1,690 research outputs found
Magnetic and orbital ordering in cuprates and manganites
The mechanisms of magnetic and orbital interactions due to double exchange
(DE) and superexchange (SE) in transition metal oxides with degenerate e_g
orbitals are presented. Specifically, we study the effective spin-orbital
models derived for the d^9 ions as in KCuF_3, and for the d^4 ions as in
LaMnO_3, for spins S=1/2 and S=2, respectively. Such models are characterized
by three types of elementary excitations: spin waves, orbital waves, and
spin-and-orbital waves. The SE interactions between Cu^{2+} (d^9) ions are
inherently frustrated, which leads to a new mechanism of spin liquid which
operates in three dimensions. The SE between Mn^{3+} (d^4) ions explains the
A-type antiferromagnetic order in LaMnO_3 which coexists with the orbital
order. In contrast, the ferromagnetic metallic phase and isotropic spin waves
observed in doped manganites are explained by DE for degenerate e_g orbitals.
It is shown that although a hole does not couple to spin excitations in
ferromagnetic planes of LaMnO_3, the orbital excitations change the energy
scale for the coherent hole propagation and cause a large redistribution of
spectral weight. Finally, we point out some open problems in the present
understanding of doped manganites.Comment: 155 pages, 66 figure
The boson-fermion model: An exact diagonalization study
The main features of a generic boson-fermion scenario for electron pairing in
a many-body correlated fermionic system are: i) a cross-over from a poor metal
to an insulator and finally a superconductor as the temperature decreases, ii)
the build-up of a finite amplitude of local electron pairing below a certain
temperature , followed by the onset of long-range phase correlations among
electron pairs below a second characteristic temperature , iii) the
opening of a pseudogap in the DOS of the electrons below , rendering these
electrons poorer and poorer quasi-particles as the temperature decreases, with
the electron transport becoming ensured by electron pairs rather than by
individual electrons. A number of these features have been so far obtained on
the basis of different many-body techniques, all of which have their built-in
shortcomings in the intermediate coupling regime, which is of interest here. In
order to substantiate these features, we investigate them on the basis of an
exact diagonalization study on rings up to eight sites. Particular emphasis has
been put on the possibility of having persistent currents in mesoscopic rings
tracking the change-over from single- to two-particle transport as the
temperature decreases and the superconducting state is approached.Comment: 7 pages, 8 figures; to be published in Phys. Rev.
Radio constraints on dark matter annihilation in the galactic halo and its substructures
Annihilation of Dark Matter usually produces together with gamma rays
comparable amounts of electrons and positrons. The e+e- gyrating in the
galactic magnetic field then produce secondary synchrotron radiation which thus
provides an indirect mean to constrain the DM signal itself. To this purpose,
we calculate the radio emission from the galactic halo as well as from its
expected substructures and we then compare it with the measured diffuse radio
background. We employ a multi-frequency approach using data in the relevant
frequency range 100 MHz-100 GHz, as well as the WMAP Haze data at 23 GHz. The
derived constraints are of the order =10^{-24} cm3 s^{-1} for a DM
mass m_chi=100 GeV sensibly depending however on the astrophysical
uncertainties, in particular on the assumption on the galactic magnetic field
model. The signal from single bright clumps is instead largely attenuated by
diffusion effects and offers only poor detection perspectives.Comment: 12 pages, 7 figures; v2: some references added, some discussions
enlarged; matches journal versio
Control of magnetism in singlet-triplet superconducting heterostructures
We analyze the magnetization at the interface between singlet and triplet
superconductors and show that its direction and dependence on the phase
difference across the junction are strongly tied to the structure of the
triplet order parameter as well as to the pairing interactions. We consider
equal spin helical, opposite spin chiral, and mixed symmetry pairing on the
triplet side and show that the magnetization vanishes at only in the
first case, follows approximately a behavior for the second, and
shows higher harmonics for the last configuration. We trace the origin of the
magnetization to the magnetic structure of the Andreev bound states near the
interface, and provide a symmetry-based explanation of the results. Our
findings can be used to control the magnetization in superconducting
heterostructures and to test symmetries of spin-triplet superconductors.Comment: 5 pages, 3 figure
Clustering properties of ultrahigh energy cosmic rays and the search for their astrophysical sources
The arrival directions of ultrahigh energy cosmic rays (UHECRs) may show
anisotropies on all scales, from just above the experimental angular resolution
up to medium scales and dipole anisotropies. We find that a global comparison
of the two-point auto-correlation function of the data with the one of
catalogues of potential sources is a powerful diagnostic tool. In particular,
this method is far less sensitive to unknown deflections in magnetic fields
than cross-correlation studies while keeping a strong discrimination power
among source candidates. We illustrate these advantages by considering ordinary
galaxies, gamma ray bursts and active galactic nuclei as possible sources.
Already the sparse publicly available data suggest that the sources of UHECRs
may be a strongly clustered sub-sample of galaxies or of active galactic
nuclei. We present forecasts for various cases of source distributions which
can be checked soon by the Pierre Auger Observatory.Comment: 11 pages, 8 figures, 4 tables; minor changes, matches published
versio
On line power spectra identification and whitening for the noise in interferometric gravitational wave detectors
In this paper we address both to the problem of identifying the noise Power
Spectral Density of interferometric detectors by parametric techniques and to
the problem of the whitening procedure of the sequence of data. We will
concentrate the study on a Power Spectral Density like the one of the
Italian-French detector VIRGO and we show that with a reasonable finite number
of parameters we succeed in modeling a spectrum like the theoretical one of
VIRGO, reproducing all its features. We propose also the use of adaptive
techniques to identify and to whiten on line the data of interferometric
detectors. We analyze the behavior of the adaptive techniques in the field of
stochastic gradient and in the
Least Squares ones.Comment: 28 pages, 21 figures, uses iopart.cls accepted for pubblication on
Classical and Quantum Gravit
Steps towards a map of the nearby universe
We present a new analysis of the Sloan Digital Sky Survey data aimed at
producing a detailed map of the nearby (z < 0.5) universe. Using neural
networks trained on the available spectroscopic base of knowledge we derived
distance estimates for about 30 million galaxies distributed over ca. 8,000 sq.
deg. We also used unsupervised clustering tools developed in the framework of
the VO-Tech project, to investigate the possibility to understand the nature of
each object present in the field and, in particular, to produce a list of
candidate AGNs and QSOs.Comment: 3 pages, 1 figure. To appear in Nucl Phys. B, in the proceedings of
the NOW-2006 (Neutrino Oscillation Workshop - 2006), R. Fogli et al. ed
Sensitivity of a VIRGO pair to stochastic GW backgrounds
The sensitivity of a pair of VIRGO interferometers to gravitational waves
backgrounds (GW) of cosmological origin is analyzed for the cases of maximal
and minimal overlap of the two detectors. The improvements in the detectability
prospects of scale-invariant and non-scale-invariant logarithmic energy spectra
of relic GW are discussed.Comment: 25 pages in RevTex style with 6 figure
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