11,214 research outputs found
Fractional charges in pyrochlore lattices
A pyrochlore lattice is considered where the average electron number of
electrons per site is half--integer, concentrating on the case of exactly half
an electron per site. Strong on-site repulsions are assumed, so that all sites
are either empty or singly occupied. Where there are in addition strong
nearest--neighbour repulsions, a tetrahedron rule comes into effect, as
previously suggested for magnetite. We show that in this case, there exist
excitations with fractional charge (+/-) e/2. These are intimately connected
with the high degeneracy of the ground state in the absence of kinetic energy
terms. When an additional electron is inserted into the system, it decays into
two point like excitations with charge -e/2, connected by a Heisenberg spin
chain which carries the electron's spin.Comment: 10 pages, 4 eps figures. To appear in Decemeber issue of Annalen der
Physi
An Exploratory Study of Field Failures
Field failures, that is, failures caused by faults that escape the testing
phase leading to failures in the field, are unavoidable. Improving verification
and validation activities before deployment can identify and timely remove many
but not all faults, and users may still experience a number of annoying
problems while using their software systems. This paper investigates the nature
of field failures, to understand to what extent further improving in-house
verification and validation activities can reduce the number of failures in the
field, and frames the need of new approaches that operate in the field. We
report the results of the analysis of the bug reports of five applications
belonging to three different ecosystems, propose a taxonomy of field failures,
and discuss the reasons why failures belonging to the identified classes cannot
be detected at design time but shall be addressed at runtime. We observe that
many faults (70%) are intrinsically hard to detect at design-time
Magnetic loop behind an interplanetary shock: Voyager, Helios and IMP-8 observations
The shock was followed by a turbulent sheath in which there were large fluctuations in both the strength and direction of the magnetic field. This in turn was followed by a region (magnetic cloud) in which the magnetic field vectors were observed to change by rotating nearly parallel to a plane, consistent with the passage of a magnetic loop. This loop extended at least 30 deg in longitude between 1-2 AU, and its radial dimension was approximately 0.5 AU. In the cloud the field strength was high and the density and temperature were relatively low. Thus, the dominant pressure in the cloud was that of the magnetic field
The kinetics of the reaction of majorite plus ferropericlase to ringwoodite: Implications for mantle upwellings crossing the 660 km discontinuity
AbstractWe have measured the kinetics of reaction between MgO and majoritic garnet at 20 GPa and 1773–2123 K as a proxy for the reaction between perovskite and ferropericlase during mantle upwelling across the 660 km seismic discontinuity. Ringwoodite forms a layer between MgO and garnet and, in the case of aluminous garnets the interface between ringwoodite and garnet develops a fingering instability resulting in a complex intergrowth at this interface. By contrast, the MgO–ringwoodite interface is always planar for an initial planar MgO–garnet interface. Two thicknesses are therefore defined; (1) a layer thickness, X1, which is the maximum thickness of ringwoodite which forms a plane-parallel bounded layer next to the MgO, and (2) an interface thickness, X2, which is the maximum extent of the intergrowth region away from the ringwoodite layer. The growth of both of these regions can be described by apparent rate constants, ki, which are Arrhenius with ln(k10)=−6.36±0.25 m2/s and E1=456±40 kJ/mol for the ringwoodite layer, and ln(k20)=−9.2±3.3 m2/s and E2=371±53 kJ/mol for the intergrowth region. The fingering instability is caused by the incompatibility of aluminium in ringwoodite and its low chemical diffusivity in garnet which results in an increase of surface area at the ringwoodite–garnet interface to minimise the aluminium concentration at the interface. The intergrowth region contains a fine-grained mixture of ringwoodite and garnet which coarsens very slowly with time. This might result in a transient weakening of upwelling regions of mantle just above the 660 km seismic discontinuity allowing some viscous decoupling between the upper and lower mantle
Parametric resonance and spin-charge separation in 1D fermionic systems
We show that the periodic modulation of the Hamiltonian parameters for 1D
correlated fermionic systems can be used to parametrically amplify their
bosonic collective modes. Treating the problem within the Luttinger liquid
picture, we show how charge and spin density waves with different momenta are
simultaneously amplified. We discuss the implementation of our predictions for
cold atoms in 1D modulated optical lattices, showing that the fermionic
momentum distribution directly provides a clear signature of spin-charge
separation.Comment: 6 pages, 3 figures, published versio
Signatures of spin in the n=1/3 Fractional Quantum Hall Effect
The activation gap Delta of the fractional quantum Hall state at constant
filling n =1/3 is measured in wide range of perpendicular magnetic field B.
Despite the full spin polarization of the incompressible ground state, we
observe a sharp crossover between a low-field linear dependence of Delta on B
associated to spin texture excitations and a Coulomb-like behavior at large B.
From the global gap-reduction we get information about the mobility edges in
the fractional quantum Hall regime.Comment: 4 pages, 3 figure
Direct Measurement of the g-Factor of Composite Fermions
The activation gap of the fractional quantum Hall states at constant
fillings and 2/5 has been measured as a function of the
perpendicular magnetic field . A linear dependence of on is
observed while approaching the spin polarization transition. This feature
allows a direct measurement of the -factor of composite fermions which
appears to be heavily renormalized by interactions and strongly sensitive to
the electronic filling factor.Comment: 4 pages, 4 figures Changed content: Fokus more on g-factors (and less
on other details
Mild cognitive impairment: a systematic review
MCI is a nosological entity proposed as an intermediate state between normal aging and dementia. The syndrome can be divided into two broad subtypes: amnestic MCI ( aMCI) characterized by reduced memory, and non- amnestic MCI ( naMCI) in which other cognitive functions rather than memory are mostly impaired. aMCI seems to represent an early stage of AD, while the outcomes of the naMCI subtypes appear more heterogeneous - including vascular dementia, frontotemporal dementia or dementia with Lewy bodies- but this aspect is still under debate. MCI in fact represents a condition with multiple sources of heterogeneity, including clinical presentation, etiology, and prognosis. To improve classification and prognosis, there is a need for more sensitive instruments specifically developed for MCI as well as for more reliable methods to determine its progression or improvement. Current clinical criteria for MCI should be updated to include restriction in complex ADL; also the diagnostic and prognostic role of behavioral symptoms and motor dysfunctions should be better defined. A multidisciplinary diagnostic approach including biological and neuroimaging techniques may probably represent the best option to predict the conversion from MCI to dementia. In this review we discuss the most recent aspects related to the epidemiological, clinical, neuropathological, neuroimaging, biochemical and therapeutic aspects of MCI, with specific attention to possible markers of conversion to dementia
- …