8,927 research outputs found
Exactly solvable time-dependent models of two interacting two-level systems
Two coupled two-level systems placed under external time-dependent magnetic
fields are modeled by a general Hamiltonian endowed with a symmetry that
enables us to reduce the total dynamics into two independent two-dimensional
sub-dynamics. Each of the sub-dynamics is shown to be brought into an exactly
solvable form by appropriately engineering the magnetic fields and thus we
obtain an exact time evolution of the compound system. Several physically
relevant and interesting quantities are evaluated exactly to disclose
intriguing phenomena in such a system.Comment: 15 pages, 13 figure
Overcharging: The Crucial Role of Excluded Volume
In this Letter we investigate the mechanism for overcharging of a single
spherical colloid in the presence of aqueous salts within the framework of the
primitive model by molecular dynamics (MD) simulations as well as
integral-equation theory. We find that the occurrence and strength of
overcharging strongly depends on the salt-ion size, and the available volume in
the fluid. To understand the role of the excluded volume of the microions, we
first consider an uncharged system. For a fixed bulk concentration we find that
upon increasing the fluid particle size one strongly increases the local
concentration nearby the colloidal surface and that the particles become
laterally ordered. For a charged system the first surface layer is built up
predominantly by strongly correlated counterions. We argue that this a key
mechanism to produce overcharging with a low electrostatic coupling, and as a
more practical consequence, to account for charge inversion with monovalent
aqueous salt ions.Comment: 7 pages, 3 figs (4 EPS files). To appear in Europhysics Letter
Influence of hydrogen on paramagnetic defects induced by UV laser exposure in natural silica
Diffusion limited reactions of point defects were investigated in amorphous
SiO2 exposed to UV laser light. Electron spin resonance and in situ absorption
measurements at room temperature evidenced the annealing of E' centers and the
growth of H(II) centers both occurring in the post-irradiation stage and
lasting a few hours. These transients are caused by reactions involving
molecular hydrogen H2, made available by dimerization of radiolytic H0.Comment: Submitted to Physica Status Solid
Behavior of block-polyampholytes near a charged surface
The behavior of polyampholytes near a charged planar surface is studied by
means of Monte Carlo simulations. The investigated polyampholytes are overall
electrically neutral and made up of oppositely charged units (called blocks)
that are highly charged and of the the same length. The influence of block
length and substrate's surface-charge-density on the adsorption behavior is
addressed. A detailed structural study, including local monomer concentration,
monomer mean height, transversal chain size, interface-bond orientation
correlation, is provided. It is demonstrated that adsorption is favored for
long enough blocks and/or high enough Coulomb interface-ion couplings. By
explicitly measuring the chain size in the bulk, it is shown that the charged
interface induces either a swelling or a shrinkage of the transversal dimension
of the chain depending, in a non trivial manner, on the block length.Comment: 23 pages - 10 eps figs. Submitted for publicatio
Time evolution of a pair of distinguishable interacting spins subjected to controllable and noisy magnetic fields
The quantum dynamics of a
-conserving
Hamiltonian model describing two coupled spins and
under controllable and fluctuating time-dependent magnetic
fields is investigated. Each eigenspace of is dynamically
invariant and the Hamiltonian of the total system restricted to any one of such
eigenspaces, possesses the SU(2) structure of the
Hamiltonian of a single fictitious spin acted upon by the total magnetic field.
We show that such a reducibility holds regardless of the time dependence of the
externally applied field as well as of the statistical properties of the noise,
here represented as a classical fluctuating magnetic field. The time evolution
of the joint transition probabilities of the two spins and
between two prefixed factorized states is examined,
bringing to light peculiar dynamical properties of the system under scrutiny.
When the noise-induced non-unitary dynamics of the two coupled spins is
properly taken into account, analytical expressions for the joint Landau-Zener
transition probabilities are reported. The possibility of extending the
applicability of our results to other time-dependent spin models is pointed
out.Comment: 11 pages, 5 figure
N-qubit states as points on the Bloch sphere
We show how the Majorana representation can be used to express the pure
states of an N-qubit system as points on the Bloch sphere. We compare this
geometrical representation of N-qubit states with an alternative one, proposed
recently by the present authors.Comment: 9 pages, 2 figures, contribution to CEWQO 2009 proceedings. v2: Minor
changes, published versio
An example of interplay between Physics and Mathematics: Exact resolution of a new class of Riccati Equations
A novel recipe for exactly solving in finite terms a class of special
differential Riccati equations is reported. Our procedure is entirely based on
a successful resolution strategy quite recently applied to quantum dynamical
time-dependent SU(2) problems. The general integral of exemplary differential
Riccati equations, not previously considered in the specialized literature, is
explicitly determined to illustrate both mathematical usefulness and easiness
of applicability of our proposed treatment. The possibility of exploiting the
general integral of a given differential Riccati equation to solve an SU(2)
quantum dynamical problem, is succinctly pointed out.Comment: 10 page
Polyelectrolyte Multilayering on a Charged Planar Surface
The adsorption of highly \textit{oppositely} charged flexible
polyelectrolytes (PEs) on a charged planar substrate is investigated by means
of Monte Carlo (MC) simulations. We study in detail the equilibrium structure
of the first few PE layers. The influence of the chain length and of a (extra)
non-electrostatic short range attraction between the polycations and the
negatively charged substrate is considered. We show that the stability as well
as the microstructure of the PE layers are especially sensitive to the strength
of this latter interaction. Qualitative agreement is reached with some recent
experiments.Comment: 28 pages; 11 (main) Figs - Revtex4 - Higher resolution Figs can be
obtained upon request. To appear in Macromolecule
- …