Bayesian analysis of a Tobit quantile regression model

This paper develops a Bayesian framework for Tobit quantile regression. Our approach is organized around a likelihood function that is based on the asymmetric Laplace dis- tribution, a choice that turns out to be natural in this context. We discuss families of prior distribution on the quantile regression vector that lead to proper posterior distributions with ¯nite moments. We show how the posterior distribution can be sampled and summarized by Markov chain Monte Carlo methods. A method for com- paring alternative quantile regression models is also developed and illustrated. The techniques are illustrated with both simulated and real data. In particular, in an em- pirical comparison, our approach out-performed two other common classical estimators

Dispersion and transitions of dipolar plasmon modes in graded plasmonic waveguides

Coupled plasmon modes are studied in graded plasmonic waveguides, which are periodic chains of metallic nanoparticles embedded in a host with gradually varying refractive indices. We identify three types of localized modes called "light", "heavy", and "light-heavy" plasmonic gradons outside the passband, according to various degrees of localization. We also demonstrate new transitions among extended and localized modes when the interparticle separation $d$ is smaller than a critical $d_c$, whereas the three types of localized modes occur for $d>d_c$, with no extended modes. The transitions can be explained with phase diagrams constructed for the lossless metallic systems.Comment: Preliminary results have been presented at ETOPIM 7. Submitted to Appl. Phys. Let

Coulomb blockade and Bloch oscillations in superconducting Ti nanowires

Quantum fluctuations in quasi-one-dimensional superconducting channels leading to spontaneous changes of the phase of the order parameter by $2\pi$, alternatively called quantum phase slips (QPS), manifest themselves as the finite resistance well below the critical temperature of thin superconducting nanowires and the suppression of persistent currents in tiny superconducting nanorings. Here we report the experimental evidence that in a current-biased superconducting nanowire the same QPS process is responsible for the insulating state -- the Coulomb blockade. When exposed to RF radiation, the internal Bloch oscillations can be synchronized with the external RF drive leading to formation of quantized current steps on the I-V characteristic. The effects originate from the fundamental quantum duality of a Josephson junction and a superconducting nanowire governed by QPS -- the QPS junction (QPSJ).Comment: 5 pages, 4 figure

Dielectric behavior of oblate spheroidal particles: Application to erythrocytes suspensions

We have investigated the effect of particle shape on the eletrorotation (ER) spectrum of living cells suspensions. In particular, we consider coated oblate spheroidal particles and present a theoretical study of ER based on the spectral representation theory. Analytic expressions for the characteristic frequency as well as the dispersion strength can be obtained, thus simplifying the fitting of experimental data on oblate spheroidal cells that abound in the literature. From the theoretical analysis, we find that the cell shape, coating as well as material parameters can change the ER spectrum. We demonstrate good agreement between our theoretical predictions and experimental data on human erthrocytes suspensions.Comment: RevTex; 5 eps figure

Many-body dipole-induced dipole model for electrorheological fluids

Theoretical investigations on electrorheological (ER) fluids usually rely on computer simulations. An initial approach for these studies would be the point-dipole (PD) approximation, which is known to err considerably when the particles approach and finally touch due to many-body and multipolar interactions. Thus various work attempted to go beyond the PD model. Being beyond the PD model, previous attempts have been restricted to either local-field effects only or multipolar effects only, but not both. For instance, we recently proposed a dipole-induced-dipole (DID) model which is shown to be both more accurate than the PD model and easy to use. This work is necessary because the many-body (local-field) effect is included to put forth the many-body DID model. The results show that the multipolar interactions can indeed be dominant over the dipole interaction, while the local-field effect may yield an important correction.Comment: RevTeX, 3 eps figure

Nonlinear ac responses of electro-magnetorheological fluids

We apply a Langevin model to investigate the nonlinear ac responses of electro-magnetorheological (ERMR) fluids under the application of two crossed dc magnetic (z axis) and electric (x axis) fields and a probing ac sinusoidal magnetic field. We focus on the influence of the magnetic fields which can yield nonlinear behaviors inside the system due to the particles with a permanent magnetic dipole moment. Based on a perturbation approach, we extract the harmonics of the magnetic field and orientational magnetization analytically. To this end, we find that the harmonics are sensitive to the degree of anisotropy of the structure as well as the field frequency. Thus, it is possible to real-time monitor the structure transformation of ERMR fluids by detecting the nonlinear ac responses.Comment: 21 pages, 4 figure

Giant enhanced optical nonlinearity of colloidal nanocrystals with a graded-index host

The effective linear and third-order nonlinear optical properties of metallic colloidal crystal immersed in a graded-index host fluid are investigated theoretically. The local electric fields are extracted self-consistently based on the layer-to-layer interactions, which are readily given by the Lekner summation method. The resultant optical absorption and nonlinearity enhancement show a series of sharp peaks, which merge in a broadened resonant band. The sharp peaks become a continuous band for increasing packing density and number of layers. We believe that the sharp peaks arise from the in-plane dipolar interactions and the surface plasmon resonance, whereas the continuous band is due to the presence of the gradient in the host refractive index. These results have not been observed in homogeneous and randomly-dispersed colloids, and thus would be of great interest in optical nanomaterial engineering.Comment: Submitted to Applied Physics Letter

Tunable Localization and Oscillation of Coupled Plasmon Waves in Graded Plasmonic Chains

The localization (confinement) of coupled plasmon modes, named as gradons, has been studied in metal nanoparticle chains immersed in a graded dielectric host. We exploited the time evolution of various initial wavepackets formed by the linear combination of the coupled modes. We found an important interplay between the localization of plasmonic gradons and the oscillation in such graded plasmonic chains. Unlike in optical superlattices, gradient cannot always lead to Bloch oscillations, which can only occur for wavepackets consisting of particular types of gradons. Moreover, the wavepackets will undergo different forms of oscillations. The correspondence can be applied to design a variety of optical devices by steering among various oscillations.Comment: Sumitted to Journal of Applied Physic

Quantum phase transition in an atomic Bose gas near a Feshbach resonance

We study the quantum phase transition in an atomic Bose gas near a Feshbach resonance in terms of the renormalization group. This quantum phase transition is characterized by an Ising order parameter. We show that in the low temperature regime where the quantum fluctuations dominate the low-energy physics this phase transition is of first order because of the coupling between the Ising order parameter and the Goldstone mode existing in the bosonic superfluid. However, when the thermal fluctuations become important, the phase transition turns into the second order one, which belongs to the three-dimensional Ising universality class. We also calculate the damping rate of the collective mode in the phase with only a molecular Bose-Einstein condensate near the second-order transition line, which can serve as an experimental signature of the second-order transition.Comment: 8 pages, 2 figures, published version in Phys. Rev.