286 research outputs found
Efficient light-emitting diodes from organic radicals with doublet emission
Organic light-emitting diodes (OLEDs) with doublet-spin radical emitters have emerged as a new route to efficient display technologies. In contrast to standard organic semiconductors, radical materials have unpaired electrons. This feature results in the most well-known examples of organic radicals being where they are reactive species in chemical reactions. Stabilized radicals can be used in optoelectronic applications, which exploit their optical and spin properties, allowing up to 100% internal quantum efficiency (IQE) for electroluminescence. Highly efficient OLEDs have been demonstrated, which operate in the doublet-spin electronic state manifold with doublet emission. The radical-based devices present a departure from the singlet- and triplet-level considerations that impose efficiency limits in OLEDs for typical organic semiconductors (25% IQE). This Perspective focuses on radical doublet emitters for optoelectronics, outlining how the photo- and spin-physics of unpaired electron systems present new avenues for research in light-emitting applications
First direct observation of the Van Hove singularity in the tunneling spectra of cuprates
In two-dimensional lattices the electronic levels are unevenly spaced, and
the density of states (DOS) displays a logarithmic divergence known as the Van
Hove singularity (VHS). This is the case in particular for the layered cuprate
superconductors. The scanning tunneling microscope (STM) probes the DOS, and is
therefore the ideal tool to observe the VHS. No STM study of cuprate
superconductors has reported such an observation so far giving rise to a debate
about the possibility of observing directly the normal state DOS in the
tunneling spectra. In this study, we show for the first time that the VHS is
unambiguously observed in STM measurements performed on the cuprate Bi-2201.
Beside closing the debate, our analysis proves the presence of the pseudogap in
the overdoped side of the phase diagram of Bi-2201 and discredits the scenario
of the pseudogap phase crossing the superconducting dome.Comment: 4 pages, 4 figure
Imaging the Two Gaps of the High-TC Superconductor Pb-Bi2Sr2CuO6+x
The nature of the pseudogap state, observed above the superconducting
transition temperature TC in many high temperature superconductors, is the
center of much debate. Recently, this discussion has focused on the number of
energy gaps in these materials. Some experiments indicate a single energy gap,
implying that the pseudogap is a precursor state. Others indicate two,
suggesting that it is a competing or coexisting phase. Here we report on
temperature dependent scanning tunneling spectroscopy of Pb-Bi2Sr2CuO6+x. We
have found a new, narrow, homogeneous gap that vanishes near TC, superimposed
on the typically observed, inhomogeneous, broad gap, which is only weakly
temperature dependent. These results not only support the two gap picture, but
also explain previously troubling differences between scanning tunneling
microscopy and other experimental measurements.Comment: 6 page
Fully gapped topological surface states in BiSe films induced by a d-wave high-temperature superconductor
Topological insulators are a new class of materials, that exhibit robust
gapless surface states protected by time-reversal symmetry. The interplay
between such symmetry-protected topological surface states and symmetry-broken
states (e.g. superconductivity) provides a platform for exploring novel quantum
phenomena and new functionalities, such as 1D chiral or helical gapless
Majorana fermions, and Majorana zero modes which may find application in
fault-tolerant quantum computation. Inducing superconductivity on topological
surface states is a prerequisite for their experimental realization. Here by
growing high quality topological insulator BiSe films on a d-wave
superconductor BiSrCaCuO using molecular beam epitaxy,
we are able to induce high temperature superconductivity on the surface states
of BiSe films with a large pairing gap up to 15 meV. Interestingly,
distinct from the d-wave pairing of BiSrCaCuO, the
proximity-induced gap on the surface states is nearly isotropic and consistent
with predominant s-wave pairing as revealed by angle-resolved photoemission
spectroscopy. Our work could provide a critical step toward the realization of
the long sought-after Majorana zero modes.Comment: Nature Physics, DOI:10.1038/nphys274
Discovery of microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x
The parent compounds of the copper oxide high-Tc superconductors are unusual
insulators. Superconductivity arises when they are properly doped away from
stoichiometry1. In Bi2Sr2CaCu2O8+x, superconductivity results from doping with
excess oxygen atoms, which introduce positive charge carriers (holes) into the
CuO2 planes, where superconductivity is believed to originate. The role of
these oxygen dopants is not well understood, other than the fact that they
provide charge carriers. However, it is not even clear how these charges
distribute in the CuO2 planes. Accordingly, many models of high-Tc
superconductors simply assume that the charge carriers introduced by doping
distribute uniformly, leading to an electronically homogeneous system, as in
ordinary metals. Here we report the observation of an electronic inhomogeneity
in the high-Tc superconductor Bi2Sr2CaCu2O8+x using scanning tunnelling
microscopy/spectroscopy. This inhomogeneity is manifested as spatial variations
in both the local density of states spectrum and the superconducting energy
gap. These variations are correlated spatially and vary on a surprisingly short
length scale of ~ 14 Angs. Analysis suggests that the inhomogeneity observed is
a consequence of proximity to a Mott insulator resulting in poor screening of
the charge potentials associated with the oxygen ions left behind in the BiO
plane after doping. Hence this experiment is a direct probe of the local nature
of the superconducting state, which is not easily accessible by macroscopic
measurements.Comment: 6 pages, 4 figure
Personality styles in patients with fibromyalgia, major depression and healthy controls
BACKGROUND: The fibromyalgia syndrome (FMS) is suggested to be a manifestation of depression or affective spectrum disorder. We measured the cognitive style of patients with FMS to assess personality styles in 44 patients with fibromyalgia syndrome (FMS) by comparing them with 43 patients with major depressive disorder (MDD) and 41 healthy controls (HC). METHODS: Personality styles were measured by the Sociotropy and Autonomy Scale (SAS) and the Dysfunctional Attitude Scale (DAS). The Structured Clinical interview for DSM Axis I was applied to Axis I disorders, while the Beck Depression Inventory was used to measure depression severity. RESULTS: Patients with FMS in general have a sociotropic personality style similar to patients with MDD, and different from HC, but FMS patients without a lifetime history of MDD had a cognitive personality style different from patients with MDD and similar to HC. CONCLUSION: These findings suggest that a depressotypic personality style is related to depressive disorder, but not to FMS
Interviews with Irish healthcare workers from different disciplines about palliative care for people with Parkinson’s disease: a definite role but uncertainty around terminology and timing
Background: An integrated palliative care approach is recommended in all life-limiting diseases, including Parkinson’s disease (PD). However research shows that people with PD have unmet palliative care needs. The study aimed to explore multidisciplinary healthcare workers’ (HCWs) views on palliative care for people with PD, identifying perceived barriers and facilitators. Methods: A qualitative design was used; data was analysed using Thematic Analysis. Semi-structured interviews were conducted with 30 HCWs, working either with people with PD or in a palliative care setting in Ireland. Results: A number of perceived barriers were evident helping to account for the previously reported unmet palliative care needs in PD. A lack of education about PD and palliative care meant that HCWs were unsure of the appropriateness of referral, and patients and carers weren’t equipped with information to seek palliative care. A lack of communication between PD and palliative care specialists was seen to impede collaboration between the disciplines. Uncertainty about the timing of palliative care meant that it was often not introduced until a crisis point, despite the recognised need for early planning due to increased prevalence of dementia. Conclusions: Most HCWs recognised a need for palliative care for people with PD; however several barriers to implementing a palliative care approach in this population need to be addressed. Implications for clinical practice and policy include the need for an integrated model of care, and education for all HCWs, patients, carers, and the public on both the nature of advanced PD, and the potential of palliative care in support of patients and their family members
Visualizing the atomic scale electronic structure of the Ca2CuO2Cl2 Mott insulator
Although the mechanism of superconductivity in the cuprates remains elusive,
it is generally agreed that at the heart of the problem is the physics of doped
Mott insulators. The cuprate parent compound has one unpaired electron per Cu
site, and is predicted by band theory to be a half-filled metal. The strong
onsite Coulomb repulsion, however, prohibits electron hopping between
neighboring sites and leads to a Mott insulator ground state with
antiferromagnetic (AF) ordering. Charge carriers doped into the CuO2 plane
destroy the insulating phase and superconductivity emerges as the carrier
density is sufficiently high. The natural starting point for tackling high Tc
superconductivity is to elucidate the electronic structure of the parent Mott
insulator and the behavior of a single doped charge. Here we use a scanning
tunneling microscope to investigate the atomic scale electronic structure of
the Ca2CuO2Cl2 parent Mott insulator of the cuprates. The full electronic
spectrum across the Mott-Hubbard gap is uncovered for the first time, which
reveals the particle-hole symmetric and spatially uniform Hubbard bands. A
single electron donated by surface defect is found to create a broad in-gap
electronic state that is strongly localized in space with spatial
characteristics intimately related to the AF spin background. The unprecedented
real space electronic structure of the parent cuprate sheds important new light
on the origion of high Tc superconductivity from the doped Mott insulator
perspective.Comment: 26 pages, 4 figures, supplementary information include
Central engine afterglow of Gamma-ray Bursts
Before 2004, nearly all GRB afterglow data could be understood in the context
of the external shocks model. This situation has changed in the past two years,
when it became clear that some afterglow components should be attributed to the
activity of the central engine; i.e., the {\it central engine afterglow}. We
review here the afterglow emission that is directly related to the GRB central
engine. Such an interpretation proposed by Katz, Piran & Sari, peculiar in
pre-{\it Swift} era, has become generally accepted now.Comment: 4 pages including 1 figure. Presented at the conference "Astrophysics
of Compact Objects" (July 1-7, 2007; Huangshan, China
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