1,036 research outputs found
Measuring the gap in ARPES experiments
Angle-resolved photoemission spectroscopy (ARPES) is considered as the only
experimental tool from which the momentum distribution of both the
superconducting and pseudo-gap can be quantitatively derived. The binding
energy of the leading edge of the photoemission spectrum, usually called the
leading edge gap (LEG), is the model-independent quantity which can be measured
in the modern ARPES experiments with the very high accuracy--better than 1 meV.
This, however, may be useless as long as the relation between the LEG and the
real gap is unknown. We present a systematic study of the LEG as a function of
a number of physical and experimental parameters. The absolute gap values which
have been derived from the numerical simulation prove, for example that the
nodal direction in the underdoped Bi-2212 in superconducting state is really
the node--the gap is zero. The other consequences of the simulations are
discussed.Comment: revtex4, 9 pages, 6 figure
Superconducting gap in the presence of bilayer splitting in underdoped Bi(Pb)2212
The clearly resolved bilayer splitting in ARPES spectra of the underdoped
Pb-Bi2212 compound rises the question of how the bonding and antibonding sheets
of the Fermi surface are gapped in the superconducting state. Here we compare
the superconducting gaps for both split components and show that within the
experimental uncertainties they are identical. By tuning the relative intensity
of the bonding and antibonding bands using different excitation conditions we
determine the precise {\bf k}-dependence of the leading edge gap. Significant
deviations from the simple cos()-cos() gap function for the
studied doping level are detected.Comment: 5 pages, 4 figures (revtex4
Tibetan sheep are better able to cope with low energy intake than Small-tailed Han sheep due to lower maintenance energy requirements and higher nutrient digestibilities
Tibetan sheep are indigenous to the Qinghai-Tibetan Plateau (QTP) and are well-adapted to and even thrive under the harsh alpine conditions. Small-tailed Han sheep were introduced to the plateau because of their high prolificacy and are maintained mainly in feedlots. Because of their different backgrounds, we hypothesised that Tibetan and Small-tailed Han sheep would differ in their utilization of energy intake and predicted that Tibetan sheep would cope better with low energy intake than Small-tailed Han sheep. To test this prediction, we determined nutrient digestibilities, energy requirements for maintenance and blood metabolite and hormone concentrations involved in energy metabolism in these breeds. Sheep of each breed (n = 24 of each, all wethers and 1.5 years of age) were distributed randomly into one of four groups and offered ad libitum diets of different digestible energy (DE) densities: 8.21, 9.33, 10.45 and 11.57 MJ DE/kg Dry matter (DM). Following 42 d of measuring feed intake, a 1-week digestion and metabolism experiment was done. DM intakes did not differ between breeds nor among treatments but, by design, DE intake increased linearly in both breeds as dietary energy level increased (P < 0.001). The average daily gain (ADG) was significantly greater in the Tibetan than Small-tailed Han sheep (P = 0.003) and increased linearly in both breeds (P < 0.001). In addition, from the regression analysis of ADG on DE intake, daily DE maintenance requirements were lower for Tibetan than for Small-tailed Han sheep (0.41 vs 0.50 MJ/BW0.75, P < 0.05). The DE and metabolizable energy (ME) digestibilities were higher in the Tibetan than Small-tailed Han sheep (P < 0.001) and increased linearly as the energy level increased in the diet (P < 0.001). At the lowest energy treatment, Tibetan sheep when compared with Small-tailed Han sheep, had: 1) higher serum glucose and glucagon, but lower insulin concentrations (P < 0.05), which indicated a higher capacity for gluconeogenesis and ability to regulate glucose metabolism; and 2) higher non-esterified fatty acids (NEFA) and lower very low density lipoprotein (VLDL) and triglyceride (TG) concentrations (P < 0.05), which indicated a higher capacity for NEFA oxidation but lower ability for triglyceride (TG) synthesis. We concluded that our prediction was supported as these differences between breeds conferred an advantage for Tibetan over Small-tailed Han sheep to cope better with low energy diets
Data Informed Model Test Design With Machine Learning â An Example in Nonlinear Wave Load on a Vertical Cylinder
Model testing is common in coastal and offshore engineering. The design of such model tests is important such that the maximal information of the underlying physics can be extrapolated with a limited amount of test cases. The design of experiments also requires considering the previous similar experimental results and the typical sea-states of the ocean environments. In this study, we develop a model test design strategy based on Bayesian sampling for a classic problem in ocean engineeringânonlinear wave loading on a vertical cylinder. The new experimental design strategy is achieved through a GP-based surrogate model, which considers the previous experimental data as the prior information. The metocean data are further incorporated into the experimental design through a modified acquisition function. We perform a new experiment, which is mainly designed by data-driven methods, including several critical parameters such as the size of the cylinder and all the wave conditions. We examine the performance of such a method when compared to traditional experimental design based on manual decisions. This method is a step forward to a more systematic way of approaching test designs with marginally better performance in capturing the higher-order force coefficients. The current surrogate model also made several âinterpretableâ decisions which can be explained with physical insights
An investigation of high-order harmonics in the pressure field around a vertical cylinder in steep wave conditions
Offshore structures, encompassing foundations for offshore wind turbines, supports for marine renewable energy devices, bridge piers, and floating vessels, are consistently subjected to severe environmental loads. These loads often dictate the design criteria. Understanding the physics and statistics of wave-structure interaction, especially under non-linear loads experienced in extreme conditions, remains a complex and partially unresolved challenge. Notably, secondary load cycles significantly contribute to the âringingâ responses in cylindrical structures, as discussed in previous studies (e.g., Grue et al. (1993), Chaplin et al. (1997)). This paper focuses on analysing loads in focused wave groups, representing short-term extreme wave conditions, on bottom-mounted vertical cylinders relevant to fixed offshore wind turbines. Pressure contour plots over the cylinderâs surface were previously examined by Ghadirian & Bredmose (2020) while studying secondary load cycles. In this research, we adopt the phase-based harmonic separation method for wave forces (Fitzgerald et al. (2014)) to analyse the pressure contour plots. This method effectively isolates harmonic pressure components from the total pressures, enabling a novel exploration of the mechanisms behind secondary load cycles from the perspective of high-order harmonics on the cylinder surface
A new Gaussian Process based model for non-linear wave loading on vertical cylinders
We aim to establish a fast and accurate model for fast prediction of nonlinear loading on vertical cylinders such as are typically used for fixed offshore wind turbines. We follow a âStokes-typeâ force model and approximate the amplitude of the higher harmonics of force by relating these to the linear force time series raised to appropriate power through amplitude and phase coefficients. We reanalyse previous experimental data and perform new experiments to expand the parameter space and establish a force coefficients database for engineering applications. A machine learning model is used to interpolate the database and make predictions on the higher order force coefficients. The machine learning model also provides a cross-validated confidence interval to indicate the prediction uncertainty and reflect model reliability. We further extend the prediction capability to unidirectional random waves with a novel force segmentation method, which localised wave groups from the random background. The new Stokes-Gaussian Process (Stokes-GP) model developed can provide engineering predictions of nonlinear wave loading on a cylinder for individual wave groups and random seas, which are straightforward to apply and fast to compute and the important higher-order loading components are considered. This will significantly improve the accuracy of the loading prediction and the ease of application for force predictions.</p
History effect in inhomogeneous superconductors
A model was proposed to account for a new kind of history effect in the
transport measurement of a sample with inhomogeneous flux pinning coupled with
flux creep. The inhomogeneity of flux pinning was described in terms of
alternating weak pinning (lower jc) and strong pinning region (higher jc). The
flux creep was characterized by logarithmic barrier. Based on this model, we
numerically observed the same clockwise V-I loops as reported in references.
Moreover, we predicted behaviors of the V-I loop at different sweeping rates of
applied current dI/dt or magnetic fields Ba, etc. Electric transport
measurement was performed in Ag-sheathed Bi2-xPbxSr2Ca2Cu3Oy tapes immersed in
liquid nitrogen with and without magnetic fields. V-I loop at certain dI/dt and
Ba was observed. It is found that the area of the loop is more sensitive to
dI/dt than to Ba, which is in agreement well with our numerical results.Comment: To appear in Phys Rev B, October 1 Issu
Quantum and Classical Integrable Systems
The key concept discussed in these lectures is the relation between the
Hamiltonians of a quantum integrable system and the Casimir elements in the
underlying hidden symmetry algebra. (In typical applications the latter is
either the universal enveloping algebra of an affine Lie algebra, or its
q-deformation.) A similar relation also holds in the classical case. We discuss
different guises of this very important relation and its implication for the
description of the spectrum and the eigenfunctions of the quantum system.
Parallels between the classical and the quantum cases are thoroughly discussed.Comment: 59 pages, LaTeX2.09 with AMS symbols. Lectures at the CIMPA Winter
School on Nonlinear Systems, Pondicherry, January 199
Confinement, the gluon propagator and the interquark potential for heavy mesons
The interquark static potential for heavy mesons described by a massive One
Gluon Exchange interaction obtained from the propagator of the truncated
Dyson-Schwinger equations does not reproduced the expected Cornell potential. I
show that no formulation based on a finite propagator will lead to confinement
of quenched QCD. I propose a mechanism based on a singular nonperturbative
coupling constant which has the virtue of giving rise to a finite gluon
propagator and (almost) linear confinement. The mechanism can be slightly
modified to produce the screened potentials of unquenched QCD.Comment: 12 pages and 7 figure
On the determination of the Fermi surface in high-Tc superconductors by angle-resolved photoemission spectroscopy
We study the normal state electronic excitations probed by angle resolved
photoemission spectroscopy (ARPES) in Bi2201 and Bi2212. Our main goal is to
establish explicit criteria for determining the Fermi surface from ARPES data
on strongly interacting systems where sharply defined quasiparticles do not
exist and the dispersion is very weak in parts of the Brillouin zone.
Additional complications arise from strong matrix element variations within the
zone. We present detailed results as a function of incident photon energy, and
show simple experimental tests to distinguish between an intensity drop due to
matrix element effects and spectral weight loss due to a Fermi crossing. We
reiterate the use of polarization selection rules in disentangling the effect
of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite
all the complications, the Fermi surface can be determined unambiguously: it is
a single large hole barrel centered about (pi,pi) in both materials.Comment: Expanded discussion of symmetrization method in Section 5, figures
remain the sam
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