6,886 research outputs found
Quantum stabilization of Z-strings, a status report on D=3+1 dimensions
We investigate an extension to the phase shift formalism for calculating
one-loop determinants. This extension is motivated by requirements of the
computation of Z-string quantum energies in D=3+1 dimensions. A subtlety that
seems to imply that the vacuum polarization diagram in this formalism is
(erroneously) finite is thoroughly investigated.Comment: Based on talk by O.S. at QFEXT07, Leipzig Sept. 2007. 8 page
Effective potential for Polyakov loops from a center symmetric effective theory in three dimensions
We present lattice simulations of a center symmetric dimensionally reduced
effective field theory for SU(2) Yang Mills which employ thermal Wilson lines
and three-dimensional magnetic fields as fundamental degrees of freedom. The
action is composed of a gauge invariant kinetic term, spatial gauge fields and
a potential for the Wilson line which includes a "fuzzy" bag term to generate
non-perturbative fluctuations. The effective potential for the Polyakov loop is
extracted from the simulations including all modes of the loop as well as for
cooled configuration where the hard modes have been averaged out. The former is
found to exhibit a non-analytic contribution while the latter can be described
by a mean-field like ansatz with quadratic and quartic terms, plus a
Vandermonde potential which depends upon the location within the phase diagram.Comment: 10 pages, 22 figures, v2: published version (minor clarifications,
update of reference list
Thermoelectric Behaviour Near Magnetic Quantum Critical Point
We use the coupled 2d-spin-3d-fermion model proposed by Rosch {\sl et. al.}
(Phys. Rev. Lett. {\bf 79}, 159 (1997)) to study the thermoelectric behaviour
of a heavy fermion compound when it is close to an antiferromagnetic quantum
critical point. When the low energy spin fluctuations are quasi two
dimensional, as has been observed in and , with a typical 2d ordering wavevector and 3d Fermi
surface, the ``hot'' regions on the Fermi surface have a finite area. Due to
enhanced scattering with the nearly critical spin fluctuations, the electrons
in the hot region are strongly renormalized. We argue that there is an
intermediate energy scale where the qualitative aspects of the renormalized hot
electrons are captured by a weak-coupling perturbative calculation. Our
examination of the electron self energy shows that the entropy carried by the
hot electrons is larger than usual. This accounts for the anomalous logarithmic
temperature dependence of specific heat observed in these materials. We show
that the same mechanism produces logarithmic temperature dependence in
thermopower. This has been observed in . We
expect to see the same behaviour from future experiments on .Comment: RevTex, two-column, 7 pages, 2 figure
Optimal embedding of a toroidal mesh in a path
We prove that the dilation of an toroidal mesh in an -vertex path equals , if and , if
A mechanism for the non-Fermi-liquid behavior in CeCu_{6-x}Au_x
We propose an explanation for the recently observed non-Fermi-liquid behavior
of metallic alloys CeCu_{6-x}Au_x: near x=0.1, the specific heat c is
proportional to T \ln (T_0/T) and the resistivity increases linearly with
temperature T over a wide range of T. These features follow from a model in
which three-dimensional conduction electrons are coupled to two-dimensional
critical ferromagnetic fluctuations near the quantum critical point, x_{c}=0.1.
This picture is motivated by the neutron scattering data in the ordered phase
(x=0.2) and is consistent with the observed phase diagram.Comment: 4 pages, LaTeX, 3 figure
The Casimir Energy for a Hyperboloid Facing a Plate in the Optical Approximation
We study the Casimir energy of a massless scalar field that obeys Dirichlet
boundary conditions on a hyperboloid facing a plate. We use the optical
approximation including the first six reflections and compare the results with
the predictions of the proximity force approximation and the semi-classical
method. We also consider finite size effects by contrasting the infinite with a
finite plate. We find sizable and qualitative differences between the new
optical method and the more traditional approaches.Comment: v2: 14 pages, 11 eps figures; typo in eq. (21) removed, clarification
added, fig. 10 improved; version published in Phys. Rev.
Phase-dependent light propagation in atomic vapors
Light propagation in an atomic medium whose coupled electronic levels form a
diamond-configuration exhibits a critical dependence on the input conditions.
In particular, the relative phase of the input fields gives rise to
interference phenomena in the electronic excitation whose interplay with
relaxation processes determines the stationary state. We integrate numerically
the Maxwell-Bloch equations and observe two metastable behaviors for the
relative phase of the propagating fields corresponding to two possible
interference phenomena. These phenomena are associated to separate types of
response along propagation, minimize dissipation, and are due to atomic
coherence. These behaviors could be studied in gases of isotopes of
alkali-earth atoms with zero nuclear spin, and offer new perspectives in
control techniques in quantum electronics.Comment: 16 pages, 11 figures, v2: typos corrected, v3: final version, to
appear in Phys. Rev.
New method for the time calibration of an interferometric radio antenna array
Digital radio antenna arrays, like LOPES (LOFAR PrototypE Station), detect
high-energy cosmic rays via the radio emission from atmospheric extensive air
showers. LOPES is an array of dipole antennas placed within and triggered by
the KASCADE-Grande experiment on site of the Karlsruhe Institute of Technology,
Germany. The antennas are digitally combined to build a radio interferometer by
forming a beam into the air shower arrival direction which allows measurements
even at low signal-to-noise ratios in individual antennas. This technique
requires a precise time calibration. A combination of several calibration steps
is used to achieve the necessary timing accuracy of about 1 ns. The group
delays of the setup are measured, the frequency dependence of these delays
(dispersion) is corrected in the subsequent data analysis, and variations of
the delays with time are monitored. We use a transmitting reference antenna, a
beacon, which continuously emits sine waves at known frequencies. Variations of
the relative delays between the antennas can be detected and corrected for at
each recorded event by measuring the phases at the beacon frequencies.Comment: 9 pages, 9 figures, 1 table, pre-print of article published in
Nuclear Inst. and Methods in Physics Research, A, available at:
http://www.sciencedirect.com/science/article/B6TJM-4Y9CF4B-4/2/37bfcb899a0f387d9875a5a0729593a
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