arXiv subjects

Saleem Ayaz Khan

Publications and source records attributed to Saleem Ayaz Khan.

13 recordsLinked to original sources

arXiv2026

Large Jahn-Teller shifts and splittings observed in halide perovskite CsPbBr3

Despite the fundamental role of Jahn-Teller effects of first and second order in shaping structural and electronic properties, there is hardly any observation in angle-resolved photoemission of solids. In oxide and halide perovskites, band structure replicas have occasionally been reported as fingerprints of Jahn-Teller effects, but no accompanying energy shifts or splittings that would allow a conclusion about their origin. In CsPbBr3, we uncover both key signatures: Upon cooling, orthorhombic replica bands emerge which had eluded earlier studies. This includes an extra valence band maximum at Γ which can even be distinguished at room temperature. Most importantly, band narrowing along Γ-M, a splitting at Γ, and the lifting of degeneracy between nonequivalent M points, all of several 100 meV, become apparent. Temperature-dependent x-ray diffraction, used as input for band structure calculations, links these effects directly to tilts and rotations of the PbBr6 octahedra. Our results uncover a strong electron-lattice interaction which is at the heart of so-far unresolved questions concerning polaronic transport, dynamic disorder, and exciton trapping in halide perovskites.

cond-mat.mtrl-sci↗

arXiv2026

Dynamical stability and multifunctional properties of Ni2+/Pr3+ co-doped CsPbCl3 perovskite: insights from first-principles lattice dynamics and carrier transport

All inorganic halide perovskites offer promising optoelectronic properties at low cost, but their structural softness and thermal instability limit applications. Density functional theory using the FP-LAPW method (WIEN2k) was used to study Ni2+/Pr3+ co-doping in CsPbCl3. Results show Ni2+ substitutes for Pb2+ at the B-site and Pr3+ for Cs at the A-site, keeping charge balance. Co-doping stabilizes the lattice, raises formation energies of halogen and metal vacancies, and reduces deep defect levels in the band gap. Phonon dispersion confirms that both pristine and co-doped CsPbCl3 are dynamically stable. Ni2+/Pr3+ co-doping suppresses low-energy vibrations and causes mode splitting in the 3 to 5 THz range, increasing phonon scattering and lowering lattice thermal conductivity. Mechanical analysis reveals higher elastic constants and bulk modulus, while ductility remains unchanged. Electronic structure calculations reveal Ni-3d and Pr-4f states at the band edges, reducing effective carrier mass and passivating vacancy states. Optical absorption is red-shifted, and the high-frequency (ε = 2.4) and low-frequency (ε = 7.4) dielectric constants are distinct. Transport analysis finds higher carrier mobility due to lighter effective masses. Altogether, Ni2+/Pr3+ co-doping reduces defect concentrations and improves the optoelectronic properties of CsPbCl3.

cond-mat.mtrl-sci↗

arXiv2025

Computational Study of Magnetic Behaviour in Ni-Adsorbed Nb2C-OF MXene using Density Functional Theory

Magnetic 2D materials have achieved significantly consideration owing to their encouraging applications. A variation of these 2D materials by occurrence of defects, by the transition-metal doping or adsorption or by the surface functionalization can initiate both the spin-polarization and magnetic properties in these materials. Density functional theory (DFT) is used to determine the electric, magnetic properties along with the electronic structures and stability of synthesized two-dimensional materials. This work describes the magnetic properties of Ni-ad-Nb2C-OF MXene. The study focuses on the computational approach based first principal calculation providing insight onto the magnetic properties of adsorbed compound and comparing it with pristine Nb2C-OF MXene. The pristine Nb2C-OF and Ni-ad-Nb2C-OF structures are simulated and optimized using Wien2k software. Using exchange-correlational functionals; spin-GGA and spin-GGA+U (for Nickel U= 6eV), Ni-ad-Nb2C-OF electronic band structure is found to be metallic having magnetic moment calculated +1.01516μ_\b{eta} showing its non-superconducting and ferromagnetic behaviour. Owing to this magnetic nature, this 2D compound can be used for new upcoming applications such as spintronics and nano magnetic data storage devices.

cond-mat.mtrl-sci↗

arXiv2025

A Density Functional Theory Study of Magnetic Transition in MnO2 adsorbed Vanadium Carbide (V$_2$C) MXene

The work reports nonmagnetic behavior (0.04 $μ$B) in two-dimensional (2D) V2C-OF MXene and ferromagnetism in MnO$_2$ adsorbed V2C-OF MXene. The density functional theory (DFT) calculations were carried out to study the magnetic moments of V$_2$C-OF and MnO$_2$@V$_2$C-OF MXene. The MXene, which is derived from the exfoliation of its parent V$_2$AlC MAX phase, shows a good potential to be a ferromagnetic when MnO$_2$ is adsorbed on it. The V$_2$C MXene and MnO$_2$ adsorbed V$_2$C MXene were successfully synthesized, as characterized using X-ray diffraction, showing an increased c-lattice parameter from 22.6Å to 27.2Å after MnO$_2$ adsorption. The DFT study confirmed that MnO$_2$ adsorbed V$_2$C MXene changed from nonmagnetic (in V$_2$C MXene) to a strong ferromagnetic with a magnetic moment of 4.48$μ$B for Mn adsorbed V$_2$C-OF MXene. The current work is a step-forward towards understanding of magnetism in two-dimensional materials for future 2D spintronics.

cond-mat.mes-hall↗

arXiv2025

Halide Perovskites as Spin-1 Dirac Materials

Halide perovskites are a promising class of materials for optoelectronic and photovoltaic applications, exhibiting high power conversion efficiency due to strong light absorption and long carrier diffusion lengths. While various aspects of their crystal and electronic structure have been studied, we identify a fundamental property previously overlooked that may significantly impact their efficiency. We demonstrate that halide perovskites realize a three-dimensional (3D) Lieb lattice, giving rise to a gapped 3D Dirac cone of spin-1 fermions. This leads to a fivefold reduction in effective mass compared to a conventional cubic structure and suppressed carrier backscattering due to Klein tunneling. Our conclusions are supported by band structure calculations and angle-resolved photoemission spectroscopy from CsPbBr$_3$ and CsSnBr$_3$. In particular, we reveal the transformation of the flat band of the Lieb lattice and the emergence of a dark corridor effect in photoemission from the Dirac cone, which increases as the band gap is decreased from CsPbBr$_3$ to CsSnBr$_3$.

cond-mat.mtrl-sci↗

arXiv2025

ARPES studies of Hf(0001) surface: flat bands formation in the dice lattice

We present the first electronic structure measurements of the Hf(0001) single-crystal surface using angle-resolved photoemission spectroscopy (ARPES). The ARPES results are supported by theoretical calculations performed using the full-potential linearized augmented plane wave (FLAPW) method and the Korringa-Kohn-Rostoker (KKR) Green function method. In addition to insight into the electronic structure of Hf(0001), our results reveal the impact of surface contamination, particularly oxygen and carbon, on the predicted surface state. Moreover, we observe a flat band induced by both, the presence of oxygen and the dice structure of the surface. The orbital texture of Hf bands is confirmed by linear dichroism studies

cond-mat.mtrl-sci↗

arXiv2025

First-principles investigation of Rb$_{2}$CaH$_{4}$ and Cs-doped Rb$_{2}$CaH$_{4}$: unveiling their potential for hydrogen storage through mechanical and optoelectronic properties

This study uses the density functional theory (DFT) approach with GGA-PBE to assess the effect of substituting alkali metals in Rb$_{2}$CaH and Cs-doped Rb$_{2}$CaH$_{4}$ on their hydrogen storage potential. To address the challenges associated with predicting accurate electronic properties in materials containing heavier elements such as cesium, spin-orbit coupling (SOC) effects have been incorporated into our calculations. The mechanical robustness of both Rb$_{2}$CaH$_{4}$ and Cs-doped Rb$_{2}$CaH$_{4}$, as demonstrated by their mechanical properties, highlights these materials as promising candidates due to their stability in hydrogen storage applications. Anisotropic factors show that all materials exhibit anisotropy, suggesting a directional dependency in their properties. The Pugh ratio indicates that Rb$_{2}$CaH$_{4}$ and Cs-doped Rb$_{2}$CaH$_{4}$ are brittle materials. Based on the calculated band gap, the electronic band structure analysis, conducted using both HSE06 and GGA-PBE, shows that Rb$_{2}$CaH$_{4}$ and Cs-doped Rb$_{2}$CaH$_{4}$ are wide-bandgap materials. Rb$_{2}$CaH$_{4}$ and Cs-doped Rb$_{2}$CaH$_{4}$ exhibit the highest optical conductivity, absorption coefficient, and energy loss function among optoelectronic materials, emphasizing their superior absorption and electron transfer capabilities. The hydrogen storage capacity has been evaluated for practical applications; Rb$_{2}$CaH$_{4}$ and Cs-doped Rb$_{2}$CaH$_{4}$ show the highest gravimetric and volumetric capacities.

cond-mat.mtrl-sci↗

arXiv2022

Electronic structure of $β$-SiAlON: effect of varying Al/O concentration at finite temperatures relevant for thermal quenchin

$β$-Si$_{6-z}$Al$_{z}$O$_{z}$N$_{8-z}$ is a prominent example of systems suitable as hosts for creating materials for light-emitting diodes. In this work, the electronic structure of a series of ordered and disordered $β$-Si$_{6-z}$Al$_{z}$O$_{z}$N$_{8-z}$ systems is investigated by means of ab initio calculations, using the FLAPW and the Green function KKR methods. Finite temperature effects are included within the alloy analogy model. We found that the trends with the Al/O doping are similar for ordered and disordered structures. The electronic band gap decreases with increasing $z$ by about 1 eV when going from $z$=0 to $z$=2. The optical gap decreases analogously as the electronic band gap. The changes in the density of states (DOS) at Si and N atoms introduced by doping $β$-Si$_{3}$N$_{4}$ with Al and O are comparable to the DOS at Al and O atoms themselves. The bottom of the conduction band in $β$-Si$_{6-z}$Al$_{z}$O$_{z}$N$_{8-z}$ is formed by extended states residing on all atomic types. Increasing the temperature leads to a shift of the bottom of the conduction band to lower energies. The amount of this shift increases with increasing doping $z$.

cond-mat.mtrl-sci↗

arXiv2022

Dependence of the electronic structure of $β$-Si$_{6-z}$Al$_{z}$O$_{z}$N$_{8-z}$ on the (Al,O) concentration $z$ and on the temperature

SiAlON is a prominent example of systems suitable as hosts for creating materials for light-emitting diodes (LEDs). In this work, the electronic structure of a series of semiordered and disordered SiAlON systems is investigated by means of ab initio calculations, using the FLAPW and Green function KKR methods. Finite temperature effects are included by averaging over thermodynamic configurations within the alloy analogy model. We found that the dependence of the electronic structure on the (Al,O) concentration $z$ is similar for semiordered and disordered structures. The electronic band gap decreases with increasing $z$ by about 1.5 eV when going from $z$=0 to $z$=2. States at the top of the valence band are mostly associated with N atoms whereas the states at the bottom of the conduction band are mostly derived from O~atoms. Increasing the temperature leads to a shift of the bottom of the conduction band to lower energies. The amount of this shift increases with increasing $z$.

cond-mat.mtrl-sci↗

arXiv2019

Co-existence of Novel Ferromagnetic/Anti-ferromagnetic Phases in Two-dimensional Ti3C2 MXene

This study reports first synthesis of MXene-derived co-existing phases. New family of two-dimensional materials such as Ti3C2 namely MXene, having transition metal forming hexagonal structure with carbon atoms have attracted tremendous interest now a days. We have reported structural, optical and magnetic properties of undoped and La-doped Ti3C2Tx MXene synthesized using co-precipitation method. The c-lattice parameters (c-LP) calculated for La-MXene is c=18.3Å which is slightly different from the parent un-doped MXene (c=19.2Å), calculated from X-ray diffraction data. The doping of La+3 ions shrinks Ti3C2Tx layers perpendicular to the planes but expands slightly the in-plane lattice parameters. The band gap for MXene is calculated to be 1.06 eV which is increased to 1.44 eV after the doping of La+3 ion that shows its good semiconducting nature. The experimental results for magnetic properties of both the samples have been presented and discussed, indicating the presence of ferromagnetic-antiferromagnetic phases co-existing. The results presented here are unique and first report on magnetic properties of two-dimensional carbides for magnetic data storage applications.

cond-mat.mes-hall↗

arXiv2019

Local geometry around B atoms in B/Si(111) from polarized x-ray absorption spectroscopy

The arrangement of B~atoms in a doped Si(111)-$(\sqrt{3}\times\sqrt{3})R30^{\circ}$:B system was studied using near-edge x-ray absorption fine structure (NEXAFS). Boron atoms were deposited via segregation from the bulk by flashing the sample repeatedly. The positions of B~atoms are determined by comparing measured polarized (angle-dependent) NEXAFS spectra with spectra calculated for various structural models based on ab-initio total energy calculations. It is found that most of boron atoms are located in sub-surface L$_{1}^{c}$ positions, beneath a Si atom. However, depending on the preparation method a significant portion of B~atoms may be located elsewhere. A possible location of these non-L$_{1}^{c}$-atoms is at the surface, next to those Si atoms which form the $(\sqrt{3}\times\sqrt{3})R30^{\circ}$ reconstruction.

cond-mat.mtrl-sci↗

arXiv2016

Magnetocrystalline anisotropy of FePt: a detailed view

To get a reliable ab-initio value for the magneto-crystalline anisotropy (MCA) energy of FePt, we employ the full-potential linearized augmented plane wave (FLAPW) method and the full-potential Korringa-Kohn-Rostoker (KKR) Green function method. The MCA energies calculated by both methods are in a good agreement with each other. As the calculated MCA energy significantly differs from experiment, it is clear that many-body effects beyond the local density approximation are essential. It is not really important whether relativistic effects for FePt are accounted for by solving the full Dirac equation or whether the spin-orbit coupling (SOC) is treated as a correction to the scalar-relativistic Hamiltonian. From the analysis of the dependence of the MCA energy on the magnetization angle and on the SOC strength it follows that the main mechanism of MCA in FePt can be described within second order perturbation theory. However, a distinct contribution not accountable for by second order perturbation theory is present as well.

cond-mat.mtrl-sci↗

arXiv2016

Local environment effects in the magnetic properties and electronic structure of disordered FePt

Local aspects of magnetism of disordered FePt are investigated by ab initio fully relativistic full potential calculations, employing the supercell approach and the coherent potential approximation (CPA). The focus is on trends of the spin and orbital magnetic moments with chemical composition and with bond lengths around the Fe and Pt atoms. A small but distinct difference between average magnetic moments obtained when using the supercells and when relying on the CPA is identified and linked to the neglect of the Madelung potential in the CPA.

cond-mat.mtrl-sci↗