Field-dependent conductivity at low electric fields in pressed pellets of doped poly (3-methylthiophene): evidence of charge-density wave depinning

Field-dependent conductivity at low electric fields was observed from low to room temperature in pressed pellets of doped poly(3-methylthiophene). The room temperature data showed good agreement with Bardeen's theory of charge-density wave depinning and the values of the parameters obtained are consistent with a strong electron-phonon interaction as expected for quasi-one dimensional systems. (C) 2003 Elsevier B.V. Ltd. All rights reserved.

Depinning and vortex-glass-like transition in Nb with uncorrelated disorder

Studies of the third harmonic of the AC-susceptibility were conducted to detect the boundaries of the linear regime of the magnetic response of granular Nb samples. These studies reveal the extent of the region, on the phase diagram, where the magnetic response is linear, which corresponds to the disordered phase of Vortex Matter. The present work addresses the correlation between a linear response and experimental parameters such as the frequency and the amplitude of the excitation field. The order-disorder border has been extracted from the onset temperature of the third harmonic measured at low-frequencies and low-excitation fields in the presence of dc magnetic fields. © 2008 IOP Publishing Ltd.

Solid-liquid transition in Nb powder determined by third harmonic susceptibility

Measurements of the third harmonic of the AC-susceptibility were employed to determine the boundaries of the linear regime of the magnetic response of Nb powder. Non-linear contributions to the magnetic response reveal the occurrence of a structured phase, disappearing as the vortex lattice melts to the liquid state. A systematic study of the third harmonic was conducted to determine how its onset temperature depends on experimental parameters, such as the frequency and amplitude of the excitation field. The melting line (ML) has been extracted from the onset temperature measured at low-frequencies and low-excitation fields in the presence of DC magnetic fields. The study indicates that the ML can be described by a 3D vortex-glass model, except at lower fields, where the system experiences a depinning crossover, and the best description of the experimental data is provided by a 3D Bose-glass model. (c) 2008 Elsevier B. V. All rights reserved.

Metal-insulator transitions in pressed pellets of BF4 - doped poly(3-methylthiophene)

From Electron Spin Resonance (ESR) data in pressed pellets of BF4 - doped Poly(3-methylthiophene) (P3MT) we obtained simultaneously the paramagnetic susceptibility and. the microwave conductivity. We observed a transition from a high-temperature insulator state to a room-temperature metallic state. Around 240K. evidence of a Peierls transition is observed, but if the sample is slowly cooled, this transition is partially suppressed. DC conductivity data taken with the sample quenched to 79 K show a non-linear I-V response for very small electric fields, suggesting depinning of Charge-Density Wave (CDW). The data for heating and cooling the system above room temperature, indicate the formation of bipolarons.

Estudo das propriedades elétricas não lineares de polímeros conjugados

Pós-graduação em Física - IGCE

Non ohmic gigahertz conductivity in pressed pellets of ClO4 doped poly(3-methylthiophene)

ESR measurements In pressed pellets of doped Poly(3-Methylthiophene)(P3MT) were performed at 10 K and 50 K after cooling the system slowly from room temperature to 110 K, quenching to 77 K and then to 10 K. ESR line asymmetry (A/B) as a function of microwave power was observed and 9.4 GHz conductivity was obtained from Dyson's theory. The data is discussed in terms of Charge-Density Wave (CDW) depinning. At 50 K the threshold electric field is estimated to be less than 1 V/cm. At 10 K a subtle pinning of the CDW was observed around 15 mW.

Espectroscopia de impedância em Fe3O2BO3 e α-MnO2 dopado com cobre

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)