144 research outputs found
Electrostatic Modulation of the Electronic Properties of Dirac Semimetal Na3Bi
Large-area thin films of topological Dirac semimetal NaBi are grown on
amorphous SiO:Si substrates to realise a field-effect transistor with the
doped Si acting as back gate. As-grown films show charge carrier mobilities
exceeding 7,000 cm/Vs and carrier densities below 3 10
cm, comparable to the best thin-film NaBi. An ambipolar field effect
and minimum conductivity are observed, characteristic of Dirac electronic
systems. The results are quantitatively understood within a model of
disorder-induced charge inhomogeneity in topological Dirac semimetals. Due to
the inverted band structure, the hole mobility is significantly larger than the
electron mobility in NaBi, and when present, these holes dominate the
transport properties.Comment: 5 pages, 4 figures; minor corrections and revisions for readabilit
Low temperature transport on surface conducting diamond
Magneto-transport measurements were performed on surface conducting
hydrogen-terminated diamond (100) hall bars at temperatures between 0.1-5 K in
magnetic fields up to 8T.Comment: 2 pages Optoelectronic and Microelectronic Materials & Devices
(COMMAD), 2012 Conferenc
A New Test of the Einstein Equivalence Principle and the Isotropy of Space
Recent research has established that nonsymmetric gravitation theories like
Moffat's NGT predict that a gravitational field singles out an orthogonal pair
of polarization states of light that propagate with different phase velocities.
We show that a much wider class of nonmetric theories encompassed by the formalism predict such violations of the Einstein equivalence principle.
This gravity-induced birefringence of space implies that propagation through a
gravitational field can alter the polarization of light. We use data from
polarization measurements of extragalactic sources to constrain birefringence
induced by the field of the Galaxy. Our new constraint is times sharper
than previous ones.Comment: 21 pages, Latex, 3 Postscript figure
Defects, band bending and ionization rings in MoS2
Chalcogen vacancies in transition metal dichalcogenides are widely
acknowledged as both donor dopants and as a source of disorder. The electronic
structure of sulphur vacancies in MoS2 however is still controversial, with
discrepancies in the literature pertaining to the origin of the in-gap features
observed via scanning tunneling spectroscopy (STS) on single sulphur vacancies.
Here we use a combination of scanning tunnelling microscopy (STM) and STS to
study embedded sulphur vacancies in bulk MoS2 crystals. We observe
spectroscopic features dispersing in real space and in energy, which we
interpret as tip position- and bias-dependent ionization of the sulphur vacancy
donor due to tip induced band bending (TIBB). The observations indicate that
care must be taken in interpreting defect spectra as reflecting in-gap density
of states, and may explain discrepancies in the literature.Comment: 7 pages, 5 figure
Electric Field-Tuned Topological Phase Transition in Ultra-Thin Na3Bi - Towards a Topological Transistor
The electric field induced quantum phase transition from topological to
conventional insulator has been proposed as the basis of a topological field
effect transistor [1-4]. In this scheme an electric field can switch 'on' the
ballistic flow of charge and spin along dissipationless edges of the
two-dimensional (2D) quantum spin Hall insulator [5-9], and when 'off' is a
conventional insulator with no conductive channels. Such as topological
transistor is promising for low-energy logic circuits [4], which would
necessitate electric field-switched materials with conventional and topological
bandgaps much greater than room temperature, significantly greater than
proposed to date [6-8]. Topological Dirac semimetals(TDS) are promising systems
in which to look for topological field-effect switching, as they lie at the
boundary between conventional and topological phases [3,10-16]. Here we use
scanning probe microscopy/spectroscopy (STM/STS) and angle-resolved
photoelectron spectroscopy (ARPES) to show that mono- and bilayer films of TDS
Na3Bi [3,17] are 2D topological insulators with bulk bandgaps >400 meV in the
absence of electric field. Upon application of electric field by doping with
potassium or by close approach of the STM tip, the bandgap can be completely
closed then re-opened with conventional gap greater than 100 meV. The large
bandgaps in both the conventional and quantum spin Hall phases, much greater
than the thermal energy kT = 25 meV at room temperature, suggest that ultrathin
Na3Bi is suitable for room temperature topological transistor operation
Visualization of Strain-Induced Landau Levels in a Graphene - Black Phosphorus Heterostructure
Strain-induced pseudo magnetic fields offer the possibility of realizing zero
magnetic field Quantum Hall effect in graphene, possibly up to room
temperature, representing a promising avenue for lossless charge transport
applications. Strain engineering on graphene has been achieved via random
nanobubbles or artificial nanostructures on the substrate, but the highly
localized and non-uniform pseudomagnetic fields can make spectroscopic probes
of electronic structure difficult. Heterostructure engineering offers an
alternative approach: By stacking graphene on top of another van der Waals
material with large lattice mismatch at a desired twist angle, it is possible
to generate large strain-induced pseudo magnetic fields uniformly over the
entire heterostructure. Here, we report using nano-angle resolved photoemission
spectroscopy (nano-ARPES) to probe the electronic bandstructure of a
graphene/black phosphorus heterostructure (G/BP). By directly measuring the
iso-energy contours of graphene and black phosphorus we determine a twist angle
of 20-degrees in our heterostructure. High-resolution nano-ARPES of the
graphene bands near the Fermi level reveals the emergence of flat bands located
within the Dirac cone. The spacing of the flat bands is consistent with Landau
level formation in graphene, and corresponds to a pseudo-field of 11.36 T. Our
work provides a new way to study quantum Hall phases induced by strain in 2D
materials and heterostructures
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