Optical traps for dark excitons

We propose a mechanism for optical trapping of dark excitons by linearly polarized unabsorbed standing waves, with a potential depth of the order of a few meV. Since this trapping, based on carrier exchanges with virtual excitons coupled to unabsorbed photons, equally acts on bright and dark states, Bose-Einstein condensation of excitons--which occurs in dark states--must appear as dark spots in a cloud of bright excitons, at the trap potential minima, when the temperature decreases.     

Dispersion and localization of electronic states at a ferrocene/Cu(111) interface

Low-temperature scanning tunneling microscopy and spectroscopy combined with first-principles simulations reveal a nondissociative physisorption of ferrocene molecules on a Cu(111) surface, giving rise to ordered molecular layers. At the interface, a 2D-like electronic band is found, which shows an identical dispersion as the Cu(111) Shockley surface-state band. Subsequent deposition of Cu atoms forms charged organometallic compounds that localize interface-state electrons.     

An NMR thermometer for cryogenic magic-angle spinning NMR: the spin-lattice relaxation of (127)I in cesium iodide

The accurate temperature measurement of solid samples under magic-angle spinning (MAS) is difficult in the cryogenic regime. It has been demonstrated by Thurber et al. (J. Magn. Reson., 196 (2009) 84-87) [10] that the temperature dependent spin-lattice relaxation time constant of ⁷⁹Br in KBr powder can be useful for measuring sample temperature under MAS over a wide temperature range (20–296 K). However the value of T₁ exceeds 3 min at temperatures below 20 K, which is inconveniently long. In this communication, we show that the spin-lattice relaxation time constant of ¹²⁷I in CsI powder can be used to accurately measure sample temperature under MAS within a reasonable experimental time down to 10 K.     

Recollections about our 1996 JMR paper on diffusion anisotropy

We provide scientific background information and personal accounts relating to our publication of “Microstructural and Physiological Features of Tissues Elucidated by Quantitative-Diffusion-Tensor MRI” in the Journal of Magnetic Resonance B. This paper provided a framework for measuring and mapping intrinsic features of diffusion anisotropy obtained from diffusion tensor MRI (DTI) data.     

Hydrogen abstraction from H‐donor substrates by the 6‐CF3‐benzotriazol‐N‐oxyl radical (TFNO)

The aminoxyl radical 6‐trifluoromethyl‐benzotriazol‐N‐oxyl (TFNO) has been generated from the parent hydroxylamine 6‐CF₃‐1‐hydroxy‐benzotriazole (TFBT) by one‐electron oxidation with a Ce^IV salt and characterized by spectrophotometry and cyclic voltammetry (CV). Rate constants of H‐abstraction (k_H) by TFNO from a number of H‐donor benzylic substrates have been determined spectrophotometrically in MeCN solution at 25 °C. A radical H‐atom transfer (HAT) route of oxidation is substantiated for TFNO by several pieces of evidence. The kinetic data also testify the relevance of stereoelectronic effects upon the HAT reactivity of TFNO. Copyright © 2010 John Wiley & Sons, Ltd.     

On the spectrum of stiffness matrices arising from isogeometric analysis

We study the spectral properties of stiffness matrices that arise in the context of isogeometric analysis for the numerical solution of classical second order elliptic problems. Motivated by the applicative interest in the fast solution of the related linear systems, we are looking for a spectral characterization of the involved matrices. In particular, we investigate non-singularity, conditioning (extremal behavior), spectral distribution in the Weyl sense, as well as clustering of the eigenvalues to a certain (compact) subset of \(\mathbb C\) . All the analysis is related to the notion of symbol in the Toeplitz setting and is carried out both for the cases of 1D and 2D problems.     

Sequential smoothing for turning point detection with application to financial decisions

A fundamental problem in financial trading is the correct and timely identification of turning points in stock value series. This detection enables to perform profitable investment decisions, such as buying‐at‐low and selling‐at‐high. This paper evaluates the ability of sequential smoothing methods to detect turning points in financial time series. The novel idea is to select smoothing and alarm coefficients on the gain performance of the trading strategy. Application to real data shows that recursive smoothers outperform two‐sided filters at the out‐of‐sample level. Copyright © 2012 John Wiley & Sons, Ltd.     

High-order moments conservation in thermostatted kinetic models

Recently the thermostatted kinetic framework has been proposed as mathematical model for studying nonequilibrium complex systems constrained to keep constant the total energy. The time evolution of the distribution function of the system is described by a nonlinear partial integro-differential equation with quadratic type nonlinearity coupled with the Gaussian isokinetic thermostat. This paper is concerned with further developments of this thermostatted framework. Specifically the term related to the Gaussian thermostat is adjusted in order to ensure the conservation of even high-order moments of the distribution function. The derived framework that constitutes a new paradigm for the derivation of specific models in the applied sciences, is analytically investigated. The global in time existence and uniqueness of the solution to the relative Cauchy problem is proved. Existence and moments conservation of stationary solutions are also performed. Suitable applications and research perspectives are outlined in the last section of the paper.     

An isoperimetric result for the fundamental frequency via domain derivative

The Faber–Krahn deficit $\delta \lambda $ of an open bounded set $\Omega $ is the normalized gap between the values that the first Dirichlet Laplacian eigenvalue achieves on $\Omega $ and on the ball having same measure as $\Omega $ . For any given family of open bounded sets of $\mathbb R ^N$ ( $N\ge 2$ ) smoothly converging to a ball, it is well known that both $\delta \lambda $ and the isoperimetric deficit $\delta P$ are vanishing quantities. It is known as well that, at least for convex sets, the ratio $\frac{\delta P}{\delta \lambda }$ is bounded by below by some positive constant (Brandolini et al., Arch Math (Basel) 94(4): 391–400, 2010; Payne and Weinberger, J Math Anal Appl 2:210–216, 1961), and in this note, using the technique of the shape derivative, we provide the explicit optimal lower bound of such a ratio as $\delta P$ goes to zero.