3,344 research outputs found
Fermi gases with imaginary mass imbalance and the sign problem in Monte Carlo calculations
Fermi gases in strongly coupled regimes, such as the unitary limit, are
inherently challenging for many-body methods. Although much progress has been
made with purely analytic methods, quantitative results require ab initio
numerical approaches, such as Monte Carlo (MC) calculations. However,
mass-imbalanced and spin-imbalanced gases are not accessible to MC calculations
due to the infamous sign problem. It was recently pointed out that the sign
problem, for finite spin imbalance, can be circumvented by resorting to
imaginary polarizations and analytic continuation. Large parts of the phase
diagram spanned by temperature and polarization then become accessible to MC
calculations. We propose to apply a similar strategy to the mass-imbalanced
case, which opens up the possibility to study the associated phase diagram with
MC calculations. In particular, our analysis suggests that a detection of a
(tri-)critical point in this phase diagram is possible. We also discuss
calculations in the zero-temperature limit with our approach.Comment: 5 pages, 3 figure
Diffeomorphism Invariant Integrable Field Theories and Hypersurface Motions in Riemannian Manifolds
We discuss hypersurface motions in Riemannian manifolds whose normal velocity
is a function of the induced hypersurface volume element and derive a second
order partial differential equation for the corresponding time function
at which the hypersurface passes the point . Equivalently, these
motions may be described in a Hamiltonian formulation as the singlet sector of
certain diffeomorphism invariant field theories. At least in some (infinite
class of) cases, which could be viewed as a large-volume limit of Euclidean
-branesmoving in an arbitrary -dimensional Riemannian manifold, the
models are integrable: In the time-function formulation the equation becomes
linear (with a harmonic function on the embedding Riemannian
manifold). We explicitly compute solutions to the large volume limit of
Euclidean membrane dynamics in \Real^3 by methods used in electrostatics and
point out an additional gradient flow structure in \Real^n. In the
Hamiltonian formulation we discover infinitely many hierarchies of integrable,
multidimensional, -component theories possessing infinitely many
diffeomorphism invariant, Poisson commuting, conserved charges.Comment: 15 pages, LATE
A no-ghost theorem for the bosonic Nappi-Witten string
We prove a no-ghost theorem for a bosonic string propagating in Nappi-Witten
spacetime. This is achieved in two steps. We first demonstrate unitarity for a
class of NW/U(1) modules: the norm of any state which is primary with respect
to a chosen timelike U(1) is non-negative. We then show that physical states -
states satisfying the Virasoro constraints - in a class of modules of an
affinisation of the Nappi-Witten algebra are contained in the NW/U(1) modules.
Similar to the case of strings on , in order to saturate the spectrum
obtained in light-cone quantization we are led to include modules with energy
not bounded from below, which are related to modules with energy bounded from
below by spectral flow automorphisms.Comment: 24 pages, 1 figur
Imaginary polarization as a way to surmount the sign problem in ab initio calculations of spin-imbalanced Fermi gases
From ultracold atoms to quantum chromodynamics, reliable ab initio studies of
strongly interacting fermions require numerical methods, typically in some form
of quantum Monte Carlo calculation. Unfortunately, (non)relativistic systems at
finite density (spin polarization) generally have a sign problem, such that
those ab initio calculations are impractical. It is well-known, however, that
in the relativistic case imaginary chemical potentials solve this problem,
assuming the data can be analytically continued to the real axis. Is this
feasible for nonrelativistic systems? Are the interesting features of the phase
diagram accessible in this manner? By introducing complex chemical potentials,
for real total particle number and imaginary polarization, the sign problem is
avoided in the nonrelativistic case. To give a first answer to the above
questions, we perform a mean-field study of the finite-temperature phase
diagram of spin-1/2 fermions with imaginary polarization.Comment: 5 pages, 2 figures; published versio
Directional emission from asymmetric resonant cavities
Asymmetric resonant cavities (ARCs) with highly non-circular but convex
cross-sections are predicted theoretically to have high-Q whispering gallery
modes with very anisotropic emission. We develop a ray dynamics model for the
emission pattern and present numerical and experimental confirmation of the
theory.Comment: 7 pages LaTeX, 3 postscript figure
Multiple high-pressure phase transitions in BiFeO3
We investigate the high-pressure phase transitions in BiFeO3 by single
crystal and powder x-ray diffraction, as well as single crystal Raman
spectroscopy. Six phase transitions are reported in the 0-60 GPa range. At low
pressures, up to 15 GPa, 4 transitions are evidenced at 4, 5, 7 and 11 GPa. In
this range, the crystals display large unit cells and complex domain
structures, which suggests a competition between complex tilt systems and
possibly off-center cation displacements. The non polar Pnma phase remains
stable over a large pressure range between 11 and 38 GPa, where the distortion
(tilt angles) changes only little with pressure. The two high-pressure phase
transitions at 38 and 48 GPa are marked by the occurence of larger unit cells
and an increase of the distorsion away from the cubic parent perovskite cell.
We find no evidence for a cubic phase at high pressure, nor indications that
the structure tends to become cubic. The previously reported insulator-to-metal
transition at 50 GPa appears to be symmetry breaking.Comment: 11 pages, 8 figure
Mathematical modeling of proteome constraints within metabolism
Genome-scale metabolic models (GEMs) are widely used to predict phenotypes with the aid of constraint-based modeling. In order to improve the predictive power of these models, there have been many efforts on imposing biological constraints, among which proteome constraints are of particular interest. Here we describe the concept of proteome constraints and review proteome-constrained GEMs, as well as their advantages and applications. In addition, we discuss a key issue in the field, i.e., low coverage of enzyme-specific turnover rates, and subsequently provide a few solutions to solve it. We end with a discussion on the trade-off between model complexity and capability
In vitro turnover numbers do not reflect in vivo activities of yeast enzymes
Turnover numbers (kcat values) quantitatively represent the activity of enzymes, which are mostly measured in vitro. While a few studies have reported in vivo catalytic rates (kapp values) in bacteria, a large-scale estimation of kapp in eukaryotes is lacking. Here, we estimated kapp of the yeast Saccharomyces cerevisiae under diverse conditions. By comparing the maximum kapp across conditions with in vitro kcat we found a weak correlation in log scale of R2 = 0.28, which is lower than for Escherichia coli (R2 = 0.62). The weak correlation is caused by the fact that many in vitro kcat values were measured for enzymes obtained through heterologous expression. Removal of these enzymes improved the correlation to R2 = 0.41 but still not as good as for E. coli, suggesting considerable deviations between in vitro and in vivo enzyme activities in yeast. By parameterizing an enzyme-constrained metabolic model with our kapp dataset we observed better performance than the default model with in vitro kcat in predicting proteomics data, demonstrating the strength of using the dataset generated here
Topologic mixing on a microfluidic chip
Mixing two liquids on a microfluidic chip is notoriously hard because the small dimensions and velocities on the chip effectively prevent turbulence. We present a topological mixing scheme that exploits the laminarity of the flow to repeatedly fold the flow and exponentially increase the concentration gradients to obtain fast and efficient mixing by diffusion. It is based on helical flow channels with opposite chiralities that split, rotate, and recombine the fluid stream in a topology reminiscent of a series of Möbius bands. This geometry is realized in a simple six-stage, two-layer elastomer structure with a footprint of 400 μm×300 μm400μm×300μm per stage that mixes two solutions efficiently at Reynolds numbers between 0.1 and 2. This represents more than an order of magnitude reduction in the size of microfluidic mixers that can be manufactured in standard multilayer soft lithography techniques. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69745/2/APPLAB-84-12-2193-1.pd
- …