Implementation of an optically active material on silicon has been a persistent technological challenge. For tandem photovoltaics using a Si bottom cell, as well as for other optoelectronic applications, there has been a longstanding need for optical
ly active, wide band gap materials that can be integrated with Si. ZnSiP$_2$ is a stable, wide band gap (2.1 eV) material that is lattice matched with silicon and comprised of inexpensive elements. As we show in this paper, it is also a defect-tolerant material. Here, we report the first ZnSiP$_2$ photovoltaic device. We show that ZnSiP$_2$ has excellent photoresponse and high open circuit voltage of 1.3 V, as measured in a photoelectrochemical configuration. The high voltage and low band gap-voltage offset are on par with much more mature wide band gap III-V materials. Photoluminescence data combined with theoretical defect calculations illuminate the defect physics underlying this high voltage, showing that the intrinsic defects in ZnSiP$_2$ are shallow and the minority carrier lifetime is 7 ns. These favorable results encourage the development of ZnSiP$_2$ and related materials as photovoltaic absorber materials.
(abridged) We present a dynamical analysis of the extended stellar stream encircling NGC 1097. Within a statistical framework, we model its surface brightness using mock streams as in Amorisco (2015) and deep imaging data from the CHART32 telescope (
Stellar Tidal Stream Survey). We reconstruct the post-infall evolution of the progenitor, which has experienced 3 pericentric passages and lost more than 2 orders of magnitude in mass. At infall, $5.4pm0.6$ Gyr ago, the progenitor was a disky dwarf with mass of $log_{10}[m(<3.4pm1 {rm kpc})/ M_odot]=10.35pm0.25$. We illustrate how the 90$^circ$ turn in the stream, identifying the `dog leg, is the signature of the progenitors prograde rotation. Today, the remnant is a nucleated dwarf, with a LOS velocity of $v_{rm p, los}^{rm obs}=-30pm 30$ kms$^{-1}$, and a luminosity of $3.3times 10^7 L_{V,odot}$ (Galianni et al. 2010). Our independent analysis predicts $v_{rm p, los}=-51^{-17}_{+14}$ kms$^{-1}$, and measures $log_{10}(m/ M_odot)=7.4^{+0.6}_{-0.8}$, so that the compact nucleus is soon becoming a low-luminosity UCD. We find that NGC 1097 has a mass of $M_{200}=1.8^{+0.5}_{-0.4} times 10^{12}; M_{odot}$, and its concentration $c_{200}=6.7^{+2.4}_{-1.3}$ is in agreement with LCDM. The stream is described almost down to the noise in a spherical host potential, we find this would not be possible if the halo was substantially triaxial at large radii. Its morphology shows that the slope of the total density profile bends from an inner $gamma(r_{rm peri})=1.5pm0.15$. The progenitors orbit reaches $r_{rm apo}=150pm 15$ kpc, more than a half of the virial radius of the host, so that, for the first time on an individual extragalactic halo, we measure the outer density slope, $gamma(0.6r_{200,c})=3.9pm0.5$. This demonstrates the promise of the newborn field of detailed, statistical modelling of extragalactic tidal streams.
A classic measure of ecological stability describes the tendency of a community to return to equilibrium after small perturbation. While many advances show how the network structure of these communities severely constrains such tendencies, few if any
of these advances address one of the most fundamental properties of network structure: heterogeneity among nodes with different numbers of links. Here we systematically explore this property of degree heterogeneity and find that its effects on stability systematically vary with different types of interspecific interactions. Degree heterogeneity is always destabilizing in ecological networks with both competitive and mutualistic interactions while its effects on networks of predator-prey interactions such as food webs depend on prey contiguity, i.e., the extent to which the species consume an unbroken sequence of prey in community niche space. Increasing degree heterogeneity stabilizes food webs except those with the most contiguity. These findings help explain previously unexplained observations that food webs are highly but not completely contiguous and, more broadly, deepens our understanding of the stability of complex ecological networks with important implications for other types of dynamical systems.
We present 3D kinematic observations of stars within the central 0.5 pc of the Milky Way nuclear star cluster using adaptive optics imaging and spectroscopy from the Keck telescopes. Recent observations have shown that the cluster has a shallower sur
face density profile than expected for a dynamically relaxed cusp, leading to important implications for its formation and evolution. However, the true three dimensional profile of the cluster is unknown due to the difficulty in de-projecting the stellar number counts. Here, we use spherical Jeans modeling of individual proper motions and radial velocities to constrain for the first time, the de-projected spatial density profile, cluster velocity anisotropy, black hole mass ($M_mathrm{BH}$), and distance to the Galactic center ($R_0$) simultaneously. We find that the inner stellar density profile of the late-type stars, $rho(r)propto r^{-gamma}$ to have a power law slope $gamma=0.05_{-0.60}^{+0.29}$, much more shallow than the frequently assumed Bahcall $&$ Wolf slope of $gamma=7/4$. The measured slope will significantly affect dynamical predictions involving the cluster, such as the dynamical friction time scale. The cluster core must be larger than 0.5 pc, which disfavors some scenarios for its origin. Our measurement of $M_mathrm{BH}=5.76_{-1.26}^{+1.76}times10^6$ $M_odot$ and $R_0=8.92_{-0.55}^{+0.58}$ kpc is consistent with that derived from stellar orbits within 1$^{primeprime}$ of Sgr A*. When combined with the orbit of S0-2, the uncertainty on $R_0$ is reduced by 30% ($8.46_{-0.38}^{+0.42}$ kpc). We suggest that the MW NSC can be used in the future in combination with stellar orbits to significantly improve constraints on $R_0$.
We observe variations on the surface potential of graphite samples that we attribute to the adsorption physisorption of tetracene isomers.
We present a method for identifying localized secondary populations in stellar velocity data using Bayesian statistical techniques. We apply this method to the dwarf spheroidal galaxy Ursa Minor and find two secondary objects in this satellite of the
Milky Way. One object is kinematically cold with a velocity dispersion of $4.25 pm 0.75 kms$ and centered at $(9.1arcmin pm 1.5, 7.2arcmin pm 1.2)$ in relative RA and DEC with respect to the center of Ursa Minor. The second object has a large velocity offset of $-12.8^{+1.75}_{-1.5} kms$ compared to Ursa Minor and centered at $(-14.0arcmin^{+2.4}_{-5.8}, -2.5arcmin^{+0.4}_{-1.0})$. The kinematically cold object has been found before using a smaller data set but the prediction that this cold object has a velocity dispersion larger than $2.0 kms$ at 95% C.L. differs from previous work. We use two and three component models along with the information criteria and Bayesian evidence model selection methods to argue that Ursa Minor has one or two localized secondary populations. The significant probability for a large velocity dispersion in each secondary object raises the intriguing possibility that each has its own dark matter halo, that is, it is a satellite of a satellite of the Milky Way.
We have performed an experimental study of the crystal structure, lattice-dynamics, and optical properties of PbCrO4 (the mineral crocoite) at ambient and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band
-gap have been accurately determined. X-ray-diffraction, Raman, and optical-absorption experiments have allowed us also to completely characterize two pressure-induced structural phase transitions. The first transition is isostructural, maintaining the monoclinic symmetry of the crystal, and having important consequences in the physical properties; among other a band-gap collapse is induced. The second one involves an increase of the symmetry of the crystal, a volume collapse, and probably the metallization of PbCrO4. The results are discussed in comparison with related compounds and the effects of pressure in the electronic structure explained. Finally, the room-temperature equation of state of the low-pressure phases is also obtained.
We present the results of deep imaging obtained at the CFHT with MegaCam in the Anticenter direction at two different heights above the Galactic disk. We detect the presence of the Monoceros ring in both fields as a conspicuous and narrow Main Sequen
ce feature which dominates star counts over a large portion of the color-magnitude diagram down to g~24. The comparison of the morphology and density of this feature with a large variety of Galactic models excludes the possibility that it can be due to a flare of the Galactic disk, supporting an extra-Galactic origin for this ring-like structure.
We report the detection of a pair of degree-long tidal tails associated with the globular cluster Palomar 14, using images obtained at the CFHT. We reveal a power-law departure from a King profile at large distances to the cluster center. The density
map constructed with the optimal matched filter technique shows a nearly symmetrical and elongated distribution of stars on both sides of the cluster, forming a S-shape characteristic of mass loss. This evidence may be the telltale signature of tidal stripping in action. This, together with its large Galactocentric distance, imposes strong constraints on its orbit and/or origin: i) it must follow an external orbit confined to the peripheral region of the Galactic halo and/or ii) it formed in a satellite galaxy later accreted by the Milky Way.
