18,931 research outputs found
Optimization of an Electromagnetic Energy Harvesting Device
This paper presents the modeling and optimization of an electromagnetic-based generator for generating power from ambient vibrations. Basic equations describing such generators are presented and the conditions for maximum power generation are described. Two-centimeter scale prototype generators, which consist of magnets suspended on a beam vibrating relative to a coil, have been built and tested. The measured power and modeled results are compared. It is shown that the experimental results confirm the optimization theory
Spin polarized STM spectra of Dirac Fermions on the surface of a topological insulator
We provide a theory for the tunneling conductance of Dirac Fermions on
the surface of a topological insulator as measured by a spin-polarized scanning
tunneling microscope tip for low bias voltages . We show that
exhibits an unconventional dependence on the direction of magnetization of the
tip and can be used to measure the magnitude of the local out-of-plane spin
orientation of the Dirac Fermions on the surface. We also demonstrate that if
the in-plane rotational symmetry on the surface of the topological insulator is
broken by an external field, then acquires a dependence on the azimuthal
angle of the magnetization of the tip. We explain the role of the Dirac
Fermions in this unconventional behavior and suggest experiments to test our
theory.Comment: 7 pages, 5 Fig
On the Natural Convection Boundary Layer Adjacent to an Inclined Flat Plate Subject to Ramp Heating
An investigation of the natural convection boundary layer adjacent to an inclined semi-infinite plate subject to a temperature boundary condition which follows a ramp function up until some specified time and then remains constant is reported. The development of the flow from start-up to a steadystate has been described based on scaling analyses and verified by numerical simulations. Attention in this study has been given to fluids having a Prandtl number Pr less than unity. The boundary layer flow depends on the comparison of the time at which the ramp heating is completed and the time at which the boundary layer completes its growth. If the ramp time is long compared with the steady state time, the layer reaches a quasi steady mode in which the growth of the layer is governed solely by the thermal balance between convection and conduction. On the other hand, if the ramp is completed before the layer becomes steady; the subsequent growth is governed by the balance between buoyancy and inertia, as for the case of instantaneous heating
Probability for Primordial Black Holes in Higher Derivative Theories
The probability for quantum creation of an inflationary universe with a pair
of black holes in higher derivative theories has been studied. Considering a
gravitational action which includes quadratic () and/or cubic
term () in scalar curvature in addition to a cosmological constant
() in semiclassical approximation with Hartle-Hawking boundary
condition, the probability has been evaluated. The action of the instanton
responsible for creating such a universe, with spatial section with
topology, is found to be less than that with a spatial
topology, unless in -theory. In the
theory, however, there exists a set of solutions without a cosmological
constant when and which admit
primordial black holes (PBH) pair in an inflationary universe scenario. We note
further that when , one gets PBH pairs in the two cases :
(i) with and both positive and (ii) with positive
and negative satisfying a constraint .
However, the relative probability for creation of an inflationary universe with
a pair of black holes in the -theory suppresses when or . However, if the above
constraints are relaxed one derives interesting results leading to a universe
with PBH in -theory without cosmological constant. PACS No(s). :
04.20.Jb, 04.60.+n, 98.80.HwComment: 15 pages, No figures. accepted in Int. J. Mod. Phys. D (2001
Tunable electronic anisotropy in single-crystal A2Cr3As3 (A = K, Rb) quasi-one-dimensional superconductors
Single crystals of A2Cr3As3 (A = K, Rb) were successfully grown using a
self-flux method and studied via structural, transport and thermodynamic
measurement techniques. The superconducting state properties between the two
species are similar, with critical temperatures of 6.1 K and 4.8 K in K2Cr3As3
and Rb2Cr3As3, respectively. However, the emergence of a strong normal state
electronic anisotropy in Rb2Cr3As3 suggests a unique electronic tuning
parameter is coupled to the inter-chain spacing in the A2Cr3As3 structure,
which increases with alkali metal ionic size while the one-dimensional
[(Cr3As3)^{2-}]_{\infty} chain structure itself remains essentially unchanged.
Together with dramatic enhancements in both conductivity and magnetoresistance
(MR), the appearance of a strong anisotropy in the MR of Rb2Cr3As3 is
consistent with the proposed quasi-one-dimensional character of band structure
and its evolution with alkali metal species in this new family of
superconductors.Comment: 6 pages, 8 figures; to appear in Phys. Rev.
Role of p-f Hybridization in the Metal-Non-Metal Transition of PrRu4P12
Electronic state evolution in the metal-non-metal transition of PrRu4P12 has
been studied by X-ray and polarized neutron diffraction experiments. It has
been revealed that, in the low-temperature non-metallic phase, two inequivalent
crystal-field (CF) schemes of Pr3+ 4f^2 electrons with Gamma_1 and Gamma_4^(2)
ground states are located at Pr1 and Pr2 sites forming the bcc unit cell
surrounded by the smaller and larger cubic Ru-ion sublattices, respectively.
This modulated electronic state can be explained by the p-f hybridization
mechanism taking two intermediate states of 4f^1 and 4f^3. The p-f
hybridization effect plays an important role for the electronic energy gain in
the metal-non-metal transition originated from the Fermi surface nesting.Comment: 5 pages, 5 figures. Accepted by J. Phys. Soc. Jp
Na2V3O7, a frustrated nanotubular system with spin-1/2 diamond rings
Following the recent discussion on the puzzling nature of the interactions in
the nanotubular system Na2V3O7, we present a detailed ab-initio microscopic
analysis of its electronic and magnetic properties. By means of a non-trivial
downfolding study we propose an effective model in terms of tubes of nine-site
rings with the geometry of a spin-diamond necklace with frustrated inter-ring
interactions. We show that this model provides a quantitative account of the
observed magnetic behavior.Comment: 5 pages, 5 figures. Phys. Rev. Lett. (in press
Space-time evolution induced by spinor fields with canonical and non-canonical kinetic terms
We study spinor field theories as an origin to induce space-time evolution.
Self-interacting spinor fields with canonical and non-canonical kinetic terms
are considered in a Friedman-Robertson-Walker universe. The deceleration
parameter is calculated by solving the equation of motion and the Friedman
equation, simultaneously. It is shown that the spinor fields can accelerate and
decelerate the universe expansion. To construct realistic models we discuss the
contributions from the dynamical symmetry breaking.Comment: 16 pages, 19 figure
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