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Search Results: 1 - 10 of 233836 matches for " X. R. "
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An Expansion of Boundary Theory and the Application of Joint Condition  [PDF]
R. Song, S. X. Wu
Open Journal of Applied Sciences (OJAppS) , 2015, DOI: 10.4236/ojapps.2015.512082
Abstract: The manuscript reviews the history and quo of the theory of Timoshenko’s method in stability analysis of compressive levers first, taking an example to explain the m-simulation method and putting forward the 3rd-7th boundary conditions demonstrating their superiorities in improving the precision through examples, followed by proposing and applying the join conditions in the stability analysis of combined axial force compressive levers gaining success. Through a brief example showing the effect of some related theories in a simple structural stability analysis, its application prospect is discussed.
A Control Strategy for Smoothing Active Power Fluctuation of Wind Farm with Flywheel Energy Storage System Based on Improved Wind Power Prediction Algorithm  [PDF]
J. C. Wang, X. R. Wang
Energy and Power Engineering (EPE) , 2013, DOI: 10.4236/epe.2013.54B075
Abstract: The fluctuation of active power output of wind farm has many negative impacts on large-scale wind power integration into power grid. In this paper, flywheel energy storage system (FESS) was connected to AC side of the doubly-fed induction generator (DFIG) wind farm to realize smooth control of wind power output. Based on improved wind power prediction algorithm and wind speed-power curve modeling, a new smooth control strategy with the FESS was proposed. The requirement of power system dispatch for wind power prediction and flywheel rotor speed limit were taken into consideration during the process. While smoothing the wind power fluctuation, FESS can track short-term planned output of wind farm. It was demonstrated by quantitative analysis of simulation results that the proposed control strategy can smooth the active power fluctuation of wind farm effectively and thereby improve power quality of the power grid.
Light Emitting Diodes of Inverse Spin Valves
X. R. Wang
Physics Research International , 2008, DOI: 10.1155/2008/434936
Abstract: Light emitting diodes made out of inverse spin valves of a ferromagnetic half metal sandwiched between two nonmagnetic metals are proposed. Based on a giant spin-dependent chemical potential difference created under an external bias, the inverse spin valves are possible to emit light when electrons with the higher chemical potential flip their spins and become the electrons of the opposite spin with the lower chemical potential. The frequency of this type of light emitting diodes is tunable by the bias.
Assimilating water column and satellite data for marine export production estimation
X. Yao,R. Schlitzer
Geoscientific Model Development Discussions , 2013, DOI: 10.5194/gmdd-6-2045-2013
Abstract: Recent advances in satellite retrieval methodology now allow estimation of particular organic carbon (POC) concentration in ocean surface waters directly from satellite-based optical data. Because of the good coverage these data reveal small-scale spatial and temporal concentration gradients and document the evolution of surface water POC as well as the underlying driving biogeochemical processes throughout the seasons. Water column nutrient data also reveal biogeochemical activity. However, because of the scarcity of data the deduction of temporal changes of particle production and export are not possible in most parts of the ocean. Here we present first results from a new study combining both data streams thereby exploiting the high spatio-temporal resolution of surface POC concentrations from satellite optical sensors with water column nutrient data having sparser coverage but providing information throughout the entire water column. We use a medium-resolution global model with steady-state 3-D circulation that has been optimized by fitting to a large number of hydrographic parameters and tracers, including CFCs and natural radiocarbon. Production and export of POC is allowed to vary monthly, and the magnitudes of the monthly export fluxes are determined by fitting the model to satellite POC data as well as water column nutrient data using the adjoint method. Two cases have been investigated: (1) the production rate of POC is set to be proportional to export production EP and the seasonal changes are assumed sinusoidal (meridionally varying amplitude and phase) and (2) the POC production rate is linked to primary production rates (literature). Both cases were run with the same initial state and model settings, and show total cost function decreases of 12 and 95%, respectively. The POC misfit term alone decreased by 75 and 99%. The integrated annual global POC exports of the two cases are 9.9 Gt C yr 1 and 12.3 Gt C yr 1, respectively. Overall, the remaining POC and phosphate misfits of both solutions are considered too large and the difference fields still exhibit significant systematic geographical patterns. This indicates that the present model runs are too simplistic and do not fully explain the data. Further, more refined model setups are needed.
Plane-chain coupling in YBa$_{2}$Cu$_{3}$O$_{7}$ : impurity effect on the critical temperature
R. Combescot,X. Leyronas
Physics , 1996, DOI: 10.1103/PhysRevB.54.4320
Abstract: We have studied the effect of impurities on $T_{c}$ for a model of YBCO involving pairing both in the CuO$_{2}$ planes and in the CuO chains. In this model pairing in the planes is due to phonons, while Coulomb repulsion induces in the chains an order parameter with opposite sign. Due to the anticrossing produced by hybridization between planes and chains, the order parameter changes sign on a single sheet of the Fermi surface resulting in nodes in the gap. We find that $T_{c}$ is much less sensitive to impurities than in standard d-wave models. One reason is that impurities produce essentially plane-plane and chain-chain scattering, which does not affect the critical temperature. $T_{c } $ is reduced by plane-chain scattering, which is smaller by a factor of order hybridization coupling over Fermi energy. In the specific case which we have studied in details and which reduces to the two-band model, we have found a further reduction of the sensitivity of $T_{c}$ to impurities with a behaviour which can vary continuously from s-wave like to d-wave like depending on the parameters. We expect a similar reduction to occur in the general case.
The London penetration depth of strongly coupled isotropic superconductors : low temperature behaviour
X. Leyronas,R. Combescot
Physics , 1996, DOI: 10.1103/PhysRevB.54.3482
Abstract: We proceed to a systematic exploration of the low temperature dependence of the London penetration depth of isotropic superconductors within strong coupling theory in the clean limit. For a sizeable range of parameters, we find that strong coupling effects can reasonably simulate a power law dependence, sometimes with an excellent precision. In such cases it would be quite difficult to distinguish experimentally between a pure power law and the strong coupling result. Physically we have been able to ascribe this temperature dependence to low frequency phonons which produce a quasi elastic scattering for electrons. The presence of these low frequency phonons requires rather wide phonon spectra and their effectiveness in scattering implies fairly strong coupling.
Comment on "Collective excitations of a degenerate gas at the BEC-BCS crossover"
R. Combescot,X. Leyronas
Physics , 2004, DOI: 10.1103/PhysRevLett.93.138901
Abstract: Very recent experiments have studied for the first time collective excitations of an ultracold $^6$Li gas covering in particular the BEC-BCS crossover domain. We point out that the results for the axial mode, through hydrodynamics, give direct access to the (3D) equation of state of the strongly interacting gas, mostly near the unitarity limit. On the other hand the surprising results found for the radial mode are actually not necessarily in contradiction with the expectations from superfluid hydrodynamics.
Plane - Chain coupling in YBa_{2}Cu_{3}O_{7} : temperature dependence of the penetration depth
X. Leyronas,R. Combescot
Physics , 1996, DOI: 10.1016/S0921-4534(97)01647-X
Abstract: We have studied the penetration depth for a model of $YBa_{2}Cu_{3}O_{7}$ involving pairing both in the $CuO_{2}$ planes and in the CuO chains. In this model pairing in the planes is due to an attractive interaction, while Coulomb repulsion induces in the chains an order parameter with opposite sign. Due to the anticrossing produced by hybridization between planes and chains, one obtains a d-wave like order parameter which changes sign on a single sheet of the Fermi surface and has nodes in the gap. We find that our model accounts quite well for the anisotropy of the penetration depth and for the absolute values. We reproduce fairly well the whole temperature dependence for both the a and the b directions, including the linear dependence at low temperature. We use a set of parameters which are all quite reasonable physically. Our results for the c direction are also satisfactory, although the situation is less clear both experimentally and theoretically.
Coupling between planes and chains in YBa2Cu3O7 : a possible solution for the order parameter controversy
R. Combescot,X. Leyronas
Physics , 1995, DOI: 10.1103/PhysRevLett.75.3732
Abstract: We propose to explain the contradictory experimental evidence about the symmetry of the order parameter in $YBa_{2}Cu_{3}O_{7} $ by taking into account the coupling between planes and chains. This leads to an anticrossing of the plane and chain band. We include an attractive pairing interaction within the planes and a repulsive one between planes and chains, leading to opposite signs for the order parameter on planes and chains, and to nodes of the gap because of the anticrossing. Our model blends s-wave and d-wave features, and provides a natural explanation for all the contradictory experiments
Instability of a trapped ultracold Fermi gas with attractive interactions : quantum effects
X. Leyronas,R. Combescot
Physics , 2004, DOI: 10.1140/epjd/e2004-00169-0
Abstract: We consider the possible mechanical instability of an ultracold Fermi gas due to the attractive interactions between fermions of different species. We investigate how the instability, predicted by a mean field calculation for an homogeneous system, is modified when the gas is trapped in a harmonic potential and quantum effects are included.
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