764 research outputs found
Polarons and bipolarons in a two-dimensional square lattice
Quasiparticles and their interactions are a key part of our understanding of
quantum many-body systems. Quantum simulation experiments with cold atoms have
in recent years advanced our understanding of isolated quasiparticles, but so
far they have provided limited information regarding their interactions and
possible bound states. Here, we show how exploring mobile impurities immersed
in a Bose-Einstein condensate (BEC) in a two-dimensional lattice can address
this problem. First, the spectral properties of individual impurities are
examined, and in addition to the attractive and repulsive polarons known from
continuum gases, we identify a new kind of quasiparticle stable for repulsive
boson-impurity interactions. The spatial properties of polarons are calculated
showing that there is an increased density of bosons at the site of the
impurity both for repulsive and attractive interactions. We then derive an
effective Schr\"odinger equation describing two polarons interacting via the
exchange of density oscillations in the BEC, which takes into account strong
impurity-boson two-body correlations. Using this, we show that the attractive
nature of the effective interaction between two polarons combined with the
two-dimensionality of the lattice leads to the formation of bound states --
i.e. bipolarons. The wave functions of the bipolarons are examined showing that
the ground state is symmetric under particle exchange and therefore relevant
for bosonic impurities, whereas the first excited state is doubly degenerate
and odd under particle exchange making it relevant for fermionic impurities.
Our results show that quantum gas microscopy in optical lattices is a promising
platform to explore the spatial properties of polarons as well as to finally
observe the elusive bipolarons.Comment: Submission to SciPos
Longitudinal dependence of B and D meson nuclear modifications in heavy-ion collisions at RHIC and the LHC
It is widely acknowledged that heavy flavor probes are sensitive to the
properties of the quark-gluon plasma and are often considered an important tool
for the plasma tomography studies. Forward rapidity observables can provide
further insight on the dynamics of the medium due to the interplay between the
medium size and the differences in the production spectra of heavy quark
probes. In this proceedings we present the nuclear modification factor
's for B and D mesons, as well as heavy flavor leptons, in the
rapidity range obtained from relativistic Langevin equation
with gluon radiation coupled with a (3+1)-dimensional viscous hydrodynamics
medium background. We present comparison with experimental data at mid-rapidity
as well as predictions for different rapidity ranges.Comment: 4 pages, 4 figures, contribution to the proceedings of Quark Matter -
XXVIII international conference on ultrarelativistic heavy-ion collisions
(Wuhan, China, 4-9 November, 2019
Herschel observations of EXtraordinary Sources: Analysis of the full Herschel/HIFI molecular line survey of Sagittarius B2(N)
A sensitive broadband molecular line survey of the Sagittarius B2(N)
star-forming region has been obtained with the HIFI instrument on the Herschel
Space Observatory, offering the first high-spectral resolution look at this
well-studied source in a wavelength region largely inaccessible from the ground
(625-157 um). From the roughly 8,000 spectral features in the survey, a total
of 72 isotopologues arising from 44 different molecules have been identified,
ranging from light hydrides to complex organics, and arising from a variety of
environments from cold and diffuse to hot and dense gas. We present an LTE
model to the spectral signatures of each molecule, constraining the source
sizes for hot core species with complementary SMA interferometric observations,
and assuming that molecules with related functional group composition are
cospatial. For each molecule, a single model is given to fit all of the
emission and absorption features of that species across the entire 480-1910 GHz
spectral range, accounting for multiple temperature and velocity components
when needed to describe the spectrum. As with other HIFI surveys toward massive
star forming regions, methanol is found to contribute more integrated line
intensity to the spectrum than any other species. We discuss the molecular
abundances derived for the hot core, where the local thermodynamic equilibrium
approximation is generally found to describe the spectrum well, in comparison
to abundances derived for the same molecules in the Orion KL region from a
similar HIFI survey.Comment: Accepted to ApJ. 64 pages, 14 figures. Truncated abstrac
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