arXiv subjects

Sayantika Bhowal

Publications and source records attributed to Sayantika Bhowal.

At least 19 recordsLinked to original sources

arXiv2026

Room-Temperature Electric-Field Control of Anomalous Hall Effect in Py/BTO/LSMO Heterostructures

We demonstrate room temperature electric field control of the anomalous Hall effect in epitaxial Ni80Fe20 (Py) BaTiO3 (BTO) La0.7Sr0.3MnO3 (LSMO) thin film heterostructures grown on MgO and LaAlO3 substrates. Substrate induced strain states generate distinct magnetic anisotropies, enabling voltage driven tuning between anomalous and topological Hall contributions. Robust ferroelectric polarization in BTO, confirmed by piezoresponse force microscopy, couples strongly to interfacial orbital reconstruction and carrier redistribution. As a result, Hall resistivity exhibits giant low voltage tunability, with up to nearly 93 percent modulation at operating voltages of only 0.5 tand 2 V. Density functional theory calculations further reveal polarization controlled Rashba spin splitting, establishing a direct link between ferroelectric order and emergent quantum transport. These findings establish Py/BTO/LSMO heterostructures as promising candidates for low-power multifunctional spintronic devices, where substrate engineering enables control over emergent quantum transport phenomena.

cond-mat.mtrl-sci↗

arXiv2026

Interface engineering of the anomalous Hall effect in Ni-based heterostructures

Using a combined experimental and first-principles theoretical approach, we demonstrate interface engineering of the anomalous Hall effect in Ni-based epitaxial thin-film heterostructures. Ferromagnetic Ni thin films are grown on (001)-oriented single-crystal LaAlO$_3$, SrTiO$_3$, and MgO substrates, which impose different biaxial tensile strains of 0.3%, 0.6%, and 0.8%, respectively. Our room-temperature Hall transport measurements reveal a pronounced substrate-dependent modulation of the anomalous Hall conductivity. Interestingly, our calculations show that strain alone cannot account for the experimentally observed trends. Instead, we identify interfacial inversion-symmetry breaking, which induces Rashba spin-orbit interaction, as the key mechanism governing the anomalous Hall conductivity across different interfaces. Building on this understanding, we further demonstrate both theoretically and experimentally that the anomalous Hall conductivity can be continuously tuned by an external electric field. These findings establish the critical role of substrate-induced interfacial effects in controlling the anomalous Hall effect in engineered heterostructures and provide a viable pathway toward electrically tunable room-temperature spintronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Orbital and Spin Nernst Effects in Monolayers of Transition Metal Dichalcogenides

In recent years, orbitronic effects have attracted growing attention as complementary counterparts to the well-established spintronic phenomena. In this work, we demonstrate that monolayers of transition metal dichalcogenides provide an excellent platform for the observation of the orbital Nernst effect, a relatively less explored phenomenon describing the generation of a transverse orbital current in response to an applied temperature gradient. We show that, similar to its electrical counterpart, viz., the orbital Hall effect, the orbital Nernst effect does not require the presence of spin-orbit coupling. Analytical results based on a low-energy valley model offer key insights into the underlying mechanisms, highlighting in particular the crucial role of electronic states at the Fermi energy for the emergence of this effect. The inclusion of spin-orbit coupling further gives rise to a spin Nernst effect, which scales with the strength of spin-orbit coupling and vanishes in its absence. We substantiate our analytical findings with full Brillouin-zone tight-binding results for two representative systems, monolayer 2H MoS$_2$ and 2H NbS$_2$. Our results show that while both orbital and spin Nernst conductivities in MoS$_2$ require electron or hole doping, both effects are intrinsically present in metallic NbS$_2$. Our work reveals the central role of orbital and spin Berry curvatures, identifies doping as an effective route for tuning orbital and spin Nernst responses, and proposes a possible experimental setup for detecting these effects in monolayer transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2026

Interplay of Valley, Orbital, Spin, and Layer Degrees of Freedom in Ta$_2$CS$_2$ MXene

We show that the MXene Ta$_2$CS$_2$ provides an excellent platform for hosting multiple coupled degrees of freedom, viz., valley, spin, orbital, and layer. The interplay among these degrees of freedom gives rise to a range of intriguing properties in reciprocal space, including valley-orbital and orbital-layer coupling. In the presence of spin-orbit interaction, these couplings lead to valley-dependent and layer-dependent spin splitting of the electronic bands. We further show that the intrinsic electric polarization in Ta$_2$CS$_2$ introduces an additional tuning parameter, enabling control over these coupled degrees of freedom and resulting in switchable valley-dependent orbital moments and Zeeman-like spin splitting. We demonstrate that these nontrivial orbital and spin textures manifest in the orbital and spin Hall effects, respectively. Our results establish noncentrosymmetric MXenes as a promising platform for exploring the interplay among multiple degrees of freedom, their tunability, and the resulting orbital and spin transport phenomena in these two-dimensional materials, thereby paving the way for next-generation spin-orbitronic devices.

cond-mat.str-el↗

arXiv2026

Why is the $d$-Wave spin splitting in CuF$_2$ bulk-like?

With the advent of nonrelativistic spin splitting in collinear compensated antiferromagnets, several candidate materials have also been proposed, among which the family of transition-metal difluorides stands out as a prominent example. Within this family, most members exhibit planar $d$-wave spin splitting, whereas CuF$_2$ shows bulk $d$-wave splitting with an explicit $k_z$ dependence. In this work, we show that this transition from planar to bulk $d$-wave splitting in CuF$_2$ is primarily driven by the antipolar displacements of the F ions, which are absent in the tetragonal rutile structure of the other family members. Our calculations reveal that these additional structural distortions introduce an extra plane of anisotropic magnetization density, giving rise to an additional totally symmetric component of the magnetic octupole tensor. The $k$-space representation of this octupole component, consequently, dictates an additional direction of spin splitting, thereby transforming the $d$-wave spin splitting pattern from planar to bulk-like. We further analyze the effect of spin-orbit coupling on the magnetic octupoles and the resulting spin splitting in the band structure. Our work highlights the possibility of controlling the pattern of nonrelativistic spin splitting through structural modifications, for example, via the application of external pressure.

cond-mat.mtrl-sci↗

arXiv2025

Non-relativistic spin splitting: Features and Functionalities

Recently, spin splitting of non-relativistic origin in compensated antiferromagnets has drawn growing attention in condensed matter research. Although many materials, now known to exhibit such spin splitting, have been studied for decades, their manifestation along non-high-symmetry momentum directions initially hindered their recognition. In recent years, significant progress has been made in uncovering the symmetry principles that allow non-relativistic spin splitting in the absence of net magnetization, revealing the unconventional physics arising from their coexistence. In this review, we provide a concise overview of non-relativistic spin splitting in compensated antiferromagnets with various spin configurations, including collinear, coplanar, and non-coplanar spin arrangements. We summarize practical identification guidelines, highlight characteristic features in electronic band structures, and discuss the emerging functionalities, with an emphasis on promising directions for future exploration.

cond-mat.mtrl-sci↗

arXiv2025

A Universal Framework for Controlling Non-Relativistic Spin Splitting

Non-relativistic spin splitting in antiferromagnets has recently attracted considerable attention. Here we present a universal framework for controlling such spin splitting by identifying and manipulating the key atomic distortions that govern it through external perturbations. We demonstrate this concept by tuning the spin-splitting energy in three representative materials with diverse symmetries, inversion-symmetric MnF$_2$, ferroelectric BaCuF$_4$, and LaMnO$_3$/RMnO$_3$ superlattices. Our results emphasize the essential role of higher-order magnetic multipoles and the intrinsic structure-spin correlations in these systems, thereby advancing current efforts to control spin splitting in real materials and motivating future experimental studies.

cond-mat.mtrl-sci↗

arXiv2025

Rational Control of Magnonic and Electronic Band Splittings

We provide a theoretical demonstration of controllable non-relativistic spin splitting in both electronic and magnonic bands via targeted structural distortions tied to specific phonon modes. Using MnF$_2$ as a model system, we identify a $d$-wave magnon band splitting between magnon modes of specific handedness, directly correlated with the non-relativistic spin splitting observed in the electronic structure. Crucially, we show that structural distortions associated with the A$_{2u}$ and A$_{1g}$ phonon modes (8.52 and 9.74 THz) modulate these splittings without altering the antiferromagnetic order. The effect originates from changes in the nonmagnetic ligand environment, highlighting the key role of lattice degrees of freedom in governing spin dynamics. Our findings establish a novel route for structure-mediated control of spin splitting, opening possibilities for tunable magnonic and spintronic functionalities in antiferromagnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

Designing Non-Relativistic Spin Splitting in Oxide Perovskites

We investigate the role of atomic distortions in non-relativistic spin splitting in perovskite oxides with Pbnm symmetry. Using LaMnO3 as a representative material, we analyze its non-relativistic spin splitting through a combined phonon and multipolar analysis. Our study provides key insights into how structural distortions and magnetic ordering drive ferroically ordered magnetic multipoles, which, in turn, give rise to non-relativistic spin splitting. Based on these findings, we propose three strategies for engineering non-relativistic spin splitting: modifying the A-site cation size, strain engineering, and electric field control in superlattice structures. Our work establishes a framework for designing non-relativistic spin splitting in the Brillouin zone of oxide perovskites.

cond-mat.mtrl-sci↗

arXiv2025

Giant topological Hall effect in epitaxial Ni$_{80}$Fe$_{20}$/La$_{0.65}$Sr$_{0.35}$MnO$_3$ thin film heterostructures

The emergence of new physical properties at the interfaces between complex oxides has always been of both fundamental and practical importance. Here, we report the observation of a giant topological Hall resistivity of $\sim 2.8 μΩ$ \text{cm} at room temperature in an epitaxial thin-film heterostructure of permalloy (Py, Ni$_{80}$Fe$_{20}$) and the half-metallic ferromagnet La$_{0.65}$Sr$_{0.35}$MnO$_3$ (LSMO). This large magnitude of the topological Hall effect in the Py/LSMO heterostructure, compared to a single-layer Py thin film, is attributed to the optimized combination of ferromagnetism in LSMO and the strong spin-orbit-coupling-driven Rashba interaction at the interface. The introduction of a ferroelectric BaTiO$_3$ (BTO) sandwich layer in the Py/LSMO heterostructure also leads to an enhanced topological Hall resistivity compared to the single-layer Py thin film. Interestingly, magnetic force microscopy measurements reveal skyrmion-like features, suggesting the origin of the topological Hall effect. Our theoretical model calculations for the skyrmion lattice further indicate that the Rashba interaction, driven by the broken inversion symmetry in the Py/LSMO films, can account for the observed changes in the topological Hall effect at the interface. Our work opens the door for the potential use of Py/LSMO thin films in spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Emergent surface multiferroicity

We show that the surface of a centrosymmetric, collinear, compensated antiferromagnet, which hosts bulk ferroically ordered magnetic octupoles, exhibits a linear magnetoelectric effect, a net magnetization, and a net electric dipole moment. Thus, the surface satisfies all the conditions of a multiferroic, in striking contrast to the bulk, which is neither polar nor exhibits any net magnetization or linear magnetoelectric response. Of particular interest is the case of non-relativistic $d$-wave spin split antiferromagnets, in which the bulk magnetic octupoles and consequently the surface multiferroicity exist even without spin-orbit interaction. We illustrate our findings using first-principles calculations, taking FeF$_2$ as an example material. Our work underscores the bulk-boundary correspondence in these unconventional antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2024

GdAlSi: An antiferromagnetic topological Weyl semimetal with non-relativistic spin splitting

Spintronics has emerged as a viable alternative to traditional electronics based technologies in the past few decades. While on one hand, the discovery of topological phases of matter with protected spin-polarized states has opened up exciting prospects, recent revelation of intriguing non-relativistic spin splitting in collinear antiferromagnetic materials with unique symmetries facilitate a wide possibility of realizing both these features simultaneously. In this work, we report the co-existence of these two intriguing properties within a single material: GdAlSi. It crystallizes in a body-centered tetragonal structure with a non-centrosymmetric space group $I4_{1}md$ ($109$), which is confirmed using detailed structural analysis through X-ray diffraction (XRD) and optical second harmonic generation (SHG) measurements. The magnetization data indicates AFM ordering with an ordering temperature ($T_N$) $\sim$ 32 K. Ab-initio calculations reveal GdAlSi to be a collinear antiferromagnetic Weyl semimetal with an unconventional, momentum-dependent spin splitting, also referred to as altermagnet. Angle-resolved photoemission spectroscopy measurements on GdAlSi single crystals subsequently confirm the presence of Fermi arcs, a distinctive hallmark of Weyl semimetals. Electric and magnetic multipole analysis provides a deeper understanding of the symmetry-mediated, momentum-dependent spin splitting, which has strictly non-relativistic origin. To the best of our knowledge, such co-existence of unconventional antiferromagnetic order and non-trivial topology is unprecedented and has never been observed before in a single material, rendering GdAlSi a special and promising candidate material. We propose a device harnessing these features, poised to enable practical and efficient topotronic applications.

cond-mat.str-el↗

arXiv2024

Electric toroidal dipole order and hidden spin polarization in ferroaxial materials

We investigate the role of electric toroidal dipoles in the prototypical ferroaxial materials NiTiO$_3$ and K$_2$Zr(PO$_4$)$_2$, which undergo ferroaxial structural phase transitions of order-disorder and displacive type, respectively. Using first-principles electronic structure theory, we compute the evolution across the ferroaxial transitions of the local electric toroidal dipole moments, defined both in terms of the vortices formed by local dipoles, as well as as the cross product of orbital and spin angular momenta. Our calculations confirm that the electric toroidal dipole acts as the order parameter for these ferroaxial transitions and highlight the importance of spin-orbit coupling in generating a non-zero atomic-site electric toroidal dipole moment. We find that, while the ferroaxial phases of NiTiO$_3$ and K$_2$Zr(PO$_4$)$_2$ preserve global inversion symmetry, they contain inversion-symmetry-broken sub-units that generate vortices of local electric dipole moments. In addition to causing the net electric toroidal dipole moment, these vortices induce a hidden spin polarization in the band structure.

cond-mat.mtrl-sci↗

arXiv2023

Universal responses in nonmagnetic polar metals

We demonstrate that two phenomena, the kinetic magneto-electric effect and the non-linear Hall effect, are universal to polar metals, as a consequence of their coexisting and contraindicated polarization and metallicity. We show that measurement of the effects provides a complete characterization of the nature of the polar metal, in that the non-zero response components indicate the direction of the polar axis, and the coefficients change sign on polarization reversal and become zero in the non-polar phase. We illustrate our findings for the case of electron-doped PbTiO$_3$ using a combination of density functional theory and model Hamiltonian-based calculations. Our model Hamiltonian analysis provides crucial insight into the microscopic origin of the effects, showing that they originate from inversion-symmetry-breaking-induced inter-orbital hoppings, which cause an asymmetric charge density quantified by odd-parity charge multipoles. Our work both heightens the relevance of the kinetic magneto-electric and non-linear Hall effects, and broadens the platform for investigating and detecting odd-parity charge multipoles in metals.

cond-mat.mtrl-sci↗

arXiv2023

Surface Magnetization in Antiferromagnets: Classification, example materials, and relation to magnetoelectric responses

We use symmetry analysis and density functional theory to characterize antiferromagnetic (AFM) materials which have a finite equilibrium magnetization density on particular surface terminations. A nonzero magnetic dipole moment per unit area or "surface magnetization" can arise on particular surfaces of many AFMs due to the bulk magnetic symmetries. Such surface magnetization plays an essential role in numerous device applications, from random-access magnetoelectric (ME) memory to exchange bias. However, at this point a universal description of AFM surface magnetization is lacking. We first introduce a classification system based on whether the surface magnetization is sensitive or robust to roughness, and on whether the surface of interest is magnetically compensated or uncompensated in the bulk magnetic ground state. We show that uncompensated surface magnetization can be conveniently described in terms of ME multipoles at the local-moment, unit cell level, and demonstrate that the symmetry of the multivalued "multipolization lattice" distinguishes between roughness-robust and roughness-sensitive surface magnetization. We then demonstrate that magnetization on bulk-compensated surfaces arises due to ME multipoles (in addition to higher-order magnetic terms) at the atomic site level. These can further be understood in terms of bulk ME responses, arising from the effective electric field resulting from the surface termination. We also show with density functional calculations that nominally compensated surfaces in Cr2O3 and FeF2 develop a finite magnetization density at the surface, in agreement with our predictions based on both group theory and the linear and higher-order ME response tensors. Our analysis provides a comprehensive basis for understanding the surface magnetic properties in AFMs, and has important implications for phenomena such as exchange bias coupling.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic octupoles as the order parameter for unconventional antiferromagnetism

We show that time-reversal symmetry broken, centrosymmetric antiferromagnets with nonrelativistic spin-splitting are conveniently described in terms of the ferroic ordering of magnetic octupoles. The magnetic octupoles are the lowest-order ferroically ordered magnetic quantity in this case, and so are the natural order parameter for the transition into the magnetically ordered state. They provide a unified description of the broken time-reversal symmetry and the non-relativistic spin splitting as well as a platform for manipulating the latter, and account for other phenomena, such as piezomagnetism, characteristic of this class of antiferromagnets. Unusually for antiferromagnets, we show that the magnetic octupoles cause a non-zero magnetic Compton scattering, providing a route for their direct experimental detection. We illustrate these concepts using density-functional and model calculations for the prototypical non-relativistic spin-split antiferromagnet, rutile-structure manganese difuoride, MnF2.

cond-mat.str-el↗

arXiv2022

Polar metals: Principles and Prospects

We review the class of materials known as polar metals, in which polarity and metallicity coexist in the same phase. While the notion of polar metals was first invoked more than 50 years ago, their practical realization has proved challenging, since the itinerant carriers required for metallicity tend to screen any polarization. Huge progress has been made in the last decade, with many mechanisms for combining polarity and metallicity proposed, and the first examples, LiOsO3 and WTe2, identified experimentally. The availability of polar metallic samples has opened a new paradigm in polar metal research, with implications in the fields of topology, ferroelectricity, magnetoelectricity, spintronics, and superconductivity. Here, we review the principles and techniques that have been developed to design and engineer polar metals and describe some of their interesting properties, with a focus on the most promising directions for future work.

cond-mat.mtrl-sci↗

arXiv2022

Orbital Hall effect in bilayer transition metal dichalcogenides: From the intra-atomic approximation to the Bloch states orbital magnetic moment approach

Using an effective Dirac model, we study the orbital Hall effect (OHE) in bilayers of transition metal dichalcogenides with 2H stacking (2H-TMD). We use first-order perturbation theory in the interlayer coupling of the bilayer system to obtain analytical expressions for the orbital Hall conductivity in the linear response regime. We use two distinct descriptions of the orbital angular momentum (OAM) operator: The first one is the intra-atomic approximation that considers only the intrasite contribution to the OAM [Cysne et al. Phys. Rev. Lett. 126, 056601 (2021)]. The second one uses the Berry-phase formula of the orbital (valley) magnetic moment to describe the OAM operator [Bhowal and Vignale, Phys. Rev. B 103, 195309 (2021)]. This approach includes both intersite and intrasite contributions to the OAM. Our results suggest that the two approaches agree qualitatively in describing the OHE in bilayers of 2H-TMDs, although they present some quantitative differences. We also show that interlayer coupling plays an essential role in understanding the OHE in the unbiased bilayer of 2H-TMD. This coupling causes the Bloch states to become bonding (antibonding) combinations of states of individual layers, demanding the consideration of the non-Abelian structure of the orbital magnetic moment to the occurrence of OHE. As we discuss throughout the work, the emerging picture of transport of OAM in the unbiased bilayer of 2H-TMDs based on OHE is very different from the usual picture based on the valley Hall effect, shedding new lights on previous experimental results. We also discuss the effect of the inclusion of a gate-voltage bias in the bilayer system. Our work gives support to recent theoretical predictions on OHE in two-dimensional materials.

cond-mat.mes-hall↗