P. Kacman

Method for suppression of stacking faults in Wurtzite III-V nanowires.

The growth of wurtzite GaAs and InAs nanowires with diameters of a few tens of nanometers with negligible intermixing of zinc blende stacking is reported. The suppression of the number of stacking faults was obtained by a procedure within the vapor-liquid-solid growth, which exploits the theoretical result that nanowires of small diameter ( approximately 10 nm) adopt purely wurtzite structure and are observed to thicken (via lateral growth) once the axial growth exceeds a certain length.

Spin interactions of interstitial Mn ions in ferromagnetic GaMnAs

The recently reported Rutherford backscattering and particle-induced x-ray emission experiments have revealed that in low-temperature molecular beam epitaxy grown Ga 1 - x Mn x As a significant part of the incorporated Mn atoms occupies tetrahedral interstitial sites in the lattice. Here we study the magnetic properties of these interstitial (Mn 1 ) ions. We show that they do not participate in the hole-induced ferromagnetism. Moreover, Mn 1 double donors may form pairs with the nearest substitutional (Mn G a ) acceptors-our calculations evidence that the spins in such pairs are antiferromagnetically coupled by the superexchange. We also show that for the Mn ion in another, hexagonal, interstitial position (which seems to be the case in the Ga 1 - x - y Mn x Be y As samples) the p-d interactions with the holes, responsible for the ferromagnetism, are very much suppressed.

Modelling the structure of GaAs and InAs nanowires

Using ab initio methods, we study the stability of thin (diameters up to 10 nm) GaAs and InAs nanowires. Modelled nanowires were constructed using bulk atomic positions along six different crystallographic directions of either zinc-blende or wurtzite structures. Taking into account the reconstruction of the nanowire surfaces, we have found that, for diameters of up to 50 A, the most stable nanowires adopt the wurtzite (0001) structure—for such diameters the free energy of zinc-blende nanowires along any crystallographic axis is always larger than that of the wurtzite (0001) ones. To calculate the free energy in nanowires with larger diameters, we have approximated their total energy by the sum of congruous bulk and bulk surface energies. In these nanowires the interplay between the wurtzite and zinc-blende structures was demonstrated. The band structure and the density of charge in the nanowires have also been calculated.

Antiferromagnetic interlayer coupling in ferromagnetic semiconductor EuS/PbS(001) superlattices

Antiferromagnetic coupling between ferromagnetic layers has been observed for the first time in an all-semiconductor superlattice structure EuS/PbS(001), by neutron scattering and magnetization measurements. Spin-dependent superlattice band structure effects are invoked to explain the possible origin and the strength of the observed coupling.

Structure-Dependent Ferromagnetism in Mn-Doped III–V Nanowires

The electronic and magnetic properties of (Ga,Mn)As and (In,Mn)As nanowires are studied by ab initio methods. The results suggest that, in contrast to the bulk, in nanowires (In,Mn)As may exhibit better ferromagnetic behavior than (Ga,Mn)As. Moreover, the calculations show that in one-dimensional diluted magnetic semiconductors the distribution of Mn ions and the magnetic order depend crucially on the crystallographic structure. Since the growth of III-V nanowires of a given, either zinc blende or wurtzite, crystal structure is nowadays well controlled, these results can help to find the preferable material and conditions for the growth of ferromagnetic semiconductor nanowires.

Interlayer exchange coupling in (Ga,Mn)As-based superlattices

The interlayer coupling between (Ga,Mn)As ferromagnetic layers in all-semiconductor superlattices is studied theoretically within a tight-binding model, which takes into account the crystal, band and magnetic structure of the constituent superlattice components. It is shown that the mechanism originally introduced to describe the spin correlations in antiferromagnetic EuTe/PbTe superlattices, explains the experimental results observed in ferromagnetic semiconductor structures, i.e., both the antiferromagnetic coupling between ferromagnetic layers in IV-VI (EuS/PbS and EuS/YbSe) superlattices as well as the ferromagnetic interlayer coupling in III-V ((Ga,Mn)As/GaAs) multilayer structures. The model allows also to predict (Ga,Mn)As-based structures, in which an antiferromagnetic interlayer coupling could be expected.

Interlayer exchange coupling mediated by valence-band electrons

The interlayer exchange coupling mediated by valence-band electrons in all-semiconductor IV-VI magnetic/nonmagnetic superlattices is studied theoretically. A three-dimensional tight-binding model, accounting for the band and magnetic structure of the constituent superlattice components is used to calculate the spin-dependent part of the total electronic energy. The antiferromagnetic coupling between ferromagnetic layers in EuS/PbS superlattices is obtained, in agreement with the experimental evidences. The results obtained for the coupling between antiferromagnetic layers in EuTe/PbTe superlattices are also presented.

Topological crystalline insulator (Pb,Sn)Te: Surface states and their spin polarization

Using a tight-binding approach we study theoretically the nature of surface states in Pb0.4Sn0.6Te - the newly discovered topological-crystalline-insulator. Apart from the studied before (001) surface states, two other surface families, {011} and {111}, in which the mirror symmetry of the crystals rock-salt structure plays the same role in topological protection, are considered. Our calculations show that while in (111) surface states of (Pb,Sn)Te four single topologically protected Dirac-cones should appear, for the (110) surface states the protection is lifted for two L points. In this case, instead of the Dirac points energy gaps occur in the surface states, due to the interaction between the two L valleys. In all studied cases a chiral spin texture is obtained.

Crystal Structure and Transport in Merged InAs Nanowires MBE Grown on (001) InAs

Molecular beam epitaxy growth of merging InAs nanowire intersections, that is, a first step toward the realization of a network of such nanowires, is reported. While InAs nanowires play already a leading role in the search for Majorana fermions, a network of these nanowires is expected to promote their exchange and allow for further development of this field. The structural properties of merged InAs nanowire intersections have been investigated using scanning and transmission electron microscope imaging. At the heart of the intersection, a sharp change of the crystal structure from wurtzite to perfect zinc blende is observed. The performed low-temperature conductance measurements demonstrate that the intersection does not impose an obstacle to current transport.

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.