arXiv subjects

S. S. Ali

Publications and source records attributed to S. S. Ali.

5 recordsLinked to original sources

arXiv2026

Quasi-Two-Dimensional Quantum Antiferromagnetism in the Distorted Honeycomb Compound KCuIn(PO4)2

We investigate the electronic structure and magnetic properties of the distorted honeycomb lattice compound KCuInP2O8 through a combination of experimental measurements, first principles calculations and quantum monte carlo simulations. Density functional theory calculations within the GGA+U framework establishes KCuInP2O8 as an indirect gap insulator with Cu2+ local moments and finite magnetocrystalline anisotropy arising from spin orbit coupling. A microscopic evaluation of magnetic exchange interactions using the magnetic force theorem reveals a pronounced hierarchy of couplings, with the next nearest neighbor interaction dominating over the nearest neighbor exchange, while interlayer couplings remain negligible. This exchange hierarchy naturally maps the system onto weakly coupled antiferromagnetic spin chains embedded in a distorted honeycomb lattice. Motivated by the ab initio estimated exchange interactions, we construct an effective spin half Hamiltonian and investigate its magnetic response using large scale quantum Monte Carlo simulations. The calculated temperature dependent susceptibility and field dependent magnetization quantitatively reproduce the experimental behavior and capture key signatures of low dimensional quantum magnetism, including a broad susceptibility maximum and a field induced saturation at low temperatures. Our results establish KCuInP2O8 as a quasi-two-dimensional quantum antiferromagnet composed of coupled spin chains, providing a consistent theoretical framework that links electronic structure, exchange interactions, and collective magnetic behavior.

cond-mat.str-el↗

arXiv2021

Evolution of the Ultraviolet Upturn at $0.3<z<1$: exploring helium rich stellar populations

We measure the near-UV (rest-frame $\sim 2400$Å) to optical color for early-type galaxies in 12 clusters at $0.3 < z < 1.0$. We show that this is a suitable proxy for the more common far-ultraviolet bandpass used to measure the ultraviolet upturn and find that the upturn is detected to $z=0.6$ in these data, in agreement with previous work. We find evidence that the strength of the upturn starts to wane beyond this redshift and largely disappears at $z=1$. Our data is most consistent with models where early-type galaxies contain minority stellar populations with non-cosmological helium abundances, up to around 46\%, formed at $z \geq 3$, resembling multiple stellar population globular clusters in our Galaxy. This suggests that elliptical galaxies and globular clusters share similar chemical evolution and star formation histories. The vast majority of the stellar mass in these galaxies also must have been in place at $z > 3$.

astro-ph.GA↗

arXiv2020

Galaxy And Mass Assembly (GAMA): Defining Passive Galaxy Samples and Searching for the UV Upturn

We use data from the GAMA and GALEX surveys to demonstrate that the UV upturn, an unexpected excess of ultraviolet flux from a hot stellar component, seen in the spectra of many early-type galaxies, arises from processes internal to individual galaxies with no measurable influence from the galaxies' larger environment. We first define a clean sample of passive galaxies without a significant contribution to their UV flux from low-level star formation. We confirm that galaxies with the optical colours of red sequence galaxies often have signs of residual star formation, which, without other information, would prevent a convincing demonstration of the presence of UV upturns. However, by including (NUV$-u$) and {\it WISE} (W2-W3) colours, and FUV data where it exists, we can convincingly constrain samples to be composed of non-star-forming objects. Using such a sample, we examine GALEX photometry of low redshift GAMA galaxies in a range of low-density environments, from groups to the general field, searching for UV upturns. We find a wide range of (NUV$-r$) colours, entirely consistent with the range seen -- and attributed to the UV upturn -- in low-redshift red sequence cluster galaxies. The range of colours is independent of group multiplicity or velocity dispersion, with isolated passive galaxies just as likely to have blue UV-to-optical colours, implying significant upturn components, as those in richer groups and in the previous data on clusters. This is supported by equivalent results for (FUV$-r$) colours which are clear indicators of upturn components.

astro-ph.GA↗

arXiv2020

Synthesis and characterization of Zn doped Mn ferrites nanostructures

Zn doped Mn ferrites nanoparticles were fabricated by using Co-precipitation. Variation in structure, magnetic and optical properties of MnZn ferrites has been discussed. First of all, samples were synthesized, annealed at different temperatures and then characterized. The as-synthesized and annealed samples were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy Dispersive spectroscopy (EDX), Ultra Violet visible spectrometry (Uv-Vis spectrometry) and Vibrating sample magnetometer (VSM). The average crystallite size of MnZn ferrites nanoparticles determined from XRD were in the range of 42 to 60 nm. These nanoparticles possess normal spinel structure. The SEM images showed the physical shape of the samples, which showed that the as prepared samples are more agglomerated and having flake like shape rather than annealed at 700C while the samples have longitudinal or rod like shape on annealing at 700C. The coercivity (Hc), saturation magnetization (Ms), and remanence (Mr) of Nps were also calculated. The (Ms) value is increasing from 26 to 65 emu/g, the coercivity (Hc) is varying from 13 to 193 Oe and remanence (Mr) has also showing increasing trend although very less, from 0.031 to 0.798 emu/g which are a little part of their bulk counter parts. The band gap energy of the samples was showing decreasing trend as with the increase of particle size which is of the order of 3.5 to 2.9 eV.

cond-mat.mtrl-sci↗

arXiv2018

UV SEDs of early-type cluster galaxies: A new look at the UV upturn

Using GALEX, UVOT and optical photometry, we explore the prevalence and strength of the UV-upturn in the spectra of quiescent early type galaxies in several nearby clusters. Even for galaxies with completely passive optical colours, there is a large spread in vacuum UV colour consistent with almost all having some UV upturn component. Combining GALEX and UVOT data below 3000Å, we generate for the first time comparatively detailed UV SEDs for Coma cluster galaxies. Fitting the UV upturn component with a blackbody, twenty six of these show a range of characteristic temperatures (10000--21000K) for the UV upturn population. Assuming a single temperature to explain GALEX-optical colours could underestimate the fraction of galaxies with UV upturns and mis-classify some as systems with residual star formation. The UV upturn phenomenon is not an exclusive feature found only in giant galaxies; we identify galaxies with similar (or even bluer) $FUV-V$ colours to the giants with upturns over a range of fainter luminosities. The temperature and strength of the UV upturn are correlated with galaxy mass. Under the plausible hypothesis that the sources of the UV upturn are blue horizontal branch stars, the most likely mechanism for this is the presence of a substantial (between 4\% and 20\%) Helium rich ($Y > 0.3$) population of stars in these galaxies, potentially formed at $z\sim 4$ and certainly at $z>2$; this plausibly sets a lower limit of $\sim {\rm 0.3 - 0.8} \times 10^{10}$ $M_{\odot}$ to the {\it in situ} stellar mass of $\sim L^*$ galaxies at this redshift.

astro-ph.GA↗