2,652 research outputs found
Accurate determination of elastic parameters for multi-component membranes
Heterogeneities in the cell membrane due to coexisting lipid phases have been
conjectured to play a major functional role in cell signaling and membrane
trafficking. Thereby the material properties of multiphase systems, such as the
line tension and the bending moduli, are crucially involved in the kinetics and
the asymptotic behavior of phase separation. In this Letter we present a
combined analytical and experimental approach to determine the properties of
phase-separated vesicle systems. First we develop an analytical model for the
vesicle shape of weakly budded biphasic vesicles. Subsequently experimental
data on vesicle shape and membrane fluctuations are taken and compared to the
model. The combined approach allows for a reproducible and reliable
determination of the physical parameters of complex vesicle systems. The
parameters obtained set limits for the size and stability of nanodomains in the
plasma membrane of living cells.Comment: (*) authors contributed equally, 6 pages, 3 figures, 1 table; added
insets to figure
Off-diagonal Wave Function Monte Carlo Studies of Hubbard Model I
We propose a Monte Carlo method, which is a hybrid method of the quantum
Monte Carlo method and variational Monte Carlo theory, to study the Hubbard
model. The theory is based on the off-diagonal and the Gutzwiller type
correlation factors which are taken into account by a Monte Carlo algorithm. In
the 4x4 system our method is able to reproduce the exact results obtained by
the diagonalization. An application is given to investigate the half-filled
band case of two-dimensional square lattice. The energy is favorably compared
with quantum Monte Carlo data.Comment: 9 pages, 11 figure
Quantum Monte Carlo diagonalization for many-fermion systems
In this study we present an optimization method based on the quantum Monte
Carlo diagonalization for many-fermion systems. Using the Hubbard-Stratonovich
transformation, employed to decompose the interactions in terms of auxiliary
fields, we expand the true ground-state wave function. The ground-state wave
function is written as a linear combination of the basis wave functions. The
Hamiltonian is diagonalized to obtain the lowest energy state, using the
variational principle within the selected subspace of the basis functions. This
method is free from the difficulty known as the negative sign problem. We can
optimize a wave function using two procedures. The first procedure is to
increase the number of basis functions. The second improves each basis function
through the operators, , using the Hubbard-Stratonovich
decomposition. We present an algorithm for the Quantum Monte Carlo
diagonalization method using a genetic algorithm and the renormalization
method. We compute the ground-state energy and correlation functions of small
clusters to compare with available data
Superconductivity, magnetic order, and quadrupolar order in the filled skutterudite system PrNdOsSb
Superconductivity, magnetic order, and quadrupolar order have been
investigated in the filled skutterudite system
PrNdOsSb as a function of composition in magnetic
fields up to 9 tesla and at temperatures between 50 mK and 10 K. Electrical
resistivity measurements indicate that the high field ordered phase (HFOP),
which has been identified with antiferroquadruoplar order, persists to
0.5. The superconducting critical temperature of PrOsSb
is depressed linearly with Nd concentration to 0.55, whereas the
Curie temperature of NdOsSb is depressed linearly with Pr
composition to () 0.45. In the superconducting region, the upper
critical field is depressed quadratically with in the range 0
0.3, exhibits a kink at 0.3, and then
decreases linearly with in the range 0.3 0.6. The
behavior of appears to be due to pair breaking caused by the
applied magnetic field and the exhange field associated with the polarization
of the Nd magnetic moments, in the superconducting state. From magnetic
susceptibility measurements, the correlations between the Nd moments in the
superconducting state appear to change from ferromagnetic in the range 0.3
0.6 to antiferromagnetic in the range 0
0.3. Specific heat measurements on a sample with 0.45
indicate that magnetic order occurs in the superconducting state, as is also
inferred from the depression of with .Comment: 7 pages, 7 figures, currently submitted to Phys. Rev.
Minimal Trinification
We study the trinified model, SU(3)_C x SU(3)_L x SU(3)_R x Z_3, with the
minimal Higgs sector required for symmetry breaking. There are five Higgs
doublets, and gauge-coupling unification results if all five are at the weak
scale, without supersymmetry. The radiative see-saw mechanism yields sub-eV
neutrino masses, without the need for intermediate scales, additional Higgs
fields, or higher-dimensional operators. The proton lifetime is above the
experimental limits, with the decay modes p -> \bar\nu K^+ and p -> \mu^+ K^0
potentially observable. We also consider supersymmetric versions of the model,
with one or two Higgs doublets at the weak scale. The radiative see-saw
mechanism fails with weak-scale supersymmetry due to the nonrenormalization of
the superpotential, but operates in the split-SUSY scenario.Comment: 23 pages, uses axodra
Optical Control of Field-Emission Sites by Femtosecond Laser Pulses
We have investigated field emission patterns from a clean tungsten tip apex
induced by femtosecond laser pulses. Strongly asymmetric modulations of the
field emission intensity distributions are observed depending on the
polarization of the light and the laser incidence direction relative to the
azimuthal orientation of tip apex. In effect, we have realized an ultrafast
pulsed field-emission source with site selectivity on the 10 nm scale.
Simulations of local fields on the tip apex and of electron emission patterns
based on photo-excited nonequilibrium electron distributions explain our
observations quantitatively.Comment: 4 pages, submitted to Physical Review Letter
Madelung Energy of the Valence Skipping Compound BaBiO
Several elements show valence skip fluctuation, for instance, Tl forms the
compounds in valence states +1 and +3, and Bi forms in +3 and +5 states. This
kind of fluctuation gives rise to a negative effective attractive interaction
and the Kondo effect. In the compounds of valence skipping elements, the
carrier doping will induce superconductivity with high critical temperature.
For example, BaKBiO shows high which is unlikely from the
conventional electron-phonon mechanism. The reason for the missing of some
valence states in such valence skip compounds remains a mystery. We have
performed the evaluation of the Madelung potential for BaBiO, and have
shown for the first time that charge-ordered state is stabilized if we take
into account the polarization of the oxygen charge. We argue that the effective
Coulomb interaction energy may be negative evaluating the local excitation
energy
Guanosine diphosphatase is required for protein and sphingolipid glycosylation in the Golgi lumen of Saccharomyces cerevisiae
Current models for nucleotide sugar use in the Golgi apparatus predict a critical role for the lumenal nucleoside diphosphatase. After transfer of sugars to endogenous macromolecular acceptors, the enzyme converts nucleoside diphosphates to nucleoside monophosphates which in turn exit the Golgi lumen in a coupled antiporter reaction, allowing entry of additional nucleotide sugar from the cytosol. To test this model, we cloned the gene for the S. cerevisiae guanosine diphosphatase and constructed a null mutation. This mutation should reduce the concentrations of GDP-mannose and GMP and increase the concentration of GDP in the Golgi lumen. The alterations should in turn decrease mannosylation of proteins and lipids in this compartment. In fact, we found a partial block in O- and N-glycosylation of proteins such as chitinase and carboxypeptidase Y and underglycosylation of invertase. In addition, mannosylinositolphosphorylceramide levels were drastically reduced
Stability, Gain, and Robustness in Quantum Feedback Networks
This paper concerns the problem of stability for quantum feedback networks.
We demonstrate in the context of quantum optics how stability of quantum
feedback networks can be guaranteed using only simple gain inequalities for
network components and algebraic relationships determined by the network.
Quantum feedback networks are shown to be stable if the loop gain is less than
one-this is an extension of the famous small gain theorem of classical control
theory. We illustrate the simplicity and power of the small gain approach with
applications to important problems of robust stability and robust
stabilization.Comment: 16 page
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