66 research outputs found
High-transition-temperature superconductivity in the absence of the magnetic-resonance mode
The fundamental mechanism that gives rise to high-transition-temperature
(high-Tc) superconductivity in the copper oxide materials has been debated
since the discovery of the phenomenon. Recent work has focussed on a sharp
'kink' in the kinetic energy spectra of the electrons as a possible signature
of the force that creates the superconducting state. The kink has been related
to a magnetic resonance and also to phonons. Here we report that infrared
spectra of Bi2Sr2CaCu2O(8+d), (Bi-2212) show that this sharp feature can be
separated from a broad background and, interestingly, weakens with doping
before disappearing completely at a critical doping level of 0.23 holes per
copper atom. Superconductivity is still strong in terms of the transition
temperature (Tc approx 55 K), so our results rule out both the magnetic
resonance peak and phonons as the principal cause of high-Tc superconductivity.
The broad background, on the other hand, is a universal property of the copper
oxygen plane and a good candidate for the 'glue' that binds the electrons.Comment: 4 pages, 3 figure
Seismic reflection images of a near-axis melt sill within the lower crust at the Juan de Fuca ridge
Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 460 (2009): 89-93, doi:10.1038/nature08095.The oceanic crust extends over two thirds of the Earth’s solid surface and is
generated along mid-ocean ridges from melts derived from the upwelling mantle.
The upper and mid crust are constructed by dyking and seafloor eruptions
originating from magma accumulated in mid-crustal lenses at the spreading axis,
but the style of accretion of the lower oceanic crust is actively debated. Models
based on geological and petrological data from ophiolites propose that the lower
oceanic crust is accreted from melt sills intruded at multiple levels between the
Moho transition zone (MTZ) and the mid-crustal lens, consistent with
geophysical studies that suggest the presence of melt within the lower crust.
However, seismic images of molten sills within the lower crust have been elusive.
To date only seismic reflections from mid-crustal melt lenses and sills within
the MTZ have been described, suggesting that melt is efficiently transported
through the lower crust. Here we report deep crustal seismic reflections off the
southern Juan de Fuca Ridge that we interpret as originating from a molten sill
presently accreting the lower oceanic crust. The sill sits 5-6 km beneath the
seafloor and 850-900 m above the MTZ, and it is located 1.4-3.2 km off thespreading axis. Our results provide evidence for the existence of low permeability
barriers to melt migration within the lower section of modern oceanic crust
forming at intermediate-to-fast spreading rates, as inferred from ophiolite
studies.This research was supported by grants form the US NSF
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
Central role of detachment faults in accretion of slow-spreading oceanic lithosphere
Author Posting. © Macmillan Publishers, 2008. This is the author's version of the work. It is posted here by permission of Macmillan Publishers for personal use, not for redistribution. The definitive version was published in Nature 455 (2008): 790-794, doi:10.1038/nature07333.The formation of oceanic detachment faults is well established from inactive,
corrugated fault planes exposed on seafloor formed along ridges spreading at less
than 80 km/My1-4. These faults can accommodate extension for up to 1-3 Myrs5,
and are associated with one of two contrasting modes of accretion operating along
the northern Mid-Atlantic Ridge (MAR). The first is symmetrical accretion,
dominated by magmatic processes with subsidiary high-angle faulting and
formation of abyssal hills on both flanks. The second is asymmetrical accretion
involving an active detachment fault6 along one ridge flank. An examination of
~2500 km of the MAR between 12.5 and 35°N reveals asymmetrical accretion
along almost half of the ridge. Hydrothermal activity identified to date in the study
region is closely associated with asymmetrical accretion, which also exhibits high-levels
of near continuous hydroacoustically and teleseismically recorded seismicity.
Enhanced seismicity is probably generated along detachment faults
accommodating a sizeable proportion of the total plate separation. In contrast,
symmetrical segments have lower levels of seismicity, which concentrates primarily
at their ends. Basalts erupted along asymmetrical segments have compositions that
are consistent with crystallization at higher pressures than basalts from
symmetrical segments, and with lower extents of partial melting of the mantle.
Both seismic and geochemical evidence indicate that the axial lithosphere is thicker
and colder at asymmetrical sections of the ridge, either because associated
hydrothermal circulation efficiently penetrates to greater depths, or because the
rising mantle is cooler. We suggest that much of the variability in seafloor
morphology, seismicity and basalt chemistry found along slow-spreading ridges
can be thus attributed to the frequent involvement of detachments in oceanic
lithospheric accretion.Supported by CNRS (JE), NSF (DKS, HS, JC, CL and SE), WHOI (JE, DKS,
HS and JC), Harvard University (JE, CL and SE), Univ. of Leeds (JC), and MIT (JE)
Quasiparticle interference and strong electron-mode coupling in the quasi-one-dimensional bands of Sr2RuO4
The single-layered ruthenate SrRuO has attracted a great deal of
interest as a spin-triplet superconductor with an order parameter that may
potentially break time reversal invariance and host half-quantized vortices
with Majorana zero modes. While the actual nature of the superconducting state
is still a matter of controversy, it has long been believed that it condenses
from a metallic state that is well described by a conventional Fermi liquid. In
this work we use a combination of Fourier transform scanning tunneling
spectroscopy (FT-STS) and momentum resolved electron energy loss spectroscopy
(M-EELS) to probe interaction effects in the normal state of SrRuO. Our
high-resolution FT-STS data show signatures of the \beta-band with a distinctly
quasi-one-dimensional (1D) character. The band dispersion reveals surprisingly
strong interaction effects that dramatically renormalize the Fermi velocity,
suggesting that the normal state of SrRuO is that of a 'correlated
metal' where correlations are strengthened by the quasi 1D nature of the bands.
In addition, kinks at energies of approximately 10meV, 38meV and 70meV are
observed. By comparing STM and M-EELS data we show that the two higher energy
features arise from coupling with collective modes. The strong correlation
effects and the kinks in the quasi 1D bands may provide important information
for understanding the superconducting state. This work opens up a unique
approach to revealing the superconducting order parameter in this compound
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
Diversity of Meiofauna from the 9°50′N East Pacific Rise across a Gradient of Hydrothermal Fluid Emissions
Background: We studied the meiofauna community at deep-sea hydrothermal vents along a gradient of vent fluid emissions in the axial summit trought (AST) of the East Pacific Rise 9 degrees 50'N region. The gradient ranged from extreme high temperatures, high sulfide concentrations, and low pH at sulfide chimneys to ambient deep-sea water conditions on bare basalt. We explore meiofauna diversity and abundance, and discuss its possible underlying ecological and evolutionary processes.
Methodology/Principal Findings: After sampling in five physico-chemically different habitats, the meiofauna was sorted, counted and classified. Abundances were low at all sites. A total of 52 species were identified at vent habitats. The vent community was dominated by hard substrate generalists that also lived on bare basalt at ambient deep-sea temperature in the axial summit trough (AST generalists). Some vent species were restricted to a specific vent habitat (vent specialists), but others occurred over a wide range of physico-chemical conditions (vent generalists). Additionally, 35 species were only found on cold bare basalt (basalt specialists). At vent sites, species richness and diversity clearly increased with decreasing influence of vent fluid emissions from extreme flow sulfide chimney (no fauna), high flow pompei worm (S: 4-7, H-loge': 0.11-0.45), vigorous flow tubeworm (S: 8-23; H-loge': 0.44-2.00) to low flow mussel habitats (S: 28-31; H-loge': 2.34-2.60).
Conclusions/Significance: Our data suggest that with increasing temperature and toxic hydrogen sulfide concentrations and increasing amplitude of variation of these factors, fewer species are able to cope with these extreme conditions. This results in less diverse communities in more extreme habitats. The finding of many species being present at sites with and without vent fluid emissions points to a non endemic deep-sea hydrothermal vent meiofaunal community. This is in contrast to a mostly endemic macrofauna but similar to what is known for meiofauna from shallow-water vents
Understanding Novel Superconductors with Ab Initio Calculations
This chapter gives an overview of the progress in the field of computational
superconductivity.
Following the MgB2 discovery (2001), there has been an impressive
acceleration in the development of methods based on Density Functional Theory
to compute the critical temperature and other physical properties of actual
superconductors from first-principles. State-of-the-art ab-initio methods have
reached predictive accuracy for conventional (phonon-mediated) superconductors,
and substantial progress is being made also for unconventional superconductors.
The aim of this chapter is to give an overview of the existing computational
methods for superconductivity, and present selected examples of material
discoveries that exemplify the main advancements.Comment: 38 pages, 10 figures, Contribution to Springer Handbook of Materials
Modellin
The Temperature Dependence of the Electron Mean Free Path in the Radiofrequency Size Effect of Potassium
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