1,461 research outputs found
Stick-slip instability for viscous fingering in a gel
The growth dynamics of an air finger injected in a visco-elastic gel (a
PVA/borax aqueous solution) is studied in a linear Hele-Shaw cell. Besides the
standard Saffmann-Taylor instability, we observe - with increasing finger
velocities - the existence of two new regimes: (a) a stick-slip regime for
which the finger tip velocity oscillates between 2 different values, producing
local pinching of the finger at regular intervals, (b) a ``tadpole'' regime
where a fracture-type propagation is observed. A scaling argument is proposed
to interpret the dependence of the stick-slip frequency with the measured
rheological properties of the gel.Comment: 7 pages, 4 figures. Submitted to Europhysics Letter
Failure time and microcrack nucleation
The failure time of samples of heterogeneous materials (wood, fiberglass) is
studied as a function of the applied stress. It is shown that in these
materials the failure time is predicted with a good accuracy by a model of
microcrack nucleation proposed by Pomeau. It is also shown that the crack
growth process presents critical features when the failure time is approached.Comment: 13 pages, 4 figures, submitted to Europhysics Letter
Compensated electron and hole pockets in an underdoped high Tc superconductor
We report quantum oscillations in the underdoped high Tc YBCO over a wide
range in magnetic field 28<B<85 T corresponding to ~12 oscillations, enabling
the Fermi surface topology to be mapped to high resolution. As earlier reported
by Sebastian et al., we find a Fermi surface comprising multiple pockets, as
revealed by the additional distinct quantum oscillation frequencies and
harmonics reported in this work. We find the originally reported broad low
frequency Fourier peak at 535 T to be clearly resolved into three separate
peaks at 460 T, 532 T and 602 T. Our increased resolution and angle-resolved
measurements identify these frequencies to originate from two similarly sized
pockets with greatly contrasting degrees of interlayer corrugation. The
spectrally dominant frequency originates from a pocket (\alpha) that is almost
ideally two-dimensional in form. In contrast, the newly resolved weaker
adjacent spectral features originate from a deeply corrugated pocket (\gamma).
On comparison with band structure, the d-wave symmetry of the interlayer
dispersion locates the minimally corrugated \alpha pocket at the 'nodal' point
where holes are located in a translational symmetry-broken scenario, and the
significantly corrugated \gamma pocket at the 'antinodal' point in the
Brillouin zone, where electrons are located in a translational symmetry-broken
scenario. Translational symmetry breaking by an SDW is suggested from the
strong suppression of Zeeman splitting for the spectrally dominant pocket,
additional evidence for which is provided from the harmonics we resolve in the
present experiments. Given the similarity in \alpha and \gamma pocket sizes,
their opposite carrier type and the previous report of a diverging effective
mass, we discuss the possibility of a secondary Fermi surface instability at
low dopings of the excitonic insulator type, associated with the
metal-insulator QCP.Comment: Expanded abstract, added two new figures to establish the enhanced
resolution of the current measurements and provide further support for
earlier reported suppression of Zeeman splitting
Label-Free 3D Imaging of Development of Cell Patterns in Drosophila melanogaster Wing Imaginal Disc
Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7-August 11, 201
Specific Heat of Zn-Doped YBa_{2}Cu_3O_{6.95}: Possible Evidence for Kondo Screening in the Superconducting State
The magnetic field dependence of the specific heat of Zn-doped single
crystals of YBa_{2}Cu_3O_{6.95} was measured between 2 and 10 K and up to 8
Tesla. Doping levels of 0, 0.15%, 0.31%, and 1% were studied and compared. In
particular we searched for the Schottky anomaly associated with the Zn-induced
magnetic moments.Comment: 5 pages, 6 figure
Phenomenology of a-axis and b-axis charge dynamics from microwave spectroscopy of highly ordered YBa2Cu3O6.50 and YBa2Cu3O6.993
Extensive measurements of the microwave conductivity of highly pure and
oxygen-ordered \YBCO single crystals have been performed as a means of
exploring the intrinsic charge dynamics of a d-wave superconductor. Broadband
and fixed-frequency microwave apparatus together provide a very clear picture
of the electrodynamics of the superconducting condensate and its thermally
excited nodal quasiparticles. The measurements reveal the existence of very
long-lived excitations deep in the superconducting state, as evidenced by sharp
cusp-like conductivity spectra with widths that fall well within our
experimental bandwidth. We present a phenomenological model of the microwave
conductivity that captures the physics of energy-dependent quasiparticle
dynamics in a d-wave superconductor which, in turn, allows us to examine the
scattering rate and oscillator strength of the thermally excited quasiparticles
as functions of temperature. Our results are in close agreement with the
Ferrell-Glover-Tinkham sum rule, giving confidence in both our experiments and
the phenomenological model. Separate experiments for currents along the and directions of detwinned crystals allow us to isolate the role
of the CuO chain layers in \YBCO, and a model is presented that incorporates
both one-dimensional conduction from the chain electrons and two-dimensional
transport associated with the \cuplane plane layers.Comment: 17 pages, 13 figure
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