Dynamical properties of single-electron devices and molecular magnets

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Doctoral thesis (article-based)
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52, [53]
Helsinki University of Technology publications in engineering physics. A, 805
This doctoral dissertation consists of theoretical studies of a number of nanometer-scale structures. In papers [1]-[5], the emphasis is on tiny devices based on conducting materials, i.e., metals and doped semiconductors. Depending on the feature size, geometry, and the electronic density of states, charging effects and quantization of single-electron states may occur. These properties can be utilized to control charge and electric current with the precision of fractions of the electronic charge e - hence the name single-electron device. In paper [6] the focus is on the rich magnetization dynamics of the molecular magnet Mn acetate. At low temperature the molecules of this material acquire a magnetic single-spin ground state with S=10. Interestingly, the quantum tunneling of the molecular spins between the different spin states is manifest even in the magnetization relaxation of macroscopic samples. In order to study or utilize the quantized states in a given nanostructure, this needs to be coupled to some measuring device. The coupling always gives rise to exchange of particles and/or heat between the nanostructure and its environment and the stronger the coupling the more the environment affects the smaller system. This may lead to modifications in the quantized states, interference effects, and dissipation. In some cases, even completely new many-body states such as the Kondo resonances observed in ultrasmall quantum dots are found to emerge. All these effects are of great fundamental as well as of nanoengineering interest. In this thesis, a theoretical model applicable to all the above systems is developed and used. The real-time diagrammatic technique is well suited for describing the various strong-coupling effects between a set of localized states and its fermionic and/or bosonic environment. This approach also allows the description of the nonequilibrium conditions attained in single-electron devices.
mesoscopic, nanoelectronics, nanophysics, single-electron transistor, quantum dot, Kondo effect, molecular magnet, quantum dissipation, quantum transport theory, real-time diagrams, nonequilibrium phenomena, quantum computing, Anderson model
  • Teemu Pohjola, Jürgen König, Herbert Schoeller, and Gerd Schön, Strong Tunneling in Double-Island Structures, Physical Review B<strong>59</strong>, 7579 (1999).
  • Teemu Pohjola, Jürgen König, Martti Salomaa, Jörg Schmid, Herbert Schoeller, and Gerd Schön, Resonant Tunneling through a Two-Level Dot and Double Quantum Dots, Europhysics Letters <strong>40</strong>, 189 (1997).
  • Teemu Pohjola, Daniel Boese, Jürgen König, Herbert Schoeller, and Gerd Schön, Strong Tunneling in Small Quantum Dots: Kondo Effect in Two Model Systems, Journal of Low Temperature Physics <strong>118</strong>, 391 (2000).
  • Teemu Pohjola, Herbert Schoeller, and Gerd Schön, Orbital and Spin Kondo Effects in a Double Quantum Dot, HUT Report Series, TKK-F-A802 (submitted for publication).
  • Jürgen König, Teemu Pohjola, Herbert Schoeller, and Gerd Schön, Transport through Quantum Dots and the Kondo Problem, invited contribution to the Proceedings of the NATO ASI "Quantum Mesoscopic Phenomena and Mesoscopic Devices in Microelectronics", Eds. I.O. Kulik and R. Ellialtioglu, Kluwer Academic Publishers, Dordrecht, NATO Science Series C, vol. <strong>559</strong>, pp. 161-167 (2000).
  • Teemu Pohjola and Herbert Schoeller, Spin Dynamics of Mn12-acetate in the Thermally-Activated Tunneling Regime: ac-Susceptibility and Magnetization Relaxation, Physical Review B <strong>62</strong>, 15026 (2000).
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