22 research outputs found
Hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet
Superconductivity in layered copper-oxide compounds emerges when charge
carriers are added to antiferromagnetically-ordered CuO2 layers. The carriers
destroy the antiferromagnetic order, but strong spin fluctuations persist
throughout the superconducting phase and are intimately linked to
super-conductivity. Neutron scattering measurements of spin fluctuations in
hole-doped copper oxides have revealed an unusual `hour-glass' feature in the
momentum-resolved magnetic spectrum, present in a wide range of superconducting
and non-superconducting materials. There is no widely-accepted explanation for
this feature. One possibility is that it derives from a pattern of alternating
spin and charge stripes, an idea supported by measurements on stripe-ordered
La1.875Ba0.125CuO4. However, many copper oxides without stripe order also
exhibit an hour-glass spectrum$. Here we report the observation of an
hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the
family of superconducting copper oxides. Our system has stripe correlations and
is an insulator, which means its magnetic dynamics can conclusively be ascribed
to stripes. The results provide compelling evidence that the hour-glass
spectrum in the copper-oxide superconductors arises from fluctuating stripes.Comment: 13 pages, 4 figures, to appear in Natur
Normal-State Spin Dynamics and Temperature-Dependent Spin Resonance Energy in an Optimally Doped Iron Arsenide Superconductor
The proximity of superconductivity and antiferromagnetism in the phase
diagram of iron arsenides, the apparently weak electron-phonon coupling and the
"resonance peak" in the superconducting spin excitation spectrum have fostered
the hypothesis of magnetically mediated Cooper pairing. However, since most
theories of superconductivity are based on a pairing boson of sufficient
spectral weight in the normal state, detailed knowledge of the spin excitation
spectrum above the superconducting transition temperature Tc is required to
assess the viability of this hypothesis. Using inelastic neutron scattering we
have studied the spin excitations in optimally doped BaFe1.85Co0.15As2 (Tc = 25
K) over a wide range of temperatures and energies. We present the results in
absolute units and find that the normal state spectrum carries a weight
comparable to underdoped cuprates. In contrast to cuprates, however, the
spectrum agrees well with predictions of the theory of nearly antiferromagnetic
metals, without complications arising from a pseudogap or competing
incommensurate spin-modulated phases. We also show that the temperature
evolution of the resonance energy follows the superconducting energy gap, as
expected from conventional Fermi-liquid approaches. Our observations point to a
surprisingly simple theoretical description of the spin dynamics in the iron
arsenides and provide a solid foundation for models of magnetically mediated
superconductivity.Comment: 8 pages, 4 figures, and an animatio
Local antiferromagnetic exchange and collaborative Fermi surface as key ingredients of high temperature superconductors
Cuprates, ferropnictides and ferrochalcogenides are three classes of
unconventional high-temperature superconductors, who share similar phase
diagrams in which superconductivity develops after a magnetic order is
suppressed, suggesting a strong interplay between superconductivity and
magnetism, although the exact picture of this interplay remains elusive. Here
we show that there is a direct bridge connecting antiferromagnetic exchange
interactions determined in the parent compounds of these materials to the
superconducting gap functions observed in the corresponding superconducting
materials. High superconducting transition temperature is achieved when the
Fermi surface topology matches the form factor of the pairing symmetry favored
by local magnetic exchange interactions. Our result offers a principle guide to
search for new high temperature superconductors.Comment: 12 pages, 5 figures, 1 table, 1 supplementary materia
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
Strength of the Spin-Fluctuation-Mediated Pairing Interaction in a High-Temperature Superconductor
Theories based on the coupling between spin fluctuations and fermionic
quasiparticles are among the leading contenders to explain the origin of
high-temperature superconductivity, but estimates of the strength of this
interaction differ widely. Here we analyze the charge- and spin-excitation
spectra determined by angle-resolved photoemission and inelastic neutron
scattering, respectively, on the same crystals of the high-temperature
superconductor YBa2Cu3O6.6. We show that a self-consistent description of both
spectra can be obtained by adjusting a single parameter, the spin-fermion
coupling constant. In particular, we find a quantitative link between two
spectral features that have been established as universal for the cuprates,
namely high-energy spin excitations and "kinks" in the fermionic band
dispersions along the nodal direction. The superconducting transition
temperature computed with this coupling constant exceeds 150 K, demonstrating
that spin fluctuations have sufficient strength to mediate high-temperature
superconductivity.Comment: 25 pages, 7 figures, including supplementary information, accepted
for publication in Nature Physic
Intense paramagnon excitations in a large family of high-temperature superconductors
In the search for the mechanism of high-temperature superconductivity,
intense research has been focused on the evolution of the spin excitation
spectrum upon doping from the antiferromagnetic insulating to the
superconducting states of the cuprates. Because of technical limitations, the
experimental investigation of doped cuprates has been largely focused on
low-energy excitations in a small range of momentum space. Here we use resonant
inelastic x-ray scattering to show that a large family of superconductors,
encompassing underdoped YBaCuO and overdoped YBaCuO,
exhibits damped spin excitations (paramagnons) with dispersions and spectral
weights closely similar to those of magnons in undoped cuprates. %The results
are in excellent agreement with the spin excitations obtained by exact
diagonalization of the Hamiltonian on finite-sized clusters. The
comprehensive experimental description of this surprisingly simple spectrum
permits quantitative tests of magnetic Cooper pairing models. A numerical
solution of the Eliashberg equations for the magnetic spectrum of
YBaCuO reproduces its superconducting transition temperature
within a factor of two, a level of agreement comparable to Eliashberg theories
of conventional superconductors.Comment: Main text (11 pages, 4 figures) + supplementary information (4 pages,
4 figures, 1 table). An updated version will appear in Nature Physic
Emergence of coherent magnetic excitations in the high temperature underdoped La2-xSrxCuO4 superconductor at low temperatures
We use inelastic neutron scattering to measure the magnetic excitations in
the underdoped superconductor La2-xSrxCuO4 (x=0.085, Tc=22 K) over energy and
temperatures ranges 5 < E < 200 meV and 5 < T < 300 K respectively. At high
temperature (T = 300 K), we observe strongly damped excitations with a
characteristic energy scale of approximately 50 meV. As the temperature is
lowered to T = 30 K, and we move into the pseudogap state, the magnetic
excitations become highly structured in energy and momentum below about 60 meV.
This change appears to be associated with the development of the pseudogap in
the electronic excitations
Spin Waves in the (pi, 0) Magnetically Ordered Iron Chalcogenide Fe1.05Te
We use inelastic neutron scattering to show that the spin waves in the iron
chalcogenide FeTe display novel dispersion clearly different from
those in the related iron pnictide systems. By fitting the spin waves to a
Heisenberg Hamiltonian, we extract magnetic exchange couplings that are
dramatically different from both predictions by density functional calculations
and measurements on the iron pnictide CaFeAs. While the
nearest-neighbor exchange couplings in CaFeAs and FeTe are
quite different, their next-nearest-neighbor exchange couplings are similar.
These results suggest that superconductivity in the pnictides and chalcogenides
share a common magnetic origin that is intimately associated with the
next-nearest-neighbor magnetic coupling between the irons.Comment: Includes supplementary information as appendice