arXiv subjects

N. Chandra

Publications and source records attributed to N. Chandra.

3 recordsLinked to original sources

arXiv2024

Knots of Darkness in Atmospheric Turbulence

Topology, which originated as a mathematical discipline, nowadays advances the understanding of many branches of science and technology from elementary particle physics and cosmology to condensed matter physics. In optics, the topology of light and darkness facilitates new degrees of freedom for sculpting optical beams beyond conventionally used amplitude, phase, and polarization. This fundamentally new, spatial dimension opens new opportunities for several optical applications, ranging from optical manipulation, trapping, data processing, optical sensing and metrology, enhanced imaging, and microscopy, to classical and quantum communications. While topological stability of mathematical knots implying robustness to perturbations suggests their potential as information carriers, the behavior of optical knots in perturbative environments such as atmospheric turbulence is largely unexplored. Here, we experimentally and theoretically investigate the effects of atmospheric turbulence of optical knot stability and demonstrate that the number of crossing (the topological invariant) is preserved in the weak-turbulence regime, but may not be conserved in the stronger turbulence conditions. The turbulent medium is simulated in the laboratory using phase screens, which carry the refractive index changes associated with the Kolmogorov power spectrum, encoded in a spatial light modulator. The optical knots are reconstructed by single-shot measurements of the complex field, and the resilience of the knot topology is analyzed for various realistic turbulence strengths. These studies may give rise to entirely new approaches to the three-dimensional (3D) spatially resolved probing of turbulence.

physics.optics↗

arXiv2016

Solitons in an effective theory of CP violation

We study an effective field theory describing CP-violation in a scalar meson sector. We write the simplest interaction that we can imagine, $${\cal L}\sim ε_{i_1\cdots i_5}ε^{μ_1\cdotsμ_4}ϕ_{i_1}\partial_{μ_1}ϕ_{i_2}\partial_{μ_2}ϕ_{i_3}\partial_{μ_3}ϕ_{i_4}\partial_{μ_4}ϕ_{i_5}$$ which involves 5 scalar fields. The theory describes CP-violation only when it contains scalar fields representing mesons such as the $K^*_0$, sigma, $f_0$ or $a_0$. If the fields represent pseudo-scalar mesons, such as B, K and $π$ mesons then the Lagrangian describes anomalous processes such as $KK\to πππ$. We speculate that the field theory contains long lived excitations corresponding to $Q$-ball type domain walls expanding through space-time. In an 1+1 dimensional, analogous, field theory we find an exact, analytic solution corresponding to such solitons. The solitons have a U(1) charge $Q$, which can be arbitrarily high, but oddly, the energy behaves as $Q^{2/3}$ for large charge, thus the configurations are stable under disintegration into elementary charged particles of mass $m$ with $Q=1$. We also find analytic complex instanton solutions which have finite, positive Euclidean action.

hep-th↗

arXiv1999

Nearly universal crossing point of the specific heat curves of Hubbard models

A nearly universal feature of the specific heat curves C(T,U) vs. T for different U of a general class of Hubbard models is observed. That is, the value C_+ of the specific heat curves at their high-temperature crossing point T_+ is almost independent of lattice structure and spatial dimension d, with C_+/k_B \approx 0.34. This surprising feature is explained within second order perturbation theory in U by identifying two small parameters controlling the value of C_+: the integral over the deviation of the density of states N(ε) from a constant value, characterized by δN=\int dε|N(ε)-1/2|, and the inverse dimension, 1/d.

cond-mat.str-el↗