45,788 research outputs found
High Resolution Ionization of Ultracold Neutral Plasmas
Collective effects, such as waves and instabilities, are integral to our
understanding of most plasma phenomena. We have been able to study these in
ultracold neutral plasmas by shaping the initial density distribution through
spatial modulation of the ionizing laser intensity. We describe a relay imaging
system for the photoionization beam that allows us to create higher resolution
features and its application to extend the observation of ion acoustic waves to
shorter wavelengths. We also describe the formation of sculpted density
profiles to create fast expansion of plasma into vacuum and streaming plasmas
Conductivity of suspended and non-suspended graphene at finite gate voltage
We compute the DC and the optical conductivity of graphene for finite values
of the chemical potential by taking into account the effect of disorder, due to
mid-gap states (unitary scatterers) and charged impurities, and the effect of
both optical and acoustic phonons. The disorder due to mid-gap states is
treated in the coherent potential approximation (CPA, a self-consistent
approach based on the Dyson equation), whereas that due to charged impurities
is also treated via the Dyson equation, with the self-energy computed using
second order perturbation theory. The effect of the phonons is also included
via the Dyson equation, with the self energy computed using first order
perturbation theory. The self-energy due to phonons is computed both using the
bare electronic Green's function and the full electronic Green's function,
although we show that the effect of disorder on the phonon-propagator is
negligible. Our results are in qualitative agreement with recent experiments.
Quantitative agreement could be obtained if one assumes water molelcules under
the graphene substrate. We also comment on the electron-hole asymmetry observed
in the DC conductivity of suspended graphene.Comment: 13 pages, 11 figure
Creating Non-Maxwellian Velocity Distributions in Ultracold Plasmas
We present techniques to perturb, measure and model the ion velocity
distribution in an ultracold neutral plasma produced by photoionization of
strontium atoms. By optical pumping with circularly polarized light we promote
ions with certain velocities to a different spin ground state, and probe the
resulting perturbed velocity distribution through laser-induced fluorescence
spectroscopy. We discuss various approaches to extract the velocity
distribution from our measured spectra, and assess their quality through
comparisons with molecular dynamic simulationsComment: 13 pages, 8 figure
Absence of Klein's paradox for massive bosons coupled by nonminimal vector interactions
A few properties of the nonminimal vector interactions in the
Duffin-Kemmer-Petiau theory are revised. In particular, it is shown that the
space component of the nonminimal vector interaction plays a peremptory role
for confining bosons whereas its time component contributes to the leakage.
Scattering in a square step potential with proper boundary conditions is used
to show that Klein's paradox does not manifest in the case of a nonminimal
vector coupling
Velocity Relaxation in a Strongly Coupled Plasma
Collisional relaxation of Coulomb systems is studied in the strongly coupled
regime. We use an optical pump-probe approach to manipulate and monitor the
dynamics of ions in an ultracold neutral plasma, which allows direct
measurement of relaxation rates in a regime where common Landau-Spitzer theory
breaks down. Numerical simulations confirm the experimental results and display
non-Markovian dynamics at early times.Comment: 5 pages, 5 figure
On the contribution of nearly-critical spin and charge collective modes to the Raman spectra of high-Tc cuprates
We discuss how Raman spectra are affected by nearly-critical spin and charge
collective modes, which are coupled to charge carriers near a stripe quantum
critical point. We show that specific fingerprints of nearly-critical
collective modes can indeed be observed in Raman spectra and that the
selectivity of Raman spectroscopy in momentum space may also be exploited to
distinguish the spin and charge contribution. We apply our results to discuss
the spectra of high-Tc superconducting cuprates finding that the collective
modes should have masses with substantial temperature dependence in agreement
with their nearly critical character. Moreover spin modes should be more
diffusive than charge modes indicating that in stripes the charge is nearly
ordered, while spin modes are strongly overdamped and fluctuate with high
frequency.Comment: 5 pages, 3 figure
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