Abstract
In this thesis, electron transport in mesoscopic semiconductor nanostructures, in particular socalled electron billiards or quantum dots, is studied. The thesis can be divided into two areas: the linear response regime of transport, and the nonlinear response regime of transport.
The classical and quantum mechanical electron dynamics are studied for triangular and squareshaped electron billiards in the linear response regime of transport. It is shown that the temperature averaged magnetoconductance can be well understood in a semiclassical singleelectron picture, in which the elecrons move on classical trajectories determined by the shape of the cavity. Studied in the phase coherent regime as a function of magnetic field or Fermi energy, the conductance fluctuations, which are due to wave interference, can also be related to certain classical orbits.
In the nonlinear regime of transport, it is found that the conductance of asymmetric quantum dots is asymmetric with respect to zero bias. This leads to an average net current, when an AC voltage is applied over the device, and the devices are often referred to as quantum ratchets. In the phase coherent regime, the asymmetry of the conductance is clearly related to the effect of an electric field on the electron states inside the dot. The observed asymmetry is sensitively dependent on the amplitude of the applied bias, on the magnetic field, and on the Fermi energy. The importance of the designed dot geometry is significant, as compared to unintentional imperfections, such as impurities and fabrication inaccuracies. In the tunneling regime, the net current direction can be predicted from the shape of the tunneling barrier, and the direction can be changed by simply changing the temperature.
Rectification is also studied in artificial materials, consisting of nanometersized asymmetric scatterers, arranged in a twodimensional lattice. At room temperature, the output is determined by ballistic scattering and electric field collimation, whereas at liquid helium temperatures, the output is believed to be dominated by the changed angular distribution, due to mode opening, and collimation caused by the applied voltage.
The classical and quantum mechanical electron dynamics are studied for triangular and squareshaped electron billiards in the linear response regime of transport. It is shown that the temperature averaged magnetoconductance can be well understood in a semiclassical singleelectron picture, in which the elecrons move on classical trajectories determined by the shape of the cavity. Studied in the phase coherent regime as a function of magnetic field or Fermi energy, the conductance fluctuations, which are due to wave interference, can also be related to certain classical orbits.
In the nonlinear regime of transport, it is found that the conductance of asymmetric quantum dots is asymmetric with respect to zero bias. This leads to an average net current, when an AC voltage is applied over the device, and the devices are often referred to as quantum ratchets. In the phase coherent regime, the asymmetry of the conductance is clearly related to the effect of an electric field on the electron states inside the dot. The observed asymmetry is sensitively dependent on the amplitude of the applied bias, on the magnetic field, and on the Fermi energy. The importance of the designed dot geometry is significant, as compared to unintentional imperfections, such as impurities and fabrication inaccuracies. In the tunneling regime, the net current direction can be predicted from the shape of the tunneling barrier, and the direction can be changed by simply changing the temperature.
Rectification is also studied in artificial materials, consisting of nanometersized asymmetric scatterers, arranged in a twodimensional lattice. At room temperature, the output is determined by ballistic scattering and electric field collimation, whereas at liquid helium temperatures, the output is believed to be dominated by the changed angular distribution, due to mode opening, and collimation caused by the applied voltage.
Original language  English 

Qualification  Doctor 
Awarding Institution 

Supervisors/Advisors 

Award date  2002 Sep 27 
Publisher  
Print ISBNs  9162853631 
Publication status  Published  2002 
Bibliographical note
Defence detailsDate: 20020927
Time: 13:15
Place: Sal B, Fysikum
External reviewer(s)
Name: Kristensen, Anders
Title: Dr
Affiliation: Danmark

Article: H. Linke, K. F. Berggren, L. Christensson, P. E. Lindelof, A. Löfgren, P. Omling, M. Yousefi and I. V. Zozoulenko, Triangular ballistic quantum dots: classical, semiclassical and wave mechanical dynamics, Semicond. Sci. and Technol. 13, A24 (1998)
Article: A. Löfgren, H. Linke, P. Omling, and P. E. Lindelof, Energy level spectroscopy of triangular electron billiards, Manuscript (2002)
Article: H. Linke, W. D. Sheng, A. Löfgren, H. Q. Xu, P. Omling, and P. E. Lindelof, A quantum dot ratchet: Experiment and theory, Europhys. Lett. 44, 341 (1998)
Article: H. Linke, H. Q. Xu, A. Löfgren, W. D. Sheng, A. Svensson, P. Omling, and P. E. Lindelof, R. Newbury, and R. P. Taylor, Voltage and temperature limits for the operation of a quantum dot ratchet, Physica B 272, 61 (1999)
Article: H. Linke, W. D. Sheng, A. Svensson, A. Löfgren, L. Christensson, H. Q. Xu, P. Omling, and P. E. Lindelof, Asymmetric nonlinear conductance of quantum dots with broken inversion symmetry, Phys. Rev. B 61, 15914 (2000)
Article: A. Löfgren, H. Linke, T. E. Humphrey, P. Omling, R. Newbury, and P. E. Lindelof, Rectification in quantum dots as a function of dot symmetry, Manuscript (2002)
Article: H. Linke, T. E. Humphrey, A. Löfgren, A. O. Sushkov, R. Newbury, R. P. Taylor, and P. Omling, Experimental tunneling ratchets, Science 286, 2314 (1999)
Article: A. Löfgren, I. Shorubalko, P. Omling and A. Song, Reversed and oscillatory output of nanostructured artificial materials, Manuscript (2002)
Subject classification (UKÄ)
 Condensed Matter Physics
Keywords
 magnetoconductance
 mesoscopic
 electron billiard
 quantum dot
 quantum ratchets
 nonlinear
 symmetries
 nonsymmetric conduction
 rectification
 artificial material
 Halvledarfysik
 Fysicumarkivet A:2002:Löfgren
 ballistic transport
 quantum transport
 2DEG
 Semiconductory physics
 InP/InGaAs
 GaAs/AlGaAs