arXiv subjects

M. Agarwal

Publications and source records attributed to M. Agarwal.

6 recordsLinked to original sources

arXiv2023

Collective spin oscillations in a magnetized graphene sheet

We investigate collective spin excitations of graphene electrons with short-ranged interactions and subject to the external Zeeman magnetic field. We find that in addition to the familiar Silin spin wave, a collective spin-flip excitation that reduces to the uniform precession when the wave's momentum approaches zero, the magnetized graphene supports another collective mode visible in the transverse spin susceptibility: a collective spin-current mode. Unlike the Silin wave, this mode is not dictated by the spin-rotational symmetry but rather owns its existence to the pseudo-spin structure of the graphene lattice. We find the new collective excitation to become sharply defined in a finite interval of wave's momenta, the range of which is determined by the interaction and the magnetization.

cond-mat.mes-hall↗

arXiv2022

Characterization of hot populations of Melotte 66 open cluster using Swift/UVOT

Ultraviolet (UV) wavelength observations have made a significant contribution to our understanding of hot stellar populations of star clusters. Multi-wavelength spectral energy distributions (SEDs) of stars, including ultraviolet observations, have proven to be an excellent tool for discovering unresolved hot companions in exotic stars such as blue straggler stars (BSS), thereby providing helpful clues to constrain their formation mechanisms. Melotte 66 is a 3.4 Gyr old open cluster located at a distance of 4810 pc. We identify the cluster members by applying the ML-MOC algorithm on Gaia EDR3 data. Based on our membership identification, we find 1162 members, including 14 BSS candidates, 2 yellow straggler candidates (YSS), and one subdwarf B candidate (sdB). We generated SEDs for 11 BSS candidates and the sdB candidate using Swift/UVOT data combined with other archival data in the optical and IR wavelengths. We discover a hot companion of one BSS candidate, BSS3, with temperature of 38000$_{-6000}^{+7000}$ K, luminosity of 2.99$_{-1.86}^{+5.47}$ L$_\odot$, and radius of 0.04$_{-0.005}^{+0.008}$ R$_\odot$. This hot companion is a likely low-mass WD with an estimated mass of 0.24 $\text{-}$ 0.44 M$_\odot$. We report one BSS candidate, BSS6, as an Algol-type eclipsing binary with a period of 0.8006 days, based on the Gaia DR3 variability classification. We suggest that BSS3 is formed via either the Case A or Case B mass-transfer channel, whereas BSS6 is formed via the Case A mass transfer.

astro-ph.GA↗

arXiv2020

Dynamic response functions of two-dimensional Dirac fermions with screened Coulomb and short-range interactions

We consider a screened Coulomb interaction between electrons in graphene and determine their dynamic response functions, such as a longitudinal and a transverse electric conductivity and a polarization function and compare them to the corresponding quantities in the short-range interaction model. The calculations are performed to all orders for short-range interaction by taking into account the self-energy renormalization of the electron velocity and using a ladder approximation to account for the vertex corrections, ensuring that the Ward identity (charge conservation law) is satisfied. Our findings predict a resonant response of interacting electron-hole pairs at a particular frequency below the threshold $qv=ω$ and further predict an instability for sufficiently strong interactions.

cond-mat.mes-hall↗

arXiv2019

Binding Energy and Lifetime of Excitons in Metallic Nanotubes

The difficulty of describing excitons in semiconducting SWNTs analytically lies with the fact that excitons can neither be considered strictly 1D nor 2D objects. However, the situation changes in the case of metallic nanotubes where, by virtue of screening from gapless metallic subbands, the radius of the exciton becomes much larger than the radius of the nanotube $R_\text{ex}\gg R$. Taking advantage of this, we develop the theory of excitons in metallic nanotubes, determining that their binding energy is about $0.08v/R$, in agreement with the existing experimental data. Additionally, because of the presence of the gapless subbands, there are processes where bound excitons are scattered into unbound electron-hole pairs belonging to the gapless subbands. Such processes lead to a finite exciton lifetime and the broadening of its spectral function. We calculate the corresponding decay rate of the excitons.

cond-mat.mes-hall↗

arXiv2018

Potential and spin-exchange interaction between Anderson impurities in graphene

The effective interaction between resonant magnetic Anderson impurities in graphene, mediated by conduction electrons, is studied as a function of the strength of the onsite energy level of the impurities and the amplitude of coupling to conduction electrons. The sign and character of the interaction depend on whether the impurities reside on the same or opposite sublattices. For the same (opposite) sublattice, the potential interaction is attractive (repulsive) in the weak coupling limit with $1/R^3$ dependence on the distance; the interaction reverses sign and becomes repulsive (attractive) in the strong coupling limit and displays $1/R$ behavior. The spin-exchange coupling is ferromagnetic (antiferromagnetic) at both large and small distances, but reverses sign and becomes anti-ferromagnetic (ferromagnetic) for intermediate distances. For opposite sublattices, the effective spin exchange coupling is resonantly enhanced at distances where the energy levels cross the Dirac points.

cond-mat.mes-hall↗

arXiv2016

Long-range exchange interaction between magnetic impurities in graphene

The effective spin exchange coupling between impurities (adatoms) on graphene mediated by conduction electrons is studied as a function of the strength of the potential part of the on-site energy $U$ of the electron-adatom interaction. With increasing $U$, the exchange coupling becomes long-range, determined largely by the impurity levels with energies close to the Dirac points. When adatoms reside on opposite sublattices, their exchange coupling, normally antiferromagnetic, becomes ferromagnetic and resonantly enhanced at a specific distance where an impurity level crosses the Dirac point.

cond-mat.mes-hall↗