arXiv subjects

Soumen Das

Publications and source records attributed to Soumen Das.

9 recordsLinked to original sources

arXiv2024

Lever rule violation and pressure imbalance in a driven granular system

We study a monolayer of metal balls under periodic chiral driving in the horizontal plane. Energy dissipation occurs in this system via (i) inelastic collisions and (ii) frictional interaction with the substrate. We show that below a density-dependent critical drive, the system phase separates into a fluid phase coexisting with a solid phase. Unlike ordinary coexistence, however, the system does not obey the lever rule, as the fluid-phase density depends on the overall particle density. Additionally, the pressure is discontinuous across the fluid-solid interface, accompanied by a chiral edge current at the interface.

cond-mat.soft↗

arXiv2024

Role of kinematic constraints in the time reversal symmetry breaking of a model active matter

Active-matter systems are inherently out-of-equilibrium and perform mechanical work by utilizing their internal energy sources. Breakdown of time-reversal symmetry (BTRS) is a hallmark of such dissipative non-equilibrium dynamics. We introduce a robust, experimentally accessible, noninvasive, quantitative measure of BTRS in terms of the Kullback-Leibler divergence in collision events, demonstrated in our novel artificial active matter, comprised of battery-powered spherical rolling robots whose energetics in different modes of motion can be measured with high precision. Our dimensionless measure characterizes how dissipation and internal energetics are influenced by kinematic constraints from interactions with the environment. We propose this measure of BTRS as an empirical estimate of the distance from equilibrium. An energetic insight into this departure of active matter from equilibrium comes from our demonstration of a non-trivial fluctuation symmetry, which reveals a potentially universal thermodynamic characteristic of active energetics. As a many-body consequence of BTRS in our experimental active system, we demonstrate the emergence of activity-induced herding, which has no equilibrium analogue.

cond-mat.soft↗

arXiv2022

Charge transfer mediated giant photo-amplification in air-stable $α$-CsPbI$_3$ nanocrystals decorated 2D-WS$_2$ photo-FET with asymmetric contacts

Hybrid heterostructure based phototransistors are attractive owing to their high gain induced by photogating effect. However, the absence of an in-plane built-in electric field in the single channel layer transistor results in a relatively higher dark current and require a large operating gate voltage of the device. Here, we report novel air-stable cesium lead iodide/tungsten di-sulfide (CsPbI$_3$/WS$_2$) mixed dimensional heterostructure based photo-field-effect-transistors (photo-FETs) with asymmetric metal electrodes (Cr/WS$_2$/Au), exhibiting extremely low dark current (~10-12 A) with a responsivity of ~102 A/W at zero gate bias. The Schottky barrier (WS$_2$/Au interface) induced rectification characteristics in the channel accompanied by the excellent photogating effect from solution-processed $α$-phase CsPbI$_3$ NCs sensitizers, resulting in gate-tunable broadband photodetection with a very high responsivity (~104 A/W) and excellent sensitivity (~106). Most interestingly, the device shows superior performance even under high humidity (50-65%) conditions owing to the formation of cubic $α$-phase CsPbI$_3$ nanocrystals with a relatively smaller lattice constant (a = 6.2315 Å) and filling of surface vacancies (Pb2+ centres) with the sulfur atoms from WS$_2$ layer, thus protecting it from environmental degradation. These results emphasise a novel strategy for developing mixed dimensional hybrid heterostructure based phototransistors for futuristic integrated nano-optoelectronic systems.

cond-mat.mtrl-sci↗

arXiv2022

Squigglebot: a battery powered spherical rolling robot as a model active matter system to measure its energetics

Active matter systems use their internal or ambient source of energy and dissipate them at the scale of individual constituent particles to generate motion. Direct measurement of the energy influx for individual particles has not been achieved in the experiments. Here we present "Squigglebot" - a battery powered spherical rolling robot based on open source hardware as an artificial active matter system whose energy consumption as well as the energy dissipation into different modes of motion both can be measured experimentally. This can serve as a prototype system to study a number of interesting problems in non-equilibrium statistical physics, where details of the energetics are required.

cond-mat.soft↗

arXiv2022

Self-similar inhomogeneous stationary states under constrained dynamics

The dynamics of $n$ rigid objects, each having $d$ degrees of freedom, is played out in the configuration space of dimension $nd$. Being rigid, there are additional constraints at work that render a portion of the configuration space inaccessible. In this paper, we make the assertion that treating the overall dynamics as a Markov process whose states are defined by the number of contacts made between the rigid objects provides an effective coarse grained characterization of the otherwise complex phenomenon. This coarse graining reduces the dimensionality of the space from $nd$ to one. We test this assertion for a one dimensional array of curved squares each of which is undergoing a biased diffusion in its angular orientation.

cond-mat.stat-mech↗

arXiv2022

State-dependent driving: A route to non-equilibrium stationary states

We study three different experiments that involve dry friction and periodic driving, and which employ both single and many-particle systems. These experimental set-ups, besides providing a playground for investigation of frictional effects, are relevant in broad areas of science and engineering. Across all these experiments, we monitor the dynamics of objects placed on a substrate that is being moved in a horizontal manner. The driving couples to the degrees of freedom of the substrate, and this coupling in turn influences the motion of the objects. Our experimental findings suggest emergence of stationary-states with non-trivial features. We invoke a minimalistic phenomenological model to explain our experimental findings. Within our model, we treat the injection of energy into the system to be dependent on its dynamical state, whereby energy injection is allowed only when the system is in its suitable-friction state. Our phenomenological model is built on the fact that such a state-dependent driving results in a force that repeatedly toggles the frictional states in time, and serves to explain our experimental findings.

cond-mat.stat-mech↗

arXiv2021

Substrate Dependent Synergistic Many-Body Effects in Atomically Thin Two Dimensional WS$_2$

Mott transition has been realized in atomically thin monolayer (ML) of two dimensional semiconductors (WS$_2$) via optically excited carriers above a critical carrier density through many body interactions. The above nonlinear optical transition occurs when excited electron hole pairs in ML WS2 continuum heavily interact with each other followed by transformation into a collective electron hole plasma phase (EHP), by losing their identity as individual quasiparticles. This is manifested by the alluring red-shift-blue-shift crossover (RBC) phenomena of the excitonic peaks in the emission spectra, resulting from the synergistic attraction-repulsion processes at the Mott-transition point. A systematic investigation of many-body effects is reported on ML WS$_2$, while considering the modulated dielectric screening of three different substrates, viz., silicon dioxide, sapphire, and gold. Substrate doping effects on ML WS$_2$ are discussed using the Raman fingerprints and PL spectral weight, which are further corroborated using theoretical DFT calculations. Further the substrate dependent excitonic Bohr radius of ML WS$_2$ is extracted via modelling the emission energy shift with Lennard-Jones potential. The variation of Mott point as well as excitonic Bohr radius is explained via substrate induced dielectric screening effect for both the dielectric substrates, which is however absent in ML WS$_2$ on Au. Our study therefore reveals diverse many-body ramifications in 2D semiconductors and offers decisive outlooks on selecting the impeccable substrate materials for innovative device engineering.

cond-mat.mtrl-sci↗

arXiv2020

Recharging and rejuvenation of decontaminated N95 masks

N95 respirators comprise a critical part of the personal protective equipment used by frontline health-care workers, and are typically meant for one-time usage. However, the recent COVID-19 pandemic has resulted in a serious shortage of these masks leading to a worldwide effort to develop decontamination and re-use procedures. A major factor contributing to the filtration efficiency of N95 masks is the presence of an intermediate layer of charged polypropylene electret fibers that trap particles through electrostatic or electrophoretic effects. This charge can degrade when the mask is used. Moreover, simple decontamination procedures (e.g. use of alcohol) can degrade any remaining charge from the polypropylene, thus severely impacting the filtration efficiency post decontamination. In this report, we summarize our results on the development of a simple laboratory setup allowing measurement of charge and filtration efficiency in N95 masks. In particular, we propose and show that it is possible to recharge the masks post-decontamination and recover filtration efficiency.

physics.med-ph↗

arXiv2020

Tuning of friction noise by accessing the rolling-sliding option

Variable power transmission in mechanical systems is often achieved by devices, e.g., clutches and brakes, that use dry friction. In these systems, the variability in power transmission is brought about by engaging and disengaging the friction plates. Though commonly used, this method of making the coupling noisy is not as versatile as their electrical analog. An alternative method would be to intermittently vary the frictional force. In this paper, we demonstrate a self-organized way to tune the noise in the frictional coupling between two surfaces which are in relative motion with each other. This is achieved by exploiting the complexity that arises from the frictional interaction of the balls which are placed in a circular groove between the surfaces. The extent of floppiness in the coupling is related to the rate at which the balls make transitions between their rolling and sliding states. If the moving surface is soft and the static surface is hard we show that with increasing filling fraction of the balls the transitions between rolling and sliding against the static surface give way to the transitions between rolling and sliding against the moving surface. As a consequence, the noise in the coupling is large for both small and large filling fraction with a dip in the middle. In contrast, the sliding with the static surface is suppressed if the moving pate is hard and the noise in the coupling decreases monotonically with the filling fraction of the balls.

cond-mat.soft↗