24 research outputs found
All-wurtzite (In,Ga)As-(Ga,Mn)As core-shell nanowires grown by molecular beam epitaxy
Structural and magnetic properties of (In,Ga)As-(Ga,Mn)As core-shell
nanowires grown by molecular beam epitaxy on GaAs(111)B substrate with gold
catalyst have been investigated.(In,Ga)As core nanowires were grown at high
temperature (500 {\deg}C) whereas (Ga,Mn)As shells were deposited on the
{1-100} side facets of the cores at much lower temperature (220 {\deg}C). High
resolution transmission electron microscopy images and high spectral resolution
Raman scattering data show that both the cores and the shells of the nanowires
have wurtzite crystalline structure. Scanning and transmission electron
microscopy observations show smooth (Ga,Mn)As shells containing 5% of Mn
epitaxially deposited on (In,Ga)As cores containing about 10% of In, without
any misfit dislocations at the core-shell interface. With the In content in the
(In,Ga)As cores larger than 5% the (In,Ga)As lattice parameter is higher than
that of (Ga,Mn)As and the shell is in the tensile strain state. Elaborated
magnetic studies indicate the presence of ferromagnetic coupling in (Ga,Mn)As
shells at the temperatures in excess of 33 K. This coupling is maintained only
in separated mesoscopic volumes resulting in an overall superparamagnetic
behavior which gets blocked below ~17 K.Comment: 37 pages, 8 figure
Fragility of the Dirac Cone Splitting in Topological Crystalline Insulator Heterostructures
The 'double Dirac cone' 2D topological interface states found on the (001)
faces of topological crystalline insulators such as PbSnSe
feature degeneracies located away from time reversal invariant momenta, and are
a manifestation of both mirror symmetry protection and valley interactions.
Similar shifted degeneracies in 1D interface states have been highlighted as a
potential basis for a topological transistor, but realizing such a device will
require a detailed understanding of the intervalley physics involved. In
addition, the operation of this or similar devices outside of ultra-high vacuum
will require encapsulation, and the consequences of this for the topological
interface state must be understood. Here we address both topics for the case of
2D surface states using angle-resolved photoemission spectroscopy. We examine
bulk PbSnSe(001) crystals overgrown with PbSe, realizing
trivial/topological heterostructures. We demonstrate that the valley
interaction that splits the two Dirac cones at each is extremely
sensitive to atomic-scale details of the surface, exhibiting non-monotonic
changes as PbSe deposition proceeds. This includes an apparent total collapse
of the splitting for sub-monolayer coverage, eliminating the Lifshitz
transition. For a large overlayer thickness we observe quantized PbSe states,
possibly reflecting a symmetry confinement mechanism at the buried topological
interface
Ferromagnetism in semiconductors and oxides: prospects from a ten years' perspective
Over the last decade the search for compounds combining the resources of
semiconductors and ferromagnets has evolved into an important field of
materials science. This endeavour has been fuelled by continual demonstrations
of remarkable low-temperature functionalities found for ferromagnetic
structures of (Ga,Mn)As, p-(Cd,Mn)Te, and related compounds as well as by ample
observations of ferromagnetic signatures at high temperatures in a number of
non-metallic systems. In this paper, recent experimental and theoretical
developments are reviewed emphasising that, from the one hand, they disentangle
many controversies and puzzles accumulated over the last decade and, on the
other, offer new research prospects.Comment: review, 13 pages, 8 figures, 109 reference
Ferromagnetic semiconductors
The current status and prospects of research on ferromagnetism in
semiconductors are reviewed. The question of the origin of ferromagnetism in
europium chalcogenides, chromium spinels and, particularly, in diluted magnetic
semiconductors is addressed. The nature of electronic states derived from 3d of
magnetic impurities is discussed in some details. Results of a quantitative
comparison between experimental and theoretical results, notably for Mn-based
III-V and II-VI compounds, are presented. This comparison demonstrates that the
current theory of the exchange interactions mediated by holes in the valence
band describes correctly the values of Curie temperatures T_C magnetic
anisotropy, domain structure, and magnetic circular dichroism. On this basis,
chemical trends are examined and show to lead to the prediction of
semiconductor systems with T_C that may exceed room temperature, an expectation
that are being confirmed by recent findings. Results for materials containing
magnetic ions other than Mn are also presented emphasizing that the double
exchange involving hoping through d states may operate in those systems.Comment: 18 pages, 8 figures; special issue of Semicon. Sci. Technol. on
semiconductor spintronic
Epitaxial Metal Halide Perovskites by InkjetâPrinting on Various Substrates
Metalâhalideâperovskites revolutionized the field of thinâfilm semiconductor technology, due to their favorable optoelectronic properties and facile solution processing. Further improvements of perovskite thinâfilm devices require structural coherence on the atomic scale. Such perfection is achieved by epitaxial growth, a method that is based on the use of highâend deposition chambers. Here epitaxial growth is enabled via a â1000 times cheaper device, a single nozzle inkjet printer. By printing, singleâcrystal microâ and nanostructure arrays and crystalline coherent thin films are obtained on selected substrates. The heteroâepitaxial structures of methylammonium PbBr3 grown on lattice matching substrates exhibit similar luminescence as bulk single crystals, but the crystals phase transitions are shifted to lower temperatures, indicating a structural stabilization due to interfacial lattice anchoring by the substrates. Thus, the inkjetâprinting of metalâhalide perovskites provides improved material characteristics in a highly economical way, as a future cheap competitor to the highâend semiconductor growth technologies.DFG, 404984854, Bleifreie Perovksite fĂŒr die RöntgendetektionDFG, 399073171, GRK 2495: Energiekonvertierungssysteme: von Materialien zu Bauteile