467 research outputs found
Bulk contribution to magnetotransport properties of low defect-density BiTe topological insulator thin films
An important challenge in the field of topological materials is to carefully
disentangle the electronic transport contribution of the topological surface
states from that of the bulk. For BiTe topological insulator samples,
bulk single crystals and thin films exposed to air during fabrication processes
are known to be bulk conducting, with the chemical potential in the bulk
conduction band. For BiTe thin films grown by molecular beam epitaxy,
we combine structural characterization (transmission electron microscopy),
chemical surface analysis as function of time (x-ray photoelectron
spectroscopy) and magnetotransport analysis to understand the low defect
density and record high bulk electron mobility once charge is doped into the
bulk by surface degradation. Carrier densities and electronic mobilities
extracted from the Hall effect and the quantum oscillations are consistent and
reveal a large bulk carrier mobility. Because of the cylindrical shape of the
bulk Fermi surface, the angle dependence of the bulk magnetoresistance
oscillations is two-dimensional in nature.Comment: 12 pages, 5 figure
MgB2 tunnel junctions and SQUIDs
Recent advances in the realization and understanding of MgB2 tunnel junctions and SQUIDs are surveyed. High quality MgB2 junctions with suitable tunnel barriers have been realized based on both oriented and epitaxial thin MgB2 films. Multiband transport properties, such as the existence of two energy gaps, phonon spectra and anisotropy have been investigated with these junctions. We review the suitability of different barrier materials and recent advances in obtaining reproducible all-MgB2 Josephson junctions for superconducting electronic circuitry. The development of epitaxial thin films has also led to high-quality multiband MgB2 SQUIDs and magnetometers that operate at high temperatures. The multiband nature of MgB2 provides new phenomena such as the Leggett mode. Manipulating the different phases of the condensates could lead to novel MgB2 devices with phase degrees of freedom.\ud
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Stabilization of the perovskite phase in the Y-Bi-O system by using a BaBiO buffer layer
A topological insulating phase has theoretically been predicted for the
thermodynamically unstable perovskite phase of YBiO. Here, it is shown
that the crystal structure of the Y-Bi-O system can be controlled by using a
BaBiO buffer layer. The BaBiO film overcomes the large lattice
mismatch of 12% with the SrTiO substrate by forming a rocksalt structure
in between the two perovskite structures. Depositing an YBiO film
directly on a SrTiO substrate gives a fluorite structure. However, when
the Y-Bi-O system is deposited on top of the buffer layer with the correct
crystal phase and comparable lattice constant, a single oriented perovskite
structure with the expected lattice constants is observed.Comment: 8 pages, 7 figures + 4 pages supporting informatio
Thickness-Dependent Band Gap Modification in BaBiO
The material BaBiO is known for its insulating character. However, for
thin films, in the ultra-thin limit, metallicity is expected because
BaBiO is suggested to return to its undistorted cubic phase where the
oxygen octahedra breathing mode will be suppresse as reported recently. Here,
we confirm the influence of the oxygen breathing mode on the size of the band
gap. The electronic properties of a BaBiO thickness series are studied
using \textit{in-situ} scanning tunneling microscopy. We observe a wide-gap
(~ 1.2 V) to small-gap~(~ 0.07 eV)
semiconductor transition as a function of a decreasing BaBiO film
thickness. However, even for an ultra-thin BaBiO film, no metallic state
is present. The dependence of the band gap size is found to be coinciding with
the intensity of the Raman response of the breathing phonon mode as a function
of thickness
Conduction spectroscopy of a proximity induced superconducting topological insulator
The combination of superconductivity and the helical spin-momentum locking at
the surface state of a topological insulator (TI) has been predicted to give
rise to p-wave superconductivity and Majorana bound states. The
superconductivity can be induced by the proximity effect of a an s-wave
superconductor (S) into the TI. To probe the superconducting correlations
inside the TI, dI/dV spectroscopy has been performed across such S-TI
interfaces. Both the alloyed BiSbTeSe and the
stoichiometric BiSbTeSe have been used as three dimensional TI. In the case
of BiSbTeSe, the presence of disorder induced
electron-electron interactions can give rise to an additional zero-bias
resistance peak. For the stoichiometric BiSbTeSe with less disorder, tunnel
barriers were employed in order to enhance the signal from the interface. The
general observations in the spectra of a large variety of samples are
conductance dips at the induced gap voltage, combined with an increased sub-gap
conductance, consistent with p-wave predictions. The induced gap voltage is
typically smaller than the gap of the Nb superconducting electrode, especially
in the presence of an intentional tunnel barrier. Additional uncovered
spectroscopic features are oscillations that are linearly spaced in energy, as
well as a possible second order parameter component.Comment: Semiconductor Science and Technology; Special Issue on Hybrid Quantum
Materials and Device
Artificial oxide heterostructures with non-trivial topology
In the quest for topological insulators with large band gaps,
heterostructures with Rashba spin-orbit interactions come into play. Transition
metal oxides with heavy ions are especially interesting in this respect. We
discuss the design principles for stacking oxide Rashba layers. Assuming a
single layer with a two-dimensional electron gas (2DEG) on both interfaces as a
building block, a two-dimensional topological insulating phase is present when
negative coupling between the 2DEGs exists. When stacking multiple building
blocks, a two-dimensional or three-dimensional topological insulator is
artificially created, depending on the intra- and interlayer coupling strengths
and the number of building blocks. We show that the three-dimensional
topological insulator is protected by reflection symmetry, and can therefore be
classified as a topological crystalline insulator. In order to isolate the
topological states from bulk states, the intralayer coupling term needs to be
quadratic in momentum. It is described how such a quadratic coupling could
potentially be realized by taking buckling within the layers into account. The
buckling, thereby, brings the idea of stacked Rashba system very close to the
alternative approach of realizing the buckled honeycomb lattice in
[111]-oriented perovskite oxides.Comment: Accepted for publication in Journal of Physics: Condensed Matte
Interaction between counter-propagating quantum Hall edge channels in the 3D topological insulator BiSbTeSe
The quantum Hall effect is studied in the topological insulator BiSbTeSe.
By employing top- and back-gate electric fields at high magnetic field, the
Landau levels of the Dirac cones in the top and bottom topological surface
states can be tuned independently. When one surface is tuned to the
electron-doped side of the Dirac cone and the other surface to the hole-doped
side, the quantum Hall edge channels are counter-propagating. The opposite edge
mode direction, combined with the opposite helicities of top and bottom
surfaces, allows for scattering between these counter-propagating edge modes.
The total Hall conductance is integer valued only when the scattering is
strong. For weaker interaction, a non-integer quantum Hall effect is expected
and measured
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