44,773 research outputs found
Tunable Quantum Fluctuation-Controlled Coherent Spin Dynamics
Temporal evolution of a macroscopic condensate of ultra cold atoms is usually
driven by mean field potentials, either due to scattering between atoms or due
to coupling to external fields; and coherent quantum dynamics have been
observed in various cold-atom experiments. In this article, we report results
of studies of a class of quantum spin dynamics which are purely driven by zero
point quantum fluctuations of spin collective coordinates. Unlike the usual
mean-field coherent dynamics, quantum fluctuation-controlled spin dynamics or
QFCSD studied here are very sensitive to variation of quantum fluctuations and
can be tuned by four to five order of magnitude using optical lattices. They
have unique dependence on optical lattice potential depths and quadratic Zeeman
fields. QFCSD can be potentially used to calibrate quantum fluctuations and
investigate correlated fluctuations and various universal scaling properties
near quantum critical points.Comment: 14 pages, 12 figures included; including detailed discussions on
thermal effects, trapping potentials and spin exchange losses. (To appear in
PRA
Quantum fluctuation-induced uniaxial and biaxial spin nematics
It is shown that zero point quantum fluctuations (ZPQFs) completely lift the
accidental continuous degeneracy that is found in mean field analysis of
quantum spin nematic phases of hyperfine spin 2 cold atoms. The result is two
distinct ground states which have higher symmetries: a uniaxial spin nematic
and a biaxial spin nematic with dihedral symmetry . There is a novel
first order quantum phase transition between the two phases as atomic
scattering lengths are varied. We find that the ground state of atoms
should be a uniaxial spin nematic. We note that the energy barrier between the
phases could be observable in dynamical experiments.Comment: 4 pages, 2 figures included; published versio
Enhanced thermionic-dominated photoresponse in graphene Schottky junctions
Vertical heterostructures of van der Waals materials enable new pathways to
tune charge and energy transport characteristics in nanoscale systems. We
propose that graphene Schottky junctions can host a special kind of
photoresponse which is characterized by strongly coupled heat and charge flows
that run vertically out of the graphene plane. This regime can be accessed when
vertical energy transport mediated by thermionic emission of hot carriers
overwhelms electron-lattice cooling as well as lateral diffusive energy
transport. As such, the power pumped into the system is efficiently extracted
across the entire graphene active area via thermionic emission of hot carriers
into a semiconductor material. Experimental signatures of this regime include a
large and tunable internal responsivity with a non-monotonic
temperature dependence. In particular, peaks at electronic
temperatures on the order of the Schottky potential and has a large
upper limit ( when ). Our proposal opens up new approaches for engineering the
photoresponse in optically-active graphene heterostructures.Comment: 6 pages, 2 figure
THEORETICAL STUDIES OF BILIPROTEIN CHROMOPHORES AND RELATED BILE PIGMENTS BY MOLECULAR ORBITAL AND RAMACHANDRAN TYPE CALCULATIONS
Ramachandran calculations have been used to gain insight into steric hindrance in bile
pigments related to biliprotein chromophores. The high optical activity of denatured phycocyanin, as
compared to phycoerythrin, has been related to the asymmetric substitution at ring A, which shifts the
equilibrium towards the P-helical form of the chromophore. Geometric effects on the electronic structures
and transitions have then been studied by molecular orbital calculations for several conjugation
systems including the chromophores of phycocyanin. phytochrome P,, cations, cation radicals and
tautomeric forms. For these different chromophores some general trends can be deduced. For instance,
for a given change in the gross shape (e.g. either unfolding of the molecule from a cyclic-helical to a fully
extended geometry, or upon out-of-plane twists of the pyrrole ring A) of the molecules under study, the
predicted absorption spectra all change in a simikar way. Nonetheless, there are characteristic distinctions
between the different n-systems, both in the transition energies and the charge distribution, which
can be related to their known differences in spectroscopic properties and their reactivity
Wetting and bonding characteristics of selected liquid-metals with a high power diode laser treated alumina bioceramic
Changes in the wettability characteristics of an alumina bioceramic occasioned by high power diode laser (HPDL) surface treatment were apparent from the observed reduction in the contact angle. Such changes were due to the HPDL bringing about reductions the surface roughness, increases in the surface O2 content and increases in the polar component of the surface energy. Additionally, HPDL treatment of the alumina bioceramic surface was found to effect an improvement in the bonding characteristics by increasing the work of adhesion. An electronic approach was used to elucidate the bonding characteristics of the alumina bioceramic before and after HPDL treatment. It is postulated that HPDL induced changes to the alumina bioceramic produced a surface with a reduced bandgap energy which consequently increased the work of adhesion by increasing the electron transfer at the metal/oxide interface and thus the metal-oxide interactions. Furthermore, it is suggested that the increase in the work of adhesion of the alumina bioceramic after HPDL treatment was due to a correlation existing between the wettability and ionicity of the alumina bioceramic; for it is believed that the HPDL treated surface is less ionic in nature than the untreated surface and therefore exhibits better wettability characteristics
On the predominant mechanisms active during the high power diode laser modification of the wettability characteristics of an SiO2/Al2O3-based ceramic material
The mechanisms responsible for modifications to the wettability characteristics of a SiO2/Al2O3-based ceramic material in terms of a test liquid set comprising of human blood, human blood plasma, glycerol and 4-octonol after high power diode laser (HPDL) treatment have been elucidated. Changes in the contact angle, , and hence the wettability characteristics of the SiO2/Al2O3-based ceramic were attributed primarily to: modifications to the surface roughness of the ceramic resulting from HPDL interaction which accordingly effected reductions in ; the increase in the surface O2 content of the ceramic after HPDL treatment; since an increase in surface O2 content intrinsically brings about a decrease in , and vice versa and the increase in the polar component of the surface energy, due to the HPDL induced surface melting and resolidification which consequently created a partially vitrified microstructure that was seen to augment the wetting action. However, the degree of influence exerted by each mechanism was found to differ markedly. Isolation of each of these mechanisms permitted the magnitude of their influence to be qualitatively determined. Surface energy, by way of microstructural changes, was found to be by far the most predominant element governing the wetting characteristics of the SiO2/Al2O3-based ceramic. To a much lesser extent, surface O2 content, by way of process gas, was also seen to influence to a changes in the wettability characteristics of the SiO2/Al2O3-based ceramic, whilst surface roughness was found to play a minor role in inducing changes in the wettability characteristics
Lattice Boltzmann Approach to High-Speed Compressible Flows
We present an improved lattice Boltzmann model for high-speed compressible
flows. The model is composed of a discrete-velocity model by Kataoka and
Tsutahara [Phys. Rev. E \textbf{69}, 056702 (2004)] and an appropriate
finite-difference scheme combined with an additional dissipation term. With the
dissipation term parameters in the model can be flexibly chosen so that the von
Neumann stability condition is satisfied. The influence of the various model
parameters on the numerical stability is analyzed and some reference values of
parameter are suggested. The new scheme works for both subsonic and supersonic
flows with a Mach number up to 30 (or higher), which is validated by well-known
benchmark tests. Simulations on Riemann problems with very high ratios
() of pressure and density also show good accuracy and stability.
Successful recovering of regular and double Mach shock reflections shows the
potential application of the lattice Boltzmann model to fluid systems where
non-equilibrium processes are intrinsic. The new scheme for stability can be
easily extended to other lattice Boltzmann models.Comment: Figs.11 and 12 in JPEG format. Int. J. Mod. Phys. C (to appear
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