Dominant Spin Relaxation Mechanisms in Organic Semiconductor Alq3

Dominant Spin Relaxation Mechanisms in Organic Semiconductor Alq3

Sridhar Patibandla (Virginia Commonwealth University, USA), Bhargava Kanchibotla (Virginia Commonwealth University, USA), Sandipan Pramanik (University of Alberta, Canada), Supriyo Bandyopadhyay (Virginia Commonwealth University, USA) and Marc Cahay (University of Cincinnati, USA)
DOI: 10.4018/978-1-60960-186-7.ch017
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Abstract

We have measured the longitudinal (T1) and transverse (T2) spin relaxation times in the organic semiconductor tris(8-hydroxyquinolinolato aluminum) - also known as Alq3 - at different temperatures and under different electric fields driving current. These measurements shed some light on the spin relaxation mechanisms in the organic. The two most likely mechanisms affecting T1 are hyperfine interactions between carrier and nuclear spins, and the Elliott-Yafet mode. On the other hand, the dominant mechanism affecting T2 of delocalized electrons in Alq3 remains uncertain, but for localized electrons (bound to defect or impurity sites), the dominant mechanism is most likely spin-phonon coupling.
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Introduction

Recent interest in the field of “spintronics” stems primarily from the desire to use the spin degree of freedom of a single electron, or a collection of electrons, to store, process, detect and communicate information. Digital information (in the form of binary bits 0 and 1) is encoded in the spin polarization of electron(s), then “processed” using spin-spin or spin-orbit interactions, subsequently communicated over long distances using spin waves or spin chains, and finally sensed using techniques that are able to measure spin polarizations of single or multiple electrons.

A well known embodiment of this idea is the Single Spin Logic (SSL) paradigm (Bandyopadhyay, et al., 1994) where a single electron’s spin polarization is rendered “bistable” by placing it in a static magnetic field. The polarization can then point either parallel to the direction of the magnetic field, or anti-parallel to it, since only these two polarizations are allowed eigenstates of the Hamiltonian describing the electron. By engineering the exchange interactions between nearest neighbor spins, it is possible to implement (classical) digital Boolean logic gates and combinational logic circuits for universal computation (Bandyopadhyay, et al., 1994, Bandyopadhyay, 2005, Agarwal and Bandyopadhyay, 2007). These circuits have the advantage of being extremely energy efficient and amenable to high levels of integration, which results in enhanced computational prowess (Cahay and Bandyopadhyay, 2009).

Evidently, the most important concern in such approaches is preserving the fidelity of the data that is being processed. The processed information must remain intact during the entire computational cycle, which can happen only if spin does not flip spontaneously while computation takes place. Coupling of an electron’s spin with the environment (phonons, magnons, etc.) can randomly flip the spin, leading to errors in the computation. The probability of such an error occurring during one computational cycle is, (1) where T is the duration of a computational cycle (typically the period of the clock that drives the computation) and Ts is the spin relaxation time. In order to make the error probability as low as possible, we will have to make Ts as long as possible and/or T as short as possible.

There are two distinct types of spin relaxation time Ts that matter. To understand them, consider the fact that an electron’s spin is a quantum mechanical entity and can exist in a state that is a coherent superposition of two mutually anti-parallel polarizations, which we will label as “up” and “down”. An arbitrary spin can therefore be written as, (2) wheredenotes the “up” polarization and denotes the “down” polarization. The coefficients a and b are complex quantities. Because of this property, an electron’s spin is able to represent a quantum bit (or “qubit”) which is a coherent superposition of the classical binary bits 0 and 1. If we encode the classical bit 0 in the up-polarization and the classical bit 1 in the down-polarization, then a spin can represent the qubit

. (3)

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