531 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
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
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.+
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
A numerical study of multi-soliton configurations in a doped antiferromagnetic Mott insulator
We evaluate from first principles the self-consistent Hartree-Fock energies
for multi-soliton configurations in a doped, spin-1/2, antiferromagnetic Mott
insulator on a two-dimensional square lattice. We find that nearest-neighbor
Coulomb repulsion stabilizes a regime of charged meron-antimeron vortex soliton
pairs over a region of doping from 0.05 to 0.4 holes per site for intermediate
coupling 3 < U/t <8. This stabilization is mediated through the generation of
``spin-flux'' in the mean-field antiferromagnetic (AFM) background. Holes
cloaked by a meron-vortex in the spin-flux AFM background are charged bosons.
Our static Hartree-Fock calculations provide an upper bound on the energy of a
finite density of charged vortices. This upper bound is lower than the energy
of the corresponding charged stripe configurations. A finite density of charge
carrying vortices is shown to produce a large number of unoccupied electronic
levels in the Mott-Hubbard charge transfer gap. These levels lead to
significant band tailing and a broad mid-infrared band in the optical
absorption spectrum as observed experimentally. At very low doping (below 0.05)
the doping charges create extremely tightly bound meron-antimeron pairs or even
isolated conventional spin-polarons, whereas for very high doping (above 0.4)
the spin background itself becomes unstable to formation of a conventional
Fermi liquid and the spin-flux mean-field is energetically unfavorable. Our
results point to the predominance of a quantum liquid of charged, bosonic,
vortex solitons at intermediate coupling and intermediate doping
concentrations.Comment: 12 pages, 25 figures; added references, modified/eliminated some
figure
On magnetic catalysis in even-flavor QED3
In this paper, we discuss the role of an external magnetic field on the
dynamically generated fermion mass in even-flavor QED in three space-time
dimensions. Based on some reasonable approximations, we present analytic
arguments on the fact that, for weak fields, the magnetically-induced mass
increases quadratically with increasing field, while at strong fields one
crosses over to a mass scaling logarithmically with the external field. We also
confirm this type of scaling behavior through quenched lattice calculations
using the non-compact version for the gauge field. Both the zero and finite
temperature cases are examined. A preliminary study of the fermion condensate
in the presence of magnetic flux tubes on the lattice is also included.Comment: 38 pages latex, 18 figures and a style file (axodraw) incorporated
(some clarifying remarks concerning the validity of the approximations made
and some references were added correcting an earlier version; no effect on
conclusions; version to appear in Phys. Rev. D.
Plasmonic excitations in noble metals: The case of Ag
The delicate interplay between plasmonic excitations and interband
transitions in noble metals is described by means of {\it ab initio}
calculations and a simple model in which the conduction electron plasmon is
coupled to the continuum of electron-hole pairs. Band structure effects,
specially the energy at which the excitation of the -like bands takes place,
determine the existence of a subthreshold plasmonic mode, which manifests
itself in Ag as a sharp resonance at 3.8 eV. However, such a resonance is not
observed in the other noble metals. Here, this different behavior is also
analyzed and an explanation is provided.Comment: 9 pages, 8 figure
Using patient-reported outcome measures during the management of patients with end-stage kidney disease requiring treatment with haemodialysis (PROM-HD): a qualitative study
Objectives Patients undergoing haemodialysis report elevated symptoms and reduced health-related quality of life, and often prioritise improvements in psychosocial well-being over long-term survival. Systematic collection and use of patient-reported outcomes (PROs) may help support tailored healthcare and improve outcomes. This study investigates the methodological basis for routine PRO assessment, particularly using electronic formats (ePROs), to maximise the potential of PRO use, through exploration of the experiences, views and perceptions of patients and healthcare professionals (HCPs) on implementation and use of PROs in haemodialysis settings.
Study design Qualitative study.
Setting and participants Semistructured interviews with 22 patients undergoing haemodialysis, and 17 HCPs in the UK.
Analytical approach Transcripts were analysed deductively using the Consolidated Framework for Implementation Research (CFIR) and inductively using thematic analysis.
Results For effective implementation, the potential value of PROs needs to be demonstrated empirically to stakeholders. Any intervention must remain flexible enough for individual and aggregate use, measuring outcomes that matter to patients and clinicians, while maintaining operational simplicity. Any implementation must sit within a wider framework of education and support for both patients and clinicians who demonstrate varying previous experience of using PROs and often confuse related concepts. Implementation plans must recognise the multidimensionality of end-stage kidney disease and treatment by haemodialysis, while acknowledging the associated challenges of delivering care in a highly specialised environment. To support implementation, careful consideration needs to be given to barriers and facilitators including effective leadership, the role of champions, effective launch and ongoing evaluation.
Conclusions Using the CFIR to explore the experiences, views and perceptions of key stakeholders, this study identified key factors at organisational and individual levels which could assist effective implementation of ePROs in haemodialysis settings. Further research will be required to evaluate subsequent ePRO interventions to demonstrate the impact and benefit to the dialysis community
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