417 research outputs found
Quantum limit of photothermal cooling
We study the problem of cooling a mechanical oscillator using the
photothermal (bolometric) force. Contrary to previous attempts to model this
system, we take into account the noise effects due to the granular nature of
photon absorption. This allows us to tackle the cooling problem down to the
noise dominated regime and to find reasonable estimates for the lowest
achievable phonon occupation in the cantilever
Physical approximations for the nonlinear evolution of perturbations in dark energy scenarios
The abundance and distribution of collapsed objects such as galaxy clusters
will become an important tool to investigate the nature of dark energy and dark
matter. Number counts of very massive objects are sensitive not only to the
equation of state of dark energy, which parametrizes the smooth component of
its pressure, but also to the sound speed of dark energy as well, which
determines the amount of pressure in inhomogeneous and collapsed structures.
Since the evolution of these structures must be followed well into the
nonlinear regime, and a fully relativistic framework for this regime does not
exist yet, we compare two approximate schemes: the widely used spherical
collapse model, and the pseudo-Newtonian approach. We show that both
approximation schemes convey identical equations for the density contrast, when
the pressure perturbation of dark energy is parametrized in terms of an
effective sound speed. We also make a comparison of these approximate
approaches to general relativity in the linearized regime, which lends some
support to the approximations.Comment: 15 pages, 2 figure
Insights into the Structure of Dot@Rod and Dot@Octapod CdSe@CdS Heterostructures
CdSe@CdS dot@rods with diameter around 6 nm and length of either
20, 27, or 30 nm and dot@octapods with pod diameters of ?15 nm and lengths of ?50
nm were investigated by X-ray absorption spectroscopy. These heterostructures are
prepared by seed-mediated routes, where the structure, composition, and morphology of
the CdSe nanocrystals used as a seed play key roles in directing the growth of the second
semiconducting domain. The local structural environment of all the elements in the
CdSe@CdS heterostructures was investigated at the Cd, S, and Se K-edges by taking
advantage of the selectivity of X-ray absorption spectroscopy, and was compared to pure
reference compounds. We found that the structural features of dot@rods are
independent of the size of the rods. These structures can be described as made of a
CdSe dot and a CdS rod, both in the wurtzite phase with a high crystallinity of both the
core and the rod. This result supports the effectiveness of high temperature colloidal
synthesis in promoting the formation of core@shell nanocrystals with very low
defectivity. On the other hand, data on the CdSe@CdS with octapod morphology suggest the occurrence of a core composed of
a CdSe cubic sphalerite phase with eight pods made of CdS wurtzite phase. Our findings are compared to current models
proposed for the design of functional heterostructures with controlled nanoarchitecture
Influence of the Ion Coordination Number on Cation Exchange Reactions with Copper Telluride Nanocrystals
Cu2-xTe nanocubes were used as starting seeds to access metal telluride
nanocrystals by cation exchanges at room temperature. The coordination number
of the entering cations was found to play an important role in dictating the
reaction pathways. The exchanges with tetrahedrally coordinated cations (i.e.
with coordination number 4), such as Cd2+ or Hg2+, yielded monocrystalline CdTe
or HgTe nanocrystals with Cu2-xTe/CdTe or Cu2-xTe/HgTe Janus-like
heterostructures as intermediates. The formation of Janus-like architectures
was attributed to the high diffusion rate of the relatively small tetrahedrally
coordinated cations, which could rapidly diffuse in the Cu2-xTe NCs and
nucleate the CdTe (or HgTe) phase in a preferred region of the host structure.
Also, with both Cd2+ and Hg2+ ions the exchange led to wurtzite CdTe and HgTe
phases rather than the more stable zinc-blende ones, indicating that the anion
framework of the starting Cu2- xTe particles could be more easily deformed to
match the anion framework of the metastable wurtzite structures. As hexagonal
HgTe had never been reported to date, this represents another case of
metastable new phases that can only be accessed by cation exchange. On the
other hand, the exchanges involving octahedrally coordinated ions (i.e. with
coordination number 6), such as Pb2+ or Sn2+, yielded rock-salt polycrystalline
PbTe or SnTe nanocrystals with Cu2-xTe@PbTe or Cu2-xTe@SnTe core@shell
architectures at the early stages of the exchange process. In this case, the
octahedrally coordinated ions are probably too large to diffuse easily through
the Cu2-xTe structure: their limited diffusion rate restricts their initial
reaction to the surface of the nanocrystals, where cation exchange is initiated
unselectively, leading to core@shell architectures.Comment: 11 pages, 7 figures in J. Am. Chem. Soc, 13 May 201
Fermionized photons in an array of driven dissipative nonlinear cavities
We theoretically investigate the optical response of a one-dimensional array
of strongly nonlinear optical microcavities. When the optical nonlinearity is
much larger than both losses and inter-cavity tunnel coupling, the
non-equilibrium steady state of the system is reminiscent of a strongly
correlated Tonks-Girardeau gas of impenetrable bosons. Signatures of strong
correlations are identified in the absorption spectrum of the system, as well
as in the intensity correlations of the emitted light. Possible experimental
implementations in state-of-the-art solid-state devices are discussed
Optical properties of atomic Mott insulators: from slow light to dynamical Casimir effects
We theoretically study the optical properties of a gas of ultracold,
coherently dressed three-level atoms in a Mott insulator phase of an optical
lattice. The vacuum state, the band dispersion and the absorption spectrum of
the polariton field can be controlled in real time by varying the amplitude and
the frequency of the dressing beam. In the weak dressing regime, the system
shows unique ultra-slow light propagation properties without absorption. In the
presence of a fast time modulation of the dressing amplitude, we predict a
significant emission of photon pairs by parametric amplification of the
polaritonic zero-point fluctuations. Quantitative considerations on the
experimental observability of such a dynamical Casimir effect are presented for
the most promising atomic species and level schemes
The coherent interaction between matter and radiation - A tutorial on the Jaynes-Cummings model
The Jaynes-Cummings (JC) model is a milestone in the theory of coherent
interaction between a two-level system and a single bosonic field mode. This
tutorial aims to give a complete description of the model, analyzing the
Hamiltonian of the system, its eigenvalues and eigestates, in order to
characterize the dynamics of system and subsystems. The Rabi oscillations,
together with the collapse and revival effects, are distinguishing features of
the JC model and are important for applications in Quantum Information theory.
The framework of cavity quantum electrodynamics (cQED) is chosen and two
fundamental experiments on the coherent interaction between Rydberg atoms and a
single cavity field mode are described.Comment: 22 pages, 7 figures. Tutorial. Submitted to a special issue of EPJ -
ST devoted to the memory of Federico Casagrand
Sensitivity and Insensitivity of Galaxy Cluster Surveys to New Physics
We study the implications and limitations of galaxy cluster surveys for
constraining models of particle physics and gravity beyond the Standard Model.
Flux limited cluster counts probe the history of large scale structure
formation in the universe, and as such provide useful constraints on
cosmological parameters. As a result of uncertainties in some aspects of
cluster dynamics, cluster surveys are currently more useful for analyzing
physics that would affect the formation of structure than physics that would
modify the appearance of clusters. As an example we consider the Lambda-CDM
cosmology and dimming mechanisms, such as photon-axion mixing.Comment: 24 pages, 8 eps figures. References added, discussion of scatter in
relations between cluster observables lengthene
Verification of a hatchery protocol for green mussel Perna viridis spat production in the Philippines using industry-scale facilities
Hatchery seed production of mussels has been identified as a sustainable method to address the problem of low production due to insufficient seed supply. We conducted the first hatchery production trials of the Asian green mussel Perna viridis spats in the Philippines to demonstrate the feasibility of producing a sufficient and reliable seed supply for grow-out operations. However, results of small-scale experiments cannot be directly rolled-out commercially unless these are verified in bigger-scale facilities approximating those of commercial hatcheries. Thus, techniques on broodstock collection, spawning, and larval rearing, developed by the project in the laboratory during the experimental trials, were applied and verified in a production run using industry-scale tank facilities. Mature broodstocks collected from traditional mussel growing areas were successfully spawned in the hatchery. Eggs were fertilized, and these developed into D-hinged larvae, pediveliger, and metamorphosed into early spat before fully developing into the spat stage. Successful larval rearing up to the spat stage required the use of appropriate algal species and rates of feeding, close monitoring of larval stages, and water quality management. The survival from eggs to D-hinged larvae, D-hinged larvae to pediveliger, pediveliger to early spat, D-hinged larvae to early spat, and early spat (1 mm) to fully grown spat was 77, 64, 6.4, 3.1, and 72%, respectively. This study has established the feasibility of producing P. viridis in commercial-scale hatchery facilities
Value of clinical, ultrasonographic and MRI signs as diagnostic differentiators of non-benign lipomatous tumours
Suspicion of malignant change within a lipoma is a common and increasing workload within the UK Sarcoma multidisciplinary team (MDT) network, and a source of considerable patient anxiety. Currently, there is no lipoma-specific data, with regard to which clinical or radiographic features predict non-benign histology, or calculate an odds-ratio specific to a lipomatous lesion being non-benign. We performed a 9-year, double-blind, unmatched cohort study, comparing post-operative histology outcomes (benign versus non-benign) versus 15 signs across three domains: Clinical (size of tumour, depth, growth noticed by patient, previous lipoma, patient felt pain), Ultrasonographic (size, depth, vascularity, heterogenous features, septae) and MRI (size, depth, vascularity, heterogenous features, septae, complete fat signal suppression). Receiver operating characteristic (ROC) analysis, odds ratios and binary logistic regression analysis was performed double-blind. When each sign is considered independently, (ROC analysis, followed by binary logistic regression) only Ultrasound depth is a significant predictor (p = 0.044) of a histologically non-benign lipoma. Ultrasonographically determined vascularity and septation were not statistically significant predictors. None of the clinical signs were statistically significant (p > 0.05). Of the MRI signs none was statistically significant (p > 0.05). However, heterogeneous MRI features fared better than MRI depth. Ultrasound signs (Pseudo R-Square = 0.105) are more predictive of the post-operation histology outcome than Clinical signs (Pseudo R-Square = 0.082) or MRI tests (Pseudo R-Square = 0.052) Ultrasound and Clinical tests combined (Pseudo R-Square = 0.147) are more predictive of the post-operation histology outcome than MRI tests (Pseudo R-Square = 0.052). This work challenges the traditional perception of “red-flag” signs when applied to lipomatous tumours. We provide accurate data upon which an informed choice can be made, and provides a robust bases for expedited risk/benefit. The importance of an experienced and cohesive MDT network is emphasised.peer-reviewe
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