arXiv subjects

Monirul Shaikh

Publications and source records attributed to Monirul Shaikh.

8 recordsLinked to original sources

arXiv2026

Polar Charge-Ordered States in BiFeO$_3$/CaFeO$_3$ Superlattice

Oxide superlattices represent a potent avenue for tailoring emergent electronic phases through sophisticated interfacial charge transfer and dynamic lattice distortions. This study systematically investigates the structural and electronic attributes of the BiFeO$_3$/CaFeO$_3$ superlattice, leveraging a comprehensive approach that integrates first-principles computations with detailed symmetry-mode analysis. The strategic integration of polar bismuth ferrite alongside charge-transfer calcium ferrite instigates profound lattice instabilities, notably manifest in octahedral rotations and cooperative FeO$_6$ breathing modes that might not necessarily be soft. However, their synergistic coupling stabilizes a non-centrosymmetric $Pc$ ground state that intrinsically features polar charge ordering of Fe ions. This resultant phase ingeniously unifies C-type antiferromagnetism with robust ferroelectric semiconductor characteristics, exhibiting a calculated indirect band gap of about 0.6 eV. Our discoveries firmly establish ferrite superlattices as an exceptionally versatile and tunable platform for the rational design of next-generation multifunctional materials, offering precise control over polarization, charge ordering phenomena, and electronic transport behavior via advanced interface and strain engineering techniques.

cond-mat.mtrl-sci↗

arXiv2025

Ferroelectric metal-organic frameworks as wide band gap materials

Wide band gap materials are particularly relevant at high temperatures. The band gap shrinkage at higher temperatures prevents device applications with narrow band gap semiconductors. Considering $\alpha$-phase strontium cyanurate as a prototype structure, we identify a group of metal-organic frameworks (MOFs) that exhibit ultra-wide band gaps ranging from 5.5 to 5.7 eV. Recently, a strontium cyanurate compound was found to undergo a phase transition from a high-symmetry $\beta$-phase to a low-symmetry ferroelectric $\alpha$-phase when the temperature was reduced. In the present study, utilizing group theory techniques, we unravel that a zone-center $\Gamma_2^-$ phonon mode modifies our structures from high-symmetry $\beta$-phase to a low-symmetry $\alpha$-phase for A$_3$(O$_3$C$_3$N$_3$)$_2$ MOFs with A = Mg, Ca, Sr, and Ba. We implement first-principles calculations to investigate structural, ferroelectric, and optical properties of these compounds in $\alpha$-phase. The switching barriers between bistable polar states are also estimated. Further, to realize their feasibility, we examine the dynamical and thermal stabilities for all of these MOFs.

cond-mat.mtrl-sci↗

arXiv2025

Electric Polarization-Driven Modulation of Fe Adatoms on Ferroelectric $\alpha$-In$_2$Se$_3$

The interplay among structural, electronic, and magnetic properties of Fe adatoms on the surface of two-dimensional ferroelectric {\alpha}-In$_2$Se$_3$ is investigated using first-principles electronic structure calculations, with a focus on how these properties are modulated by the direction of the electric polarization of the substrate. We identify two competing adsorption sites for Fe adatoms, whose relative stability depends on the adatom concentration and can be reversed by switching the electric polarization of {\alpha}-In$_2$Se$_3$. The calculated energy barrier for thermally activated hopping between these sites is approximately 0.4 eV, corresponding to a blocking temperature of around 100 K. The hybridization between Fe and In$_2$Se$_3$ orbitals strongly depends on the adsorption site and polarization direction, driven by variations in the local adatom geometry. As a result, the electronic configuration of adatom, magnetic moment, and magnetic anisotropy exhibit a pronounced site dependence and can be effectively modulated by switching the electric polarization of the In$_2$Se$_3$ layer. In particular, at higher adatom concentrations, an exceptionally large perpendicular magnetic anisotropy, exceeding 200 meV per Fe atom, emerges for one polarization direction, but is largely diminished when the polarization is reversed. These findings indicate that ferroelectric substrates offer a promising route for voltage-controlled tuning of magnetic adatom properties via reversible polarization switching.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling Insulating Ferro and Ferrimagnetism in Double-Double Perovskite Oxides

The emergence of ferro- and ferrimagnetic behavior in insulating materials is uncommon, largely due to Hund's rules. Utilizing symmetry analysis, first-principles methods, and classical Monte Carlo simulations, \textcolor{black}{we report technologically important insulating ferro and ferrimagnetic double-double perovskite oxides. Our study predicts LaA$^{\prime}$MnNiO$_6$ (A$^{\prime}$ = V, Cr, Mn, Co, and Ni) as promising candidates for spintronic and optical applications exhibiting band gaps between 1.3 eV and 1.9 eV. We explain the mechanisms driving band gap openings and magnetic exchange interactions in these ferro and ferrimagnetic compounds. Monte Carlo simulations, together with state-of-the-art orbital-decomposed exchange parameter analysis, reveal intriguing variations in magnetic transition temperatures (up to 242 K) and the corresponding exchange mechanisms in all LaA$^{\prime}$MnNiO$_6$ compounds.} In addition, we assess the thermodynamic and dynamic stability of these compounds to comment on the feasibility of these systems.

cond-mat.mtrl-sci↗

arXiv2024

Low-Energy Magnetic States of Tb Adatom on Graphene

Electronic structure and magnetic interactions of a Tb adatom on graphene are investigated from first principles using combination of density functional theory and multiconfigurational quantum chemistry techniques including spin-orbit coupling. We determine that the six-fold symmetry hollow site is the preferred adsorption site and we investigate electronic spectrum for different adatom oxidation states including Tb$^{3+}$, Tb$^{2+}$, Tb$^{1+}$, and Tb$^{0}$. For all charge states, the Tb $4f^8$ configuration is retained with other adatom valence electrons being distributed over $5d_{xy}$, $5d_{x2+y2}$, and $6s/5d_0$ single-electron orbitals. We find strong intra-site adatom exchange coupling that ensures that the $5d6s$ spins are parallel to the $4f$ spin. For Tb$^{3+}$, the energy levels can be described by the $J=6$ multiplet split by the graphene crystal field. For other oxidation states, the interaction of $4f$ electrons with spin and orbital degrees of freedom of $6s5d$ electrons in the presence of spin-orbit coupling results in the low-energy spectrum composed closely lying effective multiplets that are split by the graphene crystal field. Stable magnetic moment is predicted for Tb$^{3+}$ and Tb$^{2+}$ adatoms due to uniaxial magnetic anisotropy and effective anisotropy barrier around 440 cm$^{-1}$ controlled by the temperature assisted quantum tunneling of magnetization through the third excited doublet. On the other hand, in-plane magnetic anisotropy is found for Tb$^{1+}$ and Tb$^{0}$ adatoms. Our results indicate that the occupation of the $6s5d$ orbitals can dramatically affect the magnetic anisotropy and magnetic moment stability of rare earth adatoms.

cond-mat.mtrl-sci↗

arXiv2024

Design of Magnetic Polar Double-Double Perovskite Oxides through Cation Ordering

Commencing from the centrosymmetric MnRMnSbO$_6$ compound, we explore the realm of magnetic polar double-double perovskite oxides characterized by significant ferroelectric polarization. Employing symmetry operations, first-principles methodologies, and Monte Carlo simulations, our investigation delves into the structural, magnetic, ferroelectric, and electronic attributes of the polar LaFeMnNiO$_6$ and LaTiMnNiO$_6$ compounds. The structural analysis uncovers that the paraelectric-ferroelectric phase transition is intricately linked to the Fe/Ti-displacement of square planar Fe/TiO$_4$. Notably, the magnetic LaFeMnNiO$_6$ and LaTiMnNiO$_6$ compounds demonstrate robust ferroelectric polarizations, measuring 20.0 $\mu$C/cm$^2$ and 21.8 $\mu$C/cm$^2$, respectively, accompanied by minimalist forbidden energy gaps of 1.40 eV and 1.18 eV using the GGA+U method. Furthermore, we pinpoint elevated magnetic transition temperatures for these compounds. Additionally, our study scrutinizes the energies associated with diverse spin configurations and identifies potential minimum decomposition pathways into stable oxides. This comprehensive analysis ensures the meticulous formation of the LaFeMnNiO$_6$ and LaTiMnNiO$_6$ compounds.

cond-mat.mtrl-sci↗

arXiv2022

Insights into cation ordering of double perovskite oxides from machine learning and causal relations

This work investigates the origins of cation ordering of double perovskites using first-principles theory computations combined with machine learning (ML) and causal relations. We have considered various oxidation states of A, A', B, and B' from the family of transition metal ions to construct a diverse compositional space. A conventional framework employing traditional ML classification algorithms such as Random Forest (RF) coupled with appropriate features including geometry-driven and key structural modes leads to highly accurate prediction (~98%) of A-site cation ordering. We have evaluated the accuracy of ML models by entailing analyses of decision paths, assignments of probabilistic confidence bound, and finally introducing a direct non-Gaussian acyclic structural equation model to investigate causality. Our study suggests that the structural modes are the most important features for classifying layered, columnar and rock-salt ordering. For clear layered ordering, the charge difference between the A and A' is the most important feature which in turn depends on the B, B' charge separation. Based on the outputs from ML models, we have designed functional forms with these features to derive energy differences forming clear layered ordering. The trilinear coupling between tilt, rotation, and A-site antiferroelectric displacement in Landau free-energy expansion becomes the necessary condition behind the formation of A-site cation ordering.

cond-mat.mtrl-sci↗

arXiv2022

Engineering multifunctionality at oxide interfaces by multimode coupling

We employed first-principles density functional theory calculations guided by group-theoretical analysis and demonstrated the control of insulator-metal-insulator transition, polarization and two sublattice magnetization in (LaFeO$_3$)$_1$/(CaFeO$_3$)$_1$ superlattice via. multi structural mode coupling i.e., 'multimode coupling'. We have discovered a polar A-type charge disproportionation mode, Q$_{ACD}$ (analogous to the A-type antiferromagnetic ordering), and found that it couples with the trilinear coupling, $Q_{Tri}$ mode (common in $Pnma$ perovskite oxides and involves three structural modes), and lowers the symmetry further. By tuning the strength of the coupling between the participating modes, the polar metallic phase, polar zero bandgap semiconducting, and polar insulating phases can be obtained. Here, $Q_{Tri}$ switches the polarization direction, whereas, Q$_{ACD}$ can trigger insulator-metal-insulator transition along with the polarization switching. The mechanism is true for any transition metal superlattices constituted with $Pnma$ building blocks and with partially filled $e_g$ or $t_{2g}$ electron(s) at the transition metal sites.

cond-mat.mtrl-sci↗