30 research outputs found
Holographic Superconductor/Insulator Transition at Zero Temperature
We analyze the five-dimensional AdS gravity coupled to a gauge field and a
charged scalar field. Under a Scherk-Schwarz compactification, we show that the
system undergoes a superconductor/insulator transition at zero temperature in
2+1 dimensions as we change the chemical potential. By taking into account a
confinement/deconfinement transition, the phase diagram turns out to have a
rich structure. We will observe that it has a similarity with the RVB
(resonating valence bond) approach to high-Tc superconductors via an emergent
gauge symmetry.Comment: 25 pages, 23 figures; A new subsection on a concrete string theory
embedding added, references added (v2); Typos corrected, references added
(v3
Magnetism and its microscopic origin in iron-based high-temperature superconductors
High-temperature superconductivity in the iron-based materials emerges from,
or sometimes coexists with, their metallic or insulating parent compound
states. This is surprising since these undoped states display dramatically
different antiferromagnetic (AF) spin arrangements and Nel
temperatures. Although there is general consensus that magnetic interactions
are important for superconductivity, much is still unknown concerning the
microscopic origin of the magnetic states. In this review, progress in this
area is summarized, focusing on recent experimental and theoretical results and
discussing their microscopic implications. It is concluded that the parent
compounds are in a state that is more complex than implied by a simple Fermi
surface nesting scenario, and a dual description including both itinerant and
localized degrees of freedom is needed to properly describe these fascinating
materials.Comment: 14 pages, 4 figures, Review article, accepted for publication in
Nature Physic
Two-Particle-Self-Consistent Approach for the Hubbard Model
Even at weak to intermediate coupling, the Hubbard model poses a formidable
challenge. In two dimensions in particular, standard methods such as the Random
Phase Approximation are no longer valid since they predict a finite temperature
antiferromagnetic phase transition prohibited by the Mermin-Wagner theorem. The
Two-Particle-Self-Consistent (TPSC) approach satisfies that theorem as well as
particle conservation, the Pauli principle, the local moment and local charge
sum rules. The self-energy formula does not assume a Migdal theorem. There is
consistency between one- and two-particle quantities. Internal accuracy checks
allow one to test the limits of validity of TPSC. Here I present a pedagogical
review of TPSC along with a short summary of existing results and two case
studies: a) the opening of a pseudogap in two dimensions when the correlation
length is larger than the thermal de Broglie wavelength, and b) the conditions
for the appearance of d-wave superconductivity in the two-dimensional Hubbard
model.Comment: Chapter in "Theoretical methods for Strongly Correlated Systems",
Edited by A. Avella and F. Mancini, Springer Verlag, (2011) 55 pages.
Misprint in Eq.(23) corrected (thanks D. Bergeron
Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides
In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors,
superconductivity occurs when electrons form coherent Cooper pairs below the
superconducting transition temperature Tc. Although the kinetic energy of
paired electrons increases in the superconducting state relative to the normal
state, the reduction in the ion lattice energy is sufficient to give the
superconducting condensation energy. For iron pnictide superconductors derived
from electron or hole doping of their antiferromagnetic (AF) parent compounds,
the microscopic origin for supercnductivity is unclear. Here we use neutron
scattering to show that high-Tc superconductivity only occurs for iron
pnictides with low-energy itinerant electron-spin excitation coupling and high
energy spin excitations. Since our absolute spin susceptibility measurements
for optimally hole-doped iron pnictide reveal that the change in magnetic
exchange energy below and above Tc can account for the superconducting
condensation energy, we conclude that the presence of both high-energy spin
excitations giving rise to a large magnetic exchange coupling J and low-energy
spin excitations coupled to the itinerant electrons is essential for high-Tc
superconductivity in iron pnictides.Comment: 9 pages, 4 figures in the main article; 11 pages, 13 figures in the
supplementary material