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Nirmal Roy

Publications and source records attributed to Nirmal Roy.

15 recordsLinked to original sources
arXiv2026

Chiral, Electronically Decoupled Layers of 1T'-WS2 Topological Insulator via Neutral-Molecule Intercalation

Monolayer 1T'-WS2 is predicted to be a two-dimensional topological insulator, but its intrinsic electronic properties are masked by strong interlayer coupling in its metallic and superconducting bulk parent phase, 2M-WS2. Isolating monolayers by mechanical exfoliation is also hindered by this coupling, preventing experimental examination of monolayer properties. Here we show that 2M-WS2 undergoes amine intercalation through a simple wet-chemical reaction, yielding superlattices in which the 1T' layers are structurally preserved but electronically decoupled by neutral molecular spacers. Intercalation expands the interlayer spacing from 0.5 to 1-4 nm and reconstructs the stacking while preserving the intralayer 1T' framework. Controlled (de)intercalation reversibly switches the system between a superconducting metal and an insulator with an activation gap matching that of the isolated monolayer. Density functional theory indicates that the electronically decoupled layers retain the nontrivial Z2 topology of the monolayer. Chiral amine intercalation further induces chiroptical activity in WS2 electronic transitions. Overall, the successful intercalation challenges the long-held view that group VIB dichalcogenides are inert toward neutral-molecule intercalation and demonstrates molecular intercalation as a general chemical route for realizing monolayer-like topological-insulator physics and enabling chiral van der Waals superlattices in bulk single crystals.

cond-mat.mtrl-sci↗
arXiv2026

Polytype-Dependent Upconversion Photoluminescence in 3R-MoS2

Ferroelectric van der Waals materials offer switchable polarization states, yet optical readout of their stacking configurations remains challenging. Here, building on the resonant exciton-exciton annihilation (EEA) mechanism recently identified in 2H-phase TMDs, we report the first observation of upconversion photoluminescence (UPL) in rhombohedral MoS$_2$ and demonstrate that this many-body process is strongly polytype-dependent. Using low-temperature spectroscopy, we observe anti-Stokes emission with superlinear power dependence characteristic of EEA. Beyond serving as an accurate layer-number sensor due to discrete bandgap variations, UPL provides a sensitive probe of stacking order across thicknesses. The two neutral trilayer polytypes, which remain indistinguishable by surface potential measurements and second harmonic generation, exhibit markedly different UPL intensities. This sensitivity persists in thicker samples where multiple configurations coexist. First-principles calculations suggest that the intensity contrast originates primarily from the layer confinement of the annihilating excitons, while energy matching to the $\Gamma$ final-state manifold provides additional intensity selectivity. Power-dependent spectroscopy further disentangles two distinct annihilation channels originating from different dark exciton valleys, identified through their contrasting intensity scaling and opposite density-induced energy shifts. Crucially, the annihilation process doubles the energy separation of nearly degenerate dark excitons while converting their weak emission into bright signal, providing experimental access to valley-specific responses that are obscured in direct dark-exciton spectroscopy. Our findings demonstrate that ferroelectric configurations provide a new degree of freedom for controlling nonlinear optical processes.

cond-mat.mtrl-sci↗
arXiv2025

Switching Graphitic Polytypes in Elastically Coupled Islands

Van der Waals polytypes are commensurate configurations of two-dimensional layers with discrete crystalline symmetries and distinct stacking-dependent properties. In graphitic polytypes, the different stacking arrangements of graphene sheets exhibit rich electronic phases, such as intrinsic electric polarizations, orbital magnetizations, superconductivity, and anomalous fractional Hall states. Switching between these metastable periodic configurations by controlling interlayer shifts unlocks intriguing multiferroic responses. Here, we report super-lubricant arrays of polytypes (SLAP) devices, with nanometer-scale islands of Bernal polytypes that switch into Rhombohedral crystals and vice versa under a shear force as low as 6 nano-Newtons. We assemble these four-layer SLAP structures by aligning a pair of graphene bilayers above and under circular cavities in a misaligned spacer layer. Using local current measurements, we detect the shifts between the active bilayers and reveal long-range elastic relaxations outside the cavities that enable efficient nucleation and spontaneous sliding of stacking dislocation inside the islands. We demonstrate configurable, deterministic, and robust polytype switching by confining these boundary strips in narrow cavity channels that connect the islands. Such controlled switching between elastically-coupled single-crystalline islands is appealing for novel multiferroic SlideTronic applications.

cond-mat.mes-hall↗
arXiv2024

Learning When to Retrieve, What to Rewrite, and How to Respond in Conversational QA

Augmenting Large Language Models (LLMs) with information retrieval capabilities (i.e., Retrieval-Augmented Generation (RAG)) has proven beneficial for knowledge-intensive tasks. However, understanding users' contextual search intent when generating responses is an understudied topic for conversational question answering (QA). This conversational extension leads to additional concerns when compared to single-turn QA as it is more challenging for systems to comprehend conversational context and manage retrieved passages over multiple turns. In this work, we propose a method for enabling LLMs to decide when to retrieve in RAG settings given a conversational context. When retrieval is deemed necessary, the LLM then rewrites the conversation for passage retrieval and judges the relevance of returned passages before response generation. Operationally, we build on the single-turn SELF-RAG framework (Asai et al., 2023) and propose SELF-multi-RAG for conversational settings. SELF-multi-RAG demonstrates improved capabilities over single-turn variants with respect to retrieving relevant passages (by using summarized conversational context) and assessing the quality of generated responses. Experiments on three conversational QA datasets validate the enhanced response generation capabilities of SELF-multi-RAG, with improvements of ~13% measured by human annotation.

cs.CL↗
arXiv2024

Switchable Crystalline Islands in Super Lubricant Arrays

Expanding the performance of field effect devices is a key challenge of the ever-growing chip industry at the core of current technologies. A highly desired nonvolatile response in tiny multiferroic transistors is expected by electric field control of atomic movements rather than the typical electronic redistribution. Recently, such field effect control of structural transitions was established in commensurate stacking configurations of honeycomb van der Waals (vdW) polytypes by sliding narrow boundary dislocations between oppositely polarized domains. The interfacial ferroelectric response, however, relied on preexisting boundary strips between relatively large micron-scale domains, severely limiting practical implementations. Here, we report the robust switching of single-domain polytypes in nm-scale islands embedded in super lubricant vdW arrays. We etch cavities into a thin layered spacer and then encapsulate it with parallel functional flakes. The incommensurate flakes above and under the spacer sag and touch at each cavity to form uniform crystalline islands free from interlayer deformations. By imaging the polytypes' ferroelectric response, we observe reversible nucleation and annihilation of boundary strips and geometry-adaptable hysteresis loops. Using mechanical stress, we accurately position the boundary strip, modify the interlayer twist angle, and nucleate intermediate polar domain patterns. By precisely designing the size, shape, symmetry, and distribution of the islands in these Super Lubricant Arrays of Polytype (SLAP), we envision numerous device functionalities and SlideTronics applications. These range from ultra-sensitive detectors of atomic-scale shifts to nonvolatile multi-ferroic tunneling transistors with tunable coercive switching fields, and even elastically-coupled memory cells for neuromorphic architectures.

cond-mat.mtrl-sci↗
arXiv2024

Is Interpretable Machine Learning Effective at Feature Selection for Neural Learning-to-Rank?

Neural ranking models have become increasingly popular for real-world search and recommendation systems in recent years. Unlike their tree-based counterparts, neural models are much less interpretable. That is, it is very difficult to understand their inner workings and answer questions like how do they make their ranking decisions? or what document features do they find important? This is particularly disadvantageous since interpretability is highly important for real-world systems. In this work, we explore feature selection for neural learning-to-rank (LTR). In particular, we investigate six widely-used methods from the field of interpretable machine learning (ML) and introduce our own modification, to select the input features that are most important to the ranking behavior. To understand whether these methods are useful for practitioners, we further study whether they contribute to efficiency enhancement. Our experimental results reveal a large feature redundancy in several LTR benchmarks: the local selection method TabNet can achieve optimal ranking performance with less than 10 features; the global methods, particularly our G-L2X, require slightly more selected features, but exhibit higher potential in improving efficiency. We hope that our analysis of these feature selection methods will bring the fields of interpretable ML and LTR closer together.

cs.IR↗
arXiv2024

Impact of Co2C Nanoparticles on Enhancing the Critical Current Density of Bi-2223 Superconductor

We have investigated the superconducting properties of nanocomposite pellets made from Bi-2223 and Co2C powders. There is loss of superconducting fraction in the nanocomposites, but the retained superconducting fraction exhibits robust bulk superconducting properties, having Tc ~ 109 K which was found to be comparable to that of the pure Bi-2223 pellet. We found that the composites net magnetization response is a superposition of ferromagnetic and superconducting fractions contributions. We also found the surviving superconducting fraction exhibits a robust Meissner response. In the nanocomposite the irreversibility field of the superconducting fraction at 77 K is found to increase by almost three times compared to the pristine material, thereby showing strong vortex pinning features. We also find a broadened magnetic field regime over which we observe a single vortex pinning regime sustained in the nanocomposite. The critical current density, Jc, of the nanocomposite was found to be approximately five times higher than that of the pristine Bi-2223 pellet at low T. In fact, the enhancement in Jc is most significant in the high T regime, where at temperatures close to Tc in the nanocomposite we see almost two orders of magnitude increase of Jc compared to the pristine Bi-2223 pellet. The larger sized agglomeration of magnetic nanoparticles of Co2C leads to loss of superconductivity in the nanocomposite. However, there are also unagglomerated Co2C nanoparticles distributed uniformly throughout the nanocomposite which acts as efficient pinning centres allowing for collective vortex pinning centres to be retained, even upto temperatures near Tc, and these nanoparticles also do not compromise the bulk Tc of the superconducting fraction. Our study shows that these nanocomposites exhibit enhanced Jc especially in the high T regime are potentially useful for high current applications.

cond-mat.supr-con↗
arXiv2023

Probing the strongly correlated magnetic state of Co$_2$C nanoparticles at low temperatures using $μ$SR

Co$_2$C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. A recent study by Nirmal Roy et al. [1] showed that a collection of Co$_2$C NPs exhibit an exchange bias (EB) effect below T$_{EB}$ = 50 K and also a spin glass (SG) state below T$_{SG}$ = 5 K. We use magnetic, electrical transport, specific heat, and muon spin rotation ($μ$SR) measurements to explore further the magnetic properties of these NPs. We uncover the onset of Kondo localization at Kondo temperature T$_K$ (= 40.1 K), near the onset of EB effect. A crossover from the Kondo-screened scenario to an RKKY interaction-dominated regime is also observed for T < T$_K$. Specific heat measurements confirm Kondo localization and heavy fermionic nature in Co$_2$C at low T. At low T, zero field $μ$SR spectra reveal a dominant magnetically disordered fraction with slow relaxation and a smaller fraction with short-range order exhibiting fast relaxation, with no evidence of long-range magnetic order. We observe an increase in this fast relaxation rate between T$_{EB}$ and T$_{SG}$, suggesting a slowing down of the fluctuating local magnetic environment around muons. Transverse field $μ$SR spectra show the emergence of a stable, multi-peaked local magnetic field distribution below T$_{EB}$. Longitudinal field $μ$SR spectra shows distinct changes in the dynamics of fluctuations suggesting the presence of a frozen glassy like state below 6 K. Our results suggest that below T$_{EB}$, Co$_2$C NPs pellet develops a magnetic interface, separating disordered and short-range order fractions. The Exchange interaction that sets in below T$_{EB}$ at the interface couples them and suppresses the fluctuations. With the suppression of magnetic fluctuations below T$_{EB}$, strong correlation effects in the electronic state of Co$_2$C lead to Kondo localization.

cond-mat.str-el↗
arXiv2023

Spontaneous Electric Polarization in Graphene Polytypes

A crystalline solid is a periodic sequence of identical cells, each containing one or more atoms. If the constituting unit cell is not centrosymmetric, charge may distribute unevenly between the atoms, resulting in internal electric polarization. This effect serves as the basis for numerous ferroelectric, piezoelectric, and pyroelectric phenomena. In nearly all polar materials, including multilayered van der Waals stacks that were recently found to exhibit interfacial polarization, inversion symmetry is broken by having two or more atomic species within the unit cell. Here, we show that even elemental crystals, consisting of one type of atom, and composed of non-polar centrosymmetric layers, exhibit electric polarization if arranged in an appropriate three-dimensional architecture. This concept is demonstrated here for inversion and mirror asymmetric mixed-stacking tetra-layer polytypes of non-polar graphene sheets. Furthermore, we find that the room temperature out-of-plane electric polarization increases with external electrostatic doping, rather than decreases owing to screening. Using first-principles calculations, as well as tight-binding modeling, we unveil the origin of polytype-induced polarization and its dependence on doping. Extension of this idea to graphene multilayers suggests that solely by lateral shifts of constituent monolayers one can obtain multiple meta-stable interlayer stacking sequences that may allow for even larger electrical polarization.

cond-mat.mtrl-sci↗
arXiv2023

Hear Me Out: A Study on the Use of the Voice Modality for Crowdsourced Relevance Assessments

The creation of relevance assessments by human assessors (often nowadays crowdworkers) is a vital step when building IR test collections. Prior works have investigated assessor quality & behaviour, though into the impact of a document's presentation modality on assessor efficiency and effectiveness. Given the rise of voice-based interfaces, we investigate whether it is feasible for assessors to judge the relevance of text documents via a voice-based interface. We ran a user study (n = 49) on a crowdsourcing platform where participants judged the relevance of short and long documents sampled from the TREC Deep Learning corpus-presented to them either in the text or voice modality. We found that: (i) participants are equally accurate in their judgements across both the text and voice modality; (ii) with increased document length it takes participants significantly longer (for documents of length > 120 words it takes almost twice as much time) to make relevance judgements in the voice condition; and (iii) the ability of assessors to ignore stimuli that are not relevant (i.e., inhibition) impacts the assessment quality in the voice modality-assessors with higher inhibition are significantly more accurate than those with lower inhibition. Our results indicate that we can reliably leverage the voice modality as a means to effectively collect relevance labels from crowdworkers.

cs.IR↗
arXiv2022

Users and Contemporary SERPs: A (Re-)Investigation Examining User Interactions and Experiences

The Search Engine Results Page (SERP) has evolved significantly over the last two decades, moving away from the simple ten blue links paradigm to considerably more complex presentations that contain results from multiple verticals and granularities of textual information. Prior works have investigated how user interactions on the SERP are influenced by the presence or absence of heterogeneous content (e.g., images, videos, or news content), the layout of the SERP (list vs. grid layout), and task complexity. In this paper, we reproduce the user studies conducted in prior works-specifically those of Arguello et al. [4] and Siu and Chaparro [29]-to explore to what extent the findings from research conducted five to ten years ago still hold today as the average web user has become accustomed to SERPs with ever-increasing presentational complexity. To this end, we designed and ran a user study with four different SERP interfaces: (i) a heterogeneous grid; (ii) a heterogeneous list; (iii) a simple grid; and (iv) a simple list. We collected the interactions of 41 study participants over 12 search tasks for our analyses. We observed that SERP types and task complexity affect user interactions with search results. We also find evidence to support most (6 out of 8) observations from [4 , 29] indicating that user interactions with different interfaces and to solve tasks of different complexity have remained mostly similar over time.

cs.IR↗
arXiv2021

Searching to Learn with Instructional Scaffolding

Search engines are considered the primary tool to assist and empower learners in finding information relevant to their learning goals-be it learning something new, improving their existing skills, or just fulfilling a curiosity. While several approaches for improving search engines for the learning scenario have been proposed, instructional scaffolding has not been studied in the context of search as learning, despite being shown to be effective for improving learning in both digital and traditional learning contexts. When scaffolding is employed, instructors provide learners with support throughout their autonomous learning process. We hypothesize that the usage of scaffolding techniques within a search system can be an effective way to help learners achieve their learning objectives whilst searching. As such, this paper investigates the incorporation of scaffolding into a search system employing three different strategies (as well as a control condition): (I) AQE_{SC}, the automatic expansion of user queries with relevant subtopics; (ii) CURATED_{SC}, the presenting of a manually curated static list of relevant subtopics on the search engine result page; and (iii) FEEDBACK_{SC}, which projects real-time feedback about a user's exploration of the topic space on top of the CURATED_{SC} visualization. To investigate the effectiveness of these approaches with respect to human learning, we conduct a user study (N=126) where participants were tasked with searching and learning about topics such as `genetically modified organisms'. We find that (I) the introduction of the proposed scaffolding methods does not significantly improve learning gains. However, (ii) it does significantly impact search behavior. Furthermore, (iii) immediate feedback of the participants' learning leads to undesirable user behavior, with participants focusing on the feedback gauges instead of learning.

cs.IR↗
arXiv2020

Exploring the non-equilibrium fluctuation relation for quantum mechanical tunneling of electrons across a modulating barrier

We experimentally explore the phenomenon of electron tunneling across a modulated tunneling barrier which is created between an STM tip and an Au film deposited on a vibrating piezo surface. Measurements of the time series of the quantum mechanical tunneling current across the modulating barrier show large fluctuations. Analysis of the average work done in establishing tunneling current in finite time interval shows a distribution of both positive and negative work events. The negative work events suggest tunneling against the bias voltage direction. We show that these distributions obey the Gallavotti Cohen Non-equilibrium Fluctuation Relations (GC-NEFR) valid for systems driven through a dissipating environment. Typically, while the GC-NEFR has been shown for non -equilibrium classical systems we show its validity for the quantum mechanical tunneling process too. The GC-NEFR analysis also gives us a way to measure the dissipation present in this quantum tunneling system. We propose the modulated barrier behaves like a lossy scattering medium for the tunneling electrons resulting in a tendency to randomize of the tunneling process.

cond-mat.mes-hall↗
arXiv2020

The Exchange Bias effect in pure Co2C nanoparticles

We study the low temperature magnetic properties of nanoparticles of pure transition metal carbide, viz., Co2C, with an average particle diameter of $40 \pm 10$ nm. These Co2C nanoparticles are ferromagnetic (FM) up to room temperature with blocking temperatures above room temperature. The coercive field shows abrupt deviation from the Kneller law below 50 K. In this low temperature regime the magnetization hysteresis loop shows shifts due to exchange bias (EB) effect, with an exchange field of ~ 250 Oe. Analysis of training of the EB effect and ac and dc magnetic measurements suggest that EB arises in the nanoparticles due to a core-shell structure with a FM core and a cluster glass shell. The shell contains uncompensated spins, some of which are freely rotatable while some are frozen. DFT calculations of structural and magnetic properties of small Co2C clusters of diameter of few Angstroms confirm a core-shell structure, where the structurally ordered core has uniform magnetic moment distribution and the structurally disordered shell has non-uniform moment distribution.

cond-mat.str-el↗
arXiv2019

Localized spin waves at low temperatures in a Cobalt Carbide nanocomposite

We study magnetic, transport and thermal properties of Cobalt carbide nanocomposite with a mixture of Co2C and Co3C phases in 1:1 ratio, with an average particle diameter of 40$\pm 15$ nm. We show that the behavior of the nanocomposite is completely different from that of either Co3C or Co2C. We observed that with decreasing temperature the saturation magnetization MS(T) increases, however, below 100 K, there is a steep rise. A detail analysis shows the increase in MS(T) down to 100 K is explained via the surface spin freezing model. However, below 100 K the steep increase in MS(T) is explained by a finite size effect related to a confinement of spin waves within the nano particles. The measurement of heat capacity shows broad peak at 100 K along with presence of another anomaly at a lower temperature 43 K(=Tex). Resistance measurement in the nanocomposite shows metallic behavior at high T with an unusual anomaly appearing at Tex, which is near the T regime where MS(T) begins to increase steeply. A measurement of the temperature gradients across the sample thickness indicates an abrupt change in thermal conductivity at Tex which suggests a phase transition at Tex. Our results are explained in terms of a transformation from a magnetically coupled state with a continuous spectrum of spin waves into a magnetically decoupled state below 100 K with confined spin waves.

cond-mat.str-el↗