395 research outputs found
Four-fold structure of vortex core states in Bi2Sr2CaCu2O8 (Bi2212)
We present a detailed study of vortex core spectroscopy in slightly overdoped
Bi2Sr2CaCu2O8 using a low temperature scanning tunneling microscope. Inside the
vortex core we observe a four-fold symmetric modulation of the local density of
states with an energy-independent period of (4.3\pm 0.3)a0. Furthermore we
demonstrate that this square modulation is related to the vortex core states
which are located at ~6 meV. Since the core-state energy is proportional to the
superconducting gap magnitude, our results strongly suggest the existence of a
direct relation between the superconducting state and the local electronic
modulations in the vortex core.Comment: 5 pages, 4 figures. Submitted to Physical Review Letter
Preeminent role of the Van Hove singularity in the strong-coupling analysis of scanning tunneling spectroscopy for two-dimensional cuprates
In two dimensions the non-interacting density of states displays a Van Hove
singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent
in three dimensions. We show that due to this VHS the strong-coupling analysis
of tunneling spectra in high- superconductors must be reconsidered. Based
on a microscopic model which reproduces the experimental data with great
accuracy, we elucidate the peculiar role played by the VHS in shaping the
tunneling spectra, and show that more conventional analyses of strong-coupling
effects can lead to severe errors.Comment: 5 pages, 4 figure
Peroxynitrite Inhibits Glutamate Transporter Subtypes
The reuptake of glutamate in neurons and astrocytes terminates excitatory signals and prevents the persistence of excitotoxic levels of glutamate in the synaptic cleft. This process is inhibited by oxygen radicals and hydrogen peroxide (H2O2). Here we show that another biological oxidant, peroxynitrite (ONOO-), formed by combination of superoxide (O2-) and nitric oxide (NO), potently inhibits glutamate uptake by purified or recombinant high affinity glutamate transporters reconstituted in liposomes. ONOO- reduces selectively the Vmax of transport; its action is fast (reachingor = 90% within 20 s), dose-dependent (50% inhibition at 50 microM), persistent upon ONOO- (or by product) removal, and insensitive to the presence of the lipid antioxidant vitamin E in the liposomal membranes. Therefore, it likely depends on direct interaction of ONOO- with the glutamate transporters. Three distinct recombinant glutamate transporters from the rat brain, GLT1, GLAST, and EAAC1, exhibit identical sensitivity to ONOO . H2O2 also inhibits reconstituted transport, and its action matches that of ONOO- on all respects; however, this is observed only with 5-10 mM H202 and after prolonged exposure (10 min) in highly oxygenated buffer. NO, released from NO donors (up to 10 mM), does not modify reconstituted glutamate uptake, although in parallel conditions it promotes cGMP formation in synaptosomal cytosolic fraction. Overall, our results suggest that the glutamate transporters contain conserved sites in their structures conferring vulnerability to ONOO- and other oxidants
Trigeminal microvascular decompression for short-lasting unilateral neuralgiform headache attacks
A significant proportion of patients with short-lasting unilateral neuralgiform headache attacks (SUNHA) are refractory to medical treatments. Neuroimaging studies have suggested a role for ipsilateral trigeminal neurovascular conflict with morphological changes in the pathophysiology of this disorder. We present the outcome of an uncontrolled open-label prospective single centre study conducted between 2012 and 2020, to evaluate the efficacy and safety of trigeminal microvascular decompression in refractory chronic SUNHA with magnetic resonance imaging evidence of trigeminal neurovascular conflict ipsilateral to the pain side. Primary endpoint was the proportion of patients who achieved an "excellent response", defined as 90-100% weekly reduction in attack frequency, or "good response", defined as a reduction in weekly headache attack frequency between 75% and 89% at final follow-up, compared to baseline. These patients were defined as responders. The study group consisted of 47 patients of whom 31 had SUNCT and 16 had SUNA (25 females, mean age ± SD 55.2 years ± 14.8). Participants failed to respond or tolerate a mean of 8.1 (±2.7) preventive treatments pre-surgery. Magnetic resonance imaging of the trigeminal nerves (n = 47 patients, n = 50 symptomatic trigeminal nerves) demonstrated ipsilateral neurovascular conflict with morphological changes in 39/50 (78.0%) symptomatic nerves and without morphological changes in 11/50 (22.0%) symptomatic nerves. Post-operatively, 37/47 (78.7%) patients obtained either an excellent or a good response. Ten patients (21.3%, SUNCT = 7 and SUNA = 3) reported no post-operative improvement. The mean post-surgery follow-up was 57.4 ± 24.3 months (range 11-96 months). At final follow-up, 31 patients (66.0%) were excellent/good responders. Six patients experienced a recurrence of headache symptoms. There was no statistically significant difference between SUNCT and SUNA in the response to surgery (p = 0.463). Responders at the last follow-up were however more likely not to have interictal pain (77.42% vs 22.58%, p = 0.021) and to show morphological changes on the magnetic resonance imaging (78.38% vs 21.62%, p = 0.001). The latter outcome was confirmed in the Kaplan Meyer analysis, where patients with no morphological changes were more likely to relapse overtime compared to those with morphological changes (p = 0.0001). All but one patient who obtained an excellent response without relapse, discontinued their preventive medications. Twenty-two post-surgery adverse events occurred in 18 patients (46.8%) but no mortality or severe neurological deficit was seen. Trigeminal microvascular decompression may be a safe and effective long-term treatment for short-lasting unilateral neuralgiform headache attacks patients with magnetic resonance evidence of neurovascular conflict with morphological changes
Establishing non-thermal regimes in pump-probe electron-relaxation dynamics
Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the
electronic structure of solids under optical excitation, and is a powerful
technique for studying the coupling between electrons and collective modes. One
approach to infer electron-boson coupling is through the relaxation dynamics of
optically-excited electrons, and the characteristic timescales of energy
redistribution. A common description of electron relaxation dynamics is through
the effective electronic temperature. Such a description requires that
thermodynamic quantities are well-defined, an assumption that is generally
violated at early delays. Additionally, precise estimation of the non-thermal
window -- within which effective temperature models may not be applied -- is
challenging. We perform TR-ARPES on graphite and show that Boltzmann rate
equations can be used to calculate the time-dependent electronic occupation
function, and reproduce experimental features given by non-thermal electron
occupation. Using this model, we define a quantitative measure of non-thermal
electron occupation and use it to define distinct phases of electron relaxation
in the fluence-delay phase space. More generally, this approach can be used to
inform the non-thermal-to-thermal crossover in pump-probe experiments.Comment: 18 pages, 10 figure
A versatile laser-based apparatus for time-resolved ARPES with micro-scale spatial resolution
We present the development of a versatile apparatus for a 6.2 eV laser-based
time and angle-resolved photoemission spectroscopy with micrometer spatial
resolution (time-resolved -ARPES). With a combination of tunable spatial
resolution down to 11 m, high energy resolution (11 meV),
near-transform-limited temporal resolution (280 fs), and tunable 1.55 eV
pump fluence up to 3 mJ/cm, this time-resolved -ARPES system
enables the measurement of ultrafast electron dynamics in exfoliated and
inhomogeneous materials. We demonstrate the performance of our system by
correlating the spectral broadening of the topological surface state of
BiSe with the spatial dimension of the probe pulse, as well as
resolving the spatial inhomogeneity contribution to the observed spectral
broadening. Finally, after in-situ exfoliation, we performed time-resolved
-ARPES on a 30 m few-layer-thick flake of transition metal
dichalcogenide WTe, thus demonstrating the ability to access ultrafast
electron dynamics with momentum resolution on micro-exfoliated and twisted
materials
Eddy-driven pulses of respiration in the Sargasso Sea
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 56 (2009): 1242-1250, doi:10.1016/j.dsr.2009.03.001.An analysis of nine years of data from the NW subtropical Atlantic reveals that variability in heterotrophic processes associated with (sub)mesoscale features has a major impact on the balance between photosynthesis and respiration. Higher indirect estimates of net community production (NCPe) are associated with the center of Mode Water Eddies (MWE) and frontal regions between cyclonic and anticyclonic eddies (CA). The increase in NCPe observed at the center of MWE is driven mainly by an increase in autotrophic production, whereas in CA enhanced NCPe rates are the result of an important reduction in bacterial respiration. Both features also exhibit a decrease in nitrate concentration, consistent with nutrient consumption, and relative increases in oxygen anomaly and particulate and dissolved organic carbon in the upper 200 m. Plankton community composition in CA and MWE is characterized by the reduction in bacterial biomass, and the dominance of Prochlorococcus and Synechococcus in CA, and diatoms and dinoflagellates in MWE. Contrary to a common assumption, these results show for the first time that in ecosystems influenced by (sub)mesoscale dynamics, respiration can be as variable as photosynthesis.Support by a Fulbright postdoctoral fellowship and the Juan de la Cierva program from the Spanish Ministry of Science and Education. Support of this activity by the U.S. National Science Foundation and the National Aeronautics and Space Administration is greatly appreciated
Direct determination of mode-projected electron-phonon coupling in the time-domain
Ultrafast spectroscopies have become an important tool for elucidating the
microscopic description and dynamical properties of quantum materials. In
particular, by tracking the dynamics of non-thermal electrons, a material's
dominant scattering processes -- and thus the many-body interactions between
electrons and collective excitations -- can be revealed. Here we present a new
method for extracting the electron-phonon coupling strength in the time domain,
by means of time and angle-resolved photoemission spectroscopy (TR-ARPES). This
method is demonstrated in graphite, where we investigate the dynamics of
photo-injected electrons at the K point, detecting quantized energy-loss
processes that correspond to the emission of strongly-coupled optical phonons.
We show that the observed characteristic timescale for spectral-weight-transfer
mediated by phonon-scattering processes allows for the direct quantitative
extraction of electron-phonon matrix elements, for specific modes, and with
unprecedented sensitivity.Comment: 19 pages, 4 figure
Unveiling the underlying interactions in Ta2NiSe5 from photo-induced lifetime change
We present a generic procedure for quantifying the interplay of electronic
and lattice degrees of freedom in photo-doped insulators through a comparative
analysis of theoretical many-body simulations and time- and angle-resolved
photoemission spectroscopy (TR-ARPES) of the transient response of the
candidate excitonic insulator Ta2NiSe5. Our analysis demonstrates that the
electron-electron interactions dominate the electron-phonon ones. In
particular, a detailed analysis of the TRARPES spectrum enables a clear
separation of the dominant broadening (electronic lifetime) effects from the
much smaller bandgap renormalization. Theoretical calculations show that the
observed strong spectral broadening arises from the electronic scattering of
the photo-excited particle-hole pairs and cannot be accounted for in a model in
which electron-phonon interactions are dominant. We demonstrate that the
magnitude of the weaker subdominant bandgap renormalization sensitively depends
on the distance from the semiconductor/semimetal transition in the
high-temperature state, which could explain apparent contradictions between
various TR-ARPES experiments. The analysis presented here indicates that
electron-electron interactions play a vital role (although not necessarily the
sole one) in stabilizing the insulating state
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