1,115 research outputs found
Influence of Hybridization on the Properties of the Spinless Falicov-Kimball Model
Without a hybridization between the localized f- and the conduction (c-)
electron states the spinless Falicov-Kimball model (FKM) is exactly solvable in
the limit of high spatial dimension, as first shown by Brandt and Mielsch. Here
I show that at least for sufficiently small c-f-interaction this exact
inhomogeneous ground state is also obtained in Hartree-Fock approximation. With
hybridization the model is no longer exactly solvable, but the approximation
yields that the inhomogeneous charge-density wave (CDW) ground state remains
stable also for finite hybridization V smaller than a critical hybridization
V_c, above which no inhomogeneous CDW solution but only a homogeneous solution
is obtained. The spinless FKM does not allow for a ''ferroelectric'' ground
state with a spontaneous polarization, i.e. there is no nonvanishing
-expectation value in the limit of vanishing hybridization.Comment: 7 pages, 6 figure
Fracture strength of cusp-replacing fibre-strengthened composite restorations
INTRODUCTION : Fracture of composite restorations, especially when one or more cusps are replaced, is a common reason for failure. Finite element analysis has shown that crack propagation at the tension side of the restoration signals the failure.
AIMS AND OBJECTIVES : The strengthening effect of placing a fibre substructure on the tension side was investigated and the results compared with the fracture strengths of a conventional posterior composite without a substructure (control) and of a composite reinforced with fibres incorporated within the composite.
DESIGN : The study was an in vitro experimental blind study.
METHODS : 75 extracted lower first molars were divided into three groups of 25 teeth each to allow for the comparisons and the restorations were placed. All specimens were thermocycled for 500 cycles between 5°C and 55°C with a dwell time of 30 seconds. Each restoration was subjected to loading on a Universal testing machine at a 30° angle to the long axis of the tooth, until fracture occurred. Maximum force before failure (Fmax in N) was recorded.
RESULTS : The results indicated a significantly higher strength for the composite resin restorations placed on a fibre substructure.
CONCLUSION : A uni-directional fibre substructure is recommended to achieve greatest strength.http://www.sada.co.zaam201
Fracture behaviour patterns of cusp-replacing fibre strengthened composite restorations
OBJECTIVES : To investigate and compare, in vitro, the fracture
behaviours of three types of cusp-replacing posterior
composite resin restorations.
METHODS : Standard preparations for posterior composite
restoration of the mesio-lingual cusp were cut on seventy-
five extracted lower left first and second molars and
restorations placed. Group A (control, n =25) with a conventional
posterior composite resin, Group B (n=25): resin
reinforced with nano-scale electrospun glass fibres Group
C (n=25) :resin reinforced with a fibre substructure. Specimens
were thermocycled for 500 cycles between 5°C and
55°C with a dwell time of 30 seconds, then embedded
in plastic cylinders in acrylic resin. The specimens were
loaded in a universal testing machine at a 30° angle to
the long axis of the tooth until fracture occurred. Fracture
patterns were highlighted by staining, studied under
a microscope and classified as favourable (restorable) or
unfavourable (non-restorable). Sub-classification included
adhesive and cohesive failures.
RESULTS : Group C exhibited significantly more “restorable”
fractures. Group B displayed significantly more “nonrestorable”
fractures. Fracture patterns differed significantly
between the two fibre-strengthening techniques. (Fisher’s
Exact Test p = 0.05)
CLINICAL SIGNIFICANCE : Resin restorations reinforced with
glass-fibre substructures are more readily repaired after fracture, saving tooth structure, and reducing costs to the
patient.http://www.sada.co.zaam201
General structure of the photon self-energy in non-commutative QED
We study the behavior of the photon two point function, in non-commutative
QED, in a general covariant gauge and in arbitrary space-time dimensions. We
show, to all orders, that the photon self-energy is transverse. Using an
appropriate extension of the dimensional regularization method, we evaluate the
one-loop corrections, which show that the theory is renormalizable. We also
prove, to all orders, that the poles of the photon propagator are gauge
independent and briefly discuss some other related aspects.Comment: 16 pages, revtex4. This is the final version to be published in Phys.
Rev.
Nonlinear excitations in CsNiF3 in magnetic fields perpendicular to the easy plane
Experimental and numerical studies of the magnetic field dependence of the
specific heat and magnetization of single crystals of CsNiF3 have been
performed at 2.4 K, 2.9 K, and 4.2 K in magnetic fields up to 9 T oriented
perpendicular to the easy plane. The experimental results confirm the presence
of the theoretically predicted double peak structure in the specific heat
arising from the formation of nonlinear spin modes. The demagnetizing effects
are found to be negligible, and the overall agreement between the data and
numerical predictions is better than reported for the case when the magnetic
field was oriented in the easy plane. Demagnetizing effects might play a role
in generating the difference observed between theory and experiment in previous
work analyzing the excess specific heat using the sine-Gordon model.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
Magnetic and Dynamic Properties of the Hubbard Model in Infinite Dimensions
An essentially exact solution of the infinite dimensional Hubbard model is
made possible by using a self-consistent mapping of the Hubbard model in this
limit to an effective single impurity Anderson model. Solving the latter with
quantum Monte Carlo procedures enables us to obtain exact results for the one
and two-particle properties of the infinite dimensional Hubbard model. In
particular we find antiferromagnetism and a pseudogap in the single-particle
density of states for sufficiently large values of the intrasite Coulomb
interaction at half filling. Both the antiferromagnetic phase and the
insulating phase above the N\'eel temperature are found to be quickly
suppressed on doping. The latter is replaced by a heavy electron metal with a
quasiparticle mass strongly dependent on doping as soon as . At half
filling the antiferromagnetic phase boundary agrees surprisingly well in shape
and order of magnitude with results for the three dimensional Hubbard model.Comment: 32 page
Weak-coupling Treatment of Electronic (Anti-)Ferroelectricity in the Extended Falicov-Kimball Model
We study the (spinless) Falicov-Kimball model extended by a finite band width
(hopping ) of the localized (f-) electrons in infinite dimensions in the
weak-coupling limit of a small local interband Coulomb correlation for half
filling. In the case of overlapping conduction- and f-bands different kinds of
ordered solutions are possible, namely charge-density wave (CDW) order,
electronic ferroelectricity (EFE) and electronic antiferroelectricity (EAFE).
The order parameters are calculated as a function of the model parameters and
of the temperature. There is a first-order phase transition from the CDW-phase
to the EFE- or EAFE-phase. The total energy is calculated to determine the
thermodynamically stable solution. The quantum phase diagrams are calculated.Comment: 7 pages, 8 figure
Kontsevich product and gauge invariance
We analyze the question of gauge invariance in a flat
non-commutative space where the parameter of non-commutativity,
, is a local function satisfying Jacobi identity (and
thereby leading to an associative Kontsevich product). We show that in this
case, both gauge transformations as well as the definitions of covariant
derivatives have to modify so as to have a gauge invariant action. We work out
the gauge invariant actions for the matter fields in the fundamental and the
adjoint representations up to order while we discuss the gauge
invariant Maxwell theory up to order . We show that despite the
modifications in the gauge transformations, the covariant derivative and the
field strength, Seiberg-Witten map continues to hold for this theory. In this
theory, translations do not form a subgroup of the gauge transformations
(unlike in the case when is a constant) which is reflected in
the stress tensor not being conserved.Comment: 7 page
Superconductivity from correlated hopping
We consider a chain described by a next-nearest-neighbor hopping combined
with a nearest-neighbor spin flip. In two dimensions this three-body term
arises from a mapping of the three-band Hubbard model for CuO planes to a
generalized model and for large O-O hopping favors resonance-valence-bond
superconductivity of predominantly -wave symmetry. Solving the ground state
and low-energy excitations by analytical and numerical methods we find that the
chain is a Luther-Emery liquid with correlation exponent , where is the particle density.Comment: 10 pages, RevTeX 3.0 + 2 PostScript figs. Accepted for publication in
Phys.Rev.
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