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Low conductance of the nickel atomic junctions in hydrogen atmosphere

The low conductance of nickel atomic junctions in the hydrogen environment is studied using the nonequilibrium Green's function theory combined with first-principles calculations. The Ni junction bridged by a $H_2$ molecule has a conductance of approximately 0.7 $G_0$. This conductance is contributed by the anti-bonding state of the $H_2$ molecule, which forms a bonding state with the $3d$ orbitals of the nearby Ni atoms. In contrast, the Ni junction bridged by the two single H atoms has a conductance of approximately 1 $G_0$, which is weakly spin-polarized. The spin-up channels were found to contribute mostly to the conductance at a small junction gap, while the spin-down channels play a dominant role at a larger junction gap.