716 research outputs found
Quiet SDS Josephson Junctions for Quantum Computing
Unconventional superconductors exhibit an order parameter symmetry lower than
the symmetry of the underlying crystal lattice. Recent phase sensitive
experiments on YBCO single crystals have established the d-wave nature of the
cuprate materials, thus identifying unambiguously the first unconventional
superconductor. The sign change in the order parameter can be exploited to
construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting
a degenerate ground state and a double-periodic current-phase characteristic.
Here we discuss how to make use of these special junction characteristics in
the construction of a quantum computer. Combining such junctions together with
a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit
--- a solid state implementation of a quantum bit which remains optimally
isolated from its environment.Comment: 4 pages, 2 ps-figure
One-dimensional Topological Edge States of Bismuth Bilayers
The hallmark of a time-reversal symmetry protected topologically insulating
state of matter in two-dimensions (2D) is the existence of chiral edge modes
propagating along the perimeter of the system. To date, evidence for such
electronic modes has come from experiments on semiconducting heterostructures
in the topological phase which showed approximately quantized values of the
overall conductance as well as edge-dominated current flow. However, there have
not been any spectroscopic measurements to demonstrate the one-dimensional (1D)
nature of the edge modes. Among the first systems predicted to be a 2D
topological insulator are bilayers of bismuth (Bi) and there have been recent
experimental indications of possible topological boundary states at their
edges. However, the experiments on such bilayers suffered from irregular
structure of their edges or the coupling of the edge states to substrate's bulk
states. Here we report scanning tunneling microscopy (STM) experiments which
show that a subset of the predicted Bi-bilayers' edge states are decoupled from
states of Bi substrate and provide direct spectroscopic evidence of their 1D
nature. Moreover, by visualizing the quantum interference of edge mode
quasi-particles in confined geometries, we demonstrate their remarkable
coherent propagation along the edge with scattering properties that are
consistent with strong suppression of backscattering as predicted for the
propagating topological edge states.Comment: 15 pages, 5 figures, and supplementary materia
Topological Surface States Protected From Backscattering by Chiral Spin Texture
Topological insulators are a new class of insulators in which a bulk gap for
electronic excitations is generated by strong spin orbit coupling. These novel
materials are distinguished from ordinary insulators by the presence of gapless
metallic boundary states, akin to the chiral edge modes in quantum Hall
systems, but with unconventional spin textures. Recently, experiments and
theoretical efforts have provided strong evidence for both two- and
three-dimensional topological insulators and their novel edge and surface
states in semiconductor quantum well structures and several Bi-based compounds.
A key characteristic of these spin-textured boundary states is their
insensitivity to spin-independent scattering, which protects them from
backscattering and localization. These chiral states are potentially useful for
spin-based electronics, in which long spin coherence is critical, and also for
quantum computing applications, where topological protection can enable
fault-tolerant information processing. Here we use a scanning tunneling
microscope (STM) to visualize the gapless surface states of the
three-dimensional topological insulator BiSb and to examine their scattering
behavior from disorder caused by random alloying in this compound. Combining
STM and angle-resolved photoemission spectroscopy, we show that despite strong
atomic scale disorder, backscattering between states of opposite momentum and
opposite spin is absent. Our observation of spin-selective scattering
demonstrates that the chiral nature of these states protects the spin of the
carriers; they therefore have the potential to be used for coherent spin
transport in spintronic devices.Comment: to be appear in Nature on August 9, 200
Strain-controlled criticality governs the nonlinear mechanics of fibre networks
Disordered fibrous networks are ubiquitous in nature as major structural
components of living cells and tissues. The mechanical stability of networks
generally depends on the degree of connectivity: only when the average number
of connections between nodes exceeds the isostatic threshold are networks
stable (Maxwell, J. C., Philosophical Magazine 27, 294 (1864)). Upon increasing
the connectivity through this point, such networks undergo a mechanical phase
transition from a floppy to a rigid phase. However, even sub-isostatic networks
become rigid when subjected to sufficiently large deformations. To study this
strain-controlled transition, we perform a combination of computational
modeling of fibre networks and experiments on networks of type I collagen
fibers, which are crucial for the integrity of biological tissues. We show
theoretically that the development of rigidity is characterized by a
strain-controlled continuous phase transition with signatures of criticality.
Our experiments demonstrate mechanical properties consistent with our model,
including the predicted critical exponents. We show that the nonlinear
mechanics of collagen networks can be quantitatively captured by the
predictions of scaling theory for the strain-controlled critical behavior over
a wide range of network concentrations and strains up to failure of the
material
Molecular profiling of signet ring cell colorectal cancer provides a strong rationale for genomic targeted and immune checkpoint inhibitor therapies
We would like to thank all patients whose samples were used in this study. We are also thankful to the Northern Ireland Biobank and Grampian Biorepository for providing us with tissue blocks and patient data; and Dr HG Coleman (Queen’s University Belfast) for her advice on statistical analyses. This work has been carried out with financial support from Cancer Research UK (grant: C11512/A18067), Experimental Cancer Medicine Centre Network (grant: C36697/A15590 from Cancer Research UK and the NI Health and Social Care Research and Development Division), the Sean Crummey Memorial Fund and the Tom Simms Memorial Fund. The Northern Ireland Biobank is funded by HSC Research and Development Division of the Public Health Agency in Northern Ireland and Cancer Research UK through the Belfast CRUK Centre and the Northern Ireland Experimental Cancer Medicine Centre; additional support was received from Friends of the Cancer Centre. The Northern Ireland Molecular Pathology Laboratory which is responsible for creating resources for the Northern Ireland Biobank has received funding from Cancer Research UK, Friends of the Cancer Centre and Sean Crummey Foundation.Peer reviewedPublisher PD
Multivariate characterization of white matter heterogeneity in autism spectrum disorder
The complexity and heterogeneity of neuroimaging findings in individuals with autism spectrum disorder has suggested that many of the underlying alterations are subtle and involve many brain regions and networks. The ability to account for multivariate brain features and identify neuroimaging measures that can be used to characterize individual variation have thus become increasingly important for interpreting and understanding the neurobiological mechanisms of autism. In the present study, we utilize the Mahalanobis distance, a multidimensional counterpart of the Euclidean distance, as an informative index to characterize individual brain variation and deviation in autism. Longitudinal diffusion tensor imaging data from 149 participants (92 diagnosed with autism spectrum disorder and 57 typically developing controls) between 3.1 and 36.83 years of age were acquired over a roughly 10-year period and used to construct the Mahalanobis distance from regional measures of white matter microstructure. Mahalanobis distances were significantly greater and more variable in the autistic individuals as compared to control participants, demonstrating increased atypicalities and variation in the group of individuals diagnosed with autism spectrum disorder. Distributions of multivariate measures were also found to provide greater discrimination and more sensitive delineation between autistic and typically developing individuals than conventional univariate measures, while also being significantly associated with observed traits of the autism group. These results help substantiate autism as a truly heterogeneous neurodevelopmental disorder, while also suggesting that collectively considering neuroimaging measures from multiple brain regions provides improved insight into the diversity of brain measures in autism that is not observed when considering the same regions separately. Distinguishing multidimensional brain relationships may thus be informative for identifying neuroimaging-based phenotypes, as well as help elucidate underlying neural mechanisms of brain variation in autism spectrum disorders
Determination of Vitamin A and its Metabolites in Rat Testis: Possible Involvement of Vitamin A in Testicular Toxicity Caused by Molinate
This study was conducted to evaluate the effect of molinate on retinoids homeostasis in rat testis. Molinate was administrated to male Sprague–Dawley rats (200 mg kg−1 in corn oil, ip). Retinoid measurements were made at 6, 12, 48 and 168 h time points after administration. Testis levels of retinoic acid decreased (32 %) in a statistically significant manner at the 12 and 48 h time points. However, retinol and retinaldehyde were not significantly affected by molinate. These results suggest that molinate affects retinoic acid synthesis in testis and could contribute to understanding the molecular mechanism of molinate involved testicular toxicity
Interacting with the public as a risk factor for employee psychological distress
Background: The 1-month prevalence of any mental disorder in employees ranges from 10.5% to 18.5%. Mental disorders are responsible for substantial losses in employee productivity in both absenteeism and presenteeism. Potential work related factors contributing to mental difficulties are of increasing interest to employers. Some data suggests that being sales staff, call centre operator, nurse or teacher increases psychological distress. One aspect of these occupations is that there is an interaction with the public. The aim of this study is to evaluate whether employees who interact with the public are at greater risk of psychological distress
Radio frequency measurements of tunnel couplings and singlet–triplet spin states in Si:P quantum dots
Spin states of the electrons and nuclei of phosphorus donors in silicon are strong candidates for quantum information processing applications given their excellent coherence times. Designing a scalable donor-based quantum computer will require both knowledge of the relationship between device geometry and electron tunnel couplings, and a spin readout strategy that uses minimal physical space in the device. Here we use radio frequency reflectometry to measure singlet–triplet states of a few-donor Si:P double quantum dot and demonstrate that the exchange energy can be tuned by at least two orders of magnitude, from 20 μeV to 8 meV. We measure dot–lead tunnel rates by analysis of the reflected signal and show that they change from 100 MHz to 22 GHz as the number of electrons on a quantum dot is increased from 1 to 4. These techniques present an approach for characterizing, operating and engineering scalable qubit devices based on donors in silicon
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