76 research outputs found
Theory of the in-plane anisotropy of magnetic excitations in YBa_{2}Cu_{3}O_{6+y}
A pronounced xy-anisotropy was observed in recent neutron scattering
experiments for magnetic excitations in untwinned YBa_{2}Cu_{3}O_{6+y} (YBCO).
The small anisotropy of the bare band structure due to the orthorhombic crystal
symmetry seems to be enhanced by correlation effects. A natural possibility is
that the system is close to a Pomeranchuk instability associated with a d-wave
Fermi surface deformation (dFSD). We investigate this possibility in the
bilayer t-J model within a self-consistent slave-boson mean-field theory. We
show that the dFSD correlations drive a pronounced xy-anisotropy of magnetic
excitations at low doping and at relatively high temperatures, providing a
scenario for the observed xy-anisotropy in optimally doped as well as
underdoped YBCO, including the pseudogap phase.Comment: magnetic excitations in the even channel for YBCO are presented; the
proceedings of the M2S-HTSC VIII conferenc
Theory of reduced singlet pairing without the underlying state of charge stripes in the high-temperature superconductor YBa_2Cu_3O_6.45
Recently, a strongly enhanced xy anisotropy of magnetic excitations was
observed in YBa_2Cu_3O_y (YBCO_y) with y=6.45 and Tc=35 K [Science 319, 597
(2008)]. Unlike the observation in YBCO_6.6 and YBCO_6.85, the anisotropy grows
to be pronounced at lower temperature and at lower energy, and is not
suppressed by the onset of superconductivity. We propose that the effect of
singlet pairing is substantially reduced in YBCO_6.45. This reduction
concomitantly enhances an order competing with singlet pairing, a strong
tendency of the so-called d-wave Pomeranchuk instability, leading to the
magnetic excitations observed experimentally.Comment: 11 pages, 4 figures, published versio
Structure of the pairing gap from orbital nematic fluctuations
We study superconducting instability from orbital nematic fluctuations in a
minimal model consisting of the and orbitals, and choose
model parameters which capture the typical Fermi surface geometry observed in
iron-based superconductors. We solve the Eliashberg equations down to low
temperatures with keeping the renormalization function and a full momentum
dependence of the pairing gap. When superconductivity occurs in the tetragonal
phase, we find that the pairing gap exhibits a weak momentum dependence over
the Fermi surfaces. The superconducting instability occurs also inside the
nematic phase. When the orbital is occupied more than the
orbital in the nematic phase, a larger (smaller) gap is realized on the
Fermi-surface parts, where the () orbital component is
dominant, leading to a substantial momentum dependence of the pairing gap on
the hole Fermi surfaces. On the other hand, the momentum dependence of the gap
is weak on the electron Fermi surfaces. We also find that while the leading
instability is the so-called -wave symmetry, the second leading one is
-wave symmetry. In particular, these two states are nearly
degenerate in the tetragonal phase whereas such quasi-degeneracy is lifted in
the nematic phase and the -wave symmetry changes to highly
anisotropic -wave symmetry.Comment: 19 pages, 8 figure
Singular non-ordering susceptibility at a Pomeranchuk instability
We study magnetic susceptibilities of two-dimensional itinerant electron
systems exhibiting symmetry-breaking Fermi surface distortions, the so-called
d-wave Pomeranchuk instability, in a magnetic field. In a pure forward
scattering model, the longitudinal susceptibility chi^{zz} is found to exhibit
a jump at a critical point. The magnitude of this jump diverges at a
tricritical point. When scattering processes involving finite momentum
transfers are allowed for, chi^{zz} is expected to diverge also at a critical
point. The system displays multiple critical fluctuations. We argue that the
features of chi^{zz} are general properties associated with singularities of a
non-ordering susceptibility, leading to implications for a variety of materials
including Sr_3Ru_2O_7.Comment: 5 pages, 4 figure
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