arXiv subjects

Deobrat Singh

Publications and source records attributed to Deobrat Singh.

15 recordsLinked to original sources

arXiv2026

Strain-tunable interface electrostatics in Janus MoSSe/silk vdW heterostructure for triboelectric nanogeneration

Understanding and engineering interfacial electrostatics in hybrid two-dimensional (2D) and biomolecular material systems is essential for advancing high-performance triboelectric nanogenerators (TENGs). In this work, we systematically investigate the strain-dependent electronic structure and triboelectric response of Janus MoSSe, silk fibroin, and their van der Waals (vdW) heterostructure using first-principles calculations. Tensile strain induces a pronounced band-gap reduction in the MoSSe/silk interface, exceeding that of the isolated constituents and indicating enhanced interlayer electronic coupling. The vdW heterostructure exhibits a significant work-function shift and a substantially larger dipole moment compared to MoSSe and silk alone, revealing strong interfacial charge redistribution driven by Fermi-level alignment and asymmetric polarization. This enhanced polarization directly amplifies the triboelectric surface charge density, producing values more than double those of pristine MoSSe and several orders of magnitude higher than silk. Consequently, the open-circuit voltage and overall triboelectric output are markedly improved across all strain levels. These results demonstrate that synergistic interfacial polarization and strain engineering can effectively elevate charge separation, storage, and transfer efficiencies, establishing the MoSSe/silk vdW heterostructure as a promising material for next-generation high-efficiency TENGs.

cond-mat.mtrl-sci↗

arXiv2026

Electrochemical stability and lithium insertion at the Li|Li3OCl solid electrolyte interface

Solid-state lithium batteries have attracted considerable attention due to their potential to provide improved safety and higher energy density compared with conventional liquid electrolyte batteries. However, the stability of the interface between Li metal anodes and solid electrolytes remains a critical issue that strongly influences battery performance. In this work, first-principles density functional theory calculations are performed to investigate the interfacial properties of a solid-state battery system composed of Li metal anode and Li3OCl solid electrolyte. The structural stability, electronic structure, and electrochemical behavior of the Li|Li3OCl interface are systematically analyzed. Several interface orientations are constructed and compared in order to identify the most energetically favorable configuration. The electronic properties and interfacial charge redistribution are further examined to understand the nature of the interaction between Li metal and the Li3OCl electrolyte. Our results indicate that the Li|Li3OCl interface exhibits stable structural and electronic characteristics, with localized charge redistribution occurring near the interface region. The electrochemical stability against the insertion of an additional Li atom is also evaluated, showing that Li incorporation is energetically unfavorable in most layers of the electrolyte. These results suggest that the Li3OCl electrolyte maintains good electrochemical stability in contact with Li metal. The present study provides atomic-scale insight into the interfacial behavior of Li|Li3OCl and highlights the potential of Li3OCl as a promising solid electrolyte for solid-state lithium batteries.

cond-mat.mtrl-sci↗

arXiv2025

Investigating the interaction of a Cosmic String with an Accreting Black Hole

Rotating black holes when attached to a cosmic string have their rotational energy extracted leading to a change in its spin and mass. The spin of a black hole can be measured using various methods for an accreting black hole in an X-ray binary system. Accretion disks around black holes have an innermost stable circular orbit (ISCO) whose location is directly dependent on spin and mass of the black hole. The orbit's location changes as the black hole's spin changes and hence can be a method to detect the presence of cosmic strings. This study investigates this change and suggests the ejection of accretion material as black hole spin approaches maximum for prograde motion and material falling into the black hole for retrograde motion, regardless of the presence of cosmic string. However, in the presence of cosmic string, the spin-up process due to accretion is found out to be slower, even with high accretion rates and is detectable. There is a transition phase that occurs as the black hole approaches maximum spin, where even small changes in spin result in significant changes in the ISCO's position. Accreting black holes attached to a large string never reach this transition phase and this absence serves as potential evidence for the existence of a cosmic string.

hep-th↗

arXiv2025

Blandford-Znajek Jets and the Total Angular Momentum Evolution of a Black Hole Connected to a Cosmic String

Rotating black holes with strong magnetic fields lead to an outward energy flux in the form of jets governed by the Blandford-Znajek mechanism. These jets depend on factors such as accretion rate, magnetic flux and the spin of the black hole. When such rotating black holes get attached to a cosmic string, it leads to a further rotational energy extraction, leading to a reduced spin. We consider such a system and investigate the effect this reduced spin has on the jet power and its dependence on the cosmic string tension, $μ$. It is shown that for a constant magnetic flux and accretion rate, the jet energy flux is inversely proportional to $μ^2$. Interestingly, the rate of this energy flux varies with time and is again dependent on $μ$. We also study the total angular momentum evolution of the black hole by considering four major effects: accretion, jets, cosmic string energy extraction and the Bardeen-Petterson effect. Further, we attempt to analyse the condition for the spin-down of a black hole due to these effects and find out that it is possible for both small and large string tensions, with a higher possibility for larger string tensions. Another interesting phenomenon that has been proposed is the alignment of the jet with the cosmic string. Additionally, the Bardeen-Petterson effect also leads to alignment or misalignment of the inner and outer disks depending on the alignment of the string. In this manuscript we propose that these results might have an observable effect and hence could serve as a potential detection method for cosmic strings.

gr-qc↗

arXiv2025

Probing Cosmic Strings via Black Hole Quasinormal Modes in Gravitational Wave Astronomy

Black holes, the simplest solution to Einstein's field equations, do not emit light, making their observations a major challenge for researchers. However, discovery of binary black holes (BBHs) in 2015 by LIGO has transformed the study of compact objects, with over 300 BBHs recorded, providing a new avenue for probing new physics. GWs remain a prominent and precise method of observing not only BBHs, but also dark matter and cosmic strings. Cosmic strings -- hypothetical one dimensional topological defects formed in the early universe, are yet to be observed, with multiple detection methods such as particle radiation, gravitational waves and lensing being proposed. Here we present a novel framework to search for cosmic strings by modeling them as perturbations within non-rotating black hole spacetime, focusing on their imprint on the spectrum of quasinormal modes (QNMs). Our numerical simulations identify a lower limit on perturbation strength, $λ\sim 10^{-10}$ for uncharged string and $λ\sim 10^{-7}$ for charged string, below which cosmic string effects become unobservable in QNM signals. By analyzing eigenvalue splitting and centers, we show that cosmic string properties impart distinct and detectable features to GW signals. Our results establish QNM analysis as a powerful, alternative observational strategy for constraining or detecting cosmic strings, and offer an inverse approach to estimate string energy or charge if a signal is detected. With upgrades in LIGO technologies and advanced multimessenger astronomy under development, these findings highlight new potential for detecting cosmic strings.

gr-qc↗

arXiv2025

Black Holes as Catalysts for Cosmic String Detection and Axion Dark Matter Genesis

The global $U_{PQ}(1)$ Peccei-Quinn (PQ) symmetry, proposed to resolve the strong CP problem, predicts the existence of the axion, a pseudo Nambu-Goldstone boson and a leading dark matter candidate. The spontaneous breaking of this symmetry generates global strings that decay predominantly via the emission of massive axions and gravitational waves. In this work, we investigate the decay of cosmic axion strings in the vicinity of a Schwarzschild black hole and estimate the corresponding energy losses and decay timescales of the resulting string loops. For primordial black holes (PBHs) with masses as small as $10^{-16} M_\odot$, the total radiated energy by the string is found to be on the order of $10^{27}$ GeV, encompassing both axion emission and gravitational waves. A key finding is that the presence of a central black hole significantly accelerates the decay of cosmic string loops, substantially reducing their lifetimes. We present these results as an initial estimate of axion radiation from PBH-cosmic string systems along with the decay time as an important observational signature for axions strings.

hep-ph↗

arXiv2025

Impact on orbital period of X-ray Binary system attached to a cosmic string

Cosmic strings attached to rotating black holes extract its rotational energy, resulting in a mass loss and reduced spin. In this paper we discuss the proposed methods to detect these phenomena and present a novel methodology based on existing literature, by considering a Low Mass X-ray binary system. We investigate the impact of a cosmic string interacting with a black hole in an X-ray binary system and attempt to explain the observations of unexpected orbital period changes in such systems by proposing mass loss by cosmic strings to be a potential cause. For a period change of order $10^{-10}$, the string tension is $\sim 10^{-17}$, lying in the predicted range for cosmic string tension. An analysis of multiple low mass X-ray binary systems is carried out and it is shown that a significant and observable change occurs for a string tension $\sim 10^{-11}$.

gr-qc↗

arXiv2022

Stability of and conduction in single-walled Si$_2$BN nanotubes

We explore the possibility and potential benefit of rolling a Si2BN sheet into single-walled nanotubes (NTs). Using density functional theory (DFT), we consider both structural stability and the impact on the nature of chemical bonding and conduction. The structure is similar to carbon NTs and hexagonal boron-nitride (hBN) NTs and we consider both armchair and zigzag Si2BN configurations with varying diameters. The stability of these Si$_2$BN NTs is confirmed by first-principles molecular dynamics calculations, by an exothermal formation, an absence of imaginary modes in the phonon spectra. Also, we find the nature of conduction varies semiconducting, from semi-metallic to metallic, reflecting differences in armchair/zigzag-type structures, curvature effects, and the effect of quantum confinement. We present the detailed characterization of how these properties lead to differences in both the bonding nature and electronic structures

cond-mat.mes-hall↗

arXiv2020

Optical excitations and thermoelectric properties of 2D holey graphene

Recently, holey graphene (HG) has successfully synthesized at atomic precision of hole size and shape. This shows interesting physical and chemical properties for energy and environmental applications. Shaping of the pores also transforms semimetallic graphene to semiconductor holey graphene, which opens new door for its use in electronic applications. We systematically investigated the structural, electronic, optical and thermoelectric properties of HG structure using first-principles calculations. HG was found to have a direct band gap with 0.65 eV (PBE functional), 0.95 eV (HSE06 functional) and HSE06 functional is in good agreement with experimental results. For the optical properties, we use single-shot G0W0 calculations by solving the Bethe-Salpeter equation to determining the intralayer excitonic effects. From the absorption spectrum, we obtained the optical gap of 1.28 eV and a week excitonic binding energy of 80 meV. We have found the large values of thermopower of 1662.59 $μ$V/K and better electronic figure of merit, ZT$_{e}$ as 1.13 from the investigated thermoelectric properties. Our investigations exhibit strong and broad optical absorption in the visible light region, which makes HG monolayer a promising candidate for optoelectronic and thermoelectric applications.

cond-mat.mes-hall↗

arXiv2019

Orbital hybridization induced band offset phenomena in NixCd1-xO thin films

We present the cationic impurity assisted band offset phenomena in NixCd1-xO (x= 0, 0.02, 0.05, 0.1, 0.2, 0.4, 0.8, 1) thin films and further discussed in the light of orbital hybridization modification. Compositional and structural studies revealed that cationic substitution of Cd2+ by Ni2+ ions leads to a monotonic shift in (220) diffraction peak, indicating the suppression of lattice distortion while evolution of local strain with increasing Ni concentration mainly associated to the mismatch in electro-negativity of Cd2+ and Ni2+ ion. In fact, Fermi level pinning towards conduction band minima takes place with increasing Ni concentration at the cost of electronically compensated oxygen vacancies, resulting modification in the distribution of carrier concentration which eventually affects the band edge effective mass of conduction band electrons and further endorses band gap renormalization. Besides that, the appearance of longitudinal optical (LO) mode at 477 cm-1 as manifested by Raman spectroscopy also indicate the active involvement of electron-phonon scattering whereas modification in local coordination environment particularly anti-crossing interaction in conjunction with presence of satellite features and shake-up states with Ni doping is confirmed by X-ray absorption near-edge and X-ray photoelectron spectroscopy studies. These results manifest the gradual reduction of orbital hybridization with Ni incorporation, leading to decrement in the band edge effective mass of electron. Finally, molecular dynamics simulation reflects 13% reduction in lattice parameter for NiO thin film as compared to undoped one while projected density of states calculation further supports the experimental observation of reduced orbital hybridization with increasing Ni concentration.

cond-mat.mtrl-sci↗

arXiv2019

High temperature mediated rocksalt to wurtzite phase transformation in cadmium oxide nano-sheets and their theoretical evidence

In the paper, high temperature induced phase transformation in chemically grown CdO thin films has been demonstrated whereas their corresponding electronic origin is further investigated by density functional theory. In particular, cubic rocksalt to hexagonal wurtzite PT in 900 degree centigrade annealed CdO thin films is confirmed by Xray diffraction , consistent with High Resolution Transmission Electron Microscopy. Such high temperature treatment also leads to significant enhancement in optical band gap from 2.2 to 3.2 eV as manifested by UV-Visible spectroscopy. Moreover, atomic force microscopy and scanning electron microscopy clearly evidence the structural evolution via formation of nano-sheet network in wurtzite phased CdO films. Furthermore, Xray Absorption spectra at oxygen k edge revealed a notable shift in inflection point of absorption edge while X-ray Photoelectron spectroscopy of Cd 3d and O 1s suggested the gradual reduction in CdO2 phase with increasing annealing temperature. In addition, different complementary techniques including Rutherford Backscattering, Raman Spectroscopy have also been exploited to understand the aforementioned PT and their structural correlation. Finally, molecular dynamics simulation along with density functional theory calculations suggest that symmetry modification at Brillouin zone boundary provides a succinct signature for such PT in CdO thin film.

cond-mat.mtrl-sci↗

arXiv2019

Emergent D-instanton as a source of Dark Energy

We revisit a non-perturbative formulation leading to a vacuum created gravitational pair of (33)-brane by a Poincare dual higher form U (1) gauge theory on a D4 -brane. In particular, the analysis has revealed a dynamical geometric torsion H 3 for an on-shell Neveu-Schwarz (NS) form on a fat 4-brane. We argue that a D-instanton can be a viable candidate to incorporate the quintessence correction hidden to an emergent (3 + 1)-dimensional brane universe. It is shown that a dynamical non-perturbative correction may be realized with an axionic scalar QFT on an emergent anti 3-brane within a gravitational pair. The theoretical tool provokes thought to believe for an extra instantaneous dimension transverse to our classical brane-universe in an emergent scenario. Interestingly a D-instanton correction, sourced by an axion on an anti 3-brane, may serve as a potential candidate to explain the accelerated rate of expansion of our 3-brane universe and may provide a clue to the origin of dark energy.

gr-qc↗

arXiv2018

Prediction of new multiferroic and magnetoelectric material Fe3Se4

Nowdays, multiferroic materials with magnetoelectric coupling have many real-world applications in the fields of novel memory devices. It is challenging is to create multiferroic materials with strongly coupled ferroelectric and ferrimagnetic orderings at room temperature. The single crystal of ferric selenide (Fe3Se4) shows type-II multiferroic due to the coexistence of ferroelectric as well as magnetic ordering at room temperature. We have investigated the lattice instability, electronic structure, ferroelectric, ferrimagnetic ordering and transport properties of ferroelectric metal Fe3Se4. The density of states shows considerable hybridization of Fe-3d and Se-4p states near the Fermi level confirming its metallic behavior. The magnetic moments of Fe cations follow a type-II ferrimagnetic and ferroelectric ordering with a calculated total magnetic moment of 4.25 per unit cell (Fe6Se8). The strong covalent bonding nature of Fe-Se leads to its ferroelectric properties. In addition, the symmetry analysis suggests that tilting of Fe sub-lattice with 3d-t2g orbital ordering is due to the Jahn-Teller (JT) distortion. This study provides further insight in the development of spintronics related technology using multiferroic materials.

cond-mat.mtrl-sci↗

arXiv2016

${\mathbf{{CFT}_6}}$ Bulk/Boundary ${\mathbf{{AdS}^Q_5}}$ Correspondence and Emergent Gravity

We revisit a non-perturbation theory of quantum gravity in $1.5$ order underlying an emergent gravitational pair of $(4{\bar 4})$-brane with a renewed interest. In particular the formulation is governed by a geometric torsion ${\cal H}_3$ in second order with an on-shell NS form in first order. Interestingly the gravitational pair is sourced by a Kalb-Ramond two form CFT on a $D_5$-brane in $d$$=$$10$ type IIB superstring theory. We show that a generic form theory containing a CFT sector in $d=6$ bulk may be described by a boundary ${\rm AdS}_5$ with a quintessence Q. Analysis reveals that the bulk/boundary duality in emergent gravity can be a potential tool to explore the quintessential cosmology.

hep-th↗

arXiv2014

Quantum effects in topological and Schwarzschild de Sitter brane: Aspects of torsion on ${\mathbf{(D{\bar D})_4}}$-brane universe

We investigate an effective torsion curvature in a second order formalism underlying a two form world-volume dynamics in a $D_5$-brane. In particular, we consider the two form in presence of a background (open string) metric in a $U(1)$ gauge theory. Interestingly the formalism may be viewed via a non-coincident pair of $(D{\bar D})_5$-brane with a global NS two form on an anti brane and a local two form on a brane. The energy-momentum tensor is computed in the six dimensional CFT. It is shown to source a metric fluctuation on a vacuum created pair of $(D{\bar D})_4$-brane at a cosmological horizon by the two form quanta in the gauge theory. The emergent gravity scenario is shown to describe a low energy (perturbative) string vacuum in $6D$ with a (non-perturbative) quantum correction by a lower ($p<5$) dimensional $D_p$ brane or an anti brane in the formalism. A closed string exchange between a pair of $(D{\bar D})_4$-brane, underlying a closed/open string duality, is argued to describe the Einstein vacuum in a low energy limit. We obtain topological de Sitter and Schwarzschild brane universe in six dimensions. The brane/anti-brane geometries are analyzed to explore some of their characteristic and thermal behaviours in presence of the quantum effects. They reveal an underlying nine dimensional type IIA and IIB superstring theories on $S^1$.

hep-th↗