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A. P. Shah

Publications and source records attributed to A. P. Shah.

11 recordsLinked to original sources
arXiv2026

Delayed current sheet formation due to an external field in pulsed-power-driven reconnection experiments

We present results from pulsed-power-driven magnetic reconnection experiments, in which we drove two exploding wire arrays in parallel to produce colliding plasma flows with anti-parallel magnetic fields of 1.2$\pm$0.2 T. The experimental volume was surrounded by a Helmholtz coil pair capable of externally applying a field of up to 2 T, parallel to the reconnecting electric field. We diagnosed these experiments using laser interferometric imaging in the direction of the anti-parallel magnetic fields, gated extreme ultraviolet pinhole imaging, and in situ inductive probes. For zero and weak (0.5 T) external fields, we reproduce previous observations in which a dense reconnection layer forms between the two wire arrays. However, when we apply a strong external field (2 T), we observe a void between the arrays rather than a dense layer, and we hypothesise that the external field is frozen out of the plasma and provides a back-pressure which decelerates the flows. Our experimental results are compared with three-dimensional magnetohydrodynamic simulations of the experiment, which qualitatively support this hypothesis. These simulations allow us to study the pressure balance and dynamics of the current sheet aspect ratio, demonstrating the delayed formation of the reconnection layer due to the external field.

physics.plasm-ph↗
arXiv2024

Quadrupolar Density Structures in Driven Magnetic Reconnection Experiments with a Guide Field

Magnetic reconnection is a ubiquitous process in plasma physics, driving rapid and energetic events such as coronal mass ejections. Reconnection between magnetic fields with arbitrary shear can be decomposed into an anti-parallel, reconnecting component, and a non-reconnecting guide-field component which is parallel to the reconnecting electric field. This guide field modifies the structure of the reconnection layer and the reconnection rate. We present results from experiments on the MAIZE pulsed-power generator (500 kA peak current, 200 ns rise-time) which use two exploding wire arrays, tilted in opposite directions, to embed a guide field in the plasma flows with a relative strength $b\equiv B_g/B_{rec}=\text{0, 0.4, or 1}$. The reconnection layers in these experiments have widths which are less than the ion skin depth, $d_i=c/ω_{pi}$, indicating the importance of the Hall term, which generates a distinctive quadrupolar magnetic field structure along the separatrices of the reconnection layer. Using laser imaging interferometry, we observe quadrupolar structures in the line-integrated electron density, consistent with the interaction of the embedded guide field with the quadrupolar Hall field. Our measurements extend over much larger length scales ($40 d_i$) at higher $β$ ($\sim 1$) than previous experiments, providing an insight into the global structure of the reconnection layer.

physics.plasm-ph↗
arXiv2011

Polarization sensitive solar-blind detector based on a-plane AlGaN

We report polarization-sensitive solar-blind metal-semiconductor-metal UV photodetectors based on (11-20) a-plane AlGaN. The epilayer shows anisotropic optical properties confirmed by polarization-resolved transmission and photocurrent measurements, in good agreement with band structure calculations.

cond-mat.mtrl-sci↗
arXiv2011

Anisotropic structural and optical properties of a-plane (11-20) AlInN nearly-lattice-matched to GaN

We report epitaxial growth of a-plane (11-20) AlInN layers nearly-lattice-matched to GaN. Unlike for c-plane oriented epilayers, a-plane Al_{1-x}In_{x}N cannot be simultaneously lattice-matched to GaN in both in-plane directions. We study the influence of temperature on indium incorporation and obtain nearly-lattice-matched Al_{0.81}In_{0.19}N at a growth temperature of 760^{o}C. We outline a procedure to check in-plane lattice mismatch using high resolution x-ray diffraction, and evaluate the strain and critical thickness. Polarization-resolved optical transmission measurements of the Al_{0.81}In_{0.19}N epilayer reveal a difference in bandgap of ~140 meV between (electric field) E_parallel_c [0001]-axis and E_perpendicular_c conditions with room-temperature photoluminescence peaked at 3.38 eV strongly polarized with E_parallel_c, in good agreement with strain-dependent band-structure calculations.

cond-mat.mtrl-sci↗
arXiv2007

The role of hydrostatic stress in determining the bandgap of InN epilayers

We establish a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy we find that the films are under primarily hydrostatic stress. This results in a shift in the band edge to higher energy. The effect is significant, and may be responsible for some of the variations in InN bandgap reported in the literature.

cond-mat.mtrl-sci↗
arXiv2007

Influence of growth parameters on structural properties and bandgap of InN epilayers deposited in a showerhead MOVPE system

From a detailed analysis of InN epilayers deposited in a close-coupled showerhead metalorganic vapour phase epitaxy (MOVPE) system under various conditions we investigate the effect of growth parameters on the lattice constants of the InN layer. The layers are under significant internal hydrostatic stress which influences the optical properties. Samples typically fall into two broad categories of stress, with resultant luminescence emission around 0.8eV and 1.1eV. We can correlate the internal stress in the layer and the value of the optical absorption edge, and the PL emission wavelength.

cond-mat.mtrl-sci↗
arXiv2003

The effects of macroscopic inhomogeneities on the magneto transport properties of the electron gas in two dimensions

In experiments on electron transport the macroscopic inhomogeneities in the sample play a fundamental role. In this paper and a subsequent one we introduce and develop a general formalism that captures the principal features of sample inhomogeneities (density gradients, contact misalignments) in the magneto resistance data taken from low mobility heterostructures. We present detailed assessments and experimental investigations of the different regimes of physical interest, notably the regime of semiclassical transport at weak magnetic fields, the plateau-plateau transitions as well as the plateau-insulator transition that generally occurs at much stronger values of the external field only. It is shown that the semiclassical regime at weak fields plays an integral role in the general understanding of the experiments on the quantum Hall regime. The results of this paper clearly indicate that the plateau-plateau transitions, unlike the the plateau-insulator transition, are fundamentally affected by the presence of sample inhomogeneities. We propose a universal scaling result for the magneto resistance parameters. This result facilitates, amongst many other things, a detailed understanding of the difficulties associated with the experimental methodology of H.P. Wei et.al in extracting the quantum critical behavior of the electron gas from the transport measurements conducted on the plateau-plateau transitions.

cond-mat.mes-hall↗
arXiv2002

A model for anomalous moisture diffusion through a polymer-clay nanocomposite

Experimental data are reported on moisture diffusion and the elastoplastic response of an intercalated nanocomposite with vinyl ester resin matrix and montmorillonite clay filler at room temperature. Observations in diffusion tests show that water transport in the neat resin is Fickian, whereas it becomes anomalous (non-Fickian) with the growth of the clay content. This transition is attributed to immobilization of penetrant molecules on the surfaces of hydrophilic clay layers. Observations in uniaxial tensile tests demonstrate that the response of vinyl ester resin is strongly elastoplastic, whereas an increase in the clay content results in a severe decrease of plastic strains observed as a noticeable reduction of curvatures of the stress--strain diagrams. This is explained by slowing down of molecular mobility in the host matrix driven by confinement of chains in galleries between platelets. Constitutive equations are developed for the anomalous moisture diffusion through and the elastoplastic behavior of a nanocomposite. Adjustable parameters in these relations are found by fitting the experimental data. Fair agreement is demonstrated between the observations and the results of numerical simulation. A striking similarity is revealed between changes in diffusivity, ultimate water uptake and the rate of plastic flow with an increase in the clay content.

cond-mat.mtrl-sci↗
arXiv2002

The elastoplastic response of and moisture diffusion through a vinyl ester resin-clay nanocomposite

Experimental data are reported on the elastoplastic response of and moisture diffusion through a vinyl ester resin-montmorillonite clay nanocomposite with various amounts of filler. Two simple models are developed for the elastoplastic behavior of a nanocomposite and for the anomalous diffusion of penetrant molecules. Adjustable parameters in the constitutive equations are found by fitting the observations. It is revealed that some critical concentration of filler exists (about 1 wt.-%): in the sub-critical region of concentrations, molecular mobility of the host polymer strongly decreases with the clay content, whereas in the post-critical domain, the filler fraction weakly affects mobility of chains.

cond-mat.mtrl-sci↗
arXiv1999

Observation of Type I/II Transition in GaAs/InGaP Heterostructure by C-V Profiling

It is known in the literature that InGaP alloy synthesized within a certain range of growth temperature (520 - 720 degree Centrigrade) shows Cu-Pt crystal ordering. This ordering reduces the band gap energy . It was predicted that the ordering induced modification of energy levels of InGaP lowers its conduction band edge, which could change a type I band alignment at a GaAs/InGaP heterojunction to type II . Here we report the first observation of this ordering related type I/II transition for a GaAs/InGaP heterojunction by C-V profiling experiments done at room temperature .

cond-mat.mtrl-sci↗