Jeremy L. O'Brien

Towards the fabrication of phosphorus qubits for a silicon quantum computer

The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices, however, requires atomic scale manipulation-an immense technological challenge. We demonstrate that it is possible to fabricate an atomically precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a silicon-based quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth.

STM characterization of the Si-P heterodimer

We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to study the behavior of adsorbed phosphine (PH) on Si(001), as a function of annealing temperature, paying particular attention to the formation of the Si-P heterodimer. Dosing the Si(001) surface with ∼0.002 langmuirs of PH results in the adsorption of PH (x=2,3) onto the surface and etching of Si to form individual Si ad-dimers. Annealing to 350 °C results in the incorporation of P into the surface layer to form Si-P heterodimers and the formation of short one-dimensional Si dimer chains and monohydrides. In filled state STM images, isolated Si-P heterodimers appear as zigzag features on the surface due to the static dimer buckling induced by the heterodimer. In the presence of a moderate coverage of monohydrides this static buckling is lifted, rending the Si-P heterodimers invisible in filled state images. However, we find that we can image the heterodimer at all H coverages using empty state imaging. The ability to identify single P atoms incorporated into Si(001) will be invaluable in the development of nanoscale electronic devices based on controlled atomic-scale doping of Si.

Uranium superconductivity redux

Since the discovery of heavy-fermion superconductivity in uranium compounds in the early 1980s, other uranium compounds have been discovered that are fully as interesting to study. However, as we look forward in the year 2000, we now have higher-purity, single crystals of the element itself. Preliminary resistivity and ac susceptibility measurements show the improved quality of the samples and thus hold the promise of understanding many aspects of its superconductivity, which have remained untouched for almost 25 years.

Quantization and chiral edge state properties in nearly 3D quantum well structures

We report magneto-transport measurements including Hall, Rxy, longitudinal, Rxx, and vertical, Rzz, magnetoresistance on nearly 3-dimensional (3D) 200 layer GaAs/AlGaAs quantum well structures. Although the interlayer bandwidth is nearly 20% of the Fermi energy, we still observe complete quantization of the Hall resistance for the 3D quantum Hall state. The temperature dependence of the Rxx minimum shows two unusual features: initially, at higher temperatures 1 K where the quantum Hall state develops, a gap with an activation energy much smaller than the Landau gap is observed; in the low temperature limit 0.030 K a variable range hopping behavior takes over with a residual resistivity limit. Independent measurements of Gzz (in 3D ≈1/Rzz) where the chiral edge states dominate the vertical transport show the same temperature dependence.

Dirac series experiments in 800 T fields:: Innovations for transport measurements

Abstract Low-temperature transport measurements in extreme pulsed magnetic fields generated by implosive flux compression techniques are challenging due to the short (micro-second) duration of the pulse, and consequent problems associated with pick-up and eddy current heating arising from the large d B /d t (peak value ∼10 9 xa0T/s). For the `Dirac series of experiments at Los Alamos, we have developed microlithographic processing techniques to fabricate thin coplanar microwave transmission lines (CTLs) directly onto semiconductor and superconductor samples. The thin metal reduces eddy current heating in the CTLs, allowing measurements at liquid-helium temperatures, while providing excellent electrical coupling to electrons in the sample. For samples with a conducting surface, a thin, robust insulating layer of Si 3 N 4 was deposited before the CTLs to provide capacitive coupling.

Nanoscale phosphorus atom arrays created using STM for the fabrication of a silicon based quantum computer

Quantum computers offer the promise of formidable computational power for certain tasks. Of the various possible physical implementations of such a device, silicon based architectures are attractive for their scalability and ease of integration with existing silicon technology. These designs use either the electron or nuclear spin state of single donor atoms to store quantum information. Here we describe a strategy to fabricate an array of single phosphorus atoms in silicon for the construction of such a silicon based quantum computer. We demonstrate the controlled placement of single phosphorus bearing molecules on a silicon surface. This has been achieved by patterning a hydrogen mono-layer resist with a scanning tunneling microscope (STM) tip and exposing the patterned surface to phosphine (PH3) molecules. We also describe preliminary studies into a process to incorporate these surface phosphorus atoms into the silicon crystal at the array sites.

L-K treatment of the SdH oscillations in the SDW state of Q1D materials

Abstract We have applied Lifshitz-Kosevich theory to directly fit the high field oscillatory magnetoresistance in the Q-1-dimensional Bechgaard salts in (TMTSF) 2 X (X = AsF 6 or ClO 4 [Q-state]) in their spin density wave states. Values of m * , Dingle temperature, and SdH frequencies are obtained, and the anomalous temperature dependence of the oscillation amplitudes are discussed.