108 research outputs found
Diffusion doping of cobalt in rod-shape anatase TiO\u3csub\u3e2\u3c/sub\u3e nanocrystals leads to antiferromagnetism†
Cobalt(II) ions were adsorbed to the surface of rod-shape anatase TiO2 nanocrystals and subsequently heated to promote ion diffusion into the nanocrystal. After removal of any remaining surface bound cobalt, a sample consisting of strictly cobalt-doped TiO2 was obtained and characterized with powder Xray diffraction, transmission electron microscopy, UV-visible spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectroscopy, SQUID magnetometry, and inductively-coupled plasma atomic emission spectroscopy. The nanocrystal morphology was unchanged in the process and no new crystal phases were detected. The concentration of cobalt in the doped samples linearly correlates with the initial loading of cobalt(II) ions on the nanocrystal surface. Thin films of the cobalt doped TiO2 nanocrystals were prepared on indium-tin oxide coated glass substrate, and the electrical conductivity increased with the concentration of doped cobalt. Magnetic measurements of the cobalt-doped TiO2 nanocrystals reveal paramagnetic behavior at room temperature, and antiferromagnetic interactions between Co ions at low temperatures. Antiferromagnetism is atypical for cobalt-doped TiO2 nanocrystals, and is proposed to arise from interstitial doping that may be favored by the diffusional doping mechanism
Random Mass Dirac Fermions in Doped Spin-Peierls and Spin-Ladder systems: One-Particle Properties and Boundary Effects
Quasi-one-dimensional spin-Peierls and spin-ladder systems are characterized
by a gap in the spin-excitation spectrum, which can be modeled at low energies
by that of Dirac fermions with a mass. In the presence of disorder these
systems can still be described by a Dirac fermion model, but with a random
mass. Some peculiar properties, like the Dyson singularity in the density of
states, are well known and attributed to creation of low-energy states due to
the disorder. We take one step further and study single-particle correlations
by means of Berezinskii's diagram technique. We find that, at low energy
, the single-particle Green function decays in real space like
. It follows that at these energies the
correlations in the disordered system are strong -- even stronger than in the
pure system without the gap. Additionally, we study the effects of boundaries
on the local density of states. We find that the latter is logarithmically (in
the energy) enhanced close to the boundary. This enhancement decays into the
bulk as and the density of states saturates to its bulk value on
the scale . This scale is different from
the Thouless localization length . We
also discuss some implications of these results for the spin systems and their
relation to the investigations based on real-space renormalization group
approach.Comment: 26 pages, LaTex, 9 PS figures include
AdS Vacua, Attractor Mechanism and Generalized Geometries
We consider flux vacua attractor equations in type IIA string theory
compactified on generalized geometries with orientifold projections. The
four-dimensional N=1 superpotential in this compactification can be written as
the sum of the Ramond-Ramond superpotential and a term described by
(non)geometric flux charges. We exhibit a simple model in which supersymmetric
AdS and Minkowski solutions are classified by means of discriminants of the two
superpotentials. We further study various configurations without Ramond-Ramond
flux charges. In this case we find supersymmetric AdS vacua both in the case of
compactifications on generalized geometries with SU(3) x SU(3) structures and
on manifolds with an SU(3)-structure without nongeometric flux charges. In the
latter case, we have to introduce correction terms into the prepotential in
order to realize consistent vacua.Comment: 35 pages, accepted version in JHE
Type II compactifications on manifolds with SU(2) x SU(2) structure
We study compactifications of type II theories on SU(2) x SU(2) structure
manifolds to six, five and four spacetime dimensions. We use the framework of
generalized geometry to describe the NS-NS sector of such compactifications and
derive the structure of their moduli spaces. We show that in contrast to SU(3)
x SU(3) structure compactifications, there is no dynamical SU(2) x SU(2)
structure interpolating between an SU(2) structure and an identity structure.
Furthermore, we formulate type II compactifications on SU(2) x SU(2) structures
in the context of exceptional generalized geometry which makes the U-duality
group manifest and naturally incorporates the scalar degrees of freedom arising
in the Ramond-Ramond sector. Via this formalism we derive the structure of the
moduli spaces as it is expected from N=4 supergravity.Comment: 69 pages, v2 published versio
Elementary Excitations in Dimerized and Frustrated Heisenberg Chains
We present a detailed numerical analysis of the low energy excitation
spectrum of a frustrated and dimerized spin Heisenberg chain. In
particular, we show that in the commensurate spin--Peierls phase the ratio of
the singlet and triplet excitation gap is a universal function which depends on
the frustration parameter only. We identify the conditions for which a second
elementary triplet branch in the excitation spectrum splits from the continuum.
We compare our results with predictions from the continuum limit field theory .
We discuss the relevance of our data in connection with recent experiments on
, , and .Comment: Corrections to the text + 1 new figure, will appear in PRB (august
98
Low Energy Supersymmetry from Non-Geometry
We study a class of flux compactifications that have all the moduli
stabilised, a high (GUT) string scale and a low (TeV) gravitino mass that is
generated dynamically. These non-geometric compactifications correspond to type
II string theories on SU(3)xSU(3) structure orientifolds. The resulting
superpotentials admit, excluding non-perturbative effects, supersymmetric
Minkowski vacua with any number of moduli stabilised. We argue that
non-perturbative effects are present and introduce terms in the superpotential
that are exponentially suppressed by the same moduli that appear
perturbatively. These deform the supersymmetric Minkowski vacua to
supersymmetric AdS vacua with an exponentially small gravitino mass. The
resulting vacua allow for low scale supersymmetry breaking which can be
realised by a number of mechanisms.Comment: 36pp; v2 references added, minor clarifications, JHEP versio
E7(7) formulation of N=2 backgrounds
In this paper we reformulate N=2 supergravity backgrounds arising in type II
string theory in terms of quantities transforming under the U-duality group
E7(7). In particular we combine the Ramond--Ramond scalar degrees of freedom
together with the O(6,6) pure spinors which govern the Neveu-Schwarz sector by
considering an extended version of generalised geometry. We give
E7(7)-invariant expressions for the Kahler and hyperkahler potentials
describing the moduli space of vector and hypermultiplets, demonstrating that
both correspond to standard E7(7) coset spaces. We also find E7(7) expressions
for the Killing prepotentials defining the scalar potential, and discuss the
equations governing N=1 vacua in this formalism.Comment: 40 pages, final version to appear in JHE
Haldane-Gapped Spin Chains as Luttinger Liquids: Correlation Functions at Finite Field
We study the behavior of Heisenberg, antiferromagnetic, integer-spin chains
in the presence of a magnetic field exceeding the attendant spin gap. For
temperatures much smaller than the gap, the spin chains exhibit Luttinger
liquid behavior. We compute exactly both the corresponding Luttinger parameter
and the Fermi velocity as a function of magnetic field. This enables the
computation of a number of correlators from which we derive the spin
conductance, the expected form of the dynamic structure factor relevant to
inelastic neutron scattering experiments, and NMR relaxation rates. We also
comment upon the robustness of the magnetically induced gapless phase both to
finite temperature and finite couplings between neighbouring chains.Comment: 32 pages, 8 figures; published version includes additions discussing
the robustness of the magnetically induced gapless phase to ordering between
chains as well as the relationship between the spin-1 chains and spin-1/2
ladders in the presence of a magnetic fiel
T-duality, Generalized Geometry and Non-Geometric Backgrounds
We discuss the action of O(d,d), and in particular T-duality, in the context
of generalized geometry, focusing on the description of so-called non-geometric
backgrounds. We derive local expressions for the pure spinors descibing the
generalized geometry dual to an SU(3) structure background, and show that the
equations for N=1 vacua are invariant under T-duality. We also propose a local
generalized geometrical definition of the charges f, H, Q and R appearing in
effective four-dimensional theories, using the Courant bracket. We then address
certain global aspects, in particular whether the local non-geometric charges
can be gauged away in, for instance, backgrounds admitting a torus action, as
well as the structure of generalized parallelizable backgrounds.Comment: 33 page
Seeing through the String Landscape - a String Hunter's Companion in Particle Physics and Cosmology
In this article we will overview several aspects of the string landscape,
namely intersecting D-brane models and their statistics, possible model
independent LHC signatures of intersecting brane models, flux compactification,
moduli stabilization in type II compactifications, domain wall solutions and
brane inflation.Comment: 94 pages, Review paper invited and accepted for publication by JHEP,
revised version contains several new references and other minor modification
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