Zlatko Tesanovic

Multiband magnetism and superconductivity in Fe-based compounds

Recent discovery of superconductivity in Fe-based layered compounds may have opened a new pathway to the room temperature superconductivity. A model Hamiltonian describing FeAs layers is introduced, highlighting the crucial role of puckering of As atoms in promoting d electron itinerancy and warding off large local-moment magnetism of Fe ions, the main enemy of superconductivity. Quantum many-particle effects in charge, spin and multiband channels are explored and a nesting-induced spin density-wave order is found in the parent compund. We argue that this largely itinerant antiferromagnetism and high Tc itself are essentially tied to the multiband nature of the Fermi surface.

BERRY PHASE THEORY OF THE ANOMALOUS HALL EFFECT : APPLICATION TO COLOSSAL MAGNETORESISTANCE MANGANITES

We show that the anomalous Hall effect (AHE) observed in colossal magnetoresistance manganites is a manifestation of Berry phase effects caused by carrier hopping in a nontrivial spin background. We determine the magnitude and temperature dependence of the Berry phase contribution to the AHE, finding that it increases rapidly in magnitude as the temperature is raised from zero through the magnetic transition temperature Tc, peaks at a temperature Tmax . Tc, and decays as a power of T , in agreement with experimental data. We suggest that our theory may be relevant to the anomalous Hall effect in conventional ferromagnets as well.

A BCS-like gap in the superconductor SmFeAsO0.85F0.15

Since the discovery of superconductivity in the high-transition-temperature (high-Tc) copper oxides two decades ago, it has been firmly established that the CuO2 plane is essential for superconductivity and gives rise to a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO1-xFx has recently been discovered and the conspicuous lack of the CuO2 planes raises the tantalizing question of a different pairing mechanism in these oxypnictides. The superconducting gap (its magnitude, structure, and temperature dependence) is intimately related to pairing. Here we report the observation of a single gap in the superconductor SmFeAsO0.85F0.15 with Tc = 42 K as measured by Andreev spectroscopy. The gap value of 2Δ = 13.34 ± 0.3 meV gives 2Δ/kBTc = 3.68 (where kB is the Boltzmann constant), close to the Bardeen–Cooper–Schrieffer (BCS) prediction of 3.53. The gap decreases with temperature and vanishes at Tc in a manner consistent with the BCS prediction, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, the t-J model, and the like, originally designed for the high-Tc copper oxides.

Spin Gap and Resonance at the Nesting Wave Vector in Superconducting FeSe0:4Te0:6

Neutron scattering is used to probe magnetic excitations in FeSe_{0.4}Te_{0.6} (T_{c} = 14 K). Low energy spin fluctuations are found with a characteristic wave vector (1/21/2L) that corresponds to Fermi surface nesting and differs from Q_{m} = (delta01/2) for magnetic ordering in Fe_{1+y}Te. A spin resonance with variant Plancks over 2piOmega_{0} = 6.51(4) meV approximately 5.3k_{B}T_{c} and variant Plancks over 2piGamma = 1.25(5) meV develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with s_{+/-} superconductivity and imperfect quasi-2D Fermi surface nesting.

Superconductivity in oxides: toward a unified picture

We present a combined overview of current experimental knowledge and theoretical understanding of high temperature superconductivity based on the charge transfer excitation model. Our model is capable of rationalizing a great variety of experimental information and suggests directions to follow in designing even higherTc materials.

Self-Consistent Electronic Structure of a d x 2 − y 2 and a d x 2 − y 2 + id xy Vortex

We investigate quasiparticle states associated with an isolated vortex in a d-wave superconductor using a self-consistent Bogoliubov-de Gennes formalism. For a pure

Quasiparticles in the Vortex Lattice of Unconventional Superconductors: Bloch Waves or Landau Levels?

d_{x^2-y^2}

Critical Fluctuations in Superconductors and the Magnetic Field Penetration Depth

superconductor we find that there exist no bound states in the core; all the states are extended with continuous energy spectrum. This result is inconsistent with the existing experimental data on cuprates. We propose an explanation for this data in terms of a magnetic-field-induced

de Haas-van Alphen oscillations in a superconducting state at high magnetic fields.

d_{x^2-y^2}+id_{xy}

Effect of a velocity barrier on the ballistic transport of Dirac fermions

state recently invoked in connection with the thermal conductivity measurements on Bi