Interacting quantum wires: A possible explanation for the 0.7 anomalous conductance

We investigate an effective one-dimensional conducting channel considering both the contact umklapp and the Coulomb electron-electron interaction. We show that, at low electronic density, the proximity to the Wigner crystal reproduces the anomaly in conductance at 0.7G₀. The crucial ingredient of our theory is the fact that the gate voltage acts as a bias controlling the intensity of the umklapp term. At large gate voltages, the umklapp vanishes and we obtain a conducting quantum wire with a perfect conductance. At low gate voltages, the Wigner crystal is pinned by the umklapp term, giving rise to an insulating behavior with vanishing conductance. This crossover pattern has a transition point which can be identified with the anomalous conductance around 0.7G₀. This picture is obtained within the framework of a renormalization group calculation. The conductance static regime is achieved by taking first the limit of finite length and then the limit of zero frequency.     

Rapid crack growth in a thermoelastic solid under mixed-mode thermomechanical loading

A two-dimensional steady-sate analysis of semi-infinite brittlecrack growth at a constant subcritical rate in an unboundedfully-coupled thermoelastic solid under mixed-mode thermomechanicalloading is made. The loading consists of normal and shear tractionsand heat fluxes applied as point sources (line loads in theout-of-plane direction). A related problem is solved exactly in an integral transformspace, and robust asymptotic forms used to reduce the originalproblem to a set of integral equations. The equations are partiallycoupled and exhibit operators of both Cauchy and Abel types,yet can be solved analytically. The temperature change field at a distance from the moving crackedge is then constructed, and its dominant term is found tobe controlled by the imposed heat fluxes. The role of this termis, indeed, enhanced if the heat fluxes serve to render thecrack as a net heat source/sink for the solid, as opposed tobeing a transmitter of heat across its plane. More generally,the influence of the thermoelastic coupling on this field, aswell as other functions, is found to increase with crack speed.     

Structure et stabilité thermique des deux formes du formiate de magnesium dihydraté

We have prepared and studied a new form of dehydrated magnesium formate. This form is orthorhomic (a = 8.710(2), Å, b = 8.427(2), Å, c = 7.477(2)Å, Z = 4, Pbca). The structure (R = 0.060, R_w = 0.049) shows that the Mg are bonded together by formate bridges, thus forming sheets parallel to the x0y plane. Tridimensional cohesion is secured by water molecules inserted between sheets; these water molecules are bonded to the Mg of one sheet and, by hydrogen bonds, to two formate groups of the next neighbor sheet. From the structural comparison of the two forms (monoclinic and orthorhomic), a lower stability of the orthorhombic variety was predicted. This was confirmed by study of the thermal decomposition under argon atmosphere as observed by thermogravimetric and thermodifferential analysis, powder X-ray diffraction and infrared spectra.     

Synthetic magnetic opals

We present studies of novel nanocomposites of BiNi impregnated into the structure of opals as well as inverse opals. Atomic force microscopy and high resolution elemental analyses show a highly ordered structure and uniform distribution of the BiNi filler in the matrix. These BiNi-based nanocomposites are found to exhibit distinct ferromagnetic-like ordering with transition temperature of about 675 K. As far as we know there exists no report in literature on any BiNi compound which is magnetic.     

Noise-driven nonlinear sigma model

We present a noise-driven model for obtaining the gap and line-width as functions of the temperature in the nonlinear sigma model. The method is phenomenological and rests on the following physical idea: a classical external stochastic field is introduced representing the coupling of the sigma field with a noise source. Moreover, we assume that the inelastic scattering length is much longer than the elastic one, justifying the neglect of dissipation for temperatures such that the nonlinear sigma model is a good approximation for antiferromagnetic spin chains. This phenomenological approach is justified by comparison with other theoretical predictions and with experiment.     

Microstructure mapping of a friction stir welded AA2050 Al–Li–Cu in the T8 state

The heterogeneous precipitate microstructure of a AA2050 Al–Li–Cu in the T8 state after friction stir welding has been mapped by small-angle X-ray scattering (SAXS). 2D resolved maps of the fraction and size of both T₁ platelets precipitates and clusters/GP zones formed at room temperature are provided. TEM micrographs of selected zone confirm the interpretation of SAXS intensities. This microstructure mapping is compared to microhardness mapping and a direct correlation is shown. Short duration heat treatments made in a salt bath help understanding precipitate stability and suggest that the temperature exploration alone explains to a large extent the distribution of the precipitates microstructure across the welded structure.     

Resonance Raman scattering in graphene: Probing phonons and electrons

In this work, by using different laser excitation energies, we obtain important electronic and vibrational properties of mono- and bi-layer graphene. For monolayer graphene, we determine the phonon dispersion near the Dirac point for the in-plane transverse optical (iTO) mode. This result is compared with recent calculations that take into account electron–electron correlations for the phonon dispersion around the K point. For bilayer graphene we extract the Slonczewski–Weiss–McClure band parameters and compare them with recent infrared measurements. We also analyze the second-order feature in the Raman spectrum for trilayer graphene.     

Optical study of porphyrin-doped carbon nanotubes

Porphyrin-doped carbon nanotubes in sodium dodecylbenzenesulfonate (NaDDBS) aqueous solution were obtained from NaDDBS/carbon nanotube aqueous dispersions. Several phonon-assisted absorption and recombination processes seem to occur, including one-phonon and two-phonon processes, and remain present even upon porphyrin doping. Power-law scaling of exciton binding energies and environmental dielectric screening effects are used to infer the doping from photoluminescence maps. The dielectric constant of the 5,10,15,20-tetrakis(4-trimethylammonium phenyl) porphyrin (H₂TTMAPP) in NaDDBS aqueous solution seems to be higher than the one of NaDDBS/aqueous solution apparently because the counterions have opposite net charges.     

Sine-Gordon Model: Renormalization Group Solution and Applications

The sine-Gordon model is discussed and analyzed within the framework of the renormalization group theory. A perturbative renormalization group procedure is described, in which the sine-Gordon field is decomposed into slow and fast modes. An effective theory for the slow modes is derived and rescaled to yield the flow equations for the model. The resulting Kosterlitz–Thouless phase diagram is obtained and discussed in detail. The gap in this theory is estimated in terms of the sine-Gordon model parameters. The mapping between the sine-Gordon model and one-dimensional interacting-electron models, such as the g-ology and Hubbard models, is discussed. On the basis of the results borrowed from previous renormalization-group results for the sine-Gordon model, different aspects of Luttinger liquid systems are described, such as the nature of the excitations and phase transitions. The calculations are thoroughly and pedagogically described, to even reach the reader with no previous experience with the sine-Gordon model or the renormalization group approach.     

Transmission electron microscopy investigation of dislocation slip during superelastic cycling of Ni-Ti wires

Superelastic deformation of thin Ni-Ti wires containing various nanograined microstructures was investigated by tensile cyclic loading with in situ evaluation of electric resistivity. Defects created by the superelastic cycling in these wires were analyzed by transmission electron microscopy. The role of dislocation slip in superelastic deformation is discussed. Ni-Ti wires having finest microstructures (grain diameter <100 nm) are highly resistant against dislocation slip, while those with fully recrystallized microstructure and grain size exceeding 200 nm are prone to dislocation slip. The density of the observed dislocation defects increases significantly with increasing grain size. The upper plateau stress of the superelastic stress-strain curves is largely grain size independent from 10 up to 1000 nm. It is hence claimed that the Hall-Petch relationship fails for the stress-induced martensitic transformation in this grain size range. It is proposed that dislocation slip taking place during superelastic cycling is responsible for the accumulated irreversible strains, cyclic instability and degradation of functional properties. No residual martensite phase was found in the microstructures of superelastically cycled wires by TEM and results of the in situ electric resistance measurements during straining also indirectly suggest that none or very little martensite phase remains in the studied cycled superelastic wires after unloading. The accumulation of dislocation defects, however, does not prevent the superelasticity. It only affects the shape of the stress-strain response, makes it unstable upon cycling and changes the deformation mode from localized to homogeneous. The activity of dislocation slip during superelastic deformation of Ni-Ti increases with increasing test temperature and ultimately destroys the superelasticity as the plateau stress approaches the yield stress for slip. Deformation twins in the austenite phase ({1 1 4} compound twins) were frequently found in cycled wires having largest grain size. It is proposed that they formed in the highly deformed B19′ martensite phase during forward loading and are retained in austenite after unloading. Such twinning would represent an additional deformation mechanism of Ni-Ti yielding residual irrecoverable strains.