Browsing by Author "Rimberg, Alexander J."
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Item Development and study of charge sensors for fast charge detection in quantum dots(2007) Thalakulam, Madhu; Rimberg, Alexander J.Charge detection at microsecond time-scales has far reaching consequences in both technology and in our understanding of electron dynamics in nanoscale devices such as quantum dots. Radio-frequency superconducting single electron transistors (RF-SET) and quantum point contacts (QPC) are ultra sensitive charge sensors operating near the quantum limit. The operation of RF-SETs outside the superconducting gap has been a topic of study; the sub-gap operation, especially in the presence of large quantum fluctuations of quasiparticles remains largely unexplored, both theoretically and experimentally. We have investigated the effects of quantum fluctuations of quasiparticles on the operation of RF-SETs for large values of the quasiparticle cotunneling parameter alpha = 8EJ/Ec, where EJ and Ec are the Josephson and charging energies. We find that, for alpha > 1, sub-gap RF-SET operation is still feasible despite quantum fluctuations that wash out quasiparticle tunneling thresholds. Such RF-SETs show linearity and signal-to-noise ratio superior to those obtained when quantum fluctuations are weak, while still demonstrating excellent charge sensitivity. We have operated a QPC charge detector in a radio frequency mode that allows fast charge detection in a bandwidth of several megahertz. The noise limiting the sensitivity of the charge detector is not the noise of a secondary amplifier, but the non-equilibrium device noise of the QPC itself. The noise power averaged over a measurement bandwidth of about 10MHz around the carrier frequency is in agreement with the theory of photon-assisted shot noise. Frequency-resolved measurements, however show several significant discrepancies with the theoretical predictions. The measurement techniques developed can also be used to investigate the noise of other semiconductor nanostructures such as quantum dots in the Kondo regime. A study of the noise characteristics alone can not determine whether the device is operating at the quantum limit; a characterization of back action is also necessary. The inelastic current through a double quantum dot system (DQD) is sensitive to the spectral density of voltage fluctuations in its electromagnetic environment. Electrical transport studies on a DQD system electrostatically coupled to an SET shows qualitative evidence of back-action of SET. The design and fabrication of a few electron DQD device with integrated RF-SET and QPC charge sensors for the study of back action of the sensors and real-time electron dynamics in the DQD are also discussed.Item Double quantum dot coupled to a radio-frequency single-electron transistor: Quantum measurement and back action(2004) Thalakulam, Madhu; Rimberg, Alexander J.The radio-frequency superconducting single electron transistor (RF-SET) is a highly sensitive and fast electrometer operating near the quantum limit. The back-action current noise of the SET induces voltage fluctuations on the system it is coupled to, dephasing its charge states. Here we propose to study the back action of the RF-SET by means of a double quantum dot (DQD) and, an RF-SET electrostatically coupled to one of the two dots. The inelastic current through the DQD is very sensitive to the spectral density of the voltage fluctuations in the nearby environment. By properly choosing the dot size and inter-dot tunnel barrier thickness, one can cause inelastic process to dominate the transport processes. A measurement of the inelastic current through the double dot system can then be used to calculate the spectral density of noise associated with the RF-SET and hence its back action. The design, fabrication and characterization of an RF-SET/DQD system is discussed in this work. Real-time studies of electron dynamics in coupled dots are also addressed.Item Electrical transport in a single-electron transistor coupled to a tunable environment(1999) Lu, Wei; Rimberg, Alexander J.A novel model system is developed to study the effects of the environment on transport properties of a superconducting single-electron transistor (S-SET). The impedance of a two-dimensional electron gas (2DEG) 50 nm below the sample surface serving as the environment can be tuned in situ. A quantum dot is readily formed in the 2DEG. Josephson tunneling processes in the SET are suppressed and quasiparticle tunneling processes are enhanced as the 2DEG is confined. Important energetic parameters of the SET such as the charging energy $E\sb{c}$ and superconducting energy gap $\triangle$ remain almost unchanged in this process.Item Engineering the electromagnetic environment in a nanostructure to study single-electron tunneling(2004) Sarkar, Joy; Rimberg, Alexander J.In this research the electromagnetic environment in a ballistic semiconductor nanostructure is engineered to study single electron tunneling oscillations: the repetitive tunneling of individual electrons through an energy barrier causing a saw-tooth variation in the voltage across the barrier. In this work, novel use of staggered arrays of quantum point contacts as ballistic resistors in a two-dimensional electron gas provides a high impedance environment for a tunnel barrier placed between the arrays. Since instantaneous establishment of charge equilibrium with the potential source is prevented, strong Coulomb charging is observed for the barrier. This approach circumvents problems faced by metallic thin-film resistors, for which stray capacitive shunting occurs at the high frequencies relevant to single electron tunneling. By using an integrated Radio frequency Single Electron Transistor as a fast electrometer, single electron tunneling oscillations may therefore be directly observed.Item Lithographic techniques for and electrical transport in single-walled carbon nanotubes(2000) Cox, Michael Ellis; Rimberg, Alexander J.A technique for positioning single-walled carbon nanotubes (SWNT) at a specific location on a substrate has been developed. Self-assembled monolayers were used in conjunction with electron-beam lithography to produce patterned regions of --NH2 terminated organosilanes. SWNTs adhere to the --NH2 terminated patterns, allowing these positioned tubes to be electrically contacted with macroscopic gold leads. I-V Characteristics were measured for both annealed and nonannealed SWNTs contacted in this fashion. The lithographic technique works extremely well with nonannealed nanotubes; however, such tubes exhibit highly insulating electrical characteristics. Conversely SWNTs annealed at 1100°C for 30 minutes have electrical characteristics in agreement with predictions, but are not attracted to the --NH 2 terminated patterns.Item Single-electron transistor: Effects of the environment and detecting electron motion in real time(2003) Lu, Wei; Rimberg, Alexander J.This thesis will be divided into two parts. In the first part, theory and results of a novel system in which a superconducting single-electron transistor (S-SET) coupled to a two-dimensional electron gas (2DEG) serving as a tunable electromagnetic environment for the S-SET will be discussed, including effects of dissipation, resonant tunneling with photon emission, and photon-assisted tunneling. In the second part, we discuss the techniques for which the SET is incorporated in an RF resonant circuit, resulting in an ultra high charge sensitivity and bandwidth. After the 2DEG is confined into a quantum dot, random telegraph signals (RTS) caused by individual electrons tunneling on and off the dot have been observed. In the equilibrium configuration, the occupational probabilities of the charge states of the dot can be directly measured from the RTS and were found to follow a Fermi distribution. In the non-equilibrium configuration, the RTS correctly detected the onset of the current through the dot.Item Study of the radio frequency single electron transistor: Principles and applications(2005) Ji, Zhongqing; Rimberg, Alexander J.This thesis will discuss the principles, techniques and applications of the Radio Frequency Single Electron Transistor (RF-SET). In the first part, the operating principles of Single Electron Transistors (SETS) in the normal and superconducting states will be introduced. The general techniques of fabricating and calibrating SETs will also be introduced. In the second part, two of our recent experiments are reviewed. One is related to the sensitivity and linearity of superconducting RF-SETs. We found that the RF-SET achieves the best balance of charge sensitivity and linearity in the subgap regime, as opposed to the usual preferred working point in the above-gap regime. The second experiment relates to the real-time counting of single electrons. We demonstrated that the RF-SET can be used as a fast and ultra-sensitive electrometer which can even detect tunneling of a single electron inside a tunable quantum dot (QD) formed in a two dimensional electron gas (2DEG).