Front Stability in Mean Field Models of Diffusion Limited Growth

We present calculations of the stability of planar fronts in two mean field models of diffusion limited growth. The steady state solution for the front can exist for a continuous family of velocities, we show that the selected velocity is given by marginal stability theory. We find that naive mean field theory has no instability to transverse perturbations, while a threshold mean field theory has such a Mullins-Sekerka instability. These results place on firm theoretical ground the observed lack of the dendritic morphology in naive mean field theory and its presence in threshold models. The existence of a Mullins-Sekerka instability is related to the behavior of the mean field theories in the zero-undercooling limit.Comment: 26 pp. revtex, 7 uuencoded ps figures. submitted to PR

Transient quantum transport in double-dot Aharonov-Bohm interferometers

Real-time nonequilibrium quantum dynamics of electrons in double-dot Aharonov-Bohm (AB) interferometers is studied using an exact solution of the master equation. The building of the coherence between the two electronic paths shows up via the time-dependent amplitude of the AB oscillations in the transient transport current, and can be enhanced by varying the applied bias on the leads, the on-site energy difference between the dots and the asymmetry of the coupling of the dots to the leads. The transient oscillations of the transport current do not obey phase rigidity. The circulating current has an anti-symmetric AB oscillation in the flux. The non-degeneracy of the on-site energies and the finite bias cause the occupation in each dot to have an arbitrary flux dependence as the coupling asymmetry is varied.Comment: 11 pages, 5 figure

Effect of periodic parametric excitation on an ensemble of force-coupled self-oscillators

We report the synchronization behavior in a one-dimensional chain of identical limit cycle oscillators coupled to a mass-spring load via a force relation. We consider the effect of periodic parametric modulation on the final synchronization states of the system. Two types of external parametric excitations are investigated numerically: periodic modulation of the stiffness of the inertial oscillator and periodic excitation of the frequency of the self-oscillatory element. We show that the synchronization scenarios are ruled not only by the choice of parameters of the excitation force but depend on the initial collective state in the ensemble. We give detailed analysis of entrainment behavior for initially homogeneous and inhomogeneous states. Among other results, we describe a regime of partial synchronization. This regime is characterized by the frequency of collective oscillation being entrained to the stimulation frequency but different from the average individual oscillators frequency.Comment: Comments and suggestions are welcom

Mean Field Theory of the Morphology Transition in Stochastic Diffusion Limited Growth

We propose a mean-field model for describing the averaged properties of a class of stochastic diffusion-limited growth systems. We then show that this model exhibits a morphology transition from a dense-branching structure with a convex envelope to a dendritic one with an overall concave morphology. We have also constructed an order parameter which describes the transition quantitatively. The transition is shown to be continuous, which can be verified by noting the non-existence of any hysteresis.Comment: 16 pages, 5 figure

Probing Quantum Hall Pseudospin Ferromagnet by Resistively Detected NMR

Resistively Detected Nuclear Magnetic Resonance (RD-NMR) has been used to investigate a two-subband electron system in a regime where quantum Hall pseudo-spin ferromagnetic (QHPF) states are prominently developed. It reveals that the easy-axis QHPF state around the total filling factor $\nu =4$ can be detected by the RD-NMR measurement. Approaching one of the Landau level (LL) crossing points, the RD-NMR signal strength and the nuclear spin relaxation rate $1/T_{1}$ enhance significantly, a signature of low energy spin excitations. However, the RD-NMR signal at another identical LL crossing point is surprisingly missing which presents a puzzle

Structure and energetics of the Si-SiO_2 interface

Silicon has long been synonymous with semiconductor technology. This unique role is due largely to the remarkable properties of the Si-SiO_2 interface, especially the (001)-oriented interface used in most devices. Although Si is crystalline and the oxide is amorphous, the interface is essentially perfect, with an extremely low density of dangling bonds or other electrically active defects. With the continual decrease of device size, the nanoscale structure of the silicon/oxide interface becomes more and more important. Yet despite its essential role, the atomic structure of this interface is still unclear. Using a novel Monte Carlo approach, we identify low-energy structures for the interface. The optimal structure found consists of Si-O-Si "bridges" ordered in a stripe pattern, with very low energy. This structure explains several puzzling experimental observations.Comment: LaTex file with 4 figures in GIF forma

Intrinsic noise-induced phase transitions: beyond the noise interpretation

We discuss intrinsic noise effects in stochastic multiplicative-noise partial differential equations, which are qualitatively independent of the noise interpretation (Ito vs. Stratonovich), in particular in the context of noise-induced ordering phase transitions. We study a model which, contrary to all cases known so far, exhibits such ordering transitions when the noise is interpreted not only according to Stratonovich, but also to Ito. The main feature of this model is the absence of a linear instability at the transition point. The dynamical properties of the resulting noise-induced growth processes are studied and compared in the two interpretations and with a reference Ginzburg-Landau type model. A detailed discussion of new numerical algorithms used in both interpretations is also presented.Comment: 9 pages, 8 figures, to be published in Phys. Rev.

Flexible and Stretchable Self-Powered Multi-Sensors Based on the N-Type Thermoelectric Response of Polyurethane/Na-x(Ni-ett)(n) Composites

Flexible and stretchable electronic devices have a broad range of potential uses, from biomedicine, soft robotics, and health monitoring to the internet‐of‐things. Unfortunately, finding a robust and reliable power source remains challenging, particularly in off‐the‐grid and maintenance‐free applications. A sought‐after development overcome this challenge is the development of autonomous, self‐powered devices. A potential solution is reported exploiting a promising n‐type thermoelectric compound, poly nickel‐ethenetetrathiolates (Na_{x}(Ni‐ett)_{n}). Highly stretchable n‐type composite films are obtained by combining Nax(Ni‐ett)n with commercial polyurethane (Lycra). As high as 50 wt% Na_{x}(Ni‐ett)_{n} content composite film can withstand deformations of ≈500% and show conductivities of ≈10^{-2} S cm^{-1} and Seebeck coefficients of approx. −40 µV K^{-1}. These novel materials can be easily synthesized on a large scale with continuous processes. When subjected to a small temperature difference (<20 °C), the films generate sufficient thermopower to be used for sensing strain (gauge factor ≈20) and visible light (sensitivity factor ≈36% (kW m^{-2})^{-1}), independent of humidity (sensitivity factor ≈0.1 (%RH)^{-1}. As a proof‐of‐concept, a wearable self‐powered sensor is demonstrated by using n‐type Na_{x}(Ni‐ett)_{n}/Lycra and PEDOT:PSS/Lycra elements, connected in series by hot pressing, without employing any metal connections, hence preserving good mechanical ductility and ease of processing

Priming of adult incision response by early life injury: neonatal microglial inhibition has persistent but sexually dimorphic effects in adult rats

Neonatal hindpaw incision primes developing spinal nociceptive circuitry, resulting in enhanced hyperalgesia following re-injury in adulthood. Spinal microglia contribute to this persistent effect and microglial inhibition at the time of adult re-incision blocks the enhanced hyperalgesia. Here, we pharmacologically inhibited microglial function with systemic minocycline or intrathecal SB203580 at the time of neonatal incision and evaluated sex-dependent differences following adult re-incision. Incision in adult male and female rats induced equivalent hyperalgesia and spinal dorsal horn expression of genes associated with microglial proliferation (Emr1) and transformation to a reactive phenotype (Irf8). In control adults with prior neonatal incision, the enhanced degree and duration of incision-induced hyperalgesia and spinal microglial responses to re-incision were equivalent in males and females. However, microglial inhibition at the time of the neonatal incision revealed sex-dependent effects: the persistent mechanical and thermal hyperalgesia following re-incision in adulthood was prevented in males but unaffected in females. Similarly, re-incision induced Emr1 and Irf8 gene expression was downregulated in males, but not in females following neonatal incision with minocycline. To evaluate the distribution of re-incision hyperalgesia, prior neonatal incision was performed at different body sites. Hyperalgesia was maximal when the same paw was re-incised, and was increased following prior incision at ipsilateral, but not contralateral sites; supporting a segmentally restricted spinal mechanism. These data highlight the contribution of spinal microglial mechanisms to persistent effects of early-life injury in males, and sex-dependent differences in the ability of microglial inhibition to prevent the transition to a persistent pain state spans developmental stages.Significant Statement: Following the same surgery, some patients develop persistent pain. Contributory mechanisms are not fully understood, but early-life experience and sex/gender may influence the transition to chronic pain. Surgery and painful procedural interventions in vulnerable preterm neonates are associated with long-term alterations in somatosensory function and pain that differ in males and females. Surgical injury in neonatal rodents primes the developing nociceptive system and enhances re-injury response in adulthood. Neuroimmune interactions are critical mediators of persistent pain, but sex-dependent differences in spinal neuroglial signaling influence the efficacy of microglial inhibitors following adult injury. Neonatal microglial inhibition has beneficial long-term effects on re-injury response in adult males only, emphasizing the importance of evaluating sex-dependent differences at all ages in pre-clinical studies