4,639 research outputs found
Nambu-Goldstone Mechanism in Real-Time Thermal Field Theory
In a one-generation fermion condensate scheme of electroweak symmetry
breaking, it is proven based on Schwinger-Dyson equation in the real-time
thermal field theory in the fermion bubble diagram approximation that, at
finite temperature below the symmetry restoration temperature , a
massive Higgs boson and three massless Nambu-Goldstone bosons could emerge from
the spontaneous breaking of electroweak group
if the two fermion flavors in the one generation are mass-degenerate, thus
Goldstone Theorem is rigorously valid in this case. However, if the two fermion
flavors have unequal masses, owing to "thermal flactuation", the Goldstone
Theorem will be true only approximately for a very large momentum cut-off
in zero temperature fermion loop or for low energy scales. All
possible pinch singularities are proven to cancel each other, as is expected in
a real-time thermal field theory.Comment: 11 pages, revtex, no figure, Phys. Rev. D, to appea
Cloud for Gaming
Cloud for Gaming refers to the use of cloud computing technologies to build
large-scale gaming infrastructures, with the goal of improving scalability and
responsiveness, improve the user's experience and enable new business models.Comment: Encyclopedia of Computer Graphics and Games. Newton Lee (Editor).
Springer International Publishing, 2015, ISBN 978-3-319-08234-
Collapse of Charge Gap in Random Mott Insulators
Effects of randomness on interacting fermionic systems in one dimension are
investigated by quantum Monte-Carlo techniques. At first, interacting spinless
fermions are studied whose ground state shows charge ordering. Quantum phase
transition due to randomness is observed associated with the collapse of the
charge ordering. We also treat random Hubbard model focusing on the Mott gap.
Although the randomness closes the Mott gap and low-lying states are created,
which is observed in the charge compressibility, no (quasi-) Fermi surface
singularity is formed. It implies localized nature of the low-lying states.Comment: RevTeX with 3 postscript figure
Spectroscopic Constraints on the Surface Magnetic Field of the Accreting Neutron Star EXO 0748-676
Gravitationally redshifted absorption lines of Fe XXVI, Fe XXV, and O VIII
were inferred recently in the X-ray spectrum of the bursting neutron star EXO
0748-676. We place an upper limit on the stellar magnetic field based on the
iron lines. The oxygen absorption feature shows a multiple component profile
that is consistent with Zeeman splitting in a magnetic field of ~(1-2)x10^9
gauss, and for which the corresponding Zeeman components of the iron lines are
expected to be blended together. In other systems, a field strength >5x10^{10}
gauss could induce a blueshift of the line centroids that would counteract
gravitational redshift and complicate the derivation of constraints on the
equation of state of the neutron star.Comment: 5 pages, submitted to Phys. Rev. Let
Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate NaCoOHO
We report a precise Co nuclear quadrupolar resonance (NQR) measurement
on the recently discovered cobalt oxyhydrate NaCoOHO superconductor from =40 K down to 0.2 K. We find that in the
normal state the spin-lattice relaxation rate follows a Curie-Weiss
type temperature () variation, , with =-42 K,
suggesting two-dimensional antiferromagnetic spin correlations. Below =3.9
K, decreases with no coherence peak and follows a dependence with
2.2 down to 2.0 K but crosses over to a
variation below =1.4 K, which suggests non s-wave superconductivity. The
data in the superconducting state are most consistent with the existence of
line nodes in the gap function.Comment: submitted for publication in June '0
Vortex-induced topological transition of the bilinear-biquadratic Heisenberg antiferromagnet on the triangular lattice
The ordering of the classical Heisenberg antiferromagnet on the triangular
lattice with the the bilinear-biquadratic interaction is studied by Monte Carlo
simulations. It is shown that the model exhibits a topological phase transition
at a finite-temperature driven by topologically stable vortices, while the spin
correlation length remains finite even at and below the transition point. The
relevant vortices could be of three different types, depending on the value of
the biquadratic coupling. Implications to recent experiments on the triangular
antiferromagnet NiGaS is discussed
Transition of the Stellar Initial Mass Function Explored with Binary Population Synthesis
The stellar initial mass function (IMF) plays a crucial role in determining
the number of surviving stars in galaxies, the chemical composition of the
interstellar medium, and the distribution of light in galaxies. A key unsolved
question is whether the IMF is universal in time and space. Here we use
state-of-the-art results of stellar evolution to show that the IMF of our
Galaxy made a transition from an IMF dominated by massive stars to the
present-day IMF at an early phase of the Galaxy formation. Updated results from
stellar evolution in a wide range of metallicities have been implemented in a
binary population synthesis code, and compared with the observations of
carbon-enhanced metal-poor (CEMP) stars in our Galaxy. We find that applying
the present-day IMF to Galactic halo stars causes serious contradictions with
four observable quantities connected with the evolution of AGB stars.
Furthermore, a comparison between our calculations and the observations of CEMP
stars may help us to constrain the transition metallicity for the IMF which we
tentatively set at [Fe/H] = -2. A novelty of the current study is the inclusion
of mass loss suppression in intermediate-mass AGB stars at low-metallicity.
This significantly reduces the overproduction of nitrogen-enhanced stars that
was a major problem in using the high-mass star dominated IMF in previous
studies. Our results also demonstrate that the use of the present day IMF for
all time in chemical evolution models results in the overproduction of Type I.5
supernovae. More data on stellar abundances will help to understand how the IMF
has changed and what caused such a transition.Comment: 8 pages, 2 figures, accepted by MNRAS Lette
Anisotropic Behavior of Knight Shift in Superconducting State of Na_xCoO_2yH_2O
The Co Knight shift was measured in an aligned powder sample of
Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The
Knight-shift components parallel (K_c) and perpendicular to the c-axis (along
the ab plane K_{ab}) were measured in both the normal and superconducting (SC)
states. The temperature dependences of K_{ab} and K_c are scaled with the bulk
susceptibility, which shows that the microscopic susceptibility deduced from
the Knight shift is related to Co-3d spins. In the SC state, the Knight shift
shows an anisotropic temperature dependence: K_{ab} decreases below 5 K,
whereas K_c does not decrease within experimental accuracy. This result raises
the possibility that spin-triplet superconductivity with the spin component of
the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.Comment: 5 pages, 5 figures, to be published in Journal of Physical Society of
Japan vol. 75, No.
CoO2-Layer-Thickness Dependence of Magnetic Properties and Possible Two Different Superconducting States in NaxCoO2.yH2O
In order to understand the experimentally proposed phase diagrams of
NaxCoO2.yH2O, we theoretically study the CoO2-layer-thickness dependence of
magnetic and superconducting (SC) properties by analyzing a multiorbital
Hubbard model using the random phase approximation. When the Co valence (s) is
+3.4, we show that the magnetic fluctuation exhibits strong layer-thickness
dependence where it is enhanced at finite (zero) momentum in the thicker
(thinner) layer system. A magnetic order phase appears sandwiched by two SC
phases, consistent with the experiments. These two SC phases have different
pairing states where one is the singlet extended s-wave state and the other is
the triplet p-wave state. On the other hand, only a triplet p-wave SC phase
with dome-shaped behavior of Tc is predicted when s=+3.5, which is also
consistent with the experiments. Controversial experimental results on the
magnetic properties are also discussed.Comment: 5 pages, 4 figures. Submitted to Journal of the Physical Society of
Japa
Development of Cesium-Free Deuterium/Hydrogen Negative-Ion Source with Metal Catalectic-Ionization Method
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