678 research outputs found
Dynamical Flavour Symmetry Breaking by a Magnetic Field in Lattice QED_3
We perform a lattice study, in the quenched approximation, of dynamical mass
generation in a system of relativistic (Dirac) fermions, coupled to an Abelian
gauge field in (2+1)-dimensions, in the presence of an external (constant)
magnetic field, perpendicular to the spatial planes. It is shown that a strong
magnetic field catalyzes chiral symmetry breaking, in agreement with results in
the continuum. The r\^ole of the higher-Landau poles in inducing a critical
temperature above which the phenomenon disappears is pointed out. We also
discuss the implications of this model on the opening of a gap in doped
antiferromagnetic superconductors.Comment: 18 pages, latex, 9 figures, uses psfig and epsf; minor typos in
eqs.(10) and (17) correcte
Titanium additions to MgB2 conductors
A series of doping experiments are reported for MgB2 conductors that have
been synthesized using doped boron fibers prepared by chemical vapor
deposition(CVD) methods. Undoped MgB2 samples prepared from CVD prepared fibers
consistently give critical current densities, Jc, in the range of 500,000
A/cm^2 in low field at 5K. These values fall by a factor of about 100 as the
magnetic field increases to 3T. For heavily Ti-doped boron fibers where the
B/Ti ratio is comparable to 1, there is a substantial suppression of both Tc,
superconducting volume fraction, and Jc values. If, however, a sample with a
few percent Ti in B is deposited on a carbon coated SiC substrate and reacted
at 1100 degrees C for 15 min, then Tc is suppressed only a couple of degrees
Kelvin and critical current densities are found to be approximately 2-5 x 10^6
A/cm^2 for superconducting layers ranging from 4-10 micrometers thick. These
materials show Jc values over 10,000 A/cm^2 at 25K and 1.3 T.Comment: 10 pages, 6 figure
Functional Integral Approach to the Single Impurity Anderson Model
Recently, a functional integral representation was proposed by Weller
(Weller, W.: phys.~stat.~sol.~(b) {\bf 162}, 251 (1990)), in which the
fermionic fields strictly satisfy the constraint of no double occupancy at each
lattice site. This is achieved by introducing spin dependent Bose fields. The
functional integral method is applied to the single impurity Anderson model
both in the Kondo and mixed-valence regime. The f-electron Green's function and
susceptibility are calculated using an Ising-like representation for the Bose
fields. We discuss the difficulty to extract a spectral function from the
knowledge of the imaginary time Green's function. The results are compared with
NCA calculations.Comment: 11 pages, LaTeX, figures upon request, preprint No. 93/10/
An implantable monophasic/biphasic atrial defibrillation system using transcutaneo RF power delivery
The effect of pore-former morphology on the electrochemical performance of solid oxide fuel cells under combined fuel cell and electrolysis modes
The effect of the pore-former used in the Ni-YSZ fuel electrode on the electrochemical performance of solid oxide cells is studied. Three cells with the configuration of Ni-YSZ/YSZ/Nd2NiO4+d-YSZ were fabricated with different pore-formers, such as graphite, PMMA (polymethyl methacrylate) or an equal mixture of both, which were added to the Ni-YSZ support during the fabrication process. The results show that the Ni-YSZ support containing graphite leads to a more porous support and formation of coarser pores in the vicinity of the electrolyte. This leads to a reduction in the triple phase boundary (TPB) length with a corresponding increase of activation polarization and, as a consequence, the overall cell performance decreases in both fuel cell and electrolysis modes. The cell having PMMA delivered the highest performance under both operation modes (818 and -713 mAcm-2 were obtained in SOFC and SOEC modes at 800 °C), due to finer pores next to the electrolyte. The cell having the mixture of both pore-formers delivered intermediate results. All the cells show similar concentration polarization values meaning that even the least porous cell (PMMA) provided sufficient porosity for gas flow. In addition, long term reversible experiments were performed, showing no degradation for a period above 400 h
Strangeness in constituent quarks and one-spin asymmetry in inclusive -meson production
On the basis of the mechanism proposed earlier for one-spin asymmetries in
inclusive hadron production we consider OZI--suppressed process of
-meson production and asymmetry
in this process. The main role in generation of this asymmetry
belongs to the orbital angular momentum of strange quark-antiquark cloud in
internal structure of constituent quarks.Comment: 8 pages, plain LaTeX, no figure
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
Oscillations of the magnetic polarization in a Kondo impurity at finite magnetic fields
The electronic properties of a Kondo impurity are investigated in a magnetic
field using linear response theory. The distribution of electrical charge and
magnetic polarization are calculated in real space. The (small) magnetic field
does not change the charge distribution. However, it unmasks the Kondo cloud.
The (equal) weight of the d-electron components with their magnetic moment up
and down is shifted and the compensating s-electron clouds don't cancel any
longer (a requirement for an experimental detection of the Kondo cloud). In
addition to the net magnetic polarization of the conduction electrons an
oscillating magnetic polarization with a period of half the Fermi wave length
is observed. However, this oscillating magnetic polarization does not show the
long range behavior of Rudermann-Kittel-Kasuya-Yosida oscillations because the
oscillations don't extend beyond the Kondo radius. They represent an internal
electronic structure of the Kondo impurity in a magnetic field. PACS: 75.20.Hr,
71.23.An, 71.27.+
The Hubbard model with smooth boundary conditions
We apply recently developed smooth boundary conditions to the quantum Monte
Carlo simulation of the two-dimensional Hubbard model. At half-filling, where
there is no sign problem, we show that the thermodynamic limit is reached more
rapidly with smooth rather than with periodic or open boundary conditions. Away
from half-filling, where ordinarily the simulation cannot be carried out at low
temperatures due to the existence of the sign problem, we show that smooth
boundary conditions allow us to reach significantly lower temperatures. We
examine pairing correlation functions away from half-filling in order to
determine the possible existence of a superconducting state. On a
lattice for , at a filling of and an inverse
temperature of , we did find enhancement of the -wave correlations
with respect to the non-interacting case, a possible sign of -wave
superconductivity.Comment: 16 pages RevTeX, 9 postscript figures included (Figure 1 will be
faxed on request
Asymptotically exact mean field theory for the Anderson model including double occupancy
The Anderson impurity model for finite values of the Coulomb repulsion is
studied using a slave boson representation for the empty and doubly occupied
-level. In order to avoid well known problems with a naive mean field theory
for the boson fields, we use the coherent state path integral representation to
first integrate out the double occupancy slave bosons. The resulting effective
action is linearized using {\bf two-time} auxiliary fields. After integration
over the fermionic degrees of freedom one obtains an effective action suitable
for a -expansion. Concerning the constraint the same problem remains as
in the infinite case. For and
exact results for the ground state properties are recovered in the saddle point
approximation. Numerical solutions of the saddle point equations show that even
in the spindegenerate case the results are quite good.Comment: 19, RevTeX, cond-mat/930502
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