Effects of Coherence and Correlations on Transport through Nanostructures

Sammanfattning: Popular Abstract in English For several decades the electronic industry has developed towards smaller and smaller electronic components. The idea behind this development is quite simple: Small components have faster response times, which enables faster devices. Also, smaller components make it possible to decrease the size of the devices. A common example is today's mobile phones, that actually are quite powerful computers. This development was predicted a long time ago by Gordon E. Moore in 1965. Moore's law states that in integrated circuits the number of transistors doubles every second year, and so far the prediction is true. A transistor can be viewed as the electronic equivalent to the biological cell, in that it is the smallest building block in electronic circuits. A modern computer contains several billions of these small components. They work as switches, where a channel can either conduct current or block it. This is the origin of zeros and ones used in electronics. Intel's latest generation of transistors, named Ivy Bridge, measures 22 nm, which corresponds to less than 100 silicon atoms in length. As the transistors shrink in size, also the current passing through them will decrease. At some point, already passed by today's electronic industry, the current can no longer be viewed as continuous, but must instead be considered on the level of electrons, i.e. the electronic charge is quantized. In the ultimate limit a transistor only allows one electron to pass at a time, a so called single electron transistor. Such devices can be built using nano-electronics. One possible implementation is quantum dots. These objects, which can be thought of as artificial atoms, are small regions in space where the electrons are confined. An example of a quantum dot investigated in this thesis can be seen in Fig.~1. Here a restricted region in space is formed between two metal stripes, source and drain, in a very small wire. These wires are referred to as nanowires, and the one shown in the figure is made of indium antomonide (InSb). Single electron transistors can also be manufactured using molecules, or even single atoms. As a result of the quantized charge transport, the classical Ohm's law that states that the current is given by the bias divided by the resistance I=V/R, no longer holds. One must instead use Quantum mechanics, which is the physical tool used in this thesis. A central concept in Quantum mechanics is coherence. Coherence means that an electron can be in two different positions at the same time. To understand this somewhat mind blowing concept one should think of the electron as a wave. Like a wave hitting a double slit, the electron can pass through both openings at the same time. On the other side of the slit, the electron, like a wave, interferes constructively or destructively with itself. This effect can be observed in e.g. quantum dots or molecules. In these there can be different pathways, acting as slits, that the electron can use to when transported through such devices. This effect, not present in classical electronics, allows us to construct better devices. One example considered in this thesis is in the field of thermopower. Due to coherence, it is possible to construct quantum dots or molecules working as filters that only allow electrons with a high temperature to pass. This allows for conversion of heat into electrical current, something that would be very useful in today's industrialized world. As approximately 90% of the worlds energy is generated by heat engines that use fossil fuels, and these typically operate at 30-40% efficiency, roughly 15 terawatts of heat is constantly lost to the environment.