arXiv subjects

Meysam Bagheri Tagani

Publications and source records attributed to Meysam Bagheri Tagani.

At least 19 recordsLinked to original sources

arXiv2026

Rank-Selective Optical Tomography of Higher-Wave Altermagnetism

Identifying the spatial rank of higher-wave altermagnetic order optically is challenging because local electric-dipole response does not uniquely resolve distinct continuum harmonics. We show that finite photon momentum turns one-photon spin-resolved absorption into a rank-selective tomography. For the planar $|m|=\ell$ sector of an even-parity $\ell$-wave component, a joint Fourier projection in polarization and momentum angle isolates $\mathcal T_\ell\proptoη_\ell q^{\ell-2}$, yielding the hierarchy $d:q^0$, $g:q^2$, and $i:q^4$; phase changes track rotations of the selected magnetic harmonic. A Ward-consistent finite-$q$ microscopic calculation reproduces these powers without imposing them. Although discrete crystal symmetry can generate lower-order local aliases, they are orthogonal to the selected momentum harmonic and cannot contaminate it below $q^{\ell-2}$. In MnTe, whose nonrelativistic parent order is three-dimensional $g$ wave while spin--orbit coupling lowers the exact relativistic spin-momentum-locking symmetry, first-principles calculations show that more than $99.9\%$ of the Fourier power of the Néel-projected $A$-region spin-energy contrast remains in the parent $g$-wave-derived $m=3$ harmonic. Structured near fields place the required momentum window within experimental reach.

cond-mat.mes-hall↗

arXiv2026

Ferroelectric Altermagnetic Chern Insulator in magnetic field: electrical control of the Chern number

We investigate electrically controllable Chern topology in a two-dimensional compensated (d)-wave altermagnet described by a lattice-regularized Bernevig--Hughes--Zhang model. In the altermagnetic reference state, the two Kramers sectors acquire momentum-dependent spin splitting and opposite sector Chern numbers, while the combined $C_{4z}\mathcal T$ symmetry enforces a vanishing total charge Chern number. We show that this hidden topological structure can be activated by orbital-selective magnetic coupling and independently tuned by ferroelectric orbital hybridization. The magnetic coupling removes the sector cancellation and generates $C=\pm1$ and $\pm2$ phases, whereas the polar distortion shifts the Dirac gap closing away from high-symmetry momenta and enables electrical transitions such as $C=0\rightarrow-1$ and $C=1\rightarrow2$. The resulting phases exhibit the expected chiral edge-state multiplicity and quantized anomalous Hall conductivity. Their topology is further reflected in the orbital magnetization through the in-gap relation $\partial_μ\widetilde M_z=-C$. Finally, we show that the Chern phases remain robust against transverse Kramers-sector mixing and symmetry-allowed inversion-asymmetric spin--orbit coupling. These results establish a symmetry-based route to electrically tunable Chern insulating phases in compensated altermagnets.

cond-mat.mes-hall↗

arXiv2026

Defect Geometry Selects Polar and Anomalous Hall Phases in Two-Dimensional Altermagnets

Point defects in altermagnets can create phases absent in the pristine host by selectively breaking crystal symmetries. Combining symmetry analysis, first-principles calculations, and Hamiltonian modeling, we identify how point impurities modify the altermagnetic phase. Using the pristine d- wave altermagnetic monolayer V2Se2O as a testbed, we identify three distinct classes of impurities: those that preserve spin-momentum locking, those that induce a hybrid-parity state associated with Edelstein spin conversion, and those that produce a metallic ferrimagnetic state with an anomalous Hall effect. We further discuss the robustness of two-dimensional altermagnets against point impurities. Results for other two-dimensional systems, such as Mn4N2 and 2H-FeBr3, reveal the same symmetry-based control across distinct lattices and parent spin harmonics, establishing defect geometry as a general route for engineering spin textures and transport properties.

cond-mat.mtrl-sci↗

arXiv2026

Berry-Curvature Activation by Orbital Flux in a Kagome Altermagnet

We investigate topological electronic responses in a kagome altermagnetic metal hosting a compensated coplanar $120^\circ$ magnetic texture. Using a minimal tight-binding model containing nearest-neighbor hopping, noncollinear exchange coupling, intrinsic spin--orbit coupling, and a time-reversal-odd loop-current order, we disentangle the magnetic, orbital, and relativistic mechanisms governing the electronic response. The exchange field produces pronounced momentum-dependent spin splitting and spin-polarized Fermi surfaces without generating a net magnetization. Nevertheless, in the absence of loop-current order, a hidden antiunitary symmetry $\mathcal{T}C_{2z}$ enforces vanishing Berry curvature and intrinsic anomalous Hall conductivity, even for finite spin--orbit coupling. A directed imaginary bond order breaks this protection and activates finite Berry curvature and a sizable, strongly filling-dependent Hall response already in the nonrelativistic limit. Spin--orbit coupling subsequently reconstructs the avoided crossings and redistributes the Berry curvature, enhancing or suppressing the Hall response depending on filling. For sufficiently strong loop-current order and spin--orbit coupling, a global gap opens at $n_e=3$, and the Hall conductivity approaches $2e^2/h$, consistent with an occupied-band Chern number of magnitude two. Parameter-space and filling-dependent calculations further demonstrate that the Hall-active regime extends over broad ranges of exchange coupling, spin--orbit coupling, and chemical potential and remains robust against symmetry-preserving longer-range hopping. These results identify orbital-current order as an independent route for converting a Hall-silent kagome altermagnet into an anomalous Hall metal or a gapped topological phase without net magnetization, noncoplanar spin order, or scalar spin chirality.

cond-mat.mtrl-sci↗

arXiv2026

Machine Learning and Deep Learning in Quantum Materials: Symmetry, Topology, and the Rise of Altermagnets

The landscape of condensed matter physics is facing an unprecedented data surge driven by high-throughput ab initio workflows and rapidly expanding experimental datasets. Traditional first-principles methods such as Density Functional Theory (DFT), despite their foundational role, suffer from cubic scaling, creating a major bottleneck when exploring the vast chemical space of quantum materials. This review analyzes how Machine Learning (ML) and Deep Learning (DL) are overcoming these limitations and accelerating the discovery of exotic phases of matter. We examine the shift from rigid descriptor-based models to flexible, symmetry-aware architectures, particularly E(3)-equivariant Graph Neural Networks (GNNs) that respect rotational and translational invariance. A central focus is the automated identification of topological phases, where ML models exploit symmetry indicators and elementary band representations to diagnose non-trivial topology without costly band structure integrations. The discussion culminates in a case study of the Altermagnet, a recently identified third class of magnetism beyond the ferromagnetic, antiferromagnetic dichotomy. We highlight how specialized AI search engines, combining graph theory with crystallographic symmetry analysis, have uncovered d-wave, g-wave, and even i-wave altermagnets, expanding the known landscape of magnetic order. The review concludes by addressing the interpretability gap and advocates for symbolic regression and active-learning frameworks to connect black-box predictions with experimentally verifiable mechanisms.

cond-mat.mes-hall↗

arXiv2026

Quantum anomalous Hall conductivity in altermagnets under applied magnetic field

We investigate the emergence of quantum anomalous Hall conductivity in a two-dimensional $d$-wave altermagnet on a Lieb lattice under an external magnetic field. Altermagnetic order induces momentum-dependent spin splitting without net magnetization in the relativistic limit, producing distinct spin-resolved bands at the $X$ and $Y$ valleys. The phase diagram features a normal insulator and a spin Chern insulator separated by an accidental Dirac semimetal. The magnetic field breaks rotational symmetry between valleys while maintaining vanishing total magnetization, enabling independent valley contributions to topology. One valley supports Chern numbers $C=-1$ or $0$, while the other hosts $C=0$ or $+1$, governed by field strength and bandwidth. This competition yields valley-dependent topology. Berry curvature analysis reveals fully gapped phases with total Chern numbers $C=\pm1$, separated by valley-selective gap closings. We uncover a mechanism for rapid magnetic control of the quantum anomalous Hall effect near the semimetal phase and highlight key distinctions from ferro-valleytronic and quantum spin Hall systems.

cond-mat.mes-hall↗

arXiv2026

Long-Lived Interlayer Excitons and Type-II Band Alignment in Janus MoTe2/CrSBr van der Waals Heterostructures

Identifying two-dimensional heterostructures with exceptional electronic and optical properties remains an active area of research in advanced optoelectronics. Here, we present a comprehensive first-principles investigation of the electronic, optical, and excitonic properties of a MoTe2/CrSBr van der Waals heterostructure using density functional theory combined with fully relativistic GW and Bethe-Salpeter equation calculations. The close lattice matching between the two monolayers enables the formation of stable heterobilayers with two inequivalent interfaces (Te-S and Te-Br) arising from the Janus nature of CrSBr. Both interfaces are dynamically and thermally stable and exhibit type-II band alignment with a direct quasiparticle gap, promoting efficient spatial separation of electrons and holes. The heterostructure hosts interlayer excitons with lifetimes 18-45 ps significantly longer than those of the intralayer excitons in the isolated MoTe2, 3.6 ps, and CrSBr, 8.1 ps, monolayers. Moreover, the optical gap, exciton binding energy, and exciton lifetime of the heterostructure are strongly modulated by the built-in electric field associated with the Janus layer. These results establish the MoTe2/CrSBr heterostructure as a versatile platform for engineering long-lived interlayer excitons and highlight its potential for next-generation optoelectronic and light-harvesting applications.

cond-mat.mtrl-sci↗

arXiv2026

Kagome goldene with flat bands and Dirac nodal line fermions via line-graph epitaxy

The kagome lattice has emerged as a promising platform for investigating exotic quantum phases. However, achieving a single-atomic-layer kagome lattice in elemental materials remains a significant challenge. Here, we introduce line-graph epitaxy, a novel approach that enables the atomic-scale synthesis of goldene, a monolayer of elemental gold atoms arranged in a kagome lattice. Through scanning tunneling microscopy/spectroscopy (STM/STS), and density functional theory (DFT) calculations, we demonstrate the formation of kagome goldene, featuring a flat band with a van Hove singularity approximately 1.1 eV below the Fermi level, signaling strong electron correlation effects. Notably, the flat band is disrupted at the zigzag edges of goldene nanoflakes, revealing substantial edge effects. Furthermore, our calculations show that weak interlayer interactions between goldene and the underlying Au2Ge substrate generate dual Dirac nodal lines through a proximity effect. These findings offer not only a novel strategy for constructing elemental kagome lattices, but also a generalizable framework for fabricating and controlling line-graph materials. This research advances the exploration of quantum phases driven by strong correlations and the design of materials for next-generation quantum technologies.

cond-mat.mes-hall↗

arXiv2025

First-Principles Insights into Excitonic and Electron-Phonon Effects in van der Waals Heterostructures

Motivated by the successful synthesis of isolated ZrS2 and HfS2 transition metal dichalcogenide (TMD) monolayers and inspired by their nearly identical lattice constants, we construct and investigate a vertical ZrS2/HfS2 van der Waals (vdW) heterostructure. Using first-principles calculations based on density functional theory (DFT) and many-body perturbation theory (MBPT), we explore its electronic, optical, and excitonic properties, with particular emphasis on excitonic effects and their temperature dependence. Based on the GW method, the ZrS2/HfS2 vdW heterostructure exhibits an indirect band gap of 2.60 eV with a Type-I band alignment. The optical gap of the heterostructure is found to be 2.64 eV, with an exciton binding energy of 0.71 eV, both reduced compared to those in the isolated monolayers. Moreover, we investigate the temperature-dependent optoelectronic behavior of the heterostructure, considering electron-phonon coupling. A zero-point renormalization of 0.04 eV in the direct band gap is observed. While the direct band gap decreases monotonically with temperature from 0 K to 400 K, the indirect band gap displays a non-monotonic trend. As a result, the absorption spectrum undergoes a meaningful redshift with increasing temperature. At room temperature, the optical gap of the heterostructure is reduced to 2.51 eV and the exciton binding energy to 0.63 eV. Our findings highlight the important role of electron-phonon interaction in the optoelectronic response of ZrS2/HfS2 vdW heterostructure, supporting its use in high-performance optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2024

CoF3: a g-wave Altermagnet

Altermagnetism, a novel magnetic phase bridging ferromagnetism and antiferromagnetism, exhibits zero net magnetization due to its unique alternating spin arrangements, which cancel out macroscopic magnetization. This phase is characterized by robust time-reversal symmetry breaking and spin-momentum locking, leading to distinct electronic properties advantageous for spintronic applications. In this study, we explore the possibility of altermagnetism in cobalt trifluoride (CoF3) using density functional theory (DFT) with Hubbard U correction combined with spin group theory. Our findings reveal that CoF3 exhibits zero net magnetization similar to a g-type antiferromagnet but with spin degeneracy breaking without spin-orbit coupling, akin to a ferromagnet. The optimized structure of CoF3, characterized by a rhombohedral lattice with centrosymmetric symmetry group R3c, shows significant spin splitting in both valence and conduction bands, reaching up to 45 meV. This spin splitting is attributed to the electric crystal potential and the anisotropy of the spin density, leading to the breaking of Kramers degeneracy.

cond-mat.mtrl-sci↗

arXiv2024

Origin and properties of the flat band in NbOCl2 monolayer

The existence of a flat band near the Fermi level can be a suitable platform for the emergence of interesting phenomena in condensed matter physics. Recently, NbOCl2 monolayer has been experimentally synthesized [Nature 613 (2023) 53], which has a flat and isolated valence band. We show that monolayers based on other elements of group 5 of the periodic table, including the V and Ta atoms, also have a flat band. Motivated by the recent experiment, we investigate the origin of the flat band as well as the electronic, optical, photocatalytic, and magnetic properties of the monolayer by combining density functional theory and many-body quantum perturbation theory. Our results show that the flat and isolated band of this monolayer is caused by the interplay between the Peierls distortion and the electronic configuration of Nb atoms. The investigation of the bandwidth of the monolayer under the biaxial and uniaxial strains reveals that this material can be grown on substrates with a larger lattice constant by maintaining the flat band. Examining the material's response to the linearly polarized light not only reveals the presence of weak optical anisotropy, but also shows the existence of a bright exciton with a binding energy of about 0.94 eV. Hole doping can result in a flat band-induced phase transition from semiconductor to ferromagnet. By adjusting the amount of doping, a bipolar magnetic semiconductor or a half-metal can be created. The interaction between the nearest Nb atoms is ferromagnetic, while an antiferromagnetic interaction appears between the second neighbors, which grows significantly with increasing doping. Our results demonstrate that NbOCl2 monolayer has suitable potential for spintronic applications in addition to electronic and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Moiré-induced bandgap tuning by varying electric dipole in InSe/CuSe vertical heterostructure

The stacked two layered materials with a lattice constant mismatch and/or with twist angle relative to each other can create a moiré pattern, modulating the electronic properties of the pristine materials. Here, we combine scanning tunneling microscopy/spectroscopy and density functional theory calculations to investigate the moiré potential induced bandgap tuning in InSe/CuSe vertical heterostructure synthesized by a two-step of molecular beam epitaxy. Scanning tunneling microscopy measurements demonstrate the heterostructure with a superlattice periodicity of ~3.48nm and a twist angle of about 11° between the monolayers. Scanning tunneling spectroscopy record on the different stacking sites of the heterostructure reveals the bandgap of the InSe is location-dependent and a variation of 400 meV is observed. Density functional theory calculations reveal that the moiré-induce electric dipole in the monolayer InSe is the key factor for tuning the bandgap. Besides, charge transfer between CuSe and InSe also contributes to the bandgap variation due to its stacking related. We also show that the moiré potential not only can tune the bandgap of InSe but also can vanish the Dirac nodal line of CuSe in some stackings. Our explorations provide valuable information in understanding the electronic properties of the twodimensional moiré materials.

cond-mat.mtrl-sci↗

arXiv2022

Bilayer borophene: The effects of substrate and stacking

Bilayer borophene has recently attracted much interest due to its outstanding mechanical and electronic properties. The interlayer interactions of these bilayers are reported differently in theoretical and experimental studies. Herein, we design and investigate bilayer beta12-borophene, by first-principles calculations. Our results show that the interlayer distance of the relaxed AA-stacked bilayer is about 2.5 A, suggesting a van der Waals (vdW) interlayer interaction. However, this is not supported by previous experiments, therefore by constraining the interlayer distance, we propose a preferred model which is close to experimental records. This preferred model has one covalent interlayer bond in every unit cell (single-pillar). Further, we argue that the preferred model is nothing but the relaxed model under a 2% compression. Additionally, we designed three substrate-supported bilayers on the Ag, Al, and Au substrates, which lead to double-pillar structures. Afterward, we investigate the AB stacking, which forms covalent bonds in the relaxed form, without the need for compression or substrate. Moreover, phonon dispersion shows that, unlike the AA stacking, the AB stacking is stable in freestanding form. Subsequently, we calculate the mechanical properties of the AA and AB stackings. The ultimate strengths of the AA and the AB stackings are 29.72 N/m at 12% strain and 23.18 N/m at 8% strain, respectively. Moreover, the calculated Young's moduli are 419 N/m and 356 N/m for the AA and the AB stackings, respectively. These results show the superiority of bilayer borophene over bilayer MoS2 in terms of stiffness and compliance. Our results can pave the way for future studies on bilayer borophene structures.

cond-mat.mtrl-sci↗

arXiv2022

Optoelectronic properties of the CuI, AgI and Janus Cu2BrI, and Ag2BrI monolayers by many-body perturbation theory

In an outstanding experimental advance in the field of two-dimensional nanomaterials, cuprous iodide (CuI) and silver iodide (AgI) monolayers have been grown via a novel graphene encapsulation synthesis approach [Adv.Mater.2022, 34, 2106922]. Inspired by this accomplishment, we conduct first-principles calculations to investigate the elastic, phononic thermal transport, electronic, and optical properties of the native CuI and AgI and Janus Cu2BrI and Ag2BrI monolayers. Electronic and excitonic optical properties are elaborately studied using the many-body perturbation theory on the basis of GW approximation. Our results indicate that these novel systems are stable but with soft elastic modulus and ultralow lattice thermal conductivity. It is also shown that the studied monolayers are wide-gap semiconductors with exciton binding energies close to 1 eV. The effects of mechanical straining and electric field on the resulting electronic and optical properties are also analyzed. The presented first-principles results provide a deep understanding of the stability, phononic transport, and tunable optoelectronic properties of the native CuI and AgI and Janus Cu2BrI and Ag2BrI monolayers, which can serve as a guide for the oncoming studies.

cond-mat.mtrl-sci↗

arXiv2022

Mechanical, optical, and thermoelectric properties of semiconducting ZnIn2X4 (X= S, Se, Te) monolayers

Mechanical stability of the ZnIn2X4 monolayers. The ZnIn2S4 and ZnIn2Se4 are semiconductors with direct band gaps of 3.94 and 2.77 eV, respectively whereas the ZnIn2Te4 shows an indirect band gap of 1.84 eV at the G0W0 level. The optical properties achieved from the solution of the Bethe-Salpeter equation predict the exciton binding energy of the ZnIn2S4, ZnIn2Se4, and ZnIn2Te4 monolayers to be 0.51, 0.41, and 0.34 eV, respectively, suggesting the high stability of the excitonic states against thermal dissociation. Using the iterative solutions of the Boltzmann transport equation accelerated by machine learning interatomic potentials, the room-temperature lattice thermal conductivity of the ZnIn2S4, ZnIn2Se4, and ZnIn2Te4 monolayers is predicted to be remarkably low as 5.8, 2.0, and 0.4 W/mK, respectively. Due to the low lattice thermal conductivity, high thermopower, and large figure of merit, we propose the ZnIn2Se4 and ZnIn2Te4 monolayers as promising candidates for thermoelectric energy conversion systems. This study provides an extensive vision concerning the intrinsic physical properties of the ZnIn2X4 nanosheets and highlights their characteristics for energy conversion and optoelectronics applications.

cond-mat.mtrl-sci↗

arXiv2021

Electronic and Excitonic Properties of Semi-Hydrogenated Borophene Sheets

Borophene has triggered a surge of interest due to its outstanding properties including mechanical flexibility, polymorphism, and opto-electrical anisotropy. Very recently, a novel semi-hydrogenated borophene, called $α'$-4H, was synthesized in large-scale freestanding samples, which exhibits excellent air-stability and semiconducting nature. Herein, using the density functional theory (DFT) and many-body perturbation theory (MBPT), we investigate the electronic and excitonic optical properties of $α'$-4H borophene. The DFT results reveal that by breaking the mirror symmetry and increasing the buckling height of pure $α'$-borophene, hydrogenation causes an orbital hybridization and opens an indirect band gap of 1.49 eV in $α'$-4H borophene. This value is corrected to be 1.98, 2.23, and 2.52 eV under the G0W0, GW0, and GW levels of theory, respectively. The optical spectrum achieved from solving the Bethe-Salpeter equation shows an optical band gap of 2.40 eV, which corresponds to a strongly bound and stable bright exciton with a binding energy of 1.18 eV. More importantly, the excitonic states are robust against tension up to 10%, where the monolayer is dynamically stable. We also design and study the bilayer $α'$-4H borophene with different stackings. For the weak van der Waals interactions between the layers, the bilayer can preserve most of the structural and electronic properties of the monolayer. Our study exposes the underlying physics behind the structural, electronic, and optical properties of $α'$-4H borophene and suggests it as a very promising candidate for flexible optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Prediction of group IV-V hexagonal binary monolayers

Group IV and V monolayers are very crucial 2D materials for their high carrier mobilities, tunable band gaps, and optical linear dichroism. Very recently, a novel group IV-V binary compound, Sn2Bi, has been synthesized on silicon substrate, and has shown very interesting electronic properties. Further investigations have revealed that the monolayer would be stable in freestanding form by hydrogenation. Inspired by this, by means of first-principles calculations, we systematically predict and investigate eight counterparts of Sn2Bi, namely Si2P, Si2As, Si2Sb, Si2Bi, Ge2P, Ge2As, Ge2Sb, and Ge2Bi. The cohesive energies, phonon dispersions, and AIMD calculations show that, similar to Sn2Bi, all of these freestanding monolayers are stable in hydrogenated form. These hydrogenated monolayers are semiconductors with wide band gaps, which are favorable for opto-electronic purposes. The Si2YH2 and Ge2YH2 structures possess indirect and direct band gaps, respectively. They represent very interesting optical characteristics, such as good absorption in the visible region and linear dichroism, which are crucial for solar cell and beam-splitting devices, respectively. Finally, the Si2SbH2 and Si2BiH2 monolayers have suitable band gaps and band edge positions for photocatalytic water splitting. Summarily, our investigations offer very interesting and promising properties for this family of binary compounds. We hope that our predictions open ways to new experimental studies and fabrication of suitable 2D materials for next generation opto-electronic and photocatalytic devices.

cond-mat.mtrl-sci↗

arXiv2020

Antimonene/Bismuthene Vertical Van-der Waals Heterostructure: A Computational Study

In this paper, the structural, electronic, mechanical and optical properties of antimonene-bismuthene Van-der Waals heterostructure (Sb-Bi HS) were calculated based on the first principle density functional theory. We explored different stacks of Sb-Bi HS to find the most and the least stable staking for this heterostructure. At the GGA level of theory, the most stable model is a semiconductor with an indirect band gap of 159 meV. However, when the spin-orbit (SO) interaction is considered, the VBM and CBM touch the Fermi level and the HS becomes a semimetal. Our results also show that the electronic properties of the HS are robust against the external electric field and biaxial strain. Young modulus was calculated as 64.3N/M which predicts this HS as a resistant material against being stretched or compressed. The calculated optical properties, similar to monolayer antimonene, are completely dependent on the polarization of incident light and differ when parallel or perpendicular polarization is considered. Moreover, the absorption coefficient for perpendicular polarization in the visible region is significantly increased in comparison with the monolayer antimonene. High structural stability, electronic and mechanical robustness against electric field and strain, along with polarization-dependent optical properties of this HS, promise for its applications in beam splitters and nano-scale mirrors.

cond-mat.mes-hall↗