1,197 research outputs found
B NMR detection of the magnetic field distribution in the mixed superconducting state of MgB
The temperature dependence of the magnetic field distribution in the mixed
superconducting phase of randomly oriented MgB powder was probed by B NMR spectroscopy. Below the temperature of the second critical () field, K, our spectra reveal two NMR signal
components, one mapping the magnetic field distribution in the mixed
superconducting state and the other one arising from the normal state. The
complementary use of bulk magnetization and NMR measurements reveals that
MgB is an anisotropic superconductor with a Tesla
anisotropy parameter
Electroweak pinch technique to all orders
The generalization of the pinch technique to all orders in the electroweak
sector of the Standard Model within the class of the renormalizable 't Hooft
gauges, is presented. In particular, both the all-order PT gauge-boson-- and
scalar--fermions vertices, as well as the diagonal and mixed gauge-boson and
scalar self-energies are explicitly constructed. This is achieved through the
generalization to the Standard Model of the procedure recently applied to the
QCD case, which consist of two steps: (i) the identification of special Green's
functions, which serve as a common kernel to all self-energy and vertex
diagrams, and (ii) the study of the (on-shell) Slavnov-Taylor identities they
satisfy. It is then shown that the ghost, scalar and scalar--gauge-boson
Green's functions appearing in these identities capture precisely the result of
the pinching action at arbitrary order. It turns out that the aforementioned
Green's functions play a crucial role, their net effect being the non-trivial
modification of the ghost, scalar and scalar--gauge-boson diagrams of the
gauge-boson-- or scalar--fermions vertex we have started from, in such a way as
to dynamically generate the characteristic ghost and scalar sector of the
background field method. The pinch technique gauge-boson and scalar
self-energies are also explicitly constructed by resorting to the method of the
background-quantum identities.Comment: 48 pages, 8 figures; v2: typos correcte
Low-Energy Constraints on New Physics Revisited
It is possible to place constraints on non-Standard-Model gauge-boson
self-couplings and other new physics by studying their one-loop contributions
to precisely measured observables. We extend previous analyses which constrain
such nonstandard couplings, and we present the results in a compact and
transparent form. Particular attention is given to comparing results for the
light-Higgs scenario, where nonstandard effects are parameterized by an
effective Lagrangian with a linear realization of the electroweak symmetry
breaking sector, and the heavy-Higgs/strongly interacting scenario, described
by the electroweak chiral Lagrangian. The constraints on nonstandard
gauge-boson self-couplings which are obtained from a global analysis of
low-energy data and LEP/SLC measurements on the Z pole are updated and improved
from previous studies. Replaced version: tables and figures of Section VIb
recalculated. There were roundoff problems, especially in Fig. 8. Text
unchanged.Comment: \documentstyle[preprint,aps,floats,psfig]{revtex}, 10 figures,
postscript version available from ftp://ftp.kek.jp/kek/preprints/TH/TH-51
Magnetic field-dependent interplay between incoherent and Fermi liquid transport mechanisms in low-dimensional tau phase organic conductors
We present an electrical transport study of the 2-dimensional (2D) organic
conductor tau-(P-(S,S)-DMEDT-TTF)_2(AuBr)_2(AuBr_2)_y (y = 0.75) at low
temperatures and high magnetic fields. The inter-plane resistivity rho_zz
increases with decreasing temperature, with the exception of a slight anomaly
at 12 K. Under a magnetic field B, both rho_zz and the in-plane resistivity
plane rho_xx show a pronounced negative and hysteretic magnetoresistance with
Shubnikov de Haas (SdH)oscillations being observed in some (high
quality)samples above 15 T. Contrary to the predicted single, star-shaped,
closed orbit Fermi surface from band structure calculations (with an expected
approximate area of 12.5% of A_FBZ), two fundamental frequencies F_l and F_h
are detected in the SdH signal. These orbits correspond to 2.4% and 6.8% of the
area of the first Brillouin zone(A_FBZ), with effective masses F_l = 4.0 +/-
0.5 and F_h = 7.3 +/- 0.1. The angular dependence, in tilted magnetic fields of
F_l and F_h, reveals the 2D character of the FS and Angular dependent
magnetoresistance (AMRO) further suggests a FS which is strictly 2-D where the
inter-plane hopping t_c is virtually absent or incoherent. The Hall constant
R_xy is field independent, and the Hall mobility increases by a factor of 3
under moderate magnetic fields. Our observations suggest a unique physical
situation where a stable 2D Fermi liquid state in the molecular layers are
incoherently coupled along the least conducting direction. The magnetic field
not only reduces the inelastic scattering between the 2D metallic layers, but
it also reveals the incoherent nature of interplane transport in the AMRO
spectrum. The apparent ferromagnetism of the hysteretic magnetoresistance
remains an unsolved problem.Comment: 33 pages, 11 figure
The pinch technique at two-loops: The case of mass-less Yang-Mills theories
The generalization of the pinch technique beyond one loop is presented. It is
shown that the crucial physical principles of gauge-invariance, unitarity, and
gauge-fixing-parameter independence single out at two loops exactly the same
algorithm which has been used to define the pinch technique at one loop,
without any additional assumptions. The two-loop construction of the pinch
technique gluon self-energy, and quark-gluon vertex are carried out in detail
for the case of mass-less Yang-Mills theories, such as perturbative QCD. We
present two different but complementary derivations. First we carry out the
construction by directly rearranging two-loop diagrams. The analysis reveals
that, quite interestingly, the well-known one-loop correspondence between the
pinch technique and the background field method in the Feynman gauge persists
also at two-loops. The renormalization is discussed in detail, and is shown to
respect the aforementioned correspondence. Second, we present an absorptive
derivation, exploiting the unitarity of the -matrix and the underlying BRS
symmetry; at this stage we deal only with tree-level and one-loop physical
amplitudes. The gauge-invariant sub-amplitudes defined by means of this
absorptive construction correspond precisely to the imaginary parts of the
-point functions defined in the full two-loop derivation, thus furnishing a
highly non-trivial self-consistency check for the entire method. Various future
applications are briefly discussed.Comment: 29 pages, uses Revtex, 22 Figures in a separate ps fil
Gluon and ghost propagators in the Landau gauge: Deriving lattice results from Schwinger-Dyson equations
We show that the application of a novel gauge invariant truncation scheme to
the Schwinger-Dyson equations of QCD leads, in the Landau gauge, to an infrared
finite gluon propagator and a divergent ghost propagator, in qualitative
agreement with recent lattice data.Comment: 9 pages, 2 figures; v3: typos corrected; v2: discussion on numerical
results expanded, considerations about the Kugo-Ojima confinement criterion
adde
- …