Person: Churchill, Hugh Olen Hill
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Churchill
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Hugh Olen Hill
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Churchill, Hugh Olen Hill
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Publication Spin-orbit effects in carbon-nanotube double quantum dots(American Physical Society (APS), 2010) Weiss, Sarah; Rashba, Emmanuel; Kuemmeth, Ferdinand; Churchill, Hugh Olen Hill; Flensberg, K.We study the energy spectrum of symmetric double quantum dots in narrow-gap carbon nanotubes with one and two electrostatically confined electrons in the presence of spin-orbit and Coulomb interactions. Compared to GaAs quantum dots, the spectrum exhibits a much richer structure because of the spin-orbit interaction that couples the electron’s isospin to its real spin through two independent coupling constants. In a single dot, both constants combine to split the spectrum into two Kramers doublets while the antisymmetric constant solely controls the difference in the tunneling rates of the Kramers doublets between the dots. For the two-electron regime, the detailed structure of the spin-orbit split energy spectrum is investigated as a function of detuning between the quantum dots in a 22-dimensional Hilbert space within the framework of a single-longitudinal-mode model. We find a competing effect of the tunneling and Coulomb interaction. The former favors a left-right symmetric two-particle ground state while in the regime where the Coulomb interaction dominates over tunneling, a left-right antisymmetric ground state is found. As a result, ground states on both sides of the (11)-(02) degeneracy point may possess opposite left-right symmetry, and the electron dynamics when tuning the system from one side of the (11)-(02) degeneracy point to the other is controlled by three selection rules (in spin, isospin, and left-right symmetry). We discuss implications for the spin-dephasing and Pauli blockade experiments.Publication Quantum Dots in Gated Nanowires and Nanotubes(2012-08-17) Churchill, Hugh Olen Hill; Marcus, Charles MasamedThis 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.Publication Carbon Nanotubes for Coherent Spintronics(Elsevier Science Limited, 2010) Kuemmeth, Ferdinand; Churchill, Hugh Olen Hill; Herring, Patrick Kenichi; Marcus, CCarbon nanotubes bridge the molecular and crystalline quantum worlds, and their extraordinary electronic, mechanical and optical properties have attracted enormous attention from a broad scientific community. We review the basic principles of fabricating spin-electronic devices based on individual, electrically-gated carbon nanotubes, and present experimental efforts to understand their electronic and nuclear spin degrees of freedom, which in the future may enable quantum applications.Publication Relaxation and Dephasing in a Two-Electron \(^{13}C\) Nanotube Double Quantum Dot(American Physical Society, 2009) Churchill, Hugh Olen Hill; Kuemmeth, Ferdinand; Harlow, Jennifer W.; Bestwick, Andrew J.; Rashba, Emmanuel; Flensberg, Karsten; Stwertka, Carolyn H.; Taychatanapat, Thiti; Watson, Susan K.; Marcus, CWe 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.Publication Electron–Nuclear Interaction in \(^{13}C\) Nanotube Double Quantum Dots(Nature Publishing Group, 2009) Churchill, Hugh Olen Hill; Bestwick, Andrew J.; Harlow, Jennifer W.; Kuemmeth, Ferdinand; Marcos, David; Stwertka, Carolyn H.; Watson, Susan K.; Marcus, CFor 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.