4,018 research outputs found
Continuum Theory of Polymer Crystallization
We present a kinetic model of crystal growth of polymers of finite molecular
weight. Experiments help to classify polymer crystallization broadly into two
kinetic regimes. One is observed in melts or in high molar mass polymer
solutions and is dominated by nucleation control with , where is the growth rate and is the super-cooling. The
other is observed in low molar mass solutions (as well as for small molecules)
and is diffusion controlled with , for small . Our
model unifies these two regimes in a single formalism. The model accounts for
the accumulation of polymer chains near the growth front and invokes an
entropic barrier theory to recover both limits of nucleation and diffusion
control. The basic theory applies to both melts and solutions, and we
numerically calculate the growth details of a single crystal in a dilute
solution. The effects of molecular weight and concentration are also determined
considering conventional polymer dynamics. Our theory shows that entropic
considerations, in addition to the traditional energetic arguments, can capture
general trends of a vast range of phenomenology. Unifying ideas on
crystallization from small molecules and from flexible polymer chains emerge
from our theory.Comment: 37 double-spaced pages including 8 figures, submitted to the Journal
of Chemical Physic
Cooling Techniques for Trapped Ions
This book chapter gives an introduction to, and an overview of, methods for
cooling trapped ions. The main addressees are researchers entering the field.
It is not intended as a comprehensive survey and historical account of the
extensive literature on this topic. We present the physical ideas behind
several cooling schemes, outline their mathematical description, and point to
relevant literature useful for a more in-depth study of this topic.Comment: Part of the Proceedings of the Les Houches Winter School on the
Physics with Trapped Charged Particles held in January 2012. References
updated in mid 201
Error-resistant Single Qubit Gates with Trapped Ions
Coherent operations constitutive for the implementation of single and
multi-qubit quantum gates with trapped ions are demonstrated that are robust
against variations in experimental parameters and intrinsically indeterministic
system parameters. In particular, pulses developed using optimal control theory
are demonstrated for the first time with trapped ions. Their performance as a
function of error parameters is systematically investigated and compared to
composite pulses.Comment: 5 pages 5 figure
Seeding of supercooled polyethylene with extended chain crystals
Seeding of supercooled polyethylene with extended chain crystal
Measuring entanglement in condensed matter systems
We show how entanglement may be quantified in spin and cold atom many-body
systems using standard experimental techniques only. The scheme requires no
assumptions on the state in the laboratory and a lower bound to the
entanglement can be read off directly from the scattering cross section of
Neutrons deflected from solid state samples or the time-of-flight distribution
of cold atoms in optical lattices, respectively. This removes a major obstacle
which so far has prevented the direct and quantitative experimental study of
genuine quantum correlations in many-body systems: The need for a full
characterization of the state to quantify the entanglement contained in it.
Instead, the scheme presented here relies solely on global measurements that
are routinely performed and is versatile enough to accommodate systems and
measurements different from the ones we exemplify in this work.Comment: 6 pages, 2 figure
Enhancement of laser cooling by the use of magnetic gradients
We present a laser cooling scheme for trapped ions and atoms using a
combination of laser couplings and a magnetic gradient field. In a
Schrieffer-Wolff transformed picture, this setup cancels the carrier and blue
sideband terms completely resulting in an improved cooling behaviour compared
to standard cooling schemes (e.g. sideband cooling) and allowing cooling to the
vibrational ground state. A condition for optimal cooling rates is presented
and the cooling behaviour for different Lamb-Dicke parameters and spontaneous
decay rates is discussed. Cooling rates of one order of magnitude less than the
trapping frequency are achieved using the new cooling method. Furthermore the
scheme turns out to be robust under deviations from the optimal parameters and
moreover provides good cooling rates also in the multi particle case.Comment: 14 pages, 8 figure
Current induced local spin polarization due to the spin-orbit coupling in a two dimensional narrow strip
The current induced local spin polarization due to weak Rashba spin-orbit
coupling in narrow strip is studied. In the presence of longitudinal charge
current, local spin polarizations appear in the sample. The spin polarization
perpendicular to the plane has opposite sign near the two edges. The in-plane
spin polarization in the direction perpendicular to the sample edges also
appears, but does not change sign across the sample. From our scaling analysis
based on increasing the strip width, the out-of-plane spin polarization is
important mainly in a system of mesoscopic size, and thus appears not to be
associated with the spin-Hall effect in bulk samples.Comment: 4 pages, 4 figure
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