Electronic switches with nanoscale dimensions satisfy an urgent demand for further device miniaturization. A recent heavily investigated approach for nanoswitches is the use of molecular junctions that employ photochromic molecules that toggle between two distinct isoforms. In contrast to the reports on this approach, we demonstrate that the conductance switch behavior can be realized with only a bare metallic contact without any molecules under light illumination. We demonstrate that the conductance of bare metallic quantum contacts can be reversibly switched over eight orders of magnitude, which substantially exceeds the performance of molecular switches. After the switch process, the gap size between two electrodes can be precisely adjusted with subangstrom accuracy by controlling the light intensity or polarization. Supported by simulations, we reveal a more general and straightforward mechanism for nanoswitching behavior, i.e., atomic switches can be realized by the expansion of nanoelectrodes due to plasmonic heating.
For a (molecular) graph, the first and second Zagreb indices (M 1 and M 2 ) are two well-known topological indices, first introduced in 1972 by Gutman and Trinajstić. The first Zagreb index M 1 is equal to the sum of the squares of the degrees of the vertices, and the second Zagreb index M 2 is equal to the sum of the products of the degrees of pairs of adjacent vertices. Let K p n 1 ,n 2 with n 1 ≤ n 2 , n 1 + n 2 = n and p < n 1 be the set of bipartite graphs obtained by deleting p edges from complete bipartite graph Kn 1 ,n 2 . In this paper, we determine sharp upper and lower bounds on Zagreb indices of graphs from K p n 1 ,n 2 and characterize the corresponding extremal graphs at which the upper and lower bounds on Zagreb indices are attained. As a corollary, we determine the extremal graph from K p n 1 ,n 2 with respect to Zagreb coindices. Moreover a problem has been proposed on the first and second Zagreb indices.
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