6 research outputs found
Magnetism, superconductivity, and pairing symmetry in Fe-based superconductors
We analyze antiferromagnetism and superconductivity in novel based
superconductors within the itinerant model of small electron and hole pockets
near and . We argue that the effective interactions in both
channels logarithmically flow towards the same values at low energies, {\it
i.e.}, antiferromagnetism and superconductivity must be treated on equal
footings. The magnetic instability comes first for equal sizes of the two
pockets, but looses to superconductivity upon doping. The superconducting gap
has no nodes, but changes sign between the two Fermi surfaces (extended s-wave
symmetry). We argue that the dependencies of the spin susceptibility and
NMR relaxation rate for such state are exponential only at very low , and
can be well fitted by power-laws over a wide range below .Comment: 12 pages, 9 figures. References added, the formula for the
susceptibility correcte
Properties and Detection of Spin Nematic Order in Strongly Correlated Electron Systems
A spin nematic is a state which breaks spin SU(2) symmetry while preserving
translational and time reversal symmetries. Spin nematic order can arise
naturally from charge fluctuations of a spin stripe state. Focusing on the
possible existence of such a state in strongly correlated electron systems, we
build a nematic wave function starting from a t-J type model. The nematic is a
spin-two operator, and therefore does not couple directly to neutrons. However,
we show that neutron scattering and Knight shift experiments can detect the
spin anisotropy of electrons moving in a nematic background. We find the mean
field phase diagram for the nematic taking into account spin-orbit effects.Comment: 13 pages, 11 figures. (v2) References adde
Impurity states in antiferromagnetic Iron Arsenides
We explore theoretically impurity states in the antiferromagnetic
spin-density wave state of the iron arsenide. Two types of impurity models are
employed: one has only the intraband scattering while the other has both the
intraband and interband scattering with the equal strength. Interestingly, the
impurity bound state is revealed around the impurity site in the energy gap for
both models. However, the impurity state is doubly degenerate with respect to
spin for the first case; while the single impurity state is observed in either
the spin-up or spin-down channel for the second one. The impurity-induced
variations of the local density of states are also examined.Comment: 4 pages, 2 figure
Fractionalization patterns in strongly correlated electron systems: Spin-charge separation and beyond
We discuss possible patterns of electron fractionalization in strongly
interacting electron systems. A popular possibility is one in which the charge
of the electron has been liberated from its Fermi statistics. Such a
fractionalized phase contains in it the seed of superconductivity. Another
possibility occurs when the spin of the electron, rather than its charge, is
liberated from its Fermi statistics. Such a phase contains in it the seed of
magnetism, rather than superconductivity. We consider models in which both of
these phases occur and study possible phase transitions between them. We
describe other fractionalized phases, distinct from these, in which fractions
of the electron themselves fractionalize, and discuss the topological
characterization of such phases. These ideas are illustrated with specific
models of p-wave superconductors, Kondo lattices, and coexistence between
d-wave superconductivity and antiferromagnetism.Comment: 28 pages, 11 fig
Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictides
Weak-coupling approaches to the pairing problem in the iron pnictide
superconductors have predicted a wide variety of superconducting ground states.
We argue here that this is due both to the inadequacy of certain approximations
to the effective low-energy band structure, and to the natural near-degeneracy
of different pairing channels in superconductors with many distinct Fermi
surface sheets. In particular, we review attempts to construct two-orbital
effective band models, the argument for their fundamental inconsistency with
the symmetry of these materials, and the comparison of the dynamical
susceptibilities in two- and five-orbital models. We then present results for
the magnetic properties, pairing interactions, and pairing instabilities within
a five-orbital Random Phase Approximation model. We discuss the robustness of
these results for different dopings, interaction strengths, and variations in
band structure. Within the parameter space explored, an anisotropic,
sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to
the near nesting of Fermi surface sheets.Comment: 17 pages, 23 figure