arXiv subjects

Soumik Das

Publications and source records attributed to Soumik Das.

11 recordsLinked to original sources

arXiv2026

Chaos in Liquid Crystal Directrons

Biological systems often operate at the boundary between order and chaos, transitioning from directed to irregular dynamics to achieve adaptability and robustness. Reproducing such transitions in artificial soft matter remains a central challenge. Here, we report a biomimetic regime of directron dynamics in achiral nematic liquid crystals, in which coherent, directed motion collectively evolves into chaos. Driven by multi-directron interactions, the system develops coexisting directron families with competing trajectories, displaying randomized motion, dynamic assembly formation and spontaneous fission of high energy to low energy daughter directrons - all of which mimics the phenotypic diversity observed in biological groups. Above a critical electric field, these interactions drive the system into a chaotic state that is distinct from the directed behaviours reported previously. We further introduce a minimal dipole-based model that qualitatively captures the underlying physics of this transition. Together, our results establish an artificial active system in which chaos emerges intrinsically from interactions, offering a versatile platform to study biological dynamics and opening new avenues for liquid-crystal-based soft-matter applications involving adaptive transport, cargo delivery, and energy transduction

cond-mat.soft↗

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↗

arXiv2025

Magnetodielectric Properties in Two Dimensional Magnetic Insulators

Magnetodielectric (MD) materials are important for their ability to spin-charge conversion, magnetic field control of electric polarization and vice versa. Among these, two-dimensional (2D) van der Waals (vdW) magnetic materials are of particular interest due to the presence of magnetic anisotropy (MA) originating from the interaction between the magnetic moments and the crystal field. Also, these materials indicate a high degree of stability in the long-range spin order and may be described using suitable spin Hamiltonians of the Heisenberg, XY, or Ising type. Recent reports have suggested effective interactions between magnetization and electric polarization in 2D magnets. However, MD coupling studies on layered magnetic materials are still few. This review covers the fundamentals of magnetodielectric coupling by explaining related key terms. It includes the necessary conditions for having this coupling and sheds light on the possible physical mechanisms behind this coupling starting from phenomenological descriptions. Apart from that, this review classifies 2D magnetic materials into several categories for reaching out each and every class of materials. Additionally, this review summarizes recent advancements of some pioneer 2D magnetodielectric materials. Last but not the least, the current review provides possible research directions for enhancing magnetodielectric coupling in those and mentions the possibilities for future developments.

cond-mat.mtrl-sci↗

arXiv2024

Tailored 1D/2D Van der Waals Heterostructures for Unified Analog and Digital Electronics

We report a sequential two-step vapor deposition process for growing mixed-dimensional van der Waals (vdW) materials, specifically Te nanowires (1D) and MoS$_2$ (2D), on a single SiO$_2$ wafer. Our growth technique offers a unique potential pathway to create large scale, high-quality, defect-free interfaces. The assembly of samples serves a twofold application: first, the as-prepared heterostructures (Te NW/MoS$_2$) provide insights into the atomically thin depletion region of a 1D/2D vdW diode, as revealed by electrical transport measurements and density functional theory-based quantum transport calculations. The charge transfer at the heterointerface is confirmed using Raman spectroscopy and Kelvin probe force microscopy (KPFM). We also observe modulation of the rectification ratio with varying applied gate voltage. Second, the non-hybrid regions on the substrate, consisting of the as-grown individual Te nanowires and MoS$_2$ microstructures, are utilized to fabricate separate p- and n-FETs, respectively. Furthermore, the ionic liquid gating helps to realize low-power CMOS inverter and all basic logic gate operations using a pair of n- and p- field-effect transistors (FETs) on Si/SiO$_2$ platform. This approach also demonstrates the potential for unifying diode and CMOS circuits on a single platform, opening opportunities for integrated analog and digital electronics.

cond-mat.mes-hall↗

arXiv2024

Electron-Magnon Coupling Mediated Magnetotransport in Antiferromagnetic van der Waals Heterostructure

Electron-magnon coupling reveals key insights into the interfacial properties between non-magnetic metals and magnetic insulators, influencing charge transport and spin dynamics. Here, we present temperature-dependent Raman spectroscopy and magneto-transport measurements of few-layer graphene (FLG)/antiferromagnetic FePS$_3$ heterostructures. The magnon mode in FePS$_3$ softens below 40 K, and effective magnon stiffness decreases with cooling. Magnetotransport measurements show that FLG exhibits negative magnetoresistance (MR) in the heterostructure at low fields ($\pm 0.2 \, \text{T}$), persisting up to 100 K; beyond this, MR transitions to positive. Notably, as layer thickness decreases, the coupling strength at the interface reduces, leading to a suppression of negative MR. Additionally, magnetodielectric measurements in the FLG/FePS$_3$/FLG heterostructure show an upturn at temperatures significantly below ($T_\text{N}$), suggesting a role for the magnon mode in capacitance, as indicated by hybridization between magnon and phonon bands in pristine FePS$_3$ \textit{via} magnetoelastic coupling.

cond-mat.mes-hall↗

arXiv2024

Pressure induced insulator-to-metal transition in few-layer FePS$_3$ at 1.5 GPa

In two-dimensional (2D) van der Waals (vdW) layered materials the application of pressure often induces a giant lattice collapse, which can subsequently drive an associated Mott transition. Here, we investigate room-temperature layer-dependent insulator-metal transition (IMT) and probable spin-crossover (SCO) in vdW magnet, FePS$_3$, under high-pressure using micro-Raman scattering. Experimentally obtained spectra, in agreement with the computed Raman modes, indicate evidence of IMT of FePS$_3$ started with a thickness-dependent critical pressure ($P_c$) which reduces to 1.5 GPa in trilayer flakes compared to 10.8 GPa for the bulk counterpart. Using a phenomenological model, we argue that strong structural anisotropy in few-layer flakes enhances the in-plane strain under applied pressure and is, therefore, ultimately responsible for reducing the critical pressure for the IMT with decreasing layer numbers. Reduction of the critical pressure for phase transition in vdW magnets to 1-2 GPa marks the possibility of using intercalated few-layers in the field-effect transistor device architecture, and thereby, avoiding the conventional use of the diamond anvil cell (DAC).

cond-mat.mes-hall↗

arXiv2024

In-Plane Anisotropy-Driven Directional Charge Transport in van der Waals p-n Heterojunction

Low-symmetry two-dimensional (2D) van der Waals (vdW) materials enable anisotropic charge transport, crucial for polarization-sensitive optoelectronics. In this study, a $p$-GeS/$n$-MoS$_2$ heterostructure diode is investigated, where the anisotropic band dispersion of GeS, revealed by angle-resolved photoemission spectroscopy (ARPES), governs directional charge flow. Angle-resolved Raman spectroscopy confirms the crystallographic orientation, and transport measurements in GeS field-effect transistors (FETs) show a mobility anisotropy of $\sim 3.4$. The heterojunction exhibits orientation-dependent diode characteristics, anti-ambipolar transport, and a type-II band alignment, leading to anisotropic optoelectronic response. These findings establish a pathway for utilizing electronic anisotropy in vdW heterostructures for energy-efficient rectification.

cond-mat.mes-hall↗

arXiv2024

Negative Capacitance for Stabilizing Logic State in Tunnel Field-Effect Transistor

The study investigates the influence of negative capacitance on the transfer characteristics of vdW FETs on the heterophase of CIPS ferroelectric. Notably, a less pronounced NC resulting from the spatial distribution of the ferroelectric and paraelectric phases plays crucial role in stabilizing of n-channel-conductance. This results into the emergence of a non-volatile logic state, between the two binary states of TFETs. Concerned study proposed NC-TFETs based on ferroionic crystals as promising devices for generating a stable logic state below Vth.

physics.app-ph↗

arXiv2022

Generation and propagation of solitary waves in nematic liquid crystals

Solitons in nematic liquid crystals offer intriguing opportunities for transport and sensing in microfluidic systems. Little is known about the elementary conditions that are needed to create solitons in nematic materials. In this work, theory, simulations and experiments are used to study the generation and propagation of solitary waves (or "solitons") in nematic liquid crystals upon the application of an alternating current (AC) electric field. We find that these solitary waves exhibit "butterfly"-like or "bullet"-like structures that travel in the direction perpendicular to the applied electric field. Such structures propagate over long distances without losing their initial shape. The theoretical model adopted here serves to identify some of the key requirements that are needed to generate solitons in the absence of electrostatic interactions. These include surface imperfections that introduce a twist in the director, unequal elastic constants, and negative anisotropic dielectric permittivity. The results of simulations are shown to be in good agreement with our own experimental observations, serving to establish the validity of the theoretical concepts advanced in this work.

cond-mat.soft↗

arXiv2022

A minimal model of solitons in nematic liquid crystals

Solitons in liquid crystals have generated considerable interest. Several hypotheses of varying complexity have been advanced to explain how they emerge, and a consensus has not emerged yet about the underlying forces responsible for their formation or their structure. In this work, we present a minimal model for soliton structures in achiral nematic liquid crystals, which reveals the key requirements needed to generate traveling solitons in the absence of added charges. These include a surface imperfection or inhomogeneity capable of producing a twist, flexoelectricity, dielectric contrast, and an applied AC electric field that can couple to the director's orientation. Our proposed model is based on a tensorial representation of a confined liquid crystal, and it predicts the formation of "butterfly" structures, quadrupolar in character, in regions of a slit channel where the director is twisted by the surface imperfection. As the applied electric field is increased, solitons (or "bullets") become detached from the wings of the butterfly, which then rapidly propagate throughout the system. The main observations that emerge from the model, including the formation and structure of butterflies, bullets, and stripes, as well as the role of surface imperfections and the strength of the applied field, are consistent with our own experimental findings presented here for nematic LCs confined between two chemically treated parallel plates.

cond-mat.soft↗

arXiv2015

Preliminary study of feasibility of an experiment looking for excited state double beta transitions in tin

An attempt to study the feasibility of a new experiment to search for double beta decay in $^{112}$Sn and $^{124}$Sn was carried out by using ultra-low background HPGe detector (244 cm$^{3}$) inside the Gran Sasso National Laboratory (LNGS) of the INFN (Italy). A small sample of natural Sn was examined for 2367.5 h. The radioactive contamination of the sample has been estimated. The data has also been considered to calculate the present sensitivity for the proposed search; half-life limits $\sim$ $10^{17} - 10^{18}$ years for $β^{+}$EC and EC-EC processes in $^{112}$Sn and $\sim$ $10^{18}$ years for $β^{-}β^{-}$ transition in $^{124}$Sn were measured. In the last section of the paper the enhancement of the sensitivity for a proposed experiment with larger mass to reach theoretically estimated values of half-lives is discussed.

nucl-ex↗