Reply to “Comment on ‘Optical determination of flexoelectric coefficients and surface polarization in a hybrid aligned nematic cell’ ”

A. Mazzulla, F. Ciuchi, and J. Roy Sambles, Physical Review E, Vol. 68, article 023702 (2003). "Copyright © 2003 by the American Physical Society."In their Comment [G. Barbero and L. R. Evangelista, Phys. Rev. E 68, 023701] on our paper [A. Mazzulla, F. Ciuchi, and J. R. Sambles, Phys. Rev. E 64, 021708 (2001)], Barbero and Evangelista conclude that the procedure followed by us to fit the reflectivity data from the half leaky guided mode technique is questionable. In the absence of a model that is able to reproduce the experimentally obtained tilt angle profiles, their argument is unsubstantiated. To further refute their arguments, we also illustrate and discuss additional experimental data (that were not shown in our paper) that strongly support our conclusions

The Charge Ordered State from Weak to Strong Coupling

We apply the Dynamical Mean Field Theory to the problem of charge ordering. In the normal state as well as in the Charge Ordered (CO) state the existence of polarons, i.e. electrons strongly coupled to local lattice deformation, is associated to the qualitative properties of the Lattice Polarization Distribution Function (LPDF). At intermediate and strong coupling a CO state characterized by a certain amount of thermally activated defects arise from the spatial ordering of preexisting randomly distributed polarons. Properties of this particular CO state gives a qualitative understanding of the low frequency behavior of optical conductivity of $Ni$ perovskites.Comment: 4 pages, 3 figures, to be published in J. of Superconductivity (proceedings Stripes 98

Current saturation and Coulomb interactions in organic single-crystal transistors

Electronic transport through rubrene single-crystal field effect transistors (FETs) is investigated experimentally in the high carrier density regime (n ~ 0.1 carrier/molecule). In this regime, we find that the current does not increase linearly with the density of charge carriers, and tends to saturate. At the same time, the activation energy for transport unexpectedly increases with increasing n. We perform a theoretical analysis in terms of a well-defined microscopic model for interacting Frohlich polarons, that quantitatively accounts for our experimental observations. This work is particularly significant for our understanding of electronic transport through organic FETs.Comment: Extended version with 1 additional figure and an appendix explaining the consistency of the theoretical calculatio

Electrophoretic deposition of bilayer composite films based on CoFe2O4 and Nb-doped PZT

The magnetoelectric (ME) composites with piezoelectric and magnetostrictive material are of interest in the smart manufacturing and mechatronics fields as actuators/transducers, sensors, antennas, filters, non-volatile memories, etc. Spinel cobalt ferrite (CFO), a highly magnetostrictive material and niobium-doped lead zirconate titanate (PZTN), a piezoelectric perovskite phase are chosen as constituent phases for ME composite. The production process is designed in order to avoid the chemical reaction between the piezoelectric and magnetostrictive materials, to prevent the formation of percolation chains of the magnetostrictive phase, and to maximize the mechanical coupling at the interface between the two phases. In this view, the electrophoretic deposition (EPD) is a low cost and flexible technique to shape nanoparticles into multilayered heterostructures. The combination of different materials by EPD, showing promising ME coupling, can be regarded as a useful, preliminary approach in the search of novel ME materials for many applications, potentially with great industrial and technological benefits. In this work, composite bilayer CFO/PZTN thick films were deposited on platinum coated alumina by EPD from ethanol-based colloidal suspensions. Good adhesion and compaction of the green film were achieved by optimization of deposition voltage and time, and high density of the film and minimized interphase reactions occurred after sintering. The chemical activity between the two layers was controlled through the batches composition and it could lead to the synthesis of complex engineered structures. The deposited volume, the mixing of dielectric and magnetic phases and the density and ordering of the films have been verified by electron scanning microscopy after heat treatment. The ferroelectric, piezoelectric and magnetic properties were tested on the sintered films

Formation and observation of a quasi-two-dimensional dxyd_{xy} electron liquid in epitaxially stabilized Sr2−x_{2-x}Lax_{x}TiO4_{4} thin films

We report the formation and observation of an electron liquid in Sr$_{2-x}$La$_{x}$TiO$_4$, the quasi-two-dimensional counterpart of SrTiO$_3$, through reactive molecular-beam epitaxy and {\it in situ} angle-resolved photoemission spectroscopy. The lowest lying states are found to be comprised of Ti 3$d_{xy}$ orbitals, analogous to the LaAlO$_3$/SrTiO$_3$ interface and exhibit unusually broad features characterized by quantized energy levels and a reduced Luttinger volume. Using model calculations, we explain these characteristics through an interplay of disorder and electron-phonon coupling acting co-operatively at similar energy scales, which provides a possible mechanism for explaining the low free carrier concentrations observed at various oxide heterostructures such as the LaAlO$_3$/SrTiO$_3$ interface

Dynamical mean-field theory of the small polaron

A dynamical mean-field theory of the small polaron problem is presented, which becomes exact in the limit of infinite dimensions. The ground state properties and the one-electron spectral function are obtained for a single electron interacting with Einstein phonons by a mapping of the lattice problem onto a polaronic impurity model. The one-electron propagator of the impurity model is calculated through a continued fraction expansion (CFE), both at zero and finite temperature, for any electron-phonon coupling and phonon energy. In contrast to the ground state properties such as the effective polaron mass, which have a smooth behaviour, spectral properties exhibit a sharp qualitative change at low enough phonon frequency: beyond a critical coupling, one energy gap and then more and more open in the density of states at low energy, while the high energy part of the spectrum is broad and can be explained by a strong coupling adiabatic approximation. As a consequence narrow and coherent low-energy subbands coexist with an incoherent featureless structure at high energy. The subbands denote the formation of quasiparticle polaron states. Also, divergencies of the self-energy may occur in the gaps. At finite temperature such effect triggers an important damping and broadening of the polaron subbands. On the other hand, in the large phonon frequency regime such a separation of energy scales does not exist and the spectrum has always a multipeaked structure.Comment: 21 Pages Latex, 19 PostScript figure

Tailoring the molecular structure to suppress extrinsic disorder in organic transistors

In organic field-effect transistors, the structure of the constituent molecules can be tailored to minimize the disorder experienced by charge carriers. Experiments on two perylene derivatives show that disorder can be suppressed by attaching longer core substituents - thereby reducing potential fluctuations in the transistor channel and increasing the mobility at low temperature - without altering the intrinsic transport properties