3,499 research outputs found
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
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 BiSe topological insulator. At low
temperatures ( 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 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)
Single-dopant resonance in a single-electron transistor
Single dopants in semiconductor nanostructures have been studied in great
details recently as they are good candidates for quantum bits, provided they
are coupled to a detector. Here we report coupling of a single As donor atom to
a single-electron transistor (SET) in a silicon nanowire field-effect
transistor. Both capacitive and tunnel coupling are achieved, the latter
resulting in a dramatic increase of the conductance through the SET, by up to
one order of magnitude. The experimental results are well explained by the rate
equations theory developed in parallel with the experiment.Comment: 16 pages, 8 figure
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