arXiv subjects

Mohammad Amirabbasi

Publications and source records attributed to Mohammad Amirabbasi.

15 recordsLinked to original sources

arXiv2026

Altermagnetism of ultrathin CrSb slabs

Altermagnets exhibit momentum-dependent spin splitting without net magnetization, combining characteristics of both ferromagnets and antiferromagnets, making them highly interesting for spintronics applications. CrSb is a prime candidate with a high Néel temperature ($\sim700$~K) and a large exchange-driven splitting of $\sim0.6$--1~eV. Using ab-initio calculations, we consider slabs of various orientations in the ultrathin limit. We find that (100) oriented slabs have spin-degenerate bands. In (0001) oriented slabs, the exchange-driven altermagnetic spin splitting collapses, but including spin-orbit coupling restores a residual anisotropic splitting of $\sim70$~meV. In contrast, the (110) oriented slabs show an altermagnetic spin splitting of $\sim400$~meV, and emerges as a robust candidate for realizing large, exchange-driven altermagnetism

cond-mat.str-el↗

arXiv2026

Electron Polaron at Neutral 180$^\circ$ Domain Wall in PbTiO$_3$: Stability, Trapping Energies, and Transverse Polarization

We use density-functional theory with a Hubbard correction to investigate Ti-centered electron polarons at neutral PbO-centered $180^\circ$ domain walls in tetragonal PbTiO$_{3}$. The Hubbard parameter for Ti $3d$ states is determined using the finite-size-corrected polaronic energy-level alignment procedure, yielding stable electron-polaron formation in bulk PbTiO$_3$ with a trapping energy of $-$0.06 eV. In the domain-wall supercell, the excess electron localizes on Ti and forms a Ti$^{3+}$ center with an occupied $d_{xy}$ orbital in-gap state. Comparison of bulk-like and near-wall Ti sites shows that their trapping energies differ by only about 0.01 eV, indicating that this neutral domain wall does not provide a significant thermodynamic driving force for electron-polaron segregation. While the Ising-like reversal of the out-of-plane polarization is preserved, the localized electron induces a finite transverse polarization component normal to the wall, enhancing a local Néel-like distortion that is strongest when the polaron is located at the wall. These results show that neutral $180^\circ$ domain walls in PbTiO$_3$ do not substantially alter the stability of Ti-centered electron polarons, but they can couple to the polaron-induced lattice distortion through a localized transverse polarization response.

cond-mat.mtrl-sci↗

arXiv2026

First-principles investigation of small polarons in rhombohedral NaNbO$_{3}$

Sodium niobate (NaNbO$_{3}$) is a perovskite oxide and a key component of emerging lead-free antiferroelectric capacitors for high-energy-density applications. However, its performance can be hindered by irreversible phase transitions and leakage currents associated with low electrical resistivity. Defect and doping engineering offers a potential way to overcome these problems, but its use requires a detailed understanding of electronic, ionic, and polaron charge-compensation mechanisms, where the role of polarons remains largely unexplored. Here, we investigate the stability of small hole and electron polarons in rhombohedral NaNbO$_{3}$, which is a structurally well-defined model system that avoids lattice-dynamical instabilities. Trapping energies are calculated using density-functional theory corrected by a Hubbard $U$, using the enforced-piecewise-linearity approach including finite-size scaling. For the small hole-polaron centered on O-2$p$ orbital, we find a trapping energy of $-$0.65 (eV) and an adiabatic migration barrier of 0.32 (eV) determined by nudged-elastic-band calculations. In contrast, we show that excess electrons do not self-trap on Nb-4$d$ orbitals, reflecting weak electron-phonon coupling in the conduction band manifold. These results identify oxygen as an intrinsic hole trap in NaNbO$_{3}$ and highlight the importance of including hole polarons in defect models of NaNbO$_{3}$-based electroceramics.

cond-mat.mtrl-sci↗

arXiv2026

Why hole polaron formation on oxygen is limiting the Fermi level in Fe acceptor doped BaTiO$_{3}$ under oxidizing conditions

Oxidizing Fe-doped BaTiO$_3$ is commonly expected to convert substitutional Fe$^{3+}$ acceptors into formal Fe$^{4+}$ centers. Yet, the experimentally accessible picture based on electron-paramagnetic resonance (EPR) is dominated by Fe$^{3+}$-related signatures, while Fe$^{4+}$ is not a straightforward observable. Here we show that this apparent discrepancy reflects the preferred location of the oxidizing hole: not on Fe, but on oxygen. Using density-functional theory with with occupation-matrix control and a piecewise-linearity-based Hubbard correction (DFT+$U$) for O-2$p$ states, we find that an oxygen-centered hole polaron is forming a Fe$^{3+}$-O$^{-}$ complex that is lower in energy than the formal Fe$^{4+}$ configuration. Our results identify ligand-hole formation as a favorable charge-compensation mechanism in oxidized Fe-doped BaTiO$_3$ and provide an explanation for the predominance of Fe$^{3+}$-based centers in spectroscopy. More broadly, they show how oxygen polarons can limit Fermi-level shifts and control the electronic response of acceptor-doped ferroelectric perovskites.

cond-mat.mtrl-sci↗

arXiv2026

How semiconducting are ferroelectrics: The fundamental, optical and transport gaps of Na$_{0.5}$Bi$_{0.5}$TiO$_3$-BaTiO$_3$ and NaNbO$_{3}$

The energy gap is a fundamental property of materials, directly related to their optical and electronic properties. The energy gap of ferroelectric compounds and its adjustment by compositional variation has particularly attracted attention in recent years due to potential application in energy conversion and/or catalytic devices. It is demonstrated that it is necessary to distinguish between the fundamental gap, $E_{\rm g}^{0}$, the optical gap, $E_{\rm g}^{\rm opt}$, and the transport gap, $E_{\rm g}^{\rm tr}$, of ferroelectrics, which can differ significantly. The situation is comparable to those in organic semiconductors and emerges from the presence of localized charges. The fundamental gap is a ground state property, i.e.\ the energy difference between the maximum of the fully occupied valence band and the minimum of the completely empty conduction band. In contrast, the optical and transport gaps are excited state properties involving localized (polaronic) electrons and/or holes at energies considerably different from the band edges. This work illustrates how the different energy gaps of ferroelectrics can be determined by combining optical measurements, X-ray photoelectron spectroscopy and temperature and oxygen partial pressure dependent electrical conductivity measurements. We determine fundamental gaps of $\approx 4.5\,$eV for both materials, optical gaps of $3.25-3.45\,$eV/$3.5\,$eV and electrical gaps of $\approx 1.4\,$eV/$3.3\,$eV for Na$_{0.5}$Bi$_{0.5}$TiO$_3$-BaTiO$_3$/NaNbO$_{3}$, respectively.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic, Structural, and Electronic Properties of CrOCl with the PBE Functional

CrOCl is a van der Waals-layered insulator with an antiferromagnetic ground state, making it a promising platform for exfoliation and the exploration of low-dimensional magnetism. An accurate ab initio description is therefore essential. Previous density-functional studies have shown that DFT+$U$ calculations may erroneously favor ferromagnetic order depending on the choice of parametrization, an issue that cannot be remedied by simply adjusting the value of $U$. Here, we demonstrate that an explicit Hubbard correction is unnecessary: the PBE functional correctly reproduces the AFM ground state while simultaneously improving the description of structural properties. Moreover, PBE provides a reliable account of the electronic structure. These findings clarify the role of correlation effects in CrOCl and identify PBE as a robust starting point for future ab initio studies of CrOCl-based materials.

cond-mat.mtrl-sci↗

arXiv2025

Plasma engineered Hydroxyl Defects in NiO a DFTSupported-Spectroscopic Analysis of Oxygen Hole States and Implications for Water Oxidation

Controlling lattice oxygen reactivity in earth abundant OER catalysts requires precise tuning of defect chemistry in the oxide lattice. Here, we combine DFT+U calculations with plasma assisted synthesis to show how O2 and H2O in the discharge govern vacancy formation, electronic structure, and catalytic predisposition in NiO thin films. Oxygen rich plasmas generate isolated and clustered Ni vacancies that stabilize oxygen ligand hole states and produce shallow O 2p Ni 3d hybrid levels, enhancing Ni O covalency. In contrast, introducing H2O during growth drives local hydroxylation that compensates vacancy induced Ni3+ centers, restoring Ni2+ like coordination, suppressing deep divacancy derived in gap states, and introducing shallow Ni O H derived valence-band tails. EXAFS confirms that hydroxylation perturbs only the local environment while preserving the medium-range NiO lattice, and Ni L-edge spectroscopy shows a persistent but redistributed ligand-hole population. These complementary vacancy and hydroxylation driven pathways provide a plasma controlled route to pre define electronic defect landscapes in NiO and to tune its activation toward OER relevant NiOOH formation.

cond-mat.mtrl-sci↗

arXiv2025

DFT-Guided Operando Raman Characterization of Ni-Based Phases Relevant to Electrochemical Systems

We present a phase-resolved investigation of Ni-based oxides and hydroxides relevant to the oxygen evolution reaction (OER), combining ground-state DFT+U calculations with operando and in situ Raman spectroscopy, supported by high-resolution TEM. Five crystalline phases-cubic and hexagonal NiO, monoclinic and trigonal Ni(OH)2, and NiOOH-are systematically characterized in terms of their vibrational and electronic structure. Although the DFT models are idealized (0 K, defect-free, no solvation), they serve as clean, phase-specific references for interpreting complex experimental spectra. Cubic NiO is confirmed to be dynamically and electronically stable, consistent with dominant Raman modes observed experimentally. Despite dynamic instabilities in phonon dispersions, hexagonal NiO is structurally verified via TEM, suggesting substrate- or defect-stabilized metastability. Ni(OH)2 polymorphs are both vibrationally stable semiconductors, with the trigonal phase exhibiting stronger spin polarization. NiOOH exhibits spin-polarized electronic states across the Brillouin zone, consistent with its asymmetric band structure under ferromagnetic ordering. Independently, phonon calculations reveal soft modes near the Gamma-point, indicating dynamic instability under idealized conditions, yet operando Raman spectra align closely with calculated zone-center modes. However, introducing 0.03 Angstrom symmetry-breaking displacements relaxes the NiOOH lattice off its saddle point, removing imaginary phonon modes and stabilizing the phase. This integrated framework demonstrates how idealized DFT can reveal intrinsic fingerprints that anchor the interpretation of vibrational and electronic responses in catalytically active, dynamically evolving Ni-based materials.

cond-mat.mtrl-sci↗

arXiv2025

Impact of Ge, Ga, and Al doping on the mechanical and electronic properties of Cr$_3$Si: insights from first-principles calculations

This study systematically investigates the effects of Ge, Ga, and Al doping on the mechanical and electronic properties of cubic Cr$_3$Si using first-principles density functional theory (DFT). Doping increases lattice constants from 4.50 Å for undoped Cr$_3$Si to 4.51-4.53 Å (Ge), 4.52-4.54 Å (Ga), and 4.51-4.54 Å (Al) as doping concentrations increase from 12.5 $\%$ to 50 $\%$. Negative formation enthalpies across all configurations confirm thermodynamic stability, with values ranging from -0.35 eV/atom for undoped Cr$_3$Si to -0.33 eV/atom (Ge), -0.31 eV/atom (Al), and -0.25 eV/atom (Ga) at 50 $\%$ doping. Mechanical properties exhibit significant degradation with increased doping: bulk modulus decreases from 248.7 GPa for undoped Cr$_3$Si to 241 GPa (12.5 $\%$), 238 GPa (25 $\%$), 235 GPa (37.5 $\%$), and 231 GPa (50 $\%$) for Ge doping, with similar trends for Ga (230 GPa at 50 $\%$) and Al (232 GPa at 50 $\%$). Shear modulus and Young's modulus follow similar reductions, with shear modulus going from 158.9 GPa to 147 GPa (Ge), 145 GPa (Ga), and 147 GPa (Al) at 50 $\%$ doping. Elastic anisotropy increases notably with Al and Ga doping, while Ge maintains a relatively isotropic behavior. The wave velocities and Debye temperatures decrease for all dopants, with Debye temperature dropping from 720 K for undoped Cr$_3$Si to 700 K (Ge), 685 K (Ga), and 690 K (Al) at 50 $\%$ doping, reflecting a softer lattice and diminished thermal conductivity. While Al and Ga doping introduce higher anisotropy and reduce mechanical rigidity, Ge doping preserves isotropic mechanical behavior, making it the most suitable dopant for applications requiring balanced mechanical and thermal properties. These findings offer critical insights into tailoring Cr$_3$Si-based alloys for high-performance applications, highlighting trade-offs between stiffness, anisotropy, and thermal performance.

cond-mat.mtrl-sci↗

arXiv2024

Effect of biquadratic magnetic exchange interaction in the 2D antiferromagnets MPS_3 (M = Mn, Fe, Co, Ni)

The two-dimensional van der Waals (vdW) materials MPS_3(M = Mn, Fe, Co, Ni) display antiferromagnetic ordering of the magnetic moments at the transition metal ions. The possibility to exfoliate thin layers that preserve the magnetic order makes these materials interesting for numerous applications in devices that require integration of flexible patches of magnetic materials, e.g. in antiferromagnetic spintronics. Hence, an improved understanding of their magnetic properties is desirable. Here, we parameterize spin Hamiltonians for a monolayer of all four materials of this class using density functional theory plus Hubbard U calculations. We provide a step-by-step guide for calculating the magnetic exchange interactions and magnetic anisotropy energy using the (non-)collinear DFT+U(+ SOC) approach with a suitably chosen U for each material. It is found that the biquadratic interactions gain in importance while moving through the 3d series. Retaining the leading terms of a Holstein-Primakoff-transformed spin Hamiltonian, the magnon spectra are calculated. While MnPS_3 is found to be an almost isotropic antiferromagnet with a tiny gap, the biquadratic interaction opens an increasingly wider gap for FePS_3, CoPS_3 and NiPS_3. In line with this observation, Monte Carlo simulations demonstrate that the biquadratic interactions contribute to a systematic rise in the Neel temperature from FePS_3 to NiPS_3.

cond-mat.mtrl-sci↗

arXiv2023

$\textit{Ab initio}$ description of magnetic properties of spin-glass pyrochlore NaSrMn$_{2}$F$_{7}$

In this study, I have investigated the magnetic and critical properties of manganese pyrochlore fluoride NaSrMn$_{2}$F$_{7}$, which exhibits a glass transition at T$_\text{f}$$=$2.5 (K) due to charge disorder. A DFT+$U$+SOC framework is used in this paper to derive spin-Hamiltonian terms, including isotropic and anisotropic exchange interactions. An optimized geometry reveals a local distortion of the F-Mn-F angle along the $ < $111$ > $ direction (95.48$^{\circ}$ and 84.51$^{\circ}$), which is considered a weak bond disorder ($δJ$). In spite of the complex structure of this material, first principle calculations show that its magnetic properties are only controlled by the nearest neighbor's Heisenberg exchange interaction, and other interactions do not affect spin arrangements in the ground state. Thus, this material is considered a suitable candidate for studying electron correlation in spin glasses. Using a replica-exchange framework, Monte Carlo simulations indicate that no phase transition is observed when magnetic susceptibility changes with temperature. Based on $δJ$ and the spin Hamiltonian, 2.6 (K) is obtained as the phase transition temperature.

cond-mat.str-el↗

arXiv2023

Lattice Distortions and Magnetic Interactions in Single-Layer VOCl

Atomically thin layers exfoliated from magnetic van der Waals layered materials are currently of high interest in solid state physics. VOCl is a quasi-two-dimensional layered antiferromagnet which was recently synthesized in monolayer form. Previous theoretical studies have assumed the high-temperature orthorhombic lattice symmetry also in the low temperature range, where the bulk system is known to be monoclinic due to a strong magnetoelastic coupling. We demonstrate from \textit{ab-initio} calulations that this monoclinic distortion is prevalent also in monolayers, which is in line with recent experimental indications of monoclinic symmetry. Our calculations also show that competing ferromagnetic and antiferromagnetic interactions give rise a frustrated two-fold magnetic superstructure where higher-order magnetic interactions play a key role to stabilize the observed magnetic ground state.

cond-mat.mtrl-sci↗

arXiv2022

Orbital and magnetic ordering in single-layer FePS3: A DFT+U study

Among the numerous 2D system that can be prepared via exfoliation, iron phosphorus trisulfide (FePS3) attracts a lot of attention recently due to its broad-range photoresponse, its unusual Ising-type magnetic order and possible applications in spintronic nano-devices. Despite various experimental and theoretical-computational reports, there are still uncertainties in identifying its magnetic ground state. In this paper, we investigate the structural and magnetic properties of single-layer FePS3 by using Density Functional Theory. Our findings show that orbital ordering leads to a variation in distance between pairs of iron atoms by 0.14 Angstrom. These lattice distortions, albeit small, trigger different (ferromagnetic and antiferromagnetic) exchange couplings so that the ground state consists of ferromagnetically aligned zigzag chains along the long Fe-Fe bonds which couple antiferromagnetically along the shorter Fe-Fe bonds. Within the DFT+U framework, we parameterize a spin Hamiltonian including Heisenberg, single-ion anisotropy, Dzyaloshinskii-Moriya and biquadratic interactions. Using U=2.22eV gives a consistent description of both the electronic band gap and the Neel temperature in 2D FePS3.

cond-mat.mtrl-sci↗

arXiv2020

Ab initio determination of magnetic ground state of pyrochlore Y$_2$Mn$_2$O$_7$

There are two discrepant experimental results on the magnetic ground state of Y$_{2}$Mn$_{2}$O$_{7}$, one study proposes a spin glass state, while another introduces the material as a ferromagnet. In this study, we attempt to resolve this issue by employing density functional theory and Monte Carlo simulations. We derive different spin models by varying the Hubbard $U$ parameter in ab initio GGA+$U$ calculations. For the most range of Hubbard $U$, We obtain that the leading terms in the spin Hamiltonian are bi-quadratic and the nearest neighbor Heisenberg exchange interactions. By comparing Monte Carlo simulations of these models with the experiments, we find a ferromagnetic ground state for Y$_{2}$Mn$_{2}$O$_{7}$ as the most compatible with experiments. We also consider Y$_{2}$Mo$_{2}$O$_{7}$ as a prototype of the defect-free pyrochlore system with spin-glass behavior and compare it with Y$_{2}$Mn$_{2}$O$_{7}$. The orbital degrees of freedom are considered as a leading factor in converting a defect-free pyrochlore such as Y$_{2}$Mn$_{2}$O$_{7}$ to a spin glass system. By changing the $d$ orbital occupations of Mo atoms, our GGA+$U$ calculations for Y$_{2}$Mo$_{2}$O$_{7}$ indicate many nearly degenerate states with different $d$ orbital orientations which reveals $d$ orbital degrees of freedom in this material. While for Y$_{2}$Mn$_{2}$O$_{7}$, we find a single ground state with a fixed orbital orientation. Consequently, all of our ab initio approaches confirm Y$_{2}$Mn$_{2}$O$_{7}$ as a ferromagnetic system.

cond-mat.mtrl-sci↗

arXiv2018

Crossover between tricritical and Lifshitz points in pyrochlore FeF$_{3}$

Pyrochlore FeF$_{3}$ (pyr-FeF$_{3}$) is a Heisenberg anti-ferromagnetic (AF) with a magnetic susceptibility deviating from the Curie-Weiss law, even at the room temperature. This compound shows a transition to a long-range ordered state with all-in all-out (AIAO) spin configuration. The critical properties of this transition have remained a matter of dispute. In this work, to gain more insight into the critical properties of pyr-FeF$_{3}$, using ab initio density functional theory (DFT), we obtain spin Hamiltonian of this material under the relative volume change with respect to the experimental volume ($\frac{ΔV}{V_0}$) from $-0.2$ to $0.2$. We show that the relevant terms in the spin Hamiltonians are the AF exchange up to third neighbors, the nearest neighbor bi-quadratic and the direct Dyzaloshinski-Moriya (DM) interactions and find how these coupling constants vary under the volume change. Then we study the effect of volume change on the finite temperature critical behavior, using classical Monte Carlo (MC) simulation. We show that the spin system undergoes a weakly first order transition to AIAO at small volumes which turns to a second order transition close to the experimental structure. However, increasing $\frac{ΔV}{V_0}$ to $\sim0.2$, systems shows a transition to a modular spin structure. This finding suggests the existence of a Lifshitz point in pyr-FeF$_{3}$ and may explain the unusual critical exponents observed for this compound.

physics.comp-ph↗