23,439 research outputs found
Nematic phase in the J-J square lattice Ising model in an external field
The J-J Ising model in the square lattice in the presence of an
external field is studied by two approaches: the Cluster Variation Method (CVM)
and Monte Carlo simulations. The use of the CVM in the square approximation
leads to the presence of a new equilibrium phase, not previously reported for
this model: an Ising-nematic phase, which shows orientational order but not
positional order, between the known stripes and disordered phases. Suitable
order parameters are defined and the phase diagram of the model is obtained.
Monte Carlo simulations are in qualitative agreement with the CVM results,
giving support to the presence of the new Ising-nematic phase. Phase diagrams
in the temperature-external field plane are obtained for selected values of the
parameter which measures the relative strength of the
competing interactions. From the CVM in the square approximation we obtain a
line of second order transitions between the disordered and nematic phases,
while the nematic-stripes phase transitions are found to be of first order. The
Monte Carlo results suggest a line of second order nematic-disordered phase
transitions in agreement with the CVM results. Regarding the stripes-nematic
transitions, the present Monte Carlo results are not precise enough to reach
definite conclusions about the nature of the transitions.Comment: 13 pages, 10 figure
Waiting time distribution for electron transport in a molecular junction with electron-vibration interaction
On the elementary level, electronic current consists of individual electron
tunnelling events that are separated by random time intervals. The waiting time
distribution is a probability to observe the electron transfer in the detector
electrode at time given that an electron was detected in the same
electrode at earlier time . We study waiting time distribution for quantum
transport in a vibrating molecular junction. By treating the electron-vibration
interaction exactly and molecule-electrode coupling perturbatively, we obtain
master equation and compute the distribution of waiting times for electron
transport. The details of waiting time distributions are used to elucidate
microscopic mechanism of electron transport and the role of electron-vibration
interactions. We find that as nonequilibrium develops in molecular junction,
the skewness and dispersion of the waiting time distribution experience
stepwise drops with the increase of the electric current. These steps are
associated with the excitations of vibrational states by tunnelling electrons.
In the strong electron-vibration coupling regime, the dispersion decrease
dominates over all other changes in the waiting time distribution as the
molecular junction departs far away from the equilibrium
Quasidegeneracy of Majorana Neutrinos and the Origin of Large Leptonic Mixing
We propose that the observed large leptonic mixing may just reflect a
quasidegeneracy of three Majorana neutrinos. The limit of exact degeneracy of
Majorana neutrinos is not trivial, as leptonic mixing and even CP violation may
occur. We conjecture that the smallness of , when compared to the
other elements of , may just reflect the fact that, in the limit of
exact mass degeneracy, the leptonic mixing matrix necessarily has a vanishing
element. We show that the lifting of the mass degeneracy can lead to the
measured value of while at the same time accommodating the observed
solar and atmospheric mixing angles. In the scenario we consider for the
breaking of the mass degeneracy there is only one CP violating phase, already
present in the limit of exact degeneracy, which upon the lifting of the
degeneracy generates both Majorana and Dirac-type CP violation in the leptonic
sector. We analyse some of the correlations among physical observables and
point out that in most of the cases considered, the implied strength of
leptonic Dirac-type CP violation is large enough to be detected in the next
round of experiments.Comment: 16 pages, 4 figures. Matches published version, references added,
improved discussion, results unchange
Evidence of Strong-Coupled Superconductivity in CaC6 from Tunneling Spectroscopy
Point-contact tunneling on CaC crystals reproducibly reveals
superconducting gaps, , of 2.30.2 meV which are ~40% larger
than earlier reports. That puts CaC into the class of very strong-coupled
superconductors since 2/kT~4.6. Thus soft Ca phonons will be
primarily involved in the superconductivity, a conclusion that explains the
large Ca isotope effect found recently for CaC. Consistency among
superconductor-insulator-normal metal (SIN), SIS and Andreev reflection (SN)
junctions reinforces the intrinsic nature of this result.Comment: 2nd version, 4 pages, 4 figures, re-submitted to Physical Review
Letter
Estimating quantum chromatic numbers
We develop further the new versions of quantum chromatic numbers of graphs
introduced by the first and fourth authors. We prove that the problem of
computation of the commuting quantum chromatic number of a graph is solvable by
an SDP algorithm and describe an hierarchy of variants of the commuting quantum
chromatic number which converge to it. We introduce the tracial rank of a
graph, a parameter that gives a lower bound for the commuting quantum chromatic
number and parallels the projective rank, and prove that it is multiplicative.
We describe the tracial rank, the projective rank and the fractional chromatic
numbers in a unified manner that clarifies their connection with the commuting
quantum chromatic number, the quantum chromatic number and the classical
chromatic number, respectively. Finally, we present a new SDP algorithm that
yields a parameter larger than the Lov\'asz number and is yet a lower bound for
the tracial rank of the graph. We determine the precise value of the tracial
rank of an odd cycle.Comment: 34 pages; v2 has improved presentation based after referees'
comments, published versio
Spin susceptibilities, spin densities and their connection to spin-currents
We calculate the frequency dependent spin susceptibilities for a
two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit
interaction. The resonances of the susceptibilities depends on the relative
values of the Rashba and Dresselhaus spin-orbit constants, which could be
manipulated by gate voltages. We derive exact continuity equations, with source
terms, for the spin density and use those to connect the spin current to the
spin density. In the free electron model the susceptibilities play a central
role in the spin dynamics since both the spin density and the spin current are
proportional to them.Comment: 6 pages, revtex4 styl
Disentangling Morphology, Star Formation, Stellar Mass, and Environment in Galaxy Evolution
We present a study of the spectroscopic and photometric properties of
galaxies in six nearby clusters. We perform a partial correlation analysis on
our dataset to investigate whether the correlation between star formation rates
in galaxies and their environment is merely another aspect of correlations of
morphology, stellar mass, or mean stellar age with environment, or whether star
formation rates vary independently of these other correlations. We find a
residual correlation of ongoing star formation with environment, indicating
that even galaxies with similar morphologies, stellar masses, and mean stellar
ages have lower star formation rates in denser environments. Thus, the current
star formation gradient in clusters is not just another aspect of the
morphology-density, stellar mass-density, or mean stellar age-density
relations. Furthermore, the star formation gradient cannot be solely the result
of initial conditions, but must partly be due to subsequent evolution through a
mechanism (or mechanisms) sensitive to environment. Our results constitute a
true ``smoking gun'' pointing to the effect of environment on the later
evolution of galaxies.Comment: 31 pages, including 5 figures; accepted for publication in Ap
Induced superfluidity of imbalanced Fermi gases near unitarity
The induced intraspecies interactions among the majority species, mediated by
the minority species, is computed for a population-imbalanced two-component
Fermi gas. Although the Feshbach-resonance mediated interspecies interaction is
dominant for equal populations, leading to singlet s-wave pairing, we find that
in the strongly imbalanced regime the induced intraspecies interaction leads to
p-wave pairing and superfluidity of the majority species. Thus, we predict that
the observed spin-polaron Fermi liquid state in this regime is unstable to
p-wave superfluidity, in accordance with the results of Kohn and Luttinger,
below a temperature that, near unitarity, we find to be within current
experimental capabilities. Possible experimental signatures of the p-wave state
using radio-frequency spectroscopy as well as density-density correlations
after free expansion are presented.Comment: 15 pages, 13 figures, submitted to Phys. Rev.
Galaxy Luminosity Functions from Deep Spectroscopic Samples of Rich Clusters
Using a new spectroscopic sample and methods accounting for spectroscopic
sampling fractions that vary in magnitude and surface brightness, we present
R-band galaxy luminosity functions (GLFs) for six nearby galaxy clusters with
redshifts 4000 < cz < 20000 km/s and velocity dispersions 700 < sigma < 1250
km/s. In the case of the nearest cluster, Abell 1060, our sample extends to
M_R=-14 (7 magnitudes below M*), making this the deepest spectroscopic
determination of the cluster GLF to date. Our methods also yield composite GLFs
for cluster and field galaxies to M_R=-17 (M*+4), including the GLFs of
subsamples of star forming and quiescent galaxies. The composite GLFs are
consistent with Schechter functions (M*_R=-21.14^{+0.17}_{-0.17},
alpha=-1.21^{+0.08}_{-0.07} for the clusters, M*_R=-21.15^{+0.16}_{-0.16},
alpha=-1.28^{+0.12}_{-0.11} for the field). All six cluster samples are
individually consistent with the composite GLF down to their respective
absolute magnitude limits, but the GLF of the quiescent population in clusters
is not universal. There are also significant variations in the GLF of quiescent
galaxies between the field and clusters that can be described as a steepening
of the faint end slope. The overall GLF in clusters is consistent with that of
field galaxies, except for the most luminous tip, which is enhanced in clusters
versus the field. The star formation properties of giant galaxies are more
strongly correlated with the environment than those of fainter galaxies.Comment: 53 pages, 8 figures, 1 ASCII table; accepted for publication in Ap
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