49,015 research outputs found
Origin of the pseudogap and its influence on superconducting state
When holes move in the background of strong antiferromagnetic correlation,
two effects with different spatial scale emerge, leading to a much reduced
hopping integral with an additional phase factor. An effective Hamiltonian is
then proposed to investigate the underdoped cuprates. We argue that the
pseudogap is the consequence of dressed hole moving in the antiferromagnetic
background and has nothing to do with the superconductivity. The momentum
distributions of the gap are qualitatively consistent with the recent ARPES
measurements both in the pseudogap and superconducting state. Two thermal
qualities are further calculated to justify our model. A two-gap scenario is
concluded to describe the relation between the two gaps.Comment: 7 pages, 5 figure
Thermodynamic properties and shear viscosity over entropy density ratio of nuclear fireball in a quantum-molecular dynamics model
Thermodynamic and transport properties of nuclear fireball created in the
central region of heavy-ion collisions below 400 MeV/nucleon are investigated
within the isospin-dependent quantum molecular dynamic (IQMD) model. These
properties including the density, temperature, chemical potential, entropy
density () and shear viscosity (), are calculated by a generalized hot
Thomas Fermi formulism and a parameterized function, which was developed by
Danielewicz. As the collision goes on, a transient minimal
occurs in the largest compression stage. Besides, the
relationship of to temperature () in the freeze-out stage displays
a local minimum which is about 9-20 times around = 8-12 MeV, which
can be argued as indicative of a liquid gas phase transition. In addition, the
influences of nucleon-nucleon (NN) cross section () and symmetry
energy coefficient () are also discussed, and it is found that the
results are sensitive to but not to .Comment: 10 pages, 13 figures; Phys. Rev. C (in press) (x-axis of Fig.1 is
corrected
Pentaquarks in semileptonic decays
In terms of as the hidden charm
pentaquark states to consist of , we study the semileptonic
decays. In our
discussion, while the main contribution to is from
the non-perturbative process via the doubly charmful
transition, we propose that the
transitions are partly contribute to the decays, in which the required pair is formed
by the sea quarks, intrinsic charm, or both. We predict that for
, which are about two orders of magnitude smaller than the
observed decay of . We also explore the angular
correlations for the and pairs. Our results of
the decay branching ratios and angular asymmetries in , accessible to the ongoing experiments at the LHCb, can be
used to improve the understanding of the hidden charm pentaquark states.Comment: 11 pages, 3 figures, accepted by Annals of Physic
Magnetism out of disorder in a J=0 compound Ba2YIrO6
We systematically investigate the magnetic properties and local structure of
Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase
boundary or clusters of antisite disorder affect the magnetism observed in this
compound. We compare the magnetic properties and atomic imaging of (1) a
slow cooled crystal, (2) a crystal quenched from 900\degree C after growth, and
(3) a crystal grown using a faster cooling rate than the slow cooled one.
Atomic imaging by scanning transmission electron microscopy (STEM) shows that
quenching from 900oC introduces antiphase boundary to the crystals, and a
faster cooling rate during crystal growth leads to clusters of Y and Ir
antisite disorder. STEM study suggests the antiphase boundary region is Ir-rich
with a composition of Ba2YIrO6. The magnetic measurements show that Ba2YIrO6
crystals with clusters of antisite defects have a larger effective moment and a
larger saturation moment than the slow-cooled crystals. Quenched crystals with
Ir-rich antiphase boundary shows a slightly suppressed saturation moment than
the slow cooled crystals, and this seems to suggest that antiphase boundary is
detrimental to the moment formation. Our DFT calculations suggest magnetic
condensation is unlikely as the energy to be gained from superexchange is small
compared to the spin-orbit gap. However, once Y is replaced by Ir in the
antisite disordered region, the picture of local non-magnetic singlets breaks
down and magnetism can be induced. This is because of (a) enhanced interactions
due to increased overlap of orbitals between sites, and, (b) increased number
of orbitals mediating the interactions. Our work highlights the importance of
lattice defects in understanding the experimentally observed magnetism in
Ba2YIrO6 and other J=0 systems
Shear viscosity of hot nuclear matter by the mean free path method
The shear viscosity of hot nuclear matter is investigated by using the mean
free path method within the framework of IQMD model. Finite size nuclear
sources at different density and temperature are initialized based on the
Fermi-Dirac distribution. The results show that shear viscosity to entropy
density ratio decreases with the increase of temperature and tends toward a
constant value for , which is consistent with the previous
studies on nuclear matter formed during heavy-ion collisions. At
, a minimum of is seen at around MeV
and a maximum of the multiplicity of intermediate mass fragment
() is also observed at the same temperature which is an
indication of the liquid-gas phase transition.Comment: 5 figs and 5 pages; accepted by Physical Review
Measurement of Change of 7Be Decay Rate in Be and Au
We have measured the possible change of the decay rate of 7Be implanted into
hosts of natural beryllium and natural gold. No difference between the 7Be
decay rates in the two hosts is observed within the experimental precision of
0.12%. This result implies that change of the decay rate of 7Be implanted in
different materials cannot be simply expected from the electron affinity
difference consideration lonely and the lattice structure of the host materials
should be taken into account.Comment: 5 pages, 2 figure
Diophantine Approach to Blind Interference Alignment of Homogeneous K-user 2x1 MISO Broadcast Channels
Although the sufficient condition for a blindly interference-aligned (BIA)
2-user 2x1 broadcast channel (BC) in homogeneous fading to achieve its maximal
4/3 DoF is well understood, its counterpart for the general K-user 2x1 MISO BC
in homogeneous block fading to achieve the corresponding 2k/(2+K-1) (DoF)
remains unsolved and is, thus, the focus of this paper. An interference channel
is said BIA-feasible if it achieves its maximal DoF only via BIA. In this
paper, we cast this general feasibility problem in the framework of finding
integer solutions for a system of linear Diophantine equations. By assuming
independent user links each of the same coherence time and by studying the
solvability of the Diophantine system, we derive the sufficient and necessary
conditions on the K users' fading block offsets to ensure the BIA feasibility
of the K-user BC. If the K offsets are independent and uniformly distributed
over a coherence block, we can further prove that 11 users are enough for one
to find, with certainty of 95%, 3 users among them to form a BIA-feasible
3-user 2x1 BC.Comment: 16 page
Implement Blind Interference Alignment over Homogeneous 3-user 2x1 Broadcast Channel
This paper first studies the homogeneous 3-user 2x1 broadcast channel (BC)
with no CSIT. We show a sufficient condition for it to achieve the optimal 3/2
degrees of freedom (DoF) by using Blind Interference Alignment (BIA). BIA
refers to the interference alignment method without the need of CSIT. It
further studies the 2x1 broadcast network in which there are K>=3 homogeneous
single-antenna users, and their coherence time offsets are independently and
uniformly distributed. We show that, if K>=11, the two-antenna transmitter can
find, with more than 95% certainty, three users to form a BIA-feasible 3-user
BC and achieve the optimal 3/2 DoF.Comment: The proof of Theorem 3 is revised. arXiv admin note: text overlap
with arXiv:1209.313
Superconducting Resonance and paring symmetry in electron-doped cuprates
The magnetic excitations in the superconducting electron-doped cuprates are
studied in the framework of spin-density-wave description. The superconducting
resonance is a natural product of the superconductivity due to the opening of
d-wave gap. Its resonance energy exhibits well linear scaling with
superconducting gap as , quantitatively consisting
with the experimental discovery. This ratio is insensitive to the selected
parameters, manifesting its universality. Another lower-energy peak below
resonance energy is predicted when the hole pocket emerges due to suppression
of spin-density wave. We further verify that the ratio of linear scales is
intimately related to the pairing symmetry. Distinct ratio can be found with
respective pairing symmetry. In comparison with the inelastic neutron
scattering data, the monotonic d-wave superconductivity is the most likely
candidate in the electron-doped cuprates. Furthermore, we proposed a new method
to check the pairing symmetry by the inelastic neutron scattering measurements.Comment: 4 Pages, 4 Figure
Raman scattering, X-ray photoemission spectra and superconductivity of a tiny Ag diffusion to MgCNi3
The tiny Ag diffusion to MgCNi3 has been prepared by solid states reaction.
Its structure was characterized by X-ray diffractometer (XRD). The results show
that a small mount of Ag substitute for Ni sites, and much of Ag are in vacancy
sites of the MgCNi3. We have further studied the surface properties of
Ag-MgCNi3 using Raman scattering spectra, and X-ray photoemission spectra
(XPS). Raman spectroscopy shows that the Ag-MgCNi3 has a special Raman peak
around 842.1 cm-1 compared to that of C. The superconductor transition
temperature (about 6.6 K) of Ag-MgCNi3 was lower than that of pure MgCNi3. It
was interpreted properly by the conventional BCS phonon mechanism.Comment: 12 pages, 7 figure
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