arXiv subjects

Sagar Sarkar

Publications and source records attributed to Sagar Sarkar.

11 recordsLinked to original sources

arXiv2026

Magnetic frustration and non-collinear textures in layered Gd magnets

Using scale-bridging simulations based on electronic structure theory and atomistic spin dynamics, we investigate the magnetic properties of skyrmionic GdRu$_2$Si$_2$ and GdRu$_2$Ge$_2$ layered rare-earth magnets and similar Gd-based compounds (GdAu$_2$Si$_2$, GdAu$_2$Ge$_2$, GdAg$_2$Si$_2$ and GdAg$_2$Ge$_2$). By studying the trends across this structural family, we confirm the importance of magnetic frustration and dipolar interactions for the stability of non-collinear and skyrmion phases. Furthermore, our calculations predict promising opportunities for chemical tuning of these magnets in terms of the balance between various exchange interactions and the character of the magnetic anisotropy. These changes lead to the formation of new types of skyrmions that are stable in a wide range of applied external magnetic field. In particular, we propose partial alkali-metal substitution of Gd in GdRu$_2$Si$_2$ leading to GdKRu$_4$Si$_4$, GdRbRu$_4$Si$_4$, GdCsRu$_4$Si$_4$ as well as GdYRu$_4$Si$_4$ compounds, and suggest that it is likely to result in an ordered layered structure, similarly to previously reported iron pnictides like CaKFe$_4$As$_4$.

cond-mat.mtrl-sci↗

arXiv2026

Orientation-driven route to an intrinsic insulating ferromagnetic state in manganite superlattices

Increasing precision in the growth of superlattices sparks hope in applications that may arise from engineering layered structures. Heterostructuring and functionalization of magnetic oxides have been very popular due to their versatility and readiness for integration in modern electronics. In this study, we provide yet another example of this phenomenology by predicting that an insulating ferromagnetic state can be realized in superlattices of LaMnO$_3$ and SrTiO$_3$ oriented along the (111) direction. In strike contrast with respect to other orientations, these properties are not of extrinsic origin but arise from the interplay of structural order, strain and quantum confinement. The bandgap is shown to be either direct and indirect, depending on the precise composition, which can be explained in terms of the geometrical properties of (111)-oriented bilayers of LaMnO$_3$. The electronic structure shows narrow bands indicating localized $e_g$ states for all the investigated superlattices. These features and the analysis of the inter-atomic magnetic coupling suggest that the investigated superlattices behave as a Kugel-Khomskii material, at least for the explored compositions. Our results provide not only a new route to an insulating ferromagnet, but also novel insight into the intricate interplay between lattice symmetry, Hubbard physics and Hund's coupling to be exploited in next-generation spintronic applications.

cond-mat.str-el↗

arXiv2026

Mechanical control of magnetic exchange and response in GdRu$_2$Si$_2$: A computational study

We present a systematic computational study of the effect of uniaxial strain on the magnetic properties of GdRu$_2$Si$_2$, a centrosymmetric material known to host a field-induced skyrmion lattice (SkL). Using first-principles density functional theory, we first demonstrate the pronounced sensitivity of the exchange and anisotropy to specific structural distortions. These DFT-derived interactions are then integrated into a classical spin model to construct comprehensive magnetic phase diagrams under both compressive and tensile strain. Our key finding is that compressive strain ($\sim 2\%$) acts as an effective tuning parameter, substantially expanding the stability region of the $\vec Q_{100}$-driven topologically nontrivial phases. This results from the shifts in the critical magnetic fields and enhancement of the energy scale of the favored magnetic wave vector. In contrast, tensile strain induces a different magnetic ground-state by promoting a different magnetic ordering vector, $\vec Q_{110}$, leading to entirely distinct phase behavior. This work not only provides a quantitative understanding of the structural-magnetic coupling in GdRu$_2$Si$_2$ but also establishes strain engineering as a powerful approach to control and optimize topologically non-trivial magnetic phases in centrosymmetric magnets.

cond-mat.mtrl-sci↗

arXiv2024

Unveiling Mysteries of GdRu$_2$Si$_2$: 3D Magnetism in a layered like Magnet

GdRu$_2$Si$_2$, a centrosymmetric magnet with a square lattice of Gd atoms, hosts a short-period skyrmion square lattice (SkL) without Dzyaloshinskii-Moriya interaction (DMI). RKKY-type exchange between the Gd moments results in an exchange frustration, which is the main source of non-collinearity in the spiral phases of the system. Previous studies focused on the in-plane $\vec{Q}$ vectors in understanding the magnetic phases of the system as they appear and have been observed on the 2D Gd layers. In this work, we calculate the Gd-Gd magnetic exchange interactions ($J_{ij}$) and perform atomic spin dynamics (ASD) simulations, providing new insights about GdRu$_2$Si$_2$. Our calculated $J_{ij}$ shows that the strongest magnetic interaction occurs between Gd atoms along the [111] body-diagonal direction of the unit cell. This, along with the body-centered tetragonal structure of the Gd sublattice, points to the presence of a hitherto ignored modulation vector, $\vec{Q}_{[111]}$, along the [111] direction in the spiral phases of the system. ASD simulations confirm this interlayer modulation, demonstrating that GdRu$_2$Si$_2$'s magnetic phases are more complex than suggested by 2D layer observations. The total magnetic order is determined by $\vec{Q}_{[111]}$ alongside intralayer $\vec{Q}_{[100]}$ and $\vec{Q}_{[010]}$, establishing GdRu$_2$Si$_2$ as a strong 3D magnet requiring comprehensive theoretical modeling. Considering these, our ASD simulations accurately reproduce experimental phase transitions and highlight the significant role of dipolar interactions (due to the large Gd moment) over a weak uniaxial anisotropy in determining the ground state. This work enhances the understanding of GdRu$_2$Si$_2$'s complex magnetism, suggesting similar interlayer effects may be important in other layered magnetic systems.

cond-mat.mtrl-sci↗

arXiv2024

Revisiting magnetic exchange interactions in transition metal doped Bi$_2$Se$_3$ using DFT+MFT

Topological insulators doped with magnetic impurities has become a promising candidate for Quantum Anomalous Hall Effect (QAHE) in the dilute doping limit. The crucial factor in realizing the QAHE in these systems is the spontaneous Ferromagnetic (FM) ordering between the doped magnetic atoms. Hence, understanding the magnetic exchange interaction between the magnetic atoms becomes essential. In this work, we use the Density functional theory (DFT) and Magnetic force theorem (MFT) to calculate the magnetic exchange interaction between magnetic impurities (V, Cr, Mn, Fe) in the host Bi2Se3. Through an orbital decomposition of the calculated exchange, we can identify the nature and origin of the exchange mechanism that depends on the type of magnetic atoms, doping concentration, host material etc. Our results show that Cr doping results in an insulating state, a prerequisite for the QAHE, that remains robust against doping concentration and local correlation. In this case, the short-ranged superexchange and long-ranged exchange via the p-orbitals of the host results in an FM order. For other doped systems (V, Mn and Fe doped), their electronic configuration and local octahedral environment open the possibility of finite carrier density at the Fermi energy. Depending on the type of this carrier (electron/hole) and their localized/delocalized nature, a short-ranged double exchange / long-ranged RKKY mechanism could occur between the magnetic atoms.

cond-mat.mtrl-sci↗

arXiv2022

Hole doping in a negative charge transfer insulator

$RE$NiO$_3$ is a negative charge transfer energy system and exhibits a temperature-driven metal-insulator transition (MIT), which is also accompanied by a bond disproportionation (BD) transition. In order to explore how hole doping affects the BD transition, we have investigated the electronic structure of single-crystalline thin films of Nd$_{1-x}$Ca$_x$NiO$_3$ by synchrotron based experiments and {\it ab-initio} calculations. For a small value of $x$, we find that the doped holes are localized on one or more Ni sites around the dopant Ca$^{2+}$ ions, while the BD state for the rest of the lattice remains intact. The effective charge transfer energy ($Δ$) increases with Ca concentration and the formation of BD phase is not favored above a critical $x$, suppressing the insulating phase. Our present study firmly demonstrates that the appearance of BD mode is essential for the MIT and settles a long-standing debate about the role of structural distortions for the MIT of the $RE$NiO$_3$ series.

cond-mat.str-el↗

arXiv2021

Understanding the ferromagnetic insulating state in Cr doped VO$_2$

Experimentally Cr doping in the rutile phase of VO$_2$ is found to stabilize a charge ordered ferromagnetic insulating state in the doping range of 10\% to 20\%. In this work, we investigated its origin at 12.5\% Cr doping using a combination of ab-initio electronic structure calculations as well as microscopic modeling. Our calculations are found to reproduce the ferromagnetic insulating state as well as a charge ordering at the V and Cr sites. The mapping of the ab-initio band structure onto a tight-binding Hamiltonian allows one to calculate the energy gain from different exchange pathways. This gain is quantified in this work for the first time and the role of charge ordering in stabilizing a ferromagnetic insulating state is understood.

cond-mat.mtrl-sci↗

arXiv2020

Strain-induced stripe phase in charge ordered single layer NbSe$_2$

Charge density waves are ubiquitous phenomena in metallic transition metal dichalcogenides. In NbSe$_2$, a triangular $3\times3$ structural modulation is coupled to a charge modulation. Recent experiments reported evidence for a triangular-stripe transition at the surface, due to strain or accidental doping and associated to a $4\times4$ modulation. We employ \textit{ab-initio} calculations to investigate the strain-induced structural instabilities in a pristine single layer and analyse the energy hierarchy of the structural and charge modulations. Our results support the observation of phase separation between triangular and stripe phases in 1H-NbSe$_2$, relating the stripe phase to compressive isotropic strain, favouring the $4\times4$ modulation. The observed wavelength of the charge modulation is also reproduced with good accuracy.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous electron transport in epitaxial NdNiO$_3$ films

The origin of simultaneous electronic, structural and magnetic transitions in bulk rare-earth nickelates ($RE$NiO$_3$) remains puzzling with multiple conflicting reports on the nature of these entangled phase transitions. Heterostructure engineering of these materials offers unique opportunity to decouple metal-insulator transition (MIT) from the magnetic transition. However, the evolution of underlying electronic properties across these decoupled transitions remains largely unexplored. In order to address this, we have measured Hall effect on a series of epitaxial NdNiO$_3$ films, spanning a variety of electronic and magnetic phases. We find that the MIT results in only partially gapped Fermi surface, whereas full insulating phase forms below the magnetic transition. In addition, we also find a systematic reduction of the Hall coefficient ($R_H$) in the metallic phase of these films with epitaxial strain and also a surprising transition to negative value at large compressive strain. Partially gapped weakly insulating, paramagnetic phase is reminiscence of pseudogap behavior of high $T_c$ cuprates. The precursor metallic phase, which undergoes transition to insulating phase is a non-Fermi liquid with the temperature exponent ($n$) of resistivity of 1, whereas the exponent increases to 4/3 in the non-insulating samples. Such nickelate phase diagram with sign-reversal of $R_H$, pseudo-gap phase and non Fermi liquid behavior are intriguingly similar to high $T_c$ cuprates, giving important guideline to engineer unconventional superconductivity in oxide heterostructure.

cond-mat.str-el↗

arXiv2017

The driving force for charge ordering in rare earth nickelates

We show that charge ordering (more precisely, two-sublattice bond disproportionation) in the rare earth nickelate perovskites is intimately related to a negative charge transfer energy. By adding an additional potential on the Ni d states we are able to vary the charge tranfer energy and compute relaxed structures within an ab-initio framework. We show that the difference in Ni-O bond lengths and the value of the ordered state magnetic moment correlate with the charge transfer energy and that the transition to the bond-disproportionated state occurs when the effective charge transfer energy becomes negative.

cond-mat.str-el↗

arXiv2016

Driving force for martensitic transformation in Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$

The martensitic transformation in Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$ alloys has been investigated within ab-initio density functional theory. The experimental trend of a martensitic transition happening beyond $x$ = 0.36 is captured within these calculations. The microscopic considerations leading to this are traced to increased Ni-Mn hybridization which results from the Ni atom experiencing a resultant force along a lattice parameter and moving towards the Mn atoms above a critical concentration. The presence of the lone pair electrons on Sn forces the movement of Ni atoms away from Sn. While band Jahn Teller effects have been associated with this transition, we show quantitatively that atleast in this class of compounds they have a minor role.

cond-mat.mtrl-sci↗