arXiv subjects

Ravi Shanker

Publications and source records attributed to Ravi Shanker.

9 recordsLinked to original sources

arXiv2026

Imprints of $U_A(1)$ chiral anomaly and disorder in the Dirac eigenspectrum of QCD at finite temperature

We perform a comprehensive study of the properties of Dirac eigenvalue spectrum in QCD as a function of temperature on the lattice. In addition to effects due to interplay between interactions and disorder inherently present in a many-body system, the Dirac spectrum also contains crucial information about the effective restoration of different subgroups of almost exact two-flavor chiral symmetry in QCD. We calculate the infrared eigenvalues of the overlap Dirac operator on 2+1 flavor QCD ensembles generated using domain wall fermion discretization, on a large volume lattice. From the normalized level spacing ratios, we identify those eigenvalues that have intermediate level statistics, distinctly different from the majority in the bulk spectrum that follow universal level fluctuations similar to a random matrix of Gaussian unitary type. We provide an explanation of these intermediate level ratios in terms of a specific random matrix model and quantify the correlation between these eigenstates and disorder in the gauge fields manifested in the renormalized Polyakov loop values. Whereas the existence of intermediate eigenmodes is intimately connected to the effective restoration of different subgroups of chiral symmetry close to chiral crossover transition, their origin can be traced to random uncorrelated disorder at higher temperatures when the $U_A(1)$ is effectively restored. We also, for the first time, calculate the Thouless conductance for the Dirac spectrum that quantifies the structural rigidity of the eigenvectors, and use it as a diagnostic tool to understand the restoration of the anomalous $U_A(1)$ subgroup of chiral symmetry and localization driven by disorder.

hep-lat↗

arXiv2025

Uniform and Tunable Structural Colors based on Ultrathin Lignin Optical Coatings

Structural coloration offers a sustainable and non-fading alternative to conventional pigment- and dye-based colorants. In this study, we present a scalable strategy for generating vivid, tunable structural colors using ultrathin films of industrial kraft lignin. By employing solvent fractionation to isolate a methanol-soluble lignin fraction, followed by spin-coating onto silicon substrates, we achieve highly uniform films with controllable thickness. The observed colors originate from thin-film interference and span the entire visible spectrum by adjusting lignin concentration and spin speed. Spectroscopic ellipsometry and transfer matrix modeling confirm that the interference is modulated by the film thickness. The lignin films exhibit robust mechanical properties, limited angular color shift, and fast, reversible optical response to humidity changes, enabling real-time environmental sensing. This approach circumvents the need for nanoparticle synthesis or self-assembly and highlights the potential of lignin, a widely available and underutilized biopolymer, as a functional photonic material for sustainable optical coatings and colorimetric sensors.

physics.optics↗

arXiv2025

Thermal SU(2) lattice gauge theory for intertwined orders and hole pockets in the cuprates

The cuprate pseudogap phase displays Fermi arc spectral weight in photoemission and scanning tunneling microscopy (STM), while recent magnetotransport observations yield evidence for the existence of hole pockets of fractional area $p/8$, where $p$ is the doping density. We present a Monte Carlo study of a thermal SU(2) lattice gauge theory which can reconcile these observations. Our simulation includes the SU(2) gauge field $U$ of a $\pi$-flux spin liquid, and a SU(2) fundamental charge $e$ Higgs boson $B$. There is a Yukawa coupling between $B$, the fermionic spinons of the spin liquid, and the hole pockets of a fractionalized Fermi liquid. At the higher temperatures of the pseudogap, the finite-doping sign problem is evaded by including only thermal fluctuations of $B$ and $U$, while the fermions are diagonalized exactly for each boson background. Our study also yields a fractionalized description of intertwined orders at lower temperatures, including the onset of $d$-wave superconductivity by the expulsion of vortices with flux $h/(2e)$, each with charge-order halos. We discuss conditions under which quantum oscillations in the density of states from hole pockets of area $p/8$ could be observable in clean under-hole-doped cuprates.

cond-mat.str-el↗

arXiv2024

Aspects of the chiral crossover transition in (2+1)-flavor QCD with M\"{o}bius domain-wall fermions

The non-singlet part of the chiral symmetry in QCD with two light flavors is known to be restored through a crossover transition at a pseudo-critical temperature. However, the temperature dependence of the singlet part of the chiral symmetry and whether it is effectively restored at the same temperature is not well understood. Using (2+1)-flavor QCD configurations generated using the M\"{o}bius domain-wall discretization on an $N_\tau=8$ lattice, we construct suitable observables where the singlet and non-singlet chiral symmetries are disentangled in order to study their temperature dependence across the crossover transition. From the peak of the disconnected part of the chiral susceptibility, we obtain a pseudo-critical temperature $T_{pc}=158.7{}_{{}-2.3}^{{}+2.6}$ MeV where the non-singlet part of the chiral symmetry is effectively restored. From a calculation of the topological susceptibility and its temperature dependence we find that the singlet $U_A(1)$ part of the chiral symmetry is not effectively restored at $T<186$ MeV.

hep-lat↗

arXiv2024

Understanding the approach to thermalization from the eigenspectrum of non-Abelian gauge theories

We study some interesting aspects of the spectral properties of SU(3) gauge theory, both with and without dynamical quarks (QCD) at thermal equilibrium using lattice gauge theory techniques. By calculating the eigenstates of a massless overlap Dirac operator on the gauge configurations, we implement a gauge-invariant method to study spectral properties of non-Abelian gauge theories. We have unambiguously categorized Dirac eigenvalues into different regimes based on a quantity defined in terms of the ratios of nearest neighbor spacings. While majority of these eigenstates below the magnetic scale are similar to those of random matrices belonging to the Gaussian Unitary ensemble at temperatures much higher than the chiral crossover transition in QCD, a few among them start to become prominent only near the crossover. These form fractal-like clusters with the median value for their fractal dimensions hinting at the universality class of the chiral transition in QCD. We further demonstrate that momentum modes below the magnetic scale in a particular non-equilibrium state of QCD are classically chaotic and estimate an upper bound on the thermalization time $\sim 1.44$ fm/c by matching this magnetic scale with that of a thermal state at $\sim 600$ MeV.

hep-lat↗

arXiv2023

Eigenvalues of the QCD Dirac matrix with improved staggered quarks in the continuum limit

We calculate the eigenmodes of the Highly Improved Staggered Quark (HISQ) matrix near the chiral crossover transition in QCD with $2+1$ flavors with the aim to gain more insights into its temperature dependence. On performing the continuum extrapolation, we do not observe any gap opening up in the infrared part of the eigenvalue density of the QCD Dirac operator; instead we observe a peak. The existence of the peak and oscillations of the infrared eigenmodes can be understood in terms of an interacting ensemble of instantons. From the properties of the continuum extrapolated eigenspectrum we further show that the anomalous $U_A(1)$ part of the chiral symmetry is not effectively restored simultaneously along with its non-singlet counterpart. We provide an explanation for this observation, further showing interesting connections between the anomalous $U_A(1)$ restoration and the change in the infrared part of the eigenvalue distribution.

hep-lat↗

arXiv2021

Photon Walk in Transparent Wood: Scattering and Absorption in Hierarchically Structured Materials

The optical response of hierarchical materials is convoluted, which hinders their direct study and property control. Transparent wood (TW) is an emerging biocomposite in this category, which adds optical function to the structural properties of wood. Nano- and microscale inhomogeneities in composition, structure and at interfaces strongly affect light transmission and haze. While interface manipulation can tailor TW properties, the realization of optically clear wood requires detailed understanding of light-TW interaction mechanisms. Here we show how material scattering and absorption coefficients can be extracted from a combination of experimental spectroscopic measurements and a photon diffusion model. Contributions from different length scales can thus be deciphered and quantified. It is shown that forward scattering dominates haze in TW, primarily caused by refractive index mismatch between the wood substrate and the polymer phase. Rayleigh scattering from the wood cell wall and absorption from residual lignin have minor effects on transmittance, but the former affects haze. Results provide guidance for material design of transparent hierarchical composites towards desired optical functionality; we demonstrate experimentally how transmittance and haze of TW can be controlled over a broad range.

physics.optics↗

arXiv2021

Redox-tunable structural colour images based on UV-patterned conducting polymers

Precise manipulation of light-matter interaction has enabled a wide variety of approaches to create bright and vivid structural colours. Techniques utilizing photonic crystals, Fabry-Pérot cavities, plasmonics, or high-refractive index dielectric metasurfaces have been studied for applications ranging from optical coatings to reflective displays. However, complicated fabrication procedures for sub-wavelength nanostructures, limited active areas, and inherent absence of tunability with these approaches significantly impede their further developments towards flexible, large-scale, and switchable devices compatible with facile and cost-effective production. Herein, we present a way to generate structural colours based on conducting polymer thin films prepared on metallic surfaces via vapour phase polymerization and ultraviolet (UV) light patterning. Varying the UV dose leads to synergistic variation of film absorption and thickness, which generates controllable colours from violet to red. Together with greyscale photomasks this enables fabrication of high-resolution colour images using single exposure steps. We further demonstrate spatiotemporal tuning of the structurally coloured surfaces and images via electrochemical modulation of the polymer redox state. The simple structure, facile fabrication, wide colour gamut, and dynamic colour tuning make this concept competitive for future multi-functional and smart displays.

physics.optics↗

arXiv2020

Tunable and saturated structural colors from carbon nanotubes incorporated highly ordered photonic opals

Synthetic opals, based on self-assembly of polymeric nanoparticles generally produces fainted/pale structural colors due to too many lattice flaws in the structures. Here we produces carbon nanotubes (CNTs) incorporated high quality 3D photonic opals (PC-CNT) by evaporative self-assembly. Although the CNTs make up only 0.01% of the fabricated photonic opal, their controlled incorporation has dramatic effect to change the color of the photonic crystals from milky white to intense red. Microscopic study suggest that CNT incorporation did not affect the lattice ordering of the photonic crystals. The tunability of structural colors, as a function of incident angle, were tested and varied against Bragg-Snell law. Furthermore, we tested mechanochromic sensing of the photonic opals, demonstrating their potential as visual indicators. This tunable PC-CNT brings many possibilities including strain sensing or structural health care monitoring, as well as being of fundamental interest.

physics.app-ph↗