arXiv subjects

Swarup Deb

Publications and source records attributed to Swarup Deb.

13 recordsLinked to original sources

arXiv2026

Mismatch between Raman shear modes and ferroelectric polarization in 3R-MoS$_{2}$

Sliding ferroelectricity in parallel-stacked two-dimensional van der Waals materials enables a broad range of novel device concepts, but exploiting it requires reliable, non-destructive assignment of the underlying stacking order and polarization state. Here, we combine Kelvin-probe force microscopy (KPFM) with low-frequency Raman spectroscopy to probe the polarization domains and stacking configurations of a exfoliated trilayer 3R-MoS$_{2}$ flake on a hBN substrate. We find that ABA and BAB - both stackings with zero net polarization - are indistinguishable in KPFM, yet show drastically different low-frequency shear modes. This observation is reproduced across multiple flakes and is corroborated by low-temperature photoluminescence. Notably, the standard bond-polarizability model does not account for the difference in shear-mode activity between the ABA and BAB configurations, indicating that the interlayer Raman response of these stackings is governed by physics beyond a simple polarizability picture. Our results show that none of the here-used individual techniques alone is sufficient to assign sliding-ferroelectric stacking order and motivate a combined spectroscopic-scanning-probe approach.

cond-mat.mes-hall↗

arXiv2025

Investigating the Ferroelectric Potential Landscape of 3R-MoS$_2$ through Optical Measurements

In recent years, sliding ferroelectricity has emerged as a topic of significant interest due to its possible application in non-volatile, reconfigurable storage devices. This phenomenon is unique to two-dimensional van der Waals materials, where out-of-plane ferroelectric polarization switching is induced by relative in-plane sliding of adjacent layers. The intrinsic stacking order influences the resulting polarization, creating distinct polarization regions separated by domain walls. These regions and the domain walls can be manipulated using an applied vertical electric field, enabling a switchable system that retains the environmental robustness of van der Waals materials under ambient conditions. This study investigates 3R-MoS$_2$ using various optical measurement techniques at room temperature. The spatially resolved optical measurements reveal apparent signal changes corresponding to different ferroelectric stacking orders and variations in layer count. Our findings demonstrate that fast optical mapping at room temperature is a reliable method for probing ferroelectric potential steps in 3R-stacked MoS$_2$ samples, thereby facilitating the identification of the ferroelectric configuration. This approach does not require a conductive substrate or an electrical contact to the sample, making it more versatile than traditional atomic force probe techniques.

cond-mat.mtrl-sci↗

arXiv2025

Ferroelectric Control of Interlayer Excitons in 3R-MoS$_{2}$ / MoSe$_{2}$ Heterostructures

We investigate the interaction between interlayer excitons and ferroelectric domains in hBN-encapsulated 3R-MoS$_2$/MoSe$_2$ heterostructures, combining photoluminescence experiments with density functional theory and many-body Green's function calculations. Low-temperature photoluminescence spectroscopy reveals a strong redshift of the interlayer exciton energy with increasing MoS$_2$ layer thickness, attributed to band renormalization and dielectric effects. We observe local variations in exciton energy that correlate with local ferroelectric domain polarization of the 3R-MoS$_2$ layer, showcasing distinct domain-dependent interlayer exciton transition energies. Gate voltage experiments demonstrate that the interlayer exciton energy can be tuned by electrically induced domain switching. These results highlight the potential for interlayer exciton control by local ferroelectric order and establish a foundation for future ferroelectric optoelectronic devices based on van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

Excitonic signatures of ferroelectric order in parallel-stacked MoS$_2$

Interfacial ferroelectricity, prevalent in various parallel-stacked layered materials, allows switching of out-of-plane ferroelectric order by in-plane sliding of adjacent layers. Its resilience against doping potentially enables next-generation storage and logic devices. However, studies have been limited to indirect sensing or visualization of ferroelectricity. For transition metal dichalcogenides, there is little knowledge about the influence of ferroelectric order on their intrinsic valley and excitonic properties. Here, we report direct probing of ferroelectricity in few-layer 3R-MoS$_2$ using reflectance contrast spectroscopy. Contrary to a simple electrostatic perception, layer-hybridized excitons with out-of-plane electric dipole moment remain decoupled from ferroelectric ordering, while intralayer excitons with in-plane dipole orientation are sensitive to it. Ab initio calculations identify stacking-specific interlayer hybridization leading to this asymmetric response. Exploiting this sensitivity, we demonstrate optical readout and control of multi-state polarization with hysteretic switching in a field-effect device. Time-resolved Kerr ellipticity reveals a direct correspondence between spin-valley dynamics and stacking order.

cond-mat.mtrl-sci↗

arXiv2024

Polarization Saturation in Multi-layered Interfacial Ferroelectrics

Van der Waals (vdW) polytypes of broken inversion and mirror symmetries were recently shown to exhibit switchable electric polarization even at the ultimate two-layer thin limit. Their out-of-plane polarization was found to accumulate in a ladder-like fashion with each successive layer, offering 2D building blocks for the bottom-up construction of 3D ferroelectrics. Here, we demonstrate experimentally that beyond a critical stack thickness, the accumulated polarization in rhombohedral polytypes of molybdenum disulfide (r-MoS2) saturates. The underlying saturation mechanism, deciphered via DFT and self-consistent Poisson-Schr\"odinger calculations, point to a purely electronic redistribution involving: (1) polarization-induced bandgap closure that allows for cross-stack charge transfer and the emergence of free surface charge; (2) reduction of the polarization saturation value, as well as the critical thickness at which it is obtained, by the presence of free carriers. The resilience of polar layered structures to atomic surface reconstruction, which is essentially unavoidable in polar 3D crystals, potentially allows for the design of new devices with mobile surface charges. Our findings, which are of general nature, should be accounted for when designing switching and/or conductive devices based on ferroelectric layered materials.

cond-mat.mtrl-sci↗

arXiv2024

Effect of spin-dependent tunneling in a MoSe$_2$/Cr$_2$Ge$_2$Te$_6$ van der Waals heterostructure on exciton and trion emission

We study van der Waals heterostructures consisting of monolayer MoSe$_2$ and few-layer Cr$_2$Ge$_2$Te$_6$ fully encapsulated in hexagonal Boron Nitride using low-temperature photoluminescence and polar magneto-optic Kerr effect measurements. Photoluminescence characterization reveals a partial quenching and a change of the exciton-trion emission ratio in the heterostructure as compared to the isolated MoSe$_2$ monolayer. Under circularly polarized excitation, we find that the exciton-trion emission ratio depends on the relative orientation of excitation helicity and Cr$_2$Ge$_2$Te$_6$ magnetization, even though the photoluminescence emission itself is unpolarized. This observation hints at an ultrafast, spin-dependent interlayer charge transfer that competes with exciton and trion formation and recombination.

cond-mat.mes-hall↗

arXiv2023

Rapid spin depolarization in the layered 2D Ruddlesden Popper perovskite (BA)(MA)PbI

We report temperature-dependent spectroscopy on the layered (n=4) two-dimensional (2D) Ruddlesden-Popper perovskite (BA)(MA)PbI. Helicity-resolved steady-state photoluminescence (PL) reveals no optical degree of polarization. Time-resolved PL shows a photocarrier lifetime on the order of nanoseconds. From simultaneaously recorded time-resolved differential reflectivity (TR$\Delta$R) and time-resolved Kerr ellipticity (TRKE), a photocarrier lifetime of a few nanoseconds and a spin dephasing time on the order of picoseconds was found. This stark contrast in lifetimes clearly explains the lack of spin polarization in steady-state PL. While we observe clear temperature-dependent effects on the PL dynamics that can be related to structural dynamics, the spin dephasing is nearly T-independent. Our results highlight that spin dephasing in 2D (BA)(MA)PbI occurs at time scales faster than the exciton recombination time, which poses a bottleneck for applications aimingto utilize this degree of freedom.

cond-mat.mes-hall↗

arXiv2023

Shaping Exciton Polarization Dynamics in 2D Semiconductors by Tailored Ultrafast Pulses

The ultrafast formation of strongly bound excitons in two-dimensional semiconductors provide a rich platform for studying fundamental physics as well as developing novel optoelectronic technologies. While extensive research has explored the excitonic coherence, many-body interactions, and nonlinear optical properties, the potential to study these phenomena by directly controlling their coherent polarization dynamics has not been fully realized. In this work, we use a sub-10fs pulse shaper to study how temporal control of coherent exciton polarization affects the generation of four-wave mixing in monolayer WSe2 under ambient conditions. By tailoring multiphoton pathway interference, we tune the nonlinear response from destructive to constructive interference, resulting in a 2.6-fold enhancement over the four-wave mixing generated by a transform-limited pulse. This demonstrates a general method for nonlinear enhancement by shaping the pulse to counteract the temporal dispersion experienced during resonant light-matter interactions. Our method allows us to excite both 1s and 2s states, showcasing a selective control over the resonant state that produces nonlinearity. By comparing our results with theory, we find that exciton-exciton interactions dominate the nonlinear response, rather than Pauli blocking. This capability to manipulate exciton polarization dynamics in atomically thin crystals lays the groundwork for exploring a wide range of resonant phenomena in condensed matter systems and opens up new possibilities for precise optical control in advanced optoelectronic devices.

physics.optics↗

arXiv2022

Cumulative Polarization Coexisting with Conductivity at Interfacial Ferroelectrics

Ferroelectricity in atomically thin bilayer structures has been recently predicted1 and measured[2-4] in two-dimensional (2D) materials with hexagonal non-centrosymmetric unit-cells. Interestingly, the crystal symmetry translates lateral shifts between parallel 2D layers to a change of sign in their out-of-plane electric polarization, a mechanism referred to as "Slide-Tronics"[4]. These observations, however, have been restricted to switching between only two polarization states under low charge carrier densities[5-12], strongly limiting the practical application of the revealed phenomena[13]. To overcome these issues, one needs to explore the nature of the polarization that arises in multi-layered van der Waals (vdW) stacks, how it is governed by intra- and inter-layer charge redistribution, and to which extent it survives the introduction of mobile charge carriers, all of which are presently unknown14. To explore these questions, we conduct surface potential measurements of parallel WSe2 and MoS2 multi-layers with aligned and anti-aligned configurations of the polar interfaces. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields, which provide means to design multi-state "ladder ferroelectrics". Furthermore, we find that the internal polarization remains significant upon electrostatic doping of a mobile charge carrier density as high as 1013 cm-2, with substantial in-plane conductivity. Using first-principles calculations based on density functional theory (DFT), we trace the extra charge redistribution in real and momentum space and identify an eventual doping-induced depolarization mechanism.

cond-mat.mtrl-sci↗

arXiv2022

Effect of manganese incorporation on the excitonic recombination dynamics in monolayer MoS$_2$

Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy techniques we investigate the incorporation of Manganese (Mn) in monolayer (1L)-MoS$_2$ grown on sapphire substrates by microcavity based chemical vapor deposition (CVD) method. These layers are coated with different amount of Mn by pulsed laser deposition (PLD) technique and temperature dependent photo-luminescence (PL) spectroscopic study has helped us in understanding how such deposition affects the dynamics of excitonic recombination in this system. The study further reveals two distinctly different Mn-incorporation regimes. Below a certain critical deposition amount of Mn, thin Mn-coating with large area coverage is found on MoS$_2$ layers and in this regime, substitution of Mo ions by Mn is detected through XPS. Dewetting takes place when Mn-deposition crosses the critical mark, which results in the formation of Mn-droplets on MoS$_2$ layers. In this regime, substitutional incorporation of Mn is suppressed, while the Raman study suggests an enhancement of disorder in the lattice with the Mn-deposition time. From PL investigation, it has been found that the increase of the amount of Mn-deposition not only enhances the density of non-radiative recombination channels for the excitons but also raises the barrier height for such recombination to take place. The study attributes these non-radiative transitions to certain Mo related defects (either Mo-vacancies or distorted Mo-S bonds), which are believed to be generated in large numbers during Mn-droplet formation stage as a result of the withdrawal of Mn ions from the Mo-substitutional sites.

cond-mat.mtrl-sci↗

arXiv2020

Spin transport in polarization induced two dimensional confinement of carriers in wedge shaped \textit{c}-GaN nanowalls

Spin transport property of polarization induced two-dimensional electron gas channel formed in the central vertical plane of a wedge-shaped \textit{c}-oriented GaN nanowall is investigated theoretically. Since the confining potential preserves the spatial symmetry between the conduction and valence band, the Rashba effect is suppressed in this system even when the shape of the wedge is asymmetric. It has been found that the relaxation of the electron spin oriented along the direction of the confinement via D'yakonov-Perel' (DP) mechanism, which is the dominant process of relaxation in this high mobility channel, is entirely switched off at low temperatures. Spin relaxation can be turned on by applying a suitable bias at the gate. Exploiting this remarkable effect, a novel all electrically driven spin-transistor has been proposed.

cond-mat.mes-hall↗

arXiv2020

Tunability of electrical and thermoelectrical properties of monolayer MoS$_2$ through oxygen passivation

Electric and thermoelectric properties of strictly monolayer MoS$_2$ films, which are grown using a novel micro-cavity based CVD growth technique, have been studied under diverse environmental and annealing conditions. Resistance of a thermoelectric device that is fabricated on a continuous monolayer MoS$_2$ layer using photolithography technique has been found to reduce by about six orders of magnitudes upon annealing in vacuum at 525 K. Seebeck coefficient of the layer also reduces by almost an order of magnitude upon annealing. When the sample is exposed to oxygen atmosphere, these parameters return to their previous values. In fact, it has been found that the electron concentration, mobility as well as the thermoelectric power of the material can be tuned by controlling the temperature of annealing and oxygen exposure. Once established, these values are maintained as long as the layer is not exposed to oxygen environment. This can offer a unique way to control doping in the material provided an effective encapsulation method is devised. Such control is an important step forward for device application. The effect has been attributed to the passivation of di-sulfur vacancy donors present in the MoS$_2$ film by physisorbed oxygen molecules. Band structural calculations using density functional theory have been carried out, results of which indeed validate this picture.

cond-mat.mes-hall↗

arXiv2018

Demonstration of defect-defect ferromagnetic coupling in Gd doped GaN epitaxial films: A polarization selective magneto-photoluminescence study

Magnetic field dependent polarization selective photoluminescence(PL) study has been carried out at 1.5~K on Gd-doped GaN epitaxial layers grown on c-SiC substrates by molecular beam epitaxy technique. It has been found that the incorporation of Gd in GaN leads to the generation of three types of donor like defects that result in neutral donor bound excitonic features in low temperature PL. The study reveals that the rate of spin-flip scattering for all the three excitonic features becomes almost $B$-independent suggesting that these signals must be stemming from defects which are ferromagnetically coupled with each other. This is further confirmed by the study carried out on a GaN sample co-doped with Si and Gd, where defects are found to be ferromagnetically coupled, while Si-donors do not show any involvement in coupling.

cond-mat.mtrl-sci↗