10,691 research outputs found
Second Harmonic Generation from Phononic Epsilon-Near-Zero Berreman Modes in Ultrathin Polar Crystal Films
Immense optical field enhancement was predicted to occur for the Berreman
mode in ultrathin films at frequencies in the vicinity of epsilon near zero
(ENZ). Here, we report the first experimental proof of this prediction in the
mid-infrared by probing the resonantly enhanced second harmonic generation
(SHG) at the longitudinal optic phonon frequency from a deeply
subwavelength-thin aluminum nitride (AlN) film. Employing a transfer matrix
formalism, we show that the field enhancement is completely localized inside
the AlN layer, revealing that the observed SHG signal of the Berreman mode is
solely generated in the AlN film. Our results demonstrate that ENZ Berreman
modes in intrinsically low-loss polar dielectric crystals constitute a
promising platform for nonlinear nanophotonic applications
Strong Ramsey Games in Unbounded Time
For two graphs and the strong Ramsey game on the
board and with target is played as follows. Two players alternately
claim edges of . The first player to build a copy of wins. If none of
the players win, the game is declared a draw. A notorious open question of Beck
asks whether the first player has a winning strategy in
in bounded time as . Surprisingly, in a recent paper Hefetz
et al. constructed a -uniform hypergraph for which they proved
that the first player does not have a winning strategy in
in bounded time. They naturally ask
whether the same result holds for graphs. In this paper we make further
progress in decreasing the rank.
In our first result, we construct a graph (in fact )
and prove that the first player does not have a winning strategy in
in bounded time. As an application of this
result we deduce our second result in which we construct a -uniform
hypergraph and prove that the first player does not have a winning
strategy in in bounded time. This improves the
result in the paper above.
An equivalent formulation of our first result is that the game
is a draw. Another reason for interest
on the board is a folklore result that the disjoint
union of two finite positional games both of which are first player wins is
also a first player win. An amusing corollary of our first result is that at
least one of the following two natural statements is false: (1) for every graph
, is a first player win; (2) for every graph
if is a first player win, then
is also a first player win.Comment: 18 pages, 46 figures; changes: fully reworked presentatio
Gapless Hamiltonians for the toric code using the PEPS formalism
We study Hamiltonians which have Kitaev's toric code as a ground state, and
show how to construct a Hamiltonian which shares the ground space of the toric
code, but which has gapless excitations with a continuous spectrum in the
thermodynamic limit. Our construction is based on the framework of Projected
Entangled Pair States (PEPS), and can be applied to a large class of
two-dimensional systems to obtain gapless "uncle Hamiltonians".Comment: 8 pages, 2 figure
Switchgrass Cultivar/Ecotype Selection and Management for Biofuels in the Upper Southeast USA
Switchgrass (Panicum virgatum L.), a perennial warm-season grass indigenous to the eastern USA, has potential as a biofuels feedstock. The objective of this study was to investigate the performance of upland and lowland switchgrass cultivars under different environments and management treatments. Four cultivars of switchgrass were evaluated from 2000 to 2001 under two management regimes in plots established in 1992 at eight locations in the upper southeastern USA. Two management treatments included 1) a single annual harvest (in late October to early November) and a single application of 50âkg N/ha/yr and 2) two annual harvests (in midsummer and November) and a split application of 100âkgâN/ha/yr. Biomass yields averaged 15âMg/ha/yr and ranged from 10 to 22âMg/ha/yr across cultivars, managements, locations, and years. There was no yield advantage in taking two harvests of the lowland cultivars (Alamo and Kanlow). When harvested twice, upland cultivars (Cave-in-Rock and Shelter) provided yields equivalent to the lowland ecotypes. Tiller density was 36% lower in stands cutting only once per year, but the stands appeared vigorous after nine years of such management. Lowland cultivars and a one-cutting management (after the tops have senesced) using low rates of applied N (50âkg/ha) are recommended
Preface: Special Issue on Environmental Impact of NatureâBased Tourism.
Tourism is growing rapidly throughout the world, including natureâbased tourism, but natural habitats are shrinking. How do we avoid damaging what is left as an increasing number of visitors wish to experience nature or enjoy adventure or relaxation in natural surroundings? This issue explores various environmental impact factors, from the physical effects of trampling through the disturbance of wildlife to the attitudes of visitors and how wellârun tourism ventures might modify their behavior
Strong Coupling of Epsilon-Near-Zero Phonon Polaritons in Polar Dielectric Heterostructures
We report the first observation of epsilon near zero (ENZ) phonon polaritons
in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP)
supported by the adjacent SiC substrate. Employing a strong coupling model for
the analysis of the dispersion and electric field distribution in these
hybridized modes, we show that they share the most prominent features of the
two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly
propagative character and deeply sub-wavelength light confinement can be
utilized as building blocks for future infrared and terahertz (THz)
nanophotonic integration and communication devices
The On/Off Nature of Star-Planet Interactions
Evidence suggesting an observable magnetic interaction between a star and its
hot Jupiter appears as a cyclic variation of stellar activity synchronized to
the planet's orbit. In this study, we monitored the chromospheric activity of 7
stars with hot Jupiters using new high-resolution echelle spectra collected
with ESPaDOnS over a few nights in 2005 and 2006 from the CFHT. We searched for
variability in several stellar activity indicators (Ca II H, K, the Ca II
infrared triplet, Halpha, and He I). HD 179949 has been observed almost every
year since 2001. Synchronicity of the Ca II H & K emission with the orbit is
clearly seen in four out of six epochs, while rotational modulation with
P_rot=7 days is apparent in the other two seasons. We observe a similar
phenomenon on upsilon And, which displays rotational modulation (P_rot=12 days)
in September 2005, in 2002 and 2003 variations appear to correlate with the
planet's orbital period. This on/off nature of star-planet interaction (SPI) in
the two systems is likely a function of the changing stellar magnetic field
structure throughout its activity cycle. Variability in the transiting system
HD 189733 is likely associated with an active region rotating with the star,
however, the flaring in excess of the rotational modulation may be associated
with its hot Jupiter. As for HD 179949, the peak variability as measured by the
mean absolute deviation for both HD 189733 and tau Boo leads the sub-planetary
longitude by 70 degrees. The tentative correlation between this activity and
the ratio of Mpsini to the planet's rotation period, a quantity proportional to
the hot Jupiter's magnetic moment, first presented in Shkolnik et al. 2005
remains viable. This work furthers the characterization of SPI, improving its
potential as a probe of extrasolar planetary magnetic fields.Comment: Accepted for publication in the Astrophysical Journa
Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201
We present the detection and characterization of a peculiar low-mass
protostar (IRAS 22129+7000) located ~0.4 pc from Ced 201 Photodissociation
Region (PDR) and ~0.2 pc from the HH450 jet. The cold circumstellar envelope
surrounding the object has been mapped through its 1.2 mm dust continuum
emission with IRAM-30m/MAMBO. The deeply embedded protostar is clearly detected
with Spitzer/MIPS (70 um), IRS (20-35 um) and IRAC (4.5, 5.8, and 8 um) but
also in the K_s band (2.15 um). Given the large "near- and mid-IR excess" in
its spectral energy distribution, but large submillimeter-to-bolometric
luminosity ratio (~2%), IRAS 22129+7000 must be a transition Class 0/I source
and/or a multiple stellar system. Targeted observations of several molecular
lines from CO, 13CO, C18O, HCO+ and DCO+ have been obtained. The presence of a
collimated molecular outflow mapped with the CSO telescope in the CO J=3-2 line
suggests that the protostar/disk system is still accreting material from its
natal envelope. Indeed, optically thick line profiles from high density tracers
such as HCO+ J=1-0 show a red-shifted-absorption asymmetry reminiscent of
inward motions. We construct a preliminary physical model of the circumstellar
envelope (including radial density and temperature gradients, velocity field
and turbulence) that reproduces the observed line profiles and estimates the
ionization fraction. The presence of both mechanical and (non-ionizing)
FUV-radiative input makes the region an interesting case to study triggered
star formation
The environmental dependence of the structure of outer galactic discs in STAGES spiral galaxies
We present an analysis of V-band radial surface brightness profiles for
spiral galaxies from the field and cluster environments using Hubble Space
Telescope/Advanced Camera for Surveys imaging and data from the Space Telescope
A901/2 Galaxy Evolution Survey (STAGES). We use a large sample of ~330 face-on
to intermediately inclined spiral galaxies and assess the effect of the galaxy
environment on the azimuthally averaged radial surface brightness mu profiles
for each galaxy in the outer stellar disc (24 < mu < 26.5 mag per sq arcsec).
For galaxies with a purely exponential outer disc (~50 per cent), we determine
the significance of an environmental dependence on the outer disc scalelength
h_out. For galaxies with a broken exponential in their outer disc, either
down-bending (truncation, ~10 per cent) or up-bending (anti-truncation, ~40 per
cent), we measure the strength T (outer-to-inner scalelength ratio,
log_10(h_out/h_in) of the mu breaks and determine the significance of an
environmental dependence on break strength T. Surprisingly, we find no evidence
to suggest any such environmental dependence on either outer disc scalelength
h_out or break strength T, implying that the galaxy environment is not
affecting the stellar distribution in the outer stellar disc. We also find that
for galaxies with small effective radii (r_e < 3 kpc) there is a lack of outer
disc truncations in both the field and cluster environments. Our results
suggest that the stellar distribution in the outer disc of spiral galaxies is
not significantly affected by the galaxy environment.Comment: Accepted to MNRAS. Appendix A available at
http://www.nottingham.ac.uk/~ppxdtm/STAGES_profiles_appendix.pd
The fate of murine double minute X (MdmX) is dictated by distinct signaling pathways through murine double minute 2 (Mdm2)
Mouse double minute 2 (Mdm2) and MdmX dimerize in response to low levels of genotoxic stress to function in a ubiquitinating complex, which signals for destabilization of p53. Under growth conditions, Mdm2 functions as a neddylating ligase, but the importance and extent of MdmX involvement in this process are largely unknown. Here we show that when Mdm2 functions as a neddylating enzyme, MdmX is stabilized. Furthermore, we demonstrate that under growth conditions, MdmX enhances the neddylation activity of Mdm2 on p53 and is a substrate for neddylation itself. Importantly, MdmX knockdown in MCF-7 breast cancer cells resulted in diminished neddylated p53, suggesting that MdmX is important for Mdm2-mediated neddylation. Supporting this finding, the lack of MdmX in transient assays or in p53/MdmX-/- MEFs results in decreased or altered neddylation of p53 respectively; therefore, MdmX is a critical component of the Mdm2-mediated neddylating complex. c-Src is the upstream activator of this Mdm2-MdmX neddylating pathway and loss of Src signaling leads to the destabilization of MdmX that is dependent on the RING (Really Interesting New Gene) domain of MdmX. Treatment with a small molecule inhibitor of neddylation, MLN4924, results in the activation of Ataxia Telangiectasia Mutated (ATM). ATM phosphorylates Mdm2, converting Mdm2 to a ubiquitinating enzyme which leads to the destabilization of MdmX. These data show how distinct signaling pathways engage neddylating or ubiquitinating activities and impact the Mdm2-MdmX axis
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