@phdthesis{Hartmann2008,
  author    = {Hartmann, David},
  title     = {Elektrisches und magnetisches Schalten im nichtlinearen mesoskopischen Transport},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-29175},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {Im Rahmen dieser Arbeit wurden Transporteigenschaften von Nanostrukturen basierend auf modulationsdotierten GaAs/AlGaAs Hetero{\"u}berg{\"a}ngen untersucht. Derartige Heterostrukturen zeichnen sich durch ein hochbewegliches zweidimensionales Elektronengas (2DEG) aus, das sich wenige 10 nm unterhalb der Probenoberfl{\"a}che ausbildet. Mittels Elektronenstrahl-Lithographie und nasschemischer {\"A}tztechnik wurde dieses Ausgangsmaterial strukturiert. Eindimensionale Leiter mit Kanalweiten von wenigen 10 nm wurden auf diese Weise hergestellt. Die Vorz{\"u}ge derartiger Strukturen zeigen sich im ballistischen Elektronentransport {\"u}ber mehrere 10 µm und einer hohen Elektronenbeweglichkeit im Bereich von 10^6cm^2/Vs. Als nanoelektronische Basiselemente wurden eingehend eindimensionale Quantendr{\"a}hte sowie y-f{\"o}rmig verzweigte Strukturen untersucht, deren Kanalleitwert {\"u}ber seitliche Gates kontrolliert werden kann. Dabei wurden die Transportmessungen {\"u}berwiegend im stark nichtlinearen Transportregime bei Temperaturen zwischen 4,2 K und Raumtemperatur durchgef{\"u}hrt. Der Fokus dieser Arbeit lag insbesondere in der Untersuchung von Verst{\"a}rkungseigenschaften und kapazitiven Kopplungen zwischen Nanodr{\"a}hten, der Realisierung von komplexen Logikfunktionen wie Z{\"a}hler- und Volladdiererstrukturen, dem Einsatz von Quantengates sowie der Analyse von rauschaktiviertem Schalten, stochastischen Resonanzph{\"a}nomenen und Magnetfeldasymmetrien des nichtlinearen mesoskopischen Leitwertes.},
  subject      = {Niederdimensionales Elektronengas},
  language  = {de}
}
@phdthesis{Scheibner2007,
  author    = {Scheibner, Ralf},
  title     = {Thermoelectric Properties of Few-Electron Quantum Dots},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26699},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2007},
  abstract  = {This thesis presents an experimental study of the thermoelectrical properties of semiconductor quantum dots (QD). The measurements give information about the interplay between first order tunneling and macroscopic quantum tunneling transport effects in the presence of thermal gradients by the direct comparison of the thermoelectric response and the energy spectrum of the QD. The aim of the thesis is to contribute to the understanding of the charge and spin transport in few-electron quantum dots with respect to potential applications in future quantum computing devices. It also gives new insight into the field of low temperature thermoelectricity. The investigated QDs were defined electrostatically in a two dimensional electron gas (2DEG) formed with a GaAs/(Al,Ga)As heterostructure by means of metallic gate electrodes on top of the heterostructure. Negative voltages with respect to the potential of the 2DEG applied to the gate electrodes were used to deplete the electron gas below them and to form an isolated island of electron gas in the 2DEG which contains a few ten electrons. This QD was electrically connected to the 2DEG via two tunneling barriers. A special electron heating technique was used to create a temperature difference between the two connecting reservoirs across the QD. The resulting thermoelectric voltage was used to study the charge and spin transport processes with respect to the discrete energy spectrum and the magnetic properties of the QD. Such a two dimensional island usually exhibits a discrete energy spectrum, which is comparable to that of atoms. At temperatures below a few degrees Kelvin, the electrostatic charging energy of the QDs exceeds the thermal activation energy of the electrons in the leads, and the transport of electrons through the QD is dominated by electron-electron interaction effects. The measurements clarify the overall line shape of thermopower oscillations and the observed fine structure as well as additional spin effects in the thermoelectrical transport. The observations demonstrate that it is possible to control and optimize the strength and direction of the electronic heat flow on the scale of a single impurity and create spin-correlated thermoelectric transport in nanostructures, where the experimenter has a close control of the exact transport conditions. The results support the assumption that the performance of thermoelectric devices can be enhanced by the adjustment of the QD energy levels and by exploiting the properties of the spin-correlated charge transport via localized, spin-degenerate impurity states. Within this context, spin entropy has been identified as a driving force for the thermoelectric transport in the spin-correlated transport regime in addition to the kinetic contributions. Fundamental considerations, which are based on simple model assumptions, suggest that spin entropy plays an important role in the presence of charge valence fluctuations in the QD. The presented model gives an adequate starting point for future quantitative analysis of the thermoelectricity in the spin-correlated transport regime. These future studies might cover the physics in the limit of single electron QDs or the physics of more complex structures such as QD molecules as well as QD chains. In particular, it should be noted that the experimental investigations of the thermopower of few-electron QDs address questions concerning the entropy transport and entropy production with respect to single-bit information processing operations. These questions are of fundamental physical interest due to their close connection to the problem of minimal energy requirements in communication, and thus ultimately to the so called "Maxwell's demon" with respect to the second law of thermodynamics.},
  subject      = {Quantenpunkt},
  language  = {en}
}