arXiv subjects

Gul Rahman

Publications and source records attributed to Gul Rahman.

At least 19 recordsLinked to original sources

arXiv2019

CrAs monolayer: Low buckled two-dimensional half-metal ferromagnet

\textit{Ab-initio} calculations based on density functional theory (DFT) are performed to study the structural, electronic, and magnetic properties of two-dimensional (2D) free-standing honeycomb CrAs. We show that CrAs has low buckled stable structure. Magnetic CrAs has larger buckling than non-magnetic CrAs. 2D-CrAs is a ferromagnetic semiconductor for lattice constant $a \leq 3.71$\AA, and above this lattice constant CrAs is a half-metal ferromagnet. 2D-CrAs is shown to be half-metal ferromagnetic with magnetic moment of 3.0$\mu_{\rm{B}}$ per unit cell, at equilibrium structure. The $d_{z}^{2}$ orbital of $e_{g}$ band is completely empty in the spin-down state whereas it is almost occupied in the spin-up state, and the magnetic moment in the $e_{g}$ band is mainly dominated by the $d_{z}^{2}$ orbital of Cr. The $d_{zx}/d_{zy}$ and $d_{xy}$ orbitals of $t_{2g}$ band are partially occupied in the spin-up state and behaves as metal whereas they are insulator in the spin-down state. Phonon calculations confirm the thermodynamic stability of 2D-CrAs. The ferromagnetic (FM) and antiferromagnetic (AFM) interaction between the Cr atoms reveal that the FM state is more stable than the AFM state of 2D-CrAs.

cond-mat.mtrl-sci↗

arXiv2018

Defects-driven magnetism in bulk $\alpha$-Li$ _{3}$N

\textit{Ab-initio} calculations based on density functional theory with local spin density approximation are used to study defects-driven magnetism in bulk $\alpha$-Li$ _{3}$N. Our calculations show that bulk Li$ _{3} $N is a non-magnetic semiconductor. Two types of Li vacancies (Li-I and Li-II) are considered, and Li-vacancies (either Li-I or Li-II type) can induce magnetism in Li$ _{3}$N with a total magnetic moment of 1.0 $\mu_{\rm B}$ which arises mainly due to partially occupied N-$p$-orbitals around the Li vacancies. The defect formation energies dictate that Li-II vacancy, which is in the Li$ _{2}$N plane, is thermodynamically more stable as compared with Li-I vacancy. The electronic structures of Li-vacancies show half-metallic behavior. On the other hand N-vacancy does not induce magnetism and has a larger formation energy than Li-vacancies. N vacancy derived bands at the Fermi energy are mainly contributed by the Li atoms. Carbon is also doped at Li-I and Li-II sites, and it is expected that doping C at Li-I site is thermodynamically more stable as compared with Li-II site. Carbon can induce metallicity with zero magnetic moment when doped at Li-I site, whereas magnetism is observed when Li-II site is occupied by the C impurity atom and C-driven magnetism is spread over the N atoms as well. Carbon can also induce half-metallic magnetism when doped at N site in Li$ _{3}$N, and has a smaller defect formation energy as compared with Li-II site doping. The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the C atoms is also investigated, and we conclude that FM state is more stable than the AFM state.

cond-mat.mtrl-sci↗

arXiv2018

Enhanced band edge luminescence of ZnO nanorods after surface passivation with ZnS

We report on the passivation of surface defects of ZnO nanorods by surface layer deposition. ZnO nanorods and ZnS-ZnO hybrid nanostructures are grown on FTO coated glass substrate by chemical bath deposition method. XRD spectrum of ZnO nanorods shows the preferential growth along the c-axis. SEM analysis confirms the nearly aligned growth of the ZnO nanorods with a hexagonal shape. XPS measurements were performed to confirm the deposition of the surface layer and surface stoichiometry. Room temperature photoluminescence of ZnO nanorods showed two emission bands, viz. the band edge emission and the blue-green emission, with the latter being associated with the defect states arising from the surface of ZnO nanorods. The band edge emission is significantly increased as compared to blue-green emission after ZnS surface layer deposition on ZnO nanorods. The quenching of blue-green emission is explained in terms of reduced surface defects after ZnS deposition. Density functional theory (DFT) calculations are used to understand the mechanisms of defect passivation in ZnS-ZnO nanostructures and we show that the S atoms prefer the O site as compared with the Zn and interstitial sites

cond-mat.mes-hall↗

arXiv2017

Magnetic monolayer Li$_{2}$N: Density Functional Theory Calculations

Density functional theory (DFT) calculations are used to investigate the electronic and magnetic structures of a two-dimensional (2D) monolayer Li$_{2}$N. It is shown that bulk Li$_{3}$N is a non-magnetic semiconductor. The non-spinpolarized DFT calculations show that $p$ electrons of N in 2D Li$_{2}$N form a narrow band at the Fermi energy $E_{\rm{F}}$ due to a low coordination number, and the density of states at the Fermi energy ($g(E_{\rm{F}}$)) is increased as compared with bulk Li$_{3}$N. The large $g(E_{\rm{F}}$) shows instability towards magnetism in Stoner's mean field model. The spin-polarized calculations reveal that 2D Li$_{2}$N is magnetic without intrinsic or impurity defects. The magnetic moment of 1.0\,$\mu_{\rm{B}}$ in 2D Li$_{2}$N is mainly contributed by the $p_{z}$ electrons of N, and the band structure shows half-metallic behavior. {Dynamic instability in planar Li$_{2}$N monolayer is observed, but a buckled Li$_{2}$N monolayer is found to be dynamically stable.} The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the N atoms is also investigated to access the exchange field strength. {We found that planar (buckled) 2D Li$_{2}$N is a ferromagnetic material with Curie temperature $T_{c}$ of 161 (572) K.}

cond-mat.mtrl-sci↗

arXiv2016

Ground state structure of BaFeO$_{3}$: Density Functional Theory Calculations

Using density functional theory calculations, the ground state structure of BaFeO$_3$ (BFO) is investigated with local spin density approximation (LSDA). Cubic, tetragonal, orthorhombic, and rhombohedral types BFO are considered to calculate the formation enthalpy. The formation enthalpies reveal that cubic is the most stable structure of BFO. Small energy difference between the cubic and tetragonal suggests a possible tetragonal BFO. Ferromagnetic(FM) and anitiferromagnetic (AFM) coupling between the Fe atoms show that all the striochmetric BFO are FM. The energy difference between FM and AFM shows room temperature ferromagnetism in cubic BFO in agreement with the experimental work. The LSDA calculated electronic structures are metallic in all studied crystallographic phases of BFO. Calculations including the Hubbard potential $U,i.e.$ LSDA+$U$, show that all phases of BFO are half-metallic consistent with the integer magnetic moments. The presence of half-metallicity is discussed in terms of electronic band structures of BFO.

cond-mat.mtrl-sci↗

arXiv2015

Strain and correlation induced half-metallic ferromagnetism in orthorhombic BaFeO$_{3}$

Using first-principles calculations, the electronic and magnetic properties of orthorhombic BaFeO$_{3}$ (BFO) are investigated with local spin density approximation (LSDA). The calculations reveal that at the optimized lattice volume BFO has a lower energy in ferromagnetic state as compared with antiferromagnetic state. At the equilibrium volume, BFO shows metallic behavior, however, under a large tensile strain ($\sim25\%$), BFO shows half-metallic behavior consistent with the integer magnetic moment of $4.0μ_{\rm{B}}$/fu mainly caused by the $t_{2g}$ and $e_{g}$ electrons of Fe. Including a Hubbard-like contribution $U$ (LSDA$+U$) on Fe $d$ states induced half-metallic bahvior without external strain, which indicates that $U$ can be used to tune the electronic structure of BFO. The magnetic moments remained robust against $\sim 10\%$ compressive and tensile strain. At large compressive (tensile) strain, the half-metallicity of BFO is mainly destroyed by the Fe-$d$ (O-$p$) electrons in agreement with the non-integer value of the magnetic moments of BFO.

cond-mat.mtrl-sci↗

arXiv2015

Nitrogen-induced Ferromagnetism in BaO

Density functional theory with local spin density approximation has been used to propose possible room temperature ferromagnetism in N-doped NaCl-type BaO. Pristine BaO is a wide bandgap semiconductor, however, N induces a large density of states at the Fermi level in the nonmagnetic state, which suggests magnetic instability within the Stoner mean field model. The spin-polarized calculations show that N-doped BaO is a true half- metal, where N has a large magnetic moment, which is mainly localized around the N atoms and a small polarization at the O sites is also observed. The origin of magnetism is linked to the electronic structure. The ferromagnetic(FM) and antiferromagnetic (AFM) coupling between the N atoms in BaO reveal that doping N atoms have a FM ground state, and the calculated transition temperature ($T_{C}$), within the Heisenberg mean field theory, theorizes possible room temperature FM in N-doped BaO. Nitrogen also induces ferromagnetism when doping occurs at surface O site and has a smaller defect formation energy than the bulk N-doped BaO. The magnetism of N-doped BaO is also compared with Co-doped BaO, and we believe that N has a greater potential for tuning magnetism in BaO than Co.

cond-mat.mtrl-sci↗

arXiv2014

Elastic and magnetic properties of cubic Fe$_{4}$C from first-principles

First-principles based on density functional theory is used to study the phase stability, elastic, magnetic, and electronic properties of cubic (c)-Fe$_4$C. Our results show that c-Fe$_{4}$C has a ferromagnetic (FM) ground state structure compared with antiferromagnetic (AFM) and nonmagnetic (NM)states. To study the phase stability of c-Fe$_4$C, BCC Fe$_4$C, FCC Fe$_4$C, and BCC Fe$_{16}$C, where C is considered at tetrahedral and octahedral interstitial sites, are also considered. Although, the formation energy of c-Fe$_4$C is smaller than BCC Fe$_4$C, but the shear moduli of c-Fe$_4$C is negative in the FM and AFM states indicating that c-Fe$_4$C is dynamically not stable in the magnetic (FM/AFM) states. However, NM state has positive shear moduli which illustrates that instability in c-Fe$_4$C is due to magnetism and can lead to soft phonon modes. The calculated formation energy also shows that c-Fe$_4$C has higher formation energy compared with the FCC Fe$_4$C indicating no possibility of c-Fe$_4$C in low carbon steels at low temperature. The magnetic moment of Fe in c-Fe$_4$C is also sensitive to lattice deformation. The electronic structure reveals the itinerant nature of electrons responsible for metallic behavior of c-Fe$_4$C.

cond-mat.mtrl-sci↗

arXiv2014

Spin-polarized surface state in Li-doped SnO$_{2}$(001)

Using LDA+$U$, we investigate Li-doped rutile SnO$_2$(001) surface. The surface defect formation energy shows that it is easier for Li to be doped at surface Sn site than bulk Sn site in SnO$_2$. Li at surface and sub-surface Sn sites has a magnetic ground state, and the induced magnetic moments are not localized at Li site, but spread over Sn and O sites. The surface electronic structures show that Li at surface Sn site shows $100%$ spin-polarization (half metallic), whereas Li at sub-surface Sn site does not have half metallic state due to Li-Sn hybridized orbitals. The spin-polarized surface has a ferromagnetic ground state, therefore, ferromagnetism is expected in Li-doped SnO$_2$(001) surface.

cond-mat.mtrl-sci↗

arXiv2014

Distortion and electric field control of band structure of silicene

Density functional theory with local density approximation for exchange and correlation functional is used to tune the electronic band structure of silicene monolayer. The cohesive energy of free standing monolayer is increasing (decreasing) with external electric field (distortion). Electrons in silicene behave like Dirac fermions, when the bond angle between the Si atoms is larger than $\sim 102^{0}$. Large distortions destroy the electronic structure of silicene and silicene is no longer a semi-metallic material, and the distorted silicene acts like an $n$-doped system. Electric field opens a band gap around $K$ point in the Brillouin zone, which increases with electric field. The bond angle between the Si atoms is a key player to determine the presence or absence of Dirac cones in silicene.

cond-mat.mtrl-sci↗

arXiv2013

Jahn-Teller distortion induced magnetic phase transition in cubic BaFeO$_{3}$

Using density functional theory (DFT) with local density approximation (LDA) and generalized gradient approximation (GGA) correlation functionals, the electronic and magnetic structures of cubic BaFeO$_{3}$ in the ferromagnetic (FM) and antiferromagnetic (AFM) states are studied. Our LDA/GGA and LDA$+U$/GGA$+U$ results show that cubic BFO has a FM ground state, in agreement with recent experimental works. Two types of Jahn-Teller (JT) distortions, denoted as JT1 and JT2, are considered. We find FM to ferrimagnetic (FIM) and FM to AFM magnetic phase transitionn in the JT1 and JT2 type of distortions, respectively. Larger strains are required for the FM-AFM transition as compared to the FM-FIM. DFT$+U$ calculations also show that the magnetic moments dramatically decrease at large strains due to strong overlapping between the Fe and O atoms. The origins of these transitions is discussed in terms of a competition between double exchange and superexchange interactions. Oxygen and Fe displacements are therefore responsible for the magnetic phase transitions and the reduction of the magnetic moments.

cond-mat.mtrl-sci↗

arXiv2013

Stabilizing intrinsic defects in SnO$_{2}$

TThe magnetism and electronic structure of Li-doped SnO$_{2}$ are investigated using first-principles LDA/LDA$+U$ calculations. We find that Li induces magnetism in SnO$_{2}$ when doped at the Sn site but becomes non-magnetic when doped at the O and interstitial sites. The calculated formation energies show that Li prefers the Sn site as compared with the O site, in agreement with previous experimental works. The interaction of Li with native defects (Sn V$_\mathrm{Sn}$ and O V$_\mathrm{O}$ vacancies) is also studied, and we find that Li not only behaves as a spin polarizer, but also a vacancy stabilizer, i.e. Li significantly reduces the defect formation energies of the native defects and helps the stabilization of magnetic oxygen vacancies. The electronic densities of states reveals that these systems, where the Fermi level touches the conduction (valence) band, are non-magnetic (magnetic).cancies. The electronic densities of states reveal that those systems, where the Fermi levels touch the conduction (valence) band, are non-magnetic (magnetic).

cond-mat.mtrl-sci↗

arXiv2013

Role of vacancies in the magnetic and electronic properties of SiC nanoribbons: an ab initio study

Using ab-initio calculations based on density functional theory, we investigate the effects of vacancies on the electronic and magnetic properties of zigzag SiC nanoribbons (Z-SiCNR). Single (V_C and V_Si) and double (V_SiV_Si and V_SiV_C) vacancies are observed to induce magnetism in Z-SiCNRs. The presence of a single V_Si does not affect the half-metallic behavior of pristine Z-SiCNRs; however, a single V_C leads to a transition from half-metallic to metallic behavior in Z-SiCNRs due to the edge Si p orbitals and the atoms surrounding the vacancy. The interactions of vacancies with foreign impurity atoms (B and N) are also investigated and it is observed that V_SiN_C does not only suppress the oscillatory type magnetism of V_SiV_C, but also retains the half-metallic character of the pristine Z-SiCNRs. The defect formation energies of vacancies can be reduced by substitutional B and N atoms. We believe that ferromagnetism is expected if Z-SiCNR are grown under suitable conditions.

cond-mat.mtrl-sci↗

arXiv2012

Intrinsic Magnetism in Nanosheets of SnO$_{2}$: A First-principles Study

We propose intrinsic magnetism in nanosheets of SnO$_{2}$, based on first-principles calculations. The electronic structure and spin density reveal that $p$ orbitals of the oxygen atoms, surrounding Sn vacancies, have a non itinerant nature which gives birth to localized magnetism. A giant decrease in defect formation energies of Sn vacancies in nanosheets is observed. We, therefore, believe that native defects can be stabilized without any chemical doping. Nanosheets of different thicknesses are also studied, and it is found that it is easier to create vacancies, which are magnetic, at the surface of the sheets. SnO$_{2}$ nanosheets can, therefore, open new opportunities in the field of spintronics.

cond-mat.mtrl-sci↗

arXiv2012

A first-principles investigation on the effects of magnetism on the Bain transformation of $α$-phase FeNi systems

The effects of magnetism on the Bain transformation of $α$-phase FeNi systems are investigated by using the full potential linearized augmented plane wave (FLAPW) method based on the generalized gradient approximation (GGA). We found that Ni impurity in bcc Fe increases the lattice constant in ferromagnetic (FM) states, but not in the nonmagnetic (NM) states. The shear modulus $G$ and Young's modulus $E$ of bcc Fe are also increased by raising the concentration of nickel. All the compositions considered show high shear anisotropy and the ratio of the bulk to shear modulus is greater than 1.75 implying ductility. The mean sound velocities in the $[100]$ directions are greater than in the $[110]$ directions. The Bain transformation, which is a component of martensitic transformation, has also been studied to reveal that Ni$_{x}$Fe$_{1-x}$ alloys are elastically unstable in the NM states, but not so in the FM states. The electronic structures explain these results in terms of the density of states at the Fermi level. It is evident that magnetism cannot be neglected when dealing with the Bain transformation in iron and its alloys.

cond-mat.mtrl-sci↗

arXiv2012

Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe

External pressure driven phase transitions of FeSe are predicted using \textit{ab initio} calculations. The calculations reveal that $α$-FeSe takes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from NiAs-type to MnP-type and CsCl-type FeSe is also predicted. MnP-type FeSe is also found to be able to transform to CsCl-type FeSe, which is easier from $α$-FeSe than the transition to MnP-type FeSe, but comparable to the transition from NiAs-type FeSe. The calculated electronic structures show that all phases of FeSe are metallic, but the ionic interaction between Fe-Se bonds becomes stronger and the covalent interaction becomes weaker when the structural phase transition occurs from $α$-FeSe to the other phases of FeSe. The experimentally observed decrease in $T_{c}$ of superconducting $α$-FeSe at high pressure may be due to a structural/magnetic instability, which exists at high pressure. The results suggest us to increase the $T_{c}$ of $α$-FeSe if such phase transitions are frustrated by suitable methods.

cond-mat.mtrl-sci↗

arXiv2010

First-principles prediction of spin-density-reflection symmetry driven magnetic transition of CsCl-type FeSe

Based on results of density functional theory (DFT) calculations with the local spin density approximation (LSDA) and the generalized gradient approximation (GGA), we propose a new magnetic material, CsCl-type FeSe. The calculations reveal the existence of ferromagnetic (FM) and antiferromagnetic (AFM) states over a wide range of lattice constants. At 3.12\,Å in the GGA, the equilibrium state is found to be AFM with a local Fe magnetic moment of $\pm 2.69\,μ_\mathrm{B}$. A metastable FM state with Fe and Se local magnetic moments of $2.00\,μ_\mathrm{B}$ and $-0.032\,μ_\mathrm{B}$, respectively, lies 171.7\,{meV} above the AFM state. Its equilibrium lattice constant is $\sim 2$\,{\%} smaller than that of the AFM state, implying that when the system undergoes a phase transition from the AFM state to the FM one, the transition is accompanied by volume contraction. Such an AFM-FM transition is attributed to spin-density $z$-reflection symmetry; the symmetry driven AFM-FM transition is not altered by spin-orbit coupling. The relative stability of different magnetic phases is discussed in terms of the local density of states. We find that CsCl-type FeSe is mechanically stable, but the magnetic states are expected to be brittle.

cond-mat.mtrl-sci↗

arXiv2010

First-principles investigation of magnetism and electronic structures of substitutional $3d$ transition-metal impurities in bcc Fe

The magnetic and electronic structures of $3d$ impurity atoms from Sc to Zn in ferromagnetic body-centered cubic iron are investigated using the all-electron full-potential linearized augmented plane-wave method based on the generalized gradient approximation (GGA). We found that in general, the GGA results are closer to the experimental values than those of the local spin density approximation. The calculated formation enthalpy data indicate the importance of a systematic study on the ternary Fe-C-$X$ systems rather than the binary Fe-$X$ systems, in steel design. The lattice parameters are optimized and the conditions for spin polarization at the impurity sites are discussed in terms of the local Stoner model. Our calculations, which are consistent with previous work, imply that the local spin-polarizations at Sc, Ti, V, Cu, and Zn are induced by the host Fe atoms. The early transition-metal atoms couple antiferromagnetically, while the late transition-metal atoms couple ferromagnetically, to the host Fe atoms. The calculated total magnetization ($M$) of bcc Fe is reduced by impurity elements from Sc to Cr as a result of the antiferromagnetic interaction, with the opposite effect for solutes which couple ferromagnetically. The changes in $M$ are attributed to nearest neighbor interactions, mostly between the impurity and host atoms. The atom averaged magnetic moment is shown to follow generally the well-known Slater-Pauling curve, but our results do not follow the linearity of the Slater-Pauling curve. We attribute this discrepancy to the weak ferromagnetic nature of bcc Fe. The calculated Fermi contact hyperfine fields follow the trend of the local magnetic moments. The effect of spin-orbit coupling is found not to be significant although it comes into prominence at locations far from the impurity sites.

cond-mat.mtrl-sci↗