2,106 research outputs found
Interaction and thermodynamics of spinons in the XX chain
The mapping between the fermion and spinon compositions of eigenstates in the
one-dimensional spin-1/2 XX model on a lattice with N sites is used to describe
the spinon interaction from two different perspectives: (i) For finite N the
energy of all eigenstates is expressed as a function of spinon momenta and
spinon spins, which, in turn, are solutions of a set of Bethe ansatz equations.
The latter are the basis of an exact thermodynamic analysis in the spinon
representation of the XX model. (ii) For N -> infinity the energy per site of
spinon configurations involving any number of spinon orbitals is expressed as a
function of reduced variables representing momentum, filling, and magnetization
of each orbital. The spins of spinons in a single orbital are found to be
coupled in a manner well described by an Ising-like equivalent-neighbor
interaction, switching from ferromagnetic to antiferromagnetic as the filling
exceeds a critical level. Comparisons are made with results for the
Haldane-Shastry model.Comment: 16 pages, 3 figure
Scattering and Bound State Green's Functions on a Plane via so(2,1) Lie Algebra
We calculate the Green's functions for the particle-vortex system, for two
anyons on a plane with and without a harmonic regulator and in a uniform
magnetic field. These Green's functions which describe scattering or bound
states (depending on the specific potential in each case) are obtained exactly
using an algebraic method related to the SO(2,1) Lie group. From these Green's
functions we obtain the corresponding wave functions and for the bound states
we also find the energy spectra.Comment: 21 Latex pages. Typos corrected. Results unchanged. Version to appear
in JM
Higher dimensional radiation collapse and cosmic censorship
We study the occurrence of naked singularities in the spherically symmetric
collapse of radiation shells in a higher dimensional spacetime. The necessary
conditions for the formation of a naked singularity or a black hole are
obtained. The naked singularities are found to be strong in the Tipler's sense
and thus violating cosmic censorship conjecture.Comment: 4 pages, ReVTeX, Phys Rev D Vol 62 107502 (2000
Introduction to dynamical horizons in numerical relativity
This paper presents a quasi-local method of studying the physics of dynamical
black holes in numerical simulations. This is done within the dynamical horizon
framework, which extends the earlier work on isolated horizons to
time-dependent situations. In particular: (i) We locate various kinds of
marginal surfaces and study their time evolution. An important ingredient is
the calculation of the signature of the horizon, which can be either spacelike,
timelike, or null. (ii) We generalize the calculation of the black hole mass
and angular momentum, which were previously defined for axisymmetric isolated
horizons to dynamical situations. (iii) We calculate the source multipole
moments of the black hole which can be used to verify that the black hole
settles down to a Kerr solution. (iv) We also study the fluxes of energy
crossing the horizon, which describes how a black hole grows as it accretes
matter and/or radiation.
We describe our numerical implementation of these concepts and apply them to
three specific test cases, namely, the axisymmetric head-on collision of two
black holes, the axisymmetric collapse of a neutron star, and a
non-axisymmetric black hole collision with non-zero initial orbital angular
momentum.Comment: 20 pages, 16 figures, revtex4. Several smaller changes, some didactic
content shortene
Room temperature spin coherence in ZnO
Time-resolved optical techniques are used to explore electron spin dynamics
in bulk and epilayer samples of n-type ZnO as a function of temperature and
magnetic field. The bulk sample yields a spin coherence time T2* of 20 ns at T
= 30 K. Epilayer samples, grown by pulsed laser deposition, show a maximum T2*
of 2 ns at T = 10 K, with spin precession persisting up to T = 280 K.Comment: 3 pages, 3 figure
Biomechanical stability of a supra-acetabular pedicle screw Internal Fixation device (INFIX) vs External Fixation and plates for vertically unstable pelvic fractures
Abstract
Background
We have recently developed a subcutaneous anterior pelvic fixation technique (INFIX). This internal fixator permits patients to sit, roll over in bed and lie on their sides without the cumbersome external appliances or their complications. The purpose of this study was to evaluate the biomechanical stability of this novel supraacetabular pedicle screw internal fixation construct (INFIX) and compare it to standard internal fixation and external fixation techniques in a single stance pelvic fracture model.
Methods
Nine synthetic pelves with a simulated anterior posterior compression type III injury were placed into three groups (External Fixator, INFIX and Internal Fixation). Displacement, total axial stiffness, and the stiffness at the pubic symphysis and SI joint were calculated. Displacement and stiffness were compared by ANOVA with a Bonferroni adjustment for multiple comparisons
Results
The mean displacement at the pubic symphysis was 20, 9 and 0.8 mm for external fixation, INFIX and internal fixation, respectively. Plate fixation was significantly stiffer than the INFIX and external Fixator (P = 0.01) at the symphysis pubis. The INFIX device was significantly stiffer than external fixation (P = 0.017) at the symphysis pubis. There was no significant difference in SI joint displacement between any of the groups.
Conclusions
Anterior plate fixation is stiffer than both the INFIX and external fixation in single stance pelvic fracture model. The INFIX was stiffer than external fixation for both overall axial stiffness, and stiffness at the pubic symphysis. Combined with the presumed benefit of minimizing the complications associated with external fixation, the INFIX may be a more preferable option for temporary anterior pelvic fixation in situations where external fixation may have otherwise been used
Multicentricity of breast cancer: whole-organ analysis and clinical implications
We studied the spatial relationship within the breast between multicentric foci (MCF) and the primary tumour in 30 modified radical mastectomy specimens using Egan's correlated pathological-radiological method using 5 mm slices of the whole breast. The relative positions within the breast of the primary tumour and MCF were used to calculate the relative distribution of primary tumour and MCF in the four quadrants of the breast and the per cent breast volume that would be required to be excised to include all MCF. Nineteen (63%) breast harboured MCF. The relative distribution of primary tumour and MCF in the four breast quadrants was significantly different (P = 0.034). MCF were present beyond the index quadrant (25% of breast volume including the tumour) in as many as 79% (15/19) of breasts that harboured MCF; and in half the cases (15/30) when all breast were considered. This is in variance with the suggestion put forward previously that MCF are contained within the index quadrant in 90% of cases. Although the number of patients in the present series is small, the probability of our finding being due to play of chance is 1 in 1500. In a large series of breast conservation studies > 90% of early breast recurrences have been found to occur in the index quadrant. Our finding, that in half the patients (15/30) MCF are present in quadrants other than the index quadrant, suggests that MCF do not give rise to early breast recurrence
Spontaneous Symmetry Breaking in Twisted Noncommutative Quantum Theories
We analyse aspects of symmetry breaking for Moyal spacetimes within a
quantisation scheme which preserves the twisted Poincar\'e symmetry. Towards
this purpose, we develop the LSZ approach for Moyal spacetimes. The latter
gives a formula for scattering amplitudes on these spacetimes which can be
obtained from the corresponding ones on the commutative spacetime. This formula
applies in the presence of spontaneous breakdown of symmetries as well.We also
derive Goldstone's theorem on Moyal spacetime. The formalism developed here can
be directly applied to the twisted standard model.Comment: +13 pages; minor changes in references,To appear in Phys Rev
Gravitational and electromagnetic fields of a charged tachyon
An axially symmetric exact solution of the Einstein-Maxwell equations is
obtained and is interpreted to give the gravitational and electromagnetic
fields of a charged tachyon. Switching off the charge parameter yields the
solution for the uncharged tachyon which was earlier obtained by Vaidya. The
null surfaces for the charged tachyon are discussed.Comment: 8 pages, LaTex, To appear in Pramana- J. Physic
Field of a Radiation Distributuion
General relativistic spherically symmetric matter field with a vanishing
stress energy scalar is analyzed. Procedure for generating exact solutions of
the field equations for such matter distributions is given. It is further
pointed out that all such type I spherically symmetric fields with distinct
eignvalues in the radial two space can be treated as a mixture of isotropic and
directed radiations. Various classes of exact solutions are given. Junction
conditions for such a matter field to the possible exterior solutions are also
discussed.Comment: Latex file, 13 pages, no figures. Accepted for publication in Phys.
Rev.
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