6 research outputs found
Extended d_{x^2 - y^2}-wave superconductivity
Remarkable anisotropic structures have been recently observed in the order
parameter Delta_k of the underdoped superconductor Bi-2212. Such findings are
strongly suggestive of deviations from a simple d_{x^2 - y^2}-wave picture of
high-Tc superconductivity, i.e. Delta_k ~ cos (k_x) - cos (k_y). In particular,
flatter nodes in Delta_k are observed along the k_x = (+/-) k_y directions in
k-space, than within this simple model for a d-wave gap. We argue that
nonlinear corrections in the k-dependence of Delta_k near the nodes introduce
new energy scales, which would lead to deviations in the predicted power-law
asymptotic behaviour of several measurable quantities, at low or intermediate
temperatures. We evaluate such deviations, either analytically or numerically,
within the interlayer pair-tunneling model, and within yet another
phenomenological model for a d-wave order parameter. We find that such
deviations are expected to be of different sign in the two cases. Moreover, the
doping dependence of the flatness of the gap near the nodes is also
attributable to Fermi surface effects, in addition to possible screening
effects modifying the in-plane pairing kernel, as recently proposed.Comment: 7 pages, 4 embedded PostScript figures. Uses svjour, epsfig, amsmath,
amssymb, xspace. To be published in Eur. Phys. J.
Interplay among critical temperature, hole content, and pressure in the cuprate superconductors
Within a BCS-type mean-field approach to the extended Hubbard model, a
nontrivial dependence of T_c on the hole content per unit CuO_2 is recovered,
in good agreement with the celebrated non-monotonic universal behaviour at
normal pressure. Evaluation of T_c at higher pressures is then made possible by
the introduction of an explicit dependence of the tight-binding band and of the
carrier concentration on pressure P. Comparison with the known experimental
data for underdoped Bi2212 allows to single out an `intrinsic' contribution to
d T_c / d P from that due to the carrier concentration, and provides a
remarkable estimate of the dependence of the inter-site coupling strength on
the lattice scale.Comment: REVTeX 8 pages, including 5 embedded PostScript figures; other
required macros included; to be published in Phys. Rev. B (vol. 54
Role of H Distribution on Coherent Quantum Transport of Electrons in Hydrogenated Graphene
Using quantum mechanical methods, in the framework of non-equilibrium Green’s function (NEGF) theory, we discuss the effects of the real space distribution of hydrogen adatoms on the electronic properties of graphene. Advanced methods for the stochastic process simulation at the atomic resolution are applied to generate system configurations in agreement with the experimental realization of these systems as a function of the process parameters (e.g., temperature and hydrogen flux). We show how these Monte Carlo (MC) methods can achieve accurate predictions of the functionalization kinetics in multiple time and length scales. The ingredients of the overall numerical methodology are highlighted: the ab initio study of the stability of key configurations, on lattice matching of the energetic configuration relation, accelerated algorithms, sequential coupling with the NEGF based on calibrated Hamiltonians and statistical analysis of the transport characteristics. We demonstrate the benefit to this coupled MC-NEGF method in the study of quantum effects in manipulated nanosystems