593 research outputs found
Microcavity quantum-dot systems for non-equilibrium Bose-Einstein condensation
We review the practical conditions required to achieve a non-equilibrium BEC
driven by quantum dynamics in a system comprising a microcavity field mode and
a distribution of localised two-level systems driven to a step-like population
inversion profile. A candidate system based on eight 3.8nm layers of
In(0.23)Ga(0.77)As in GaAs shows promising characteristics with regard to the
total dipole strength which can be coupled to the field mode.Comment: 4 pages, 4 figures, to be published in J. Phys. Conf. Ser. for QD201
Quantum state preparation in semiconductor dots by adiabatic rapid passage
Preparation of a specific quantum state is a required step for a variety of
proposed practical uses of quantum dynamics. We report an experimental
demonstration of optical quantum state preparation in a semiconductor quantum
dot with electrical readout, which contrasts with earlier work based on Rabi
flopping in that the method is robust with respect to variation in the optical
coupling. We use adiabatic rapid passage, which is capable of inverting single
dots to a specified upper level. We demonstrate that when the pulse power
exceeds a threshold for inversion, the final state is independent of power.
This provides a new tool for preparing quantum states in semiconductor dots and
has a wide range of potential uses.Comment: 4 pages, 4 figure
Dynamics of glass-forming liquids. XVI. Observation of ultrastable glass transformation via dielectric spectroscopy
The transformation of vapor-deposited ultrastable glasses of indomethacin (IMC) into the supercooled liquid state near Tg is monitored by means of dielectric spectroscopy. Films with thickness between 400 and 800 nm are deposited on differential interdigitated electrode cells and their loss profiles are measured during isothermal annealing using a dual-channel impedance technique for frequencies between 0.03 and 100 Hz. All dielectric loss spectra observed during the transformation process can be explained by a volume fraction of the supercooled liquid that increases linearly with time. From the early stages of the transformation to the liquid that is formed via complete annealing of the ultrastable glass, the average dielectric relaxation time as well as the distribution of relaxation times of the liquid component are identical to those of the conventional liquid obtained by cooling the melt. The dependence of the transformation rate on the film thickness is consistent with a growth front mechanism for the direct conversion from the ultrastable glass to the equilibrium supercooled liquid. We conclude that the IMC liquid recovered from the ultrastable glass is structurally and dynamically identical to the conventional supercooled state
Correlations of Structure and Dynamics in an Aging Colloidal Glass
We study concentrated colloidal suspensions, a model system which has a glass
transition. Samples in the glassy state show aging, in that the motion of the
colloidal particles slows as the sample ages from an initial state. We study
the relationship between the static structure and the slowing dynamics, using
confocal microscopy to follow the three-dimensional motion of the particles.
The structure is quantified by considering tetrahedra formed by quadruplets of
neighboring particles. We find that while the sample clearly slows down during
aging, the static properties as measured by tetrahedral quantities do not vary.
However, a weak correlation between tetrahedron shape and mobility is observed,
suggesting that the structure facilitates the motion responsible for the sample
aging.Comment: Submitted to Solid State Communication
Finite-temperature critical point of a glass transition
We generalize the simplest kinetically constrained model of a glass-forming
liquid by softening kinetic constraints, allowing them to be violated with a
small finite rate. We demonstrate that this model supports a first-order
dynamical (space-time) phase transition, similar to those observed with hard
constraints. In addition, we find that the first-order phase boundary in this
softened model ends in a finite-temperature dynamical critical point, which we
expect to be present in natural systems. We discuss links between this critical
point and quantum phase transitions, showing that dynamical phase transitions
in dimensions map to quantum transitions in the same dimension, and hence
to classical thermodynamic phase transitions in dimensions. We make these
links explicit through exact mappings between master operators, transfer
matrices, and Hamiltonians for quantum spin chains.Comment: 10 pages, 5 figure
Reply to ``Comment on `Hole-burning experiments within glassy models with infinite range interactions' ''
This is a reply to the comments by Richter and Chamberlin, and Diezemann and
Bohmer to our paper (Phys. Rev. Lett. 85, 3448 (2000)). As further evidence for
the claims in this Letter, we here reproduce the nonlinear spectral
hole-burning experimental protocol in an equilibrated fully connected
spin-glass model and we exhibit frequency selectivity, together with a shift in
the base of the spectral hole.Comment: 1 page, two figures, to appear in Phys. Rev. Let
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
Glassy freezing of orbital dynamics in FeCr2S4 and FeSc2S4
We report on a thorough dielectric investigation of the glass-like freezing
of the orbital reorientation-dynamics, recently found for the crystalline
sulpho-spinels FeCr2S4 and FeSc2S4. As the orbital reorientations are coupled
to a rearrangement of the surrounding ionic lattice via the Jahn-Teller effect,
the freezing of the orbital moments is revealed by a relaxational behaviour of
the complex dielectric permittivity. Additional conductivity (both dc and ac)
and contact contributions showing up in the spectra are taken into account by
an equivalent circuit description. The orbital relaxation dynamics continuously
slows down over six decades in time, before at the lowest temperatures the
glass transition becomes suppressed by quantum tunnelling.Comment: J. Non-Cryst. Solids, in press. 6 pages, 4 figure
Subdiffusion and the cage effect studied near the colloidal glass transition
The dynamics of a glass-forming material slow greatly near the glass
transition, and molecular motion becomes inhibited. We use confocal microscopy
to investigate the motion of colloidal particles near the colloidal glass
transition. As the concentration in a dense colloidal suspension is increased,
particles become confined in transient cages formed by their neighbors. This
prevents them from diffusing freely throughout the sample. We quantify the
properties of these cages by measuring temporal anticorrelations of the
particles' displacements. The local cage properties are related to the
subdiffusive rise of the mean square displacement: over a broad range of time
scales, the mean square displacement grows slower than linearly in time.Comment: submitted to Chemical Physics, special issue on "Strange Kinetics
Absorption and photoluminescence spectroscopy on a single self-assembled charge-tunable quantum dot
We have performed detailed photoluminescence (PL) and absorption spectroscopy
on the same single self-assembled quantum dot in a charge-tunable device. The
transition from neutral to charged exciton in the PL occurs at a more negative
voltage than the corresponding transition in absorption. We have developed a
model of the Coulomb blockade to account for this observation. At large
negative bias, the absorption broadens as a result of electron and hole
tunneling. We observe resonant features in this regime whenever the quantum dot
hole level is resonant with two-dimensional hole states located at the capping
layer-blocking barrier interface in our structure.Comment: 6 pages, 6 figure
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