5,604 research outputs found
SU(N) Coherent States
We generalize Schwinger boson representation of SU(2) algebra to SU(N) and
define coherent states of SU(N) using bosonic harmonic
oscillator creation and annihilation operators. We give an explicit
construction of all (N-1) Casimirs of SU(N) in terms of these creation and
annihilation operators. The SU(N) coherent states belonging to any irreducible
representations of SU(N) are labelled by the eigenvalues of the Casimir
operators and are characterized by (N-1) complex orthonormal vectors describing
the SU(N) manifold. The coherent states provide a resolution of identity,
satisfy the continuity property, and possess a variety of group theoretic
properties.Comment: 25 pages, LaTex, no figure
Proposal for an Experiment to Test a Theory of High Temperature Superconductors
A theory for the phenomena observed in Copper-Oxide based high temperature
superconducting materials derives an elusive time-reversal and rotational
symmetry breaking order parameter for the observed pseudogap phase ending at a
quantum-critical point near the composition for the highest . An
experiment is proposed to observe such a symmetry breaking. It is shown that
Angle-resolved Photoemission yields a current density which is different for
left and right circularly polarized photons. The magnitude of the effect and
its momentum dependence is estimated. Barring the presence of domains of the
predicted phase an asymmetry of about 0.1 is predicted at low temperatures in
moderately underdoped samples.Comment: latex, 2 figure
Coherent States For SU(3)
We define coherent states for SU(3) using six bosonic creation and
annihilation operators. These coherent states are explicitly characterized by
six complex numbers with constraints. For the completely symmetric
representations (n,0) and (0,m), only three of the bosonic operators are
required. For mixed representations (n,m), all six operators are required. The
coherent states provide a resolution of identity, satisfy the continuity
property, and possess a variety of group theoretic properties. We introduce an
explicit parameterization of the group SU(3) and the corresponding integration
measure. Finally, we discuss the path integral formalism for a problem in which
the Hamiltonian is a function of SU(3) operators at each site.Comment: 18 pages, LaTeX, no figure
Phase Diagram of Half Doped Manganites
An analysis of the properties of half-doped manganites is presented. We build
up the phase diagram of the system combining a realistic calculation of the
electronic properties and a mean field treatment of the temperature effects.
The electronic structure of the manganites are described with a double exchange
model with cooperative Jahn-Teller phonons and antiferromagnetic coupling
between the core spins. At zero temperature a variety of electronic phases
as ferromagnetic (FM) charge ordered (CO) orbital ordered (OO), CE-CO-OO and FM
metallic, are obtained. By raising the temperature the CE-CO-OO phase becomes
paramagnetic (PM), but depending on the electron-phonon coupling and the
exchange coupling the transition can be direct or trough intermediate states: a
FM disorder metallic, a PM-CO-OO or a FM-CO-OO. We also discus the nature of
the high temperature PM phase in the regime of finite electron phonon coupling.
In this regime half of the oxygen octahedra surrounding the ions are
distorted. In the weak coupling regime the octahedra are slightly deformed and
only trap a small amount of electronic charge, rendering the system metallic
consequentially. However in the strong coupling regime the octahedra are
strongly distorted, the charge is fully localized in polarons and the system is
insulator.Comment: 10 pagses, 9 figures include
Probing black hole accretion in quasar pairs at high redshift
Models and observations suggest that luminous quasar activity is triggered by
mergers, so it should preferentially occur in the most massive primordial dark
matter haloes, where the frequency of mergers is expected to be the highest.
Since the importance of galaxy mergers increases with redshift, we identify the
high-redshift Universe as the ideal laboratory for studying dual AGN. Here we
present the X-ray properties of two systems of dual quasars at z=3.0-3.3
selected from the SDSS-DR6 at separations of 6-8 arcsec (43-65kpc) and observed
by Chandra for 65ks each. Both members of each pair are detected with good
photon statistics to allow us to constrain the column density, spectral slope
and intrinsic X-ray luminosity. We also include a recently discovered dual
quasar at z=5 (separation of 21 arcsec, 136kpc) for which XMM-Newton archival
data allow us to detect the two components separately. Using optical spectra we
derived bolometric luminosities, BH masses and Eddington ratios that were
compared to those of luminous SDSS quasars in the same redshift ranges. We find
that the brighter component of both pairs at z=3.0-3.3 has high luminosities
compared to the distribution of SDSS quasars at similar redshift, with J1622A
having an order magnitude higher luminosity than the median. This source lies
at the luminous end of the z~3.3 quasar luminosity function. While we cannot
conclusively state that the unusually high luminosities of our sources are
related to their having a close companion, for J1622A there is only a 3%
probability that it is by chance.Comment: MNRAS, in pres
Searching for tetraquarks on the lattice
We address the question whether the lightest scalar mesons sigma and kappa
are tetraquarks. We present a search for possible light tetraquark states with
J^PC=0^++ and I=0, 1/2, 3/2, 2 in the dynamical and the quenched lattice
simulations using tetraquark interpolators. In all the channels, we unavoidably
find lowest scattering states pi(k)pi(-k) or K(k)pi(-k) with back-to-back
momentum k=0,2*pi/L,.. . However, we find an additional light state in the I=0
and I=1/2 channels, which may be related to the observed resonances sigma and
kappa with a strong tetraquark component. In the exotic repulsive channels I=2
and I=3/2, where no resonance is observed, we find no light state in addition
to the scattering states.Comment: 3 pages, 1 figure, proceedings of Lepton-Photon 2009, Hambur
The Quantum Physics of Black Holes: Results from String Theory
We review recent progress in our understanding of the physics of black holes.
In particular, we discuss the ideas from string theory that explain the entropy
of black holes from a counting of microstates of the hole, and the related
derivation of unitary Hawking radiation from such holes.Comment: 49 pages, Latex, 4 figures, (Review article
Effect of attention control on sustained attention during induced anxiety
Anxiety has wide-reaching and complex effects on cognitive performance. Although it can intrude on cognition and interfere with performance, it can also facilitate information processing and behavioural responses. In a previous study, we showed that anxiety induced by threat of shock facilitates performance on the Sustained Attention to Response Task, a vigilance test, which probes response inhibition to infrequent nogo stimuli. The present study sought to identify factors that may have contributed to such improved performance, including on- and off-task thinking (assessed with thought probes) and individual differences in attention control, as measured with the Attention Control Scale. Replicating our prior finding, we showed that shock threat significantly reduced errors of commission on the nogo trials. However, we extended this finding in demonstrating that this effect was driven by subjects with low attention control. We therefore confirm that anxiety increases inhibitory control of prepotent responses-a mechanism which is adaptive under threat-and show that this effect is greater in those who rely more upon such prepotent responding, i.e., those with low attentional control
Precise time and frequency intercomparison between NPL, India and PTB, Federal Republic of Germany via satellite symphonie-1
A time and frequency intercomparison experiment conducted using Earth stations in New Delhi, India and Raisting, FRG is described. The NPL clock was placed at New Delhi Earth Station and the Raisting Clock was calibrated with PTB/Primary standard via LORAN-C and travelling clocks. The random uncertainity of time comparisons, represented by two sample Allan Variance sigma (30 seconds), was less than 10 nanoseconds. The relative frequency difference between the NPL and Raisting Clocks, SNPL, RAIS, as measured over the 44 days period was found to be -15.7 x 10 to the -13th power. The relative frequency difference between PTB Primary Standard and Raisting Clock, SPTB, RAIS, during this period, was measured to be -22.8 x 10 to the -13th power. The relative frequency difference between NPL clock and PTB Primary Standard, SNPL, PTB, thus, is +7.1 x 10 to the -13th power. The clock rate (UTC, India) of +7.1 + or - 0.5 x 10 to the -13th power, agrees well with that obtained via VLF phase measurements over one year period and with USNO travelling clock time comparisons made in September, 1980
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