Scholarly Work - Physics
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Item Dielectric and EPR measurements of the deuterated glass D-RADA x=0.46(1994-02-25) Zbigniew, Trybula; Stefan, Waplak; Stankowski, Jan; Szymon, Łoś; Schmidt, V. Hugo; Drumheller, John E.The results of the complex dielectric permittivity measurements and the spin-lattice relaxation times of the deuterated glass D-RADA x = 0.46, by the dielectric and EPR techniques are reported. The transition from paraelectric to antiferroelectric phase is detected at TN = 162 K. Below 90 K the dispersion of the complex dielectric permittivity was detected. The shape of the temperature dependence of the imaginary part of the dielectric permittivity was analyzed. This analysis shows the existence of two relaxation phases forming among the antiferroelectric domains, described by Arrhenius equation, and the glass state appears after the antiferroelectric ordering has been destroyed. The dielectric results are confirmed by the spin-lattice relaxation times data.Item 75As nuclear quadrupole resonance in weakly substitutionally disordered Rb1‑x(NH4)xH2AsO4(1994-07-11) Papantopoulos, G.; Papavassiliou, G.; Milia, F.; Schmidt, V. Hugo; Drumheller, John E.; Pinto, Nicholas J.; Blinc, Robert; Zalar, B.Item Proton-glass state in K0.60(NH4)0.40H2AsO4 detected by dielectric measurements(1990-10-01) Trybula, Zbigniew; Schmidt, V. Hugo; Drumheller, John E.; Blinc, RobertDielectric permittivity measurements of ε′a and ε''a in a K0.60(NH4)0.40H2AsO4 mixed crystal have been made in the temperature and frequency ranges 3–300 K and 1 Hz–30 kHz. We find proton-glass behavior in this material similar to that observed in the Rb1−x(NH4)xH2AsO4 and Rb1−x(NH4)xH2PO4 systems. Below Tg the dielectric dispersion characteristics of the freezing of the polarization are well described phenomenologically by the Vogel-Fulcher law. The best fit gives the Vogel-Fulcher temperature T0=5.4 K, the attempt frequency ν0=2.5×1013 Hz, and the activation energy Ec/k=409 K.Item Coexistence of Proton-Glass and Ferroelectric Order in Rb1-x(NH4)xH2AsO4(1991-01-01) Trybula, Zbigniew; Schmidt, V. Hugo; Drumheller, John E.The mixed crystal Rb1−x(NH4)xH2AsO4 (RADA) has been investigated for several values of (x=0,0.12,0.15, and 0.20) by measuring the complex dielectric permittivity along the a tetragonal axis in the temperature range from 3 to 300 K and frequency range from 1 Hz to 30 kHz. We find coexistence of ferroelectric and proton-glass order in RADA for x=0.12 and probably at x=0.15. The long-range ferroelectric order is not destroyed by the onset of proton-glass ordering. This behavior is similar to that of other proton-glass systems and of the magnetic Ising glass systems reported by Wong et al. in Fe0.55Mg0.45Cl2 and by Yoshizawa et al. in FexMn1−xTiO3. The partial phase diagram for Rb1−x(NH4)xH2AsO4 including the coexisting proton-glass and ferroelectric phases is presented.Item EPR studies of AsO4−4 spin-lattice-relaxation times in antiferroelectric NH4H2AsO4 and mixed glassy Rb1−x(NH4)xH2AsO4 (x=0.35)(1990-11-01) Waplak, S.; Trybula, Z.; Drumheller, John E.; Schmidt, V. HugoThe electron spin-lattice-relaxation time T1 was measured for x-ray-induced AsO4−4 centers in antiferroelectric NH4H2AsO4 (ADA) and mixed Rb1−x(NH4)xH2AsO4 (RADA, x=0.35) in the temperature range 4.2–300 K by the saturation method. It is shown that below T0 1/T1 reflects the direct relaxation process. From above T0 to 55 K, 1/T1∝T1.6 and can be described by a Vogel-Fulcher law 1/T1∼(T-T0). It is also shown that the flatness of 1/T1 in the temperature region from 55 to 120 K is due to the tunneling motion of nitrogen in the glassy phase of RADA. Contrary to ADA, the 1/T1 behavior in RADA reflects the fracton nature of relaxation in the spin-glass state.Item Proton Glass Dielectric Susceptibility Compared with Monte Carlo and Bound Charge Semiconductor Model Predictions(1990-06-01) Schmidt, V. Hugo; Trybula, Z.; He, Di; Drumheller, John E.; Stigers, C.; Li, Zhouning; Howell, Francis L.Our latest results for dielectric permittivity and loss and protonic conductivity in RADP, RADA and DRADA proton glasses are presented. Improvements in our “bound charge semiconductor” model for dielectric behavior are discussed. Monte Carlo studies of the phase diagram and polarization decay are described. Bias order parameter vs. temperature plots from the simulation and from ND4 deuteron NMR lineshapes are compared.Item Dielectric measurements of the proton-glass state in Rb0.65(NH4)0.35H2AsO4(1988-09) Trybuła, Z.; Schmidt, V. Hugo; Drumheller, John E.; He, Di; Li, ZhouningDielectric measurements of the Rb1−x/emph>(NH4)xH2AsO4 (x=0.35) mixed crystals in the a and c tetragonal directions in the temperature range from 3 to 300 K are reported. Below Tg a dielectric dispersion of ε’(T) and ε’’(T) in the applied frequency range from 1 Hz to 30 kHz was observed. A small anisotropy of the proton-glass transition temperature Tg was detected, but we did not find the large anisotropy reported by J. Kim, N. Kim, and K. Lee [J. Phys. C 21, L-663 (1988)].Item EPR investigation of Cu2+ tetramers in ferroelectric CsH2PO4(1985-07) Waplak, S.; Schmidt, V. Hugo; Drumheller, John E.EPR study of CsH2PO4 (CDP) single crystals with 0.005 mol % of Cu2+ at room temperature reveals isolated Cu2+ (S=(1/2)) complexes with four ligand coordination and spin Hamiltonian parameters gpara=2.2575, g⊥=2.1866, Apara=30 G, and A⊥=27 G. At temperatures below 250 K the spectra were observed to have fine structure and are described as arising from a Cu2+ tetramer with effective spin S=2 and spin Hamiltonian parameters D=0.181 cm−1, E=-0.081 cm−1, a=0.001 cm−1, b=0, and c=-0.021 cm−1. Also reported are transitions within the S=1 multiplet of the tetramer, and a microscopic model of superexchange interaction has been used to evaluate the parameters Dx=0.543 cm−1 and Ex=-0.159 cm−1. A tetramer model as well as a temperature dependence of the ratio of tetramer–to–single-ion intensity is presented.Item EPR determination of three-dimensional correlations below the ferroelectric phase transition in pseudo-one-dimensional CsH2PO4:Cu2+(1986-11) Waplak, S.; Schmidt, V. Hugo; Drumheller, John E.The polarization fluctuations of ferroelectric CsH2PO4 above Tc are generally classified as a quasi-one-dimensional phenomenon. The temperature dependence of the EPR line splitting of the S=1 state of the Cu2+ tetramer, however, exhibits a critical exponent below Tc of β=0.5 indicating that the correlations associated with the order parameter are three dimensional below Tc.Item Dielectric Properties of Lithium Hydrazinium Sulfate.(1971-12) Schmidt, V. Hugo; Drumheller, John E.; Howell, Francis L.The dc conductivity and ac dielectric susceptibility of normal and deuterated lithium hydrazinium sulfate have been measured over a wide temperature range at frequencies up to 9.33 GHz. Over a very large temperature and frequency range the real and imaginary parts of the susceptibility are very large (up to ε′≃ε''≃106) and vary with frequency somewhat as f−12. This unusual behavior is shown to result form the nearly one-dimensional protonic conductivity and its extreme sensitivity to barriers caused by local structural defects. Etching studies indicate that the crystal is not ferroelectric, implying that the apparent hysteresis loops result from saturation of the ac conduction.