Gökhan Gökoğlu

Electronic structure of BSb defective monolayers and nanoribbons

In this paper, we investigate two- and one-dimensional honeycomb structures of boron antimony (BSb) using a first-principles plane wave method within the density functional theory. BSb with a two-dimensional honeycomb structure is a semiconductor with a 0.336 eV band gap. The vacancy defects, such as B, Sb, B + Sb divacancy, and B + Sb antisite disorder affect the electronic and magnetic properties of the 2D BSb sheet. All the structures with vacancies have nonmagnetic metallic characters, while the system with antisite disorder has a semiconducting band structure. We also examine bare and hydrogen-passivated quasi-one-dimensional armchair BSb nanoribbons. The effects of ribbon width (n) on an armchair BSb nanoribbon and hydrogen passivation on both B and Sb edge atoms are considered. The band gaps of bare and H passivated A-Nr-BSb oscillate with increasing ribbon width; this property is important for quantum dots. For ribbon width n = 12, the bare A-Nr-BSb is a nonmagnetic semiconductor with a 0.280 eV indirect band gap, but it becomes a nonmagnetic metal when B edge atoms are passivated with hydrogen. When Sb atoms are passivated with hydrogen, a ferromagnetic half-metallic ground state is observed with 2.09μB magnetic moment. When both B and Sb edges are passivated with hydrogen, a direct gap semiconductor is obtained with 0.490 eV band gap with disappearance of the bands of edge atoms.

Electronic structure and magnetic properties of PbMO3 (M = Fe, Co, Ni) magnetic perovskites: An ab initio study

We present the electronic, magnetic and structural properties of the magnetic transition metal oxides PbMO3 (M = Fe, Co, Ni) in cubic perovskite structure. The calculations are based on the density functional theory (DFT) within plane-wave pseudopotential method and local spin density approximation (LSDA) of the exchange-correlation functional. On-site Coulomb interaction is also included in calculations (LSDA+U). The systems are considered in ferromagnetic (FM) and G-type antiferromagnetic (G-AFM) order. FM structures are energetically more favored than G-AFM and than non-magnetic states for all the systems studied. The spin-polarized electronic band structures show that all the structures have metallic property in FM order without Hubbard-U interaction (Ueff = 0). However, the inclusion of on-site Coulomb interaction (Ueff = 7 eV) opens a semiconducting gap for majority spin channel of PbFeO3 and of PbNiO3 resulting in a half-metallic character. PbCoO3 system remains as metallic with LSDA+U scheme. Bonding features of all structures are largely determined by the hybridizations between O–p and d-states of transition metal atoms. The partial magnetic moment of Fe atom in PbFeO3 is enhanced by inclusion of Hubbard-U interaction (2.55 μB ⇒ 3.78 μB). Total magnetic moments of half-metallic PbFeO3 and of PbNiO3 compounds are very close to integer values.

Structural properties of small semiconductor-binding synthetic peptides

We have performed exhaustive multicanonical Monte Carlo simulations of three 12-residue synthetic peptides in order to investigate the thermodynamic and structural properties as well as the characteristic helix-coil transitions. In these studies, we employ a realistic model where the interactions between all atoms are taken into account. Effects of solvation are also simulated by using an implicit-solvent model.

CONFORMATION AND PHASE TRANSITION OF POLY-GLUTAMINE

In order to provide insights into the misfolding mechanism and the subsequent aggregate formation which cause what are known as the neurodegenerative polyglutamine diseases, we have simulated a 10-residue polyglutamine (poly-Q) chain in vacuum and in solvent by multicanonical method, which enabled us to study the system in a wide temperature range and discuss thermodynamic properties. It is understood that the system in vacuum shows two phase transitions, first of them occur at high temperature that is the well-known helix-coil transition and the second one is a solid-solid transition. However, the poly-Q chain in solvent is in a random coil state at higher temperatures, goes through a conformational change at T = 200 K and assumes predominantly a mixture of anti-parallel β-sheet and α-helix structures at low temperatures. One-residue glutamine dipeptide is also simulated and low-energy stable conformations are identified.

First-principles investigation of LaGaO3 and LaInO3 lanthanum perovskite oxides

Abstract Among the class of ABO3-type perovskite oxides, LaMO3 (M=Ga and In) compounds are investigated in cubic (Pm-3m), tetragonal (P4mm), hexagonal (P-3m1), rhombohedral (R-3c) and orthorhombic (Pbnm) phases using generalised gradient approximation (GGA) within the density functional theory. On-site Coulomb interaction is also included in the calculations (GGA + U). After the determination of the stable phase, phase transition pressures have also been calculated. Then, their full structural, mechanical, electronic, optical and vibrational properties have been studied in stable orthorhombic (Pbnm) phase. Both compounds are non-magnetic insulators in their ground states. The energy gaps (Eg) of LaGaO3 and LaInO3 compounds have been found as 3.14 and 2.55 eV, respectively. The calculated elastic constants and phonon dispersion curves confirm the stability of orthorhombic phase mechanically and dynamically.

Conformational Analysis Of Polyalanyl Chains

Including all-atom interactions and solution effects, we have simulated the monoalanine dipeptide and short polyalanyl chains by multicanonical algorithm, in order to determine and compare their low-lying microstates and thermodynamically stable conformations.

T-ZrS nanoribbons: structure and electronic properties

Recently, monolayer and few layers of trigonal phases of zirconium disulfide (T-ZrS) sheets were obtained experimentally on hexagonal boron nitride using an evaporation technique. On the basis of these previous results, we report the structural and electronic properties of armchair nanoribbons (ANRs) and zigzag nanoribbons (ZNRs) of T-ZrS by means of density functional theory. According to our results, both ANRs and ZNRs are nonmagnetic semiconductors similar to a two-dimensional T-ZrS monolayer. The semiconducting character is not altered by termination of the edge atoms with hydrogen. The band gaps are associated with the ribbon widths and edge structures. The band gaps of bare and H-terminated ANR-ZrS decrease exponentially, whereas the band gaps of ultra-narrow zigzag nanoribbons oscillate slightly with increasing ribbon width. Although the band gaps of bare ANRs approach that of 2D T-ZrS, other structures have larger band gaps than the monolayer with increasing ribbon width. The cohesive and formation energies of bare ANRs and ZNRs converge rapidly to that of the 2D T-ZrS structure with increasing ribbon width.