arXiv subjects

Sahar Izadi Vishkayi

Publications and source records attributed to Sahar Izadi Vishkayi.

17 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

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

Effect of polar distortions on the linear and nonlinear anomalous Hall conductivity of altermagnetic $α$-MnTe

Altermagnetic $α$-MnTe with Néel vector along the $y$-axis exhibits a finite anomalous Hall conductivity (AHC) and weak ferromagnetism along the $z$-axis. As already demonstrated in the bulk, there is the breaking of the C$_6$ symmetry by the in-plane Néel vector, leaving a C$_2$-type magnetic symmetry. The surface of $α$-MnTe breaks the C$_2$, leaving only a time-reversed mirror symmetry with respect to the $x=0$ plane. Therefore, we demonstrate that on the surface, the interplay between breaking of the crystal symmetry and Néel vector orientation produces a reduction of the space group from hexagonal P6$_3$/mmc to orthorhombic Amm2. As a result, the surface exhibits not only a polar distortion along the $z$-axis, but also a polar distortion and a weak ferrimagnetism along the $y$-axis. To describe the surface of MnTe in an accessible way, we simplify the problem and examine the effect of the in-plane electric field in bulk MnTe. Moreover, as a doped ionic semiconductor, the properties of MnTe can be influenced by lattice polarization under an applied electric field. We investigate the interplay between the intrinsic anomalous Hall effect and lattice polarization, showing that polarization effects can substantially affect the AHC. Since the electric field breaks inversion symmetry, this contribution from the lattice polarization coexists with the non-linear anomalous Hall effect, highlighting the rich transport phenomenology of altermagnets.

cond-mat.mtrl-sci↗

arXiv2026

Magnetically tunable symmetry-enforced nodal lines producing huge anomalous Hall conductivity in altermagnetic $α$-MnTe

Altermagnetic $α$-MnTe exhibits huge anomalous Hall conductivity (AHC) up to room-temperature together with weak ferromagnetism arising from spin and orbital polarizations. We clarify the origin of the large value of the AHC by identifying two sets of distinct symmetry-enforced nodal lines in the valence bands with Mn character, located at $k_z=0$ and $k_z=\fracπ{c}$, protected by mirror symmetry $M_z$ and glide symmetry $G_z = \{M_z\,|\,0,0,\tfrac{c}{2}\}$, respectively. Both nodal lines are energy-dependent with an approximate C$_6$ symmetry, which is reduced to an exact C$_2$ symmetry due to the presence of the Néel vector. The highest valence band exhibits a Mexican-hat dispersion, whereas the second-highest valence band exhibits an inverted Mexican-hat dispersion, with nodal lines at the crossing between the two bands. Within first-principles accuracy, we demonstrate that these nodal lines give rise to the large AHC observed experimentally and exhibit a strong interplay with the weak ferromagnetism. We further show that even a small spin canting strongly modifies the nodal lines and the AHC, making them both magnetically tunable. By disentangling the altermagnetic and ferromagnetic contributions to the AHC, the altermagnetic contribution dominates at small canting angles, while the ferromagnetic contribution becomes sizeable for larger values. Using linear dichroism in angle-resolved photoemission spectroscopy, we show a signature of the nodal line at the border of the Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2026

Symmetry-protected nodal planes and accidental nodal surfaces in mixed odd-even wave spin-momentum locking of relativistic altermagnets

Non-relativistic spin--momentum locking in altermagnets exhibits an even number of nodal planes. In the relativistic limit, the number of nodal planes can be lowered by symmetry reduction due to the Néel vector and spin--orbit coupling in noncentrosymmetric systems. Therefore, an analysis of the evolution of the nodal planes in relativistic altermagnets is required. While $g$-wave spin--momentum locking is straightforward to realize in non-relativistic altermagnets, this $g$-wave does not necessarily survive in the relativistic case. In this work, we investigate the relativistic spin--momentum locking of the centrosymmetric CrSb and the noncentrosymmetric wurtzite MnTe. As a first result, we show that in both systems the dominant spin component retains its $g$-wave character in the relativistic regime only when the Néel vector is oriented along the $z$-axis, while the subdominant components exhibit $d$-wave symmetry in CrSb and $p$-wave symmetry in ferroelectric wurtzite MnTe. More generally, the $g$-wave character is preserved in the relativistic limit only when both the Néel vector and the electric field associated with inversion-symmetry breaking are oriented along the $z$-axis. As a second result, we show that relativistic spin--momentum locking of ferroelectric altermagnets can exhibit $p$-wave magnetism with one symmetry-protected nodal plane and an accidental nodal surface not protected by symmetry, or can have two accidental nodal surfaces. With the Néel vector aligned along the $x$-axis, selected bands of ferroelectric altermagnet wurtzite MnTe exhibit $p$-wave magnetism. Our results establish that altermagnets can host distinct spin components that realize a mixture of angular-momentum wave symmetries in momentum space in the relativistic limit.

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↗

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↗

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

Strain and electric-field control of spin-spin interactions in monolayer CrI$_3$

We investigate the impact of mechanical strains and a perpendicular electric field on the electronic and magnetic ground-state properties of two-dimensional monolayer CrI$_3$ using density functional theory. We propose a minimal spin model Hamiltonian, consisting of symmetric isotropic exchange interactions, magnetic anisotropy energy, and Dzyaloshinskii-Moriya (DM) interactions, to capture most pertinent magnetic properties of the system. We compute the mechanical strain and electric field dependence of various spin-spin interactions. Our results show that both the amplitudes and signs of the exchange interactions can be engineered by means of strain, while the electric field affects only their amplitudes. However, strain and electric fields affect both the directions and amplitudes of the DM vectors. The amplitude of the magnetic anisotropy energy can also be substantially modified by an applied strain. We show that in comparison with an electric field, strain can be more efficiently used to manipulate the magnetic and electronic properties of the system. Notably, such systematic tuning of the spin interactions is essential for the engineering of room-temperature spintronic nanodevices.

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↗

arXiv2019

Tuning electronic and optical properties of free-standing Sn2Bi monolayer stabilized by hydrogenation

In this study, we systematically investigated the structural, mechanical, electronic and optical properties of Sn2Bi monolayer, a sheet experimentally synthesized recently [PRL, 121, 126801 (2018)] which has been hydrogenated (Sn2BiH2) to stabilize free-standing form using density functional theory (DFT). For tuning the band structure and electronic properties, the mechanical strain and electric field are used. Our investigations show that in this free-standing sample, there are electron flat bands and free hole bands like the recently synthesized sample on silicon wafer, which provide the possibility of having strongly localized electrons and free holes with high mobility. Also, the band gap of Sn2BiH2 monolayer has experienced a growth of 80% compared with the experimental sample. The relevant results to strain suggest that the band gap can be properly manipulated by biaxial strain (-13% to +21%) within a range from 0.2 to 1.6 eV. It should be mentioned that the stability and flexibility of the corresponding monolayer under tensile and compressive strain are due to the strong σ bonds between atoms. We also realized the strain can cause indirect-direct transition in the band gap. Furthermore, our optical findings indicate that the Sn2BiH2 monolayer has almost metallic properties in a specific range of UV spectrum and it is transparent in the IR and visible spectrum of electromagnetic radiation. All these tunable properties and nontrivial features portend that Sn2BiH2 monolayer has great potentials in applications as near-infrared detectors, thermoelectric devices, field-effect transistors, sensors, photocatalysis, energy harvesting, and optoelectronics.

cond-mat.mtrl-sci↗

arXiv2019

Tuning thermoelectric efficiency of polyaniline sheet by strain engineering

Two-dimensional polyaniline monolayer (C3N) has been recently synthesized as an indirect semiconductor with high electron mobility. In this research, with combination of density functional theory and Green function formalism, we investigate electrical and thermal properties of C3N sheet in details. It is observed that a tensile strain along zigzag direction can induce a transition from indirect to direct semiconductor, whereas the sheet transits from semiconductor to metal under compressive strain. Thermoelectric efficiency of unstretched C3N sheet is higher in p-doping and its maximum value is obtained when the transport is along zigzag direction. A reduction in figure of merit is found upon applying strain independent from its direction. To overcome the reduction, we show that when the electrical transport and strain are perpendicular to each other, thermoelectric efficiency of the C3N sheet can be significantly increased dependent on the kind of strain (tensile or compression). Results predict the potential application of C3N sheet in thermoelectric and optoelectronic industry by strain engineering.

cond-mat.mes-hall↗

arXiv2018

Polyaniline (C_{3}N) nanoribbons: Magnetic metal, Semiconductor, and Half-Metal

Two-dimensional polyaniline sheet has been recently synthesized and found that it is a semiconductor with indirect band gap. Polyaniline nanoribbons decomposed from two-dimensional polyaniline sheet (C3N sheet) are investigated using density functional theory. The existence of nitrogen atoms in the edge of the ribbons increases stability and magnetization of the ribbons and make them different from graphene nanoribbons. Unsaturated nanoribbons are magnetic metals so that armchair C3N nanoribbons are gap-less spin semicon- ductors in the antiferromagnetic state and half-metals in the ferromagnetic state. A transition from metal to semiconductor is observed in the armchair C3N nanoribbons when the edge atoms are passivated by hy- drogen. The band gap of hydrogen saturated armchair C3N nanoribbons can be controlled using an external transverse electric field so that its magnitude is dependent on the direction of the electric field. Being metal or semiconductor in hydrogen saturated zigzag C3N nanoribbons is strongly dependent on the edge atoms so that just ribbons having nitrogen atoms in both edges are semiconductor. An external electric field cannot induce any spin polarization in the zigzag nanoribbons which is in contrast with what was observed in zigzag

cond-mat.mes-hall↗

arXiv2017

Edge-dependent electronic and magnetic characteristics of freestanding β_12-Borophene Nanoribbons

Nanoribbons produced from cutting β_12-Borophene sheet is investigated by density functional theory. The electronic and magnetic properties of Borophene nanoribbons are studied and found that all considered ribbons are metal which is in good agreement with recent experimental results. β_12-Borophene nanoribbons have a lot of diversity due to existence of 5 Boron atoms in a unit cell of the sheet. The magnetic properties of ribbons are strongly dependent on the cutting direction and the edge profile. It is interesting that a ribbon with a specific width can be a normal or ferromagnetic metal with magnetization in just one edge or two edges. The spin anisotropy is observed in some ribbons so that magnetic moment is not the same in both edges in antiferromagnetic configuration. The effect comes from the edge asymmetry of the ribbons and results in the breaking of spin degeneracy in the bandstructure. Our findings show that β_12 nanoribbons are potential candidates for next-generation spintronic devices.

cond-mat.mes-hall↗

arXiv2017

Current-Voltage Characteristics of Borophene and Borophane Sheets

Motivated by recent experimental and theoretical research on a monolayer of Boron atoms, Borophene, current-voltage characteristics of three different Borophene sheets, 2Pmmn, 8Pmmn and 8Pmmm, are calculated using density functional theory combined with nonequilibriumGreen function formalism. Borophene sheets with two and eight atoms in a unit cell are considered and bandstructure, electron density and structural anisotropy of them are analyzed in details. Results show that 8Pmmn and 8Pmmm structures which have eight atoms in the unit cell have less anisotropy than 2Pmmn. In addition, although 8Pmmn shows a Dirac cone in the bandstructure, its current is lower than two others. We also consider a fully hydrogenated Borophene, Borophane, and find that the hydrogenation process reduces the structural anisotropy and the current significantly. Our findings reveal that the current-voltage characteristics of the Borophene sheets can be used to detect the kind and the growth direction of the sample because it is strongly dependent on the direction of the electron transport, anisotropy and details of the unit cell of the Borophene.

physics.chem-ph↗