5,416 research outputs found
Negative Pressures in QED Vacuum in an External Magnetic Field
Our aim is to study the electron-positron vacuum pressures in presence of a
strong magnetic field . To that end, we obtain a general energy-momentum
tensor, depending on external parameters, which in the zero temperature and
zero density limit leads to vacuum expressions which are
approximation-independent. Anisotropic pressures arise, and in the tree
approximation of the magnetic field case, the pressure along is positive,
whereas perpendicular to it is negative. Due to the common axial symmetry,
the formal analogy with the Casimir effect is discussed, for which in addition
to the usual negative pressure perpendicular to the plates, there is a positive
pressure along the plates. The formal correspondence between the Casimir and
black body energy-momentum tensors is analyzed. The fermion hot vacuum behavior
in a magnetic field is also briefly discussed
Remark on charge conjugation in the non relativistic limit
We study the non relativistic limit of the charge conjugation operation in the context of the Dirac equation coupled to an electromagnetic field.
The limit is well defined and, as in the relativistic case, ,
(parity) and (time reversal) are the generators of a matrix group
isomorphic to a semidirect sum of the dihedral group of eight elements and
. The existence of the limit is supported by an argument based in quantum
field theory. Also, and most important, the limit exists in the context of
galilean relativity. Finally, if one complexifies the Lorentz group and
therefore the galilean spacetime , then the explicit form of the matrix
for allows to interpret it, in this context, as the complex
conjugation of the spatial coordinates: . This result is
natural in a fiber bundle description.Comment: 8 page
Electric field of a pointlike charge in a strong magnetic field and ground state of a hydrogenlike atom
In an external constant magnetic field, so strong that the electron Larmour
length is much shorter than its Compton length, we consider the modification of
the Coulomb potential of a point charge owing to the vacuum polarization. We
establish a short-range component of the static interaction in the Larmour
scale, expressed as a Yukawa-like law, and reveal the corresponding "photon
mass" parameter. The electrostatic force regains its long-range character in
the Compton scale: the tail of the potential follows an anisotropic Coulomb
law, decreasing away from the charge slower along the magnetic field and faster
across. In the infinite-magnetic-field limit the potential is confined to an
infinitely thin string passing though the charge parallel to the external
field. This is the first evidence for dimensional reduction in the photon
sector of quantum electrodynamics. The one-dimensional form of the potential on
the string is derived that includes a delta-function centered in the charge.
The nonrelativistic ground-state energy of a hydrogenlike atom is found with
its use and shown not to be infinite in the infinite-field limit, contrary to
what was commonly accepted before, when the vacuum polarization had been
ignored. These results may be useful for studying properties of matter at the
surface of extremely magnetized neutron stars.Comment: 45 pages, 6 figures, accepted to Phys. Rev.
Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays
We study the role of dissipation and structural defects on the time evolution
of quantum dot arrays with mobile charges under external driving fields. These
structures, proposed as quantum dot cellular automata, exhibit interesting
quantum dynamics which we describe in terms of equations of motion for the
density matrix. Using an open system approach, we study the role of asymmetries
and the microscopic electron-phonon interaction on the general dynamical
behavior of the charge distribution (polarization) of such systems. We find
that the system response to the driving field is improved at low temperatures
(and/or weak phonon coupling), before deteriorating as temperature and
asymmetry increase. In addition to the study of the time evolution of
polarization, we explore the linear entropy of the system in order to gain
further insights into the competition between coherent evolution and
dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR
Size effects in the magnetic behaviour of TbAl_2 milled alloys
The study of the magnetic properties depending upon mechanical milling of the
ferromagnetic polycrystalline TbAl_2 material is reported. The Rietveld
analysis of the X-ray diffraction data reveals a decrease of the grain size
down to 14 nm and -0.15 % of variation of the lattice parameter, after 300
hours of milling time. Irreversibility in the zero field cooled - field cooled
(ZFC-FC) DC-susceptibility and clear peaks in the AC susceptibility between 5
and 300 K show that the long-range ferromagnetic structure is inhibited in
favour of a disordered spin arrangement below 45 K. This glassy behaviour is
also deduced from the variation of the irreversibility transition with the
field (H^{2/3}) and frequency. The magnetization process of the bulk TbAl_2 is
governed by domain wall thermal activation processes. By contrast, in the
milled samples, cluster-glass properties arise as a result of cooperative
interactions due to the substitutional disorder. The interactions are also
influenced by the nanograin structure of the milled alloys, showing a variation
of coercivity with the grain size, below the crossover between the multi- and
single-domain behaviours.Comment: 23 pages, 11 figures, to appear in J. Phys.: Condens. Ma
Quantum Phase Transitions in Josephson Junction Chains
We investigate the quantum phase transition in a one-dimensional chain of
ultra-small superconducting grains, considering both the self- and junction
capacitances. At zero temperature, the system is transformed into a
two-dimensional system of classical vortices, where the junction capacitance
introduces anisotropy in the interaction between vortices. This leads to the
superconductor-insulator transition of the Berezinskii-Kosterlitz-Thouless
type, as the ratios of the Josephson coupling energy to the charging energies
are varied. It is found that the junction capacitance plays a role similar to
that of dissipation and tends to suppress quantum fluctuations; nevertheless
the insulator region survives even for arbitrarily large values of the junction
capacitance.Comment: REVTeX+5 EPS figures, To appear in PRB Rapid
HATS-17b: A Transiting Compact Warm Jupiter in a 16.3 Days Circular Orbit
We report the discovery of HATS-17b, the first transiting warm Jupiter of the
HATSouth network. HATS-17b transits its bright (V=12.4) G-type
(M=1.131 0.030 M,
R=1.091 R) metal-rich ([Fe/H]=+0.3 dex)
host star in a circular orbit with a period of P=16.2546 days. HATS-17b has a
very compact radius of 0.777 0.056 R given its Jupiter-like mass of
1.338 0.065 M. Up to 50% of the mass of HATS-17b may be composed of
heavy elements in order to explain its high density with current models of
planetary structure. HATS-17b is the longest period transiting planet
discovered to date by a ground-based photometric survey, and is one of the
brightest transiting warm Jupiter systems known. The brightness of HATS-17b
will allow detailed follow-up observations to characterize the orbital geometry
of the system and the atmosphere of the planet.Comment: 12 page, 8 figures, submitted to A
CaB_6: a new semiconducting material for spin electronics
Ferromagnetism was recently observed at unexpectedly high temperatures in
La-doped CaB_6. The starting point of all theoretical proposals to explain this
observation is a semimetallic electronic structure calculated for CaB_6 within
the local density approximation. Here we report the results of parameter-free
quasiparticle calculations of the single-particle excitation spectrum which
show that CaB_6 is not a semimetal but a semiconductor with a band gap of 0.8
eV. Magnetism in La_xCa_{1-x}B_6 occurs just on the metallic side of a Mott
transition in the La-induced impurity band.Comment: 4 pages, 1 postscript figur
Mean Field Theory of Josephson Junction Arrays with Charge Frustration
Using the path integral approach, we provide an explicit derivation of the
equation for the phase boundary for quantum Josephson junction arrays with
offset charges and non-diagonal capacitance matrix. For the model with nearest
neighbor capacitance matrix and uniform offset charge , we determine,
in the low critical temperature expansion, the most relevant contributions to
the equation for the phase boundary. We explicitly construct the charge
distributions on the lattice corresponding to the lowest energies. We find a
reentrant behavior even with a short ranged interaction. A merit of the path
integral approach is that it allows to provide an elegant derivation of the
Ginzburg-Landau free energy for a general model with charge frustration and
non-diagonal capacitance matrix. The partition function factorizes as a product
of a topological term, depending only on a set of integers, and a
non-topological one, which is explicitly evaluated.Comment: LaTex, 24 pages, 8 figure
Reduction of the Yb valence in YbAl3 nanoparticles
Measurements of specific heat, dc magnetic susceptibility, and Yb LII and LIII x-ray absorption near-edge
structure XANES and extended x-ray absorption fine structure EXAFS on YbAl3 milled alloys are reported.
X-ray diffraction patterns are consistent with a reduction in particle size down to 10 nm and an increase in the
lattice strain up to 0.4% for 120 h of milling time. A decrease in the mean valence from 2.86 for the unmilled
alloy to 2.70 for 120 h milled YbAl3 is obtained from the analysis of XANES spectra. From the analysis of
spectra in the EXAFS region, an increase in the mean-square disorder of neighbor distance with milling time
is detected in good agreement with the results of x-ray diffraction. Size effects strongly influence the magnetic
and thermal properties. The value for the maximum of the magnetic susceptibility decreases around 30% for
120 h milled alloy and an excess specific heat, with a peak around 40 K in the milled samples, is derived.
These changes in the physical properties along the milled YbAl3 alloys are associated with the reduction in
particle size. Such a reduction leads to the existence of a large number of Yb2+ atoms at the surface with
respect to the bulk affecting the overall electronic state
- …