32,867 research outputs found
Kernel smoothing for spatially correlated data
Kernel smoothing is a nonparametric approach for estimating the relationship between a response variable and a set of predictors (or design variables). A major problem for kernel smoothing is the selection of the bandwidth, which controls the amount of smoothing. When data are correlated, former studies on kernel smoothing have been essentially limited to the case of a univariate predictor, with equally spaced design. In this dissertation, we discuss a more general case for correlated data, the case of multivariate predictors with random design. Three types of estimators, the Priestley-Chao estimator, the Nadaraya-Watson estimator, and the local linear estimator, are addressed, with emphasis on the local linear estimator. We will derive formulas for asymptotic mean square, errors of these kernel smoothing estimators, and formulas of asymptotically optimal bandwidth. In the presence of spatially correlated errors, we show that traditional data-driven bandwidth selection methods, such as cross-validation and generalized cross-validation, fail to provide good bandwidth values. We propose several data-driven bandwidth selection methods that account for the presence of spatial correlation. Simulation studies show that these methods are effective when the covariances between the errors are completely known. When the covariances need to be estimated from data, we consider two special cases: spatial data with repeated measurements, and spatial data collected on a grid (with only one realization). For data with repeated measurements, we propose an estimation method based on semi-variogram fitting. For data on a grid, we propose a method based on differencing, with the application of approximate Whittle likelihood estimation. Simulation studies show that these methods can provide reasonably good estimates of the covariances for the purpose of bandwidth selection
Local electronic structures on the superconducting interface
Motivated by the recent discovery of superconductivity on the heterointerface
, we theoretically investigate its local electronic
structures near an impurity considering the influence of Rashba-type spin-orbit
interaction (RSOI) originated in the lack of inversion symmetry. We find that
local density of states near an impurity exhibits the in-gap resonance peaks
due to the quasiparticle scattering on the Fermi surface with the reversal sign
of the pairing gap caused by the mixed singlet and RSOI-induced triplet
superconducting state. We also analyze the evolutions of density of states and
local density of states with the weight of triplet pairing component determined
by the strength of RSOI, which will be widely observed in thin films of
superconductors with surface or interface-induced RSOI, or various
noncentrosymmetric superconductors in terms of point contact tunneling and
scanning tunneling microscopy, and thus reveal an admixture of the spin singlet
and RSOI-induced triplet superconducting states.Comment: Phys. Rev. B 81, 144504 (2010)
Universal linear-temperature resistivity: possible quantum diffusion transport in strongly correlated superconductors
The strongly correlated electron fluids in high temperature cuprate
superconductors demonstrate an anomalous linear temperature () dependent
resistivity behavior, which persists to a wide temperature range without
exhibiting saturation. As cooling down, those electron fluids lose the
resistivity and condense into the superfluid. However, the origin of the
linear- resistivity behavior and its relationship to the strongly correlated
superconductivity remain a mystery. Here we report a universal relation
, which bridges the slope of the
linear--dependent resistivity () to the London penetration depth
at zero temperature among cuprate superconductor
BiSrCaCuO and heavy fermion superconductors
CeCoIn, where is vacuum permeability, is the Boltzmann
constant and is the reduced Planck constant. We extend this scaling
relation to different systems and found that it holds for other cuprate,
pnictide and heavy fermion superconductors as well, regardless of the
significant differences in the strength of electronic correlations, transport
directions, and doping levels. Our analysis suggests that the scaling relation
in strongly correlated superconductors could be described as a hydrodynamic
diffusive transport, with the diffusion coefficient () approaching the
quantum limit , where is the quasi-particle effective
mass.Comment: 8 pages, 2 figures, 1 tabl
Parameter-tuning Networks: Experiments and Active Walk Model
The tuning process of a large apparatus of many components could be
represented and quantified by constructing parameter-tuning networks. The
experimental tuning of the ion source of the neutral beam injector of HT-7
Tokamak is presented as an example. Stretched-exponential cumulative degree
distributions are found in the parameter-tuning networks. An active walk model
with eight walkers is constructed. Each active walker is a particle moving with
friction in an energy landscape; the landscape is modified by the collective
action of all the walkers. Numerical simulations show that the parameter-tuning
networks generated by the model also give stretched exponential functions, in
good agreement with experiments. Our methods provide a new way and a new
insight to understand the action of humans in the parameter-tuning of
experimental processes, is helpful for experimental research and other
optimization problems.Comment: 4 pages, 5 figure
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