Rate statistics for radio noise from lightning

Radio frequency noise from lightning was measured at several frequencies in the HF - VHF range at the Kennedy Space Center, Florida. The data were examined to determine flashing rate statistics during periods of strong activity from nearby storms. It was found that the time between flashes is modeled reasonably well by a random variable with a lognormal distribution

High Frequency Scattering from Arbitrarily Oriented Dielectric Disks

Calculations have been made of electromagnetic wave scattering from dielectric disks of arbitrary shape and orientation in the high frequency (physical optics) regime. The solution is obtained by approximating the fields inside the disk with the fields induced inside an identically oriented slab (i.e. infinite parallel planes) with the same thickness and dielectric properties. The fields inside the disk excite conduction and polarization currents which are used to calculate the scattered fields by integrating the radiation from these sources over the volume of the disk. This computation has been executed for observers in the far field of the disk in the case of disks with arbitrary orientation and for arbitrary polarization of the incident radiation. The results have been expressed in the form of a dyadic scattering amplitude for the disk. The results apply to disks whose diameter is large compared to wavelength and whose thickness is small compared to diameter, but the thickness need not be small compared to wavelength. Examples of the dependence of the scattering amplitude on frequency, dielectric properties of the disk and disk orientation are presented for disks of circular cross section

Structural dichroism in the antiferromagnetic insulating phase of V_2O_3

We performed near-edge x-ray absorption spectroscopy (XANES) at V K edge in the antiferromagnetic insulating (AFI) phase of a 2.8% Cr-doped V_2O_3 single crystal. Linear dichroism of several percent is measured in the hexagonal plane and found to be in good agreement with ab-initio calculations based on multiple scattering theory. This experiment definitively proves the structural origin of the signal and therefore solves a controversy raised by previous interpretations of the same dichroism as non-reciprocal. It also calls for a further investigation of the role of the magnetoelectric annealing procedure in cooling to the AFI phase.Comment: 4 pages 3 figures. To be published in Phys. Rev. B (2005

Microwave properties of Nb/PdNi/Nb trilayers. Observation of flux flow in excess of Bardeen-Stephen theory

We combine wideband (1-20 GHz) Corbino disk and dielectric resonator (8.2 GHz) techniques to study the microwave properties in Nb/PdNi/Nb trilayers, grown by UHV dc magnetron sputtering, composed by Nb layers of nominal thickness $d_S$=15 nm, and a ferromagnetic PdNi layer of thickness $d_F$= 1, 2, 8 and 9 nm. We focus on the vortex state. Magnetic fields up to $H_{c2}$ were applied. The microwave resistivity at fixed $H/H_{c2}$ increases with $d_F$, eventually exceeding the Bardeen Stephen flux flow value.Comment: 6 pages. Submitted to Journal of Superconductivity and Novel Magnetis

Polaronic state and nanometer-scale phase separation in colossal magnetoresistive manganites

High resolution topographic images obtained by scanning tunneling microscope in the insulating state of Pr0.68Pb0.32MnO3 single crystals showed regular stripe-like or zigzag patterns on a width scale of 0.4 - 0.5 nm confirming a high temperature polaronic state. Spectroscopic studies revealed inhomogeneous maps of zero-bias conductance with small patches of metallic clusters on length scale of 2 - 3 nm only within a narrow temperature range close to the metal-insulator transition. The results give a direct observation of polarons in the insulating state, phase separation of nanometer-scale metallic clusters in the paramagnetic metallic state, and a homogeneous ferromagnetic state

Gate stability of GaN-Based HEMTs with P-Type Gate

status: publishe

High-Pressure Phase Diagram in the Manganites: a Two-site Model Study

The pressure dependence of the Curie temperature $T_C$ in manganites, recently studied over a wide pressure range, is not quantitatively accounted for by the quenching of Jahn-Teller distortions, and suggests the occurrence of a new pressure-activated localizing processes. We present a theoretical calculation of $T_C$ based on a two-site double-exchange model with electron-phonon coupling interaction and direct superexchange between the $t_{2g}$ core spins. We calculate the pressure dependence of $T_C$ and compare it with the experimental phase diagram. Our results describe the experimental behavior quite well if a pressure-activated enhancement of the antiferromagnetic superexchange interaction is assumed

Gauge factor of thick film resistors: outcomes of the variable range hopping model

Despite a large amount of data and numerous theoretical proposals, the microscopic mechanism of transport in thick film resistors remains unclear. However, recent low temperature measurements point toward a possible variable range hopping mechanism of transport. Here we examine how such a mechanism affects the gauge factor of thick film resistors. We find that at sufficiently low temperatures $T$, for which the resistivity follows the Mott's law $R(T)\sim \exp(T_0/T)^{1/4}$, the gauge factor GF is proportional to $(T_0/T)^{1/4}$. Moreover, the inclusion of Coulomb gap effects leads to ${\rm GF}\sim (T_0'/T)^{1/2}$ at lower temperatures. In addition, we study a simple model which generalizes the variable range hopping mechanism by taking into account the finite mean inter-grain spacing. Our results suggest a possible experimental verification of the validity of the variable range hopping in thick film resistors.Comment: 7 pages, 3 eps figures, submitted to Journal of Applied Physic

Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone

In subduction zones, seismic slip at shallow crustal depths can lead to the generation of tsunamis. Large slip displacements during tsunamogenic earthquakes are attributed to the low coseismic shear strength of the fluid-saturated and non-lithified clay-rich fault rocks. However, because of experimental challenges in confining these materials, the physical processes responsible for the coseismic reduction in fault shear strength are poorly understood. Using a novel experimental setup, we measured pore fluid pressure during simulated seismic slip in clay-rich materials sampled from the deep oceanic drilling of the Pāpaku thrust (Hikurangi subduction zone, New Zealand). Here, we show that at seismic velocity, shear-induced dilatancy is followed by pressurisation of fluids. The thermal and mechanical pressurisation of fluids, enhanced by the low permeability of the fault, reduces the energy required to propagate earthquake rupture. We suggest that fluid-saturated clay-rich sediments, occurring at shallow depth in subduction zones, can promote earthquake rupture propagation and slip because of their low permeability and tendency to pressurise when sheared at seismic slip velocities