Correlation effects in quasi one dimensional electron wires

We explore the role of electron correlation in quasi one dimensional quantum wires as the range of the interaction potential is changed and their thickness is varied by performing exact quantum Monte Carlo simulations at various electronic densities. In the case of unscreened interactions with a long range 1/x tail there is a crossover from a liquid to a quasi Wigner crystal state as the density decreases. When this interaction is screened, quasi long range order is prevented from forming, although a significant correlation with 4 k_F periodicity is still present at low densities. At even lower electron concentration, exchange is suppressed and the spin-dependent interactions become negligible, making the electrons behave like spinless fermions. We show that this behavior is shared by the long range and screened interactions by studying the spin and charge excitations of the system in both cases. Finally, we study the effect of electron correlations in the double quantum wire experiment [Steinberg et al., Phys. Rev. B 77, 113307 (2006)], by introducing an accurate model for the screening in the experiment and explicitly including the finite length of the system in our simulations. We find that decreasing the electron density drives the system from a liquid to a state with quite strong 4 k_F correlations. This crossover takes place around $20 \mu m^{-1}$, the density where the electron localization occurs in the experiment. The charge and spin velocities are also in remarkable agreement with the experimental findings in the proximity of the crossover. We argue that correlation effects play an important role at the onset of the localization transition.Comment: minor improvements, 13 pages, 12 figure

Growing CeO2 Nanoparticles Within the Nano-Porous Architecture of the SiO2 Aerogel

In this study, new CeO2-SiO2 aerogel nanocomposites obtained by controlled growth of CeO2 nanoparticles within the highly porous matrix of a SiO2 aerogel are presented. The nanocomposites have been synthesized via a sol-gel route, employing cerium (III) nitrate as the CeO2 precursor and selected surfactants to control the growth of the CeO2 nanoparticles, which occurs during the supercritical drying of the aerogels. Samples with different loading of the CeO2 dispersed phase, ranging from 5 to 15%, were obtained. The nanocomposites showed the morphological features typical of the SiO2 aerogels such as open mesoporosity with surface area values up to 430 m2·g−1. TEM and XRD characterizations show that nanocrystals of the dispersed CeO2 nanophase grow within the aerogel already during the supercritical drying process, with particle sizes in the range of 3 to 5 nm. TEM in particular shows that the CeO2 nanoparticles are well-distributed within the aerogel matrix. We also demonstrate the stability of the nanocomposites under high temperature conditions, performing thermal treatments in air at 450 and 900°C. Interestingly, the CeO2 nanoparticles undergo a very limited crystal growth, with sizes up to only 7 nm in the case of the sample subjected to a 900°C treatment

Hexatic and mesoscopic phases in the 2D quantum Coulomb system

We study the Wigner crystal melting in a two dimensional quantum system of particles interacting via the 1/r Coulomb potential. We use quantum Monte Carlo methods to calculate its phase diagram, locate the Wigner crystal region, and analyze its instabilities towards the liquid phase. We discuss the role of quantum effects in the critical behavior of the system, and compare our numerical results with the classical theory of melting, and the microemulsion theory of frustrated Coulomb systems. We find a Pomeranchuk effect much larger then in solid helium. In addition, we find that the exponent for the algebraic decay of the hexatic phase differs significantly from the Kosterilitz-Thouless theory of melting. We search for the existence of mesoscopic phases and find evidence of metastable bubbles but no mesoscopic phase that is stable in equilibrium

Alcuni esempi di catalogazione e restauro nell’ambito della Collezione Storica degli Strumenti di Fisica dell’Università di Palermo

In questo articolo presenteremo e discuteremo della catalogazione e restauro e di alcuni strumenti scientifici di particolare interesse storico-didattico appartenenti alla Collezione Storica degli Strumenti di Fisica dell’Università di Palermo. La catalogazione è stata effettuata secondo le indicazioni fornite dell’Istituto Centrale per il Catalogo e la Documentazione, per mezzo della scheda per il patrimonio scientifico-tecnologico. Nell’articolo saranno discussi gli aspetti tecnici degli interventi di restauro effettuati e saranno indicate le linee guida generali per il proseguimento dell’attività avviata.The article deals with the cataloguing and restoration of some scientific instruments of particular historical and didactic interest belonging to the Historical Collection of the Physics Instruments of the University of Palermo. The restoration activities have been carried out side by side the cataloguing, which has been done following the instructions supplied by the Central Institute for Cataloguing and Documentation with the official form for the scientific and technological heritage. In the article, we will discuss the technical aspects of the work carried out and basic guidelines will be given for the continuation of the activity

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

Dynamical screening in strongly correlated metal SrVO3

The consequences of dynamical screening of Coulomb interaction among correlated electrons in realistic materials have not been widely considered before. In this letter we try to incorporate a frequency dependent Coulomb interaction into the state-of-the-art ab initio electronic structure computing framework of local density approximation plus dynamical mean-field theory, and then choose SrVO3 as a prototype material to demonstrate the importance of dynamical screening effect. It is shown to renormalise the spectral weight near the Fermi level, to increase the effective mass, and to suppress the t2g quasiparticle band width apparently. The calculated results are in accordance with very recent angle-resolved photoemission spectroscopy experiments and Bose factor ansatz calculations.Comment: 6 pages, 4 figures. arXiv admin note: text overlap with arXiv:1107.312

The Ultrasonic Field of Focused Trandsucers Through a Liquid-Solid Interface

This paper presents theoretical and experimental results on the ultrasonic field of focused immersion transducers. The French Atomic Energy Commission (C.E.A.) has developed a software which calculates the ultrasonic field produced by a focused (or unfocused) transducer through a liquid-solid interface at normal or oblique incidence. The radiation of the transducer is formulated by the method of the Rayleigh integral, extended to take into account the liquid-solid interface. Firstly we describe this model, then we present measurements of the ultrasonic field produced by focused transducers in steel blocks. Experiments have been made using, at low frequencies, an electrodynamic probe, and, at high frequencies, an optical probe