Application of the coherent state formalism to multiply excited states

A general expression is obtained for the matrix element of an m-body operator between coherent states constructed from multiple orthogonal coherent boson species. This allows the coherent state formalism to be applied to states possessing an arbitrarily large number of intrinsic excitation quanta. For illustration, the formalism is applied to the two-dimensional vibron model [U(3) model], to calculate the energies of all excited states in the large-N limit.Comment: LaTeX (iopart); 10 pages; to be published in J. Phys.

Weak distinction and the optimal definition of causal continuity

Causal continuity is usually defined by imposing the conditions (i) distinction and (ii) reflectivity. It is proved here that a new causality property which stays between weak distinction and causality, called feeble distinction, can actually replace distinction in the definition of causal continuity. An intermediate proof shows that feeble distinction and future (past) reflectivity implies past (resp. future) distinction. Some new characterizations of weak distinction and reflectivity are given.Comment: 9 pages, 2 figures. v2: improved and expanded version. v3: a few misprints have been corrected and a reference has been update

Hyperuniform long-range correlations are a signature of disordered jammed hard-particle packings

We show that quasi-long-range (QLR) pair correlations that decay asymptotically with scaling $r^{-(d+1)}$ in $d$-dimensional Euclidean space $\mathbb{R}^d$, trademarks of certain quantum systems and cosmological structures, are a universal signature of maximally random jammed (MRJ) hard-particle packings. We introduce a novel hyperuniformity descriptor in MRJ packings by studying local-volume-fraction fluctuations and show that infinite-wavelength fluctuations vanish even for packings with size- and shape-distributions. Special void statistics induce hyperuniformity and QLR pair correlations.Comment: 10 pages, 3 figures; changes to figures and text based on review process; accepted for publication at Phys. Rev. Let

Threading Through Macrocycles Enhances the Performance of Carbon Nanotubes as Polymer Fillers

In this work we study the reinforcement of polymers by mechanically interlocked derivatives of single-walled carbon nanotubes (SWNTs). We compare the mechanical properties of fibers made of polymers and of composites with pristine single-walled carbon nanotubes (SWNTs), mechanically interlocked derivatives of SWNTs (MINTs) and the corresponding supramolecular models. Improvements of both Young's modulus and tensile strength of up to 200 % were observed for the polystyrene-MINTs samples with an optimized loading of just 0.01 wt.%, while the supramolecular models with identical chemical composition and loading showed negligible or even detrimental influence. This behavior is found for three different types of SWNTs and two types of macrocycles. Molecular dynamics simulations show that the polymer adopts an elongated conformation parallel to the SWNT when interacting with MINT fillers, irrespective of the macrocycle chemical nature, whereas a more globular structure is taken upon facing with either pristine SWNTs or supramolecular models. The MINT composite architecture thus leads to a more efficient exploitation of the axial properties of the SWNTs and of the polymer chain at the interface, in agreement with experimental results. Our findings demonstrate that the mechanical bond imparts distinctive advantageous properties to SWNT derivatives as polymer fillers.Comment: 39 pages, 19 figure

ABC and Hamiltonian Monte-Carlo methods in COGARCH models

The analysis of financial series, assuming calendar effects and unequally spaced times over continuous time, can be studied by means of COGARCH models based on Lévy processes. In order to estimate the COGARCH model parameters, we propose to use two different Bayesian approaches. First, we suggest to use a Hamiltonian Montecarlo (HMC) algorithm that improves the performance of standard MCMC methods. Secondly, we introduce an Approximate Bayesian Computational (ABC) methodology which allows to work with analytically infeasible or computationally expensive likelihoods. After a simulation and comparison study for both methods, HMC and ABC, we apply them to model the behaviour of some NASDAQ time series and we discuss the results

Quality and technical efficiency do not evolve hand in hand in Spanish hospitals: Observational study with administrative data

Objective Recent evidence on the Spanish National Health System (SNHS) reveals a considerable margin for hospital efficiency and quality improvement. However, those studies do not consider both dimensions together. This study aims at jointly studying both technical efficiency (TE) and quality, classifying the public SNHS hospitals according to their joint performance. Methods Stochastic frontier analysis is used to estimate TE and multilevel logistic regressions to build a low-quality composite measure (LQ), which considers in-hospital mortality and safety events. All hospitalizations discharged in Spain in 2003 and 2013, in 179 acute-care general hospitals, were studied. Four scenarios of resulting performance were built setting yearly medians as thresholds for the overall sample, and according to hospital-complexity strata. Results Overall, since 2003, median TE improved and LQ reduced -from TE2003:0.89 to TE2013:0.93 and, from LQ2003:42.6 to LQ2013:27.7 per 1, 000 treated patients. The time estimated coefficient showed technical progress over the period. TE across hospitals showed scarce variability (CV2003:0.08 vs. CV2013:0.07), not so the rates of LQ (CV2003:0.64 vs. CV2013:0.76). No correlation was found between TE values and LQ rates. When jointly considering technical efficiency and quality, hospitals dealing with the highest clinical complexity showed the highest chance to be placed in optimal scenarios, also showing lesser variability between hospitals. Conclusions Efficiency and quality have improved in Spanish public hospitals. Not all hospitals experiencing improvements in efficiency equally improved their quality. The joint analysis of both dimensions allowed identifying those optimal hospitals according to this trade-off

Comparing hospital efficiency: An illustrative study of knee and hip replacement surgeries in Spain

WHO’s Health Systems Performance Assessment framework suggests monitoring a set of dimensions. This study aims to jointly assess productivity and quality using a treatment-based approach, specifically analyzing knee and hip replacement, two prevalent surgical procedures performed with consolidated technology and run in most acute-care hospitals. Focusing on the analysis of these procedures sets out a novel approach providing clues for hospital management improvements, covering an existing gap in the literature. The Malmquist index under the metafrontier context was used to estimate the productivity in both procedures and its decomposition in terms of efficiency, technical and quality change. A multilevel logistic regression was specified to obtain the in-hospital mortality as a quality factor. All Spanish public acute-care hospitals were classified according to their average severity attended, dividing them into three groups. Our study revealed a decrease in productivity mainly due to a decrease in the technological change. Quality change remained constant during the period with highest variations observed between one period to the next according to the hospital classification. The improvement in the technological gap between different levels was due to an improvement in quality. These results provide new insights of operational efficiency after incorporating the quality dimension, specifically highlighting a decreasing operational performance, confirming that the technological heterogeneity is a critical question when measuring hospital performance