83 research outputs found
A numerical model of twin disc test arrangement for the evaluation of railway wheel wear prediction methods
Twin disc tests are commonly used to study wear in railway materials. In this work the implementation of a numerical model of the twin disc arrangement is given, which reproduces the distribution of tangential forces over the contact patch between the two discs. Wear is subsequently calculated by relating the forces and creepage between the two discs using three different wear functions found in the literature. The resulting wear rates are compared with experimental data for discs made of common railway wheel and rail steels. This allows a comparison and assessment of the validity of the different wear algorithms considered
Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates
We consider the lightly doped cuprates YCaBaCuO
and LaSrCuO (with ,0.04), where the presence of a
fluctuating nematic state has often been proposed as a precursor of the stripe
(or, more generically, charge-density wave) phase, which sets in at higher
doping. We phenomenologically assume a quantum critical character for the
longitudinal and transverse nematic, and for the charge-ordering fluctuations,
and investigate the effects of these fluctuations in Raman spectra. We find
that the longitudinal nematic fluctuations peaked at zero transferred momentum
account well for the anomalous Raman absorption observed in these systems in
the channel, while the absence of such effect in the channel
may be due to the overall suppression of Raman response at low frequencies,
associated with the pseudogap. While in YCaBaCuO the
low-frequency lineshape is fully accounted by longitudinal nematic collective
modes alone, in LaSrCuO also charge-ordering modes with finite
characteristic wavevector are needed to reproduce the shoulders observed in the
Raman response. This different involvement of the nearly critical modes in the
two materials suggests a different evolution of the nematic state at very low
doping into the nearly charge-ordered state at higher doping.Comment: 12 pages with 10 figures, to appear in Phys. Rev. B 201
First-Order Type Effects in YBaCuO at the Onset of Superconductivity
We present results of Raman scattering experiments on tetragonal for doping levels between 0 and
0.07 holes/CuO. Below the onset of superconductivity at , we find evidence of a diagonal superstructure. At ,
lattice and electron dynamics change discontinuously with the charge and spin
properties being renormalized at all energy scales. The results indicate that
charge ordering is intimately related to the transition at and
that the maximal transition temperature to superconductivity at optimal doping
depends on the type of ordering at .Comment: 4 pages, 4 figure
Raman scattering evidence for a cascade-like evolution of the charge-density-wave collective amplitude mode
The two-dimensional rare-earth tri-tellurides undergo a unidirectional
charge-density-wave (CDW) transition at high temperature and, for the heaviest
members of the series, a bidirectional one at low temperature. Raman scattering
experiments as a function of temperature on DyTe and on LaTe at 6 GPa
provide a clear-cut evidence for the emergence of the respective collective CDW
amplitude excitations. In the unidirectional CDW phase, we surprisingly
discover that the amplitude mode develops as a succession of two mean-field,
BCS-like transitions in different temperature ranges
Electron interactions and charge ordering in LaSrCuO
We present results of inelastic light scattering experiments on
single-crystalline LaSrCuO in the doping range and TlBaCuO at and . The main
emphasis is placed on the response of electronic excitations in the
antiferromagnetic phase, in the pseudogap range, in the superconducting state,
and in the essentially normal metallic state at , where no
superconductivity could be observed. In most of the cases we compare B
and B spectra which project out electronic properties close to
and , respectively. In the channel of electron-hole excitations
we find universal behavior in B symmetry as long as the material
exhibits superconductivity at low temperature. In contrast, there is a strong
doping dependence in B symmetry: (i) In the doping range we observe rapid changes of shape and temperature dependence of the
spectra. (ii) In LaSrCuO new structures appear for
which are superposed on the electron-hole continuum. The temperature dependence
as well as model calculations support an interpretation in terms of
charge-ordering fluctuations. For the response from fluctuations
disappears at B and appears at B symmetry in full agreement with
the orientation change of stripes found by neutron scattering. While, with a
grain of salt, the particle-hole continuum is universal for all cuprates the
response from fluctuating charge order in the range is so
far found only in LaSrCuO. We conclude that
LaSrCuO is close to static charge order and, for this reason,
may have a suppressed .Comment: 17 pages, 15 figure
A Low Rhodium Content Smart Catalyst for Hydrogenation and Hydroformylation Reactions
Abstract: This paper describes the preparation, broad characterization and study of activity in hydrogenation and hydroformylation reactions of an easily produced 0.18% Rh/Al2O3. Analytical studies on fresh and recycled samples shed light on the smart properties of such catalyst. Results showed high activity as well as fine/excellent chemoselectivity or regioselectivity, characteristics that may suggest a wide range of applicability. Graphic Abstract: The low metal content catalyst 0.18% Rh/Al2O3 was very active in both hydrogenation and hydroformylation reactions so providing intermediates for valuable APIs, as Nabumetone and Eletriptan, and a fragrance with a fresh, green-floral smell, that recalls scent of lily of the valley.[Figure not available: see fulltext.
analysis of hmds self assembled monolayer effect on trap density in pc70bm n type thin film transistors through admittance studies
Abstract In this work, n-type organic thin film transistors (OTFTs) were fabricated in the bottom-gate bottom-contact configuration, depositing a fullerene-derived semiconductor (PC 70 BM) by drop-casting technique on SiO 2 substrates treated with a self-assembled monolayer, namely the HMDS. The influence of the deposition temperature of the HMDS on the device performance was investigated, using three different temperatures. The relationship between the properties of the resulting semiconductor films and the electrical characteristics of the transistors was evaluated through admittance measurements. The frequency response of the devices vs. the bias was interpreted applying an electrical equivalent circuit to model the properties of the semiconductor and of the transistor conductive channel. The proposed model shows the critical role played by the quality of the insulator-semiconductor interface on the traps density in the semiconductor, and therefore on the increase of the mobility and on the reduction of the threshold voltage of the transistors
pc70bm n type thin film transistors influence of hmds deposition temperature on the devices properties
This study investigates the influence of the deposition temperature of hexamethyldisilazane (HMDS) on the performances of organic thin film transistors (OTFTs) using the [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) as semiconductor. N-type OTFTs have been fabricated using this fullerene derivative, deposited from solution by drop casting technique on HMDS self-assembled monolayer (SAM) deposited at three different temperatures, 7 °C, 25 °C and 60 °C, in order to evaluate the influence of these deposition conditions on the morphology of PC70BM films and on the electrical responses of fullerene derivative-based OTFTs. The effect of the treatments of the surfaces was observed through contact angle measurements. AFM imaging of the deposited material has been used to analyse its structure and morphology. The transistors performances have been evaluated through I vs. V static characterization and parameters extraction. Contact angle vs. HMDS deposition temperature shows the minimum value at 60 °C, instead here field effect mobility presents a maximum. It has been observed that the lower hydrophobicity of the surface of the SAM induces the formation of more homogeneous surface of the PC70BM film, resulting in an increase of the OTFTs performances
Dynamical properties of charged stripes in La(2-x)SrxCuO4
Inelastic light-scattering spectra of underdoped La_2−xSr_xCuO4 single crystals are presented which provide direct evidence of the formation of quasi-one-dimensional charged structures in the two-dimensional CuO2 planes. The stripes manifest themselves in a Drude-like peak at low energies and temperatures. The selection rules allow us to determine the orientation to be along the diagonals at x=0.02 and along the principal axes at x=0.10. The electron-lattice interaction determines the correlation length which turns out to be larger in compound classes with lower superconducting transition temperatures. Temperature is the only scale of the response at different doping levels demonstrating the importance of quantum critical behavior
A study on wear evaluation of railway wheels based on multibody dynamics and wear computation
The wear evolution of railway wheels is a very important issue in railway engineering. In the past, the reprofiling intervals of railway vehicle steel wheels have been scheduled according to designers' experience. Today, more reliable and accurate tools in predicting wheel wear evolution and wheelset lifetime can be used in order to achieve economical and safety benefits. In this work, a computational tool that is able to predict the evolution of the wheel profiles for a given railway system, as a function of the distance run, is presented. The strategy adopted consists of using a commercial multibody software to study the railway dynamic problem and a purpose-built code for managing its pre- and post-processing data in order to compute the wear. The tool is applied here to realistic operation scenarios in order to assess the effect of some service conditions on the wheel wear progression
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