591 research outputs found
Probing topological transitions in HgTe/CdTe quantum wells by magneto-optical measurements
In two-dimensional topological insulators, such as inverted HgTe/CdTe quantum
wells, helical quantum spin Hall (QSH) states persist even at finite magnetic
fields below a critical magnetic field , above which only quantum
Hall (QH) states can be found. Using linear-response theory, we theoretically
investigate the magneto-optical properties of inverted HgTe/CdTe quantum wells,
both for infinite two-dimensional and finite-strip geometries, and possible
signatures of the transition between the QSH and QH regimes. In the absorption
spectrum, several peaks arise due to non-equidistant Landau levels in both
regimes. However, in the QSH regime, we find an additional absorption peak at
low energies in the finite-strip geometry. This peak arises due to the presence
of edge states in this geometry and persists for any Fermi level in the QSH
regime, while in the QH regime the peak vanishes if the Fermi level is situated
in the bulk gap. Thus, by sweeping the gate voltage, it is possible to
experimentally distinguish between the QSH and QH regimes due to this
signature. Moreover, we investigate the effect of spin-orbit coupling and
finite temperature on this measurement scheme.Comment: 14 pages, 13 figure
Proximitized Materials
Advances in scaling down heterostructures and having an improved interface
quality together with atomically-thin two-dimensional materials suggest a novel
approach to systematically design materials. A given material can be
transformed through proximity effects whereby it acquires properties of its
neighbors, for example, becoming superconducting, magnetic, topologically
nontrivial, or with an enhanced spin-orbit coupling. Such proximity effects not
only complement the conventional methods of designing materials by doping or
functionalization, but can also overcome their various limitations. In
proximitized materials it is possible to realize properties that are not
present in any constituent region of the considered heterostructure. While the
focus is on magnetic and spin-orbit proximity effects with their applications
in spintronics, the outlined principles provide also a broader framework for
employing other proximity effects to tailor materials and realize novel
phenomena.Comment: Invited Review to appear in Materials Today, 28 pages, 22 figure
Tuning Topological Superconductivity in Phase-Controlled Josephson Junctions with Rashba and Dresselhaus Spin-Orbit Coupling
Recently, topological superconductors based on Josephson junctions in
two-dimensional electron gases with strong Rashba spin-orbit coupling have been
proposed as attractive alternatives to wire-based setups. Here, we elucidate
how phase-controlled Josephson junctions based on quantum wells with [001]
growth direction and an arbitrary combination of Rashba and Dresselhaus
spin-orbit coupling can also host Majorana bound states for a wide range of
parameters as long as the magnetic field is oriented appropriately. Hence,
Majorana bound states based on Josephson junctions can appear in a wide class
of two-dimensional electron gases. We study the effect of spin-orbit coupling,
the Zeeman energies, and the superconducting phase difference to create a full
topological phase diagram and find the optimal stability region to observe
Majorana bound states in narrow junctions. Surprisingly, for equal Rashba and
Dresselhaus spin-orbit coupling, well localized Majorana bound states can
appear only for phase differences as the topological gap
protecting the Majorana bound states vanishes at . Our results show
that the ratio between Rashba and Dresselhaus spin-orbit coupling or the choice
of the in-plane crystallographic axis along which the superconducting phase
bias is applied offer additional tunable knobs to test Majorana bound states in
these systems. Finally, we discuss signatures of Majorana bound states that
could be probed experimentally by tunneling conductance measurements at the
edge of the junction.Comment: 21 pages, 12 figure
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