522 research outputs found
Abrupt Onset of Second Energy Gap at Superconducting Transition of Underdoped Bi2212
The superconducting gap - an energy scale tied to the superconducting
phenomena-opens on the Fermi surface at the superconducting transition
temperature (TC) in conventional BCS superconductors. Quite differently, in
underdoped high-TC superconducting cuprates, a pseudogap, whose relation to the
superconducting gap remains a mystery, develops well above TC. Whether the
pseudogap is a distinct phenomenon or the incoherent continuation of the
superconducting gap above TC is one of the central questions in high-TC
research. While some experimental evidence suggests they are distinct, this
issue is still under intense debate. A crucial piece of evidence to firmly
establish this two-gap picture is still missing: a direct and unambiguous
observation of a single-particle gap tied to the superconducting transition as
function of temperature. Here we report the discovery of such an energy gap in
underdoped Bi2212 in the momentum space region overlooked in previous
measurements. Near the diagonal of Cu-O bond direction (nodal direction), we
found a gap which opens at TC and exhibits a canonical (BCS-like) temperature
dependence accompanied by the appearance of the so-called Bogoliubov
quasiparticles, a classical signature of superconductivity. This is in sharp
contrast to the pseudogap near the Cu-O bond direction (antinodal region)
measured in earlier experiments. The emerging two-gap phenomenon points to a
picture of richer quantum configurations in high temperature superconductors.Comment: 16 pages, 4 figures, authors' version Corrected typos in the abstrac
Direct evidence for a competition between the pseudogap and high temperature superconductivity in the cuprates
A pairing gap and coherence are the two hallmarks of superconductivity. In a
classical BCS superconductor they are established simultaneously at Tc. In the
cuprates, however, an energy gap (pseudogap) extends above Tc. The origin of
this gap is one of the central issues in high temperature superconductivity.
Recent experimental evidence demonstrates that the pseudogap and the
superconducting gap are associated with different energy scales. It is however
not clear whether they coexist independently or compete. In order to understand
the physics of cuprates and improve their superconducting properties it is
vital to determine whether the pseudogap is friend or foe of high temperature
supercondctivity. Here we report evidence from angle resolved photoemission
spectroscopy (ARPES) that the pseudogap and high temperature superconductivity
represent two competing orders. We find that there is a direct correlation
between a loss in the low energy spectral weight due to the pseudogap and a
decrease of the coherent fraction of paired electrons. Therefore, the pseudogap
competes with the superconductivity by depleting the spectral weight available
for pairing in the region of momentum space where the superconducting gap is
largest. This leads to a very unusual state in the underdoped cuprates, where
only part of the Fermi surface develops coherence.Comment: Improved version was published in Natur
Disentangling Cooper-pair formation above Tc from the pseudogap state in the cuprates
The discovery of the pseudogap in the cuprates created significant excitement
amongst physicists as it was believed to be a signature of pairing, in some
cases well above the room temperature. In this "pre-formed pairs" scenario, the
formation of pairs without quantum phase rigidity occurs below T*. These pairs
condense and develop phase coherence only below Tc. In contrast, several recent
experiments reported that the pseudogap and superconducting states are
characterized by two different energy scales, pointing to a scenario, where the
two compete. However a number of transport, magnetic, thermodynamic and
tunneling spectroscopy experiments consistently detect a signature of
phase-fluctuating superconductivity above leaving open the question of whether
the pseudogap is caused by pair formation or not. Here we report the discovery
of a spectroscopic signature of pair formation and demonstrate that in a region
of the phase diagram commonly referred to as the "pseudogap", two distinct
states coexist: one that persists to an intermediate temperature Tpair and a
second that extends up to T*. The first state is characterized by a doping
independent scaling behavior and is due to pairing above Tc, but significantly
below T*. The second state is the "proper" pseudogap - characterized by a
"checker board" pattern in STM images, the absence of pair formation, and is
likely linked to Mott physics of pristine CuO2 planes. Tpair has a universal
value around 130-150K even for materials with very different Tc, likely setting
limit on highest, attainable Tc in cuprates. The observed universal scaling
behavior with respect to Tpair indicates a breakdown of the classical picture
of phase fluctuations in the cuprates.Comment: 9 pages, 4 figure
Nodal quasiparticle meltdown in ultra-high resolution pump-probe angle-resolved photoemission
High- cuprate superconductors are characterized by a strong
momentum-dependent anisotropy between the low energy excitations along the
Brillouin zone diagonal (nodal direction) and those along the Brillouin zone
face (antinodal direction). Most obvious is the d-wave superconducting gap,
with the largest magnitude found in the antinodal direction and no gap in the
nodal direction. Additionally, while antinodal quasiparticle excitations appear
only below , superconductivity is thought to be indifferent to nodal
excitations as they are regarded robust and insensitive to . Here we
reveal an unexpected tie between nodal quasiparticles and superconductivity
using high resolution time- and angle-resolved photoemission on optimally doped
BiSrCaCuO. We observe a suppression of the nodal
quasiparticle spectral weight following pump laser excitation and measure its
recovery dynamics. This suppression is dramatically enhanced in the
superconducting state. These results reduce the nodal-antinodal dichotomy and
challenge the conventional view of nodal excitation neutrality in
superconductivity.Comment: 7 pages, 3 figure. To be published in Nature Physic
Eustachian tube dysfunction: consensus statement on definition, types, clinical presentation and diagnosis
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Energy gaps in the failed high-Tc superconductor La1.875Ba0.125CuO4
A central issue on high-Tc superconductivity is the nature of the
normal-state gap (pseudogap) in the underdoped regime and its relationship with
superconductivity. Despite persistent efforts, theoretical ideas for the
pseudogap evolve around fluctuating superconductivity, competing order and
spectral weight suppression due to many-body effects. Recently, while some
experiments in the superconducting state indicate a distinction between the
superconducting gap and pseudogap, others in the normal state, either by
extrapolation from high-temperature data or directly from La1.875Ba0.125CuO4
(LBCO-1/8) at low temperature, suggest the ground-state pseudogap is a single
gap of d-wave form. Here we report angle-resolved photoemission (ARPES) data
from LBCO-1/8, collected with improved experimental conditions, that reveal the
ground-state pseudogap has a pronounced deviation from the simple d-wave form.
It contains two distinct components: a d-wave component within an extended
region around the node and the other abruptly enhanced close to the antinode,
pointing to a dual nature of the pseudogap in this failed high-Tc
superconductor which involves a possible precursor pairing energy scale around
the node and another of different but unknown origin near the antinode.Comment: Nature Physics advance online publication, Dec. 21st 2008; Author's
original version of the main text; for a better resolution of figures &
Supplementary Information, visit Nature Physics' websit
Interpersonal violence: an important risk factor for disease and injury in South Africa
<p>Abstract</p> <p>Background</p> <p>Burden of disease estimates for South Africa have highlighted the particularly high rates of injuries related to interpersonal violence compared with other regions of the world, but these figures tell only part of the story. In addition to direct physical injury, violence survivors are at an increased risk of a wide range of psychological and behavioral problems. This study aimed to comprehensively quantify the excess disease burden attributable to exposure to interpersonal violence as a risk factor for disease and injury in South Africa.</p> <p>Methods</p> <p>The World Health Organization framework of interpersonal violence was adapted. Physical injury mortality and disability were categorically attributed to interpersonal violence. In addition, exposure to child sexual abuse and intimate partner violence, subcategories of interpersonal violence, were treated as risk factors for disease and injury using counterfactual estimation and comparative risk assessment methods. Adjustments were made to account for the combined exposure state of having experienced both child sexual abuse and intimate partner violence.</p> <p>Results</p> <p>Of the 17 risk factors included in the South African Comparative Risk Assessment study, interpersonal violence was the second leading cause of healthy years of life lost, after unsafe sex, accounting for 1.7 million disability-adjusted life years (DALYs) or 10.5% of all DALYs (95% uncertainty interval: 8.5%-12.5%) in 2000. In women, intimate partner violence accounted for 50% and child sexual abuse for 32% of the total attributable DALYs.</p> <p>Conclusions</p> <p>The implications of our findings are that estimates that include only the direct injury burden seriously underrepresent the full health impact of interpersonal violence. Violence is an important direct and indirect cause of health loss and should be recognized as a priority health problem as well as a human rights and social issue. This study highlights the difficulties in measuring the disease burden from interpersonal violence as a risk factor and the need to improve the epidemiological data on the prevalence and risks for the different forms of interpersonal violence to complete the picture. Given the extent of the burden, it is essential that innovative research be supported to identify social policy and other interventions that address both the individual and societal aspects of violence.</p
Performance of the CMS Cathode Strip Chambers with Cosmic Rays
The Cathode Strip Chambers (CSCs) constitute the primary muon tracking device
in the CMS endcaps. Their performance has been evaluated using data taken
during a cosmic ray run in fall 2008. Measured noise levels are low, with the
number of noisy channels well below 1%. Coordinate resolution was measured for
all types of chambers, and fall in the range 47 microns to 243 microns. The
efficiencies for local charged track triggers, for hit and for segments
reconstruction were measured, and are above 99%. The timing resolution per
layer is approximately 5 ns
Decrease of upper critical field with underdoping in cuprate superconductors
The transition temperature Tc of cuprate superconductors falls when the
doping p is reduced below a certain optimal value. It is unclear whether this
fall is due to strong phase fluctuations or to a decrease in the pairing gap.
Different interpretations of photoemission data disagree on the evolution of
the pairing gap and different estimates of the upper critical field Hc2 are in
sharp contradiction. Here we resolve this contradiction by showing that
superconducting fluctuations in the underdoped cuprate Eu-LSCO, measured via
the Nernst effect, have a characteristic field scale that falls with
underdoping. The critical field Hc2 dips at p = 0.11, showing that
superconductivity is weak where stripe order is strong. In the archetypal
cuprate superconductor YBCO, Hc2 extracted from other measurements has the same
doping dependence, also with a minimum at p = 0.11, again where stripe order is
present. We conclude that competing states such as stripe order weaken
superconductivity and this, rather than phase fluctuations, causes Tc to fall
as cuprates become underdoped.Comment: Supplementary Information file available upon request; Nature Physics
(2012
High activity Rhenium-186 HEDP with autologous peripheral blood stem cell rescue: a phase I study in progressive hormone refractory prostate cancer metastatic to bone
We tested the feasibility and toxicity of high activities Rhenium-186 hydroxyethylidene diphosphonate, with peripheral blood stem cell rescue in patients with progressive hormone refractory prostate cancer metastatic to bone. Twenty-five patients received between 2500 and 5000 MBq of Rhenium-186 hydroxyethylidene diphosphonate followed 14 days later by the return of peripheral blood peripheral blood stem cells. Activity limiting toxicity was defined as grade III haematological toxicity, lasting at least 7 days, or grade IV haematological toxicity of any duration or any serious unexpected toxicity. Activity limiting toxicity occurred in two of six who received activities of 5000 MBq and maximum tolerated activity was defined at this activity level. Prostate specific antigen reductions of 50% or more lasting at least 4 weeks were seen in five of the 25 patients (20%) all of whom received more than 3500 MBq of Rhenium-186 hydroxyethylidene diphosphonate. The actuarial survival at 1 year is 54%. Administered activities of 5000 MBq of Rhenium-186 hydroxyethylidene diphosphonate are feasible using autologous peripheral blood peripheral blood stem cell rescue in patients with progressive hormone refractory prostate cancer metastatic to bone. The main toxicity is thrombocytopaenia, which is short lasting. A statistically significant activity/prostate specific antigen response was seen. We have now commenced a Phase II trial to further evaluate response rates
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