Schematic of the three devices investigated. The three molecules are constructed from benzene rings connected respectively by (a) a single bonded (SB), (b) a double bonded (DB), and (c) a triple bonded (TB) C dimer. The three molecules couple to Al(100) electrodes with a finite cross section. The scattering region and two leads are separated by vertical black lines in each structure and the Al electrodes extend to .
(a) characteristics of the three structures described in the text. The current is vanishing for a wide bias range for the SB molecule while a NDR appears for the DB and TB structures. [(b)–(d)] Three-dimensional plot of the transmission spectra as a function of bias voltages and energy. The black lines in the highlighted area denote effective transmission probability inside the bias window and the dashed lines denote transmission probability outside the bias window. (b) SB molecule, (c) DB molecule, and (d) TB molecule. [(e)–(g)] Frontier orbitals for the three structures investigated. The HOMO and LUMO states are shown in top perspective view. The HOMO states are relatively localized for all the three structures but the LUMO state of DB and TB shows nonlocal bond character.
Transmission coefficient as a function of energy of the DB junction at three bias voltages: 0.0, 0.6, and 1.1 V. These voltages correspond to thermodynamical equilibrium, current peak, and current valley, respectively. The NDR occurs between 0.6 and 1.1 V.
PDOS and charge isosurfaces of important states of the DB molecule at various biases. (a) PDOS projected over the left Al atoms, , and benzenethiol. The lead states are denoted as S1 and D1 for the left and right lead, respectively. The LUMO and LUMO-1 states of molecule are also indicated. The chemical potentials of two leads are marked as dashed lines in each figure. (b) Evolution of one state of S1 for three biases. (c) Evolution of LUMO state for three biases. (d) Evolution of LUMO-1 state for three biases.
Schematics explaining the mechanism for the NDR. The S1 and D1 states and two LUMO states are denoted in the figures. As the bias increases, the alignment of LUMO and D1 states at three particular positions in biases are shown. The bias voltages of 0.0, 0.6, and 1.1 V correspond to equilibrium, current peak, and current valley in the NDR, respectively.
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