11 research outputs found
Density-Matrix approach to a Strongly Coupled Two-Component Bose-Einstein Condensate
The time evolution equations for average values of population and relative
phase of a strongly coupled two component BEC is derived analytically. The two
components are two hyper-fine states coupled by an external laser that drives
fast Rabi oscillations between these states. Specifically, this derivation
incorporates the two-mode model proposed in [1] for the strongly coupled
hyper-fine states of Rb. The fast Rabi cycle is averaged out and rate equations
are derived that represents the slow dynamics of the system. These include the
collapse and revival of Rabi oscillations and their subsequent dependence on
detuning and trap displacement as reported in experiments of [1]. A proposal to
create stable vortices is also given.Comment: 11 Latex pages, 2 figures (Figure 3 was removed and the text chnaged
accordingly
Dirac, Anderson, and Goldstone on the Kagome
We show that there exists a long-range RVB state for the kagome lattice
spin-1/2 Heisenberg antiferromagnet for which the spinons have a massless Dirac
spectrum. By considering various perturbations of the RVB state which give mass
to the fermions by breaking a symmetry, we are able to describe a wide-ranging
class of known states on the kagome lattice, including spin-Peierls solid and
chiral spin liquid states. Using an RG treatment of fluctuations about the RVB
state, we propose yet a different symmetry breaking pattern and show how
collective excitations about this state account for the gapless singlet modes
seen experimentally and numerically. We make further comparison with numerics
for Chern numbers, dimer-dimer correlation functions, the triplet gap, and
other quantities. To accomplish these calculations, we propose a variant of the
SU(N) theory which enables us to include many of the effects of Gutzwiller
projection at the mean-field level.Comment: 18 pages, 6 figures; added references, minor correction
Quantized circular motion of a trapped Bose-Einstein condensate: coherent rotation and vortices
We study the creation of vortex states in a trapped Bose-Einstein condensate
by a rotating force. For a harmonic trapping potential the rotating force
induces only a circular motion of the whole condensate around the trap center
which does not depend on the interatomic interaction. For the creation of a
pure vortex state it is necessary to confine the atoms in an anharmonic
trapping potential. The efficiency of the creation can be greatly enhanced by a
sinusodial variation of the force's angular velocity. We present analytical and
numerical calculations for the case of a quartic trapping potential. The
physical mechanism behind the requirement of an anharmonic trapping potential
for the creation of pure vortex states is explained.
[Changes: new numerical and analytical results are added and the
representation is improved.]Comment: 13 Pages, 5 Figures, RevTe
Vortices in a Bose-Einstein condensate confined by an optical lattice
We investigate the dynamics of vortices in repulsive Bose-Einstein
condensates in the presence of an optical lattice (OL) and a parabolic magnetic
trap. The dynamics is sensitive to the phase of the OL potential relative to
the magnetic trap, and depends less on the OL strength. For the cosinusoidal OL
potential, a local minimum is generated at the trap's center, creating a stable
equilibrium for the vortex, while in the case of the sinusoidal potential, the
vortex is expelled from the center, demonstrating spiral motion. Cases where
the vortex is created far from the trap's center are also studied, revealing
slow outward-spiraling drift. Numerical results are explained in an analytical
form by means of a variational approximation. Finally, motivated by a discrete
model (which is tantamount to the case of the strong OL lattice), we present a
novel type of vortex consisting of two pairs of anti-phase solitons.Comment: 10 pages, 6 figure
Inhomogeneous d-wave superconducting state of a doped Mott insulator
Recent scanning tunneling microscope (STM) measurements discovered remarkable
electronic inhomogeneity, i.e. nano-scale spatial variations of the local
density of states (LDOS) and the superconducting energy gap, in the high-Tc
superconductor BSCCO. Based on the experimental findings we conjectured that
the inhomogeneity arises from variations in local oxygen doping level and may
be generic of doped Mott insulators which behave rather unconventionally in
screening the dopant ionic potentials at atomic scales comparable to the short
coherence length. Here, we provide theoretical support for this picture. We
study a doped Mott insulator within a generalized t-J model, where doping is
accompanied by ionic Coulomb potentials centered in the BiO plane. We calculate
the LDOS spectrum, the integrated LDOS, and the local superconducting gap, make
detailed comparisons to experiments, and find remarkable agreement with the
experimental data. We emphasize the unconventional screening in a doped Mott
insulator and show that nonlinear screening dominates at nano-meter scales
which is the origin of the electronic inhomogeneity. It leads to strong
inhomogeneous redistribution of the local hole density and promotes the notion
of a local doping concentration. We find that the inhomogeneity structure
manifests itself at all energy scales in the STM tunneling differential
conductance, and elucidate the similarity and the differences between the data
obtained in the constant tunneling current mode and the same data normalized to
reflect constant tip-to-sample distance. We also discuss the underdoped case
where nonlinear screening of the ionic potential turns the spatial electronic
structure into a percolative mixture of patches with smaller pairing gaps
embedded in a background with larger gaps to single particle excitations.Comment: 19 pages, final versio
A Recommendation for Naming Transcription Factor Proteins in the Grasses
Transcription factors are central for the exquisite temporal and spatial expression patterns of many genes. These proteins are characterized by their ability to be tethered to particular regulatory sequences in the genes that they control. While many other proteins participate in the regulation of gene expression, we limit our definition of transcription factors here to proteins that often contain a characteristic structural motif, the DNA-binding domain, which is involved in recognizing a short (usually 4–8 bp) DNA sequence. Based on the structure of the DNA-binding domain, transcription factors are classified into 50 to 60 different families, and in plants, 5% to 7% of all the protein-encoding genes are transcription factors, making them, collectively, perhaps the largest functional class of proteins..
The Physcomitrella patens chromosome-scale assembly reveals moss genome structure and evolution
The draft genome of the moss model, Physcomitrella patens, comprised approximately 2000 unordered scaffolds. In order to enable analyses of genome structure and evolution we generated a chromosome-scale genome assembly using genetic linkage as well as (end) sequencing of long DNA fragments. We find that 57% of the genome comprises transposable elements (TEs), some of which may be actively transposing during the life cycle. Unlike in flowering plant genomes, gene- and TE-rich regions show an overall even distribution along the chromosomes. However, the chromosomes are mono-centric with peaks of a class of Copia elements potentially coinciding with centromeres. Gene body methylation is evident in 5.7% of the protein-coding genes, typically coinciding with low GC and low expression. Some giant virus insertions are transcriptionally active and might protect gametes from viral infection via siRNA mediated silencing. Structure-based detection methods show that the genome evolved via two rounds of whole genome duplications (WGDs), apparently common in mosses but not in liverworts and hornworts. Several hundred genes are present in colinear regions conserved since the last common ancestor of plants. These syntenic regions are enriched for functions related to plant-specific cell growth and tissue organization. The P. patens genome lacks the TE-rich pericentromeric and gene-rich distal regions typical for most flowering plant genomes. More non-seed plant genomes are needed to unravel how plant genomes evolve, and to understand whether the P. patens genome structure is typical for mosses or bryophytes