arXiv subjects

Prasenjit Sen

Publications and source records attributed to Prasenjit Sen.

At least 19 recordsLinked to original sources

arXiv2026

Property-Guided Diffusion for Inverse Design of Crystalline Materials

Diffusion-based generative models with property guidance have emerged as a promising paradigm for inverse materials design by enabling the generation of crystalline materials with user-specified target properties. However, despite recent advances, the effectiveness of property guidance, its influence on crystallographic symmetry, and the physical viability of generated materials remain poorly understood. To address these questions, we develop a property-guided framework based on the lightweight diffusion model DiffCrysGen using parameter-efficient adapter fine-tuning and classifier-free guidance (CFG). The resulting framework enables efficient multi-property crystal generation while preserving the knowledge learned during unconditional pre-training. Using formation energy together with saturation magnetization and Vickers hardness as representative inverse-design tasks, we systematically investigate the influence of CFG across a broad range of guidance strengths. Increasing the guidance scale progressively steers the generated property distributions toward the prescribed targets while reducing the fraction of lowest-symmetry ($P1$) structures and increasing the proportion of higher-symmetry structures. To evaluate physical viability, generated structures are geometrically prescreened and subsequently validated using a machine-learning interatomic potential (MLIP)-based workflow comprising structural relaxation and thermodynamic, dynamical, and property-specific analyses. The framework identifies thermodynamically and dynamically stable magnetic and mechanically hard materials with overall success rates of 12.3\% and 3.9\%, respectively. These results establish property-guided DiffCrysGen as an efficient framework for inverse materials design while providing new insights into the role of classifier-free guidance in crystal generation.

cond-mat.mtrl-sci↗

arXiv2025

DiffCrysGen: A Generative Diffusion Model for Accelerated Design of Inorganic Crystalline Materials

Efficient exploration of the vast chemical space is a fundamental challenge in materials design and discovery, particularly for designing functional inorganic crystalline materials with targeted properties. Diffusion-based generative models have emerged as a powerful route, but most existing approaches require domain-specific constraints and separate diffusion processes for atom types, atomic positions, and lattice parameters, adding complexity and limiting efficiency. Here, we present DiffCrysGen, a fully data-driven, score-based diffusion model that generates complete crystal structures in a single, end-to-end diffusion process. This unified framework simplifies the model architecture and accelerates sampling by two to three orders of magnitude compared to existing methods without compromising chemical and structural diversity of the generated materials. In order to demonstrate the efficacy of DiffCrysGen in generating valid and useful materials, using density functional theory (DFT), we validate a number of newly generated rare earth-free magnetic materials that are energetically and dynamically stable, and are potentially synthesizable. These include ferromagnets with high saturation magnetization and large magnetocrystalline anisotropy, as also metallic antiferromagnets. These results establish DiffCrysGen as a general platform for accelerated design of functional materials.

cond-mat.mtrl-sci↗

arXiv2025

Interfacial Control of both Magnetism and Polarization in a van der Waals Ferromagnet/Ferroelectric Heterostructure

Two-dimensional multiferroic van der Waals heterostructures provide a promising platform for the simultaneous control of distinct ferroic orders, with potential applications in magnetoelectric devices and spintronics. The practical implementation of such technologies requires 2D magnets with high Curie temperatures and strong perpendicular magnetic anisotropy (PMA). Here, based on first-principles calculations, we propose a multiferroic heterostructure composed of the room-temperature ferromagnet $\text{Fe}_3\text{Ga}\text{Te}_2$ and the ferroelectric $\text{In}_2\text{Se}_3$. We show that intercalation of Fe atoms into the van der Waals gap of the $\text{Fe}_3\text{Ga}\text{Te}_2$/$\text{In}_2\text{Se}_3$ heterostructure enhances PMA by nearly an order of magnitude relative to the pristine $\text{Fe}_3\text{Ga}\text{Te}_2$ monolayer, while simultaneously allowing electric polarization to be modulated through interfacial charge redistribution. The enhancement of PMA arises from interfacial hybridization that modifies the spin-orbit coupling of Fe $d$-orbitals. Our results demonstrate an effective pathway to engineer magnetoelectric coupling in two-dimensional multiferroic heterostructures and pave the way toward energy-efficient spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

DiffCrysGen: A Score-Based Diffusion Model for Design of Diverse Inorganic Crystalline Materials

Crystal structure generation is a foundational challenge in materials discovery, particularly in designing functional inorganic crystalline materials with desired properties. Most existing diffusion-based generative models for crystals rely on complex, hand-crafted priors and modular architectures to separately model atom types, atomic positions, and lattice parameters. These methods often require customized diffusion processes and conditional denoising, which can introduce additional model complexities and inconsistencies. Here we introduce DiffCrysGen, a fully data-driven, score-based diffusion model that jointly learns the distribution of all structural components in crystalline materials. With crystal structure representation as unified 2D matrices, DiffCrysGen bypasses the need for task-specific priors or decoupled modules, enabling end-to-end generation of atom types, fractional coordinates, and lattice parameters within a single framework. Our model learns crystallographic symmetry and chemical validity directly from large-scale datasets, allowing it to scale to complex materials discovery tasks. As a demonstration, we applied DiffCrysGen to the design of rare-earth-free magnetic materials with high saturation magnetization, showing its effectiveness in generating stable, diverse, and property-aligned candidates for sustainable magnet applications.

cond-mat.mtrl-sci↗

arXiv2024

Dimension-Dependent Critical Scaling Analysis and Emergent Competing Interaction Scales in a 2D Van der Waals magnet Cr$_{2}$Ge$_{2}$Te$_{6}$

We investigate thickness-dependent transformation from a paramagnetic to ferromagnetic phase in Cr$_{2}$Ge$_{2}$Te$_{6}$ (CGT) in bulk and few-layer flake forms. 2D Ising-like critical transition in bulk CGT occurs at $T_{c}$ = 67 K with out-of-plane magnetic anisotropy. Few-layer CGT on hBN/SiO$_{2}$/Si substrate displays the same $T_{c}$ but also exhibits a new critical transition at $T^{\prime}_c$ = 14.2 K. Here, critical scaling analysis reveals the critical exponents differ significantly from those in bulk and do not align with the known universality classes. Our Density Functional Theory (DFT) and classical calculations indicate competition between magnetocrystalline and dipolar anisotropy emerges with reduced dimensions. The observed behavior is due to minor structural distortions in low dimensional CGT, which modify the balance between spin-orbit coupling, exchange interactions and dipolar anisotropy. This triggers a critical crossover at $T^{\prime}_c$. Our study shows the emergence of a complex interplay of short- and long-range interactions below $T^{\prime}_c$ as CGT approaches the 2D limit.

cond-mat.mtrl-sci↗

arXiv2023

MagGen: A graph aided deep generative model for inverse design of stable, permanent magnets

A significant development towards inverse design of materials with well-defined target properties is reported. A deep generative model based on variational autoencoder (VAE), conditioned simultaneously by two target properties, is developed to inverse design stable magnetic materials. Structure of the physics informed, property embedded latent space of the model is analyzed using graph theory, based on the idea of similarity index. The graph idea is shown to be useful for generating new materials that are likely to satisfy target properties. An impressive ~96% of the generated materials is found to satisfy the target properties as per predictions from the target learning branches. This is a huge improvement over approaches that do not condition the VAE latent space by target properties, or do not consider connectivity of the parent materials perturbing which the new materials are generated. In such models, the fraction of materials satisfying targets can be as low as ~5%. This impressive feat is achieved using a simple real-space only representation called Invertible Real-space Crystallographic Representation (IRCR), that can be directly read from material cif files. Model predictions are finally validated by performing DFT calculations on a randomly chosen subset of materials. Performance of the present model using IRCR is comparable or superior to that of the models reported earlier. This model for magnetic material generation, MagGen, is applied to the problem of designing rare earth free permanent magnets with promising results.

cond-mat.mtrl-sci↗

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↗

arXiv2017

CrCTe$_3$: Computational design of a robust two-dimensional anti-ferromagnetic semiconductor

Using density functional theory calculations we establish the hitherto unknown compound CrCTe$_3$ to be a stable anti-ferromagnetic semiconductor in the R$\bar{3}$ crystal structure with an indirect fundamental gap . Successive layers in the bulk compound are weakly bound by van der Waals forces so that individual layers can be easily exfoliated. A monolayer of CrCTe$_3$ is also an anti-ferromagnetic semiconductor. The monolayer is structurally stable over a large range of compressive and tensile strains, and the anti-ferromagnetic state is robust over this strain range. Band gap of the monolayer can be tuned by as much as 50% by applying strain in this range.

cond-mat.mtrl-sci↗

arXiv2014

Peierls transition and edge reconstruction in phosphorene nanoribbons

Atomic and electronic structures of phosphorene nanoribbons are studied within density functional theory. These novel materials present different physical phenomena expected in two very different physical systems: one dimensional metallic chains and semiconductor surfaces. While `rugged' nanoribbons are semiconducting in their layer-terminnated structures, pure `linear' and `zigzag' nanoribbons are metallic due to metallic edge states. Linear nanoribbons undergo edge reconstruction and zigzag nanoribbons beyond a certain width undergo Peierls transition leading to opening of a band gap in the electronic structure and lowering of total energy. Mixed nanoribbons with linear and zigzag edges on the two sides turn out to be a curious case that display both edge reconstruction and Peierls transition simultaneously. Most phosphoeren nanoribbons turn out to be semiconductors having important implications for their application.

cond-mat.mtrl-sci↗

arXiv2013

Designing Fe Nanostructures at Graphene/h-BN Interfaces

Tailor-made magnetic nanostructures offer a variety of functionalities useful for technological applications. In this work, we explore the possibilities of realizing Fe nanostructures at the interfaces of 2D graphene and h-BN by ab initio density functional calculations. With the aid of ab initio Born-Oppenheimer molecular dynamics simulations and diffusion barriers calculated by nudged elastic band method, we find that (i) diffusion barriers of Fe on BN are much smaller than those on graphene, (ii) the Fe adatoms form clusters within a short time interval (~2.1 ps) and (iii) Fe clusters diffuse easily across the C-N interface but become immobile at the C-B interface. The calculated magnetic exchange coupling between Fe clusters at C-B interfaces varies non-monotonically as a function of the width of BN separating the graphene parts. One may envisage design of magnetic nanostructures at the C-B interface of 2D graphene/h-BN hybrids to realize interesting applications related to spintronics.

cond-mat.mtrl-sci↗

arXiv2008

First-principles study of TMNan (TM= Cr, Mn, Fe, Co, Ni; n = 4-7) clusters

Geometry, electronic structure, and magnetic properties of TMNan (TM=Cr-Ni; n = 4-7) clusters are studied within a gradient corrected density functional theory (DFT) framework. Two complementary approaches, the first adapted to all-electron calculations on free clusters, and the second been on plane wave projector augmented wave (PAW) method within a supercell approach are used. Except for NiNan, the clusters in this series are found to retain the atomic moments of the TM atoms, and the magnetic moment presented an odd-even oscillation with respect to the number of Na atoms. The origin of these odd-even oscillations is explained from the nature of chemical bonding in these clusters. Differences and similarities between the chemical bonding and the magnetic properties of these clusters and the TMNan (TM = Sc, V and Ti; n = 4-6) clusters on one hand, and TM-doped Au and Ag clusters on the other hand, are discussed.

cond-mat.mtrl-sci↗

arXiv2007

On the question of ferromagnetism in alkali metal thin films

Electronic and magnetic structure of $(100)$ films of K and Cs are calculated within the plane-wave projector augmented wave (PAW) formalism of the density functional theory (DFT) using both local spin density approximation (LSDA) and the PW91 generalized gradient approximation (GGA). Only a 6 layer Cs film is found to have a ferromagnetic (FM) state which is degenerate with a paramagnetic (PM) state within the accuracy of these calculations. This is at variance with the results obtained from a finite thickness uniform jellium model (UJM). Implications of these results for the experiments on transition metal doped alkali metal thin films and bulk hosts are discussed.

cond-mat.mtrl-sci↗

arXiv2007

First principles study of Sc, Ti and V doped Na$_n$(n =4, 5, 6) clusters: Enhanced magnetic moments

Theoretical studies on the geometry, electronic structure and spin multiplicity of Sc, Ti and V doped Na$_n$ (n = 4, 5, 6) clusters have been carried out within a gradient corrected density functional approach. Two complementary approaches including all-electron calculations on free clusters, and supercell calculations using planewave pseudopotential and projector augmented wave formalisms have been carried out. It is shown that spin magnetic moments of the transition metal atoms, the magnitude of host polarization, and the sign of the host polarization all change with the number of alkali atoms. In particular the transition metal atoms are shown to attain spin moments that are higher than their atomic values. The role of hybridization between the transition atom d-states and the alkali $sp$-states is highlighted to account for the evolutions in the spin moments and host polarization.

cond-mat.mtrl-sci↗

arXiv2006

Peierls Instability and Electron-Phonon Coupling in a One-dimensional Sodium Wire

We have studied Peierls instability in an atomically thin wire of sodium atoms using first-principles density-functional methods. A Na wire has a stable uniform linear structure over a range of inter-atomic distances. At smaller inter-atomic distances it develops a zigzag distortion. At larger inter-atomic distances, just before breaking, a Na wire undergoes a very weak Peierls dimerization. This behavior of a Na wire is understood in terms of its electron-phonon coupling properties.

cond-mat.mtrl-sci↗

arXiv2006

Structural studies of phosphorus induced dimers on Si(001)

Renewed focus on the P-Si system due to its potential application in quantum computing and self-directed growth of molecular wires, has led us to study structural changes induced by P upon placement on Si(001)-$p(2\times 1)$. Using first-principles density functional theory (DFT) based pseudopotential method, we have performed calculations for P-Si(001) system, starting from an isolated P atom on the surface, and systematically increasing the coverage up to a full monolayer. An isolated P atom can favorably be placed on an {\bf M} site between two atoms of adjacent Si dimers belonging to the same Si dimer row. But being incorporated in the surface is even more energetically beneficial due to the participation of the {\bf M} site as a receptor for the ejected Si. Our calculations show that up to 1/8 monolayer coverage, hetero-dimer structure resulting from replacement of surface Si atoms with P is energetically favorable. Recently observed zig-zag features in STM are found to be consistent with this replacement process. As coverage increases, the hetero-dimers give way to P-P ortho-dimers on the Si dimer rows. This behavior is similar to that of Si-Si d-dimers but are to be contrasted with the Al-Al dimers, which are found between adjacent Si dimers rows and in a para-dimer arrangement. Unlike Al-Si system P-Si does not show any para to ortho transition. For both systems, the surface reconstruction is lifted at about one monolayer coverage. These calculations help us in understanding the experimental data obtained using scanning tunneling microscope.

cond-mat.mtrl-sci↗

arXiv2005

Electronic Structure and Origin of Ferromagnetism in CaB$_6$

Electronic structure calculations using quantum Monte Carlo (QMC) methods conclusively show that pure CaB$_6$ is a narrow-gap semiconductor with an $X$-point gap $\sim 1.3$ eV. This should put to rest controversies as to whether the compound is a semimetal or a semiconductor, as also theories suggesting it to be an excitonic insulator. Our extensive Hartree-Fock (HF) and density functional theory (DFT) calculations support the view that the hexaboride ferromagnetism can be induced by contamination with magnetic element atoms like Fe and that presence of La is not essential for this. La impurity, however, gives rise to a metallic state with a small electron-like Fermi surface as seen in ARPES experiments.

cond-mat.mtrl-sci↗

arXiv2003

Electronic Structure of Te and As Covered Si(211)

Electronic and atomic structures of the clean, and As and Te covered Si(211) surface are studied using pseudopotential density functional method. The clean surface is found to have (2 X 1) and rebonded (1 X 1) reconstructions as stable surface structures, but no π-bonded chain reconstruction. Binding energies of As and Te adatoms at a number of symmetry sites on the ideal and (2 X 1) reconstructed surfaces have been calculated because of their importance in the epitaxial growth of CdTe and other materials on the Si(211) surface. The special symmetry sites on these surfaces having the highest binding energies for isolated As and Te adatoms are identified. But more significantly, several sites are found to be nearly degenerate in binding energy values. This has important consequences for epitaxial growth processes. Optimal structures calculated for 0.5 ML of As and Te coverage reveal that the As adatoms dimerize on the surface while the Te adatoms do not. However, both As and Te covered surfaces are found to be metallic in nature.

cond-mat.mtrl-sci↗