Anti-ferromagnetism, spin-phonon interaction and the local-density approximation in high-TC_C superconductors

Results from different sets of band calculations for undoped and doped HgBa$_2$CuO$_4$ show that small changes in localization can lead to very different ground states. The normal LDA results are compared with 'modified' LDA results, in which different linearization energies make the O-p band more localized. The ground states in the normal calculations are far from the anti-ferromagnetic ones, while nearly AFM states are found in the modified calculations. The proximity of an AFM state in the doped system leads to increased $\lambda_{sf}$, and the modified band structure has favorable conditions for spin-phonon coupling and superconductivity mediated by spin fluctuations.Comment: 4 pages, 2 figs., Accepted in J. Physics: Condensed Matter as a lette

Temperature dependence of magnetism near defects in SrB_6

The T-dependence of magnetic moments in SrB_6 is studied through spin-polarized band calculations for a supercell of Sr_{27}B_{156} containing a B_6 vacancy. The magnetic moment decays rather quickly with T despite the fact that only electronic Fermi-Dirac effects are included. This result and the T-dependence of moments near a La impurity can hardly explain the reports of a very high Curie temperature in hexaborides, but suggest that the magnetism is caused by some other type of impurity.Comment: 3 pages, 2 figure

Fluctuation diagnostics of the electron self-energy: Origin of the pseudogap physics

We demonstrate how to identify which physical processes dominate the low-energy spectral functions of correlated electron systems. We obtain an unambiguous classification through an analysis of the equation of motion for the electron self-energy in its charge, spin and particle-particle representations. Our procedure is then employed to clarify the controversial physics responsible for the appearance of the pseudogap in correlated systems. We illustrate our method by examining the attractive and repulsive Hubbard model in two-dimensions. In the latter, spin fluctuations are identified as the origin of the pseudogap, and we also explain why $d-$wave pairing fluctuations play a marginal role in suppressing the low-energy spectral weight, independent of their actual strength.Comment: 6 pages, 2 figures + 4 pages supplementar

Why is timing of bird migration advancing when individuals are not?

Recent advances in spring arrival dates have been reported in many migratory species but the mechanism driving these advances is unknown. As population declines are most widely reported in species that are not advancing migration, there is an urgent need to identify the mechanisms facilitating and constraining these advances. Individual plasticity in timing of migration in response to changing climatic conditions is commonly proposed to drive these advances but plasticity in individual migratory timings is rarely observed. For a shorebird population that has significantly advanced migration in recent decades, we show that individual arrival dates are highly consistent between years, but that the arrival dates of new recruits to the population are significantly earlier now than in previous years. Several mechanisms could drive advances in recruit arrival, none of which require individual plasticity or rapid evolution of migration timings. In particular, advances in nest-laying dates could result in advanced recruit arrival, if benefits of early hatching facilitate early subsequent spring migration. This mechanism could also explain why arrival dates of short-distance migrants, which generally return to breeding sites earlier and have greater scope for advance laying, are advancing more rapidly than long-distance migrants

Magnetic impurity coupled to interacting conduction electrons

We consider a magnetic impurity which interacts by hybridization with a system of weakly correlated electrons and determine the energy of the ground state by means of an 1/N_f expansion. The correlations among the conduction electrons are described by a Hubbard Hamiltonian and are treated to lowest order in the interaction strength. We find that their effect on the Kondo temperature, T_K, in the Kondo limit is twofold: First, the position of the impurity level is shifted due to the reduction of charge fluctuations, which reduces T_K. Secondly, the bare Kondo exchange coupling is enhanced as spin fluctuations are enlarged. In total, T_K increases. Both corrections require intermediate states beyond the standard Varma-Yafet ansatz. This shows that the Hubbard interaction does not just provide quasiparticles, which hybridize with the impurity, but also renormalizes the Kondo coupling.Comment: ReVTeX 19 pages, 3 uuenconded postscript figure