arXiv subjects

Avadh Saxena

Publications and source records attributed to Avadh Saxena.

At least 73 records · Page 4Linked to original sources

arXiv2021

Wormhole as a Waveguide: Case of Quantum Particles with Zero Angular Momentum

We consider a static wormhole as a waveguide and determine the conditions for full transmission through the wormhole waveguide for a quantum particle with zero angular momentum. We find that the waveguide is transparent when the de Broglie wavelength of the quantum particle is an integer times twice the throat diameter of the wormhole. Such an effect may be realizable in graphene, plasmonic or optical wormholes.

quant-ph↗

arXiv2021

Kink-Antikink Collisions and Multi-Bounce Resonance Windows in Higher-Order Field Theories

We study collisions of coherent structures in higher-order field-theoretic models, such as the $ϕ^8$, $ϕ^{10}$ and $ϕ^{12}$ ones. The main distinguishing feature, of the example models considered herein, is that the collision arises due to the long-range interacting algebraic tails of these solitary waves. We extend the approach to suitably initialize the relevant kinks, in the additional presence of finite initial speed, in order to minimize the dispersive wave radiation potentially created by their slow spatial decay. We find that, when suitably initialized, these models still feature the multi-bounce resonance windows earlier found in models in which the kinks bear exponential tails, such as the $ϕ^4$ and $ϕ^6$ field theories among others. Also present is the self-similar structure of the associated windows with three- and more-bounce windows at the edges of two- and lower-bounce ones. Moreover, phenomenological, but highly accurate (and predictive) scaling relations are derived for the dependence of the time between consecutive collisions and, e.g., the difference in kinetic energy between the incoming one and the critical one for one-bounces. Such scalings are traced extensively over two-bounce collision windows throughout the three models, hinting at the possibility of an analytical theory in this direction.

hep-th↗

arXiv2021

Explicit Kink Solutions in Several One-Parameter Family of Higher Order Field Theory Models

We present several one-parameter family of higher order field theory models some of which admit explicit kink solutions with an exponential tail while others admit explicit kink solutions with a power-law tail. Various properties of these families of kink solutions are examined in detail. In particular for models harboring kink solutions with a power-law tail, we show that there is no gap between the zero mode and the beginning of the continuum in the kink stability potential. Further, by applying the recent Manton formalism, we provide estimates for the kink-kink and antikink-kink acceleration and hence the ratio of the corresponding antikink-kink and kink-kink forces.

nlin.PS↗

arXiv2020

Alignment induced re-configurable walls for patterning and assembly of liquid crystal skyrmions

Skyrmions have attracted rapidly growing interest due to their topological properties and unique aspects for potential novel applications such as data storage and soft robotics. They can also serve as key elements for materials by design, self-assembly, and functional soft materials. While not real particles, these skyrmions behave like particles-they interact with each other and can be actuated by means of electric field, surface anchoring, and light. On the other hand, they are field configurations which have properties not possessed by real particles. Here, we show that, by means of alignment induced attractive and repulsive walls, skyrmions in chiral nematic liquid crystals can be precisely controlled and programmed to serve as suitable building blocks for the realization of the above goals. Our work may stimulate new experimental efforts and concomitant applications in this direction.

cond-mat.soft↗

arXiv2020

Parity-time symmetric systems with memory

Classical open systems with balanced gain and loss, i.e. parity-time ($\mathcal{PT}$) symmetric systems, have attracted tremendous attention over the past decade. Their exotic properties arise from exceptional point (EP) degeneracies of non-Hermitian Hamiltonians that govern their dynamics. In recent years, increasingly sophisticated models of $\mathcal{PT}$-symmetric systems with time-periodic (Floquet) driving, time-periodic gain and loss, and time-delayed coupling have been investigated, and such systems have been realized across numerous platforms comprising optics, acoustics, mechanical oscillators, optomechanics, and electrical circuits. Here, we introduce a $\mathcal{PT}$-symmetric (balanced gain and loss) system with memory, and investigate its dynamics analytically and numerically. Our model consists of two coupled $LC$ oscillators with positive and negative resistance, respectively. We introduce memory by replacing either the resistor with a memristor, or the coupling inductor with a meminductor, and investigate the circuit energy dynamics as characterized by $\mathcal{PT}$-symmetric or $\mathcal{PT}$-symmetry broken phases. Due to the resulting nonlinearity, we find that energy dynamics depend on the sign and strength of initial voltages and currents, as well as the distribution of initial circuit energy across its different components. Surprisingly, at strong inputs, the system exhibits self-organized Floquet dynamics, including $\mathcal{PT}$-symmetry broken phase at vanishingly small dissipation strength. Our results indicate that $\mathcal{PT}$-symmetric systems with memory show a rich landscape.

quant-ph↗

arXiv2020

Solitary wave solutions of the 2+1 and 3+1 dimensional nonlinear Dirac equation constrained to planar and space curves

We study the effect of curvature and torsion on the solitons of the nonlinear Dirac equation considered on planar and space curves. Since the spin connection is zero for the curves considered here, the arc variable provides a natural setting to understand the role of curvature and then we can obtain the transformation for the 1+1 dimensional Dirac equation directly from the metric. Depending on the curvature, the soliton profile either narrows or expands. Our results may be applicable to yet-to-be-synthesized curved quasi-one dimensional Bose condensates.

nlin.PS↗

arXiv2020

Behavior of solitary waves of coupled nonlinear Schrödinger equations subjected to complex external periodic potentials with anti-$\mathcal{PT}$ symmetry

We discuss the response of both moving and trapped solitary wave solutions of a nonlinear two-component nonlinear Schrödinger system in 1+1 dimensions to an anti-$\mathcal{PT}$ external periodic complex potential. The dynamical behavior of perturbed solitary waves is explored by conducting numerical simulations of the nonlinear system and using a collective coordinate variational approximation. We present case examples corresponding to choices of the parameters and initial conditions involved therein. The results of the collective coordinate approximation are compared against numerical simulations where we observe qualitatively good agreement between the two. Unlike the case for a single-component solitary wave in a complex periodic $\mathcal{PT}$-symmetric potential, the collective coordinate equations do not have a small oscillation regime, and initially the height of the two components changes in opposite directions often causing instability. We find that the dynamic stability criteria we have used in the one-component case is proven to be a good indicator for the onset of dynamic instabilities in the present setup.

nlin.PS↗

arXiv2020

Anti-$\mathcal{PT}$-symmetric Qubit: Decoherence and Entanglement Entropy

We investigate a two-level spin system based anti-parity-time (anti-$\mathcal{PT}$)-symmetric qubit and study its decoherence as well as entanglement entropy properties. We compare our findings with that of the corresponding $\mathcal{PT}$-symmetric and Hermitian qubits. First we consider the time-dependent Dyson map to find the exact analytical dynamics for a general non-Hermitian qubit system coupled with a bath, then we specialize it to the case of the anti-$\mathcal{PT}$-symmetric qubit. We find that the decoherence function for the anti-$\mathcal{PT}$-symmetric qubit decays slower than the $\mathcal{PT}$-symmetric and Hermitian qubits. For the entanglement entropy we find that for the anti-$\mathcal{PT}$-symmetric qubit it grows more slowly compared to the $\mathcal{PT}$-symmetric and Hermitian qubits. Similarly, we find that the corresponding variance and area of Fisher information is much higher compared to the $\mathcal{PT}$-symmetric and Hermitian qubits. These results demonstrate that anti-$\mathcal{PT}$-symmetric qubits may be better suited for quantum computing and quantum information processing applications than conventional Hermitian or even $\mathcal{PT}$-symmetric qubits.

quant-ph↗

arXiv2020

Circular dichroism in high-order harmonic generation: Heralding topological phases and transitions in Chern insulators

Topological materials are of interest to both fundamental science and advanced technologies, because topological states are robust with respect to perturbations and dissipation. Experimental detection of topological invariants is thus in great demand, but it remains extremely challenging. Ultrafast laser-matter interactions, and in particular high-harmonic generation (HHG), meanwhile, were proposed several years ago as tools to explore the structural and dynamical properties of various matter targets. Here we show that the high-harmonic emission signal produced by a circularly-polarized laser contains signatures of the topological phase transition in the paradigmatic Haldane model. In addition to clear shifts of the overall emissivity and harmonic cutoff, the high-harmonic emission shows a unique circular dichroism, which exhibits clear changes in behavior at the topological phase boundary. Our findings pave the way to understand fundamental questions about the ultrafast electron-hole pair dynamics in topological materials via non-linear high-harmonic generation spectroscopy.

cond-mat.mes-hall↗

arXiv2020

Stability and response of trapped solitary wave solutions of coupled nonlinear Schrödinger equations in an external, $\mathcal{PT}$- and supersymmetric potential

We present trapped solitary wave solutions of a coupled nonlinear Schrödinger system in $1$+$1$ dimensions in the presence of an external, supersymmetric and complex $\mathcal{PT}$-symmetric potential. The Schrödinger system this work focuses on possesses exact solutions whose existence, stability, and spatio-temporal dynamics are investigated by means of analytical and numerical methods. Two different variational approximations are considered where the stability and dynamics of the solitary waves are explored in terms of eight and twelve time-dependent collective coordinates. We find regions of stability for specific potential choices as well as analytic expressions for the small oscillation frequencies in the collective coordinate approximation. Our findings are further supported by performing systematic numerical simulations of the nonlinear Schrödinger system.

nlin.PS↗

arXiv2020

Domain wall diode based on functionally graded Dzyaloshinskii-Moriya interaction

We present a general approach for studying the dynamics of domain walls in biaxial ferromagnetic stripes with functionally graded Dzyaloshinskii-Moriya interaction (DMI). By engineering the spatial profile of the DMI parameter we propose the concept of a diode, which implements filtering of domain walls of certain topological charge and helicity. We base our study on phenomenological Landau-Lifshitz-Gilbert equations with additional Zhang-Li spin-transfer terms using a collective variable approach. In the effective equations of motion the gradients of DMI play the role of a driving force which competes with current driving. All analytical predictions are confirmed by numerical simulations.

cond-mat.str-el↗

arXiv2019

Parametrically driven nonlinear Dirac equation with arbitrary nonlinearity

The damped and parametrically driven nonlinear Dirac equation with arbitrary nonlinearity parameter $κ$ is analyzed, when the external force is periodic in space and given by $f(x) =r\cos(K x)$, both numerically and in a variational approximation using five collective coordinates (time dependent shape parameters of the wave function). Our variational approximation satisfies exactly the low-order moment equations. Because of competition between the spatial period of the external force $λ=2 π/K$, and the soliton width $l_s$, which is a function of the nonlinearity $κ$ as well as the initial frequency $ω_0$ of the solitary wave, there is a transition (at fixed $ω_0$) from trapped to unbound behavior of the soliton, which depends on the parameters $r$ and $K$ of the external force and the nonlinearity parameter $κ$. We previously studied this phenomena when $κ=1$ (2019 J. Phys. A: Math. Theor. {\bf 52} 285201) where we showed that for $λ\gg l_s$ the soliton oscillates in an effective potential, while for $λ\ll l_s$ it moves uniformly as a free particle. In this paper we focus on the $κ$ dependence of the transition from oscillatory to particle behavior and explicitly compare the curves of the transition regime found in the collective coordinate approximation as a function of $r$ and $K$ when $κ=1/2,1,2$ at fixed value of the frequency $ω_0$. Since the solitary wave gets narrower for fixed $ω_0$ as a function of $κ$, we expect and indeed find that the regime where the solitary wave is trapped is extended as we increase $κ$.

nlin.PS↗

arXiv2019

Quantum Electron Transport in Degenerate Donor-Acceptor Systems

We develop a mathematically rigorous theory for the quantum transfer processes in degenerate donor-acceptor dimers in contact with a thermal environment. We calculate explicitly the transfer rates and the acceptor population efficiency. The latter depends critically on the initial donor state. We show that quantum coherence in the initial state enhances the transfer process. If the electron is initially shared coherently by the donor levels then the efficiency can reach values close to 100%, while an incoherent initial donor state will significantly suppress the efficiency. The results are useful for a better understanding of the quantum electron transport in many chemical, solid state, and biological systems with complex degenerate and quasi-degenerate energy landscapes.

quant-ph↗

arXiv2019

Logarithmic Potential with Super-Super-Exponential Kink Profiles and Tails

We consider a novel one dimensional model of a logarithmic potential which has super-super-exponential kink profiles as well as kink tails. We provide analytic kink solutions of the model -- it has 3 kinks, 3 mirror kinks and the corresponding antikinks. While some of the kink tails are super-super-exponential, some others are super-exponential whereas the remaining ones are exponential. The linear stability analysis reveals that there is a gap between the zero mode and the onset of continuum. Finally, we compare this potential and its kink solutions with those of very high order field theories harboring seven degenerate minima and their attendant kink solutions, specifically $ϕ^{14}$, $ϕ^{16}$ and $ϕ^{18}$.

nlin.PS↗

arXiv2019

Wide Class of Logarithmic Potentials with Power-Tower Kink Tails

We present a wide class of potentials which admit kinks and corresponding mirror kinks with either a power law or an exponential tail at the two extreme ends and a power-tower form of tails at the two neighbouring ends, i.e. of the forms $ette$ or $pttp$ where $e, p$ and $t$ denote exponential, power law and power-tower tail, respectively. We analyze kink stability equation in all these cases and show that there is no gap between the zero mode and the beginning of the continuum. Finally, we provide a recipe for obtaining logarithmic potentials with power-tower kink tails and estimate kink-kink interaction strength.

nlin.PS↗

arXiv2019

Noise amplification at spin-glass bottlenecks of quantum annealing: a solvable model

To gain better insight into the complexity theory of quantum annealing, we propose and solve a class of spin systems which contain bottlenecks of the kind expected to dominate the runtime of quantum annealing as it tries to solve difficult optimization problems. We uncover a noise amplification effect at these bottlenecks, whereby tunneling rates caused by flux-qubit noise scale in proportion to the number of qubits $N$ in the limit that $N\to \infty$. By solving the incoherent annealing dynamics exactly, we find a wide range of regimes where the probability that a quantum annealer remains in the ground-state upon exiting the bottleneck is close to one-half. We corroborate our analysis with detailed simulations of the performance of the D-Wave 2X quantum annealer on our class of computational problems.

quant-ph↗

arXiv2019

A model field theory with $(ψ\ln ψ)^2$ potential: Kinks with super-exponential profiles

We study a (1+1)-dimensional field theory based on $(ψ\ln ψ)^2$ potential. There are three degenerate minima at $ψ= 0$ and $ψ=\pm1$. There are novel, asymmetric kink solutions of the form $ψ= \mp\exp (-\exp(\pm x))$ connecting the minima at $ψ= 0$ and $ψ= \mp 1$. The domains with $ψ= 0$ repel the linear excitations, the waves (e.g. phonons). Topology restricts the domain sequences and therefore the ordering of the domain walls. Collisions between domain walls are rich for properties such as transmission of kinks and particle conversion, etc. To our knowledge this is the first example of kinks with super-exponential profiles and super-exponential tails. Finally, we provide a comparison of these results with the $ϕ^6$ model and its half-kink solution.

nlin.PS↗

arXiv2019

Family of Potentials with Power-Law Kink Tails

We provide examples of a large class of one dimensional higher order field theories with kink solutions which asymptotically have a power-law tail either at one end or at both ends. We provide analytic solutions for the kinks in a few cases but mostly provide implicit solutions. We also provide examples of a family of potentials with two kinks, both of which have power law tails either at both ends or at one end. In addition, we show that for kinks with a power law tail at one end or both the ends, there is no gap between the zero mode and the continuum of the corresponding stability equation. This is in contrast to the kinks with exponential tail at both the ends in which case there is always a gap between the zero mode and the continuum.

hep-th↗