  We first report that, for planar nematic MBBA, the electroconvection
threshold voltage has a nonmonotonic temperature dependence, with a
well-defined minimum, and a slope of about 0.12 V/degree near room temperature.
Motivated by this observation, we have designed an experiment in which a weak
continuous-wave absorbed laser beam with a diameter comparable to the pattern
wavelength generates a locally supercritical region, or pulse, in dye-doped
MBBA. Working 10-20% below the laser-free threshold voltage, we observe a
steady-state pulse shaped as an ellipse with the semimajor axis oriented
parallel to the nematic director, with a typical size of several wavelengths.
The pulse is robust, persisting even when spatially extended rolls develop in
the surrounding region, and displays rolls that counterpropagate along the
director at frequencies of tenths of Hz, with the rolls on the left (right)
side of the ellipse moving to the right (left). Systematic measurements of the
sample-voltage dependence of the pulse amplitude, spatial extent, and frequency
show a saturation or decrease when the control parameter (evaluated at the
center of the pulse) approaches ~ 0.3. We propose that the model for these
pulses should be based on the theory of control-parameter ramps, supplemented
with new terms to account for the advection of heat away from the pulse when
the surrounding state becomes linearly unstable. The advection creates a
negative feedback between the pulse size and the efficiency of heat transport,
which we argue is responsible for the attenuation of the pulse at larger
control-parameter values.
