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1
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Itinerant Theory of Exchange Constants in Diluted Co-Based Semimagnetic Semiconductors

100%
Mašek J.
Acta Physica Polonica A
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1991
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vol. 80
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issue 2
267-270
EN
The exchange constants in diluted semimagnetic semiconductors with Co are much larger than in the Mn doped systems. To explain this, we use a semiquantitative, tight binding version of the coherent potential approach and calculate the electronic structure of II-VI mixed semiconductors containing Co and Mn. The exchange constants are deduced from energies of spin reversal processes. The presence of the Co 3d states at the top of the valence band leads to the observed enhancement of the exchange coupling.
2
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Disorder-Induced Effects in III-V Semiconductors with Mn

64%
Mašek J., Máca F.
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issue 4-5
667-672
EN
The substitution of Mn in the III-V diluted magnetic semiconductors leads to a strong electron scattering on impurities. Besides the features induced in the valence band by the hybridization with the Mn d-states, also the conduction band is affected by the absence of the Mn s-states at its edge. Also the high concentration of compensating donors modifies the band structure. This is shown on the absorption coefficientε_2(ω) of GaP doped with Mn and Se. The absorption evaluated by ab initio density functional calculations starts with a smooth tail and does not show the structure typical of III-V materials. We analyze these features and the role of the donors on model systems using the tight-binding coherent potential approach.
3
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Lattice Expansion of (Ga,Mn)As: The Role of Substitutional Mn and of the Compensating Defects

64%
Mašek J., Máca F.
Acta Physica Polonica A
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2005
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vol. 108
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issue 5
789-794
EN
We apply the density-functional technique to determine the lattice constant of GaAs supercells containing Mn_{Ga}, Mn_{int}, and As_{Ga} impurities, and use a linear interpolation to describe the dependence of the lattice constant a of Ga_{1-x}Mn_{x}As on the concentrations of these impurities. The results of the supercell calculations confirm that Mn_{Ga} does not contribute to the lattice expansion. The increase in a is due to both Mn_{int} and As_{Ga}, that are both created in the as-grown (Ga,Mn)As in proportion to x, and that are most probably present in a remarkable amount also in the best annealed materials.
4
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Changes of Electronic Structure of SnTe Due to High Concentration of Sn Vacancies

64%
Mašek J., Nuzhnyj D. N.
Acta Physica Polonica A
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1997
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vol. 92
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issue 5
915-918
EN
Non-stoichiometric Sn_{1-y}Te is a strongly degenerated n-type semiconductor. This is important for understanding unusual features of magnetic behaviour of Sn_{1-x}Gd_{x}Te where the relative positions of the Fermi energy and the atomic d-level of Gd govern the exchange coupling. The influence of the Sn vacancies on the band structure cannot be neglected if their concentration reaches a few atomic percent. We address this problem by using a tight-binding coherent potential approach and show that although the character of the bands remains unchanged, they are modified so that ε_{d} can come out above the heavy-hole band.
5
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Self-Compensating Incorporation of Mn in Ga_{1-x}Mn_xAs

64%
Mašek J., Máca F.
Acta Physica Polonica A
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2001
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vol. 100
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issue 3
319-325
EN
We consider hypothetical Ga_7MnAs_8, Ga_{16}MnAs_{16}, and Ga_{14}Mn_3As_{16} crystals with Mn in a substitutional, interstitial, and both positions. Spin-polarized full-potential linearized augmented plane wave calculations were used to obtain their electronic structure. We show that the interstitial Mn acts as a double donor and compensates the holes created by two Mn atoms in substitutional positions. This explains why the number of holes in Ga_{1-x}Mn_xAs is much smaller than x. The presence of interstitial atoms may also be the reason for the lattice expansion with increasing content of Mn. The differences in electronic behavior of substitutional and interstitial Mn are discussed.
6
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Excitation of Electron Wave Packets in Low-Dimensional Semiconductor Structures by Inhomogeneous Light Pulses

64%
Král P., Mašek J.
Acta Physica Polonica A
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1992
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vol. 82
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issue 4
697-701
EN
Generation of electron wave packets by space inhomogeneous light pulses is studied in low-dimensional semiconductor systems. Examination of their free propagation for ultrashort times as well as in the stationary excitation regime is performed. Non-classical effects related to the inhomogeneity of the source are predicted.
7
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Electronic Structure of Diluted Semimagnetic Semiconductor (Cd,Co)Se

51%
Cháb V., Mašek J., Orłowski B., Kowalski B.
Acta Physica Polonica A
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1991
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vol. 80
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issue 3
333-336
EN
The electronic structure of a new semimagnetic semiconductor (Cd,Co)Se is studied by UPS and calculated by a tight-binding version of the disordered-local-moment theory. The theory accounts for the chemical disorder and for electron interactions within the Co 3d^{7} shells. Both theory and experiment show Co 3d states deep in the valence band and also at the band edge. The last state seem to be responsible for unique properties of the Co-based semimagnetic semiconductors.
8
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Comparison of Exchange Interactions in II-VI, III-V, and I-II-V Dilute Magnetic Semiconductors: Density Functional Approach

51%
Mašek J., Kudrnovský J., Máca F., Jungwirth T.
Acta Physica Polonica A
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2006
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vol. 110
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issue 2
249-254
EN
We use the coherent potential method based on the density-functional theory to compare the electronic structures and magnetic characteristics of mixed crystals (Ga,Mn)As, (Zn,Mn)Se, and Li(Zn,Mn)As. We show that, with the same degree of doping, the magnetic behavior of these materials is very similar, reflecting the similarity of their electronic structures. In particular, the superexchange in (Ga,Mn)As is as strong as in (Zn,Mn)Se and the ferromagnetic coupling in all these dilute magnetic semiconductors becomes dominant only at sufficient concentration of the holes. Remarkably, we find that ferromagnetic state appears not only in p-type, but also in n-type materials at a comparable level of doping. Although obtaining strongly n-type doped III-V dilute magnetic semiconductors is improbable, Li(Zn,Mn)As seems to be a promising candidate for a ferromagnetic n-type semiconductor.
9
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Interstitial Mn in (Ga,Mn)As: Hybridization with Conduction Band and Electron Mediated Exchange Coupling

51%
Mašek J., Kudrnovský J., Máca F., Jungwirth T.
Acta Physica Polonica A
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2007
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vol. 112
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issue 2
215-219
EN
We investigate theoretically the possibility of n-type DMS based on III-V materials with Mn impurities in interstitial instead of substitutional positions, and discuss some situations when this can happen. We show that the d-states at interstitial Mn atoms in (Ga,Mn)As hybridize with both valence and conduction bands. The hybridization is strong enough to establish an indirect ferromagnetic coupling of the Mn magnetic moments mediated either by holes or by conduction electrons. Moreover, the Curie temperatures estimated within the mean-field theory are comparable with T_c obtained for conventional materials with the same concentration of Mn_{Ga}.
10
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Correlated Doping in Semiconductors: the Role of Donors in III-V Diluted Magnetic Semiconductors

45%
Mašek J., Turek I., Drchal V., Kudrnovský J., Máca F.
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issue 4-5
673-678
EN
We investigate the compositional dependence of the total energy of the mixed crystals (Ga,Mn)As co-doped with As, Sn, and Zn. Using the ab initio linear muffin-tin orbital coherent potential approximation method we find a correlation between the incorporation of acceptors (Mn, Zn) and donors (Sn, antisite As). In particular, the formation energy of As_{Ga} is reduced by approximately 0.1 eV in the presence of Mn, and vice versa. This leads to the self-compensating behavior of (Ga,Mn)As.
11
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Compositional Dependence of the Formation Energies of Substitutional and Interstitial Mn in Partially Compensated (Ga,Mn)As

45%
Mašek J., Turek I., Kudrnovský J., Máca F., Drchal V.
Acta Physica Polonica A
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2004
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vol. 105
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issue 6
637-644
EN
We use the density-functional theory to calculate the total energy of mixed crystals (Ga,Mn)As with a small concentration of various donors. We find that the formation energy of Mn depends strongly on the partial concentrations of Mn in the substitutional and interstitial positions, and on the concentration of other dopants. The composition dependence of the formation energies represents an effective feedback mechanism, resulting in the self-compensation property of (Ga,Mn)As. We show that the partial concentrations of both substitutional and interstitial Mn increase proportionally to the total concentration of Mn.
12
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Resonant Photoemission Spectra of Zn_{1-x}Co_{x}S Valence Band

39%
Kowalski B. J., Gołacki Z., Guziewicz A., Orłowski B. A., Mašek J., Ghijsen J., Johnson R.
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EN
We report the results of a resonant photoemission study of Zn_{1-x}Co_{x}S. A Co 3d derived contribution to the valence band spectra was revealed as a 5 eV wide structure with two maxima: at the edge of the valence band and about 4 eV below the edge. The results were compared with the total DOS distribution resulting from tight binding-coherent potential approximation calculations.
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