18-07-2011, 09:48 AM
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ABSTRACT
The shotgun marriage of chemistry and engineering called “Nanotechnology” is ushering in the era of self-replicating machinery and self-assembling consumer goods made from raw atoms. Utilizing the well understood chemical properties of atoms & molecules, nanotechnology proposes the construction of novel molecular devices possessing extraordinary properties. The single electron transistor or SET is a new type of switching device that uses controlled electron tunneling to amplify current.
By using the “Electron beam lithography” and “Electromigration”, the research leads to the designing of a single atom transistor with the help of the meticulously synthesized semiconductor crystals called “quantum dots”, which embodies the electrons confined in a channel and resembles same in its properties as an real atom.
This paper presents a scenario on existing and ongoing studies on NANO ELECTRONICS with the theoretical methods relevant to their understanding. Most of the preceding discussion is premised upon the implicit assumption. That future quantum effect Nano Electronic Devices will be fabricated in Nano Metre scale using molecules. Conductance quantization in ballistic regime has been described under various conditions. The behaviour of “Coulomb Island” through which the electrons can only enter by tunneling through one of the insulators is presented.
At last, the SET presents that it is the different construction is which is based on helical logic, atomic scale motion of electrons in an applied rotating electric field.
INTRODUCTION
The discovery of the transistor has clearly had enormous impact, both intellectually and commercially, upon our lives and work. A major vein in the corpus of condensed matter physics, quite literally, owes its existence to this break through. It also led to the microminiaturization of electronics, which has permitted us to have powerful computers on our desktops that communicate easily with each other via the Internet. The resulting globalization of science, technology and culture is now transforming the ways we think and interact.
Over the past 30 years, silicon technology has been dominated by Moore’s law: the density of transistors on a silicon integrated circuit doubles about every 18 months. The same technology that allows us to shrink the sizes of devices. To continue the increasing levels of integration beyond the limits mentioned above, new approaches and architectures are required .In today’s digital integrated circuit architectures, transistors serve as circuit switches to charge and discharge capacitors to the required logic voltage levels. It is also possible to encode logic states by the positions of individual electrons (in quantum dot single-electron transistors, for example) rather than by voltages. Such structures are scaleable to molecular levels, and the performance of the device improves as the size decreases. Artificially structured single electron transistors studied to date operate only at low temperature, but molecular or atomic sized single electron transistors could function at room temperature.
The Kondo effect
The effect arises from the interactions between a single-magnetic atom, such as cobalt, and the many electrons in an otherwise nonmagnetic metal such as copper. Such as an impurity typically has an intrinsic angular momentum or spin that interacts with all the surrounding electrons in the metal. As a result, the mathematical description of the system is a difficult many-body problem.
The electrical resistance of a pure metal usually drops as its temperature is lowered, because electrons can travel through a metallic crystal more easily when the vibrations of the atoms are small. However, the resistance saturates as the temperature is lowered below about 10k due to the presence of crystal lattice defects in the material, such as vacancies, interstitial, dislocations and grain boundaries. Electrical resistance is related to the amount of back scattering from defects, which hinders the motion of the electrons through the crystal. This text book resistive behavior of metal changes dramatically when magnetic atoms, such as cobalt, are added. The electrical resistance increases as the temperature is lowered further, in contrast to that of a pure metal. This effect was first observed in the 1930s.
This behaviour does not involve any phase transition, such as a metal-insulator transition. A parameter called the Kondo temperature (roughly speaking the temperature at which the resistance starts to increase again) completely determines the low-temperature electronic properties of the material. Considering the scattering from a magnetic ion that interacts with the spins of the conducting electrons. It was found that the second term in the calculation could be much larger than the first. The result is that the resistance of a metal increases logarithmically when the temperature is lowered. Hence the name ‘Kondo effect’. However, it also makes the unphysical prediction that the resistance will be infinite at even lower temperatures. It turns out that Kondo’s result is correct only above a certain temperature, which became known as the Kondo temperature, Tk. The impurity has only one electron with energy E. In this case, the electron can quantum-mechanically tunnel from the impurity and escape, if E is greater than Fermi level of the metal. Otherwise it remains trapped. The defect has a spin of ½ and its z-component is fixed as either ‘spin up’ or ‘spin down’. However, the so-called exchange process can take place that effectively flip the spin of the impurity from spin up to spin down or vice-versa, while simultaneously creating a spin excitation in the Fermi sea. When an electron is taken from the magnetic impurity in an unoccupied energy state at the surface of the Fermi Sea. The energy needed for this process is large, between 1 and 10eV, for the magnetic impurities. Classically, it is forbidden to take an electron from the defect without putting energy into the system. In quantum mechanics, however, the Heisenberg uncertainty principle allows such a configuration to exist for a very short time-around h/E, where h is the Planck constant. Within this time scale, another electron must tunnel from the Fermi Sea back to the impurity. However, since the uncertainty principle says nothing about the spin of this electron, its z-component may point in the opposite direction. In other words, the initial and final states of the impurity can have different spins. This spin exchange qualitatively changes the energy spectrum of the system. When many such processes are taken together, one finds that a new state-known as the Kondo resonance- is generated with exactly the same energy as the Fermi level.
Such a resonance is effective at scattering electrons with energies close to the Fermi level. Since the same electrons are responsible for the low-temperature conductivity of a metal, the strong scattering from this state increases the resistance. The Kondo resonance is unusual.
In contrast, the Kondo State is generated by exchange processes between a localized electron and free electron states. Since many electrons need to be involved, the Kondo effect is many body phenomenons. It is important to note that the Kondo State is always “on resonance” since it is fixed to the Fermi energy. Even though the system may start with energy E that is very far away from the Fermi energy, the Kondo effect alters the energy of the system so that it is always on resonance. The only requirement for the effect to occur is that the metal is cooled to sufficiently low temperatures below the Kondo temperature TK.
