Quantum Dots in Gated Nanowires and Nanotubes

This thesis describes experiments on quantum dots made by locally gating one-dimensional quantum wires. The first experiment studies a double quantum dot device formed in a Ge/Si core/shell nanowire. In addition to measuring transport through the double dot, we detect changes in the charge occupancy of the double dot by capacitively coupling it to a third quantum dot on a separate nanowire using a floating gate. We demonstrate tunable tunnel coupling of the double dot and quantify the strength of the tunneling using the charge sensor. The second set of experiments concerns carbon nanotube double quantum dots. In the first nanotube experiment, spin-dependent transport through the double dot is compared in two sets of devices. The first set is made with carbon containing the natural abundance of $^{12}C$ (99%) and $^{13}C$ (1%), the second set with the 99% $^{13}C$ and 1% $^{12}C$. In the devices with predominantly $^{13}C$, we find evidence in spin-dependent transport of the interaction between the electron spins and the $^{13}C$ nuclear spins that was much stronger than expected and not present in the $^{12}C$ devices. In the second nanotube experiment, pulsed gate experiments are used to measure the timescales of spin relaxation and dephasing in a two-electron double quantum dot. The relaxation time is longest at zero magnetic field and goes through a minimum at higher field, consistent with the spin-orbit-modified electronic spectrum of carbon nanotubes. We measure a short dephasing time consistent with the anomalously strong electron-nuclear interaction inferred from the first nanotube experiment.Physic

Electron–Nuclear Interaction in 13C^{13}C Nanotube Double Quantum Dots

For coherent electron spins, hyperfine coupling to nuclei in the host material can either be a dominant source of unwanted spin decoherence or, if controlled effectively, a resource enabling storage and retrieval of quantum information. To investigate the effect of a controllable nuclear environment on the evolution of confined electron spins, we have fabricated and measured gate-defined double quantum dots with integrated charge sensors made from single-walled carbon nanotubes with a variable concentration of $^{13}C$ (nuclear spin $(I=\frac{1}{2})$ among the majority zero-nuclear-spin $^{12}C$ atoms. We observe strong isotope effects in spin-blockaded transport, and from the magnetic field dependence estimate the hyperfine coupling in $^{13}C$ nanotubes to be of the order of $100 \mu eV$, two orders of magnitude larger than anticipated. $^{13}C$-enhanced nanotubes are an interesting system for spin-based quantum information processing and memory: the $^{13}C$ nuclei differ from those in the substrate, are naturally confined to one dimension, lack quadrupolar coupling and have a readily controllable concentration from less than one to $10^5$ per electron.Physic

Relaxation and Dephasing in a Two-Electron 13C^{13}C Nanotube Double Quantum Dot

We use charge sensing of Pauli blockade (including spin and isospin) in a two-electron $^{13}C$ nanotube double quantum dot to measure relaxation and dephasing times. The relaxation time $T_1$ first decreases with a parallel magnetic field and then goes through a minimum in a field of $1.4 T$. We attribute both results to the spin-orbit-modified electronic spectrum of carbon nanotubes, which at high field enhances relaxation due to bending-mode phonons. The inhomogeneous dephasing time $T_2^*$ is consistent with previous data on hyperfine coupling strength in $^{13}C$ nanotubes.PhysicsOther Research Uni

Two-dimensional crystals: Phosphorus joins the family

Graphene was first isolated by exfoliating single layers from a graphite crystal using Scotch tape. This method was later applied to other materials with layered structures, creating a family of atomically layered materials that includes insulators such as hexagonal boron nitride, metals such as NbSe[subscript 2], and semiconductors such as MoS[subscript 2] and WSe[subscript 2]. All of these materials had been studied for decades in bulk form, but their exfoliated, two-dimensional form gave them new life and properties. Writing in Nature Nanotechnology, Xian Hui Chen, Yuanbo Zhang and co-workers have now similarly brought black phosphorus back to the spotlight, which is the most stable and least reactive form of elemental phosphorus, and was discovered in bulk form 100 years ago

Intrinsic Electronic Transport Properties of High-Quality Monolayer and Bilayer MoS[subscript 2]

We report electronic transport measurements of devices based on monolayers and bilayers of the transition-metal dichalcogenide MoS[subscript 2]. Through a combination of in situ vacuum annealing and electrostatic gating we obtained ohmic contact to the MoS[subscript 2] down to 4 K at high carrier densities. At lower carrier densities, low-temperature four probe transport measurements show a metal–insulator transition in both monolayer and bilayer samples. In the metallic regime, the high-temperature behavior of the mobility showed strong temperature dependence consistent with phonon-dominated transport. At low temperature, intrinsic field-effect mobilities approaching 1000 cm[superscript 2]/(V·s) were observed for both monolayer and bilayer devices. Mobilities extracted from Hall effect measurements were several times lower and showed a strong dependence on density, likely caused by screening of charged impurity scattering at higher densities.United States. Office of Naval Research. Multidisciplinary University Research Initiative. Graphene Approaches to Terahertz ElectronicsDavid & Lucile Packard Foundation (Fellowship