arXiv subjects

Hâldun Sevinçli

Publications and source records attributed to Hâldun Sevinçli.

7 recordsLinked to original sources

arXiv2026

Quartic energy band engineering in artificial semiconductor honeycomb lattices

Artificially engineered lattices provide a flexible platform for reproducing and extending the electronic behavior of atomic-scale materials. Artificial graphene systems, in particular, mimic graphene-like linear dispersion with tunable Dirac cones and offer a route to realizing more exotic band structures. Here we examine the emergence of quartic energy dispersion in artificial graphene heterostructures using analytical modeling and numerical solutions of the effective Hamiltonian. We identify three distinct quartic band types: Mexican-hat-shaped (MHS), purely quartic, and non-MHS quartic bands, and determine the conditions under which each arises. We find that a staggered honeycomb lattice supports all three classes of quartic dispersion, whereas its planar counterpart yields only purely quartic and non-MHS forms. These results demonstrate the feasibility of engineering quartic band edges in artificial lattices and clarify how lattice geometry can be used to tailor their characteristics.

cond-mat.mes-hall↗

arXiv2025

Doping induced magnetism and half-metallicity in nanoribbons of quartic dispersion materials

Two-dimensional (2D) quartic dispersion materials are known to develop magnetization upon doping. Here we conduct a systematic investigation of magnetization in hole-doped quartic dispersion materials (GaS, InSe, TiO$_{2}$), focusing on the effects of structural confinement from 2D monolayers to quasi-one-dimensional nanoribbons (NRs). Upon hole doping, these NRs develop itinerant magnetization across a broad range of carrier densities and display half-metallic behavior. The spin-polarization energies ($E_{sp}$) of these NRs enhance remarkably relative to their 2D counterparts, with maximum increase being in the case of TiO$_{2}$ from 31 to 103 meV/carrier. The $E_{sp}$ strongly depends on the degree of localization of the magnetic moments along the width of NRs, which is determined by edge passivation and ribbon width. Strong deformation of the topmost valence bands at higher dopings indicates deviation from the Stoner mechanism.

cond-mat.mtrl-sci↗

arXiv2025

Thermodynamic Favorability of the 1T Phase over the 1H Phase in Group III Metal Monochalcogenide Zigzag Nanoribbons

Owing to the promising optoelectronic and thermoelectric properties of two-dimensional (2D) group III-VI materials (MXs), their nanoribbons (NRs) have attracted notable attention as an emerging class of quasi-one-dimensional (quasi-1D) nanostructures. Due to the fact that the most stable 2D monolayer polymorph of MXs is the 1H phase, to date, existing studies in the literature have predominantly focused on the NRs formed from 1H phase MXs. Nevertheless, NRs of the 1T phase have received little to no attention. Employing ab initio simulations based on density functional theory, we systematically compared the thermodynamic stability of hydrogen passivated and unpassivated 1T and 1H ZNRs of GaS, GaSe, and InSe. Our results reveal that non-polar 1T phase MX ZNRs are thermodynamically more favorable than polar 1H MX ZNRs at widths up to 34 nm, a range that is realizable through contemporary experimental fabrication techniques. Furthermore, unlike metallic 1H ZNRs, 1T ZNRs remain semiconductor and retain a Mexican-hat-shaped top valence bands. Complementarily, hydrogenation energies of 1T InSe ZNRs are positive, and due to the edge-localized states, the 1T unpassivated ZNRs possess nearly flat top valence bands. Our findings serve as a compass for subsequent synthesis pathways of group III-VI NRs.

cond-mat.mtrl-sci↗

arXiv2023

Quantum transport regimes in quartic dispersion materials with Anderson disorder

Mexican-hat-shaped quartic dispersion manifests itself in certain families of single-layer twodimensional hexagonal crystals such as compounds of groups III-VI and groups IV-V as well as elemental crystals of group V. Quartic band forms the valence band edge in various of these structures, and some of the experimentally confirmed structures are GaS, GaSe, InSe, SnSb and blue phosphorene. Here, we numerically investigate strictly-one-dimensional (1D) and quasi-one dimensional (Q1D) nanoribbons with quartic dispersion and systematically study the effects of Anderson disorder on their transport properties with the help of a minimal tight-binding model and Landauer formalism. We compare the analytical expression for the scaling function with simulation data to deduce about the domains of diffusion and localization regimes. In 1D, it is shown that conductance drops dramatically at the quartic band edge compared to a quadratic band. As for the Q1D nanoribbons, a set of singularities emerge close to the band edge, which suppress conductance and lead to short mean-free-paths and localization lengths. Interestingly, wider nanoribbons can have shorter mean-free-paths because of denser singularities. However, the localization lengths do not necessarily follow the same trend. The results display the peculiar effects of quartic dispersion on transport in disordered systems.

cond-mat.dis-nn↗

arXiv2020

Dimensional crossover and enhanced thermoelectric efficiency due to broken symmetry in graphene antidot lattices

Graphene antidot lattices (GALs) are two-dimensional (2D) monolayers with periodically placed holes in otherwise pristine graphene. We investigate the electronic properties of symmetric and asymmetric GAL structures having hexagonal holes, and show that anisotropic 2D GALs can display a dimensional crossover such that effectively one-dimensional (1D) electronic structures can be realized in two-dimensions around the charge neutrality point. We investigate the transport and thermoelectric properties of these 2D GALs by using non-equilibrium Green function (NEGF) method. Dimensional crossover manifests itself as transmission plateaus, a characteristic feature of 1D systems, and enhancement of thermoelectric efficiency, where thermoelectric figure of merit, $zT$, can be as high as 0.9 at room temperature. We also study the transport properties in the presence of Anderson disorder and find that mean free paths of effectively 1D electrons of anisotropic configuration are much longer than their isotropic counterparts. We further argue that dimensional crossover due to broken symmetry and enhancement of thermoelectric efficiency can be nanostructuring strategy virtually for all 2D materials.

cond-mat.mes-hall↗

arXiv2020

Collapse of the vacuum in hexagonal graphene quantum dots: a comparative study between the tight-binding and the mean-field Hubbard models

In this paper, we perform a systematic study on the electronic, magnetic, and transport properties of the hexagonal graphene quantum dots (GQDs) with armchair edges in the presence of a charged impurity using two different configurations: (1) a central Coulomb potential and (2) a positively charged carbon vacancy. The tight binding (TB) and the half-filled extended Hubbard models are numerically solved and compared with each other in order to reveal the effect of electron interactions and system sizes. Numerical results point out that off-site Coulomb repulsion leads to an increase in the critical coupling constant to $β_{\text{c}}$ = 0.6 for a central Coulomb potential. This critical value of the $β$ is found to be independent of GQD size, reflecting its universality even in the presence of electron-electron interactions. In addition, a sudden downshift in the transmission peaks shows a clear signature of the transition from subcritical $β$ $<$ $β_{\text{c}}$ to supercritical $β$ $>$ $β_{\text{c}}$ regime. On the other hand, for a positively charged vacancy, the collapse of the lowest bound state occurs at $β_{\text{c}}$ = 0.7 for the interacting case. Interestingly, the local magnetic moment, induced by a bare carbon vacancy, is totally quenched when the vacancy is subcritically charged, whereas the valley splittings in electron and hole channels continue to exist in both regimes.

cond-mat.mes-hall↗

arXiv2018

Tuning thermal transport in graphene via combinations of molecular antiresonances

We propose a method to engineer the phonon thermal transport properties of low dimensional systems. The method relies on introducing a predetermined combination of molecular adsorbates, which give rise to antiresonances at frequencies specific to the molecular species. Despite their dissimilar transmission spectra, thermal resistances due to individual molecules remain almost the same for all species. On the other hand, thermal resistance due to combinations of different species are not additive and show large differences depending on the species. Using a toy model, the physics underlying the violation of resistance summation rule is investigated. It is demonstrated that equivalent resistance of two scatterers having the same resistances can be close to the sum of the constituents or $\sim$70\% of it depending on the relative positions of the antiresonances. The relative positions of the antiresonances determine the net change in transmission, therefore the equivalent resistance. Since the entire spectrum is involved in phonon spectrum changes in different parts of the spectrum become important. Performing extensive first-principles based computations, we show that these distinctive attributes of phonon transport can be useful to tailor the thermal transport through low dimensional materials, especially for thermoelectric and thermal management applications.

cond-mat.mes-hall↗