Jacek A. Majewski

Origin of bulk uniaxial anisotropy in zinc-blende dilute magnetic semiconductors.

It is demonstrated that the nearest-neighbor Mn pair on the GaAs (001) surface has a lower energy for the [110] direction compared to the [110] case. According to the group theory and Luttingers method of invariants, this specific Mn distribution results in bulk uniaxial in-plane and out-of-plane anisotropies. The sign and magnitude of the corresponding anisotropy energies determined by a perturbation method and ab initio computations are consistent with experimental results.

Voltage-controlled spin injection in a (Ga,Mn)As/(Al,Ga)As Zener diode

The spin polarization of the electron current in a p-(Ga,Mn)As-n-(Al,Ga)As-Zener tunnel diode, which is embedded in a light-emitting diode, has been studied theoretically. A series of self-consistent simulations determines the charge distribution, the band bending, and the current-voltage characteristics for the entire structure. An empirical tight-binding model, together with the Landauer- Buttiker theory of coherent transport has been developed to study the current spin polarization. This dual approach allows to explain the experimentally observed high magnitude and strong bias dependence of the current spin polarization.

Aggregation and magnetism of Cr, Mn, and Fe cations in GaN

A first-principles DFT-GGA+U study of the doping of GaN with Cr, Mn, or Fe confirms a strong tendency for the formation of embedded clusters occupying Ga sites of the wurtzite and zinc blende phases of GaN. Within the employed computation model, the tendency for aggregation is larger for Cr and Mn than for Fe. In contrast to previous DFT-GGA calculations, we predict a ferromagnetic ordering of the Cr and Mn clusters having more than two atoms while the Fe clusters are all antiferromagnetic. We have also investigated the magnetic ordering of nearest-neighboring ionic pairs that substitute gallium atoms at the (0001) wz-GaN surface. We find that Fe dopants tend to aggregate, whereas there is a repulsive interaction in the case of Cr and Mn. Nearest neighbor Mn and Fe pairs are coupled antiferromagnetically whereas the Cr pair is coupled ferromagnetically. The relevance of our finding to recent experimental findings is discussed.

Origin of low-temperature magnetic ordering in Ga1−xMnxN

By employing highly sensitive millikelvin SQUID magnetometry, the magnitude of the Curie temperature as a function of the Mn concentration x is determined for thoroughly characterized Ga1-xMnxN. The interpretation of the results in the frame of tight binding theory and of Monte Carlo simulations, allows us to assign the spin interaction to ferromagnetic superexchange and to benchmark the accuracy of state-of-the-art ab initio methods in predicting the magnetic characteristics of dilute magnetic insulators.

On the origin of room-temperature ferromagnetism in wide-gap semiconductors

The emerging field of semiconductor spintronics would be dramatically boosted if a semiconductor exhibiting room-temperature ferromagnetism could be found. Here, we discuss the recent stage of research, paying particular attention to the understanding of the observed room-temperature ferromagnetism in the wide-gap semiconductors GaMnN and ZnMnO. Since spinodal decomposition has been observed in these structures, we consider the possibilities of influencing density fluctuations of the alloys to obtain ferromagnetic semiconductors with required functionalities. We contrast these compounds with (In,Mn)As and (Ga,Mn)As, where the ferromagnetism is well understood, albeit well below room temperature.

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 useful electrical properties, are excellent candidates for a 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 characteristics 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 nonequilibrium 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 g...

Tight-binding model of spin-polarized tunnelling in (Ga,Mn)As-based structures

Abstract The Landauer–Buttiker formalism combined with the tight-binding transfer matrix method is used to describe the results of recent experiments: the high tunneling magnetoresistance (TMR) in (Ga,Mn)As-based trilayers and highly polarized spin injection in p-(Ga,Mn)As/n-GaAs Zener diode. For both TMR and Zener spin current polarization, the calculated values agree well with those observed experimentally. The role played in the spin dependent tunneling by carrier concentration and magnetic ion content is also studied.

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 Greens 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.

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. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Composition dependence of elastic constants in wurtzite AlGaInN alloys

In this paper, we analyze the dependence of elastic constants cij on composition for random wurtzite quaternary AlxGayIn1-x-yN alloy in the whole concentration range. The study takes as its starting point the cij 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 deviations from linear Vegard-like dependence of cij 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 AlxGayIn1-x-yN alloys are compared with the recent ab initio calculations for ternaries GaxIn1-xN and AlxIn1-xN (Gorczyca and Łepkowski, Phys. Rev. B 83 203 201, 2011) and good qualitative agreement is found.