Qin, Hua (2001): On the Dynamics of SingleElectron Tunneling in Semiconductor Quantum Dots under Microwave Radiation. Dissertation, LMU München: Faculty of Physics 

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Abstract
Efforts are made in this thesis to reveal the dynamics of singleelectron tunneling and to realize quantum bits (qubits) in semiconductor quantum dots. At low temperatures, confined single quantum dots and double quantum dots are realized in the twodimensional electron gas (2DEG) of AlGaAs/GaAs heterostructures. For transport studies, quantum dots are coupled to the drain and source contacts via tunnel barriers. Electronelectron interaction in such closed quantum dots leads to Coulombblockade (CB) effect and singleelectron tunneling (SET) through discrete quantum states. SET and its dynamics in single and double quantum dots are studied using both transport and microwave spectroscopy. In transport spectroscopy, SET is monitored by measuring the direct tunnel current through the quantum dots in both the linear and nonlinear transport regimes, where ground states and excited states of the quantum dots are resolved. In a double quantum dot, bonding and antibonding molecular states are formed. Quantum dots proved to be well controlled quantum mechanical systems. In analogy to real atoms and molecules, single quantum dots and double quantum dots are termed artificial atoms and artificial molecules, respectively. In microwave spectroscopy, continuous microwave radiation is applied to quantum dots. Photonassisted tunneling (PAT) through the ground state and excited states is observed in single quantum dots. In a double quantum dot, the molecular states can be coherently superimposed by microwave photons, inducing the Rabi oscillations and a net direct tunnel current which is experimentally measurable. A qubit is formed in a double quantum dot. Two new microwave spectroscopy techniques are developed in this thesis to explore the dynamics of PAT (SET) in quantum dots. Both techniques are called heterodyne detection of photoninduced tunnel current (photocurrent). In one method, two coherent continuous microwave sources with a slight frequency offset are combined to generate a flux of microwave photons. The photon intensity varies in time at the offset frequency. The induced alternating photocurrent at the offset frequency is detected by a lockin amplifier. The inphase component of the photocurrent reflects the tunneling strength, and the outofphase component reveals the dynamics of electron tunneling. In the other method, two coherent pulsed, i.e., broadband, microwaves are applied to irradiate the quantum dots, where the dynamic charge relaxation and the pumping by microwave pulses are studied. Both techniques allow to resolve PAT in the nonlinear transport regime. A long charge relaxation time of single quantum dots is found by using both techniques. No superposition of the ground state and the excited state is achieved.
Item Type:  Theses (Dissertation, LMU Munich) 

Subjects:  500 Natural sciences and mathematics 500 Natural sciences and mathematics > 530 Physics 
Faculties:  Faculty of Physics 
Language:  English 
Date of oral examination:  26. July 2001 
MD5 Checksum of the PDFfile:  ca8c308ca4b9bb5d4843cb94d1e3479e 
Signature of the printed copy:  0001/UMC 11658 
ID Code:  413 
Deposited On:  24. Oct 2002 
Last Modified:  24. Oct 2020 12:52 