944 research outputs found
Renormalization Group Study of Magnetic Catalysis in the 3d Gross-Neveu Model
Magnetic catalysis describes the enhancement of symmetry breaking quantum
fluctuations in chirally symmetric quantum field theories by the coupling of
fermionic degrees of freedom to a magnetic background configuration. We use the
functional renormalization group to investigate this phenomenon for interacting
Dirac fermions propagating in (2+1)-dimensional spacetime, described by the
Gross-Neveu model. We identify pointlike operators up to quartic fermionic
terms that can be generated in the renormalization group flow by the presence
of an external magnetic field. We employ the beta function for the fermionic
coupling to quantitatively analyze the field dependence of the induced spectral
gap. Within our pointlike truncation, the renormalization group flow provides a
simple picture for magnetic catalysis.Comment: 14 pages, 6 figures, typos correcte
Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary system phi Persei
Stripped-envelope stars (SESs) form in binary systems after losing mass
through Roche-lobe overflow. They bear astrophysical significance as sources of
UV and ionizing radiation in older stellar populations and, if sufficiently
massive, as stripped supernova progenitors. Binary evolutionary models predict
them to be common, but only a handful of subdwarfs (i.e., SESs) with B-type
companions are known. This could be the result of observational biases
hindering detection, or an incorrect understanding of binary evolution. We
reanalyze the well-studied post-interaction binary phi Persei. Recently, new
data improved the orbital solution of the system, which contains a ~1.2 Msun
SES and a rapidly rotating ~9.6 Msun Be star. We compare with an extensive grid
of evolutionary models using a Bayesian approach and find initial masses of the
progenitor of 7.2+/-0.4 Msun for the SES and 3.8+/-0.4 Msun for the Be star.
The system must have evolved through near-conservative mass transfer. These
findings are consistent with earlier studies. The age we obtain, 57+/-9 Myr, is
in excellent agreement with the age of the alpha Persei cluster. We note that
neither star was initially massive enough to produce a core-collapse supernova,
but mass exchange pushed the Be star above the mass threshold. We find that the
subdwarf is overluminous for its mass by almost an order of magnitude, compared
to the expectations for a helium core burning star. We can only reconcile this
if the subdwarf is in a late phase of helium shell burning, which lasts only
2-3% of the total lifetime as a subdwarf. This could imply that up to ~50 less
evolved, dimmer subdwarfs exist for each system similar to phi Persei. Our
findings can be interpreted as a strong indication that a substantial
population of SESs indeed exists, but has so far evaded detection because of
observational biases and lack of large-scale systematic searches.Comment: 11 pages, 5 figures, accepted for publication in A&
Pressure-induced isostructural phase transition of metal-doped silicon clathrates
We propose an atomistic model for the pressure-induced isostructural phase
transition of metal-doped silicon clathrates, Ba8Si46 and K8Si46, that has been
observed at 14 GPa and 23 GPa, respectively. The model explains successfully
the equation of state, transition pressure, change of Raman spectra and
dependence on the doped cations as well as the effects of substituting Si(6c)
atoms with noble metals.Comment: 5 pages, two coumn, 5 figures. See http://www.iitaka.org/down.html
for more informatio
Spontaneous, collective coherence in driven, dissipative cavity arrays
We study an array of dissipative tunnel-coupled cavities, each interacting
with an incoherently pumped two-level emitter. For cavities in the lasing
regime, we find correlations between the light fields of distant cavities,
despite the dissipation and the incoherent nature of the pumping mechanism.
These correlations decay exponentially with distance for arrays in any
dimension but become increasingly long ranged with increasing photon tunneling
between adjacent cavities. The interaction-dominated and the
tunneling-dominated regimes show markedly different scaling of the correlation
length which always remains finite due to the finite photon trapping time. We
propose a series of observables to characterize the spontaneous build-up of
collective coherence in the system.Comment: 9 pages, 4 figures, including supplemental material (with 4 pages, 1
figure). This is a shorter version with some modifications in the
supplemental material (a gap in the proof was closed and calculations
significantly generalized and improved
Design of Hybrid Conductors for Electromagnetic Forming Coils
The use of hybrid coil turns made of steel (St) and copper (Cu) is originally motivated by the
increased mechanical strength compared to monolithic copper conductors. Due to the
differing electrical conductivities of the two materials, the hybrid design also changes the
current density distribution in the conductor cross section. This affects crucial process
parameters such as the magnetic pressure and the Joule heat losses.
The effect of the hybrid conductor design on the process efficiency is investigated. An
electromagnetic sheet metal forming operation using a one-turn coil with rectangular cross
section is used as reference case. The copper layer (CuCr1Zr) was deposited on a tool steel
substrate (X40CrMoV5-1) using a selective laser melting process. The copper layer
thickness is varied ranging from a monolithic steel conductor to a monolithic copper
conductor. The workpiece (EN AW-5083, t_w = 1 mm) is formed through a drawing ring so
that the final forming height is a qualitative measure for the process efficiency. The
experimental results prove that the efficiency in case of a properly designed hybrid
conductor can exceed the efficiency of a monolithic copper coil. The current density
distribution in the hybrid cross section is investigated by means of numerical simulations.
This way a deeper insight into the physical effects of a varying copper layer thickness is
gained. The results reveal that the optimum layer thickness is not just a function of the coil
cross section and the current frequency. It is also affected by the coil length and the
resistance of the pulse generator
Experimental Investigations on the Optimum Driver Configuration for Electromagnetic Sheet Metal Forming
Electromagnetic forming is a high speed forming process especially suitable for materials
with high electrical conductivity such as copper or aluminum. In case of materials with
comparatively low electrical conductivity (e.g. stainless steel or titanium) the use of
so-called driver sheets is a common approach. Various publications proved that this way
materials with low electrical conductivity and even non-conductive materials can be
formed. Although the use of driver sheets is common practice, there are no or only
contradicting recommendations regarding the optimum driver sheet configuration.
Based on experimental investigations of the electromagnetic sheet metal forming process,
this paper investigates the optimum material and thickness of the driver sheet. The results
prove that aluminum should be favored over copper as driver material. The optimum
driver thickness was found to be dependent on thickness and electrical conductivity of the
workpiece. Even in case of a workpiece made of aluminum the use of a driver sheet could
enhance the efficiency of the process
Strong laser fields as a probe for fundamental physics
Upcoming high-intensity laser systems will be able to probe the
quantum-induced nonlinear regime of electrodynamics. So far unobserved QED
phenomena such as the discovery of a nonlinear response of the quantum vacuum
to macroscopic electromagnetic fields can become accessible. In addition, such
laser systems provide for a flexible tool for investigating fundamental
physics. Primary goals consist in verifying so far unobserved QED phenomena.
Moreover, strong-field experiments can search for new light but weakly
interacting degrees of freedom and are thus complementary to accelerator-driven
experiments. I review recent developments in this field, focusing on photon
experiments in strong electromagnetic fields. The interaction of
particle-physics candidates with photons and external fields can be
parameterized by low-energy effective actions and typically predict
characteristic optical signatures. I perform first estimates of the accessible
new-physics parameter space of high-intensity laser facilities such as POLARIS
and ELI.Comment: 7 pages, Key Lecture at the ELI Workshop and School on "Fundamental
Physics with Ultra-High Fields", 9 September - 2 October 2008 at Frauenworth
Monastery, German
INTEGRAL observations of SS433, a supercritically accreting microquasar with hard spectrum
Observations of SS433 by INTEGRAL carried out in March -- May 2003 are
presented. SS433 is evidently detected on the INTEGRAL images of the
corresponding sky region in the energy bands 25-50 and 50-100 keV. The
precessional variability of the hard X-ray flux is clearly seen. The X-ray
eclipse caused by the binary orbital motion is also detected. A possible origin
of the hard continuum is briefly discussed.Comment: 5 pages, 6 figures. Accepted to A&A INTEGRAL special volum
Exact flow equation for bound states
We develop a formalism to describe the formation of bound states in quantum
field theory using an exact renormalization group flow equation. As a concrete
example we investigate a nonrelativistic field theory with instantaneous
interaction where the flow equations can be solved exactly. However, the
formalism is more general and can be applied to relativistic field theories, as
well. We also discuss expansion schemes that can be used to find approximate
solutions of the flow equations including the essential momentum dependence.Comment: 22 pages, references added, published versio
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