arXiv subjects

Siavash Karbasizadeh

Publications and source records attributed to Siavash Karbasizadeh.

5 recordsLinked to original sources

arXiv2026

Revealing Charge Transfer in Defect-Engineered 4H$_\mathrm{b}$-TaS$_2$

We present a comprehensive first-principles investigation of defects in 4$H_b$-TaS$_2$. In this layered transition metal dichalcogenide, charge transfer between alternating Mott-insulating 1T and metallic 1H layers gives rise to exotic quantum phases such as the Kondo effect and topological superconductivity. Motivated by recent defect manipulation in 4$H_b$-TaS$_2$ via STM, we address their microscopic nature and impact on interlayer charge transfer. To this end, we systematically analyze over 90 defects using large-scale density functional theory (DFT) calculations. Our extensive dataset, compiled from STM simulations, defect formation energies, work functions, and charge transfer, establishes a foundational resource for future theoretical and experimental studies on defect engineering in 4$H_b$-TaS$_2$.

cond-mat.mtrl-sci↗

arXiv2025

Nanoscale Modulation of Flat Bands via Controllable Charge-Density-Waves Defects in 4Hb-TaS2

Electron correlation is a main driver of exotic quantum phases and their interplay. The 4Hb-TaS2 system, possessing intrinsic heterostructure of 1T- and 1H-TaS2 monolayers, offers a unique opportunity to control electron correlation by distorting the atomic lattice or tuning interlayer coupling. Here, we investigated intrinsically deformed charge-density-waves (CDW) in the 1T layer of 4Hb-TaS2 to elucidate and control their effects on flat bands using scanning tunneling microscopy and spectroscopy (STM/S) combined with first-principles calculations. We identified two types of CDW defects: Type 1 has structural distortion and locally suppressed flat bands, while Type 2 features an increased flat band filling factor of intact CDW structure. Density functional theory calculations indicate that a sulfur vacancy in the 1T layer distorts the CDW structure and gives rise to a Type 1, whereas a sulfur vacancy in the 1H layer reduces the interlayer charge transfer and lead to a Type 2. Furthermore, we demonstrated creating and erasing individual CDW defects via STM manipulation. Our findings provide a pathway to not only tune flat bands but also selectively manipulate the interaction between CDW, the atomic lattice, and interlayer coupling in strongly correlated systems with atomic precision.

cond-mat.str-el↗

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

Large Bandgap Observed on the Surfaces of EuZn2As2 Single Crystals

EuM2As2 (M = Zn, Cd, In, Sn etc.) is an excellent material system for studying topological properties, which can be easily tuned by magnetism involved. Theoretical calculations predict gapped and flat bands in EuZn2As2 but gapless structure in EuCd2As2. In this work, low-temperature (77 K) cleaved EuZn2As2 crystals are studied using scanning tunneling microscopy/spectroscopy (STM/S) and density functional theory (DFT) calculations. Defects-induced local density of states (LDOS) modification with a triangular shape helps identify the surface terminations: Eu versus AsZn surface. While large bandgaps (~1.5 eV at 77 K) are observed on both pristine surfaces, the bandgap width is found to be very sensitive to local heterogeneity, such as defects and step edges, with the tendency of reduction. Combining experimental data with DFT simulations, we conclude that the modified bandgap in the heterogeneous area arises from Zn vacancies and/or substitution by As atoms. Our investigation offers important information for reevaluating the electron topology of the EuM2As2 family.

cond-mat.mtrl-sci↗