The silicate absorption profile in the ISM towards the heavily obscured nucleus of NGC 4418

The 9.7-micron silicate absorption profile in the interstellar medium provides important information on the physical and chemical composition of interstellar dust grains. Measurements in the Milky Way have shown that the profile in the diffuse interstellar medium is very similar to the amorphous silicate profiles found in circumstellar dust shells around late M stars, and narrower than the silicate profile in denser star-forming regions. Here, we investigate the silicate absorption profile towards the very heavily obscured nucleus of NGC 4418, the galaxy with the deepest known silicate absorption feature, and compare it to the profiles seen in the Milky Way. Comparison between the 8-13 micron spectrum obtained with TReCS on Gemini and the larger aperture spectrum obtained from the Spitzer archive indicates that the former isolates the nuclear emission, while Spitzer detects low surface brightness circumnuclear diffuse emission in addition. The silicate absorption profile towards the nucleus is very similar to that in the diffuse ISM in the Milky Way with no evidence of spectral structure from crystalline silicates or silicon carbide grains.Comment: 7 Pages, 3 figures. MNRAS in pres

New Sum-Product Estimates for Real and Complex Numbers

A variation on the sum-product problem seeks to show that a set which is defined by additive and multiplicative operations will always be large. In this paper, we prove new results of this type. In particular, we show that for any finite set A of positive real numbers, it is true that |{a+bc+d:a,b,c,d∈A}|≥2|A|2-1.As a consequence of this result, it is also established that |4k-1A(k)|:=|A…A⏟ktimes+⋯+A…A⏟4k-1times|≥|A|k.Later on, it is shown that both of these bounds hold in the case when A is a finite set of complex numbers, although with smaller multiplicative constants. © 2015, Springer Science+Business Media New York

Dynamical Coulomb Blockade of Shot Noise

We observe the suppression of the finite frequency shot-noise produced by a voltage biased tunnel junction due to its interaction with a single electromagnetic mode of high impedance. The tunnel junction is embedded in a quarter wavelength resonator containing a dense SQUID array providing it with a characteristic impedance in the kOhms range and a resonant frequency tunable in the 4-6 GHz range. Such high impedance gives rise to a sizeable Coulomb blockade on the tunnel junction (roughly 30% reduction in the differential conductance) and allows an efficient measurement of the spectral density of the current fluctuations at the resonator frequency. The observed blockade of shot-noise is found in agreement with an extension of the dynamical Coulomb blockade theory

The phase diagram of 2D antiferromagnets

The magnetic phase diagram of thin-layered antiferromagnets is revealed experimentally by investigating the tunnelling conductance as a function of magnetic field. A rich magnetic behaviour in CrCl3 is uncovered, from which relevant magnetic information is extracted that is not easily available with other approaches

Bitcoin: Order Without Law in the Digital Age

Modern law makes currency a creature of the state and ultimately the value of its currency depends on the public’s trust in that state. While some nations are more capable than others at instilling public trust in the stability of their monetary institutions, it is nonetheless impossible for any legal system to make the pre-commitments necessary to completely isolate the governance of its money supply from political pressure. This proposition is true not only today, where nearly all government institutions manage their money supply in the form of central banking, but also true of past private banking regimes circulating their notes under the shadow of public law. However, bitcoin represents a potential third currency regime far more resistant to state control because it mints currency units that exist in no physical place, places a numerical ceiling on the number of units that can be created, and relies on scientific principles from cryptography to guarantee that ceiling and verify any person-to-person transfer. The trust required is not in any government but in the decentralized order of those who verify bitcoin transactions and those who create the software these verifiers choose to run on their connected computers. This Article explores the fundamental structure of bitcoin, first by demystifying it as a technology, and second by showing how its decentralized order contrasts with other currency regimes. Unlike governments that use the power of law to compel action, bitcoin relies on a system of built-in incentives to encourage behavior that benefits not only those seeking to use bitcoin, but also bitcoin miners—those who voluntarily undertake the task of maintaining the payment network. While currently bitcoin is too volatile to compete with all but the worst government-issued currencies, the qualities of this system may give bitcoin a long-term advantage over many currencies. As the bitcoin ecosystem continues to grow, its nonlegal order can help it climb the rungs of stability created by distrust in government. The technology underpinning bitcoin is the next point of innovation in the digital age—the same era that has already seen software create institutional disruption from Amazon, Facebook, and Uber, among many others. As bitcoin gains in popularity, it offers a platform for other kinds of technological alternatives to traditional legal regimes, like smart contracts. Bitcoin’s order without currency law will facilitate other forms of order with less law. This is a propitious time for fundamental examination of bitcoin. Despite experiencing significant speculation and volatility throughout late 2017 and early 2018, its ten-year history demonstrates a downward trend in volatility and an upward trend in market capitalization

The Morphologically Divided Redshift Distribution of Faint Galaxies

We have constructed a morphologically divided redshift distribution of faint field galaxies using a statistically unbiased sample of 196 galaxies brighter than I = 21.5 for which detailed morphological information (from the Hubble Space Telescope) as well as ground-based spectroscopic redshifts are available. Galaxies are classified into 3 rough morphological types according to their visual appearance (E/S0s, Spirals, Sdm/dE/Irr/Pec's), and redshift distributions are constructed for each type. The most striking feature is the abundance of low to moderate redshift Sdm/dE/Irr/Pec's at I < 19.5. This confirms that the faint end slope of the luminosity function (LF) is steep (alpha < -1.4) for these objects. We also find that Sdm/dE/Irr/Pec's are fairly abundant at moderate redshifts, and this can be explained by strong luminosity evolution. However, the normalization factor (or the number density) of the LF of Sdm/dE/Irr/Pec's is not much higher than that of the local LF of Sdm/dE/Irr/Pec's. Furthermore, as we go to fainter magnitudes, the abundance of moderate to high redshift Irr/Pec's increases considerably. This cannot be explained by strong luminosity evolution of the dwarf galaxy populations alone: these Irr/Pec's are probably the progenitors of present day ellipticals and spiral galaxies which are undergoing rapid star formation or merging with their neighbors. On the other hand, the redshift distributions of E/S0s and spirals are fairly consistent those expected from passive luminosity evolution, and are only in slight disagreement with the non-evolving model.Comment: 11 pages, 4 figures (published in ApJ

Vortex density spectrum of quantum turbulence

The fluctuations of the vortex density in a turbulent quantum fluid are deduced from local second-sound attenuation measurements. These measurements are performed with a micromachined open-cavity resonator inserted across a flow of turbulent He-II near 1.6 K. The power spectrum of the measured vortex line density is compatible with a (-5/3) power law. The physical interpretation, still open, is discussed.Comment: Submitted to Europhys. Let

Fingerprints of Inelastic Transport at the Surface of the Topological Insulator Bi2Se3: Role of Electron-Phonon Coupling

We report on electric-field and temperature dependent transport measurements in exfoliated thin crystals of Bi$_{2}$Se$_{3}$ topological insulator. At low temperatures ($< 50$ K) and when the chemical potential lies inside the bulk gap, the crystal resistivity is strongly temperature dependent, reflecting inelastic scattering due to the thermal activation of optical phonons. A linear increase of the current with voltage is obtained up to a threshold value at which current saturation takes place. We show that the activated behavior, the voltage threshold and the saturation current can all be quantitatively explained by considering a single optical phonon mode with energy $\hbar \Omega \approx 8$ meV. This phonon mode strongly interacts with the surface states of the material and represents the dominant source of scattering at the surface at high electric fields.Comment: Supplementary Material at: http://journals.aps.org/prl/supplemental/10.1103/PhysRevLett.112.086601/TIPhonon_SM.pd

Expanders with superquadratic growth

We prove several expanders with exponent strictly greater than 2. For any finite set A ⊂ ℝ, we prove the following six-variable expander results: (Formula Presented)