Anderson Localization and Metal-Insulator Transition in p - type Filmes of Pb1-xEuxTe

Neste trabalho, realizamos o estudo da transição metal-isolante e da localização de Anderson na liga de Pb1-xEuxTe do tipo p para x variando de 0 até 0.1. As propriedades de transporte nessa liga (mobilidade, concentração de portadores e resistividade elétrica) foram obtidas utilizando o método de caracterização elétrica por efeito Hall entre as temperaturas de 300 K e 10 K. Nessa região de temperatura, foi possível observar uma transição metal-isolante para x > 0.05. Verificamos que a transição é do tipo Anderson e ocorre devido à desordem presente na liga. Para baixas temperaturas (T 0.01, verificamos a presença de magnetorresistência positiva e negativa aplicando campos magnéticos de até 11T. Nas amostras metálicas, a presença de magnetorresistência negativa é causada pelo efeito conhecido como localização de Anderson (efeito de interferência quântica construtiva entre as funções de onda) e a presença de magnetorresistência positiva é causada, principalmente, pelo acoplamento spin-órbita, e é chamada de antilocalização. Nas amostras isolantes, a magnetorresistência negativa é originada pelo efeito Zeeman enquanto que a magnetoresistência positiva é causada pela redução do comprimento de localização. Assim, os valores positivos e negativos da magnetoresistência têm origens diferentes dependendo do regime de condução (metálicoou isolante). Por esse motivo, o estudo dos resultados experimentais apresentados nesse trabalho foi dividido em duas partes: uma parte que trata as amostras metálicas (região de desordem fraca) e outra parte para as amostras isolantes (região de desordem forte). A partir dessa divisão, e utilizando os modelos teóricos disponíveis na literatura, foi possível fazer uma análise das medidas experimentais de magnetotransporte. Como resultado, identificamos os principais mecanismos de interação (espalhamento inelástico, efeito Zeeman, acoplamento spin-órbita, etc.) que interferem no transporte e nos efeitos de localização e antilocalização.In this work, we investigated Anderson localization and the metal-insulator transition in p-type films of Pb1-xEuxTe for x varying from 0 up to 0.1. The transport properties of this alloy (mobility, carrier concentration and electrical resistivity) were obtained using the Hall method of electrical caracterization for temperatures ranging from 300 K down to 10 K. In this temperature range, it was possible to observe a metal-insulator transition for x > 0:05. The transition is of the Anderson type and is due to the disorder present in the alloy. For low temperatures (T 0.01, we observed positive and negative magnetoresistance for magnetic fields up to 11 T. For metallic samples, the negative magnetoresistance originates from Andersons localization (constructive quantum interference effect between the wave functions) while positive magnetoresistence is caused, mainly, by the spin-orbit scattering, and it is called antilocalization. For insulating samples, negative magnetoresistance is originated from the Zeeman effect while positive magnetoresistance is caused by the localization length reduction. Therefore, positive and negative magnetoresistance values have different origins depending on the conduction regime (metallic or insulating). For this reason, our experimental investigation, presented in this work, was separated into two parts: the first one treats the metallic samples (weak-disorder regime) and the other treats the insulating samples (strong-disorder regime). From this division, and using available theoretical models, it was possible to analyze the magnetotransport experimental measurements. As a result, we identify the main interaction mechanisms (inelastic scattering, Zeeman effect, spin-orbit coupling, etc.) that interfere on the transport and localization and antilocalization effects

Antilocalization effect on photo-generated carriers in semi-insulating GaAs sample

Magnetoresistance measurements were performed on an illuminated semi-insulating GaAs sample with intrinsic deep level defects. Electrical carriers were photo-generated under light excitation and positive magnetoresistance for B 240 K. The high disorder originated from the low temperature growth of the sample leads to the strong localization of carriers and gives rise to the positive magnetoresistance observed at temperatures as high as room temperature

Spin-Orbit Coupling in n-Type PbTe/PbEuTe Quantum Wells

Magnetoresistance measurements were performed on an n-type PbTe/PbEuTe quantum well and weak antilocalization effects were observed. This indicates the presence of spin orbit coupling phenomena and we showed that the Rashba effect is the main mechanism responsible for this spin orbit coupling. Using the model developed by Iordanskii et al., we fitted the experimental curves and obtained the inelastic and spin orbit scattering times. Thus we could compare the zero field energy spin-splitting predicted by the Rashba theory with the energy spin-splitting obtained from the analysis of the experimental curves. The final result confirms the theoretical prediction of strong Rashba effect on IV-VI based quantum wells

Antilocalization of hole carriers in Pb(1-x)Eu(x)Te alloys in the metallic regime

Magnetoresistance measurements in p-type Pb(1-x)Eu(x)Te alloys, for x varying from 0% up to 5%, have been used to investigate localization and antilocalization effects. These are attributed to both the spin-orbit scattering and to the large Zeeman splitting present in these alloys due to the large values of the effective Lande g factor. The magnetoresistance curves are analyzed using the model of Fukuyama and Hoshino, which takes into account the spin-orbit and Zeeman scattering mechanisms. The spin-orbit scattering time is found to be independent of the temperature, while the inelastic-scattering time increases with decreasing temperature suggesting the electron-phonon interaction as the main scattering mechanism.CNPqCentre National de la Recherche Scientifique - CNRS/FAPESPCAPES/COFECU

The Negative Photoconductivity of Ag/AgO Grown by Spray-Pyrolysis

The main goal of this work is to provide a general description of the negative photoconductivity effect observed in Ag/AgO films grown by the spray-pyrolysis technique. X-ray diffractograms display hybrid films with high texturized AgO and metallic Ag phases. Scanning electron microscopy images show small Ag particles on the surface. Due to its surface nature, X-ray photoelectron spectroscopy revealed the predominance of the metallic character of Ag 3d spectra as compared to Ag2+. Negative photoconductivity with photoresponse in the order of seconds is observed under several wavelengths of excitation. We found that the amplitude of the negative photoresponse is strongly dependent on the optical absorbance and enhanced by surface plasmon resonance. The low-cost technique employed and the special features regarding negative photoconductivity provide an exciting platform for developing optical-electronic devices with low power consumption