2,775 research outputs found
Dynamic effects of electromagnetic wave on a damped two-level atom
We studied the dynamic effects of an electromagnetic(EM) wave with circular
polarization on a two-level damped atom. The results demonstrate interesting ac
Stark split of energy levels of damped atom. The split levels have different
energies and lifetimes, both of which depend on the interaction and the damping
rate of atom. When the frequency of the EM wave is tuned to satisfy the
resonance condition in the strong coupling limit, the transition probability
exhibits Rabi oscillation. Momentum transfer between atom and EM wave shows
similar properties as the transition probability under resonance condition. For
a damped atom interacting with EM field, there exists no longer stable state.
More importantly, if the angular frequency of the EM wave is tuned the same as
the atomic transition frequency and its amplitude is adjusted appropriately
according to the damping coefficients, we can prepare a particular 'Dressed
State' of the coupled system between atom and EM field and can keep the system
coherently in this 'Dressed state' for a very long time. This opens another way
to prepare coherent atomic states.Comment: latex, 2 figure
Weak force detection using a double Bose-Einstein condensate
A Bose-Einstein condensate may be used to make precise measurements of weak
forces, utilizing the macroscopic occupation of a single quantum state. We
present a scheme which uses a condensate in a double well potential to do this.
The required initial state of the condensate is discussed, and the limitations
on the sensitivity due to atom collisions and external coupling are analyzed.Comment: 12 pages, 2 figures, Eq.(41) has been correcte
Collapse of ringlike structures in 2DEGs under tilted magnetic fields
In the quantum Hall regime, the longitudinal resistivity plotted
as a density--magnetic-field () diagram displays ringlike structures
due to the crossings of two sets of spin split Landau levels from different
subbands [e.g., Zhang \textit{et al.}, Phys. Rev. Lett. \textbf{95}, 216801
(2005)]. For tilted magnetic fields, some of these ringlike structures "shrink"
as the tilt angle is increased and fully collapse at . Here we theoretically investigate the topology of these structures
via a non-interacting model for the 2DEG. We account for the inter Landau-level
coupling induced by the tilted magnetic field via perturbation theory. This
coupling results in anti-crossings of Landau levels with parallel spins. With
the new energy spectrum, we calculate the corresponding diagram of
the density of states (DOS) near the Fermi level. We argue that the DOS
displays the same topology as in the diagram. For the
ring with filling factor , we find that the anti-crossings make it
shrink for increasing tilt angles and collapse at a large enough angle. Using
effective parameters to fit the data, we find a collapsing
angle . Despite this factor-of-two discrepancy with
the experimental data, our model captures the essential mechanism underlying
the ring collapse.Comment: 3 pages, 2 figures; Proceedings of the PASPS V Conference Held in
August 2008 in Foz do Igua\c{c}u, Brazi
Nanotribological Investigation of Polymer Brushes with Lithographically Defined and Systematically Varying Grafting Densities.
Following controlled photodeprotection of a 2-nitrophenylpropyloxycarbonyl-protected (aminopropyl)triethoxysilane (NPPOC-APTES) film and subsequent derivatization with a bromoester-based initiator, poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes with various grafting densities were grown from planar silicon substrates using atom transfer radical polymerization (ATRP). The grafting density correlated closely with the extent of deprotection of the NPPOC-APTES. The coefficient of friction for such PMPC brushes was measured by friction force microscopy in water and found to be inversely proportional to the grafting density due to the osmotic pressure that resists deformation. Deprotection of NPPOC-APTES via near-field photolithography using a range of writing rates enabled the fabrication of neighboring nanoscopic polymeric structures with dimensions ranging from 100 to 1000 nm. Slow writing rates enable complete deprotection to occur; hence, polymer brushes are formed with comparable thicknesses to macroscopic brushes grown under the same conditions. However, the extent of deprotection is reduced at higher writing rates, resulting in the concomitant reduction of the brush thickness. The coefficient of friction for such polymer brushes varied smoothly with brush height, with lower coefficients being obtained at slower writing rate (increasing initiator density) because the solvated brush layer confers greater lubricity. However, when ultrasharp probes were used for nanotribological measurements, the coefficient of friction increased with brush thickness. Under such conditions, the radius of curvature of the tip is comparable to the mean spacing between brush chains, allowing the probe to penetrate the brush layer leading to a relatively large contact area
Self-trapping mechanisms in the dynamics of three coupled Bose-Einstein condensates
We formulate the dynamics of three coupled Bose-Einstein condensates within a
semiclassical scenario based on the standard boson coherent states. We compare
such a picture with that of Ref. 1 and show how our approach entails a simple
formulation of the dimeric regime therein studied. This allows to recognize the
parameters that govern the bifurcation mechanism causing self-trapping, and
paves the way to the construction of analytic solutions. We present the results
of a numerical simulation showing how the three-well dynamics has, in general,
a cahotic behavior.Comment: 4 pages, 5 figure
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