224 research outputs found
Freezing of spin dynamics and omega/T scaling in underdoped cuprates
The memory function approach to spin dynamics in doped antiferromagnetic
insulator combined with the assumption of temperature independent static spin
correlations and constant collective mode damping leads to omega/T scaling in a
broad range. The theory involving a non universal scaling parameter is used to
analyze recent inelastic neutron scattering results for underdoped cuprates.
Adopting modified damping function also the emerging central peak in low-doped
cuprates at low temperatures can be explained within the same framework.Comment: 4 pages, 5 figures; to appear in Journal of Physics: Conference
Series (ICM2009 Conference, Karlsruhe, Germany
Spin susceptibility of underdoped cuprates: the case of Ortho-II YBa_2Cu_3O_{6.5}
Recent inelastic neutron scattering measurements found that the spin
susceptibility of detwinned and highly ordered ortho-II YBa_2Cu_3O_{6.5}
exhibits, in both the normal and superconducting states, one-dimensional
incommensurate modulations at low energies which were interpreted as a
signature of dynamic stripes. We propose an alternative model based on
quasiparticle transitions between the arcs of a truncated Fermi surface. Such
transitions are resonantly enhanced by scattering to the triplet spin
resonance. We show that the anisotropy in the experimental spin response is
consistent with this model if the gap at the saddle points is anisotropic.Comment: 5 fives, 3 postscript figure
Crossover from weak to strong pairing in unconventional superconductors
Superconductors are classified by their pairing mechanism and the coupling
strength, measured as the ratio of the energy gap to the critical temperature,
Tc. We present an extensive comparison of the gap ratios among many single- and
multiband superconductors from simple metals to high-Tc cuprates and iron
pnictides. Contrary to the recently suggested universality of this ratio in
Fe-based superconductors, we find that the coupling in pnictides ranges from
weak, near the BCS limit, to strong, as in cuprates, bridging the gap between
these two extremes. Moreover, for Fe- and Cu-based materials, our analysis
reveals a universal correlation between the gap ratio and Tc, which is not
found in conventional superconductors and therefore supports a common
unconventional pairing mechanism in both families. An important consequence of
this result for ferropnictides is that the separation in energy between the
excitonic spin-resonance mode and the particle-hole continuum, which determines
the resonance damping, no longer appears independent of Tc.Comment: 15 pages, 3 figures, 5 tables with an exhaustive overview of the
published gap and spin-resonance measurements in Fe-based superconductors.
New in V3: updated references. To be published in Phys. Rev.
Magnetic order in the pseudogap phase of high- superconductors
One of the leading issues in high- superconductors is the origin of the
pseudogap phase in underdoped cuprates. Using polarized elastic neutron
diffraction, we identify a novel magnetic order in the YBaCuO
system. The observed magnetic order preserves translational symmetry as
proposed for orbital moments in the circulating current theory of the pseudogap
state. To date, it is the first direct evidence of an hidden order parameter
characterizing the pseudogap phase in high- cuprates.Comment: 3 figure
Neutron scattering study of the magnetic phase diagram of underdoped YBa(2)Cu(3)O(6+x)
We present a neutron triple-axis and resonant spin-echo spectroscopy study of
the spin correlations in untwinned YBCO crystals with x= 0.3, 0.35, and 0.45 as
a function of temperature and magnetic field. As the temperature T approaches
0, all samples exhibit static incommensurate magnetic order with propagation
vector along the a-direction in the CuO2 planes. The incommensurability delta
increases monotonically with hole concentration, as it does in LSCO. However,
delta is generally smaller than in LSCO at the same doping level. The intensity
of the incommensurate Bragg reflections increases with magnetic field for
YBCO(6.45) (superconducting Tc = 35 K), whereas it is field-independent for
YBCO(6.35) (Tc = 10 K). These results suggest that YBCO samples with x ~ 0.5
exhibit incommensurate magnetic order in the high fields used for the recent
quantum oscillation experiments on this system, which likely induces a
reconstruction of the Fermi surface. We present neutron spin-echo measurements
(with energy resolution ~ 1 micro-eV) for T > 0 that demonstrate a continuous
thermal broadening of the incommensurate magnetic Bragg reflections into a
quasielastic peak centered at excitation energy E = 0, consistent with the
zero-temperature transition expected for a two-dimensional spin system with
full spin-rotation symmetry. Measurements on YBCO(6.45) with a triple-axis
spectrometer (with energy resolution ~ 100 micro-eV) yield a crossover
temperature T_SDW ~ 30 K for the onset of quasi-static magnetic order. Upon
further heating, the wavevector characterizing low-energy spin excitations
approaches the commensurate antiferromagnetic wave vector, and the
incommensurability vanishes in an order-parameter-like fashion at an
"electronic liquid-crystal" onset temperature T_ELC ~ 150 K. Both T_SDW and
T_ELC increase continuously as the Mott-insulating phase is approached with
decreasing doping level.Comment: to appear in a special issue on "Fermiology of Cuprates" of the New
Journal of Physic
Physics of -Meson Condensation and High Temperature Cuprate Superconductors
The idea of condensation of the Goldstone -meson field in nuclear matter
had been put forward a long time ago. However, it was established that the
normal nuclear density is too low, it is not sufficient to condensate
-mesons. This is why the -condensation has never been observed.
Recent experimental and theoretical studies of high temperature cuprate
superconductors have revealed condensation of Goldstone magnons, the effect
fully analogous to the -condensation. The magnon condensation has been
observed. It is clear now that quantum fluctuations play a crucial role in the
condensation, in particular they drive a quantum phase transition that destroys
the condensate at some density of fermions
Ni impurity induced enhancement of the pseudogap in cuprate high T_c superconductors
The influence of magnetic Ni and non-magnetic Zn impurities on the normal
state pseudogap (PG) in the c-axis optical conductivity of NdBa\{Cu(Ni,Zn)O crystals was studied by spectral
ellipsometry. We find that these impurities which strongly suppress
superconductivity have a profoundly different impact on the PG. Zn gives rise
to a gradual and inhomogeneous PG suppression while Ni strongly enhances the
PG. Our results challenge theories that relate the PG either to precursor
superconductivity or to other phases with exotic order parameters, such as flux
phase or d-density wave states, that should be suppressed by potential
scattering. The apparent difference between magnetic and non-magnetic
impurities instead points towards an important role of magnetic correlations in
the PG state.Comment: 11 pages and 2 figure
Fe-based superconductors: unity or diversity?
Does the high temperature superconductivity observed in the newly discovered
iron-pnictide materials represent another example of the same essential physics
responsible for superconductivity in the cuprates, or does it embody a new
mechanism?Comment: Some minor errors in the figure and in the reference in the published
version are corrected. 2 pages, 2 figure
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