546 research outputs found
Metal-insulator transition at B=0 in an ultra-low density () two dimensional GaAs/AlGaAs hole gas
We have observed a metal-insulator transition in an ultra-low density two
dimensional hole gas formed in a high quality GaAs-AlGaAs heterostructure at
B=0. At the highest carrier density studied () the hole gas is strongly metallic, with an exceptional mobility of
. The low disorder and strength of the many-body
interactions in this sample are highlighted by the observation of re-entrant
metal insulator transitions in both the fractional () and integer
() quantum Hall regimes. On reducing the carrier density the
temperature and electric field dependence of the resistivity show that the
sample is still metallic at (), becoming
insulating at . Our results indicate that
electron-electron interactions are dominant at these low densities, pointing to
the many body origins of this metal-insulator transition. We note that the
value of at the transition () is large enough to allow
the formation of a weakly pinned Wigner crystal, and is approaching the value
calculated for the condensation of a pure Wigner crystal.Comment: 4 pages, latex, 4 postscript figures, submitted to EP2DS-12 on 21st
August 1997, to appear in Physica
Dutch preposition stranding and ellipsis: 'Merchant's Wrinkle' ironed out
This paper provides an explanation for the unexpected ban on preposition stranding by wh-R-pronouns under sluicing in Dutch. After showing that previous prosodic and syntactic explanations are untenable, we propose that the observed ban is a by-product of an EPP condition that applies in the PP domain in Dutch. Our analysis revolves around the idea that ellipsis bleeds EPP-driven movement, an idea that already has empirical support from independent patterns of ellipsis found in English and in other structural domains in Dutch. Our claim is that: (1) R-pronominalization involves a pronominal argument of P moving to the periphery of its extended PP domain (PlaceP) in order to satisfy a PP-internal EPP condition, (2) this EPP-driven movement is bled under sluicing, and (3) because SpecPlaceP is the 'escape hatch' through which R-pronouns must move in order to exit the PP domain to form preposition stranding configurations, bleeding the EPP-driven movement of R-pronouns to SpecPlaceP therefore precludes R-pronouns from undergoing the wh-movement required to form a sluicing configuration.Theoretical and Experimental Linguistic
Quasi-1D dynamics and nematic phases in the 2D Emery model
We consider the Emery model of a
Cu-O plane of the high temperature superconductors. We show that in a
strong-coupling limit, with strong Coulomb repulsions between electrons on
nearest-neighbor O sites, the electron-dynamics is strictly one dimensional,
and consequently a number of asymptotically exact results can be obtained
concerning the electronic structure. In particular, we show that a nematic
phase, which spontaneously breaks the point- group symmetry of the square
lattice, is stable at low enough temperatures and strong enough coupling.Comment: 8 pages, 5 eps figures; revised manuscript with more detailed
discussions; two new figures and three edited figuresedited figures; 14
references; new appendix with a detailed proof of the one-dimensional
dynamics of the system in the strong coupling limi
The de Rham homotopy theory and differential graded category
This paper is a generalization of arXiv:0810.0808. We develop the de Rham
homotopy theory of not necessarily nilpotent spaces, using closed dg-categories
and equivariant dg-algebras. We see these two algebraic objects correspond in a
certain way. We prove an equivalence between the homotopy category of schematic
homotopy types and a homotopy category of closed dg-categories. We give a
description of homotopy invariants of spaces in terms of minimal models. The
minimal model in this context behaves much like the Sullivan's minimal model.
We also provide some examples. We prove an equivalence between fiberwise
rationalizations and closed dg-categories with subsidiary data.Comment: 47 pages. final version. The final publication is available at
http://www.springerlink.co
Residues and World-Sheet Instantons
We reconsider the question of which Calabi-Yau compactifications of the
heterotic string are stable under world-sheet instanton corrections to the
effective space-time superpotential. For instance, compactifications described
by (0,2) linear sigma models are believed to be stable, suggesting a remarkable
cancellation among the instanton effects in these theories. Here, we show that
this cancellation follows directly from a residue theorem, whose proof relies
only upon the right-moving world-sheet supersymmetries and suitable compactness
properties of the (0,2) linear sigma model. Our residue theorem also extends to
a new class of "half-linear" sigma models. Using these half-linear models, we
show that heterotic compactifications on the quintic hypersurface in CP^4 for
which the gauge bundle pulls back from a bundle on CP^4 are stable. Finally, we
apply similar ideas to compute the superpotential contributions from families
of membrane instantons in M-theory compactifications on manifolds of G_2
holonomy.Comment: 47 page
The three-dimensional random field Ising magnet: interfaces, scaling, and the nature of states
The nature of the zero temperature ordering transition in the 3D Gaussian
random field Ising magnet is studied numerically, aided by scaling analyses. In
the ferromagnetic phase the scaling of the roughness of the domain walls,
, is consistent with the theoretical prediction .
As the randomness is increased through the transition, the probability
distribution of the interfacial tension of domain walls scales as for a single
second order transition. At the critical point, the fractal dimensions of
domain walls and the fractal dimension of the outer surface of spin clusters
are investigated: there are at least two distinct physically important fractal
dimensions. These dimensions are argued to be related to combinations of the
energy scaling exponent, , which determines the violation of
hyperscaling, the correlation length exponent , and the magnetization
exponent . The value is derived from the
magnetization: this estimate is supported by the study of the spin cluster size
distribution at criticality. The variation of configurations in the interior of
a sample with boundary conditions is consistent with the hypothesis that there
is a single transition separating the disordered phase with one ground state
from the ordered phase with two ground states. The array of results are shown
to be consistent with a scaling picture and a geometric description of the
influence of boundary conditions on the spins. The details of the algorithm
used and its implementation are also described.Comment: 32 pp., 2 columns, 32 figure
Field-Induced Magnetic Ordering in the Quantum Spin System KCuCl
KCuCl is a three-dimensional coupled spin-dimer system and has a singlet
ground state with an excitation gap K. High-field
magnetization measurements for KCuCl have been performed in static magnetic
fields of up to 30 T and in pulsed magnetic fields of up to 60 T. The entire
magnetization curve including the saturation region was obtained at K.
From the analysis of the magnetization curve, it was found that the exchange
parameters determined from the dispersion relations of the magnetic excitations
should be reduced, which suggests the importance of the renormalization effect
in the magnetic excitations. The field-induced magnetic ordering accompanied by
the cusplike minimum of the magnetization was observed as in the isomorphous
compound TlCuCl. The phase boundary was almost independent of the field
direction, and is represented by the power law. These results are consistent
with the magnon Bose-Einstein condensation picture for field-induced magnetic
ordering.Comment: 9 pages, 7 figures, 9 eps files, revtex styl
Antiferromagnetic Zigzag Spin Chain in Magnetic Fields at Finite Temperatures
We study thermodynamic behaviors of the antiferromagnetic zigzag spin chain
in magnetic fields, using the density-matrix renormalization group method for
the quantum transfer matrix. We focus on the thermodynamics of the system near
the critical fields in the ground-state magnetization process(- curve):
the saturation field, the lower critical field associated with excitation gap,
and the field at the middle-field cusp singularity. We calculate magnetization,
susceptibility and specific heat of the zigzag chain in magnetic fields at
finite temperatures, and then discuss how the calculated quantities reflect the
low-lying excitations of the system related with the critical behaviors in the
- curve.Comment: accepted for publication in Physical Review
Motion-Compensation Techniques in Neonatal and Fetal MR Imaging
Fetal and neonatal MR imaging is increasingly used as a complementary diagnostic tool to sonography. MR imaging is an ideal technique for imaging fetuses and neonates because of the absence of ionizing radiation, the superior contrast of soft tissues compared with sonography, the availability of different contrast options, and the increased FOV. Motion in the normally mobile fetus and the unsettled, sleeping, or sedated neonate during a long acquisition will decrease image quality in the form of motion artifacts, hamper image interpretation, and often necessitate a repeat MR imaging to establish a diagnosis. This article reviews current techniques of motion compensation in fetal and neonatal MR imaging, including the following: 1) motion-prevention strategies (such as adequate patient preparation, patient coaching, and sedation, when required), 2) motion-artifacts minimization methods (such as fast imaging protocols, data undersampling, and motion-resistant sequences), and 3) motion-detection/correction schemes (such as navigators and self-navigated sequences, external motion-tracking devices, and postprocessing approaches) and their application in fetal and neonatal brain MR imaging. Additionally some background on the repertoire of motion of the fetal and neonatal patient and the resulting artifacts will be presented, as well as insights into future developments and emerging techniques of motion compensation
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