Copyright (c) 2009 University of Nebraska - Lincoln All rights reserved. http://digitalcommons.unl.edu/physicsducharme Recent documents in Stephen Ducharme Publications en-us Thu, 17 Sep 2009 23:40:03 PDT 3600 http://digitalcommons.unl.edu/physicsducharme/57 http://digitalcommons.unl.edu/physicsducharme/57 Wed, 16 Sep 2009 12:49:45 PDT Thermal propagation and stable hot spots (normal domains) are studied in various high Tc superconducting films (Nb3Sn, Nb, NbN and Nb3Ge). The prediction of the thermal propagation velocity of the long-standing model of Broom and Rhoderick (1960) is verified quantitatively in the regime of its validity. A new energy balance model is shown to give reasonable quantitative agreement of the dependence of the propagation velocity on the length of short normal domains. The steady state (zero velocity) measurements indicate the existence of two distinct situations for films on high thermal conductivity (sapphire) substrates. For low power per unit area the film and substrate have the same temperature, and the thermal properties of the substrate dominate. However, for higher power densities in short hot spots, the coupling is relatively weak and the thermal properties of the film alone are important. Here a connection is made between the critical current stability of superconducting films and a critical hot spot size for thermal propagation. As a result efficient heat removal is shown to dominate the stabilisation of superconducting films. These models are used to explain quantitatively self-healing of hot spots and a curious switching phenomena in a superconducting film in which a hot spot grows and collapses repeatedly over a very narrow range of current. Finally there is a discussion of the potential use of the thermal propagation model in applications of superconductors, especially switches. Kenneth E. Gray http://digitalcommons.unl.edu/physicsducharme/56 http://digitalcommons.unl.edu/physicsducharme/56 Wed, 16 Sep 2009 12:44:23 PDT The photorefractive Properties of a nominally pure single crystal of BaTiO3 were altered by treating the crystal at 650 °C in oxygen at different partial pressures. Treatment altered the effective density of photorefractive charge carriers in the crystal and couls convert an inactive crystal into an active one. Treatment at low oxygen pressure (reduction) decreased the temperature of the tetragonal-to-cubic phase transition of the crystal and also decreased the measured optical band gap, implying that oxygen vacancies had been introduced into the bulk crystal. These oxygen vacancies are associated with negative photorefractive charge sonors. Either hole transport or electron transport dominated, depending on whether the partial pressure of oxygen was greater than or less than 1/2 atmosphere during treatment. The competing roles of electrons and holes are discussed. Stephen Ducharme http://digitalcommons.unl.edu/physicsducharme/55 http://digitalcommons.unl.edu/physicsducharme/55 Wed, 16 Sep 2009 12:14:41 PDT The ferroelectric and piezoelectric properties of newly synthesized polymer systems have been studied. To date PVDF and its copolymers P(VDF-TrFE) have provided the bulk of the knowledge pertaining to ferroelectricity in polymers. Recently, ultrathin ferroelectric films of P(VDF-TrFE) 70:30 have been fabricated using the Langmuir-Blodgett technique [4]. In this study, various new polymers have been synthesized by chemically altering the PVDF structure. This alteration was performed in order to enhance the amphiphilic nature of the polymer and thus improve the LB film quality and control. Various chemical groups have been used to replace the electropositive hydrogen and electronegative fluorine found in the traditional PVDF chemical structure, including Nitrile, Ester, and Methyl groups. In all cases the resulting chemical structure provides for a net dipole moment directed from the electronegative side of the monomer to the electropositive side. However, to obtain ferroelectricity these microscopic dipoles must first pack in a manner such that a reversible macroscopic dipole is obtained. Both structural and dielectric studies have been performed on a number of newly synthesized systems. The structural properties of these new materials were probed using both temperature-dependent x-ray diffraction and differential scanning calorimetry, while dielectric properties were investigated using electric field and temperature-dependent capacitance and polarization measurements. Matthew Poulsen http://digitalcommons.unl.edu/physicsducharme/54 http://digitalcommons.unl.edu/physicsducharme/54 Wed, 16 Sep 2009 12:14:40 PDT Kristin L. Kraemer http://digitalcommons.unl.edu/physicsducharme/53 http://digitalcommons.unl.edu/physicsducharme/53 Thu, 27 Aug 2009 12:17:19 PDT The polarization switching kinetics of ferroelectric polymer nanomesas was investigated using piezoresponse force microscopy. The nanomesas were made by self-organization from Langmuir-Blodgett films of a 70% vinylidene fluoride and 30% trifluoroethylene copolymer. The polarization switching time exhibits an exponential dependence on reciprocal voltage that is consistent with nucleation-type switching dynamics. R. V. Gaynutdinov http://digitalcommons.unl.edu/physicsducharme/52 http://digitalcommons.unl.edu/physicsducharme/52 Wed, 13 May 2009 09:35:13 PDT The field of electrets encompasses a wide range of materials that maintain an external electric field due to alignment of internal electric dipoles or trapped charge, or both [1]. Polymers incorporating molecular dipoles constitute an important class of electrets, and so it is fitting that we consider the conditions under which such a polymer can also be considered a ferroelectric [2]. Further, because ferroelectric polymers have many uses, it is fruitful to consider how their properties depend on molecular structure [3]. Stephen Ducharme http://digitalcommons.unl.edu/physicsducharme/51 http://digitalcommons.unl.edu/physicsducharme/51 Mon, 26 Jan 2009 12:33:39 PST The investigation of the finite-size effect in ferroelectric crystals and films has been limited by the experimental conditions. The smallest demonstrated ferroelectric crystals had a diameter of ~ 200 A and the thinnest ferroelectric films were ~ 200 A thick, macroscopic sizes on an atomic scale. Langmuir - Blodgett deposition of films one monolayer at a time has produced high quality ferroelectric films as thin as 10 A, made from polyvinylidene fluoride and its copolymers. These ultrathin films permitted the ultimate investigation of finite-size effects on the atomic thickness scale. Langmuir ± Blodgett films also revealed the fundamental two-dimensional character of ferroelectricity in these materials by demonstrating that there is no so-called critical thickness; films as thin as two monolayers (1 nm) are ferroelectric, with a transition temperature near that of the bulk material. The films exhibit all the main properties of ferroelectricity with a first-order ferroelectric - paraelectric phase transition: polarization hysteresis (switching); the jump in spontaneous polarization at the phase transition temperature; thermal hysteresis in the polarization; the increase in the transition temperature with applied field; double hysteresis above the phase transition temperature; and the existence of the ferroelectric critical point. The films also exhibit a new phase transition associated with the two- dimensional layers. L. M. Blinov http://digitalcommons.unl.edu/physicsducharme/50 http://digitalcommons.unl.edu/physicsducharme/50 Tue, 06 Jan 2009 13:34:56 PST The switching in one monolayer of ferroelectric vinylidene fluoride-trifluoroethylene copolymer P[VDF-TrFE] is observed. The kinetics of switching is well described by Landau- Khalatnikov (LK) equation. The paper is devoted to the 75th anniversary of Prof. Lemanov's birthday. Vladimir Fridkin http://digitalcommons.unl.edu/physicsducharme/49 http://digitalcommons.unl.edu/physicsducharme/49 Tue, 06 Jan 2009 13:28:13 PST It is a great honor and responsibility to be Guest Editors for the special issue of Ferroelectrics, dedicated to the 90th Birthday of Nobel Prize Winner Prof. Vitaly Ginzburg. We have entitled our Guest Editorial "The Last Classical Physicist" because his creative work practically covers all regions of the modern physics: astronomy, astrophysics, cosmic rays, high spin elementary particles, quantum electrodynamics, solid state physics, including superconductivity and superfluidity, superdiamagnetism and ferrotoroics, crystallooptics and last (but for this journal not least) the theory of ferroelectricity and soft mode conception. Vladimir Fridkin http://digitalcommons.unl.edu/physicsducharme/47 http://digitalcommons.unl.edu/physicsducharme/47 Fri, 02 Jan 2009 08:03:47 PST Langmuir-Blodgett (LB) films are well known structures prepared as a result of successive transfer of monolayers (ML) from the gas-liquid interface onto solid substrates. One impressive possibility of LB method is the opportunity to vary the thickness of the film to an accuracy of one transfer, which could coincide with one ML. The ferroelectric properties of a copolymer of vinylidene fluoride and trifluorethylene P[VDF-TrFE] prepared by LB deposition are investigated in the region of a few monolayers. A. Tolstousov