781 research outputs found
The case for the development and use of "ecologically valid" measures of executive function in experimental and clinical neuropsychology
This article considers the scientific process whereby new and better clinical tests of executive function might be developed, and what form they might take. We argue that many of the traditional tests of executive function most commonly in use (e.g., the Wisconsin Card Sorting Test; Stroop) are adaptations of procedures that emerged almost coincidentally from conceptual and experimental frameworks far removed from those currently in favour, and that the prolongation of their use has been encouraged by a sustained period of concentration on “construct-driven” experimentation in neuropsychology. This resulted from the special theoretical demands made by the field of executive function, but was not a necessary consequence, and may not even have been a useful one. Whilst useful, these tests may not therefore be optimal for their purpose. We consider as an alternative approach a function-led development programme which in principle could yield tasks better suited to the concerns of the clinician because of the transparency afforded by increased “representativeness” and “generalisability.” We further argue that the requirement of such a programme to represent the interaction between the individual and situational context might also provide useful constraints for purely experimental investigations. We provide an example of such a programme with reference to the Multiple Errands and Six Element tests
Analytic approximation and an improved method for computing the stress-energy of quantized scalar fields in Robertson-Walker spacetimes
An improved method is given for the computation of the stress-energy tensor
of a quantized scalar field using adiabatic regularization. The method works
for fields with arbitrary mass and curvature coupling in Robertson-Walker
spacetimes and is particularly useful for spacetimes with compact spatial
sections. For massless fields it yields an analytic approximation for the
stress-energy tensor that is similar in nature to those obtained previously for
massless fields in static spacetimes.Comment: RevTeX, 8 pages, no figure
Order of Two-Dimensional Isotropic Dipolar Antiferromagnets
The question of the existence of order in two-dimensional isotropic dipolar
Heisenberg antiferromagnets is studied. It is shown that the dipolar
interaction leads to a gap in the spin-wave energy and a nonvanishing order
parameter. The resulting finite N\'eel-temperature is calculated for a square
lattice by means of linear spin-wave theory.Comment: 10 pages, REVTEX, 1 figure available upon request, TUM-CP-93-0
Hidden Symmetries and their Consequences in Cubic Perovskites
The five-band Hubbard model for a band with one electron per site is a
model which has very interesting properties when the relevant ions are located
at sites with high (e. g. cubic) symmetry. In that case, if the crystal field
splitting is large one may consider excitations confined to the lowest
threefold degenerate orbital states. When the electron hopping matrix
element () is much smaller than the on-site Coulomb interaction energy
(), the Hubbard model can be mapped onto the well-known effective
Hamiltonian (at order ) derived by Kugel and Khomskii (KK). Recently
we have shown that the KK Hamiltonian does not support long range spin order at
any nonzero temperature due to several novel hidden symmetries that it
possesses. Here we extend our theory to show that these symmetries also apply
to the underlying three-band Hubbard model. Using these symmetries we develop a
rigorous Mermin-Wagner construction, which shows that the three-band Hubbard
model does not support spontaneous long-range spin order at any nonzero
temperature and at any order in -- despite the three-dimensional lattice
structure. Introduction of spin-orbit coupling does allow spin ordering, but
even then the excitation spectrum is gapless due to a subtle continuous
symmetry. Finally we showed that these hidden symmetries dramatically simplify
the numerical exact diagonalization studies of finite clusters.Comment: 26 pages, 3 figures, 520 KB, submitted Phys. Rev.
A novel method to create a vortex in a Bose-Einstein condensate
It has been shown that a vortex in a BEC with spin degrees of freedom can be
created by manipulating with external magnetic fields. In the previous work, an
optical plug along the vortex axis has been introduced to avoid Majorana flips,
which take place when the external magnetic field vanishes along the vortex
axis while it is created. In the present work, in contrast, we study the same
scenario without introducing the optical plug. The magnetic field vanishes only
in the center of the vortex at a certain moment of the evolution and hence we
expect that the system will lose only a fraction of the atoms by Majorana flips
even in the absence of an optical plug. Our conjecture is justified by
numerically solving the Gross-Pitaevskii equation, where the full spinor
degrees of freedom of the order parameter are properly taken into account. A
significant simplification of the experimental realization of the scenario is
attained by the omission of the optical plug.Comment: 8 pages, 11 figure
A Paraconsistent Higher Order Logic
Classical logic predicts that everything (thus nothing useful at all) follows
from inconsistency. A paraconsistent logic is a logic where an inconsistency
does not lead to such an explosion, and since in practice consistency is
difficult to achieve there are many potential applications of paraconsistent
logics in knowledge-based systems, logical semantics of natural language, etc.
Higher order logics have the advantages of being expressive and with several
automated theorem provers available. Also the type system can be helpful. We
present a concise description of a paraconsistent higher order logic with
countable infinite indeterminacy, where each basic formula can get its own
indeterminate truth value (or as we prefer: truth code). The meaning of the
logical operators is new and rather different from traditional many-valued
logics as well as from logics based on bilattices. The adequacy of the logic is
examined by a case study in the domain of medicine. Thus we try to build a
bridge between the HOL and MVL communities. A sequent calculus is proposed
based on recent work by Muskens.Comment: Originally in the proceedings of PCL 2002, editors Hendrik Decker,
Joergen Villadsen, Toshiharu Waragai (http://floc02.diku.dk/PCL/). Correcte
Digital Design Considerations for Volunteer Recruitment: Making the Implicit Promises of Volunteering More Explicit
Non-profit organisations may find it difficult to demonstrate to potential volunteers what is required in their voluntary role-resulting in a mismatch between expectations and reality for volunteers. This mismatch could be perceived as a psychological contract breach. We interviewed 18 volunteers and 7 coordinators about their experiences and expectations in order to understand how the experience of volunteers can better be captured and communicated. Further, we wished to consider how future digital platforms might capture important elements of the volunteer experience to better support recruitment, retention and recognition. We present our findings and discuss digital platform implications around the four implicit 'promises' of volunteering: the social promise, the opportunity promise, the value promise and the organisational citizenship promise. We add to literature exploring the voluntary sector by assessing the feasibility of digital interventions to support various aspects of volunteer and coordinator roles
Energy-Momentum Tensor of Particles Created in an Expanding Universe
We present a general formulation of the time-dependent initial value problem
for a quantum scalar field of arbitrary mass and curvature coupling in a FRW
cosmological model. We introduce an adiabatic number basis which has the virtue
that the divergent parts of the quantum expectation value of the
energy-momentum tensor are isolated in the vacuum piece of , and
may be removed using adiabatic subtraction. The resulting renormalized
is conserved, independent of the cutoff, and has a physically transparent,
quasiclassical form in terms of the average number of created adiabatic
`particles'. By analyzing the evolution of the adiabatic particle number in de
Sitter spacetime we exhibit the time structure of the particle creation
process, which can be understood in terms of the time at which different
momentum scales enter the horizon. A numerical scheme to compute as a
function of time with arbitrary adiabatic initial states (not necessarily de
Sitter invariant) is described. For minimally coupled, massless fields, at late
times the renormalized goes asymptotically to the de Sitter invariant
state previously found by Allen and Folacci, and not to the zero mass limit of
the Bunch-Davies vacuum. If the mass m and the curvature coupling xi differ
from zero, but satisfy m^2+xi R=0, the energy density and pressure of the
scalar field grow linearly in cosmic time demonstrating that, at least in this
case, backreaction effects become significant and cannot be neglected in de
Sitter spacetime.Comment: 28 pages, Revtex, 11 embedded .ps figure
Coulomb scattering lifetime of a two-dimensional electron gas
Motivated by a recent tunneling experiment in a double quantum-well system,
which reports an anomalously enhanced electronic scattering rate in a clean
two-dimensional electron gas, we calculate the inelastic quasiparticle lifetime
due to electron-electron interaction in a single loop dynamically screened
Coulomb interaction within the random-phase-approximation. We obtain excellent
quantitative agreement with the inelastic scattering rates in the tunneling
experiment without any adjustable parameter, finding that the reported large
( a factor of six) disagreement between theory and experiment arises from
quantitative errors in the existing theoretical work and from the off-shell
energy dependence of the electron self-energy.Comment: 11 pages, RevTex, figures included. Also available at
http://www-cmg.physics.umd.edu/~lzheng
Axisymmetric versus Non-axisymmetric Vortices in Spinor Bose-Einstein Condensates
The structure and stability of various vortices in F=1 spinor Bose-Einstein
condensates are investigated by solving the extended Gross-Pitaevskii equation
under rotation. We perform an extensive search for stable vortices, considering
both axisymmetric and non-axisymmetric vortices and covering a wide range of
ferromagnetic and antiferromagnetic interactions. The topological defect called
Mermin-Ho (Anderson-Toulouse) vortex is shown to be stable for ferromagnetic
case. The phase diagram is established in a plane of external rotation Omega vs
total magnetization M by comparing the free energies of possible vortices. It
is shown that there are qualitative differences between axisymmetric and
non-axisymmetric vortices which are manifested in the Omega- and M-dependences.Comment: 9 pages, 9 figure
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