Scholarly Work - Physics
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Item ND4 Deuteron NMR and the Smearing of the Glass Transition in Rb1-x(ND4)xD2PO4(1988-05) Blinc, Robert; Dolinsek, J.; Schmidt, V. Hugo; Ailion, D.C.The broadening of the rotationally averaged ND4+ deuteron lines in Rb0.56(ND4)0.44D2PO4 due to the gradual "acid" deuteron freeze-out allows for a direct determination of the Edwards-Anderson pseudospin glass order parameter q far above the nominal glass transition temperature TG. The data provide strong evidence for a model where the basic difference between magnetic spin glasses and proton or deuteron pseudospin glasses is the presence of an inherent random field resulting from the substitutional disorder. This field acts as an ordering field conjugate to the pseudospin glass order parameter q and is responsible for the smearing of the deuteron glass transition.Item Short-range interaction explanation of ferroelectric, antiferroelectric and proton glass behavior in pure RDP, ADP, and mixed RDP-ADP crystals respectively(1985-01) Schmidt, V. Hugo; Wang, J. T.; Schnackenberg, P. T.A unified model is presented for Rb1-x(NH4)xH2PO4 crystals over the whole range x=0 (RDP) to x=1 (ADP). Two short-range interactions are postulated. One is the Slater energy ε0 which is kept at the value for RDP for all x. The other is an interaction εa between two hydrogens in O-H...O bonds across an NH4+ ion from each other. It is responsible for the off-center positions observed for ammonium ions in the antiferroelectric phase of ADP. Its strength is assumed proportional to x. By minimizing the free energy, one finds a range of x for which no transition occurs, but instead proton glass behavior sets in. Fox x near 0 and 1 respectively, first-order ferroelectric and antiferroelectric transitions are predicted. Both phase boundaries are close to those observed experimentally.Item Anisotropy in Anomalies of Hypersound Velocity and Attenuation in Ferroelectric TSCC(1985-01) Hikita, T.; Wang, J. T.; Schnackenberg, P. T.; Schmidt, V. HugoFrom Brillouin shift and linewidth of longitudinal phonons propagating along the [100] and [001] directions of TSCC, the polarization relaxation time was calculated to be τ=3.1×10-12/(Tc-T) sec below the transition temperature Tc. The anomalies in the longitudinal phonons of the [010] propagation were carefully examined using an annealed crystal of excellent quality. No essential difference was observed between the velocities of a normal and high quality crystals. The relaxation time was deduced as a function of temperature from the observed anomalies in the velocity and linewidth. Spectra are observed for nearly forward scattering from the q\varparallel[010] phonons.Item Magnetic pendulum apparatus for analog demonstration of first-order and second-order phase transitions and tricritical points(1984) Schmidt, V. Hugo; Childers, B. R.A simple apparatus is described in which the deflection of a pendulum with an electromagnet mounted at its tip is the analog of the magnitude of the order parameter in a phase transition. Its deflection is governed by the current through this electromagnet and currents through fixed electromagnets just below it. By suitable variation of these currents, the pendulum can be made to undergo gradual (second‐order transition analog) or sudden (first‐order transition analog) displacements from the vertical, as well as the intermediate situation corresponding to a tricritical point. Results for critical and tricritical exponents obtained with this apparatus are presented.Item Review of order-disorder models for KDP-family crystals(1987-03) Schmidt, V. HugoThe various order-disorder-type models for crystals in the KDP (potassium dihydrogen phosphate) family are reviewed, beginning with the Slater-Takagi-Senko model, continuing with tunneling additions by Blinc and Svetina, and analyzing the pressure-induced tricritical point in the Slater-Takagi-Senko model. The possibility of a deuteration-induced tricritical point at atmospheric pressure in RDP (rubidium dihydrogen phosphate) is explored. The Ishibashi model for ADP (ammonium dihydrogen phosphate) is reviewed, and finally various theories and computer simulations for the mixed RDP-ADP crystal system are discussed in terms of their prediction of the observed ferroelectric and antiferroelectric transitions and proton glass behavior.Item N.m.r. study of the ferroelectric phase transition in a 70/30 mol% copolymer of vinylidene fluoride (VF2) and trifluoroethylene (TrFE)(1985-07) Legrand, J. F.; Schuele, Paul J.; Schmidt, V. HugoNuclear magnetic resonance (n.m.r.) studies of 19F nuclei in a 70/30 mol% random copolymer of vinylidene fluoride and trifluoroethylene were performed at 9.14 MHz and 20.0 MHz. The free induction decays (FIDs) were analysed in terms of two T2 components attributed to the amorphous and crystalline portions of the polymer. The changes in crystallinity as well as the effects of the ferroelectric transition were observed during cycles of heating and cooling between 20°C and 140°C. The crystalline component of the FID lengthens by a factor of 2 at 100°C on heating and decreases by this factor at 60°C on cooling, thus exhibiting the thermal hysteresis of this ferroelectric transition. The spin-lattice relaxation was also investigated. From measurements at 9.14 MHz the observed longitudinal relaxation time T1 appears to be dominated by the dynamics of the amorphous phase and exhibits no anomaly through the phase transition. However, from measurements at 20 MHz, well defined minima of T1 were observed, which are associated with the ferroelectric transition (especially after repeated annealing of the samples). Results are discussed in terms of the crystalline phase structure, which appears dynamically disordered above the ferroelectric phase transition. An analogy is considered with the plastic phase transitions encountered in molecular crystals.Item Brillouin scattering near the ferroelectric phase transition in TSCC(1985-06) Hikita, T.; Schnackenberg, P. T.; Schmidt, V. HugoBrillouin spectra from longitudinal phonons in ferroelectric tris-sarcosine calcium chloride (TSCC) propagating along [100], [010] and [001] have been measured as functions of temperature. Large anomalies were found in the Brillouin shift and linewidth in the [100] and [001] phonons. These anomalies are interpreted as arising from the linear coupling of the polarization and phonons. From the the temperature where the linewidth is maximum, the relaxation time of the polarization fluctuations is estimated to be τ = 3.1×10−12/(Tc−T) sec, where Tc is the ferroelectric transition temperature. We also observed anomalies in Brillouin shift and linewidth of the [010] phonons which propagate along the ferroelectric axis. These anomalies are interpreted as coming from electrostrictive coupling. The energy relaxation time was estimated to be τE=2.5×10−10/(T−Tc) sec in the paraelectric (PE) phase and τE=1.0x10−9/(T−Tc) sec in the ferroelectric (FE) phase, by comparing our Brillouin results with those f the ultrasonic measurements.Item Dielectric studies of critical and tricritical phenomena in kdp and RD(1980-01) Schmidt, V. Hugo; Schnackenberg, P. T.; Western, Arthur B.; Baker, A. G.; Bacon, Charles R.; Crummett, W. P.Our method of obtaining and analyzing dielectric data for KDP is reviewed, which demonstrated that KDP has a tricritical point between two and three kbar pressure. We also describe our method for obtaining the critical exponents r and d. Our experimental results for KDP are summarized. New results are presented for RDP which agree with the conclusion of others that its transition is of second order at ambient pressure.Item Determination of Landau free-energy parameters by dielectric measurements in the ferroelectric TSCC(1981-05) Bornarel, J.; Schmidt, V. HugoThe authors measured the adiabatic dielectric permittivity epsilon b of tris-sarcosine calcium chloride (TSCC) at 400 Hz near the ferroelectric transition for DC bias fields up to 5000 V cm-1. They found epsilon infinity =4.2+or-0.2, Curie-Weiss constant C+=38.3+or-0.3K, T0=129.4+or-0.1K, and Landau coefficients A=(2.95+or-0.2)*109 V m A-1 s-1 K-1 and B=(3.5+or-0.1)*1015 V m5 A-3 s-3. They observed evidence of an internal bias field as reported previously by others.Item Cluster model for Tris-sarcosine calcium chloride (TSCC) describing order-disorder and displacive features of its ferroelectric transition and its pressure-induced transition to an antiferroelectric phase(1981-10) Schmidt, V. HugoFeatures of the TSCC phase diagram and its polarization and soft mode behavior are explained by a model having two-particle ferroelectric and antiferroelectric clusters in which the interacting particles reside in φ4 potential double-minimum wells.