Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi1−xMnx)2Se3 is a prototypical magnetic topological insulator with a pronounced surface band gap of ∼100 meV. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection.
For three types of colloidal magnetic nanocrystals, we
demonstrate
that postsynthetic cation exchange enables tuning of the nanocrystal’s
magnetic properties and achieving characteristics not obtainable by
conventional synthetic routes. While the cation exchange procedure,
performed in solution phase approach, was restricted so far to chalcogenide
based semiconductor nanocrystals, here ferrite-based nanocrystals
were subjected to a Fe2+ to Co2+ cation exchange
procedure. This allows tracing of the compositional modifications
by systematic and detailed magnetic characterization. In homogeneous
magnetite nanocrystals and in gold/magnetite core shell nanocrystals
the cation exchange increases the coercivity field, the remanence
magnetization, as well as the superparamagnetic blocking temperature.
For core/shell nanoheterostructures a selective doping of either the
shell or predominantly of the core with Co2+ is demonstrated.
By applying the cation exchange to FeO/CoFe2O4 core/shell nanocrystals the Neél temperature of the core
material is increased and exchange-bias effects are enhanced so that
vertical shifts of the hysteresis loops are obtained which are superior
to those in any other system.
Hole
spins have gained considerable interest in the past few years due
to their potential for fast electrically controlled qubits. Here,
we study holes confined in Ge hut wires, a so-far unexplored type
of nanostructure. Low-temperature magnetotransport measurements reveal
a large anisotropy between the in-plane and out-of-plane g-factors
of up to 18. Numerical simulations verify that this large anisotropy
originates from a confined wave function of heavy-hole character.
A light-hole admixture of less than 1% is estimated for the states
of lowest energy, leading to a surprisingly large reduction of the
out-of-plane g-factors compared with those for pure heavy holes. Given
this tiny light-hole contribution, the spin lifetimes are expected
to be very long, even in isotopically nonpurified samples.
Ferromagnetic Ge1−xMnxTe grown by molecular beam epitaxy with Mn content of xMn≈0.5 is shown to exhibit a strong tendency for phase separation. At higher growth temperatures apart from the cubic Ge0.5Mn0.5Te, a hexagonal MnTe and a rhombohedral distorted Ge0.83Mn0.17Te phase is formed. This coexistence of antiferromagnetic MnTe and ferromagnetic Ge0.5Mn0.5Te results in magnetic exchange-bias effects.
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