81,090 research outputs found
Chirality waves in two-dimensional magnets
We theoretically show that moderate interaction between electrons confined to
move in a plane and localized magnetic moments leads to formation of a
noncoplanar magnetic state. The state is similar to the skyrmion crystal
recently observed in cubic systems with the Dzyaloshinskii-Moriya interaction;
however, it does not require spin-orbit interaction. The non-coplanar magnetism
is accompanied by the ground-state electrical and spin currents, generated via
the real-space Berry phase mechanism. We examine the stability of the state
with respect to lattice discreteness effects and the magnitude of magnetic
exchange interaction. The state can be realized in a number of transition metal
and magnetic semiconductor systems
Coherent macroscopic quantum tunneling in boson-fermion mixtures
We show that the cold atom systems of simultaneously trapped Bose-Einstein
condensates (BEC's) and quantum degenerate fermionic atoms provide promising
laboratories for the study of macroscopic quantum tunneling. Our theoretical
studies reveal that the spatial extent of a small trapped BEC immersed in a
Fermi sea can tunnel and coherently oscillate between the values of the
separated and mixed configurations (the phases of the phase separation
transition of BEC-fermion systems). We evaluate the period, amplitude and
dissipation rate for Na and K-atoms and we discuss the
experimental prospects for observing this phenomenon.Comment: 4 pages, 3 figure
A note on the improvement ambiguity of the stress tensor and the critical limits of correlation functions
I study various properties of the critical limits of correlators containing
insertions of conserved and anomalous currents. In particular, I show that the
improvement term of the stress tensor can be fixed unambiguously, studying the
RG interpolation between the UV and IR limits. The removal of the improvement
ambiguity is encoded in a variational principle, which makes use of sum rules
for the trace anomalies a and a'. Compatible results follow from the analysis
of the RG equations. I perform a number of self-consistency checks and discuss
the issues in a large set of theories.Comment: 15 page
The WiggleZ Dark Energy Survey: Galaxy Evolution at 0.25 ≤ z ≤ 0.75 Using the Second Red-Sequence Cluster Survey
We study the evolution of galaxy populations around the spectroscopic WiggleZ sample of star-forming galaxies at 0.25 ≤ z ≤ 0.75 using the photometric catalog from the Second Red-Sequence Cluster Survey (RCS2). We probe the optical photometric properties of the net excess neighbor galaxies. The key concept is that the marker galaxies and their neighbors are located at the same redshift, providing a sample of galaxies representing a complete census of galaxies in the neighborhood of star-forming galaxies. The results are compared with those using the RCS WiggleZ Spare-Fibre (RCS-WSF) sample as markers, representing galaxies in cluster environments at 0.25 ≤ z ≤ 0.45. By analyzing the stacked color-color properties of the WiggleZ neighbor galaxies, we find that their optical colors are not a strong function of indicators of star-forming activities such as EW([O II]) or Galaxy Evolution Explorer (GALEX) near-UV luminosity of the markers. The galaxies around the WiggleZ markers exhibit a bimodal distribution on the color-magnitude diagram, with most of them located in the blue cloud. The optical galaxy luminosity functions (GLFs) of the blue neighbor galaxies have a faint-end slope α of ~ –1.3, similar to that for galaxies in cluster environments drawn from the RCS-WSF sample. The faint-end slope of the GLF for the red neighbors, however, is ~ –0.4, significantly shallower than the ~ –0.7 found for those in cluster environments. This suggests that the buildup of the faint end of the red sequence in cluster environments is in a significantly more advanced stage than that in the star-forming and lower galaxy density WiggleZ neighborhoods. We find that the red galaxy fraction (f_red) around the star-forming WiggleZ galaxies has similar values from z ~ 0.3 to z ~ 0.6 with f_red ~ 0.28, but drops to f_red ~ 0.20 at z gsim 0.7. This change of f_red with redshift suggests that there is either a higher rate of star-forming galaxies entering the luminosity-limited sample at z ≳ 0.7, or a decrease in the quenching rate of star formation at that redshift. Comparing to that in a dense cluster environment, the f_red of the WiggleZ neighbors is both considerably smaller and has a more moderate change with redshift, pointing to the stronger and more prevalent environmental influences on galaxy evolution in high-density regions
Efficiency of thin film photocells
We propose a new concept for the design of high-efficiency photocells based
on ultra-thin (submicron) semiconductor films of controlled thickness. Using a
microscopic model of a thin dielectric layer interacting with incident
electromagnetic radiation we evaluate the efficiency of conversion of solar
radiation into the electric power. We determine the optimal range of parameters
which maximize the efficiency of such photovoltaic element.Comment: 5 pages, 3 figure
Supersoft Supersymmetry is Super-Safe
We show that supersymmetric models with a large Dirac gluino mass can evade
much of the jets plus missing energy searches at LHC. Dirac gaugino masses
arise from "supersoft" operators that lead to finite one-loop suppressed
contributions to the scalar masses. A little hierarchy between the Dirac gluino
mass 5 - 10 times heavier than the squark masses is automatic and technically
natural, in stark contrast to supersymmetric models with Majorana gaugino
masses. At the LHC, colored sparticle production is suppressed not only by the
absence of gluino pair (or associated) production, but also because several of
the largest squark pair production channels are suppressed or absent. We recast
the null results from the present jets plus missing energy searches at LHC for
supersymmetry onto a supersoft supersymmetric simplified model (SSSM). Assuming
a massless LSP, we find the strongest bounds are: 748 GeV from a 2j + MET
search at ATLAS (4.7 fb^{-1}), and 684 GeV from a combined jets plus missing
energy search using at CMS (1.1 fb^{-1}). In the absence of a future
observation, we estimate the bounds on the squark masses to improve only
modestly with increased luminosity. We also briefly consider the further
weakening in the bounds as the LSP mass is increased.Comment: 13 pages, 8 figure
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