arXiv subjects

Ashish Shukla

Publications and source records attributed to Ashish Shukla.

33 records · Page 2Linked to original sources

arXiv2019

Mass-deformed $\mathcal{N} = 3$ Supersymmetric Chern-Simons-Matter Theory

The maximal extension of supersymmetric Chern-Simons theory coupled to fundamental matter has $\mathcal{N} = 3$ supersymmetry. In this short note, we provide the explicit form of the action for the mass-deformed $\mathcal{N} = 3$ supersymmetric $U(N)$ Chern-Simons-Matter theory. The theory admits a unique triplet mass deformation term consistent with supersymmetry. We explicitly construct the mass-deformed $\mathcal{N} = 3$ theory in $\mathcal{N} = 1$ superspace using a fundamental and an anti-fundamental superfield.

hep-th↗

arXiv2019

Structural and Magnetic Characterization of Spin Canted Mixed Ferrite-Cobaltites: LnFe0.5Co0.5O3 (Ln = Eu and Dy)

The mixed ferrite-cobaltites LnFe0.5Co0.5O3, with Ln = Eu & Dy have been prepared by a sol-gel method and the samples have been characterized using X-ray diffraction and electron microscopy. The magnetic investigations reveal that both samples ordered in canted antiferromagnetic structures near room temperature. The Dzyaloshinskii-Moriya or antisymmetric exchange interaction induces weak ferromagnetism due to canting of the antiferromagnetically ordered spins. In the case of Ln-Fe-Co orthoferrites, two magnetic sublattices (Ln3+-4f and Fe3+/Co3+-3d) generally align in opposite directions and interesting temperature dependent phenomena: e.g. uncompensated antiferromagnetic sublattices and spin-reorientations, are observed in the system. The existence of hysteresis at low temperature region has been explained in terms of the strength of magnetic interactions between Fe3+ and Co3+ ions with different A-site rare earth cations.

cond-mat.mtrl-sci↗

arXiv2018

Low temperature Magneto-dielectric coupling in nanoscale layered SmFe0.5Co0.5O3 perovskite

In this study, we determined the different physical characteristics of nanoscale layered mixed Fe-Co layers of orthoferrite SmFe0.5Co0.5O3 based on magnetic and dielectric measurements.Magnetization analyzes showed that the system was antiferromagnetic with a magnetic transition around 310 K and two spin-reorientation transitions at around 192 K and 22 K.In this phase, the antisymmetric exchange interaction induced weak ferromagnetism due to canting of the magnetic spins in a similar manner to the Ln-Fe-Co system. Importantly, this nanoscale layered perovskite also exhibited a positive magneto-dielectric effect of around 2.5 % (100 K) and the value decreased on both sides of this temperature.We analyzed the conditions related to the existence of magnetic and dielectric coupling in terms of the magnetic interactions between the cations as well as the spin-lattice interactions in the low temperature range.

cond-mat.mtrl-sci↗

arXiv2018

Nanoscale ordered Layered-Segregation of Cobalt and Iron in the Perovskite SmFe0.5Co0.5O3

The possibility to segregate cobalt and iron in a single perovskite, at a nanoscale in the form of layers has been investigated, using sol gel technic synthesis. Quite remarkably, it is shown that such a nanoscale ordering, which can only be detected by a combined HAADF-STEM and EELS study, has a significant impact upon the magnetic properties of this SmFe0.5Co0.5O3 oxide. This suggests that careful nanoscale characterization of oxides will be necessary in the future before proceeding to a viable interpretation of their physical properties, at least in the field of magnetism.

cond-mat.mtrl-sci↗

arXiv2018

Kubo formulas for thermodynamic transport coefficients

Uncharged relativistic fluids in 3+1 dimensions have three independent thermodynamic transport coefficients at second order in the derivative expansion. Fluids with a single global $U(1)$ current have nine, out of which seven are parity preserving. We derive the Kubo formulas for all nine thermodynamic transport coefficients in terms of equilibrium correlation functions of the energy-momentum tensor and the current. All parity-preserving coefficients can be expressed in terms of two-point functions in flat space without external sources, while the parity-violating coefficients require three-point functions. We use the Kubo formulas to compute the thermodynamic coefficients in several examples of free field theories.

hep-th↗

arXiv2018

On the Dynamics of Near-Extremal Black Holes

We analyse the dynamics of near-extremal Reissner-Nordström black holes in asymptotically four-dimensional Anti-de Sitter space (AdS$_4$). We work in the spherically symmetric approximation and study the thermodynamics and the response to a probe scalar field. We find that the behaviour of the system, at low energies and to leading order in our approximations, is well described by the Jackiw-Teitelboim (JT) model of gravity. In fact, this behaviour can be understood from symmetry considerations and arises due to the breaking of time reparametrisation invariance. The JT model has been analysed in considerable detail recently and related to the behaviour of the SYK model. Our results indicate that features in these models which arise from symmetry considerations alone are more general and present quite universally in near-extremal black holes.

hep-th↗

arXiv2018

Domain wall moduli in softly-broken SQCD at $\barθ=π$

We analyze the moduli space dynamics of domain walls in $SU(N)$ QCD at $\barθ=π$, by softly breaking ${\cal N}\! =\!1$ SQCD with sfermion mixing. In the supersymmetric limit, BPS domain walls between neighbouring vacua are known to possess non-translational flavour moduli that form a $\mathcal{C} P^{N-1}$ sigma model. For the simplest case with gauge group $SU(2)$ and $N_f=2$, we show that this sigma model also exhibits a Hopf term descending from the bulk Wess-Zumino term with a quantized coefficient. On soft-breaking of supersymmetry via sfermion mixing that preserves the flavour symmetry, these walls and their moduli-space dynamics survives when $\barθ=π$ so that there are two degenerate vacua.

hep-th↗

arXiv2016

Symmetry Constraints in Inflation, $α$-vacua, and the Three Point Function

The Ward identities for conformal symmetries in single field models of inflation are studied in more detail in momentum space. For a class of generalized single field models, where the inflaton action contains arbitrary powers of the scalar and its first derivative, we find that the Ward identities are valid. We also study a one-parameter family of vacua, called $α$-vacua, which preserve conformal invariance in de Sitter space. We find that the Ward identities, upto contact terms, are met for the three point function of a scalar field in the probe approximation in these vacua. Interestingly, the corresponding non-Gaussian term in the wave function does not satisfy the operator product expansion. For scalar perturbations in inflation, in the $α$-vacua, we find that the Ward identities are not satisfied. We argue that this is because the back-reaction on the metric of the full quantum stress tensor has not been self-consistently incorporated. We also present a calculation, drawing on techniques from the AdS/CFT correspondence, for the three point function of scalar perturbations in inflation in the Bunch-Davies vacuum.

hep-th↗

arXiv2016

Ward Identities for Scale and Special Conformal Transformations in Inflation

We derive the general Ward identities for scale and special conformal transformations in theories of single field inflation. Our analysis is model independent and based on symmetry considerations alone. The identities we obtain are valid to all orders in the slow roll expansion. For special conformal transformations, the Ward identities include a term which is non-linear in the fields that arises due to a compensating spatial reparametrization. Some observational consequences are also discussed.

hep-th↗

arXiv2015

Constraints from Conformal Symmetry on the Three Point Scalar Correlator in Inflation

Using symmetry considerations, we derive Ward identities which relate the three point function of scalar perturbations produced during inflation to the scalar four point function, in a particular limit. The derivation assumes approximate conformal invariance, and the conditions for the slow roll approximation, but is otherwise model independent. The Ward identities allow us to deduce that the three point function must be suppressed in general, being of the same order of magnitude as in the slow roll model. They also fix the three point function in terms of the four point function, upto one constant which we argue is generically suppressed. Our approach is based on analyzing the wave function of the universe, and the Ward identities arise by imposing the requirements of spatial and time reparametrization invariance on it.

hep-th↗

arXiv2011

Ensuring Cache Freshness in On-demand Routing Protocols for Mobile Ad Hoc Network: A Cross-layer Framework

One of the big challenges in ad hoc network design is packet routing. Studies have shown that on-demand routing protocols perform better than table-driven routing protocols. In order to avoid route discovery for each packet, on-demand routing protocols cache routes previously learnt. A node in ad hoc network learns routing information by overhearing or forwarding packets to other nodes and keep learned routes in its route cache. However, node movement results broken links and therefore increases risk of cache pollution. Ensuring cache freshness in on-demand routing protocols, therefore, presents a serious challenge. A lot of research has been done in route cache organization, however, little effort has been done for route cache timeout policy to prevent stale routes from being used. In this paper we propose a new cross-layer framework to improve route cache performance in on-demand routing protocols. The proposed framework presents novel use of Received Signal Strength Indicator (RSSI) information to choose cache timeout of individual links in route cache.

cs.NI↗

arXiv2011

A Simple Battery Aware Gossip Based Sleep Protocol for Densely Deployed Ad-hoc and Sensor Networks

Conserving power in mobile ad-hoc and sensor networks is a big challenge. Most of the nodes in these networks, in general, are battery powered, therefore, an efficient power saving protocol is required to extend the lifetime of such networks. A lot of work has been done and several protocols have been proposed to address this problem. Gossip based protocols, which are based on the results of percolation theory, significantly reduce power consumption with very little implementation overhead. However, not much work has been done to make gossiping battery aware. In this paper we introduce a simple gossip based battery aware sleep protocol. The protocol allows low battery nodes to sleep more, therefore, improves overall network lifetime.

cs.NI↗

arXiv2011

A New Route Maintenance in Dynamic Source Routing Protocol

Mobile ad-hoc network (MANET) is infrastructureless, self-organizable, multi hop packet switched network. A number of routing protocols for MANETs have been proposed in recent years. Dynamic Source Routing (DSR) protocol is one of the most popular routing protocol for ad hoc networks. This paper presents a novel method to enhance route maintenance part of DSR protocol. Our proposed route maintenance significantly increases the efficiency of the protocol at the time of route failures.

cs.NI↗

arXiv2011

On the Reduction of Broadcast Traffic in Mobile Ad Hoc Networks

Many mobile ad hoc network protocols use simple flooding, in order to adapt to changes in time varying network topology. Most of the times, a network-wide flood results in redundant packets and increases network congestion, probability of packet collision, low utilization of available bandwidth, and most important, higher power consumption. In this paper, we propose a new cross-layer broadcast scheme to minimize broadcast traffic in mobile ad hoc networks. Our scheme is based on use of received signal strength indicator, RSSI, value to reduce the number of broadcast packets. The effectiveness of the proposed technique is verified using simulations.

cs.NI↗

arXiv2009

Gauge transformation through an accelerated frame of reference

The Schrödinger equation of a charged particle in a uniform electric field can be specified in either a time-independent or a time-dependent gauge. The wave-function solutions in these two gauges are related by a phase-factor reflecting the gauge symmetry of the problem. In this article we show that the effect of such a gauge transformation connecting the two wave-functions can be mimicked by the effect of two successive extended Galilean transformations connecting the two wave-function. An extended Galilean transformation connects two reference frames out of which one is accelerating with respect to the other.

quant-ph↗