Alexandre J. C. Lanfredi

Structural characterization of indium oxide nanostructures: a Raman analysis

In this work we report on structural and Raman spectroscopy measurements of pure and Sn-doped In2O3 nanowires. Both samples were found to be cubic and high quality single crystals. Raman analysis was performed to obtain the phonon modes of the nanowires and to confirm the compositional and structural information given by structural characterization. Cubic-like phonon modes were detected in both samples and their distinct phase was evidenced by the presence of tin doping. As a consequence, disorder effects were detected evidenced by the break of the Raman selection rules.

Electron transport properties of undoped SnO2 monocrystals

Using low-resistance indium contacts, we measured some transport properties of undoped vapor-liquid-solid grown tin oxide monocrystals with a belt shape. From the transport measurements, the two following conduction mechanisms were investigated: thermal activation and variable range hopping. An energy gap of 3.8 eV was found. The energy gap was confirmed by thermally activated measurements in the range between 10 and 300 K. For high temperatures (T>300 K), the influence of the disorder caused by the superficial ions layer is measurable. The electron transport in this case was found to be governed by the well known variable range hopping mechanism and the spatial extension of carrier’s wavelength was calculated to be 4 nm.

Anomalous current-voltage behavior of CaCu3Ti4O12 ceramics

This letter reports on an investigation of dc current-voltage measured in CaCu3Ti4O12 ceramics. The experimental results show unusual I-V behaviors associated with the electrically inhomogeneous nature of the grains (mixture of semiconducting and insulating regions) and dependence on delay time. The data strongly suggest that both the localization of current in the conductive region and local Joule self-heating effects are essential elements to explain the abrupt drop in the electric field. In addition, the hysteresis behavior recorded during the I-V measurements shows strong dependence on delay time, i.e., the time needed for the polarization phenomena to occur.

One-dimensional character of Sn doped In2O3 nanowires probed by magnetotransport measurements

Magnetotransport data of individual Sn doped In2O3 nanowires were investigated at low temperatures in order to study the characteristic dimensionality of the conduction process and the electron dephasing mechanisms. The magnetoconductance data obtained in these nanowires confirm the theoretical predictions of one-dimensional quantum interference with electron–electron interaction as the dominant scattering mechanism leading to the break of the observed quantum coherence effects.

Electrical characterization of SnO2:Sb ultrathin films obtained by controlled thickness deposition

A representative study is reported on the electrical properties of SnO2:Sb ultrathin films (thickness of 40–70 nm) produced by a deposition method based on aqueous colloidal suspensions of 3–5 nm crystalline oxides. The results revealed the films’ electrical behavior in a range of 10–300 K, showing a strong dependence on dopant incorporation, with minimum resistivity values in 10 mol % of Sb content. All the samples displayed semiconductor behavior, but the transport mechanism showed a strong dependence on thickness, making it difficult to fit it to well-known models. In thicker films, the mechanism proved to be an intermediary system, with thermally activated and hopping features. Electron hopping was estimated in the range of 0.4–1.9 nm, i.e., in the same order as the particle size.

Magnetoresistance in Sn-Doped In2O3Nanowires

In this work, we present transport measurements of individual Sn-doped In2O3nanowires as a function of temperature and magnetic field. The results showed a localized character of the resistivity at low temperatures as evidenced by the presence of a negative temperature coefficient resistance in temperatures lower than 77 K. The weak localization was pointed as the mechanism responsible by the negative temperature coefficient of the resistance at low temperatures.

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm1.85Ce0.15CuO4 thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm1.85Ce0.15CuO4 thin films around Tsf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm3+ moments with the energy ħωsf = kBTsf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρres, electron-phonon ρe-ph(T) = AT, and electron-electron ρe-e(T) = BT2 contributions in addition to the fluctuation-induced contribution ρsf(T) due to thermal broadening effects (of the width ωsf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ωp = 2.1 meV, τ0−1 = 9.5 × 10−14 s−1, λ = 1.2, and EF = 0.2 eV, respectively.

The search for superconductivity in PrBa2Cu3O7−δ

Abstract The existence of superconductivity in samples of PrBa 2 Cu 3 O 7−δ (Pr-123) is still a polemic subject. To further study this problem, we have prepared high-quality polycrystalline Pr-123 samples by following a sol-gel method, associated with heat treatment in inert atmosphere. In this paper we report the structural, thermal and magnetic properties of superconducting sintered-polycrystalline Pr-123 and give a descriptive explanation about its unique dependence on the preparation procedure. The list of evidences of superconductivity in Pr-123 is further expanded with new data on a superconducting thin film. The present results confirming the occurrence of superconductivity in Pr-123 revitalizes the hypothesis that superconductivity originates outside the cuprate-plane.

Structural, thermal and magnetic properties of Pr-123 polycrystalline and thin film superconductors

Abstract We report the structural, thermal and magnetic properties of superconducting PrBa 2 Cu 3 O 7− δ (Pr-123). The existence of superconductivity in samples of Pr-123 is still a polemic subject. To further study this problem, we have measured thin films and high-quality polycrystalline bulk samples by following a sol–gel method, associated with heat treatment in argon atmosphere. The present results confirm the occurrence of superconductivity in Pr-123 polycrystalline and thin film samples.

Irreversibility line and low-field grain-boundary pinning in electron-doped superconducting thin films

ac magnetic susceptibilities of electron-doped Pr1.85Ce0.15CuO4 (PCCO) and Sm1.85Ce0.15CuO4 (SCCO) granular thin films have been measured as a function of temperature and magnetic-field strength. Depending on the level of homogeneity of our films, two different types of the irreversibility line (IL), Tirr≡Tp(H), defined as the intergrain-loss peak temperature in the imaginary part of the susceptibility and obeying the law 1−Tp∕TC∝Hq, have been found. Namely, more homogeneous PCCO films (with a grain size of the order of 2μm) are best-fitted with q=2∕3 while less homogeneous SCCO films (with a grain size of the order of 500nm) follow the IL law with q=1. The obtained results are described via the critical-state model taking into account the low-field grain-boundary pinning. The extracted pinning-force densities in more granular SCCO films turn out to be four times larger than their counterparts in less granular PCCO films.