arXiv subjects

Subhadip Das

Publications and source records attributed to Subhadip Das.

7 recordsLinked to original sources

arXiv2026

High-pressure phase transitions in the quantum spin liquid candidate Na2Co2TeO6 probed by Raman spectroscopy

The quasi-2D magnet Na2Co2TeO6 (NCTO) is a candidate for a Kitaev Quantum Spin Liquid (KQSL) state. Pressure-tuning in such materials is of interest as a potential method to tune the Kitaev exchange interactions, which are strongly dependent on bond geometry. Here we report a Raman spectroscopic study of NCTO inside a diamond anvil cell (DAC) with pressure applied up to 16.3 GPa. Based on the changes in the Raman modes, this pressure range is divided into three regions. The appearance and disappearance of several modes and changes in the polarization dependence of the representative modes, most prominently above 13.8 GPa, point to pressure-induced phase transitions in this material.

cond-mat.str-el↗

arXiv2026

Direct nanoscale mapping of band alignment in single-layer semiconducting lateral heterojunctions

Atomic-scale control over band alignment in single-layer lateral heterostructures (LHSs) of dissimilar transition metal dichalcogenides (TMDCs) is critical for nextgeneration electronic, optoelectronic, and quantum technologies. However, direct experimental access to interfacial electronic states with nanometer precision remains a significant challenge. Here, we employ angle-resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) to directly map the epitaxial alignment and valence band evolution across MoSe2-WSe2 LHSs. By combining nanoARPES with spatially resolved photoluminescence, we correlate the evolution of the valence band maximum and exciton features across both atomically sharp and compositionally graded diffusive interfaces. We identified type-II band alignments governed by both material composition and interstitial-induced modifications of band offsets, in close agreement with density functional theory calculations. These results reveal fundamental mechanisms of electronic structure modulation at 1D TMDC heterointerfaces and provide a robust platform for tailored band engineering in van der Waals materials.

cond-mat.mes-hall↗

arXiv2021

Raman and first-principles study of the pressure induced Mott-insulator to metal transition in bulk FePS$_3$

Recently discovered class of 2D materials based on transition metal phosphorous trichalcogenides exhibit antiferromagnetic ground state, with potential applications in spintronics. Amongst them, FePS$ _{3} $ is a Mott insulator with a band gap of $\sim$ 1.5 eV. This study using Raman spectroscopy along with first-principles density functional theoretical analysis examines the stability of its structure and electronic properties under pressure. Raman spectroscopy reveals two phase transitions at 4.6 GPa and 12 GPa marked by the changes in pressure coefficients of the mode frequencies and the number of symmetry allowed modes. FePS$_3$ transforms from the ambient monoclinic C2/m phase with a band gap of 1.54 eV to another monoclinic C2/m (band gap of 0.1 eV) phase at 4.6 GPa, followed by another transition at 12 GPa to the metallic trigonal P-31m phase. Our work complements recently reported high pressure X-ray diffraction studies.

cond-mat.mtrl-sci↗

arXiv2021

Thickness dependent transition from the 1T$^\prime$ to Weyl semimetal phase in ultrathin MoTe$ _{2} $: Electrical transport, Noise and Raman studies

Bulk 1T$^\prime$-MoTe$_2$ shows a structural phase transition from 1T$^\prime$ to Weyl semimetallic (WSM) $ T_{d} $ phase at $\sim$ 240 K. This phase transition and transport properties in the two phases have not been investigated on ultra-thin crystals. Here we report electrical transport, $1/f$ noise and Raman studies in ultra-thin 1T$^\prime$-MoTe$_2$ ($\sim$ 5 to 16 nm thick) field-effect transistors (FETs) devices as a function of temperature. The electrical resistivities for thickness 16 nm and 11 nm show maxima at temperatures 208 K and 178 K, respectively, making a transition from semiconducting to semi-metallic phase, hitherto not observed in bulk samples. Raman frequencies and linewidths for 11nm thick crystal show change around 178 K, attributed to additional contribution to the phonon self-energy due to enhanced electron-phonon interaction in the WSM phase. Further, the resistivity at low-temperature shows an upturn below 20 K along with the maximum in the power spectral density of the low frequency $1/f$ noise. The latter rules out the metal-insulator transition (MIT) being responsible for the upturn of resistivity below 20 K. The low temperature resistivity follows $\rho \propto 1/T$, changing to $\rho \propto T$ with increasing temperature supports electron-electron interaction physics at electron-hole symmetric Weyl nodes below 20 K. These observations will pave the way to unravel the properties of WSM state in layered ultra-thin van der Waals materials.

cond-mat.mes-hall↗

arXiv2021

Doping controlled Fano resonance in bilayer 1T$ ^{\prime} $-ReS$ _{2} $: Raman experiments and first-principles theoretical analysis

In the bilayer ReS$ _{2} $ channel of a field-effect transistor (FET), we demonstrate using Raman spectroscopy that electron doping (n) results in softening of frequency and broadening of linewidth of the in-plane vibrational modes, leaving out-of-plane vibrational modes unaffected. Largest change is observed for the in-plane Raman mode at $\sim$ 151 cm$^{-1} $, which also shows doping induced Fano resonance with the Fano parameter 1/q = -0.17 at doping concentration of $\sim 3.7\times10^{13}$ cm$^{-2} $. A quantitative understanding of our results is provided by first-principles density functional theory (DFT), showing that the electron-phonon coupling (EPC) of in-plane modes is stronger than that of out-of-plane modes, and its variation with doping is independent of the layer stacking. The origin of large EPC is traced to 1T to 1T$ ^{\prime} $ structural phase transition of ReS$ _{2} $ involving in-plane displacement of atoms whose instability is driven by the nested Fermi surface of the 1T structure. Results are also compared with the isostructural trilayer ReSe$ _{2} $.

cond-mat.mes-hall↗

arXiv2020

Tuning the structure of Skyrmion lattice system Cu2OSeO3 under pressure

The insulating ferrimagnet Cu2OSeO3 shows a rich variety of phases such as skyrmion lattice and helical magnetism controlled by interplay of different exchange interactions which can be tuned by external pressure. In this work we have investigated pressure-induced phase transitions at room temperature using synchrotron based x- ray diffraction and Raman scattering measurements. With first-principles theoretical analysis, we show that spin-spin exchange couplings in the ambient cubic phase are affected notably by hydrostatic pressure. The ambient cubic phase transforms to a monoclinic phase above 7 GPa and then to the triclinic phase above 11 GPa. Emergence of new phonon modes in the Raman spectra confirms these structural phase transitions. Notably, upon decompression, the crystal undergoes transition to a new monoclinic structure. Atomic coordinates have been refined in the low pressure cubic phase to capture the Cu-tetrahedra evolution responsible for the earlier reported magnetic behavior under pressure. Our experiments will motivate further studies of its emergent magnetic behavior under pressure.

cond-mat.mtrl-sci↗

arXiv2020

Symmetry induced phonon renormalization in few layers of 2H-MoTe$_2$ transistors: Raman and first-principles studies

Understanding of electron-phonon coupling (EPC) in two dimensional (2D) materials manifesting as phonon renormalization is essential to their possible applications in nanoelectronics. Here we report in-situ Raman measurements of electrochemically top-gated 2, 3 and 7 layered 2H-MoTe$ _{2} $ channel based field-effect transistors (FETs). While the E$ ^{1}_{2g} $ and B$ _{2g} $ phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (\textit{p}) up to $\sim$ 2.3 $\times$10$ ^{13} $/cm$ ^{2} $, A$ _{1g}$ shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the E$ ^{1}_{2g} $ mode on the number of layers in these 2D crystals confirms that hole doping occurs primarily in the top two layers, in agreement with recent predictions. We present first-principles density functional theory (DFT) analysis of bilayer MoTe$ _{2} $ that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with E$ ^{1}_{2g} $ or B$ _{2g} $ modes as compared with the A$ _{1g} $ mode originates from the in-plane orbital character and symmetry of the states at valence band maximum (VBM). The contrast between the manifestation of EPC in monolayer MoS$ _{2} $ and those observed here in a few-layered MoTe$ _{2} $ demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.

cond-mat.mes-hall↗