Department of Physics

Ashoka University is a private university located in Sonipat, Haryana, India, focusing on liberal arts and sciences. It was established in 2014.&nbsp;It is guided by an academic council of academics and scholars from around the world. The council sets the university&#39;s academic standard, advising on matters of curriculum, faculty hiring and research.&nbsp;

Ashoka University offers a full-time 3 (+ 1) year undergraduate programme. The University comprises of&nbsp;2500 plus students, with a mix of both national and international students.&nbsp; &nbsp;

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Ashoka University

The motion of glaciers over their bedrock or drops of fluid along a solid surface can vary dramatically when these substrates are lubricated. We investigate the coupled flow of a gravity current (GC) of a strain-rate softening fluid that is lubricated by a denser, lower-viscosity Newtonian fluid. We present a set of experiments in which such GCs are discharged axisymmetrically and at constant flux over a flat surface. Using imaging techniques, we follow the evolution of the front and thickness field of each fluid. We find that, unlike purely Newtonian lubricated GCs, the fronts of the non-Newtonian and lubricating Newtonian fluids have a power-law time evolution with different exponents. Each of these exponents is similar to that of a non-lubricated GC of the same fluid. Nevertheless, the fronts of our lubricated GCs evolve faster than those of the corresponding non-lubricated GCs owing to larger intercepts. In addition, in contrast with the monotonically declining thickness of non-lubricated GCs, the thickness of the lubricated, non-Newtonian fluid is nearly uniform, and that of the lubricating fluid is non-monotonic with localised spikes. Despite these complex thickness patterns, lubricated GCs remain axisymmetric as long as the flux of the lubricating fluid is sufficiently smaller than that of the non-Newtonian fluid.

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Journal of Fluid Mechanics

Lubricated gravity currents of power-law fluids

In this paper we use the spinor-helicity formalism to calculate 3-point functions involving scalar operators and spin-s conserved currents in general 3d CFTs. In spinor-helicity variables we notice that the parity-even and the parity-odd parts of a correlator are related. Upon converting spinor-helicity answers to momentum space, we show that correlators involving spin-s currents can be expressed in terms of some simple conformally invariant conserved structures. This in particular allows us to understand and separate out contact terms systematically, especially for the parity-odd case. We also reproduce some of the correlators using weight-shifting operators. © 2021, The Author(s).

Journal of High Energy Physics

Higher spin 3-point functions in 3d CFT using spinor-helicity variables

We show that general parity-violating 3d conformal field theories show a double copy structure for momentum space 3-point functions of conserved currents, stress tensor and marginal scalar operators. Splitting up the CFT correlator into two parts — called homogeneous and non-homogeneous — we show that double copy relations exist for each part separately. We arrive at similar conclusions regarding double copy structures using tree-level correlators of massless fields in dS4. We also discuss the flat space limit of these correlators. We further extend the double copy analysis to correlators involving higher-spin conserved currents, which suggests that the spin-s current correlator can be thought of as s copies of the spin one current correlator. © 2021, The Author(s).

Double copy structure of parity-violating CFT correlators

We define a generalized model for three-stranded DNA consisting of two chains of one type and a third chain of a different type. The DNA strands are modeled by random walks on the three-dimensional cubic lattice with different interactions between two chains of the same type and two chains of different types. This model may be thought of as a classical analog of the quantum three-body problem. In the quantum situation, it is known that three identical quantum particles will form a triplet with an infinite tower of bound states at the point where any pair of particles would have zero binding energy. The phase diagram is mapped out, and the different phase transitions are examined using finite-size scaling. We look particularly at the scaling of the DNA model at the equivalent Efimov point for chains up to 10 000 steps in length. We find clear evidence of several bound states in the finite-size scaling. We compare these states with the expected Efimov behavior. © 2021 Author(s).

Journal of Chemical Physics

Efimov-DNA phase diagram: Three stranded DNA on a cubic lattice

COVID-19 testing across India uses a mix of two types of tests. Rapid Antigen Tests (RATs) are relatively inexpensive point-of-care lateral-flow-assay tests, but they are also less sensitive. The reverse-transcriptase polymerase-chain-reaction (RT-PCR) test has close to 100% sensitivity and specificity in a laboratory setting, but delays in returning results, as well as increased costs relative to RATs, may vitiate this advantage. India-wide, about 49% of COVID-19 tests are RATs, but some Indian states, including the large states of Uttar Pradesh (pop. 227.9 million) and Bihar (pop. 121.3 million) use a much higher proportion of such tests. Here we show, using simulations based on epidemiological network models, that the judicious use of RATs can yield epidemiological outcomes comparable to those obtained through RT-PCR-based testing and isolation of positives, provided a few conditions are met. These are (a) that RAT test sensitivity is not too low, (b) that a reasonably large fraction of the population, of order 0.5% per day, can be tested, (c) that those testing positive are isolated for a sufficient duration, and that (d) testing is accompanied by other non-pharmaceutical interventions for increased effectiveness. We assess optimal testing regimes, taking into account test sensitivity and specificity, background seroprevalence and current test pricing. We find, surprisingly, that even 100% RAT test regimes should be acceptable, from both an epidemiological as well as a economic standpoint, provided the conditions outlined above are met. Copyright: © 2021 Cherian et al.

PLoS Computational Biology

Optimizing testing for COVID-19 in India

We study the parity-odd sector of 3-point functions comprising scalar operators and conserved currents in conformal field theories in momentum space. We use momentum space conformal Ward identities as well as spin-raising and weight-shifting operators to fix the form of some of these correlators. Wherever divergences appear we discuss their regularisation and renormalisation using appropriate counter-terms. © 2021, The Author(s).

Momentum space parity-odd CFT 3-point functions

This paper explores the use of a <a href="https://www.sciencedirect.com/topics/mathematics/cumulants">cumulant</a> method to determine the zeros of <a href="https://www.sciencedirect.com/topics/mathematics/partition-function">partition functions</a> for continuous phase transitions going up to the second zero. Unlike a first-order transition, with a uniform density of zeros near the transition point, a continuous transition shows a power law dependence of the density with a nontrivial slope for the line of zeros. Different types of models and methods of generating cumulants are used as testing grounds for the method. These include exactly solvable DNA melting problem on hierarchical <a href="https://www.sciencedirect.com/topics/mathematics/lattices">lattices</a>, heterogeneous DNA melting with randomness in sequence, <a href="https://www.sciencedirect.com/topics/mathematics/monte-carlo-simulation">Monte Carlo simulations</a> for the well-known <a href="https://www.sciencedirect.com/topics/mathematics/square-lattice">square lattice</a> <a href="https://www.sciencedirect.com/topics/physics-and-astronomy/ising-model">Ising model</a>. The method is applicable for zeros near the imaginary axis which control dynamical quantum phase transitions. In all cases, the method is found to provide the basic information about the transition, and most importantly, avoids root finding methods.

Physica A: Statistical Mechanics and its Applications

Zeros of partition function for continuous phase transitions using cumulants

One-dimensional steady-state passage of direct electric current from a binary electrolyte solution into a charge-selective solid such as a metal electrode or an ion exchange membrane is hydrodynamically unstable. Instability is preceded by concentration polarization, i.e., depletion of the electrolyte in the interface diffusion layer and it yields a microvortical flow in this layer. An ambiguity persists regarding the mechanism of this instability. The buoyant mechanisms are disregarded because instability also occurs in a gravitationally stable position and in diffusion layers too thin for buoyancy to mediate the flow. Therefore, instability is attributed to the electric forces acting in or near the interface electric double layer. These forces cause a sliplike flow known as electro-osmosis, which comes in two varieties. One is the classical equilibrium electro-osmosis related to the space charge of the electric double layer. The other is the nonequilibrium electro-osmosis related to the extended space charge that forms near the interface at high depletion. Both types of electro-osmosis may yield instability. The question is which one is at work in each particular system. The nonequilibrium electro-osmotic instability, unlike the equilibrium one, is of the short-wave type. This implies that its induced vortices are small compared to the width of the diffusion layer. Therefore, this width, which has not been clearly defined in most experiments so far, is crucial for the identification of the instability mechanism. In this paper, we report the results of our combined experimental and theoretical study of concentration polarization in a custom-designed experimental cell with a particular cation exchange membrane. As a part of our study, we investigate the recently predicted thermoelectroconvective instability. This instability is of the long-wave type and its related flow involves a pair of wide vortices spanning the diffusion layer. We experimentally retrieve this flow, which clearly marks the width of the diffusion layer. We observe that for high voltages this thermoelectroconvection is accompanied by electro-osmotic instability. Upon the background set by thermoelectroconvection, we are able to conclude that the observed electro-osmotic instability is of the short-wave type and is thus due to the nonequilibrium electro-osmosis. We suggest that a similar approach might be useful for identifying the instability mechanism in other charge-selective systems as well.

Physical Review Research

Mechanisms of hydrodynamic instability in concentration polarization

Near the melting transition the bending elastic constant κ, an emergent property of double-stranded DNA (dsDNA), is shown not to follow the rodlike scaling for small-length N. The reduction in κ with temperature is determined by the denatured bubbles for a continuous transition, e.g., when the two strands are Gaussian, but by the broken bonds near the open end in a Y-like configuration for a first-order transition as for strands with excluded volume interactions. In the latter case, a lever rule is operational, implying a phase coexistence although dsDNA is known to be a single phase. © 2020 American Physical Society.

Physical Review E

Softening of DNA near melting as disappearance of an emergent property

We show that dynamic quantum phase transitions (DQPT) in many situations involve renormalization group (RG) fixed points that are unphysical in the context of thermal phase transitions. In such cases, boundary conditions are shown to become relevant to the extent of even completely suppressing the bulk transitions. We establish these by performing an exact RG analysis of the quantum Ising model on scale-invariant lattices of different dimensions, and by analyzing the zeros of the Loschmidt amplitude. Further corroboration of boundaries affecting the bulk transition comes from the three-state quantum Potts chain, for which we also show that the DQPT corresponds to a pair of period-2 fixed points. © 2019 American Physical Society.