Shot noise in tunneling through a single quantum dot

Journal of Applied Physics, 2009

Low-temperature measurements of asymmetric carbon nanotube (CNT) quantum dots are reported. The CNTs are end-contacted with one ferromagnetic and one normal-metal electrode. The measurements show a spin-dependent rectification of the current caused by the asymmetry of the device. This rectification occurs for gate voltages for which the normal-metal lead is resonant with a level of the quantum dot. At the gate voltages at which the current is at the maximum current, a significant decrease in the current shot noise is observed. 3 Carbon nanotubes 1 are ballistic conductors that have long mean-free paths and spin diffusion lengths 2,3. CNTs can therefore be used as spacers for spin valves 4-6 or in other systems that require spin coherence over their lengths 7. Additionally, short CNT sections behave like quantum dots at low temperatures 8. Short CNT sections are therefore promising materials for charge detection 9 , the separation of spin-entangled pairs of electrons 10,11 , or the rectification of current based on the electron spin 12,13. Recently spin-based current rectification, or a spin diode effect 13-18 , has been observed in a lateral ferromagnet-CNT-normal metal device 19. Shot noise measurements can provide an additional window into the dynamics of a system 20. Noise measurements are useful for examining the correlation of tunneling events in a system, and have been studied for double-barrier tunnel junctions 21-23. Here we extend upon previous results by considering the current noise of a CNT-based spin diode. A schematic of our device is given in Figure 1 (a). It consists of a single-walled carbon nanotube (SWNT) grown by chemical vapor deposition 24 and end-contacted to alternating niobium and cobalt electrodes. Devices are measured by applying a bias voltage and a capacatively-coupled gate voltage and measuring the two-terminal current. A representative AFM image of a device is shown in Figure 1 (b). We have also measured the resistance of sections of the niobium electrode as a function of temperature, which is shown in Figure 1 (c). We observe that there the resistance drops to zero around 6.5 K as the niobium electrode becomes superconducting. Electronic transport measurements were performed at temperatures ranging from 4.2 K to 10K.

Applied Physics Letters, 2006

We study charge transport through a chain of quantum dots. The dots are fully coherent among each other and weakly coupled to metallic electrodes via the dots at the interface, thus modelling a molecular wire. If the non-local Coulomb interactions dominate over the inter-dot hopping we find strongly enhanced shot noise above the sequential tunneling threshold. The current is not enhanced in the region of enhanced noise, thus rendering the noise super-Poissonian. In contrast to earlier work this is achieved even in a fully symmetric system. The origin of this novel behavior lies in a competition of "slow" and "fast" transport channels that are formed due to the differing non-local wave functions and total spin of the states participating in transport. This strong enhancement may allow direct experimental detection of shot noise in a chain of lateral quantum dots.

Applied Physics Letters, 2007

A scanning tunneling microscopic study revealed quantum fluctuation of tunneling currents in individual Ge quantum dots ͑QDs͒ on SiO 2 / Si. This was due to the charging energy change in the QDs caused by single-electron transfer from or into the QDs. The observed electron discharging time of approximately milliseconds agreed with the propagation model of the electron wave packets from the QDs to the Si substrates by a tunneling effect rather than by passing through voids in the SiO 2 smaller than electron de Broglie wavelength.

Physical Review B, 2007

We explore the full counting statistics of single-electron tunneling through a quantum dot using a quantum point contact as noninvasive high bandwidth charge detector. The distribution of counted tunneling events is measured as a function of gate and source-drain voltages for several consecutive electron numbers on the quantum dot. For certain configurations, we observe super-Poissonian statistics for bias voltages at which excited states become accessible. The associated counting distributions interestingly show a bimodal characteristic. Analyzing the time dependence of the number of electron counts, we relate this to a slow switching between different electron configurations on the quantum dot.

Physical Review B, 2007

We present noise measurements of self-assembled InAs quantum dots at high magnetic fields. In comparison to I-V characteristics at zero magnetic field we notice a strong current overshoot which is due to a Fermi-edge singularity. We observe an enhanced suppression in the shot noise power simultaneous to the current overshoot which is attributed to the electron-electron interaction in the Fermi-edge singularity.

Physical Review B, 2006

We study the Fermi-edge singularity appearing in the current-voltage characteristics for resonant tunneling through a localized level at finite temperature. An explicit expression for the current at low temperature and near the threshold for the tunneling process is presented which allows to coalesce data taken at different temperatures to a single curve. Based on this scaling function for the current we analyze experimental data from a GaAs-AlAs-GaAs tunneling device with embedded InAs quantum dots obtained at low temperatures in high magnetic fields.

Physical Review Letters, 2006

We have investigated the noise properties of the tunneling current through vertically coupled self-assembled InAs quantum dots. We observe super-Poissonian shot noise at low temperatures. For increased temperature this effect is suppressed. The super-Poissonian noise is explained by capacitive coupling between different stacks of quantum dots.

Physical Review B, 2006

Motivated by activities of several experimental groups we investigate electron transport through two coherent, strongly coupled quantum dots ("double quantum dots"), taking into account both intra-and inter-dot Coulomb interactions. The shot noise in this system is very sensitive to the internal electronic level structure of the coupled dot system and its specific coupling to the electrodes. Accordingly a comparison between experiments and our predictions should allow for a characterization of the relevant parameters. We discuss in detail the effect of asymmetries, either asymmetries in the couplings to the electrodes or a detuning of the quantum dot levels out of resonance with each other. In the Coulomb blockade region super-Poissonian noise appears even for symmetric systems. For bias voltages above the sequential tunneling threshold super-Poissonian noise and regions of negative differential conductance develop if the symmetry is broken sufficiently strongly.

Physical Review Letters, 2005