1,966 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
Driving Topological Phases by Spatially Inhomogeneous Pairing Centers
We investigate the effect of periodic and disordered distributions of pairing
centers in a one-dimensional itinerant system to obtain the microscopic
conditions required to achieve an end Majorana mode and the topological phase
diagram. Remarkably, the topological invariant can be generally expressed in
terms of the physical parameters for any pairing center configuration. Such a
fundamental relation allows us to unveil hidden local symmetries and to
identify trajectories in the parameter space that preserve the non-trivial
topological character of the ground state. We identify the phase diagram with
topologically non-trivial domains where Majorana modes are completely
unaffected by the spatial distribution of the pairing centers. These results
are general and apply to several systems where inhomogeneous perturbations
generate stable Majorana modes.Comment: 9 pages, 5 figure
Adaptive spectral identification techniques in presence of undetected non linearities
The standard procedure for detection of gravitational wave coalescing
binaries signals is based on Wiener filtering with an appropriate bank of
template filters. This is the optimal procedure in the hypothesis of addictive
Gaussian and stationary noise. We study the possibility of improving the
detection efficiency with a class of adaptive spectral identification
techniques, analyzing their effect in presence of non stationarities and
undetected non linearities in the noiseComment: 4 pages, 2 figures, uses ws-procs9x6.cls Proceedings of "Non linear
physics: theory and experiment. II", Gallipoli (Lecce), 200
Spin-Orbital Order Modified by Orbital Dilution in Transition Metal Oxides: From Spin Defects to Frustrated Spins Polarizing Host Orbitals
We study the substitution in transition metal oxides in the cases
of doping at either or sites which realize orbital
dilution. We derive the effective (or ) superexchange in a Mott
insulator with different ionic valencies, underlining the emerging structure of
the spin-orbital coupling between the impurity and the host sites and
demonstrate that it is qualitatively different from that encountered in the
host itself. This derivation shows that the interaction between the host and
the impurity depends in a crucial way on the type of doubly occupied
orbital. One finds that in some cases, due to the quench of the orbital degree
of freedom at the impurity, the spin and orbital order within the host is
drastically modified by doping. The impurity acts either as a spin defect
accompanied by an orbital vacancy in the spin-orbital structure when the
host-impurity coupling is weak, or it favors doubly occupied active orbitals
(orbital polarons) along the bond leading to antiferromagnetic or
ferromagnetic spin coupling. This competition between different magnetic
couplings leads to quite different ground states. We find that magnetic
frustration and spin degeneracy can be lifted by the quantum orbital flips of
the host but they are robust in special regions of the incommensurate phase
diagram. The spin-orbit coupling can lead to anisotropic spin and orbital
patterns along the symmetry directions and cause a radical modification of the
order imposed by the spin-orbital superexchange. Our findings are expected to
be of importance for future theoretical understanding of experimental results
for doped transition metal oxides doped with ions. We suggest how
the local or global changes of the spin-orbital order induced by such
impurities could be detected experimentally.Comment: 27 pages, 16 figures, submitte
Novel Spin-Orbital Phases Induced by Orbital Dilution
We demonstrate that magnetic impurities with spins and no
orbital degree of freedom induce changes of spin-orbital order in a Mott
insulator with spins. Impurities act either as spin defects which
decouple from the surrounding ions, or trigger orbital polarons along -
bonds. The - superexchange in the host competes with
- superexchange --- it depends on which orbital is doubly
occupied. The spin-orbital order within the host is totally modified at doping
. Our findings provide new perspective for future theoretical and
experimental studies of doped transition-metal oxides.Comment: 4 pages, 3 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.
Topological Phases emerging from Spin-Orbital Physics
We study the evolution of spin-orbital correlations in an inhomogeneous
quantum system with an impurity replacing a doublon by a holon orbital degree
of freedom. Spin-orbital entanglement is large when spin correlations are
antiferromagnetic, while for a ferromagnetic host we obtain a pure orbital
description. In this regime the orbital model can be mapped on spinless
fermions and we uncover topological phases with zero energy modes at the edge
or at the domain between magnetically inequivalent regions.Comment: 6 pages, 5 figures, submitte
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
Doping dependence of magnetic excitations of 1D cuprates as probed by Resonant Inelastic x-ray Scattering
We study the dynamical, momentum dependent two- and four-spin response
functions in doped and undoped 1D cuprates, as probed by resonant inelastic
x-ray scattering, using an exact numerical diagonalization procedure. In the
undoped system the four-spin response vanishes at , whereas the
two-spin correlator is peaked around , with generally larger spectral
weight. Upon doping spectra tend to soften and broaden, with a transfer of
spectral weight towards higher energy. However, the total spectral weight and
average peak position of either response are only weakly affected by doping up
to a concentration of 1/8. Only the two-spin response at changes
strongly, with a large reduction of spectral weight and enhancement of
excitation energy. At other momenta the higher-energy, generic features of the
magnetic response are robust against doping. It signals the presence of strong
short-range antiferromagnetic correlations, even after doping mobile holes into
the system. We expect this to hold also in higher dimensions.Comment: 7 pages, 5 figure
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
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