arXiv subjects

Jacek A. Majewski

Publications and source records attributed to Jacek A. Majewski.

29 records · Page 2Linked to original sources

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↗

arXiv2013

Functionalization of carbon nanotubes with -CHn, -NHn fragments, -COOH and -OH groups

We present results of extensive theoretical studies concerning stability, morphology, and band structure of single wall carbon nanotubes (CNTs) covalently functionalized by -CHn(for n=2,3,4),-NHn(for n=1,2,3,4),-COOH and -OH groups. Our studies are based on ab initio calculations in the framework of the density functional theory. We determine the dependence of the binding energies on the concentration of the adsorbed molecules, critical densities of adsorbed molecules, global and local changes in the morphology, and electronic structure paying particular attention to the functionalization induced changes of the band gaps. These studies reveal physical mechanisms that determine stability and electronic structure of those systems and also provide valuable theoretical predictions relevant for application. Functionalization of CNTs causes generally their elongation and locally sp2 -> sp3 rehybridization in the neighborhood of chemisorbed groups. For adsorbants making particularly strong covalent bonds with the CNTs(-CH2), we observe formation of the 5/7 defects. In CNTs functionalized with -CH2,-NH4, and -OH, we determine critical density of molecules that could be covalently bound to CNTs. Functionalization of CNTs can be utilized for band gap engineering and also lead to changes in their metallic/semiconductor character. In semiconducting CNTs, adsorbants such as -CH3,-NH2,-OH and -COOH, introduce 'impurity' bands in the band gap of pristine CNTs. In the case of -CH3,-NH2, the induced band gaps are typically smaller than in the pure CNT and depend strongly on the concentration of adsorbants. However, functionalization of semiconducting CNTs with -OH leads to the metallization of CNTs. On the other hand, the functionalization of semi-metallic (9,0)CNT with -CH2 causes the increase of the band gap and induces semi-metal to semiconductor transition.

cond-mat.mes-hall↗

arXiv2013

Electronic structure of graphene functionalized with boron and nitrogen

We present a theoretical study of the structural and electronic properties of graphene monolayer functionalized with boron and nitrogen atoms substituting carbon atoms. Our study is based on the ab initio calculations in the framework of the density functional theory. We calculate the binding energies of the functionalized systems, changes in the morphology caused by functionalization, and further the band gap energy as a function of the concentration of dopants. Moreover, we address the problem of possible clustering of dopants at a given concentration. We define the clustering parameter to quantify the dependence of the properties of the functionalized systems on the distribution of B/N atoms. We show that clustering of B/N atoms in graphene is energetically unfavorable in comparison to the homogenous distribution of dopants. For most of the structures, we observe a nonzero energy gap that is only slightly dependent on the concentration of the substituent atoms.

cond-mat.mtrl-sci↗

arXiv2012

Composition dependence of elastic constants in wurtzite AlGaInN alloys

In this paper, we analyze the dependence of elastic constants c_ij on composition for random wurtzite quater- nary AlGayInN alloy in the whole concentration range. The study takes as its starting point the c_ij parameters for zinc blende phase calculated earlier by the authors on the basis of valence force field model. To obtain the wurtzite constants from cubic material parameters the Martin transformation is used. The de- viations from linear Vegard-like dependence of c_ij on composition are analyzed and accurate quadratic fits to calculated moduli are presented. The influence of nonlinear internal strain term in the Martin transformation is also investigated. Our general results for quaternary AlGaInN alloys are compared with the recent ab initio calculations for ternaries GaInN and AlInN (Gorczyca and Lepkowski, Phys. Rev. B 83 203201, 2011) and good qualitative agreement is found.

cond-mat.mtrl-sci↗

arXiv2012

Ordering in ternary nitride semiconducting alloys

We present a thorough theoretical study of ordering phenomena in nitride ternary alloys GaInN, AlInN, and AlGaN. Using the Monte Carlo approach and energetics based on the Keating model we analyze the influence of various factors on ordering in bulk crystals and epitaxial layers. We characterize the degree of both short range order (SRO) and long ranger order (LRO) for different compositions, temperatures and for substrates associated with different epitaxial strain. For the description of the SRO the Warren-Cowley parameters related to the first four coordination shells are used. The LRO is detected by means of the introduced sim-LRO parameter, based on the Bragg-Williams approach. The description of the observed long-range ordering patterns and conditions for their occurrence follows.

cond-mat.mtrl-sci↗

arXiv2011

Forces and atomic relaxations in the pSIC approach with ultrasoft pseudopotentials

We present the scheme that allows for efficient calculations of forces in the framework of pseudopotential self-interaction corrected (pSIC) formulation of the density functional theory. The scheme works with norm conserving and also with ultrasoft pseudopotentials and has been implemented in the plane-wave basis code {\sc quantum espresso}. We have performed tests of the internal consistency of the derived expressions for forces considering ZnO and CeO$_2$ crystals. Further, we have performed calculations of equilibrium geometry for LaTiO$_3$, YTiO$_3$, and LaMnO$_3$ perovskites and also for Re and Mn pairs in silicon. Comparison with standard DFT and DFT+U approaches shows that in the cases where spurious self-interaction matters, the pSIC approach predicts different geometry, very often closer to the experimental data.

cond-mat.str-el↗

arXiv2010

Computational study of structural and elastic properties of random AlGaInN alloys

In this work we present a detailed computational study of structural and elastic properties of cubic AlGaInN alloys in the framework of Keating valence force field model, for which we perform accurate parametrization based on state of the art DFT calculations. When analyzing structural properties, we focus on concentration dependence of lattice constant, as well as on the distribution of the nearest and the next nearest neighbour distances. Where possible, we compare our results with experiment and calculations performed within other computational schemes. We also present a detailed study of elastic constants for AlGaInN alloy over the whole concentration range. Moreover, we include there accurate quadratic parametrization for the dependence of the alloy elastic constants on the composition. Finally, we examine the sensitivity of obtained results to computational procedures commonly employed in the Keating model for studies of alloys.

cond-mat.mtrl-sci↗

arXiv2009

Experimental probing of the interplay between ferromagnetism and localisation in (Ga,Mn)As

The question whether the Anderson-Mott localisation enhances or reduces magnetic correlations is central to the physics of magnetic alloys. Particularly intriguing is the case of (Ga,Mn)As and related magnetic semiconductors, for which diverging theoretical scenarios have been proposed. Here, by direct magnetisation measurements we demonstrate how magnetism evolves when the density of carriers mediating the spin-spin coupling is diminished by the gate electric field in metal/insulator/semiconductor structures of (Ga,Mn)As. Our findings show that the channel depletion results in a monotonic decrease of the Curie temperature, with no evidence for the maximum expected within the impurity-band models. We find that the transition from the ferromagnetic to the paramagnetic state proceeds via the emergence of a superparamagnetic-like spin arrangement. This implies that carrier localisation leads to a phase separation into ferromagnetic and nonmagnetic regions, which we attribute to critical fluctuations in the local density of states, specific to the Anderson-Mott quantum transition.

cond-mat.mtrl-sci↗

arXiv2007

Ab initio calculations of third-order elastic constants and related properties for selected semiconductors

We present theoretical studies for the third-order elastic constants $C_{ijk}$ in zinc-blende nitrides AlN, GaN, and InN. Our predictions for these compounds are based on detailed ab initio calculations of strain-energy and strain-stress relations in the framework of the density functional theory. To judge the computational accuracy, we compare the ab initio calculated results for $C_{ijk}$ with experimental data available for Si and GaAs. We also underline the relation of the third-order elastic constants to other quantities characterizing anharmonic behaviour of materials, such as pressure derivatives of the second-order elastic constants and the mode Grüneisen constants for long-wavelength acoustic modes.

cond-mat.mtrl-sci↗

arXiv2007

Theory of Spin Transport Across Domain-Walls in (Ga,Mn)As

We present results of numerical calculations of domain-wall resistance in the ferromagnetic semiconductor (Ga,Mn)As. We employ Landauer-Buttiker formalism and the tight binding method. Taking into account the full valence band structure we predict the magnitude of the domain-wall resistance without disorder and compare it to experimental values. Next we add disorder to the model and study numerically both small and large disorder regime.

cond-mat.mes-hall↗