1,152 research outputs found
On Long-Range Order in Low-Dimensional Lattice-Gas Models of Nematic Liquid Crystals
The problem of the orientational ordering transition for lattice-gas models
of liquid crystals is discussed in the low-dimensional case . For
isotropic short-range interactions, orientational long-range order at finite
temperature is excluded for any packing of molecules on the lattice ; on
the other hand, for reflection-positive long-range isotropic interactions, we
prove existence of an orientational ordering transition for high packing () and low temperatures ().Comment: 11 page
Enhancing quantum transport in a photonic network using controllable decoherence
Transport phenomena on a quantum scale appear in a variety of systems,
ranging from photosynthetic complexes to engineered quantum devices. It has
been predicted that the efficiency of quantum transport can be enhanced through
dynamic interaction between the system and a noisy environment. We report the
first experimental demonstration of such environment-assisted quantum
transport, using an engineered network of laser-written waveguides, with
relative energies and inter-waveguide couplings tailored to yield the desired
Hamiltonian. Controllable decoherence is simulated via broadening the bandwidth
of the input illumination, yielding a significant increase in transport
efficiency relative to the narrowband case. We show integrated optics to be
suitable for simulating specific target Hamiltonians as well as open quantum
systems with controllable loss and decoherence.Comment: 6 pages, 3 figure
New Exactly Solvable Model of Strongly Correlated Electrons Motivated by High T_c Superconductivity
We present a new model describing strongly correlated electrons on a general
-dimensional lattice. It differs from the Hubbard model by interactions of
nearest neighbours, and it contains the - model as a special case. The
model naturally describes local electron pairs, which can move coherently at
arbitrary momentum. By using an -pairing mechanism we can construct
eigenstates of the hamiltonian with off-diagonal-long-range-order (ODLRO).
These might help to relate the model to high- superconductivity. On a
one-dimensional lattice, the model is exactly solvable by Bethe Ansatz.Comment: 10 pages, using latex, Phys.Rev.Lett. 68 (1992) 296
Mobility transition from ballistic to diffusive transport in non-Hermitian lattices
Within all physical disciplines, it is accepted that wave transport is predetermined by the existence of disorder. In this vein, it is known that ballistic transport is possible only when a structure is ordered, and that disorder is crucial for diffusion or (Anderson-)localization to occur. As this commonly accepted picture is based on the very foundations of quantum mechanics where Hermiticity of the Hamiltonian is naturally assumed, the question arises whether these concepts of transport hold true within the more general context of non-Hermitian systems. Here we demonstrate theoretically and experimentally that in ordered time-independent PT-symmetric systems, which are symmetric under space-time reflection, wave transport can undergo a sudden change from ballistic to diffusive after a specific point in time. This transition as well as the diffusive transport in general is impossible in Hermitian systems in the absence of disorder. In contrast, we find that this transition depends only on the degree of dissipation
Mobility transition from ballistic to diffusive transport in non-Hermitian lattices
Within all physical disciplines, it is accepted that wave transport is predetermined by the existence of disorder. In this vein, it is known that ballistic transport is possible only when a structure is ordered, and that disorder is crucial for diffusion or (Anderson-)localization to occur. As this commonly accepted picture is based on the very foundations of quantum mechanics where Hermiticity of the Hamiltonian is naturally assumed, the question arises whether these concepts of transport hold true within the more general context of non-Hermitian systems. Here we demonstrate theoretically and experimentally that in ordered time-independent PT-symmetric systems, which are symmetric under space-time reflection, wave transport can undergo a sudden change from ballistic to diffusive after a specific point in time. This transition as well as the diffusive transport in general is impossible in Hermitian systems in the absence of disorder. In contrast, we find that this transition depends only on the degree of dissipation
Ordered phase and phase transitions in the three-dimensional generalized six-state clock model
We study the three-dimensional generalized six-state clock model at values of
the energy parameters, at which the system is considered to have the same
behavior as the stacked triangular antiferromagnetic Ising model and the
three-state antiferromagnetic Potts model. First, we investigate ordered phases
by using the Monte Carlo twist method (MCTM). We confirmed the existence of an
incompletely ordered phase (IOP1) at intermediate temperature, besides the
completely ordered phase (COP) at low-temperature. In this intermediate phase,
two neighboring states of the six-state model mix, while one of them is
selected in the low temperature phase. We examine the fluctuation the mixing
rate of the two states in IOP1 and clarify that the mixing rate is very stable
around 1:1.
The high temperature phase transition is investigated by using
non-equilibrium relaxation method (NERM). We estimate the critical exponents
beta=0.34(1) and nu=0.66(4). These values are consistent with the 3D-XY
universality class. The low temperature phase transition is found to be of
first-order by using MCTM and the finite-size-scaling analysis
Dynamical Structure Factors of the S=1/2 Bond-Alternating Spin Chain with a Next-Nearest-Neighbor Interaction in Magnetic Fields
The dynamical structure factor of the S=1/2 bond-alternating spin chain with
a next-nearest-neighbor interaction in magnetic field is investigated using the
continued fraction method based on the Lanczos algorithm. When the plateau
exists on the magnetization curve, the longitudinal dynamical structure factor
shows a large intensity with a periodic dispersion relation, while the
transverse one shows a large intensity with an almost dispersionless mode. The
periodicity and the amplitude of the dispersion relation in the longitudinal
dynamical structure factor are sensitive to the coupling constants. The
dynamical structure factor of the S=1/2 two-leg ladder in magnetic field is
also calculated in the strong interchain-coupling regime.
The dynamical structure factor shows gapless or gapful behavior depending on
the wave vector along the rung.Comment: 8 pages, 4 figures, to appear in Journal of the Physical Society of
Japan, vol. 69, no. 10, (2000
Influence of batch or fed-batch growth on Staphylococcus epidermidis biofilm formation
Aims: To make a quantitative evaluation of the differences in biofilm formation by Staphylococcus epidermidis using
batch and fed-batch growth systems and to correlate this with production of the major biofilm polysaccharide,
poly-N-acetyl glucosamine (PNAG).
Methods and Results: Dry weight measurements of biofilms formed in batch and fed-batch conditions were
compared with haemagglutination titres, which measure the amount of PNAG produced. Strains grown in batch
systems developed less biofilm than when grown in fed-batch systems. A good correlation was found between the
amount of biofilm formed in fed-batch systems and the haemagglutination titres.
Conclusions: Differences in biofilm formation and PNAG production by S. epidermidis are dependent on the
availability of nutrients, with higher availability correlating with more biofilm and PNAG production.
Significance of and Impact of the Study: Comparisons of the formation of biofilms by S. epidermidis are
dependent on choosing an appropriate biofilm growth system. Comparability or disparity of conclusions among
different investigations will be strongly influenced by which mode S. epidermidis biofilms are formed.NIH - grant AI 46706.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” (POCTI) - POCTI/ESP/42688/2001, SFRH/BD/8676/2002
Cohort studies of Faroese children concerning potential adverse health effects after the mothers’ exposure to marine contaminants during pregnancy
Irreducibility criterion for a finite-dimensional highest weight representation of the sl(2) loop algebra and the dimensions of reducible representations
We present a necessary and sufficient condition for a finite-dimensional
highest weight representation of the loop algebra to be irreducible. In
particular, for a highest weight representation with degenerate parameters of
the highest weight, we can explicitly determine whether it is irreducible or
not. We also present an algorithm for constructing finite-dimensional highest
weight representations with a given highest weight. We give a conjecture that
all the highest weight representations with the same highest weight can be
constructed by the algorithm. For some examples we show the conjecture
explicitly. The result should be useful in analyzing the spectra of integrable
lattice models related to roots of unity representations of quantum groups, in
particular, the spectral degeneracy of the XXZ spin chain at roots of unity
associated with the loop algebra.Comment: 32 pages with no figure; with corrections on the published versio
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