Vibrational origin of the fast relaxation processes in molecular glass-formers

We study the interaction of the relaxation processes with the density fluctuations by molecular dynamics simulation of a flexible molecule model for o-terphenyl (oTP) in the liquid and supercooled phases. We find evidence, besides the structural relaxation, of a secondary vibrational relaxation whose characteristic time, few ps, is slightly temperature dependent. This i) confirms the result by Monaco et al. [Phys. Rev, E 62, 7595 (2000)] of the vibrational nature of the fast relaxation observed in Brillouin Light Scattering (BLS) experiments in oTP; and ii) poses a caveat on the interpretation of the BLS spectra of molecular systems in terms of a purely center of mass dynamics.Comment: RevTeX, 5 pages, 4 eps figure

Difficult Biliary Stones: A Comprehensive Review of New and Old Lithotripsy Techniques

Biliary stones represent the most common indication for therapeutic endoscopic retrograde cholangiopancreatography. Many cases are successfully managed with biliary sphincterotomy and stone extraction with balloon or basket catheters. However, more complex conditions secondary to the specific features of stones, the biliary tract, or patient's needs could make the stone extraction with the standard techniques difficult. Traditionally, mechanical lithotripsy with baskets has been reported as a safe and effective technique to achieve stone clearance. More recently, the increasing use of endoscopic papillary large balloon dilation and the diffusion of single-operator cholangioscopy with laser or electrohydraulic lithotripsy have brought new, safe, and effective therapeutic possibilities to the management of such challenging cases. We here summarize the available evidence about the endoscopic management of difficult common bile duct stones and discuss current indications of different lithotripsy techniques

On the Yang-Lee and Langer singularities in the O(n) loop model

We use the method of `coupling to 2d QG' to study the analytic properties of the universal specific free energy of the O(n) loop model in complex magnetic field. We compute the specific free energy on a dynamical lattice using the correspondence with a matrix model. The free energy has a pair of Yang-Lee edges on the high-temperature sheet and a Langer type branch cut on the low-temperature sheet. Our result confirms a conjecture by A. and Al. Zamolodchikov about the decay rate of the metastable vacuum in presence of Liouville gravity and gives strong evidence about the existence of a weakly metastable state and a Langer branch cut in the O(n) loop model on a flat lattice. Our results are compatible with the Fonseca-Zamolodchikov conjecture that the Yang-Lee edge appears as the nearest singularity under the Langer cut.Comment: 38 pages, 16 figure

Glassy behavior of a homopolymer from molecular dynamics simulations

We study at- and out-of-equilibrium dynamics of a single homopolymer chain at low temperature using molecular dynamics simulations. The main quantities of interest are the average root mean square displacement of the monomers below the theta point, and the structure factor, as a function of time. The observation of these quantities show a close resemblance to those measured in structural glasses and suggest that the polymer chain in its low temperature phase is in a glassy phase, with its dynamics dominated by traps. In equilibrium, at low temperature, we observe the trapping of the monomers and a slowing down of the overall motion of the polymer as well as non-exponential relaxation of the structure factor. In out-of-equilibrium, at low temperatures, we compute the two-time quantities and observe breaking of ergodicity in a range of waiting times, with the onset of aging.Comment: 11 pages, 4 figure

The worldwide air transportation network: Anomalous centrality, community structure, and cities' global roles

We analyze the global structure of the world-wide air transportation network, a critical infrastructure with an enormous impact on local, national, and international economies. We find that the world-wide air transportation network is a scale-free small-world network. In contrast to the prediction of scale-free network models, however, we find that the most connected cities are not necessarily the most central, resulting in anomalous values of the centrality. We demonstrate that these anomalies arise because of the multi-community structure of the network. We identify the communities in the air transportation network and show that the community structure cannot be explained solely based on geographical constraints, and that geo-political considerations have to be taken into account. We identify each city's global role based on its pattern of inter- and intra-community connections, which enables us to obtain scale-specific representations of the network.Comment: Revised versio

Large Scale Cross-Correlations in Internet Traffic

The Internet is a complex network of interconnected routers and the existence of collective behavior such as congestion suggests that the correlations between different connections play a crucial role. It is thus critical to measure and quantify these correlations. We use methods of random matrix theory (RMT) to analyze the cross-correlation matrix C of information flow changes of 650 connections between 26 routers of the French scientific network `Renater'. We find that C has the universal properties of the Gaussian orthogonal ensemble of random matrices: The distribution of eigenvalues--up to a rescaling which exhibits a typical correlation time of the order 10 minutes--and the spacing distribution follow the predictions of RMT. There are some deviations for large eigenvalues which contain network-specific information and which identify genuine correlations between connections. The study of the most correlated connections reveals the existence of `active centers' which are exchanging information with a large number of routers thereby inducing correlations between the corresponding connections. These strong correlations could be a reason for the observed self-similarity in the WWW traffic.Comment: 7 pages, 6 figures, final versio

Improvement of Fault Current Calculation and Static Security Risk for Droop Control of the Inverter-Interfaced DG of Grid-Connected and Isolated Microgrids

The contribution current of an inverter-interfaced distributed generator unit during a fault is one of the significant challenges for two modes: grid-connected and isolated AC microgrid. For this challenge, this article is aimed to study two methods of fault current calculation for two modes: grid-connected and isolated microgrids. These methods include a virtual equivalent impedance and a proposed method. The proposed method is a new technique for calculating the fault current contribution depending on the droop control of inverter-interfaced DG. The proposed method can control the contribution short-circuit current of DG within its limit (2 p.u.) where it is dependent on the voltage value of the DG bus to calculate the short circuit current of DG by using the control criterion. Static security risk and load shedding are calculated after fault clearance using an operation scenario in which the distribution system will be divided into small subsystems and is then gridconnected and isolated due to the removal of the faulted bus by protection devices. The proposed technique is applied to a standard IEEE 33-bus distribution network with five DGs. The results show that the contribution current of inverter-interfaced DG during the fault has more effects than the fault current of the nearest faulted bus to the DG bus. The proposed technique improves the calculated fault current value by about 30% for the grid-connected microgrid and by about 50% for the isolated microgrid from its value of the virtual impedance method. The static security risk is improved after load shedding. The static security risk improved by about 0.025%. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.Đại Học Lạc Hồng; Ministry of Higher Education, Egypt, MHEFunding: The study presented in this paper is funded in part by the Ministry of Higher Education in Egypt and in part by the Lac Hong University in Vietnam

Statistical Mechanics of Glass Formation in Molecular Liquids with OTP as an Example

We extend our statistical mechanical theory of the glass transition from examples consisting of point particles to molecular liquids with internal degrees of freedom. As before, the fundamental assertion is that super-cooled liquids are ergodic, although becoming very viscous at lower temperatures, and are therefore describable in principle by statistical mechanics. The theory is based on analyzing the local neighborhoods of each molecule, and a statistical mechanical weight is assigned to every possible local organization. This results in an approximate theory that is in very good agreement with simulations regarding both thermodynamical and dynamical properties

Survey of the American Society of Neuroradiology Membership on the Use and Value of Intracranial Vessel Wall MRI

BACKGROUND AND PURPOSE: Intracranial vessel wall MR imaging is an emerging technique for intracranial vasculopathy assessment. Our aim was to investigate intracranial vessel wall MR imaging use by the American Society of Neuroradiology (ASNR) members at their home institutions, including indications and barriers to implementation. MATERIALS AND METHODS: The ASNR Vessel Wall Imaging Study Group survey on vessel wall MR imaging use, frequency, applications, MR imaging systems and field strength used, protocol development approaches, vendor engagement, reasons for not using vessel wall MR imaging, ordering-provider interest, and impact on clinical care, was distributed to the ASNR membership between April 2 and August 30, 2019. RESULTS: There were 532 responses; 79 were excluded due to nonresponse and 42 due to redundant institutional responses, leaving 411 responses. Fifty-two percent indicated that their institution performs vessel wall MR imaging, with 71.5% performed at least 1–2 times/ month, most frequently on 3T MR imaging, and 87.7% using 3D sequences. Protocols most commonly included were T1-weighted pre- and postcontrast and TOF-MRA; 60.6% had limited contributions from vendors or were still in protocol development. Vasculopathy differentiation (94.4%), cryptogenic stroke (41.3%), aneurysm (38.0%), and atherosclerosis (37.6%) evaluation were the most common indications. For those not performing vessel wall MR imaging, interpretation (53.1%) or technical (46.4%) expertise, knowledge of applications (50.5%), or limitations of clinician (56.7%) or radiologist (49.0%) interest were the most common reasons. If technical/expertise obstacles were overcome, 56.4% of those not performing vessel wall MR imaging indicated that they would perform it. Ordering providers most frequently inquiring about vessel wall MR imaging were from stroke neurology (56.5%) and neurosurgery (25.1%), while 34.3% indicated that no providers had inquired. CONCLUSIONS: More than 50% of neuroradiology groups use vessel wall MR imaging for intracranial vasculopathy characterization and differentiation, emphasizing the need for additional technical and educational support, especially as clinical vessel wall MR imaging implementation continues to grow

Ductal Carcinoma in Situ of the Breast: MR Imaging Findings With Histopathologic Correlation.

Ductal carcinoma in situ (DCIS) is a noninvasive malignancy that is commonly encountered at routine breast imaging. It may be a primary tumor or may be seen in association with other focal higher-grade tumors. Early detection is important because of the large proportion of DCIS that can progress to invasive carcinoma. The extent of DCIS involvement is frequently underestimated at mammography, which can reliably help detect only calcified DCIS; consequently, magnetic resonance (MR) imaging evaluation can alter the course of treatment. Seven biopsy-proved cases of DCIS were evaluated with T2-weighted MR imaging sequences, as well as T1-weighted sequences performed both before and after contrast material administration. The signal intensity and enhancement patterns of the tumors were analyzed, and the findings were correlated with the relevant underlying histopathologic features. Common enhancement patterns of DCIS include clumped linear-ductal enhancement, clumped focal enhancement, and masslike enhancement. The most common enhancement distribution pattern is segmental, followed by focal, diffuse, linear-ductal, and regional patterns. At T2-weighted MR imaging, DCIS is typically isointense relative to breast parenchyma; less commonly, it is hypointense or hyperintense. The use of MR imaging in the evaluation of DCIS is controversial, and many questions remain with regard to treatment and management. However, breast MR imaging can be extremely useful in the preoperative diagnosis and evaluation of DCIS when used in conjunction with other imaging modalities