234 research outputs found
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.
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
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
Resonant magnetic excitations at high energy in superconducting
A detailed inelastic neutron scattering study of the high temperature
superconductor provides evidence of new resonant
magnetic features, in addition to the well known resonant mode at 41 meV: (i) a
commensurate magnetic resonance peak at 53 meV with an even symmetry under
exchange of two adjacent layers; and (ii) high energy
incommensurate resonant spin excitations whose spectral weight is around 54
meV. The locus and the spectral weight of these modes can be understood by
considering the momentum shape of the electron-hole spin-flip continuum of
d-wave superconductors. This provides new insight into the interplay between
collective spin excitations and the continuum of electron-hole excitations.Comment: 5 figure
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
Doping Dependence of Bilayer Resonant Spin Excitations in
Resonant magnetic modes with odd and even symmetries were studied by
inelastic neutron scattering experiments in the bilayer high-
superconductor over a wide doping range. The
threshold of the spin excitation continuum in the superconducting state,
deduced from the energies and spectral weights of both modes, is compared with
the superconducting d-wave gap, measured on the same samples by electronic
Raman scattering in the symmetry. Above a critical doping level of
, both mode energies and the continuum threshold coincide.
We find a simple scaling relationship between the characteristic energies and
spectral weights of both modes, which indicates that the resonant modes are
bound states in the superconducting energy gap, as predicted by the
spin-exciton model of the resonant mode.Comment: 4 figure
Two resonant magnetic modes in an overdoped high- superconductor
A detailed inelastic neutron scattering study of the overdoped high
temperature copper oxide superconductor
reveals two distinct magnetic resonant modes in the superconducting state. The
modes differ in their symmetry with respect to exchange between adjacent copper
oxide layers. Counterparts of the mode with odd symmetry, but not the one with
even symmetry, had been observed before at lower doping levels. The observation
of the even mode resolves a long-standing puzzle, and the spectral weight ratio
of both modes yields an estimate of the onset of particle-hole spin-flip
excitations.Comment: Submitted to PR
Pseudogap in the chain states of YBCO
As established by scanning tunneling microscopy (STM) cleaved surfaces of the
high temperature superconductor YBaCuO develop charge
density wave (CDW) modulations in the one-dimensional (1D) CuO chains. At the
same time, no signatures of the CDW have been reported in the spectral function
of the chain band previously studied by photoemission. We use soft X-ray angle
resolved photoemission (SX-ARPES) to detect a chain-derived surface band that
had not been detected in previous work. The for the new surface
band is found to be 0.55\,\AA, which matches the wave vector of the CDW
observed in direct space by STM. This reveals the relevance of the Fermi
surface nesting for the formation of CDWs in the CuO chains in
YBaCuO. In agreement with the short range nature of the
CDW order the newly detected surface band exhibits a pseudogap, whose energy
scale also corresponds to that observed by STM
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