235 research outputs found
Spin correlations in the electron-doped high-transition-temperature superconductor Nd{2-x}Ce{x}CuO{4+/-delta}
High-transition-temperature (high-Tc) superconductivity develops near
antiferromagnetic phases, and it is possible that magnetic excitations
contribute to the superconducting pairing mechanism. To assess the role of
antiferromagnetism, it is essential to understand the doping and temperature
dependence of the two-dimensional antiferromagnetic spin correlations. The
phase diagram is asymmetric with respect to electron and hole doping, and for
the comparatively less-studied electron-doped materials, the antiferromagnetic
phase extends much further with doping [1, 2] and appears to overlap with the
superconducting phase. The archetypical electron-doped compound
Nd{2-x}Ce{x}CuO{4\pm\delta} (NCCO) shows bulk superconductivity above x \approx
0.13 [3, 4], while evidence for antiferromagnetic order has been found up to x
\approx 0.17 [2, 5, 6]. Here we report inelastic magnetic neutron-scattering
measurements that point to the distinct possibility that genuine long-range
antiferromagnetism and superconductivity do not coexist. The data reveal a
magnetic quantum critical point where superconductivity first appears,
consistent with an exotic quantum phase transition between the two phases [7].
We also demonstrate that the pseudogap phenomenon in the electron-doped
materials, which is associated with pronounced charge anomalies [8-11], arises
from a build-up of spin correlations, in agreement with recent theoretical
proposals [12, 13].Comment: 5 pages, 4 figure
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
Search for time-dependent B0s - B0s-bar oscillations using a vertex charge dipole technique
We report a search for B0s - B0s-bar oscillations using a sample of 400,000
hadronic Z0 decays collected by the SLD experiment. The analysis takes
advantage of the electron beam polarization as well as information from the
hemisphere opposite that of the reconstructed B decay to tag the B production
flavor. The excellent resolution provided by the pixel CCD vertex detector is
exploited to cleanly reconstruct both B and cascade D decay vertices, and tag
the B decay flavor from the charge difference between them. We exclude the
following values of the B0s - B0s-bar oscillation frequency: Delta m_s < 4.9
ps-1 and 7.9 < Delta m_s < 10.3 ps-1 at the 95% confidence level.Comment: 18 pages, 3 figures, replaced by version accepted for publication in
Phys.Rev.D; results differ slightly from first versio
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
Adaptive Evolution of the Lactose Utilization Network in Experimentally Evolved Populations of Escherichia coli
Adaptation to novel environments is often associated with changes in gene regulation. Nevertheless, few studies have been able both to identify the genetic basis of changes in regulation and to demonstrate why these changes are beneficial. To this end, we have focused on understanding both how and why the lactose utilization network has evolved in replicate populations of Escherichia coli. We found that lac operon regulation became strikingly variable, including changes in the mode of environmental response (bimodal, graded, and constitutive), sensitivity to inducer concentration, and maximum expression level. In addition, some classes of regulatory change were enriched in specific selective environments. Sequencing of evolved clones, combined with reconstruction of individual mutations in the ancestral background, identified mutations within the lac operon that recapitulate many of the evolved regulatory changes. These mutations conferred fitness benefits in environments containing lactose, indicating that the regulatory changes are adaptive. The same mutations conferred different fitness effects when present in an evolved clone, indicating that interactions between the lac operon and other evolved mutations also contribute to fitness. Similarly, changes in lac regulation not explained by lac operon mutations also point to important interactions with other evolved mutations. Together these results underline how dynamic regulatory interactions can be, in this case evolving through mutations both within and external to the canonical lactose utilization network
A randomized, controlled trial of 3.0 mg of liraglutide in weight management
BACKGROUND Obesity is a chronic disease with serious health consequences, but weight loss is difficult to maintain through lifestyle intervention alone. Liraglutide, a glucagonlike peptide-1 analogue, has been shown to have potential benefit for weight management at a once-daily dose of 3.0 mg, injected subcutaneously. METHODS We conducted a 56-week, double-blind trial involving 3731 patients who did not have type 2 diabetes and who had a body-mass index (BMI; the weight in kilograms divided by the square of the height in meters) of at least 30 or a BMI of at least 27 if they had treated or untreated dyslipidemia or hypertension. We randomly assigned patients in a 2:1 ratio to receive once-daily subcutaneous injections of liraglutide at a dose of 3.0 mg (2487 patients) or placebo (1244 patients); both groups received counseling on lifestyle modification. The coprimary end points were the change in body weight and the proportions of patients losing at least 5% and more than 10% of their initial body weight. RESULTS At baseline, the mean (±SD) age of the patients was 45.1±12.0 years, the mean weight was 106.2±21.4 kg, and the mean BMI was 38.3±6.4; a total of 78.5% of the patients were women and 61.2% had prediabetes. At week 56, patients in the liraglutide group had lost a mean of 8.4±7.3 kg of body weight, and those in the placebo group had lost a mean of 2.8±6.5 kg (a difference of -5.6 kg; 95% confidence interval, -6.0 to -5.1; P<0.001, with last-observation-carried-forward imputation). A total of 63.2% of the patients in the liraglutide group as compared with 27.1% in the placebo group lost at least 5% of their body weight (P<0.001), and 33.1% and 10.6%, respectively, lost more than 10% of their body weight (P<0.001). The most frequently reported adverse events with liraglutide were mild or moderate nausea and diarrhea. Serious events occurred in 6.2% of the patients in the liraglutide group and in 5.0% of the patients in the placebo group. CONCLUSIONS In this study, 3.0 mg of liraglutide, as an adjunct to diet and exercise, was associated with reduced body weight and improved metabolic control. (Funded by Novo Nordisk; SCALE Obesity and Prediabetes NN8022-1839 ClinicalTrials.gov number, NCT01272219.)
Protocol for a statewide randomized controlled trial to compare three training models for implementing an evidence-based treatment
- …