arXiv subjects

Pallabi Das

Publications and source records attributed to Pallabi Das.

7 recordsLinked to original sources

arXiv2025

Vorticity-induced surfing and trapping in porous media

Microorganisms often encounter strong confinement and complex hydrodynamic flows while navigating their habitats. Combining finite-element methods and stochastic simulations, we study the interplay of active transport and heterogeneous flows in dense porous channels. We find that swimming always slows down the traversal of agents across the channel, giving rise to robust power-law tails of their exit-time distributions. These exit-time distributions collapse onto a universal master curve with a scaling exponent of $\approx 3/2$ across a wide range of packing fractions and motility parameters, which can be rationalized by a scaling relation. We further identify a new motility pattern where agents alternate between surfing along fast streams and extended trapping phases, the latter determining the power-law exponent. Unexpectedly, trapping occurs in the flow backbone itself -- not only at obstacle boundaries -- due to vorticity-induced reorientation in the highly-heterogeneous fluid environment. These findings provide a fundamentally new active transport mechanism with direct implications for biofilm clogging and the design of novel microrobots capable of operating in heterogeneous media.

cond-mat.soft↗

arXiv2024

Rare and Exotic Higgs decays at ATLAS and CMS

After the Higgs boson discovery in 2012, the experiments at the LHC are continuing to study this particle and look for physics beyond the standard model. Some of the Higgs boson properties, such as the mass, has been measured with sub-percent level accuracy. Yet the present integrated luminosity is still a limiting factor for measuring the Higgs boson self-coupling or the first generation Yukawa couplings. The current constraints on the Higgs boson couplings would still allow for a sizeable branching fraction into undetected final states, which motivates the direct searches for rare and exotic decay modes. This presentation discusses several new results from these searches utilizing advanced online selection methods or analysis techniques with the entire Run 2 data.

hep-ex↗

arXiv2022

Annealing glasses by cyclic shear deformation

A major challenge in simulating glassy systems is the ability to generate configurations that may be found in equilibrium at sufficiently low temperatures, in order to probe static and dynamic behaviour close to the glass transition. A variety of approaches have recently explored ways of surmounting this obstacle. Here, we explore the possibility of employing mechanical agitation, in the form of cyclic shear deformation, to generate low energy configurations in a model glass former. We perform shear deformation simulations over a range of temperatures, shear rates and strain amplitudes. We find that shear deformation induces faster relaxation towards low energy configurations, or overaging, in simulations at sufficiently low temperatures, consistently with previous results for athermal shear. However, for temperatures at which simulations can be run till a steady state is reached with or without shear deformation, we find that the inclusion of shear deformation does not result in any speed up of the relaxation towards low energy configurations. Although we find the configurations from shear simulations to have properties indistinguishable from an equilibrium ensemble, the cyclic shear procedure does not guarantee that we generate an equilibrium ensemble at a desired temperature. In order to ensure equilibrium sampling, we develop a hybrid Monte Carlo algorithm that employs cyclic shear as a trial generation step, and has acceptance probabilities that depend not only on the change in internal energy but also on heat dissipated (equivalently, work done). We show that such an algorithm indeed generates an equilibrium ensemble.

cond-mat.soft↗

arXiv2020

Design and analysis of electro-optic modulators based on high contrast gratings (HCGs) in AlGaN/GaN heterostructures

Recently High Electron Mobility Transistor (HEMT) inspired III-V electro-optic modulator topologies were proposed for realizing high speed electro-optic modulators leveraging plasma dispersion effect due to the 2D Electron Gas (2DEG) present at the III-V heterostructure interface. The 2DEG is highly confined at the interface, extending to very low depths in the bulk ($\approx$10nm) and therefore has limited spatial overlap with the optical mode. In this paper, we propose a novel modulator design to boost the 2DEG-light interaction, wherein the HEMT is embedded within a high contrast grating (HCG) mirror. We present an analytical model extending the conventional HCG model to multi-layer structures and observe good agreement with rigorous coupled-wave analysis (RCWA). We explore the design space for identifying optimal device topology and present geometries that produce a change in reflectivity as large as 70% for C- and L-band wavelengths. We also present results of sensitivity analysis and observe low variation in device performance due to geometry variation arising from device fabrication imperfections. The device platforms presented here are suitable for designing high efficiency electro-optic modulators by incorporating the HEMT HCG into a Fabry-Perot cavity.

physics.app-ph↗

arXiv2019

Large $d_{33}$ Piezoelectric-Polymer Composites For RF Acoustic Resonators

While piezoelectric transduction enables designing acoustic resonators operating at multi-GHz frequencies, the deposition of piezoelectric materials typically requires high temperature processes and specific crystallographic orientation of substrates, thus imposing a limitation on materials that could be used. In this paper we present a piezoelectrically transduced thickness mode acoustic resonator that employs piezoelectric (PMNPT) nanoparticles embedded in a polymer (SU8) matrix. This composite material is deposited using standard resist-spin coaters and is thus compatible with a variety of substrates. The device presented here uses a double side polished single crystal silicon wafer as the low loss acoustic substrate for the resonator and $1.7μm$ thick SU8-PMNPT composite film as the actuator, and exhibits large effective piezoelectric coefficient $(d_{33})$ of $216pm/V$, and we experimentally demonstrate efficient transduction of acoustic resonances at frequencies up to $1.5GHz$.

physics.app-ph↗

arXiv2019

Design and analysis of high electron mobility transistor (HEMT) inspired III-V electro-optic modulator topologies

III-V heterostructure based high electron mobility transistors (HEMTs) offer superior performance as compared to CMOS silicon transistors owing to the high mobility in the 2D electron gas (2DEG) channel at the heterostructure interface. Gallium nitride (GaN) based HEMTs are also suitable for high power and high temperature applications. GaN has a rich offering of material properties spanning domains of nonlinear optics, piezoelectric micro-electro-mechanical systems (MEMS), and monolithic microwave integrated circuits (MMICs). In this paper, we propose HEMT inspired III-V electro-optic modulator topologies that could potentially outperform silicon photonic modulators. We analyze the electroabsorption and electrorefraction on account of the 2DEG interaction with light and present a design framework to selectively leverage the desired mechanism of modulation. Our analysis suggests that modulation index of electrorefractive modulation in a HEMT-like structure is comparable to silicon photonic modulators, albeit with much higher electron mobility and thereby much higher modulation rates.

physics.app-ph↗

arXiv2019

Unified phase diagram of reversible-irreversible, jamming and yielding transitions in cyclically sheared soft sphere packings

Self-organization, and transitions from reversible to irreversible behaviour, of interacting particle assemblies driven by externally imposed stresses or deformation is of interest in comprehending diverse phenomena in soft matter. They have been investigated in a wide range of systems, such as colloidal suspensions, glasses, and granular matter. In different density and driving regimes, such behaviour is related to yielding of amorphous solids, jamming, and memory formation, \emph{etc.} How these phenomena are related to each other has not, however, been much studied. In order to obtain a unified view of the different regimes of behaviour, and transitions between them, we investigate computationally the response of soft sphere assemblies to athermal cyclic shear deformation over a wide range of densities and amplitudes of shear deformation. Cyclic shear deformation induces transitions from reversible to irreversible behaviour in both unjammed and jammed soft sphere packings. Well above isotropic jamming density ($\bf{ϕ_J}$), this transition corresponds to yielding. In the vicinity of the jamming point, up to a higher density limit we designate ${\bf ϕ_J^{cyc}}$, an unjammed phase emerges between a localised, \emph{absorbing} phase, and a diffusive, {\emph irreversible} phase. The emergence of the unjammed phase signals the shifting of the jamming point to higher densities as a result of annealing, and opens a window where shear jamming becomes possible for frictionless packings. Below $\bf{ϕ_J}$, two distinct localised states, termed point and loop reversibile, are observed. We characterise in detail the different regimes and transitions between them, and obtain a unified density-shear amplitude phase diagram.

cond-mat.soft↗