720 research outputs found
Nonequilibrium transport and statistics of Schwinger pair production in Weyl semimetals
The non-equilibrium dynamics beyond linear response of Weyl semimetals is
studied after a sudden switching on of a DC electric field. The resulting
current is a nonmonotonic function of time, with an initial quick increase of
polarization current followed by a power-law decay. Particle-hole creation \`a
la Schwinger dominates for long times when the conduction current takes over
the leading role, with the total current increasing again. The conductivity
estimated from a dynamical calculation within a Drude picture agrees with the
one obtained from Kubo's formula. The full distribution function of
electron-hole pairs changes from Poissonian for short perturbations to a
Gaussian in the long perturbation (Landau-Zener) regime. The vacuum persistence
probability of high energy physics manifests itself in a finite probability of
no pair creation and no induced current at all times.Comment: 7 pages, 4 figure
On the Diophantine equation
By finding all integral points on certain elliptic and hyperelliptic curves
we completely solve the Diophantine equation for
and Moreover, we present some other
observations of computational and theoretical nature concerning the title
equation
Physiological sharp wave-ripples and interictal events in vitro: What’s the difference?
Sharp wave-ripples and interictal events are physiological and pathological forms of transient high activity
in the hippocampus with similar features. Sharp wave-ripples have been shown to be essential in memory
consolidation, while epileptiform (interictal) events are thought to be damaging. It is essential to grasp the
difference between physiological sharp wave-ripples and pathological interictal events in order to
understand the failure of control mechanisms in the latter case. We investigated the dynamics of activity
generated intrinsically in the CA3 region of the mouse hippocampus in vitro, using four different types of
intervention to induce epiletiform activity. As a result, sharp wave-ripples spontaneously occurring in CA3
disappeared, and following an asynchronous transitory phase, activity reorganized into a new form of
pathological synchrony. During epileptiform events, all neurons increased their firing rate compared to sharp
wave-ripples. Different cell types showed complementary firing: parvalbumin-positive basket cells and
some axo-axonic cells stopped firing due to a depolarization block at the climax of the events in high
potassium, 4-aminopyridine and zero magnesium models, but not in the gabazine model. In contrast,
pyramidal cells started firing maximally at this stage. To understand the underlying mechanism we
measured changes of intrinsic neuronal and transmission parameters in the high potassium model. We found
that the cellular excitability increased and excitatory transmission was enhanced, whereas inhibitory
transmission was compromised. We observed a strong short-term depression in parvalbumin-positive basket
cell to pyramidal cell transmission. Thus, the collapse of pyramidal cell perisomatic inhibition appears to be
a crucial factor in the emergence of epileptiform events
NGC 7789: An Open Cluster Case Study
We have obtained high-resolution spectra of 32 giants in the open cluster NGC
7789 using the Wisconsin-Indiana-Yale-NOAO Hydra spectrograph. We explore
differences in atmospheric parameters and elemental abundances caused by the
use of the linelist developed for the Gaia-ESO Survey (GES) compared to one
based on Arcturus used in our previous work. [Fe/H] values decrease when using
the GES linelist instead of the Arcturus-based linelist; these differences are
probably driven by systematically lower (~ -0.1 dex) GES surface gravities.
Using the GES linelist we determine abundances for 10 elements - Fe, Mg, Si,
Ca, Ti, Na, Ni, Zr, Ba, and La. We find the cluster's average metallicity
[Fe/H] = 0.03 +/- 0.07 dex, in good agreement with literature values, and a
lower [Mg/Fe] abundance than has been reported before for this cluster (0.11
+/- 0.05 dex). We also find the neutron-capture element barium to be highly
enhanced - [Ba/Fe] = +0.48 +/- 0.08 - and disparate from cluster measurements
of neutron-capture elements La and Zr (-0.08 +/- 0.05 and 0.08 +/- 0.08,
respectively). This is in accordance with recent discoveries of supersolar Ba
enhancement in young clusters along with more modest enhancement of other
neutron-capture elements formed in similar environments.Comment: 15 pages, 9 figures, Table 1 typo fixe
Cryptanalysis of ITRU
ITRU cryptosystem is a public key cryptosystem and one of the known variants of NTRU cryptosystem. Instead of working in a truncated polynomial ring, ITRU cryptosystem is based on the ring of integers. The authors claimed that ITRU has better features comparing to the classical NTRU, such as having a simple parameter selection algorithm, invertibility, and successful message decryption, and better security. In this paper, we present an attack technique against the ITRU cryptosystem, and it is mainly based on a simple frequency analysis on the letters of ciphertexts
Results of Aortocoronary Bypass Surgery for Angina Pectoris
To determine the relative risks and benefits of coronary bypass surgery for angina pectoris, we examined the results in our first consecutive 360 patients operated upon between May 1970 and December 1975. The age range was 27 to 75 years; there were 309 males and 51 females. The patients were classified clinically as having stable angina in 119 cases (33%), unstable angina in 205 cases (57%) and preinfarction angina in 36 cases (10%). Unstable angina was defined as a definite recent increase in severity or frequency of chest pain, angina at rest or nocturnal angina. Preinfarction angina was defined as a syndrome of prolonged angina, poorly controlled by nitrites, occurring at rest with typical ECG changes of ischemia. Such patients underwent observation in the Coronary Intensive Care Unit followed by emergency arteriography and surgery, usually within 24 hours
Bimodal pumice populations in the 13.5 Ma Harsány ignimbrite, Bükkalja Volcanic Field, Northern Hungary: Syn-eruptive mingling of distinct rhyolitic magma batches?
Abstract
The 13.5 Ma Harsány ignimbrite, in the eastern part of the Bükkalja volcanic field, eastern-central Europe, provides a rare example of mingled rhyolite. It consists of two distinct pumice populations (‘A’- and ‘B’-type) that can be recognized only by detailed geochemical work. The pumice and the host ignimbrite have a similar mineral assemblage involving quartz, plagioclase, biotite and sporadic Kfeldspar. Zircon, allanite, apatite and ilmenite occur as accessory minerals. The distinct pumice types are recognized by their different trace element compositions and the different CaO contents of their groundmass glasses. Plagioclase has an overlapping composition; however, biotite shows bimodal composition. Based on trace element and major element modeling, a derivation of ‘A’-type rhyolite magma from the ‘B’-type magma by fractional crystallization is excluded. Thus, the two pumice types represent two isolated rhyolite magma batches, possibly residing in the same crystal mush. Coeval remobilization of the felsic magmas might be initiated by intrusion of hot basaltic magma into the silicic magma reservoir The rapid ascent of the foaming rhyolite magmas enabled only a short-lived interaction and thus, a syn-eruptive mingling between the two magma batches
Chemical Abundances in Field Red Giants from High-Resolution H-Band Spectra using the APOGEE Spectral Linelist
High-resolution H-band spectra of five bright field K, M, and MS giants,
obtained from the archives of the Kitt Peak National Observatory (KPNO) Fourier
Transform Spectrometer (FTS), are analyzed to determine chemical abundances of
16 elements. The abundances were derived via spectrum synthesis using the
detailed linelist prepared for the SDSS III Apache Point Galactic Evolution
Experiment (APOGEE), which is a high-resolution near-infrared spectroscopic
survey to derive detailed chemical abundance distributions and precise radial
velocities for 100,000 red giants sampling all Galactic stellar populations.
Measured chemical abundances include the cosmochemically important isotopes
12C, 13C, 14N, and 16O, along with Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co,
Ni, and Cu. A comparison of the abundances derived here with published values
for these stars reveals consistent results to ~0.1 dex. The APOGEE spectral
region and linelist is, thus, well-suited for probing both Galactic chemical
evolution, as well as internal nucleosynthesis and mixing in populations of red
giants using high-resolution spectroscopy.Comment: Accepted for publication in The Astrophysical Journal. 42 pages, 12
figure
Chemical tagging can work: Identification of stellar phase-space structures purely by chemical-abundance similarity
Chemical tagging promises to use detailed abundance measurements to identify
spatially separated stars that were in fact born together (in the same
molecular cloud), long ago. This idea has not yielded much practical success,
presumably because of the noise and incompleteness in chemical-abundance
measurements. We have succeeded in substantially improving spectroscopic
measurements with The Cannon, which has now delivered 15 individual abundances
for ~100,000 stars observed as part of the APOGEE spectroscopic survey, with
precisions around 0.04 dex. We test the chemical-tagging hypothesis by looking
at clusters in abundance space and confirming that they are clustered in phase
space. We identify (by the k-means algorithm) overdensities of stars in the
15-dimensional chemical-abundance space delivered by The Cannon, and plot the
associated stars in phase space. We use only abundance-space information (no
positional information) to identify stellar groups. We find that clusters in
abundance space are indeed clusters in phase space. We recover some known
phase-space clusters and find other interesting structures. This is the
first-ever project to identify phase-space structures at survey-scale by blind
search purely in abundance space; it verifies the precision of the abundance
measurements delivered by The Cannon; the prospects for future data sets appear
very good.Comment: accepted for publication in the Ap
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