19 research outputs found
Quasiparticle excitation in and around the vortex core of underdoped YBa_2Cu_4O_8 studied by site-selective NMR
We report a site-selective ^{17}O spin-lattice relaxation rate T_1^{-1} in
the vortex state of underdoped YBa_2Cu_4O_8. We found that T_1^{-1} at the
planar sites exhibits an unusual nonmonotonic NMR frequency dependence. In the
region well outside the vortex core, T_1^{-1} cannot be simply explained by the
density of states of the Doppler-shifted quasiparticles in the d-wave
superconductor. Based on T_1^{-1} in the vortex core region, we establish
strong evidence that the local density of states within the vortex core is
strongly reduced.Comment: 5 pages, 3 figure
Massless Dirac Fermions, Gauge Fields, and Underdoped Cuprates
We study 2+1 dimensional massless Dirac fermions and bosons coupled to a U(1)
gauge field as a model for underdoped cuprates. We find that the uniform
susceptibility and the specific heat coefficient are logarithmically enhanced
(compared to linear-in-T behavior) due to the fluctuation of transverse gauge
field which is the only massless mode at finite boson density. We analyze
existing data, and find good agreement in the spin gap phase. Within our
picture, the drop of the susceptibility below the superconducting T_c arises
from the suppression of gauge fluctuations.Comment: 4 pages, REVTEX, 1 eps figur
Normal-state magnetic susceptibility in a bilayer cuprate
The magnetic susceptibility of high-T_c superconductors is investigated in
the normal state using a coupled bilayer model. While this model describes in a
natural way the normal-state pseudogaps seen in c-axis optical conductivity on
underdoped samples, it predicts a weakly increasing susceptibility with
decreasing temperature and cannot explain the magnetic pseudogaps exhibited in
NMR measurements. Our result, together with some experimental evidence suggest
that the mechanism governing the c-axis optical pseudogap is different from
that for the plane magnetic pseudogap.Comment: 5 pages, 2 figure
Charge degree of freedom and single-spin fluid model in YBa_2Cu_4O_8
We present a 17O nuclear magnetic resonance study in the stoichiometric
superconductor YBa_2Cu_4O_8. A double irradiation method enables us to show
that, below around 180 K, the spin-lattice relaxation rate of plane oxygen is
not only driven by magnetic, but also significantly by quadrupolar
fluctuations, i.e. low-frequency charge fluctuations. In the superconducting
state, on lowering the temperature, the quadrupolar relaxation diminishes
faster than the magnetic one. These findings show that, with the opening of the
pseudo spin gap, a charge degree of freedom of mainly oxygen character is
present in the electronic low-energy excitation spectrum.Comment: 4 pages, 3 figures, REVTE