67,277 research outputs found
An unification of general theory of relativity with Dirac's large number hypothesis
Taking a hint from Dirac's large number hypothesis, we note the existence of
cosmologically combined conservation laws that work to cosmologically long
time. We thus modify Einstein's theory of general relativity with fixed
gravitation constant to a theory for varying , with a tensor term
arising naturally from the derivatives of in place of the cosmological
constant term usually introduced \textit{ad hoc}. The modified theory, when
applied to cosmology, is consistent with Dirac's large number hypothesis, and
gives a theoretical Hubble's relation not contradicting the observational data.
For phenomena of duration and distance short compared with that of the
universe, our theory reduces to Einstein's theory with being constant
outside the gravitating matter, and thus also passes the crucial tests of
Einstein's theory.Comment: 9 pages, 1 figur
Engineering direct-indirect band gap transition in wurtzite GaAs nanowires through size and uniaxial strain
Electronic structures of wurtzite GaAs nanowires in the [0001] direction were
studied using first-principles calculations. It was found that the band gap of
GaAs nanowires experience a direct-to-indirect transition when the diameter of
the nanowires is smaller than ~28 {\AA}. For those thin GaAs nanowires with an
indirect band gap, it was found that the gap can be tuned to be direct if a
moderate external uniaxial strain is applied. Both tensile and compressive
strain can trigger the indirect-to-direct gap transition. The critical strains
for the gap-transition are determined by the energy crossover of two states in
conduction bands.Comment: 4 pages, 4 figure
Space-Based Gravity Detector for a Space Laboratory
A space-based superconducting gravitational low-frequency wave detector is
considered. Sensitivity of the detector is sufficient to use the detector as a
partner of other contemporary low-frequency detectors like LIGO and LISA. This
device can also be very useful for experimental study of other effects
predicted by theories of gravitation.Comment: 4 pages, 4 figures
Full electrical control of Charge and Spin conductance through Interferometry of Edge States in Topological Insulators
We investigate electron interferometry of edge states in Topological
Insulators. We show that, when inter-boundary coupling is induced at two
quantum point contacts of a four terminal setup, both Fabry-P\'erot-like and
Aharonov-Bohm-like loop processes arise. These underlying interference effects
lead to a full electrically controllable system, where the magnitude of charge
and spin linear conductances can be tuned by gate voltages, without applying
magnetic fields. In particular we find that, under appropriate conditions,
inter-boundary coupling can lead to negative values of the conductance.
Furthermore, the setup also allows to selectively generate pure charge or pure
spin currents, by choosing the voltage bias configuration.Comment: 12 pages, 5 figures (expanded discussion section, corrected typos
Wormhole Effect in a Strong Topological Insulator
An infinitely thin solenoid carrying magnetic flux Phi (a `Dirac string')
inserted into an ordinary band insulator has no significant effect on the
spectrum of electrons. In a strong topological insulator, remarkably, such a
solenoid carries protected gapless one-dimensional fermionic modes when
Phi=hc/2e. These modes are spin-filtered and represent a distinct bulk
manifestation of the topologically non-trivial insulator. We establish this
`wormhole' effect by both general qualitative considerations and by numerical
calculations within a minimal lattice model. We also discuss the possibility of
experimental observation of a closely related effect in artificially engineered
nanostructures.Comment: 4 pages, 3 figures. For related work and info visit
http://www.physics.ubc.ca/~fran
Field-effect mobility enhanced by tuning the Fermi level into the band gap of Bi2Se3
By eliminating normal fabrication processes, we preserve the bulk insulating
state of calcium-doped Bi2Se3 single crystals in suspended nanodevices, as
indicated by the activated temperature dependence of the resistivity at low
temperatures. We perform low-energy electron beam irradiation (<16 keV) and
electrostatic gating to control the carrier density and therefore the Fermi
level position in the nanodevices. In slightly p-doped Bi2-xCaxSe3 devices,
continuous tuning of the Fermi level from the bulk valence band to the band-gap
reveals dramatic enhancement (> a factor of 10) in the field-effect mobility,
which suggests suppressed backscattering expected for the Dirac fermion surface
states in the gap of topological insulators
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