs) at the Fermi level (EF ) in the heavily overdoped regime to be
a universal feature of the cuprates. The evolution of these properties with
temperature and doping can be fully explained by the retreat of EF from
the vHs and the opening of a normal state pseudogap as doping is decreased.
Consequently, the pairing potential amplitude is found to be a strongly decreasing
function of hole concentration, similar to the doping dependence of
the exchange interaction, J. The pairing interaction is possibly a universal
function of the EF â EvHs with the maximum in the transition temperature
(Tc) governed by the exact magnitude of the density of states on the flanks of
the vHs. These are key new discoveries which may provide a route forward
to solving the puzzle of high-temperature superconductivity.
