arXiv subjects

Nasaru Khan

Publications and source records attributed to Nasaru Khan.

4 recordsLinked to original sources

arXiv2026

Resonantly Enhanced Multiphonon Scattering and Local Orbital-Phonon Coupling in Bulk and Thin Flakes of 2D Single Crystals of Antiferromagnetic (NixFe1-x)2P2S6

Orbital degrees of freedom play a pivotal role in shaping the physical properties of two-dimensional (2D) van der Waals magnetic systems, strongly influencing electron-phonon coupling and intermediate-state dynamics. In this study, we present a comprehensive temperature and thickness-dependent Raman investigation of the single crystals of 2D van der Waals antiferromagnetic series (NixFe1-x)2P2S6. Utilizing Raman spectroscopy, we explore the interplay between local orbital excitations and lattice vibrations, observing higher-order phonon modes extending up to the fourth order. We observe an anomalously high and weakly temperature-dependent intensity ratio of these higher-order modes relative to low-energy first-order phonons. Theoretical cluster calculations and Franck-Condon modelling reveal that these features originate from resonantly enhanced multiphonon scattering mediated by localized intermediate Ni 3d8 multiplet excitations. The local orbital occupancy of these intermediate states couples strongly to lattice coordinates, providing a selective enhancement mechanism for specific phonon channels. Notably, these higher-order features are absent in the end-member Fe2P2S6. This distinction reflects the charge-transfer character of Ni2P2S6, which places optical transitions in resonance with local multiplet states, contrasted with the Mott-Hubbard insulator regime in Fe-rich analogues. Our findings clarify the microscopic origin of high-frequency Raman scattering in transition metal thiophosphates and underscore the central role of local orbital-phonon interactions in 2D correlated magnets.

cond-mat.other↗

arXiv2025

Exciton dynamics, Fano quantum interference and d-d excitation in the single crystal of two-dimensional antiferromagnetic Fe2P2S6

Excitonic quasiparticle and their interactions with phonons, magnons and charge carriers may play a pivotal role in governing the optical properties and their correlation with magnetic interactions in two-dimensional (2D) magnetic semiconductors. Further, in transition metal compounds, d-d electronic transitions, arising from excitations between crystal-field split d-orbitals, significantly influence the optical and magnetic properties, particularly in strongly correlated and low-dimensional systems. Fe2P2S6, a layered antiferromagnetic semiconductor, offers a rich platform for studying the interplay between spin, charge, and lattice degrees of freedom in these 2D systems. In this work, we investigate the photoluminescence (PL) properties of Fe2P2S6 to probe the exciton dynamics, intra-atomic transitions, and their temperature evolution. Two prominent d-d emission peaks are observed at ~ 1.63 eV (D1) and ~ 1.80 eV (D2), attributed to the crystal field-split Fe2+ states. An excitonic emission near the band edge is also identified, which exhibits a characteristic Fano asymmetric line shape. This asymmetry is attributed to the quantum interference between the discrete excitonic state and the d-d transition induced continuum (D2), revealing a Fano resonance behaviour. This exciton peak disappears well before the Neel temperature, indicating its faster destabilization than magnetic ordering. Temperature-dependent PL measurements show a quenching of the excitonic peak with increasing temperature. Our findings provide detailed insight into the optical excitation pathways in Fe2P2S6.

cond-mat.other↗

arXiv2025

Exciton dynamics and exciton-phonon coupling in bulk and thin flakes of layered van der Waals antiferromagnet Ni$_2$P$_2$S$_6$

The Zhang-Rice (ZR) singlet is an intriguing quantum state offering potential to realize a spin-orbit-entangled bosonic quasiparticle, which gives rise to the Zhang-Rice exciton. Its formation is attributed to the correlation between a localized d-orbital of a transition metal and the p-orbitals of the neighbouring ligands. The layered two-dimensional (2D) antiferromagnetic Ni2P2S6 system provide an excellent platform to probe the ZR exciton dynamics along with the role of exciton-phonon coupling. Here, we present a comprehensive study of ZR exciton and coupling with the phonons in bulk and few-layered single crystals of Ni2P2S6 using temperature, polarization and power-dependent photoluminescence (PL) spectroscopy. At cryogenic temperatures, the PL spectra reveal distinct phonon sidebands spaced by an energy difference of nearly 117 cm-1, indicative of exciton-phonon hybridization. Polarization-resolved measurements demonstrate a strong optical anisotropy, with a linear polarization degree of ~ 40 % at 4 K. Excitation power variation highlights linear scaling of PL intensity in the low-power regime, followed by spectral deformation at higher powers attributed to the phonon-assisted recombination and exciton saturation effects. ZR exciton and phonon side bands survival temperature decreases with decreasing flake thickness suggesting their tunability. The emergence and suppression of phonon sidebands with temperature and flake thickness emphasize dimensional sensitivity and coherence limits of excitonic states. Our findings position Ni2P2S6 as a promising candidate for tunable and anisotropic optoelectronic applications, while offering insight into quasiparticle interactions in 2D magnetic systems.

cond-mat.other↗

arXiv2023

Tunable Resonance and Electron-Phonon Coupling in Layered MoS2

Resonance Raman scattering, a very effective and sensitive technique for atomically thin semiconducting transition metal dichalcogenide, can be used to observe the phonons from the entire Brillouin zone. In addition to the significance of resonance effect on the Raman spectrum it may also be used to probe the electron-phonon coupling. Our study is devoted to understand the phonons in layered MoS2, especially for very low frequency range (i.e. below 100 cm-1), as a function of temperature under the resonance effect. Understanding the phonon-phonon and electron-phonon coupling and the effects of temperature on the Raman spectrum are the central points of the present study. We observe the anomalous softening and broadening of a very low frequency phonon mode P3 (~34 cm-1) at low temperature ( i.e below 150 K). We attributed the observed anomalous trend in frequency and linewidth of this low frequency phonon to the electron-phonon coupling. Furthermore, our work also highlights the temperature induced tuning of resonance condition via understanding the intensity of phonon modes as a function of temperature.

cond-mat.mtrl-sci↗