Tuning Hole Mobility, Concentration, and Repulsion in High-TcT_c Cuprates via Apical Atoms

Using a newly developed first-principles Wannier-states approach that takes into account large on-site Coulomb repulsion, we derive the effective low-energy interacting Hamiltonians for several prototypical high-$T_c$ superconducting cuprates. The material dependence is found to originate primarily from the different energy of the apical atom $p_z$ state. Specifically, the general properties of the low-energy hole state, namely the Zhang-Rice singlet, are significantly modified by a triplet state associated with this $p_z$ state, via additional intra-sublattice hoppings, nearest-neighbor "super-repulsion", and other microscopic many-body processes. Possible implications on modulation of $T_c$, local superconducting gaps, charge distribution, hole mobility, electron-phonon interaction, and multi-layer effects are discussed.Comment: 5 pages, 3 figures, 1 tabl

Can disorder alone destroy the eg' hole pockets of NaxCoO2? A Wannier function based first-principles method for disordered systems

We investigate from first principles the proposed destruction of the controversial eg' pockets in the Fermi surface of NaxCoO2 due to Na disorder, by calculating its k-dependent configurational averaged spectral function . To this end, a Wannier function based method is developed that treats the effects of disorder beyond the mean field. Remarkable spectral broadenings of order ~eV are found for the oxygen orbitals, possibly explaining their absence in the experiments. Contrary to the current lore, however, the eg' pockets remain almost perfectly coherent. The developed method is expected to generate exciting opportunities in the study of the countless functional materials that owe their important electronic properties to disordered dopants

Mottness induced phase decoherence suggests Bose-Einstein condensation in overdoped cuprate high-temperature superconductors

Recent observations of diminishing superfluid phase stiffness in overdoped cuprate high-temperature superconductors challenges the conventional picture of superconductivity. Here, through analytic estimation and verified via variational Monte Carlo calculation of an emergent Bose liquid, we point out that Mottness of the underlying doped holes dictates a strong phase fluctuation of the superfluid at moderate carrier density. This effect turns the expected doping-increased phase stiffness into a dome shape, in good agreement with the recent observation. Specifically, the effective mass divergence due to "jamming" of the low-energy bosons reproduces the observed nonlinear relation between phase stiffness and transition temperature. Our results suggest a new paradigm, in which the high-temperature superconductivity in the cuprates is dominated by physics of Bose-Einstein condensation, as opposed to pairing-strength limited Cooper pairing.Comment: 6+3 pages, 4+1 figure

Unfolding first-principles band structures

A general method is presented to unfold band structures of first-principles super-cell calculations with proper spectral weight, allowing easier visualization of the electronic structure and the degree of broken translational symmetry. The resulting unfolded band structures contain additional rich information from the Kohn-Sham orbitals, and absorb the structure factor that makes them ideal for a direct comparison with angular resolved photoemission spectroscopy experiments. With negligible computational expense via the use of Wannier functions, this simple method has great practical value in the studies of a wide range of materials containing impurities, vacancies, lattice distortions, or spontaneous long-range orders.Comment: 4 pages, 3 figure