Local structural order in carbonic acid polymorphs: Raman and FT‐IR spectroscopy

Two different polymorphs of carbonic acid, α‐ and β‐H₂CO₃, were identified and characterized using infrared spectroscopy (FT‐IR) previously. Our attempts to determine the crystal structures of these two polymorphs using powder and thin‐film X‐ray diffraction techniques have failed so far. Here, we report the Raman spectrum of the α‐polymorph, compare it with its FT‐IR spectrum and present band assignments in line with our work on the β‐polymorph [Angew. Chem. Int. Ed. 48 (2009) 2690–2694]. The Raman spectra also contain information in the wavenumber range ∼90–400 cm⁻¹, which was not accessible by FT‐IR spectroscopy in the previous work. While the α‐polymorph shows Raman and IR bands at similar positions over the whole accessible range, the rule of mutual exclusion is obeyed for the β‐polymorph. This suggests that there is a center of inversion in the basic building block of β‐H₂CO₃ whereas there is none in α‐H₂CO₃. Thus, as the basic motif in the crystal structure we suggest the cyclic carbonic acid dimer containing a center of inversion in case of β‐H₂CO₃ and a catemer chain or a sheet‐like structure based on carbonic acid dimers not containing a center of inversion in case of α‐H₂CO₃. This hypothesis is strengthened when comparing Raman active lattice modes at < 400 cm⁻¹ with the calculated Raman spectra for different dimers. In particular, the intense band at 192 cm⁻¹ in β‐H₂CO₃ can be explained by the inter‐dimer stretching mode of the centrosymmetric RC(OHO)₂ CR entity with ROH. The same entity can be found in gas‐phase formic acid (RH) and in β‐oxalic acid (RCOOH) and produces an intense Raman active band at a very similar wavenumber. The absence of this band in α‐H₂CO₃ confirms that the difference to β‐H₂CO₃ is found in the local coordination environment and/or monomer conformation rather than on the long range. Copyright © 2011 John Wiley & Sons, Ltd.     

Dual-channel spectral subtraction algorithms based speech enhancement dedicated to a bilateral cochlear implant

In this paper, two speech enhancement algorithms (SEAs) based on spectral subtraction (SS) principle have been evaluated for bilateral cochlear implant (BCI) users. Specifically, dual-channel noise power spectral estimation algorithm using power spectral densities (PSD) and cross power spectral density (CPSD) of the observed signals was studied. The enhanced speech signals were obtained using either Dual Channel Non Linear Spectral Subtraction ‘DC-NLSS’ or Dual-Channel Multi-Band Spectral Subtraction ‘DC-MBSS’ algorithms. For performance evaluation, some objective speech assessment tests relying on Perceptual Evaluation of Speech Quality (PESQ) score and speech Itakura-Saito (IS) distortion measurement were performed to fix the optimal number of frequency band needed in DC-MBSS algorithm. In order to evaluate the speech intelligibility, subjective listening tests were assessed with 50 normal hearing listeners using a specific BCI simulator and with three deafened BCI patients. Experimental results, obtained using French Lafon database corrupted by an additive babble noise at different Signal-to-Noise Ratios (SNR), showed that DC-MBSS algorithm improves speech understanding better than DC-NLSS algorithm for single and multiple interfering noise sources.     

Ultraprecise measurement of optical frequency ratios

We developed a novel technique for frequency measurement and synthesis, based on the operation of a femtosecond comb generator as transfer oscillator. The technique can be used to measure frequency ratios of any optical signals throughout the visible and near-infrared part of the spectrum. Relative uncertainties of 10(-18) for averaging times of 100 s are possible. Using a Nd:YAG laser in combination with a nonlinear crystal we measured the frequency ratio of the second harmonic nu(SH) at 532 nm to the fundamental nu(0) at 1064 nm, nu(SH)/nu(0) = 2.000 000 000 000 000 001x (1 +/- 7x10(-19)).     

光学特异材料的设计

In this contribution we review our latest achievements of combined experimental and theoretical studies to tailor the properties of optical metamaterials（MMs） at will. We give three examples of metamaterial designs that have been realized by means of electron-beam lithography and whose spectroscopic characteristics have been comprehensively investigated. In every case, our experiments are complemented by rigorous numer ical simulations. Particular emphasis is put on the significance of such tailored effective properties of optical MMs     

A covariant representation of the Ball–Chiu vertex

In nonabelian gauge theory the three-gluon vertex function contains important structural information, in particular on infrared divergences, and is also an essential ingredient in the Schwinger–Dyson equations. Much effort has gone into analyzing its general structure, and at the one-loop level also a number of explicit computations have been done, using various approaches. Here we use the string-inspired formalism to unify the calculations of the scalar, spinor and gluon loop contributions to the one-loop vertex, leading to an extremely compact representation in all cases. The vertex is computed fully off-shell and in dimensionally continued form, so that it can be used as a building block for higher-loop calculations. We find that the Bern–Kosower loop replacement rules, originally derived for the on-shell case, hold off-shell as well. We explain the relation of the structure of this representation to the low-energy effective action, and establish the precise connection with the standard Ball–Chiu decomposition of the vertex. This allows us also to predict that the vanishing of the completely antisymmetric coefficient function S   of this decomposition is not a one-loop accident, but persists at higher-loop orders. The sum rule found by Binger and Brodsky, which leads to the vanishing of the one-loop vertex in N=4$N=4$ SYM theory, in the present approach relates to worldline supersymmetry.     

Nonaffine deformation of inherent structure as a static signature of cooperativity in supercooled liquids

We unveil the existence of nonaffinely rearranging regions in the inherent structures (IS) of supercooled liquids by numerical simulations of model glass formers subject to static shear deformations combined with local energy minimizations. In the liquid state IS, we find a broad distribution of large rearrangements which are correlated only over small distances. At low temperatures, the onset of the cooperative dynamics corresponds to much smaller displacements correlated over larger distances. This finding indicates the presence of nonaffinely rearranging domains of relevant size in the IS deformation, which can be seen as the static counterpart of the cooperatively rearranging regions in the dynamics. This idea provides new insight into possible structural signatures of slow cooperative dynamics of supercooled liquids and supports the connections with elastic heterogeneities found in amorphous solids.     

Chirality sensitive effect on surface states in chiral p-wave superconductors

We study the local density of states at the surface of a chiral p-wave superconductor in the presence of a weak magnetic field. As a result, the formation of low-energy Andreev bound states is either suppressed or enhanced by an applied magnetic field, depending on its orientation with respect to the chirality of the p-wave superconductor. Similarly, an Abrikosov vortex, which is situated not too far from the surface, leads to a zero-energy peak of the density of states, if its chirality is the same as that of the superconductor, and to a gap structure for the opposite case. We explain the underlying principle of this effect and propose a chirality sensitive test on unconventional superconductors.