Spin mapping at the nanoscale and atomic scale

We describe a novel quantum technology for possible ultra-fast, ultra-dense and ultra-low-power supercomputing. The technology utilizes single electrons as binary logic devices in which the spin of the electron encodes the bit information. Both two-dimensional cellular automata and random wired logic can be realized by laying out on a wafer specific geometric patterns of quantum dots each hosting a single electron. Various types of logic gates, combinational circuits for arithmetic logic units, and sequential circuits for memory have been designed. The technology has many advantages such as (1) the absence of physical interconnects between devices (inter-device interaction is provided by quantum mechanical spin-spin coupling between single electrons in adjacent quantum dots), (2) ultra-fast switching times of approximately 1 picosecond for individual devices, (3) extremely high bit density approaching 10 terabits cm-2, (4) non-volatile memory, (5) robustness and possible room-temperature operation with very high noise margin and reliability, (6) a very low power delay product ( approximately 10-20 J) for switching between logic levels, and (7) a very small power dissipation of a few tens of nanowatts per switching event. In spite of the above advantages, the technology also has some serious drawbacks in that the fan-out of individual logic devices may be small, wiring crossover is very problematic and the devices themselves have no inherent gain so that isolation between input and output is virtually non-existent. These are problems that plague all similar quantum technologies although they are seldom recognized as such. We will discuss these problems, and where possible, offer plausible solutions. In spite of these drawbacks, however, there are still enough attractive features of this technology to merit serious research. In this paper, we will describe how the spin-polarized single-electron logic devices work, along with the associated circuits and architecture. Finally, we will propose a new fabrication technique for realizing these chips which we believe is much more compatible with the demands of the technology than conventional nanofabrication methods.

http://iopscience.iop.org/article/10.1088/0957-4484/5/2/007/pdf

Supercomputing with spin-polarized single electrons in a quantum coupled architecture

We have studied the (001) surface of a ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ single crystal using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy. A clean surface was obtained by a combination of ${\mathrm{Ar}}^{+}$ ion sputtering, in situ annealing in ${\mathrm{O}}_{2}$ atmosphere, and further annealing in ultra high vacuum conditions. A sharp $(\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ reconstruction was observed by LEED and STM analysis at room temperature. The STM measurements were performed using a tip of antiferromagnetic MnNi alloy. Atomically resolved STM images provide evidence of a surface terminated at the octahedral plane, with rows of Fe cations running along the [110] and $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ crystallographic axes. The 3 \AA{} periodicity of the Fe rows expected for a bulk-terminated B plane is not observed. Instead, two different kinds of Fe cations with a separation of 6 \AA{} are imaged. The periodicity between Fe cations of the same kind is about 12 \AA{}. We propose a model to explain the observed symmetry in terms of charge ordering of the Fe cations on the octahedral sites of the inverse spinel lattice. We also explain the $(\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ LEED pattern in terms of charge ordering, as opposed to a structural rearrangement of the atoms on the surface. We further suggest the possibility of a spin polarized effect, in view of the anomalous corrugation observed along the [110] Fe rows on the octahedral plane, in agreement with the different spin configuration of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions.

Charge ordering on the surface of Fe 3 O 4 ( 001 )

A full magnetic characterization of bulk Cr tips has been achieved using spin-polarized scanning tunneling microscopy at low temperature. A detailed bias-dependent study of the spatial magnetic sensitivity on the system of 1.5 monolayers of Fe/W(110) reveals that all magnetic directions in space are sensed over a wide bias range, thereby indicating a canted magnetization direction being a typical feature of bulk Cr tips. Consequently, using Cr as tip material allows any standard scanning tunneling microscope setup to be extended by the spin-polarized mode.

Bulk Cr tips with full spatial magnetic sensitivity for spin-polarized scanning tunneling microscopy

This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. Measurements are presented of the circular-polarization-dependent photocurrent (the so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/ tau -MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of tau -MnAl along the tunnelling direction; the latter is corroborated by band structure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

/pdf/spin-dependent-transport-in-metal-semiconductor-tunnel-3klrjhugn8.pdf

A.A. Minakov

Papers

On the possibility of resolving quantization axes of surface spins by means of a scanning tunneling microscope with a magnetic tip