1,343 research outputs found
Philosophy with children : helping designers cooperate with children
Engaging children in design through in-depth interviews is coming to prominence in the IDC community, which increasingly engages with issues about understanding the children's world. To date, research in this area has primarily focused on engaging children using techniques somehow similar to adult-techniques (moodboards, brainstorming, laddering,...). However, questioning or interviewing children is fraught with difficulties. The proposed workshop seeks to explore where and how a philosophy with children methodology can be adapted for design, exploring themes such as Socratic Attitudes, wondering, and question types. This workshop aims to build an interdisciplinary community of researchers, designers, and practitioners to share and discuss their work and experiences
Pyrochlore Photons: The U(1) Spin Liquid in a S=1/2 Three-Dimensional Frustrated Magnet
We study the S=1/2 Heisenberg antiferromagnet on the pyrochlore lattice in
the limit of strong easy-axis exchange anisotropy. We find, using only standard
techniques of degenerate perturbation theory, that the model has a U(1) gauge
symmetry generated by certain local rotations about the z-axis in spin space.
Upon addition of an extra local interaction in this and a related model with
spins on a three-dimensional network of corner-sharing octahedra, we can write
down the exact ground state wavefunction with no further approximations. Using
the properties of the soluble point we show that these models enter the U(1)
spin liquid phase, a novel fractionalized spin liquid with an emergent U(1)
gauge structure. This phase supports gapped S^z = 1/2 spinons carrying the U(1)
``electric'' gauge charge, a gapped topological point defect or ``magnetic''
monopole, and a gapless ``photon,'' which in spin language is a gapless,
linearly dispersing S^z = 0 collective mode. There are power-law spin
correlations with a nontrivial angular dependence, as well as novel U(1)
topological order. This state is stable to ALL zero-temperature perturbations
and exists over a finite extent of the phase diagram. Using a convenient
lattice version of electric-magnetic duality, we develop the effective
description of the U(1) spin liquid and the adjacent soluble point in terms of
Gaussian quantum electrodynamics and calculate a few of the universal
properties. The resulting picture is confirmed by our numerical analysis of the
soluble point wavefunction. Finally, we briefly discuss the prospects for
understanding this physics in a wider range of models and for making contact
with experiments.Comment: 22 pages, 14 figures. Further minor changes. To appear in Phys. Rev.
Large-N expansion based on the Hubbard-operator path integral representation and its application to the model
In the present work we have developed a large-N expansion for the model
based on the path integral formulation for Hubbard-operators. Our large-N
expansion formulation contains diagrammatic rules, in which the propagators and
vertex are written in term of Hubbard operators. Using our large-N formulation
we have calculated, for J=0, the renormalized boson propagator. We
also have calculated the spin-spin and charge-charge correlation functions to
leading order 1/N. We have compared our diagram technique and results with the
existing ones in the literature.Comment: 6 pages, 3 figures, Phys.Rev.B (in press
Lattice gauge theory with baryons at strong coupling
We study the effective Hamiltonian for strong-coupling lattice QCD in the
case of non-zero baryon density. In leading order the effective Hamiltonian is
a generalized antiferromagnet. For naive fermions, the symmetry is U(4N_f) and
the spins belong to a representation that depends on the local baryon number.
Next-nearest-neighbor (nnn) terms in the Hamiltonian break the symmetry to
U(N_f) x U(N_f). We transform the quantum problem to a Euclidean sigma model
which we analyze in a 1/N_c expansion. In the vacuum sector we recover
spontaneous breaking of chiral symmetry for the nearest-neighbor and nnn
theories. For non-zero baryon density we study the nearest-neighbor theory
only, and show that the pattern of spontaneous symmetry breaking depends on the
baryon density.Comment: 31 pages, 5 EPS figures. Corrected Eq. (6.1
Ring exchange, the Bose metal, and bosonization in two dimensions
Motivated by the high-T_c cuprates, we consider a model of bosonic Cooper
pairs moving on a square lattice via ring exchange. We show that this model
offers a natural middle ground between a conventional antiferromagnetic Mott
insulator and the fully deconfined fractionalized phase which underlies the
spin-charge separation scenario for high-T_c superconductivity. We show that
such ring models sustain a stable critical phase in two dimensions, the *Bose
metal*. The Bose metal is a compressible state, with gapless but uncondensed
boson and ``vortex'' excitations, power-law superconducting and charge-ordering
correlations, and broad spectral functions. We characterize the Bose metal with
the aid of an exact plaquette duality transformation, which motivates a
universal low energy description of the Bose metal. This description is in
terms of a pair of dual bosonic phase fields, and is a direct analog of the
well-known one-dimensional bosonization approach. We verify the validity of the
low energy description by numerical simulations of the ring model in its exact
dual form. The relevance to the high-T_c superconductors and a variety of
extensions to other systems are discussed, including the bosonization of a two
dimensional fermionic ring model
Phase diagram for a class of spin-half Heisenberg models interpolating between the square-lattice, the triangular-lattice and the linear chain limits
We study the spin-half Heisenberg models on an anisotropic two-dimensional
lattice which interpolates between the square-lattice at one end, a set of
decoupled spin-chains on the other end, and the triangular-lattice Heisenberg
model in between. By series expansions around two different dimer ground states
and around various commensurate and incommensurate magnetically ordered states,
we establish the phase diagram for this model of a frustrated antiferromagnet.
We find a particularly rich phase diagram due to the interplay of magnetic
frustration, quantum fluctuations and varying dimensionality. There is a large
region of the usual 2-sublattice Ne\'el phase, a 3-sublattice phase for the
triangular-lattice model, a region of incommensurate magnetic order around the
triangular-lattice model, and regions in parameter space where there is no
magnetic order. We find that the incommensurate ordering wavevector is in
general altered from its classical value by quantum fluctuations. The regime of
weakly coupled chains is particularly interesting and appears to be nearly
critical.Comment: RevTeX, 15 figure
Possible Z2 phase and spin-charge separation in electron doped cuprate superconductors
The SU(2) slave-boson mean-field theory for the tt'J model is analyzed. The
role of next-nearest-neighbor hopping t' on the phase-diagram is studied. We
find a pseudogap phase in hole-doped materials (where t'<0). The pseudo-gap
phase is a U(1) spin liquid (the staggered-flux phase) with a U(1) gauge
interaction and no fractionalization. This agrees with experiments on hole
doped samples. The same calculation also indicates that a positive t' favors a
Z2 state with true spin-charge separation. The Z2 state that exists when t' >
0.5J can be a candidate for the pseudo-gap phase of electron-doped cuprates (if
such a phase exists). The experimental situation in electron-doped materials is
also addressed.Comment: 6 pages, 2 figures, RevTeX4. Homepage http://dao.mit.edu/~wen
Predicting Phenotype and Emerging Strains among Chlamydia trachomatis Infections
Single nucleotide polymorphisms can be used for epidemiologic and evolutionary studies worldwide
Lessons from Agriculture for the Sustainable Management of Malaria Vectors
The effectiveness of insecticide-treated bed nets and indoor insecticide sprays to control adult mosquito vectors is being threatened by the spread of insecticide resistance. We argue for expanding beyond “insecticide monotherapy” to more sustainable integrated vector management strategies that use optimal suites of control tactics. Experience in agriculture suggests that such integrated approaches can provide more effective and durable pest management. This shift will require increased investment in research and translational science. Failure to act risks a resurgence of malaria and erosion of community support and donor commitment
An Effective-Medium Tight-Binding Model for Silicon
A new method for calculating the total energy of Si systems is presented. The
method is based on the effective-medium theory concept of a reference system.
Instead of calculating the energy of an atom in the system of interest a
reference system is introduced where the local surroundings are similar. The
energy of the reference system can be calculated selfconsistently once and for
all while the energy difference to the reference system can be obtained
approximately. We propose to calculate it using the tight-binding LMTO scheme
with the Atomic-Sphere Approximation(ASA) for the potential, and by using the
ASA with charge-conserving spheres we are able to treat open system without
introducing empty spheres. All steps in the calculational method is {\em ab
initio} in the sense that all quantities entering are calculated from first
principles without any fitting to experiment. A complete and detailed
description of the method is given together with test calculations of the
energies of phonons, elastic constants, different structures, surfaces and
surface reconstructions. We compare the results to calculations using an
empirical tight-binding scheme.Comment: 26 pages (11 uuencoded Postscript figures appended), LaTeX,
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