arXiv subjects

Jacek A. Majewski

Publications and source records attributed to Jacek A. Majewski.

At least 19 recordsLinked to original sources

arXiv2026

Taming quantum interference: a route to high electrical conductance in carbon nanotube assemblies

In nanostructured networks, transport is governed by junctions between neighbouring building blocks. Improving their alignment and removing defects is the intuitive route to better electron transport. At low temperatures, when transport becomes coherent, a junction cannot always be reduced to a single effective resistance, because electron-wave interference can strongly enhance or suppress transmission even in nominally ideal junctions. Using carbon nanotube (CNT) networks as a model system, we explore coherent transport through experimentally relevant junctions, from single and multiple single-walled CNT (SWCNT) contacts to double-walled CNT (DWCNT) and multi-walled CNT (MWCNT) junctions, with atomistic tight-binding non-equilibrium Green's-function calculations, also under a perpendicular magnetic field. We use analytically solvable minimal models to identify transport regimes expected for quasi-1D nanoscale junctions, and an electron-waveguide picture to interpret their CNT-specific manifestations. For single SWCNT--SWCNT junctions, high-transmission windows are set mainly by overlap length, doping and magnetic field. Gateway states can enhance conductance when some CNT subbands are gapped, and in some cases a magnetic field can restore transmission by lifting an interference blockade. In more complex architectures, added paths become selective: multi-junctions generate resonant filtering, while additional walls redistribute transmission instead of acting as independent channels. DWCNT junctions remain outer-wall dominated and SWCNT-like, whereas MWCNT junctions redistribute transmission among coupled walls and show a more complex field response. Our ultrahigh-field measurements likewise show lower, more field-sensitive conductance in MWCNT than SWCNT fibres. This work turns microscopic interference mechanisms into design principles for high-conductance, field-stable CNT conductors.

cond-mat.mes-hall↗

arXiv2026

Quantum Limits of Electronic Transport in Nanostructured Macroscopic Conductors

Macroscopic assemblies of one- and two-dimensional materials promise to translate nanoscale electronic properties into device-scale performance, yet the microscopic principles governing charge transport in such networks remain unresolved. In these systems, conductivity is often interpreted using phenomenological models that do not explicitly connect electronic structure to macroscopic magnetotransport. Here we develop a unified atomistic framework that links quantum-coherent transport, thermal disorder and magnetic-field effects, and combine it with ultrahigh-field magnetotransport measurements up to 60 T over a broad temperature range on carbon nanotube fibres. We show that positive magnetoresistance is controlled by junction overlap length, whereas negative magnetoresistance arises predominantly from lattice-mismatched heterojunctions rather than weak localisation alone. Statistical analysis of a large-scale numerical dataset reveals that the experimentally observed positive quadratic magnetoresistance originates from junction transport. These results show that macroscopic transport in disordered low-dimensional networks is governed primarily by junction-level quantum interference rather than solely by defects or doping.

cond-mat.mtrl-sci↗

arXiv2022

The magnetic properties of the iron phthalocyanine molecule grafted to the Ti$_2$C MXene layer

The magnetic tetrapyrrole molecules (such as porphyrins and phthalocyanines) with an active transition metal atom in their centre are currently intensively studied as prosperous potential elements of devices for high-density information storage and processing. It has been recently proved that by means of external factors one could induce two stable fully controllable molecular states. Therefore, hybrid systems consisting of such magnetic molecules and suitable carriers from the family of two-dimensional materials are often considered as promising highly scalable spintronic systems that could in the near future lead to novel industrial applications. Here, we perform the spin polarised density functional theory (DFT) studies of the hybrid system, which is the iron phthalocyanine molecule (FePc) on the top of the titanium carbide Ti$_2$C MXene layer. The most relevant issue in this part is the interaction between magnetic atoms: Ti from MXene substrate and iron from FePc. Four various magnetic configurations of FePc/Ti$_2$C were considered. The significant ferromagnetic interaction between the iron atom and the upper titanium layer plays important role in the reorientation of the iron atom's magnetic moment. We also analyse a model of the system in which the FePc molecule is in a quintet state (the ground state of an isolated molecule is a triplet). To get a better understanding of the physics of the FePc/Ti$_2$C hybrid system, we studied the hybrid systems with a single iron atom and non-magnetic H$_2$Pc on the Ti$_2$C layer, Fe/Ti$_2$C and H$_2$Pc/Ti$_2$C, respectively, which nicely explains the role of the Pc ligand in the FePc/Ti$_2$C hybrid system.

cond-mat.mtrl-sci↗

arXiv2022

Energetics, electronic states, and magnetism of iron phthalocyanine on pristine and defected graphene layers

Transition metal phthalocyanines (TMPc's) are under intense scrutiny in the field of spintronics, as they may be promising storage devices. The simplicity and cheapness of such molecules increase their commercial potential. There is an active study of how the magnetic moment of the metal centre of such molecules can be changed. Here, we particularly consider the iron phthalocyanine molecule (FePc) on a graphene layer as a substrate. We study how graphene defects (the Stone-Wales defect, B-doping, N-doping, S-doping, and combined B (N, S)-doped Stone-Wales defects) change the FePc electronic structure. We present ab initio study of the systems, which is done using several approaches: based on periodic plane wave density functional theory (DFT), a linear combination of atomic orbitals (LCAO) DFT with a cluster representation of graphene, and multiconfigurational methods with the pyrene molecule presented as a miniaturised graphene cluster. The treatment of the FePc/Graphene hybrid system using multiconfigurational methods was done for the first time. It was found that the hybrid systems with B- and N- dopings have quasi-degenerate ground states and it is necessary to go beyond the approximation of one Slater determinant.

cond-mat.mtrl-sci↗

arXiv2022

Superexchange dominates in magnetic topological insulators

It has been suggested that the enlarged spin susceptibility in topological insulators, described by Van Vleck's formalism, accounts for the ferromagnetism of bismuth-antimony topological chalcogenides doped with transition metal impurities. In contrast, earlier studies of HgTe and related topological systems pointed out that the interband analog of the Ruderman-Kittel-Kasuya-Yosida interaction (the Bloembergen-Rowland mechanism) leads to antiferromagnetic coupling between pairs of localized spins. Here, we critically revisit these two approaches, show their shortcomings, and elucidate why the magnitude of the interband contribution is small even in topological systems. From the proposed theoretical approach and our computational studies of magnetism in Mn-doped HgTe and CdTe, we conclude that, in the absence of band carriers, the superexchange dominates, and its sign depends on the coordination and charge state of magnetic impurities rather than on the topological class of the host material.

cond-mat.mes-hall↗

arXiv2021

Stability, electronic structure, and magnetic moment of Vanadium phthalocyanine grafted to the Au(111) surface

The studies of electronic and magnetic properties of V-Pc molecule adsorbed onto Au(111) surface are based on ab-initio calculations in the framework of density functional theory. We compute adsorption energies, investigate interaction mechanisms between constituents of the hybrid system consisting of V-Pc molecule and Au surface, and determine geometry changes in the system, particularly in the grafted molecule. We find out that the energetically most stable configuration of the V-Pc/Au(111) occurs when V-Pc is grafted to the Au surface's fcc site, which leads to the reduction of the point group symmetry of the hybrid system in comparison to the free standing V-Pc molecule. Further, our studies reveal that the electronic structure and magnetic properties of the V-Pc change significantly after adsorption to the Au(111). Generally, these studies shed light on physical mechanisms of the V-Pc adsorption to metallic surfaces and open up new prospects for design of novel spintronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Electronic structure and magneto-optical properties of silicon-nitrogen-vacancy complexes in diamond

The silicon-vacancy (SiV) and nitrogen-vacancy (NV) centers in diamond are commonly regarded as prototypical defects for solid-state quantum information processing. Here we show that when silicon and nitrogen are simultaneously introduced into the diamond lattice these defects can strongly interact and form larger complexes. Nitrogen atoms strongly bind to Si and SiV centers and complex formation can occur. Using a combination of hybrid density functional theory (DFT) and group theory, we analyze the electronic structure and provide various useful physical properties, such as hyperfine structure, quasi-local vibrational modes, and zero-phonon line, to enable experimental identification of these complexes. We demonstrate that the presence of substitutional silicon adjacent to nitrogen significantly shifts the donor level toward the conduction band, resulting in an activation energy for the SiN center that is comparable to phosphorus. We also find that the neutral SiNV center is of particular interest due to its photon emission at $\sim$1530 nm, which falls within the C band of telecom wavelengths, and its paramagnetic nature. In addition, the optical transition associated with the SiNV$^0$ color center exhibits very small electron--phonon coupling (Huang--Rhys factor~=~0.78) resulting in high quantum efficiency (Debye-Waller factor = 46\%) for single-photon emission. These features render this new center very attractive for potential application in scalable quantum telecommunication networks.

cond-mat.mtrl-sci↗

arXiv2019

Morphology, ordering, stability, and electronic structure of carbon-doped hexagonal boron nitride

We present theoretical studies of morphology, stability, and electronic structure of monolayer hexagonal CBN alloys with rich content of h-BN and carbon concentration not exceeding 50 %. Our studies are based on the bond order type of the valence force field to account for the interactions between atomic constituents and Monte Carlo method with Metropolis algorithm to establish equilibrium distribution of atoms over the lattice. We find out that the phase separation into graphene and h-BN domains occurs in the majority of growth conditions. Only in N-rich growth conditions, it is possible to obtain quasi uniform distribution of carbon atoms over boron sublattice. We predict also that the energy gap in stoichiometric C$_x$(BN)$_{1-x}$ alloys exhibits extremely strong bowing.

cond-mat.mtrl-sci↗

arXiv2018

Nematicity of correlated systems driven by anisotropic chemical phase separation

The origin of nematicity, i.e., in-plane rotational symmetry breaking, and in particular the relative role played by spontaneous unidirectional ordering of spin, orbital, or charge degrees of freedom, is a challenging issue of magnetism, unconventional superconductivity, and quantum Hall effect systems, discussed in the context of doped semiconductor systems, such as Ga$_{1-x}$Mn$_x$As, Cu$_x$Bi$_2$Se$_3$, and Ga(Al)As/Al$_x$Ga$_{1-x}$As quantum wells, respectively. Here, guided by our experimental and theoretical results for In$_{1-x}$Fe$_x$As, we demonstrate that spinodal phase separation at the growth surface (that has a lower symmetry than the bulk) can lead to a quenched nematic order of alloy components, which then governs low temperature magnetic and magnetotransport properties, in particular the magnetoresistance anisotropy whose theory for the $C_{2v}$ symmetry group is advanced here. These findings, together with earlier data for Ga$_{1-x}$Mn$_x$As, show under which conditions anisotropic chemical phase separation accounts for the magnitude of transition temperature to a collective phase or merely breaks its rotational symmetry. We address the question to what extent the directional distribution of impurities or alloy components setting in during the growth may account for the observed nematicity in other classes of correlated systems.

cond-mat.mtrl-sci↗

arXiv2018

Free energy landscape of dissociative adsorption of methane on ideal and defected graphene from ab initio simulations

We study the dissociative adsorption of methane at the surface of graphene. Free energy profiles, which include activation energies for different steps of the reaction, are computed from constrained ab initio molecular dynamics. At 300 K, the reaction barriers are much lower than experimental bond dissociation energies of gaseous methane, strongly indicating that graphene surface acts as a catalyst of methane decomposition. On the other hand, the barriers are still much higher than on nickel surface. Methane dissociation therefore occurs at a higher rate on nickel than on graphene. This reaction is a prerequisite for graphene growth from precursor gas. Thus, the growth of the first monolayer should be a fast and efficient process while subsequent layers grow at diminished rate and in a more controllable manner. Defects may also influence reaction energetics. This is evident from our results, in which simple defects (Stone-Wales defect and nitrogen substitution) lead to different free energy landscapes at both dissociation and adsorption steps of the process.

cond-mat.mtrl-sci↗

arXiv2017

Ab initio studies of carbon dioxide affinity to carbon compounds and minerals

We have performed quantum chemical computational studies to determine carbon dioxide affinity to carbon compounds and minerals, which could be present in shales. These studies shed light on the microscopic mechanisms of the possible carbon oxide sequestration processes. Our studies reveal that the carbon oxide can be adsorbed to various forms of carbon structures and also minerals such as periclase or illite. We find out that the strongest affinity of carbon oxide towards carbon structures occurs when the carbon structures exhibit $sp^3$ bonds.

cond-mat.mtrl-sci↗

arXiv2017

Ordering effects in 2D hexagonal systems of binary and ternary BCN alloys

We present theoretical study of ordering phenomena in binary $C_{1-x}B_{x}$, $C_{1-x}N_{x}$ and ternary $B_{x}C_{1-x-y}N_{y}$ alloys forming two-dimensional, graphene-like systems. For calculating energy of big systems (20 000 atoms in the supercell with periodic boundary conditions assumed) empirical Tersoff potential was employed. In order to find equilibrium distribution of different species corresponding to minima of the energy, we use Monte Carlo approach in Metropolis regime. We take into consideration wide range of concentrations (1-50%) and temperatures (70-1500 K), to provide more complete picture. For quantitative description of order, we determine Warren-Cowley Short Range Order (SRO) parameters for the first coordination shell. This procedure allows us to determine energetically favorable compositions of all alloys, and characterize resulting types of order for both binary and ternary systems.

cond-mat.mes-hall↗

arXiv2015

(Ga,Mn)As under pressure: a first-principles investigation

Electronic and magnetic properties of Ga$_{1-x}$Mn$_{x}$As, obtained from first-principles calculations employing the hybrid HSE06 functional, are presented for $x=6.25\%$ and $12.5\%$ under pressures ranging from 0 to 15 GPa. In agreement with photoemission experiments at ambient pressure, we find for $x=6.25\%$ that non-hybridized Mn-3$d$ levels and Mn-induced states reside about 5 and 0.4 eV below the Fermi energy, respectively. For elevated pressures, the Mn-3$d$ levels, Mn-induced states, and the Fermi level shift towards higher energies, however, the position of the Mn-induced states relative to the Fermi energy remains constant due to hybridization of the Mn-3$d$ levels with the valence As-4$p$ orbitals. We also evaluate, employing Monte Carlo simulations, the Curie temperature ($T_{\rm C}$). At zero pressure, we obtain $T_{\rm C}=181$K, whereas the pressure-induced changes in $T_{\rm C}$ are d$T_{\rm C}$/d$p=+4.3$K/GPa for $x=12.5\%$ and an estimated value of d$T_{\rm C}$/d$p\approx+2.2$K/GPa for $x=6.25\%$ under pressures up to 6 GPa. The determined values of d$T_{\rm C}$/d$p$ compare favorably with d$T_{\rm C}$/d$p=+$(2-3) K/GPa at $p\leq1.2$GPa found experimentally and estimated within the $p$-$d$ Zener model for Ga$_{0.93}$Mn$_{0.07}$As in the regime where hole localization effects are of minor importance [M. Gryglas-Borysiewicz $et$ $al$., Phys. Rev. B ${\bf 82}$, 153204 (2010)].

cond-mat.mtrl-sci↗

arXiv2015

$Ab~initio$ studies of Co$_2$FeAl$_{1-x}$Si$_x$ Heusler alloys

We present results of extensive theoretical studies of Co$_2$FeAl$_{1-x}$Si$_x$ Heusler alloys, which have been performed in the framework of density functional theory employing the all-electron full-potential linearized augmented plane-wave scheme. It is shown that the Si-rich alloys are more resistive to structural disorder and as a consequence Si stabilizes the $L2_1$ structure. Si alloying changes position of the Fermi level, pushing it into the gap of the minority spin-band. It is also shown that the hyperfine field on Co nuclei increases with the Si concentration, and this increase originates mostly from the changes in the electronic density of the valence electrons.

cond-mat.mtrl-sci↗

arXiv2014

Graphene based sensors: theoretical study

Graphene, a 2-dimensional monolayer form of sp2 hybridizated carbon atoms, is attracting increasing attention due to its unique and superior physicochemical properties. Covalently functionalized graphene layers, with their modifiable chemical functionality and usefull electrical properties, are excellent candidates for broad range of sensors, suitable for biomedical, optoelectronic and environmental applications. Here, we present extensive study of transport properties of sensors based on covalently functionalized graphene monolayer (GML) with graphene electrodes. The transmissions, density of states and current-voltage characterisctics supported by analysis of charge distribution of GML functionalized by -CH3, -CH2, -NH2, -NH and -OH fragments have been calculated by means of density functional theory (DFT) and non-equilibrium Green's function (NEGF). Further, we demonstrate how to control the device sensitivity by manipulating: (i) concentration, (ii) particular arrangement, and (iii) type of surface groups. We explain the underlying detection physical mechanisms. Comparisons of the theoretical results to available experimental data are provided are made and show good agreement.

cond-mat.mtrl-sci↗

arXiv2014

Stability and electronic structure of covalently functionalized graphene layers

We present exemplary results of extensive studies of mechanical, electronic and transport properties of covalent functionalization of graphene monolayers (GML) with -NH2. We report new results of ab initio studies of covalent functionalization of GML with -NH2 groups up to 12.5% concentration. Our studies are performed in the framework of the density functional theory (DFT) and non-equilibrium Green's function (NEGF). We discuss the stability (adsorption energy), elastic moduli, electronic structure, band gaps, and effective electron masses as a function of the density of the adsorbed molecules. We also show the conductance and I(V) characteristic of these systems. Generally, the stability of the functionalized graphene layers decreases with the growing concentration of attachments and we determine the critical density of the molecules that can be chemisorbed on the surface of GLs. Because of local deformations of GLs and sp3 rehybridization of the bonds induced by fragments, elastic moduli decrease with increasing number of groups. Simultaneously, we observe that the functionalizing molecules stretch the graphenes lattice, the effect being more pronounced for higher concentration of adsorbed molecules. We find out that the GLs functionalization leads in many cases to the opening of the graphene band gap (up to 0.5302 eV for 12.5% concentration) and can be therefore utilized in graphene devices. The new HOMO and LUMO originate mostly from the impurity bands induced by the functionalization and they exhibit parabolic dispersion with electron effective masses comparable to ones in silicon or gallium nitride.

cond-mat.mtrl-sci↗

arXiv2013

Manipulating Mn--Mg$_k$ cation complexes to control the charge- and spin-state of Mn in GaN

Owing to the variety of possible charge and spin states and to the different ways of coupling to the environment, paramagnetic centres in wide band-gap semiconductors and insulators exhibit a strikingly rich spectrum of properties and functionalities, exploited in commercial light emitters and proposed for applications in quantum information. Here we demonstrate, by combining synchrotron techniques with magnetic, optical and \emph{ab initio} studies, that the codoping of GaN:Mn with Mg allows to control the Mn$^{n+}$ charge and spin state in the range $3\le n\le 5$ and $2\ge S\ge 1$. According to our results, this outstanding degree of tunability arises from the formation of hitherto concealed cation complexes Mn-Mg$_k$, where the number of ligands $k$ is pre-defined by fabrication conditions. The properties of these complexes allow to extend towards the infrared the already remarkable optical capabilities of nitrides, open to solotronics functionalities, and generally represent a fresh perspective for magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2013

Elastic Properties of Functionalized Carbon Nanotubes

We study the effects of covalent functionalization of single wall carbon nanotubes (CNT) on their elastic properties. We consider simple organic molecules -NH, -NH2, -CH2, -CH3, -OH attached to CNTs' surface at various densities. The studies are based on the first principles calculations in the framework of density functional theory. We have determined the changes in the geometry and the elastic moduli of the functionalized CNTs as a function of the density of adsorbed molecules. It turns out that elastic moduli diminish with increasing concentration of adsorbands, however, the functionalized CNTs remain strong enough to be suitable for reinforcement of composites. The strongest effect is observed for CNTs functionalized with -CH2 radical, where the Young's modulus of the functionalized system is by 30% smaller than in the pristine CNTs.

cond-mat.mtrl-sci↗