arXiv subjects

Sajjan Sheoran

Publications and source records attributed to Sajjan Sheoran.

15 recordsLinked to original sources

arXiv2026

Negative Differential Resistance and Ultra-High TMR in Altermagnetic Tunnel Junctions

Altermagnets can replace ferromagnets in tunnel junctions, yielding large tunneling magnetoresistance, ultrafast switching, and low-power functionality. While most studies explore the linear-response regime, interesting features emerge at finite bias, where the peculiar electronic structure of altermagnets gives rise to complex non-linear behaviour. Using non-equilibrium Green's functions implemented with density functional theory, we predict that a large low-bias negative differential resistance can be observed in an altermagnetic tunnel junction. Our proposed junction incorporates the orbital-ordered altermagnet KV2Se2O, whose quasi-2D Fermi surface plays a crucial role in realizing the negative differential resistance. Upon the application of a finite bias voltage, the current in the parallel configuration first increases sharply and then decreases, to be almost completely suppressed at around 0.14 V. At the same time, the antiparallel configuration displays a monotonic current-voltage curve. This behaviour, in addition to the negative differential resistance, supports a large tunneling magnetoresistance with sign inversion at 0.13 V. Our results suggest that altermagnetic tunnel junctions can be used as components in applications requiring strong non-linear response at low bias.

cond-mat.mes-hall↗

arXiv2025

Tuning spin currents in collinear antiferromagnets and altermagnets

Spin current generation through non-relativistic spin splittings, found in uncompensated magnets and d-wave altermagnets, is desirable for low-power spintronics. Such spin currents, however, are symmetry forbidden in conventional collinear antiferromagnets and higher-order altermagnets. Using spin point group analysis, we demonstrate that finite spin currents can be induced in these materials via magnetoelectric, piezomagnetic, and piezomagnetoelectric-like couplings. We utilize electric fields, strain, and their combinations to drive symmetry-lowering phase transitions into uncompensated magnetic or d-wave altermagnetic states, thereby enabling finite spin conductivity in a broader class of magnetic materials. We further substantiate this framework using density functional theory and Boltzmann transport calculations on representative magnetic materials - KV2Se2O, RuF4 , Cr2O3 , FeS2 , and MnPSe3 - spanning these different cases. The charge-to-spin conversion ratio reaches up to almost 100% via uncompensated magnetism and about 40% via d-wave altermagnetism under realistic conditions, highlighting the effectiveness of this approach for efficient spin current generation.

cond-mat.mtrl-sci↗

arXiv2025

Spontaneous Anomalous Hall Effect in Two-Dimensional Altermagnets

The anomalous Hall effect (AHE) is an efficient tool for detecting the N\'eel vector in collinear compensated magnets with spin-split bands, known as altermagnets (AMs). Here, we establish design principles for obtaining non-zero anomalous Hall conductivity in the recently proposed two-dimensional (2D) AMs using spin and magnetic group symmetry analysis. We show that only two of the seven nontrivial spin layer groups exhibit an unconventional in-plane AHE in which the N\'eel vector lies within the plane of the Hall current. Through first-principles simulations on bilayers of MnPSe$_3$ and MnSe, we demonstrate the validity of our group theoretic framework for obtaining AHE with $d$ and $i$-wave altermagnetic orders, depending on the stacking of the bilayers. We find that the spin group symmetry is successful in determining the linear and cubic dependence of anomalous Hall conductivity in N\'eel vector space, although AHE is a relativistic effect. This work shows that the AHE in 2D AMs can probe the altermagnetic order and N\'eel vector reversal, thereby facilitating the miniaturization of altermagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Nonrelativistic spin splittings and altermagnetism in twisted bilayers of centrosymmetric antiferromagnets

Magnetism-driven nonrelativistic spin splittings (NRSS) are promising for highly efficient spintronics applications. Although 2D centrosymmetric (in four-dimensional spacetime) antiferromagnets are abundant, they have not received extensive research attention owing to symmetry-forbidden spin polarization and magnetization. Here, we demonstrate a paradigm to harness NRSS by twisting the bilayer of centrosymmetric antiferromagnets with commensurate twist angles. We observe $i$-wave altermagnetism and spin-momentum locking by first-principles simulations and symmetry analysis on prototypical MnPSe$_3$ and MnSe antiferromagnets. The strength of NRSS (up to 80 meVÅ) induced by twisting is comparable to SOC-induced linear Rashba-Dresselhaus effects. The results also demonstrate how applying biaxial strain and a vertical electric field tune the NRSS. The findings reveal the untapped potential of centrosymmetric antiferromagnets and thus expand the material's horizons in spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Probing switchable valley-related Hall effects in 2D magnetic MXenes

The search for two-dimensional materials with exotic valley-dependent properties has attracted rapid attention as they are fundamentally intriguing and practically appealing for nanoscale device applications. Here, using first-principles calculations, we report the identification of promising intrinsic valley-related switchable Hall effects in Cr2CSF. With a high out-of-plane magnetic anisotropy, Cr2CSF is a ferrovalley semiconductor with spontaneously polarized valleys having a valley polarization of 27.1 meV in the conduction band. This facilitates the observation of an intrinsic anomalous valley Hall (AVH) effect that is manipulated under an in-plane electric field. The underlying physics of spontaneous valley polarization is also discussed based on the SOC Hamiltonian model. Furthermore, on application of unidirectional compressive strain, Cr2CSF is further transitioned from the AVH phase to a long-sought quasi-half valley metal state. Here, only the electrons are valley polarized such that holes and electrons carriers are separated under the in-plane electric field. Our work enriches materials with the valley-related Hall effects and provides a platform for interplay among valleytronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Accelerated Neural Network Training through Dimensionality Reduction for High-Throughput Screening of Topological Materials

Machine Learning facilitates building a large variety of models, starting from elementary linear regression models to very complex neural networks. Neural networks are currently limited by the size of data provided and the huge computational cost of training a model. This is especially problematic when dealing with a large set of features without much prior knowledge of how good or bad each individual feature is. We try tackling the problem using dimensionality reduction algorithms to construct more meaningful features. We also compare the accuracy and training times of raw data and data transformed after dimensionality reduction to deduce a sufficient number of dimensions without sacrificing accuracy. The indicated estimation is done using a lighter decision tree-based algorithm, AdaBoost, as it trains faster than neural networks. We have chosen the data from an online database of topological materials, Materiae. Our final goal is to construct a model to predict the topological properties of new materials from elementary properties.

cond-mat.mtrl-sci↗

arXiv2023

Multiple Zeeman-type Hidden Spin Splitting in $\mathcal{\hat{P}\hat{T}}$-Symmetric Layered Antiferromagnets

Centrosymmetric antiferromagnetic semiconductors, although abundant in nature, appear less favorable in spintronics owing to the lack of inherent spin polarization and magnetization. We unveil hidden Zeeman-type spin splitting (HZSS) in layered centrosymmetric antiferromagnets with asymmetric sublayer structures by employing first-principles simulations and symmetry analysis. Taking the bilayer counterpart of recently synthesized monolayer MnSe, we demonstrate that the degenerate states around specific high-symmetry points spatially segregate on different sublayers forming PT-symmetric pair. Furthermore, degenerate states exhibit uniform in-plane spin configurations with opposite orientations enforced by mirror symmetry. Bands are locally Zeeman-split up to order of 70 meV. Strikingly, a tiny electric field of a few mVA-1 along the z-direction breaks the double degeneracy forming additional Zeeman pair. Moreover, our simulations on trilayer and tetralayer MnSe show that achieved HZSS is independent of layer number. These findings establish the design principle to obtain Zeeman-type splitting in centrosymmetric antiferromagnets and significantly expand the range of materials to look for spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Vacancy-Ordered Double Perovskites Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials

Vacancy-ordered double perovskites (A$_2$BX$_6$), being one of the environmentally friendly and stable alternatives to lead halide perovskites, have garnered considerable research attention in the scientific community. However, their thermal transport has not been explored much despite their potential applications. Here, we explore Cs$_2$BI$_6$ (B = Pt, Pd, Te, Sn) as potential thermoelectric materials using the state-of-the-art first-principles based methodologies, viz., density functional theory combined with many-body perturbation theory (G$_0$W$_0$) and spin-orbit coupling. %The phonon dispersion plots and Poisson's and Pugh's ratios show the dynamical and mechanical stability of this class of perovskites. The absence of polyhedral connectivity in vacancy-ordered perovskites gives rise to additional degrees of freedom leading to lattice anharmonicity. The presence of anharmonic lattice dynamics leads to strong electron-phonon coupling, which is well captured by Fröhlich mesoscopic model. % to investigate the interaction of longitudinal optical phonon modes with the carriers that strongly influence the carrier mobility. The lattice anharmonicity is further studied using {\it ab initio} molecular dynamics and electron localization function. The maximum anharmonicity is observed in Cs$_2$PtI$_6$, followed by Cs$_2$PdI$_6$, Cs$_2$TeI$_6$ and Cs$_2$SnI$_6$. Also, the computed average thermoelectric figure of merit ($zT$) for Cs$_2$PtI$_6$, Cs$_2$PdI$_6$, Cs$_2$TeI$_6$ and Cs$_2$SnI$_6$ are 0.88, 0.85, 0.95 and 0.78, respectively, which reveals their promising renewable energy applications.

cond-mat.mtrl-sci↗

arXiv2023

Probing the uniaxial strain-dependent valley drift and Berry curvature in monolayer MoSi$_2$N$_4$

We use ab initio calculations and theoretical analysis to investigate the influence of in-plane strain field on valley drifts and Berry curvatures in the monolayer MoSi$_2$N$_4$, a prototypical septuple atomic layered two-dimensional material. The low energy electron and hole valleys drift far off the K/K' point under uniaxial strains. The direction and strength of valley drift strongly depend on the nature of the charge carrier and uniaxial strain with a more substantial response along the zigzag path. Our findings are governed by the interplay between microscopic orbital contribution and symmetry lowering. The changing geometric properties of Bloch states affect the Berry curvatures and circular dichroism. Specifically, Berry curvature dipole is significantly enhanced under the tensile strain along armchair and zigzag directions. Meanwhile, the particle-hole asymmetry arising from non-equivalent electron and hole valley drifts relax the selection rules, thus reducing the degree of circular polarization up to ~0.98. Therefore, strain engineering of valley physics in the monolayer MoSi$_2$N$_4$ is of prime importance for valleytronics.

cond-mat.mtrl-sci↗

arXiv2023

Manipulation of valley and spin properties in two-dimensional Janus WSiGeZ$_4$ (Z=N,P, As) through symmetry control

A class of septuple-atomic-layer two-dimensional (2D) materials, MA$_2$Z$_4$, is sought as an alternative to 2D hexagonal transition metal dichalcogenides in the field of valleytronics and spintronics. In these materials, the structural symmetry can be varied by changing the stacking of its three parts in the monolayer. We show that in Janus monolayer WSiGeZ$_4$ (Z=N, P, As), the Berry curvature and Rashba effect are enhanced by modifying the stacking orders. Intrinsic electric field and composition of the d orbitals play a dominant role in determining these properties. The vertical field lifts the spin degeneracy along in-plane direction, inducing the Rashba effect around $Γ$ point. The in-plane orbitals under the influence of in-plane electric field contribute to the Zeeman splitting of bands at K/K' points. Berry curvature is strengthened by up to 300% compared to its ground state through symmetry control, along with a significant increment in the Rashba coefficient. Moreover, monolayer WSiGeP$_4$ and WSiGeAs$_4$ have multiple valleys, implying another valley-dimension. We construct a symmetry adapted k.p Hamiltonian for the valleys and investigate the effect of strain and electric field on the band structure. The interesting spin-valley physics in monolayer WSiGeZ$_4$ suggests their exceptional potential for spintronics and valleytronics applications.

cond-mat.mtrl-sci↗

arXiv2022

Rashba and Dresselhaus effects in doped methylammonium lead halide perovskite MAPbI$_3$

Inorganic-organic lead halide perovskites, particularly methylammonium lead halide (MAPbI$_3$) perovskite, is perceived to be a promising material for optoelectronics and spintronics. However, lead toxicity and instability under air and moisture restrict its practical uses. Hence, it is essential to reduce lead extent by substituting appropriate alternatives. Here, we substitute Sn and Ge in cubic MAPbI$_3$ and compare various properties of hybrid perovskites by employing state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with semilocal and hybrid functional (HSE06) and generalized gradient approximation (PBE) combined with spin-orbit coupling (SOC). We mainly study the Rashba-Dresselhaus (RD) effect, which occurs here due to two major mechanisms breaking inversion symmetry, i.e., static and dynamic, and the presence of heavy elements contributing to significant SOC. We find non-negligible spin-splitting effects in the conduction band minimum (CBm) and valence band maximum (VBM) for hybrid perovskites. For a deeper understanding of the observed spin-splitting, the spin textures are analyzed and Rashba coefficients are calculated. We find that Dresselhaus effect comes into play in substituted hybrids in addition to the usual Rashba effect observed in pristine compound. We also observe that the strength of Rashba spin-splitting can be substantially tuned on application of uniaxial strain ($\pm5\%$). Also, we notice that some of the hybrids are mechanically stable and ductile. Hence these hybrid perovskites can prove to be potent for perovskite-based spintronic applications.

cond-mat.mtrl-sci↗

arXiv2022

Coupled spin-valley, Rashba effect and hidden persistent spin polarization in WSi$_2$N$_4$ family

The new two-dimensional materials, MoSi$_2$N$_4$ and WSi$_2$N$_4$ are experimentally synthesized successfully and various similar structures are predicted theoretically. Here, we report the electronic properties with a special focus on the band splitting in WA$_2$Z$_4$ (A=Si, Ge; Z=N, P, As), using state-of-the-art density functional theory and many-body perturbation theory (within the framework of G$_0$W$_0$ and BSE). Due to the broken inversion symmetry and strong spin-orbit coupling effects, we detect coupled spin-valley effects at the corners of the first Brillouin zone (BZ). Additionally, we observe cubically and linearly split bands around the $Γ$ and M points, respectively. Interestingly, the in-plane mirror symmetry ($σ_h$) and the reduced symmetry of arbitrary $k$-point, enforce the persistent spin textures (PST) to occur in full BZ. We induce the Rashba splitting by breaking the $σ_h$ through an out-of-plane external electric field (EEF). The inversion asymmetric site point group of the W atom introduces the hidden spin polarization in centrosymmetric layered bulk counterparts. Therefore, the spin-layer locking effect, namely, energy degenerate opposite spins spatially segregated in the top and bottom W layers, has been identified. Our low energy $k.p$ model demonstrates that the PST along the M-K line is robust to EEF and layer thickness, making them suitable for applications in spintronics and valleytronics.

cond-mat.mtrl-sci↗

arXiv2022

Full-plane persistent spin textures with cubic order intrinsic and anisotropic band splitting in bulk Lead-free materials

Spin-orbit coupling (SOC) effects occurring in noncentrosymmetric materials are known to be responsible for nontrivial spin configurations and a number of emergent physical phenomena such as electrical control of spin degrees of freedom and spin-to-charge conversion. The materials preserving a uniform spin configuration in the momentum-space, known as persistent spin texture (PST), provide long carrier spin lifetimes through persistent spin helix (PSH) mechanism. However, most of the PST studied till now are attributed to the linear in \textbf{\textit{k}} splitting and cease to exist locally around certain high-symmetry-point of first Brillouin Zone (FBZ). The persistent spin textures with purely cubic spin splittings have drawn attention owing to unique benefits in spin transport. Here, by using the relativistic first-principles calculations supplemented with \textbf{\textit{k.p}} analysis, we report the emergence of purely cubic splitting (PCS) belonging to $D_{3h}$ point group, which is enforced by in-plane mirror and three-fold rotation operations. In addition, the in-plane mirror symmetry operation sustains the PST in larger region (i.e. full planes) of FBZ alongside giant spin splitting. Our results also demonstrate how application of uniaxial strain could be envisaged to tune the magnitude of the PCS, preserving the PST. The observed PSTs provide a route to non-dephasing spin transport with larger spin-Hall conductivity, thus offering a promising platform for future spintronics devices.

cond-mat.mtrl-sci↗

arXiv2021

Origin of Rashba spin-splitting and strain tunability in ferroelectric bulk CsPbF$_3$

Spin-orbit coupling (SOC) in conjunction with broken inversion symmetry acts as a key ingredient for several intriguing quantum phenomena viz. persistent spin textures, topological surface states and Rashba-Dresselhaus (RD) effects. The coexistence of spontaneous polarization and the RD effect in ferroelectric materials enables the electrical control of spin degree of freedom. In light of this, we explore here the ferroelectric lead halide perovskite viz. CsPbF$_3$ as a potential candidate in the field of spintronics by employing state-of-the-art first-principles based methodologies viz. density functional theory (DFT) with semi-local and hybrid functional (HSE06) combined with spin-orbit coupling (SOC) and many-body perturbation theory (G$_0$W$_0$). For a deeper understanding of the observed spin-splitting, we have analyzed the spin textures within the combined framework of DFT and $\textbf{k.p}$ model Hamiltonian. The latter confirms that there is no out of plane spin component indicating that the Rashba splitting dominates over Dresselhaus splitting in this system. Owing to the presence of Pb-6$p$ orbital in conduction band, the large value of Rashba coefficient ($α_R$) at conduction band minimum (CBm) is noticed in comparison to that of at the valence band maximum (VBM). Notably, we also observe that strength of Rashba spin-splitting can be substancially tuned on application of uniaxial strain ($\pm5\%$). This finding will further pave the path for perovskite-based spintronics devices.

cond-mat.mtrl-sci↗

arXiv2021

Rashba dominated spin-splitting in the bulk ferroelectric oxide perovskite KIO3

The momentum-dependent Rashba and Dresselhaus spin-splitting has gained much attention for its highly promising applications in spintronics. In the present work, ab initio density functional theory calculations are performed to study the spin-splitting effect in ferroelectric oxide perovskite KIO3. Our calculations are additionally supported by symmetry adapted two-band k.p Hamiltonian. Non-negligible spin-splitting effect is observed at conduction band minimum (CBm) and valence band maximum (VBM) for rhombohedral R3m and R3c phases. Linear Rashba terms successfully explain the splitting at VBM. However, cubic terms become important in realizing spin-orientation near CBm. Our results show the enhancement in Rashba parameters on tuning the ferroelectric order parameter. Further, we have observed reversal of spin-orientation on switching the direction of polarization.

cond-mat.mtrl-sci↗