arXiv subjects

Chumki Nayak

Publications and source records attributed to Chumki Nayak.

5 recordsLinked to original sources

arXiv2026

Intrinsic Electric Field Driven High Sensitive Photodetection in Alloy TMDC MoSSe

Alloying offers an effective way to improve the functionality of transition metal dichalcogenides (TMDCs) in both fundamental research and optoelectronic applications, as it allows for engineering their electronic and optical properties. This study investigates the optoelectronic properties of CVD-synthesized alloy MoSSe, which exhibits an inherent out-of-plane dipole moment, arising from asymmetry in S and Se atoms on either side of the Mo layer, as confirmed by piezoelectric force microscopy, polarization-resolved second harmonic generation studies and theoretical first-principles calculations. Time-resolved photoluminescence measurements reveal an extended exciton radiative recombination lifetime in MoSSe, attributed to electron-hole wavefunction separation by the dipole moment, which improves photodetection by facilitating enhanced electron-hole separation before recombination. The device demonstrates significant responsivity over broad spectral range. By employing the photogating effect, the device response can be switched from slow to fast modes. These findings are further supported by illumination intensity-dependent photoluminescence and Raman measurements, underscoring the potential of polar TMDCs in future optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Spin Reorientation Driven Renormalization of Spin-Phonon Coupling in Fe$_4$GeTe$_2$

Quasi-2D van der Waals ferromagnet Fe$_4$GeTe$_2$, featuring the simultaneous presence of high Curie temperature ($T_\mathrm{C}$ $\sim 270$ K) and a spin-reorientation transition at $T_\mathrm{SR}$ $\sim 110$ K, is a rare system where strong interplay of spin dynamics, lattice vibrations, and electronic structure leads to a wide range of interesting phenomena. Here, we investigate the lattice response of exfoliated Fe$_4$GeTe$_2$ nanoflakes using temperature-dependent Raman spectroscopy. Polarization-resolved measurements reveal that, while one Raman mode exhibits a purely out-of-plane character, the rest display mixed symmetry, reflecting interlayer vibrational nonuniformity and symmetry-driven mode degeneracies. Below $T_\mathrm{C}$, phonons harden, and the linewidth narrows, consistent with reduced anharmonicity, while across the spin reorientation transition at $T_\mathrm{SR}$ they display anomalous softening, linewidth broadening, and a peak in lifetime, which are signatures of strengthened spin-phonon coupling. Complementary DFT+DMFT calculations and atomistic spin dynamical simulations reveal temperature-dependent spin excitations whose energies overlap with the Raman-active phonons, providing a natural route for the observed magnon-phonon interaction. Together, these insights establish Fe$_4$GeTe$_2$ as a versatile platform for exploring intertwined spin, lattice, and electronic degrees of freedom, with relevance for dynamic spintronic and magneto-optic functionalities near technologically meaningful temperatures.

cond-mat.mes-hall↗

arXiv2025

Alloying as a new route to generating interlayer excitons

Heterobilayers formed by stacking two-dimensional atomic crystals are particularly promising for low-dimensional semiconductor optics, as they host interlayer excitons, bound states of electrons and holes residing in different layers. They inherit the valley-contrasting physics of the individual monolayers, leading to a range of unique properties that distinguish them from other solid-state nanostructures. Here, we propose a novel route for the generation of interlayer excitons based on the synthesis of a transition metal dichalcogenide bilayer alloy material, WS$_{2x}$Se$_{2(1-x)}$. Using piezoelectric force microscopy, we demonstrate the existence of an internal electric field oriented in the out-of-plane direction. Interlayer excitons have so far been mostly observed in heterostructures with a type-II band alignment. In the presence of an internal electric field, a similar alignment occurs in the alloy bilayer resulting in an efficient generation of interlayer excitons. Photoluminescence spectroscopy measurements involving circularly polarised light come up with key observations like a negative degree of circular polarization of the interlayer excitons which increases as a function of temperature. A simple theoretical model provides a physical understanding of the major experimentally observed features. With experimentally fitted parameter values, the dominant contribution to the degree of circular polarization is shown to arise from spin polarization and not from valley polarization, a consequence of the spin-valley-layer coupling characteristic of a TMDC bilayer. The room-temperature interlayer excitonic transition in bilayer TMDCs has key implications for fundamental physics, including Bose-Einstein condensation and high-temperature superfluidity, while enabling advanced valleytronic and quantum information functionalities.

cond-mat.mes-hall↗

arXiv2025

Dimensional crossover and emergence of novel phases in puckered PdSe$_2$ under pressure

We investigate the pressure-driven structural and electronic evolution of PdSe$_2$ using powder X-ray diffraction, Raman spectroscopy, and first-principles calculations. Beyond 2.3 GPa, suppression of the Jahn-Teller distortion induces in-plane lattice expansion and metallization. Around 4.8 GPa, the interlayer $d_{z^2}-π^*$ orbital hybridization drives the dimensional crossover, facilitating the transformation from the 2D distorted to a 3D undistorted pyrite phase. At $\sim$ 9 GPa, a novel phase emerges, characterized by octahedral distortions in the $d$ orbitals of Pd. Structural analysis suggests the presence of marcasite ($Pnnm$) or arsenopyrite ($P2_1/c$) phase with orthorhombic and monoclinic configurations, respectively. Furthermore, the observed phonon anomaly and electronic structure modifications, including the emergence of flat bands in the high-pressure phases, elucidate the fundamental mechanisms underlying the previously reported exotic superconductivity with an enhanced critical temperature. These results highlight the pivotal role of dimensional crossover and structural transitions in tuning the electronic properties of puckered materials, providing pathways for novel functionalities.

cond-mat.mtrl-sci↗

arXiv2023

Valley polarization and photocurrent generation in transition metal dichalcogenide alloy MoS$_{2x}$Se$_{2(1-x)}$

Monolayer transition metal dichalcogenides (TMDCs) constitute the core group of materials in the emerging semiconductor technology of valleytronics. While the coupled spin-valley physics of pristine TMDC materials and their heterstructures has been extensively investigated, less attention was given to TMDC alloys, which could be useful in optoelectronic applications due to the tunability of their band gaps. We report here our experimental investigations of the spin-valley physics of the monolayer and bilayer TMDC alloy, MoS$_{2x}$Se$_{2(1-x)}$, in terms of valley polarization and the generation as well as electrical control of a photocurrent utilising the circular photogalvanic effect. Piezoelectric force microscopy provides evidence for an internal electric field perpendicular to the alloy layer, thus breaking the out-of-plane mirror symmetry. The experimental observation is supported by first principles calculations based on the density functional theory. A comparison of the photocurrent device, based on the alloy material, is made with similar devices involving other TMDC materials.

cond-mat.mes-hall↗