Quantum transport in Spintronics: current and shot noise via nonequilibrium Green functions.

Author(s)

Fabricio Macedo de Souza

• Document type: Doctoral Thesis
• Press: São Carlos. , gráficos, ilustrações, tabelas.
• Institution: Universidade de São Paulo (USP). Instituto de Física de São Carlos (IFSC/BT)
• Defense date: 2004-12-20

Examining board members

Jose Carlos Egues de Menezes; Enrique Victoriano Anda; Mario Norberto Baibich; Klaus Werner Capelle; Luisa Maria Scolfaro Leite

• Advisor: Jose Carlos Egues de Menezes
• Field of knowledge: Physical Sciences and Mathematics - Physics
• Indexed in: Banco de Dados Bibliográficos da USP-DEDALUS; Biblioteca Digital de Teses e Dissertações - USP
• Location: Universidade de São Paulo. Biblioteca do Instituto de Física de São Carlos; IFSC-F/Te1640

Abstract

We study spin dependent quantum transport in quantum dots and quantum well devices attached to magnetic leads. We first derive general formulas, including electron-electron interaction and spin flip, for both current and noise, using the no equilibrium Green function technique (Keldysh). From our equations we regain limiting cases in the literature - in particular the Landauer-Buttiker formula when we neglect electron-electron interaction. We apply these formulas to study three distinct systems: (1) a quantum dot attached to two ferromagnetic leads, (2) a quantum dot linked to many ferromagnetic leads, and (3) a quantum well coupled to dilute magnetic semiconductor (DMS) terminals. In the first system we consider both parallel (P) and anti-parallel (AP) ferromagnetic alignments of the leads. Coulomb interaction and spin flip scattering are also taken into account. With the formulas for the current and the noise derived here, we find, for instance, that the Coulomb interaction, combined with the magnetism of the electrodes, gives rise to a spin-dependent Coulomb blockade. This effect allows the control (intensity and sign) of the current polarization via the bias voltage. We also observe that spin flip scattering yields contrasting behavior between current and shot noise. While the current in the AP configuration increases with the spin flip, the shot noise becomes suppressed for a range of spin flip rates. Another interesting finding is the possibility to suppress the thermal noise via a gate voltage. For the dot coupled to three magnetic leads, we show that it is possible to inject current &#8593-polarized into the dot from the FM emitter, detect simultaneously &#8593 and &#8595 - polarized currents at distinct collectors. In addition, we find that the current has its polarization amplified when going from the emitter to one of the collectors. Therefore we have a device that operates as both as current polarization inverter and amplifier. Finally, we analyze the effects of DMS leads and Landau quantization on the current and noise of system (3). We and that the giant Zeeman effect in the DMS leads, due to the it s-d exchange interaction, gives rise to a spin polarized current, and for a particular bias voltage range, full suppression of one spin component. This gives rise to the possibility of tuning the current polarization via the bias voltage. We also observe oscillations in the current, the noise and the Fano factor as a function of the magnetic field. (AU)

• FAPESP's process: 00/10650-2 - Spin-dependent quantum shot noise
• Grantee: Fabricio Macedo de Souza
• Support Opportunities: Scholarships in Brazil - Doctorate