1,365 research outputs found
Singularities of the renormalization group flow for random elastic manifolds
We consider the singularities of the zero temperature renormalization group
flow for random elastic manifolds. When starting from small scales, this flow
goes through two particular points and , where the average value
of the random squared potential turnes negative ($l^{*}$) and where
the fourth derivative of the potential correlator becomes infinite at the
origin ($l_{c}$). The latter point sets the scale where simple perturbation
theory breaks down as a consequence of the competition between many metastable
states. We show that under physically well defined circumstances $l_{c} to negative values does not
take place.Comment: RevTeX, 3 page
The Amplitude Mode in the Quantum Phase Model
We derive the collective low energy excitations of the quantum phase model of
interacting lattice bosons within the superfluid state using a dynamical
variational approach. We recover the well known sound (or Goldstone) mode and
derive a gapped (Higgs type) mode that was overlooked in previous studies of
the quantum phase model. This mode is relevant to ultracold atoms in a strong
optical lattice potential. We predict the signature of the gapped mode in
lattice modulation experiments and show how it evolves with increasing
interaction strength.Comment: 4 pages, 3 figure
Characteristics of First-Order Vortex Lattice Melting: Jumps in Entropy and Magnetization
We derive expressions for the jumps in entropy and magnetization
characterizing the first-order melting transition of a flux line lattice. In
our analysis we account for the temperature dependence of the Landau parameters
and make use of the proper shape of the melting line as determined by the
relative importance of electromagnetic and Josephson interactions. The results
agree well with experiments on anisotropic YBaCuO and
layered BiSrCaCuO materials and reaffirm the validity of
the London model.Comment: 4 pages. We have restructured the paper to emphasize that in the
London scaling regime (appropriate for YBCO) our results are essentially
exact. We have also emphasized that a major controversy over the relevance of
the London model to describe VL melting has been settled by this wor
Weak- to strong pinning crossover
Material defects in hard type II superconductors pin the flux lines and thus
establish the dissipation-free current transport in the presence of a finite
magnetic field. Depending on the density and pinning force of the defects and
the vortex density, pinning is either weak-collective or strong. We analyze the
weak- to strong pinning crossover of vortex matter in disordered
superconductors and discuss the peak effect appearing naturally in this
context.Comment: 4 pages, 2 figure
Angular dependence of the magnetization of isotropic superconductors: which is the vortex direction?
We present studies of the dc magnetization of thin platelike samples of the
isotropic type II superconductor PbTl(10%), as a function of the angle between
the normal to the sample and the applied magnetic field . We determine
the magnetization vector by measuring the components both parallel
and normal to in a SQUID magnetometer, and we further decompose it in
its reversible and irreversible contributions. The behavior of the reversible
magnetization is well understood in terms of minimization of the free energy
taking into account geometrical effects. In the mixed state at low fields, the
dominant effect is the line energy gained by shortening the vortices, thus the
flux lines are almost normal to the sample surface. Due to the geometrical
constrain, the irreversible magnetization remains locked to the
sample normal over a wide range of fields and orientations, as already known.
We show that in order to undestand the angle and field dependence of the
modulus of , which is a measure of the vortex pinning, and to
correctly extract the field dependent critical current density, the knowledge
of the modulus and orientation of the induction field is required.Comment: 11 pages, 6 figure
Edge Tunneling of Vortices in Superconducting Thin Films
We investigate the phenomenon of the decay of a supercurrent due to the
zero-temperature quantum tunneling of vortices from the edge in a thin
superconducting film in the absence of an external magnetic field. An explicit
formula is derived for the tunneling rate of vortices, which are subject to the
Magnus force induced by the supercurrent, through the Coulomb-like potential
barrier binding them to the film's edge. Our approach ensues from the
non-relativistic version of a Schwinger-type calculation for the decay of the
2D vacuum previously employed for describing vortex-antivortex pair-nucleation
in the bulk of the sample. In the dissipation-dominated limit, our explicit
edge-tunneling formula yields numerical estimates which are compared with those
obtained for bulk-nucleation to show that both mechanisms are possible for the
decay of a supercurrent.Comment: REVTeX file, 15 pages, 1 Postscript figure; to appear in Phys.Rev.
Asymmetric spin echo sequences. a simple new method for obtaining NMR 1H spectral images
Journal ArticleThe nuclear magnetic resonance (NMR) signal decay produced by reversible tissue-induced dephasing of the magnetization components in the transverse plane (reversible tissue-induced dephasing) was measured and expressed as a function of a new transverse relaxation time T'2 (T2 prime) for samples of rat liver, retroperitoneal fat, inflated lung, and corn oil. Simple exponentials did not adequately describe the observed NMR signal decay. Inflated lung demonstrated the most rapid signal decay (T'2 = 4.8 ms) followed by retroperitoneal fat (T'2 = 16 ms). No reversible tissue-induced dephasing was observed in liver (T'2 immeasurably long). In tissues which contain both fat and water, the chemically shifted 'H resonance peaks from -OH and -CH- are in phase with symmetric spin echo sequences but out of phase with asymmetric sequences. The interference of these two peaks produces a beat pattern with asymmetric sequences. Subtraction images obtained from paired symmetric- and asymmetric-sequence images accurately (r = .96) reflect T'2 and can be used to indicate the presence of fat. In vivo subtraction images of ethionine-induced fatty rat livers were significantly different from similar in vivo images of normal rat livers (P<.0005). Since for each pixel of a subtraction image, the magnitude of the difference signal should be approximately proportional to the ratio of hydroxyl and alkyl protons, this simple spin echo sequence modification may obviate the need for more timeconsuming 3-dimensional Fourier transform proton chemical shift images
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