1,703 research outputs found
Quantum oscillations from Fermi arcs
When a metal is subjected to strong magnetic field B nearly all measurable
quantities exhibit oscillations periodic in 1/B. Such quantum oscillations
represent a canonical probe of the defining aspect of a metal, its Fermi
surface (FS). In this study we establish a new mechanism for quantum
oscillations which requires only finite segments of a FS to exist. Oscillations
periodic in 1/B occur if the FS segments are terminated by a pairing gap. Our
results reconcile the recent breakthrough experiments showing quantum
oscillations in a cuprate superconductor YBCO, with a well-established result
of many angle resolved photoemission (ARPES) studies which consistently
indicate "Fermi arcs" -- truncated segments of a Fermi surface -- in the normal
state of the cuprates.Comment: 8 pages, 5 figure
Observation of TeV gamma rays from the Cygnus region with the ARGO-YBJ experiment
We report the observation of TeV gamma-rays from the Cygnus region using the
ARGO-YBJ data collected from 2007 November to 2011 August. Several TeV sources
are located in this region including the two bright extended MGRO J2019+37 and
MGRO J2031+41. According to the Milagro data set, at 20 TeV MGRO J2019+37 is
the most significant source apart from the Crab Nebula. No signal from MGRO
J2019+37 is detected by the ARGO-YBJ experiment, and the derived flux upper
limits at 90% confidence level for all the events above 600 GeV with medium
energy of 3 TeV are lower than the Milagro flux, implying that the source might
be variable and hard to be identified as a pulsar wind nebula. The only
statistically significant (6.4 standard deviations) gamma-ray signal is found
from MGRO J2031+41, with a flux consistent with the measurement by Milagro.Comment: 14 pages, 4 figure
Proton-air cross section measurement with the ARGO-YBJ cosmic ray experiment
The proton-air cross section in the energy range 1-100 TeV has been measured
by the ARGO-YBJ cosmic ray experiment. The analysis is based on the flux
attenuation for different atmospheric depths (i.e. zenith angles) and exploits
the detector capabilities of selecting the shower development stage by means of
hit multiplicity, density and lateral profile measurements at ground. The
effects of shower fluctuations, the contribution of heavier primaries and the
uncertainties of the hadronic interaction models, have been taken into account.
The results have been used to estimate the total proton-proton cross section at
center of mass energies between 70 and 500 GeV, where no accelerator data are
currently available.Comment: 14 pages, 9 figure
Effect of the tetrahedral distortion on the electronic properties of iron-pnictides
We study the dependence of the electronic structure of iron pnictides on the
angle formed by the arsenic-iron bonds. Within a Slater-Koster tight binding
model which captures the correct symmetry properties of the bands, we show that
the density of states and the band structure are sensitive to the distortion of
the tetrahedral environment of the iron atoms. This sensitivity is extremely
strong in a two-orbital (d_xz, d_yz) model due to the formation of a flat band
around the Fermi level. Inclusion of the d_xy orbital destroys the flat band
while keeping a considerable angle dependence in the band structure.Comment: 5 pages, including 5 figures. Fig. 5 replaced. Minor changes in the
tex
The ARGO-YBJ Experiment Progresses and Future Extension
Gamma ray source detection above 30TeV is an encouraging approach for finding
galactic cosmic ray origins. All sky survey for gamma ray sources using wide
field of view detector is essential for population accumulation for various
types of sources above 100GeV. To target the goals, the ARGO-YBJ experiment has
been established. Significant progresses have been made in the experiment. A
large air shower detector array in an area of 1km2 is proposed to boost the
sensitivity. Hybrid detection with multi-techniques will allow a good
discrimination between different types of primary particles, including photons
and protons, thus enable an energy spectrum measurement for individual specie.
Fluorescence light detector array will extend the spectrum measurement above
100PeV where the second knee is located. An energy scale determined by balloon
experiments at 10TeV will be propagated to ultra high energy cosmic ray
experiments
Observation of CR Anisotropy with ARGO-YBJ
The measurement of the anisotropies of cosmic ray arrival direction provides
important informations on the propagation mechanisms and on the identification
of their sources. In this paper we report the observation of anisotropy regions
at different angular scales. In particular, the observation of a possible
anisotropy on scales between 10 and 30
suggests the presence of unknown features of the magnetic fields the charged
cosmic rays propagate through, as well as potential contributions of nearby
sources to the total flux of cosmic rays. Evidence of new weaker few-degree
excesses throughout the sky region R.A. is
reported for the first time.Comment: Talk given at 12th TAUP Conference 2011, 5-9 September 2011, Munich,
German
Long-term monitoring of the TeV emission from Mrk 421 with the ARGO-YBJ experiment
ARGO-YBJ is an air shower detector array with a fully covered layer of
resistive plate chambers. It is operated with a high duty cycle and a large
field of view. It continuously monitors the northern sky at energies above 0.3
TeV. In this paper, we report a long-term monitoring of Mrk 421 over the period
from 2007 November to 2010 February. This source was observed by the
satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray
band. Mrk 421 was especially active in the first half of 2008. Many flares are
observed in both X-ray and gamma-ray bands simultaneously. The gamma-ray flux
observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag
between the X-ray and gamma-ray photons longer than 1 day is found. The
evolution of the spectral energy distribution is investigated by measuring
spectral indices at four different flux levels. Hardening of the spectra is
observed in both X-ray and gamma-ray bands. The gamma-ray flux increases
quadratically with the simultaneously measured X-ray flux. All these
observational results strongly favor the synchrotron self-Compton process as
the underlying radiative mechanism.Comment: 30 pages, 8 figure
Mean Interplanetary Magnetic Field Measurement Using the ARGO-YBJ Experiment
The sun blocks cosmic ray particles from outside the solar system, forming a
detectable shadow in the sky map of cosmic rays detected by the ARGO-YBJ
experiment in Tibet. Because the cosmic ray particles are positive charged, the
magnetic field between the sun and the earth deflects them from straight
trajectories and results in a shift of the shadow from the true location of the
sun. Here we show that the shift measures the intensity of the field which is
transported by the solar wind from the sun to the earth.Comment: 6 papges,3 figure
Pairing symmetry and properties of iron-based high temperature superconductors
Pairing symmetry is important to indentify the pairing mechanism. The
analysis becomes particularly timely and important for the newly discovered
iron-based multi-orbital superconductors. From group theory point of view we
classified all pairing matrices (in the orbital space) that carry irreducible
representations of the system. The quasiparticle gap falls into three
categories: full, nodal and gapless. The nodal-gap states show conventional
Volovik effect even for on-site pairing. The gapless states are odd in orbital
space, have a negative superfluid density and are therefore unstable. In
connection to experiments we proposed possible pairing states and implications
for the pairing mechanism.Comment: 4 pages, 1 table, 2 figures, polished versio
Observation of associated near-side and away-side long-range correlations in âsNN=5.02ââTeV proton-lead collisions with the ATLAS detector
Two-particle correlations in relative azimuthal angle (ÎÏ) and pseudorapidity (Îη) are measured in âsNN=5.02ââTeV p+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1ââÎŒb-1 of data as a function of transverse momentum (pT) and the transverse energy (ÎŁETPb) summed over 3.1<η<4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2<|Îη|<5) ânear-sideâ (ÎÏâŒ0) correlation that grows rapidly with increasing ÎŁETPb. A long-range âaway-sideâ (ÎÏâŒÏ) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small ÎŁETPb, is found to match the near-side correlation in magnitude, shape (in Îη and ÎÏ) and ÎŁETPb dependence. The resultant ÎÏ correlation is approximately symmetric about Ï/2, and is consistent with a dominant cosâĄ2ÎÏ modulation for all ÎŁETPb ranges and particle pT
- âŠ