Putting spin into carbon nanoelectronics

单壁碳纳米管正证明自己是颠覆性纳米电子器件的杰出候选者。电子可以以弹道的方式在纳米管中流动，这意味着它们不会被纳米管材料的特性散射。用单壁纳米管制造晶体管是可能的，它同时具有电子和空穴导体，从而为完整的逻辑电路提供元件。甚至有可能制造出在有源区仅改变一个电子就能工作的晶体管。所有这些对于未来的电子应用都是非常重要的，但这仅仅是个开始，因为这些设备只使用电子上的电荷。还有另一种可以使用的秘密武器，以电子自旋的形式。电子自旋可以被认为是一个微小的磁铁，它可以指向两个方向中的一个（通常被称为向上和向下）。由此开辟的电子学新领域已经有了一个名字——自旋电子学，并获得了诺贝尔奖。自旋电子学目前最大的应用是在读取计算机硬盘数据的磁头上，彻底改变了这个价值数十亿美元的产业。磁性随机存取存储器现在正在制造和销售，并承诺具有更高的存取速度。研究人员正在寻找可用于自旋电子器件的新材料系统，这些设备可能会被用于新的应用。新的效应一直在被发现。例如，如果你对金属磁铁的两端施加不同的温度，电子自旋电流就会流动。我们的愿景是将自旋引入碳纳米电子学。如果我们能做到这一点，我们可能会在纳米管晶体管的基础上增加一种全新的能力。为了达到这个目的，我们将使用其他比纳米管更小的碳材料，以富勒烯分子（也被称为巴基球）的笼子形式存在。这些分子可以包含一个或多个原子，这些原子携带电子自旋。它们可以被插入到单壁纳米管中，由此产生的结构有时被称为豆荚，因为这就是它们在电子显微镜下的样子。豆荚为纳米管注入自旋提供了一种理想的方式。在我们的研究计划中，我们将制造豆荚晶体管，并在对样品损害最小的条件下，用高分辨率透射电子显微镜观察它们。这样，我们就能以原子的分辨率看到在装置中起作用的那块材料。当我们改变磁场和温度时，我们将测量通过晶体管的电流，并寻找可能对其中一个或另一个非常敏感的效应。我们将应用微波辐射，当我们通过自旋共振扫描磁场时，检测对电导率的影响。最后，我们将进行控制实验，以电测量自旋的方向。虽然这些都是基础实验，但我们希望它们能带来实际应用。这可能是通过集体激发的效应，用于纳米电子电路和传感器等应用，也可能是通过直接控制自旋态，用于更革命性的设备，如量子逻辑设备、量子存储器，甚至可能最终是量子计算。

Single walled carbon nanotubes are proving themselves to be remarkable candidates for disruptive nanoelectronic devices. Electrons can flow through them ballistically, which means that they travel without being scattered by features in the material of the nanotube. It is possible to make transistors with single walled nanotubes with both electron and hole conductors, thus providing components for complete logic circuits. It is even possible to make transistors which work with a change of only a single electron in the active region. All of this could be very significant for future electronics applications, but that would be only the beginning, because these devices use only the charge on the electron. There is another secret weapon which can be used, in the form of the electron spin.The electron spin can be thought of as a tiny magnet, which can point in one of two directions (often referred to as up and down). The new field of electronics which this opens up already has a name, spintronics, and its own Nobel Prize Winners. The biggest current application of spintronics is in the heads for reading data on hard discs in computers, revolutionizing this multibillion dollar industry. Magnetic random access memory is now being made and sold, with the promise of ever higher access speeds. The search is on for new materials systems which can be used for spintronic devices, which may in turn be exploited in new applications. New effects are being discovered all the time. For example, if you apply different temperatures to the two ends of a metallic magnet, a current of electron spins can flow.Our vision is to put spin into carbon nanoelectronics. If we can do this, we may be able to add a whole new capability to what is already possible with nanotube transistors. For this purpose we shall use other carbon materials, even smaller than nanotubes, in the form of cages called fullerene molecules (also known as Bucky balls). These molecules can each contain one or more atoms which carry a resulting electron spin. They can be inserted into single walled nanotubes, and the resulting structures are sometimes called peapods, because that is what they look like in an electron microscope. Peapods provide an ideal way to put spin into nanotubes.In our research programme, we shall fabricate peapod transistors, and look at them by high resolution transmission electron microscopy under conditions which minimise the damage to the samples. In this way we shall be able to see with atomic resolution the very piece of material which is active in the device. We shall measure the current through the transistor while we vary the magnetic field and the temperature, and look for effects which may be very sensitive to one or other of these. We shall apply microwave radiation, and detect the effect on electrical conductance as we sweep the magnetic field through the spin resonance. Finally we shall perform controlled experiments to measure electrically the direction of the spin.Although these are fundamental experiments, our hope is that they will lead to practical applications. These may be through the effects of collective excitations, for applications such as nanoelectronic circuits and sensors, or they may be through direct control of the spin states, for more revolutionary devices such as quantum logic devices, quantum memories and perhaps even eventually quantum computing.