7,982 research outputs found
Scattering by flexural phonons in suspended graphene under back gate induced strain
We have studied electron scattering by out-of-plane (flexural) phonon modes
in doped suspended graphene and its effect on charge transport. In the
free-standing case (absence of strain) the flexural branch shows a quadratic
dispersion relation, which becomes linear at long wavelength when the sample is
under tension due to the rotation symmetry breaking. In the non-strained case,
scattering by flexural phonons is the main limitation to electron mobility.
This picture changes drastically when strains above are considered. Here we study in particular the
case of back gate induced strain, and apply our theoretical findings to recent
experiments in suspended graphene.Comment: 4 pages, 3 figures, published versio
Ballistic Josephson junctions in edge-contacted graphene
Hybrid graphene-superconductor devices have attracted much attention since
the early days of graphene research. So far, these studies have been limited to
the case of diffusive transport through graphene with poorly defined and modest
quality graphene-superconductor interfaces, usually combined with small
critical magnetic fields of the superconducting electrodes. Here we report
graphene based Josephson junctions with one-dimensional edge contacts of
Molybdenum Rhenium. The contacts exhibit a well defined, transparent interface
to the graphene, have a critical magnetic field of 8 Tesla at 4 Kelvin and the
graphene has a high quality due to its encapsulation in hexagonal boron
nitride. This allows us to study and exploit graphene Josephson junctions in a
new regime, characterized by ballistic transport. We find that the critical
current oscillates with the carrier density due to phase coherent interference
of the electrons and holes that carry the supercurrent caused by the formation
of a Fabry-P\'{e}rot cavity. Furthermore, relatively large supercurrents are
observed over unprecedented long distances of up to 1.5 m. Finally, in the
quantum Hall regime we observe broken symmetry states while the contacts remain
superconducting. These achievements open up new avenues to exploit the Dirac
nature of graphene in interaction with the superconducting state.Comment: Updated version after peer review. Includes supplementary material
and ancillary file with source code for tight binding simulation
Antibiotic production in Streptomyces is organized by a division of labor through terminal genomic differentiation
One of the hallmark behaviors of social groups is division of labor, where different group members become specialized to carry out complementary tasks. By dividing labor, cooperative groups increase efficiency, thereby raising group fitness even if these behaviors reduce individual fitness. We find that antibiotic production in colonies of Streptomyces coelicolor is coordinated by a division of labor. We show that S. coelicolor colonies are genetically heterogeneous because of amplifications and deletions to the chromosome. Cells with chromosomal changes produce diversified secondary metabolites and secrete more antibiotics; however, these changes reduced individual fitness, providing evidence for a trade-off between antibiotic production and fitness. Last, we show that colonies containing mixtures of mutants and their parents produce significantly more antibiotics, while colony-wide spore production remains unchanged. By generating specialized mutants that hyper-produce antibiotics, streptomycetes reduce the fitness costs of secreted secondary metabolites while maximizing the yield and diversity of these products
Star formation histories and evolution of 35 brightest E+A galaxies from SDSS DR5
We pick out the 35 brightest galaxies from Goto's E+A galaxies catalogue
which are selected from the Sloan Digital Sky Survey Data Release 5. The
spectra of E+As are prominently characterized by the strong Balmer absorption
lines but little [Oii] or H_alpha emission lines. In this work we study the
stellar populations of the sample galaxies by fitting their spectra using
ULySS, which is a robust full spectrum fitting method. We fit each of the
sample with 1-population (a single stellar population-a SSP) and 3-population
(3 SSPs) models, separately. By 1-population fits, we obtain SSP-equivalent
ages and metallicities which correspond to the `luminosity-weighted' averages.
By 3-population fits, we divide components into three groups in age (old
stellar population-OSP, intermediate-age stellar population-ISP, and young
stellar population-YSP), and then get the optimal age, metallicity and
population fractions in both mass and light for OSP, ISP and YSP. During the
fits, both Pegase.HR/Elodie3.1 and Vazdekis/Miles are used as two independent
population models. The two models result in generally consistent conclusions as
follows: for all the sample galaxies, YSPs (< 1Gyr) make important
contributions to the light. However, the dominant contributors to mass are
OSPs. We also reconstruct the smoothing star formation histories (SFHs) by
giving star formation rate (SFR) versus evolutionary age. In addition, we fit
the E+A sample and 34 randomly selected elliptical galaxies with 2-population
(2 SSPs) model. We obtain the equivalent age of old components for each of the
E+A sample and elliptical galaxies. By comparison, the old components of E+As
are statistically much younger than those of ellipticals. From the standpoint
of the stellar population age, this probably provides an evidence for the
proposed evolutionary link from E+As to early-types (E/S0s).Comment: 16 pages, 9 figures, Accepted for publication on MNRA
Narrowband Biphotons: Generation, Manipulation, and Applications
In this chapter, we review recent advances in generating narrowband biphotons
with long coherence time using spontaneous parametric interaction in monolithic
cavity with cluster effect as well as in cold atoms with electromagnetically
induced transparency. Engineering and manipulating the temporal waveforms of
these long biphotons provide efficient means for controlling light-matter
quantum interaction at the single-photon level. We also review recent
experiments using temporally long biphotons and single photons.Comment: to appear as a book chapter in a compilation "Engineering the
Atom-Photon Interaction" published by Springer in 2015, edited by A.
Predojevic and M. W. Mitchel
Specific Heat Study of the Magnetic Superconductor HoNi2B2C
The complex magnetic transitions and superconductivity of HoNi2B2C were
studied via the dependence of the heat capacity on temperature and in-plane
field angle. We provide an extended, comprehensive magnetic phase diagram for B
// [100] and B // [110] based on the thermodynamic measurements. Three magnetic
transitions and the superconducting transition were clearly observed. The 5.2 K
transition (T_{N}) shows a hysteresis with temperature, indicating the first
order nature of the transition at B=0 T. The 6 K transition (T_{M}), namely the
onset of the long-range ordering, displays a dramatic in-plane anisotropy:
T_{M} increases with increasing magnetic field for B // [100] while it
decreases with increasing field for B // [110]. The anomalous anisotropy in
T_{M} indicates that the transition is related to the a-axis spiral structure.
The 5.5 K transition (T^{*}) shows similar behavior to the 5.2 K transition,
i.e., a small in-plane anisotropy and scaling with Ising model. This last
transition is ascribed to the change from a^{*} dominant phase to c^{*}
dominant phase.Comment: 9 pages, 11 figure
Velocity and density profiles of granular flow in channels using lattice gas automaton
We have performed two-dimensional lattice-gas-automaton simulations of
granular flow between two parallel planes. We find that the velocity profiles
have non-parabolic distributions while simultaneously the density profiles are
non-uniform. Under non-slip boundary conditions, deviation of velocity profiles
from the parabolic form of newtonian fluids is found to be characterized solely
by ratio of maximal velocity at the center to the average velocity, though the
ratio depends on the model parameters in a complex manner. We also find that
the maximal velocity () at the center is a linear function of the
driving force (g) as with non-zero in
contrast with newtonian fluids. Regarding density profiles, we observe that
densities near the boundaries are higher than those in the center. The width of
higher densities (above the average density) relative to the channel width is a
decreasing function of a variable which scales with the driving force (g),
energy dissipation parameter () and the width of the system (L) as
with exponents and . A phenomenological theory based on a scaling argument is presented to
interpret these findings.Comment: Latex, 15 figures, to appear in PR
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