Molecule Cascade

Six carbon monoxide (CO) molecules on a copper surface are shown: a lone molecule, an adjacent pair and a trio in a v-shaped "chevron" arrangement, which is metastable. Red circles indicate CO molecules and blue dots indicate atoms in the underlying copper surface.
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After a few minutes, the middle molecule in the chevron has moved to a more stable location farther away from the other two. The key to the cascade circuit is that each molecule that moves creates a new chevron (or other metastable arrangement), which in turn causes another molecule to move.
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Logic AND gate in the initial configuration. A single CO in the center becomes part of a chevron when both arms X and Y have propagated to the center. Blue circles represent CO's that are going to hop during the operation of the logic gate. Red circles are not going to change their location during the operation of the gate.
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Logic AND gate after Input X was triggered and the resulting cascade propagated to the central molecule. The remaining cascade is now a 'simple' linked chevron cascade. Green circles represent CO's that have hopped to their final location.
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Logic AND gate in the final configuration. All CO molecules have hopped to their final locations.
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Christopher Lutz
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Jay Gupta
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(L-R) Jay Gupta, Don Eigler, Andreas Heinrich, Christopher Lutz
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A wire: Linked-chevron cascade
A linked-chevron cascade in the initial configuration consists of pairs of carbon monoxide (CO) molecules at nearest neighbor sites, plus a trigger molecule. Moving the trigger molecule with a scanning tunneling microscope (STM) tip starts the cascade, which then propagates the "signal" like a wire for an arbitrary length (to the end of the cascade) without further involvement of the STM. Creating the first "chevron" configuration (three CO molecules in a slightly bent arrangement) causes the central CO molecule to tunnel outward to a new position, which sets up the second chevron. By positioning the molecules precisely, each chevron causes a molecule to move, setting up the next chevron and so on as the cascade continues. (Click mouse once to start the PowerPoint.) Click to view PPT
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A faster wire: Chevron cascade with helper molecules
A linked-chevron cascade with "helper" molecules positioned to the sides operates 70 times faster than a simple linked-chevron cascade. The helper molecules make subtle changes in the energy landscape that result in this dramatic change of propagation speed. (Click mouse once to start the PowerPoint animation.) Click to view PPT
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Logical AND gate animation
A logical AND gate consists of 2 input arms (X and Y) which meet in the center of the gate. The central carbon monoxide molecule becomes part of a chevron only after both inputs have been triggered and have propagated to this molecule. Only then is the output cascade triggered. All hops in an AND gate and in the connecting wires are based on the chevron principle. (Click mouse click to start the X input cascade; second mouse click starts Y input.) Click to view PPT
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Two-input sorter animation
A two-input sorter consists of 2 inputs (X and Y) and 2 outputs, which are simultaneously computed. Those outputs are the logic operations a) X AND Y and b) X OR Y. To achieve the simultaneous computation of AND and OR, each input signal is doubled -- split into two cascades by a "fanout" arrangement. Since these cascades operate on a surface in two dimensions, the signals must also pass through each other using the "crossover" construction in the center of the device. (Click mouse to start X input.) Click to view PPT
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Linked-Chevron Cascade animation
Click to view AVI video - 3.4MB
Click to view WMV video - 1.8MB
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3-input sorter cascade with electrical schematic
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Logic 3-input sorter in the initial configuration. Overlayed symbols (+ for OR, dot for AND) show the logic operation of the sorter in stages. The 3-input sorter comprises three 2-input sorters. This device computes the logic AND, the logic OR, and the logic MAJORITY simultaneously.
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Logic 3-input sorter in the initial configuration. The 3-input sorter comprises three 2-input sorters. This device computes the logic AND, the logic OR, and the logic MAJORITY simultaneously.
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Logic 3-input sorter after Input X was triggered and has propagated to the OR output on the right.
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Logic 2-input sorter after Input X was triggered and has propagated to the OR output. The second arm stopped at the AND gate which did not propagate to the output.
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Logic 2-input sorter after Input X and Input Y were triggered and have propagated to the outputs. The second input arm went through the AND gate to the logic AND output.
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This sorter computes the logic AND and the logic OR simulataneously and consists of one AND gate, one OR gate, two FANOUT gates, and one CROSSOVER gate in the center of the structure. The blue circles indicate the positions of carbon monoxide (CO) molecules that will move as the cascade operates. The red circles indicate the positions of CO molecules that remain stationary.
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The same scanning tunneling microscope (STM) image of the inital state of the logic 2-input sorter.
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