39,299 research outputs found
Angle Dependence of Landau Level Spectrum in Twisted Bilayer Graphene
In the context of the low energy effective theory, the exact Landau level
spectrum of quasiparticles in twisted bilayer graphene with small twist angle
is analytically obtained by spheroidal eigenvalues. We analyze the dependence
of the Landau levels on the twist angle to find the points, where the two-fold
degeneracy for twist angles is lifted in the nonzero modes and below/above
which massive/massless fermion pictures become valid. In the perpendicular
magnetic field of 10\,T, the degeneracy is removed at %angles around 3 degrees for a few low levels, specifically,
for the first pair of nonzero levels and
for the next pair. Massive quasiparticle
appears at in 10\,T, %angles less
than 1.17 degrees. which match perfectly with the recent experimental results.
Since our analysis is applicable to the cases of arbitrary constant magnetic
fields, we make predictions for the same experiment performed in arbitrary
constant magnetic fields, e.g., for B=40\,T we get and the sequence of angles for the pairs of nonzero energy levels. The symmetry restoration
mechanism behind the massive/massless transition is conjectured to be a
tunneling (instanton) in momentum space.Comment: 8 pages, 7 figures, version to appear in PR
Lattice dynamics of anharmonic solids from first principles
An accurate and easily extendable method to deal with lattice dynamics of
solids is offered. It is based on first-principles molecular dynamics
simulations and provides a consistent way to extract the best possible harmonic
- or higher order - potential energy surface at finite temperatures. It is
designed to work even for strongly anharmonic systems where the traditional
quasiharmonic approximation fails. The accuracy and convergence of the method
are controlled in a straightforward way. Excellent agreement of the calculated
phonon dispersion relations at finite temperature with experimental results for
bcc Li and bcc Zr is demonstrated
Open and Closed Loop Stability of Hingeless Rotor Helicopter Air and Ground Resonance
The air and ground resonance instabilities of hingeless rotor helicopters are examined on a relatively broad parametric basis including the effects of blade tuning, virtual hinge locations, and blade hysteresis damping, as well as size and scale effects in the gross weight range from 5,000 to 48,000 pounds. A special case of a 72,000 pound helicopter air resonance instability is also included. The study shows that nominal to moderate and readily achieved levels of blade inertial hysteresis damping in conjunction with a variety of tuning and/or feedback conditions are highly effective in dealing with these instabilities. Tip weights and reductions in pre-coning angles are also shown to be effective means for improving the air resonance instability
Quantum oscillations and a non-trivial Berry phase in the noncentrosymmetric superconductor BiPd
We report the measurements of de Haas-van Alphen (dHvA) oscillations in the
noncentrosymmetric superconductor BiPd. Several pieces of a complex multi-sheet
Fermi surface are identified, including a small pocket (frequency 40 T) which
is three dimensional and anisotropic. From the temperature dependence of the
amplitude of the oscillations, the cyclotron effective mass is (
0.1) . Further analysis showed a non-trivial -Berry phase is
associated with the 40 T pocket, which strongly supports the presence of
topological states in bulk BiPd and may result in topological superconductivity
due to the proximity coupling to other bands.Comment: 5 pages, 3 figure
Numerical Study of Wave Propagation in Uniaxially Anisotropic Lorentzian Backward Wave Slabs
The propagation and refraction of a cylindrical wave created by a line
current through a slab of backward wave medium, also called left-handed medium,
is numerically studied with FDTD. The slab is assumed to be uniaxially
anisotropic. Several sets of constitutive parameters are considered and
comparisons with theoretical results are made. Electric field distributions are
studied inside and behind the slab. It is found that the shape of the
wavefronts and the regions of real and complex wave vectors are in agreement
with theoretical results.Comment: 6 pages, figure
Who will use pre-exposure prophylaxis (PrEP) and why?: Understanding PrEP awareness and acceptability amongst men who have sex with men in the UK – a mixed methods study
Background:
Recent clinical trials suggest that pre-exposure prophylaxis (PrEP) may reduce HIV transmission by up to 86% for men who have sex with men (MSM), whilst relatively high levels of PrEP acceptability have been reported to date. This study examines PrEP awareness amongst sub-groups of MSM communities and acceptability amongst MSM in a low prevalence region (Scotland, UK), using a mixed methods design.
Methods:
Quantitative surveys of n = 690 MSM recruited online via social and sociosexual media were analysed using descriptive statistics and multivariate logistic regression. In addition, n = 10 in-depth qualitative interviews with MSM were analysed thematically.
Results:
Under one third (29.7%) of MSM had heard of PrEP, with awareness related to living in large cities, degree level education, commercial gay scene use and reporting an HIV test in the last year. Just under half of participants (47.8%) were likely to use PrEP if it were available but there was no relationship between PrEP acceptability and previous PrEP awareness. Younger men (18–25 years) and those who report higher risk UAI were significantly more likely to say they would use PrEP. Qualitative data described specific PrEP scenarios, illustrating how risk, patterns of sexual practice and social relationships could affect motivation for and nature of PrEP use.
Conclusion:
These findings suggest substantial interest PrEP amongst MSM reporting HIV risk behaviours in Scotland. Given the Proud results, there is a strong case to investigate PrEP implementation within the UK. However, it appears that disparities in awareness have already emerged along traditional indicators of inequality. Our research identifies the need for comprehensive support when PrEP is introduced, including a key online component, to ensure equity of awareness across diverse MSM communities (e.g. by geography, education, gay scene use and HIV proximity), as well as to responding to the diverse informational and sexual health needs of all MSM communities
Magnetic and spectral properties of multi-sublattice oxides SrY2O4:Er3+ and SrEr2O4
SrEr2O4 is a geometrically frustrated magnet which demonstrates rather
unusual properties at low temperatures including a coexistence of long- and
short-range magnetic order, characterized by two different propagation vectors.
In the present work, the effects of crystal fields (CF) in this compound
containing four magnetically inequivalent erbium sublattices are investigated
experimentally and theoretically. We combine the measurements of the CF levels
of the Er3+ ions made on a powder sample of SrEr2O4 using neutron spectroscopy
with site-selective optical and electron paramagnetic resonance measurements
performed on single crystal samples of the lightly Er-doped nonmagnetic
analogue, SrY2O4. Two sets of CF parameters corresponding to the Er3+ ions at
the crystallographically inequivalent lattice sites are derived which fit all
the available experimental data well, including the magnetization and dc
susceptibility data for both lightly doped and concentrated samples.Comment: 14 pages, 9 figure
The Molecular Gas in the Circumnuclear Region of Seyfert Galaxies
Sub-arcsecond IRAM Plateau de Bure mm-interferometric observations of the
12CO (2-1) line emission in the Seyfert~1 NGC 3227 and the Seyfert~2 NGC 1068
have revealed complex kinematic systems in the inner 100 pc to 300 pc that are
not consistent with pure circular motion in the host galaxies. Modeling of
these kinematic systems with elliptical orbits in the plane of the host galaxy
(representing gas motion in a bar potential) is a possible solution but does
not reproduce all features observed. A better description of the complex
kinematics is achieved by circular orbits which are tilted out of the plane of
the host galaxy. This could indicate that the thin circumnuclear gas disk is
warped. In the case of NGC 1068 the warp model suggests that at a radius of
about 70 pc, the gas disk is oriented edge-on providing material for the
obscuration of the AGN nucleus. The position-velocity diagrams show rising
rotation curves at r 2
x 10^7 M_solar for NGC 3227 and > 10^8 M_solar for NGC 1068 within the central
25 pc.Comment: 14 pages, Ap.J. letter, accepte
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