3,142 research outputs found
First principles investigation of ferroelectricity in epitaxially strained PbTiO
The structure and polarization of the as-yet hypothetical Ruddlesden-Popper
compound PbTiO are investigated within density-functional theory. Zone
enter phonons of the high-symmetry KNiF-type reference structure, space
group , were calculated. At the theoretical ground-state lattice
constants, there is one unstable infrared-active phonon. This phonon freezes in
to give the ferroelectric state. As a function of epitaxial strain, two
additional ferroelectric phases are found, with space groups and
at compressive and tensile strains, respectively.Comment: 4 pages, 4 figure
Ground state magnetic structure of MnGe
We have used spherical neutron polarimetry to investigate the magnetic
structure of the Mn spins in the hexagonal semimetal MnGe, which exhibits a
large intrinsic anomalous Hall effect. Our analysis of the polarimetric data
finds a strong preference for a spin structure with symmetry relative
to the point group. We show that weak ferromagnetism is an inevitable
consequence of the symmetry of the observed magnetic structure, and that sixth
order anisotropy is needed to select a unique ground state
A model for spin-polarized transport in perovskite manganite bi-crystal grain boundaries
We have studied the temperature dependence of low-field magnetoresistance and
current-voltage characteristics of a low-angle bi-crystal grain boundary
junction in perovskite manganite La_{2/3}Sr_{1/3}MnO_3 thin film. By gradually
trimming the junction we have been able to reveal the non-linear behavior of
the latter. With the use of the relation M_{GB} \propto M_{bulk}\sqrt{MR^*} we
have extracted the grain boundary magnetization. Further, we demonstrate that
the built-in potential barrier of the grain boundary can be modelled by
V_{bi}\propto M_{bulk}^2 - M_{GB}^2. Thus our model connects the
magnetoresistance with the potential barrier at the grain boundary region. The
results indicate that the band-bending at the grain boundary interface has a
magnetic origin.Comment: 9 pages, 5 figure
Characterization of a Plain Broadband Textile PIFA
Bandwidth characteristic of a wearable antenna is one of the major factors in determining its usability on the human body. In this work, a planar inverted-F antenna (PIFA) structure is proposed to achieve a large bandwidth to avoid serious antenna reflection coefficient detuning when placed in proximity of the body. The proposed structure is designed based on a simple structure, in order to provide practicality in application and maintain fabrication simplicity. Two different types of conductive textiles, namely Pure Copper Polyester Taffeta Fabric (PCPTF) and ShieldIt, are used in order to proof its concept, in comparison with a metallic antenna made from copper foil. The design is spaced and fabricated using a 6 mm thick fleece fabric. To cater for potential fabrication and material measurement inaccuracies, both antennas' performance are also investigated and analyzed with varying physical and material parameters. From this investigation, it is found that the proposed structure's extended bandwidth enabled the antenna to function with satisfactory on-body reflection coefficients, despite unavoidable gain and efficiency reduction
Renormalized Thermodynamic Entropy of Black Holes in Higher Dimensions
We study the ultraviolet divergent structures of the matter (scalar) field in
a higher D-dimensional Reissner-Nordstr\"{o}m black hole and compute the matter
field contribution to the Bekenstein-Hawking entropy by using the Pauli-Villars
regularization method. We find that the matter field contribution to the black
hole entropy does not, in general, yield the correct renormalization of the
gravitational coupling constants. In particular we show that the matter field
contribution in odd dimensions does not give the term proportional to the area
of the black hole event horizon.Comment: Final Revision Form as to be published in Physical Review D, ReVTeX,
No Figure
Fermi-level alignment at metal-carbon nanotube interfaces: application to scanning tunneling spectroscopy
At any metal-carbon nanotube interface there is charge transfer and the
induced interfacial field determines the position of the carbon nanotube band
structure relative to the metal Fermi-level. In the case of a single-wall
carbon nanotube (SWNT) supported on a gold substrate, we show that the charge
transfers induce a local electrostatic potential perturbation which gives rise
to the observed Fermi-level shift in scanning tunneling spectroscopy (STS)
measurements. We also discuss the relevance of this study to recent experiments
on carbon nanotube transistors and argue that the Fermi-level alignment will be
different for carbon nanotube transistors with low resistance and high
resistance contacts.Comment: 4 pages, 3 ps figures, minor corrections, accepted by Phys. Rev. Let
The effective action of (2+1)-dimensional QED: the effect of finite fermion density
The effective action of (2+1)-dimensional QED with finite fermion density is
calculated in a uniform electromagnetic field. It is shown that the integer
quantum Hall effect and de Haas-van Alphen like phenomena in condensed matter
physics are derived directly from the effective action.Comment: 10 pages, Revtex, No figure
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