arXiv subjects

S. Verma

Publications and source records attributed to S. Verma.

At least 19 recordsLinked to original sources

arXiv2026

On the horizon problem over the product of self-similar fractals

In this article, we first investigate the box dimension results for the graphs of continuous functions defined on the product of two self-similar fractals ({\mathfrak{F}}s), where the fractal {\mathfrak{F}} satisfies the open set condition. Following this, we manifest the existence of a prevalent surface, defined by a continuous function over the product of two {\mathfrak{F}}s, that satisfies the horizon property (associated with the box dimension), i.e., the box dimension of the prevalent surface is greater than that of its horizon up to a constant {\eta}, where {\eta} is the box dimension of the self-similar fractal {\mathfrak{F}}.

math.DS↗

arXiv2025

Ab initio study of mechanical and functional properties of novel CaZnC and CaZnSi half-Heusler materials

This research work introduces the DFT through FP-LAPW+lo technique in WIEN2k software to obtain information about structural, thermoelectric, and optoelectronic characteristics of CaZnC and CaZnSi materials. The structural optimization was performed using PBE-GGA functional, while the rest of the characteristics were obtained with the PBE-GGA + TB-mBJ approach. The thermoelectric parameters were evaluated using BoltzTraP software. The elastic constants and other mechanical parameters were computed by utilizing the ELAST code within the WIEN2k software, while the thermodynamic characteristics were evaluated using the Gibbs2 program. The findings show a correlation between atomic composition and lattice dimensions while finding that CaZnC has a direct ($\Gamma$-$\Gamma$) band gap of $1.186$ eV, whereas CaZnSi has an indirect ($\Gamma$-$X$) band gap of $1.067$ eV. The optical studies of the compounds show potential applications for photovoltaics while the thermoelectric results find optimized power factors and figure of merit values for energy conversion performance. The elastic parameters of CaZnC and CaZnSi demonstrate material stability and brittleness. Lastly, the thermodynamic evaluations provide information about the thermal mechanism and disorder of the materials. As a result, this research work provides significant advancements in the understanding of the fundamentals of these compounds and highlights their promising applications in renewable energy technologies.

cond-mat.mtrl-sci↗

arXiv2024

Probing Primordial Black Hole Mergers in Clusters with Pulsar Timing Data

We consider the possibility that the stochastic gravitational wave (GW) background suggested by Pulsar Timing Array (PTA) datasets is sourced by Primordial Black Holes (PBHs). Specifically, we perform a Bayesian search in the International PTA Data Release 2 (IPTA DR2) for a combined GW background arising from scalar perturbations and unresolved PBH mergers, assuming a broad PBH mass distribution. In our analysis, we incorporate constraints on the curvature power spectrum from CMB $\mu$-distortions and the overproduction of PBHs, which significantly suppress the contribution of PBH mergers to the total GW background. We find that scalar-induced GWs dominate the nHz frequency range, while PBH mergers alone cannot account for the observed signal under the standard PBH formation scenario involving Gaussian perturbations, and including only Poissonian PBH clustering. However, specific PBH models, such as those with enhanced clustering, could yield a GW background dominated by PBH mergers. Overall, we find that the IPTA DR2 strongly favors an astrophysical origin for the reported common-spectrum process over the PBH models considered in this analysis.

astro-ph.CO↗

arXiv2024

Reactor-based Search for Axion-Like Particles using CsI(Tl) Detector

The absence of conclusive signals in weakly interacting massive particle (WIMP) searches has motivated increased interest in alternative dark matter candidates such as axions and axion-like particles (ALPs), which also provide a solution to the strong CP problem. In this work, we employ a $\sim100~\mathrm{kg}$ scale CsI(Tl)-based detector operated in proximity to a nuclear reactor to achieve a sub-100 DRU (differential rate unit, expressed in counts/keV/kg/day) background level in the MeV energy range through a combination of active veto and passive shielding techniques. Such a low-background environment enables sensitivity to ALPs with axion--photon coupling $g_{a\gamma\gamma} \gtrsim 10^{-6}$ and axion--electron coupling in the range $10^{-8} < g_{aee} < 10^{-4}$ for ALP masses between 1~keV and 10~MeV. These results demonstrate that the experiment has the potential to probe previously unexplored regions of parameter space, including the so-called cosmological triangle in the ALP--photon coupling for MeV-scale ALPs.

hep-ex↗

arXiv2023

Multiple exciton generation in VO2

Multiple exciton generation (MEG) is a widely studied phenomenon in semiconductor nanocrystals and quantum dots, aimed at improving the energy conversion efficiency of solar cells. MEG is the process wherein incident photon energy is significantly larger than the band gap, and the resulting photoexcited carriers relax by generating additional electron-hole pairs, rather than decaying by heat dissipation. Here, we present an experimental demonstration of MEG in a prototype strongly correlated material, VO2, through photocurrent spectroscopy and ultrafast transient reflectivity measurements, both of which are considered the most prominent ways for detecting MEG in working devices. The key result of this paper is the observation of MEG at room temperature (in a correlated insulating phase of VO2), and the estimated threshold for MEG is 3Eg. We demonstrate an escalated photocurrent due to MEG in VO2, and quantum efficiency is found to exceed 100%. Our studies suggest that this phenomenon is a manifestation of expeditious impact ionization due to stronger electron correlations and could be exploited in a large number of strongly correlated materials.

cond-mat.str-el↗

arXiv2022

Development of a large-mass, low-threshold detector system with simultaneous measurements of athermal phonons and scintillation light

We have combined two low-threshold detector technologies to develop a large-mass, low-threshold detector system that simultaneously measures the athermal phonons in a sapphire detector while an adjacent silicon high-voltage detector detects the scintillation light from the sapphire detector. This detector system could provide event-by-event discrimination between electron and nuclear events due to the difference in their scintillation light yield. While such systems with simultaneous phonon and light detection have been demonstrated earlier with smaller detectors, our system is designed to provide a large detector mass with high amplification for the limited scintillation light. Future work will focus on at least an order of magnitude improvement in the light collection efficiency by having a highly reflective detector housing and custom phonon mask design to maximize light collection by the silicon high-voltage detector.

physics.ins-det↗

arXiv2022

MAC: A Meta-Learning Approach for Feature Learning and Recombination

Optimization-based meta-learning aims to learn an initialization so that a new unseen task can be learned within a few gradient updates. Model Agnostic Meta-Learning (MAML) is a benchmark algorithm comprising two optimization loops. The inner loop is dedicated to learning a new task and the outer loop leads to meta-initialization. However, ANIL (almost no inner loop) algorithm shows that feature reuse is an alternative to rapid learning in MAML. Thus, the meta-initialization phase makes MAML primed for feature reuse and obviates the need for rapid learning. Contrary to ANIL, we hypothesize that there may be a need to learn new features during meta-testing. A new unseen task from non-similar distribution would necessitate rapid learning in addition reuse and recombination of existing features. In this paper, we invoke the width-depth duality of neural networks, wherein, we increase the width of the network by adding extra computational units (ACU). The ACUs enable the learning of new atomic features in the meta-testing task, and the associated increased width facilitates information propagation in the forwarding pass. The newly learnt features combine with existing features in the last layer for meta-learning. Experimental results show that our proposed MAC method outperformed existing ANIL algorithm for non-similar task distribution by approximately 13% (5-shot task setting)

cs.LG↗

arXiv2022

Fractal polynomials On the Sierpi\'nski gasket and some dimensional results

In this paper, we explore some significant properties associated with a fractal operator on the space of all continuous functions defined on the Sierpi\'nski Gasket (SG). We also provide some results related to constrained approximation with fractal polynomials and study the best approximation properties of fractal polynomials defined on the SG. Further we discuss some remarks on the class of polynomials defined on the SG and try to estimate the fractal dimensions of the graph of $\alpha$- fractal function defined on the SG by using the oscillation of functions.

math.FA↗

arXiv2022

Fractal Interpolation Function On Products of the Sierpi\'nski Gaskets

In this paper, we aim to construct fractal interpolation function(FIF) on the product of two Sierpi\'nski gaskets. Further, we collect some results regarding smoothness of the constructed FIF. We prove, in particular, that the FIF are H\"older functions under specific conditions. In the final section, we obtain some bounds on the fractal dimension of FIF.

math.DS↗

arXiv2022

Large-mass, low-threshold sapphire detector for rare event searches

Low mass nuclear recoil dark matter and coherent-elastic-neutrino-nucleus-scattering (CENNS) searches confront similar challenges in choosing ultra-low threshold and large-mass detectors. We report experimental results from the first-of-its-kind 100 g single-crystal sapphire detector design with a diameter of 76 mm and thickness of 4 mm. The detector is designed to be sensitive for low-energy rare interactions with an intention to investigate the low mass region of dark matter phase-space and search for CENNS at the reactor site. Sapphire is a crystal of aluminum oxide (Al2O3) and has been found to be a good candidate for light mass spin-dependent dark matter search experiments due to its lower atomic mass compared to other detector materials such as germanium and silicon. Using the data collected from the test facility at Texas A&M University, we were able to resolve low energy lines from calibration sources and estimated that our newly developed sapphire detector has a baseline recoil energy resolution of 18 eV. These detectors are operated at 0 V with the phonon-assisted detection providing a quenching-free low-threshold operation.

physics.ins-det↗

arXiv2021

Prospects for detecting axionlike particles at the Coherent CAPTAIN-Mills experiment

We show results from the Coherent CAPTAIN Mills (CCM) 2019 engineering run which begin to constrain regions of parameter space for axion-like particles (ALPs) produced in electromagnetic particle showers in an 800 MeV proton beam dump, and further investigate the sensitivity of ongoing data-taking campaigns for the CCM200 upgraded detector. Based on beam-on background estimates from the engineering run, we make realistic extrapolations for background reduction based on expected shielding improvements, reduced beam width, and analysis-based techniques for background rejection. We obtain reach projections for two classes of signatures; ALPs coupled primarily to photons can be produced in the tungsten target via the Primakoff process, and then produce a gamma-ray signal in the Liquid Argon (LAr) CCM detector either via inverse Primakoff scattering or decay to a photon pair. ALPs with significant electron couplings have several additional production mechanisms (Compton scattering, $e^+e^-$ annihilation, ALP-bremsstrahlung) and detection modes (inverse Compton scattering, external $e^+e^-$ pair conversion, and decay to $e^+e^-$). In some regions, the constraint is marginally better than both astrophysical and terrestrial constraints. With the beginning of a three year run, CCM will be more sensitive to this parameter space by up to an order of magnitude for both ALP-photon and ALP-electron couplings. The CCM experiment will also have sensitivity to well-motivated parameter space of QCD axion models. It is only a recent realization that accelerator-based large volume liquid argon detectors designed for low energy coherent neutrino and dark matter scattering searches are also ideal for probing ALPs in the unexplored $\sim$MeV mass scale.

hep-ph↗

arXiv2021

First Leptophobic Dark Matter Search from Coherent CAPTAIN-Mills

We report the first results of a search for leptophobic dark matter (DM) from the Coherent CAPTAIN-Mills (CCM) liquid argon (LAr) detector. An engineering run with 120 photomultiplier tubes (PMTs) and $17.9 \times 10^{20}$ protons-on-target (POT) was performed in Fall 2019 to study the characteristics of the CCM detector. The operation of this 10-ton detector was strictly light-based with a threshold of 50 keV and used coherent elastic scattering off argon nuclei to detect DM. Despite only 1.5 months of accumulated luminosity, contaminated LAr, and non-optimized shielding, CCM's first engineering run already achieved sensitivity to previously unexplored parameter space of light dark matter (LDM) models with a baryonic vector portal. With an expected background of 115,005 events, we observe 115,005+16.5 events which is compatible with background expectations. For a benchmark mediator-to-dark matter mass ratio of $m_{_{V_B}}/m_{\chi}=2.1$, DM masses within the range $9\,\text{MeV} \lesssim m_\chi \lesssim 50\,\text{MeV}$ have been excluded at 90% C.L. in the leptophobic model after applying the Feldman-Cousins test statistic. CCM's upgraded run with 200 PMTs, filtered LAr, improved shielding, and ten times more POT will be able to exclude the remaining thermal relic density parameter space of this model, as well as probe new parameter space of other leptophobic DM models.

hep-ex↗

arXiv2021

First Dark Matter Search Results From Coherent CAPTAIN-Mills

This paper describes the operation of the Coherent CAPTAIN-Mills (CCM) detector located at the Lujan Neutron Science Center (LANSCE) at Los Alamos National Laboratory (LANL). CCM is a 10-ton liquid argon (LAr) detector located 20 meters from a high flux neutron/neutrino source and is designed to search for sterile neutrinos ($\nu_s$) and light dark matter (LDM). An engineering run was performed in Fall 2019 to study the characteristics of the CCM120 detector by searching for coherent scattering signals consistent with $\nu_s$'s and LDM resulting from $\pi^+$ and $\pi^0$ decays in the tungsten target. New parameter space in a leptophobic dark matter model was excluded for DM masses between $\sim2.0$ and 30 MeV. The lessons learned from this run have guided the development and construction of the new CCM200 detector that will begin operations in 2021 and significantly improve on these searches.

hep-ex↗

arXiv2021

Hausdorff dimension and infinitesimal similitudes on complete metric spaces

In this paper, we answer a question of Nussbaum, Priyadarshi, and Lunel [Positive operators and Hausdorff dimension of invariant sets, Trans. Amer. Math. Soc. 364(2) (2012) 1029-1066.]. We also show that the Hausdorff dimension and box dimension of the attractor generated by a finite set of contractive infinitesimal similitudes are the same. Further, we extend many results of dimension theory to complete metric spaces. In the last part, we fill the gaps in the proofs of some articles, which are related to the dimension theory, and hint at some possible improvements in the recent papers.

math.DS↗

arXiv2021

On bivariate fractal approximation

In this paper, the notion of dimension preserving approximation for real-valued bivariate continuous functions, defined on a rectangular domain $\rectangle$, has been introduced and several results, similar to well-known results of bivariate constrained approximation in terms of dimension preserving approximants, have been established. Further, some clue for the construction of bivariate dimension preserving approximants, using the concept of fractal interpolation functions, has been added. In the last part, some multi-valued fractal operators associated with bivariate $\alpha$-fractal functions are defined and studied.

math.CA↗

arXiv2021

Katugampola fractional integral and fractal dimension of bivariate functions

The subject of this note is the mixed Katugampola fractional integral of a bivariate function defined on a rectangular region in the Cartesian plane. This is a natural extension of the Katugampola fractional integral of a univariate function - a concept well-received in the recent literature on fractional calculus and its applications. It is shown that the mixed Katugampola fractional integral of a prescribed bivariate function preserves properties such as boundedness, continuity and bounded variation of the function. Furthermore, we estimate fractal dimension of the graph of the mixed Katugampola integral of a continuous bivariate function. Some examples for bivariate functions that are not of bounded variation but with graphs having box dimension $2$ are constructed. The findings in the current note may be viewed as a sequel to our work reported in [Appl. Math. Comp., 339, 2018, pp. 220-230].

math.CA↗

arXiv2020

Non-Riemannian gravity actions from double field theory

Non-Riemannian gravitational theories suggest alternative avenues to understand properties of quantum gravity and provide a concrete setting to study condensed matter systems with non-relativistic symmetry. Derivation of an action principle for these theories generally proved challenging for various reasons. In this technical note, we employ the formulation of double field theory to construct actions for a variety of such theories. This formulation helps removing ambiguities in the corresponding equations of motion. In particular, we embed Torsional Newton-Cartan gravity, Carrollian gravity and String Newton-Cartan gravity in double field theory, derive their actions and compare with the previously obtained results in literature.

hep-th↗

arXiv2020

Dimensional analysis of fractal interpolation functions

We provide a rigorous study on dimensions of fractal interpolation function defined on a closed and bounded interval of $\mathbb{R}$ which is associated to a continuous function with respect to a base function, scaling functions and a partition of the interval. In particular, we provide an exact estimation of the box dimension of $\alpha$-fractal functions.

math.DS↗