Correlation between transport properties and atomic configuration of atomic contacts of zinc by low-temperature measurements

We report low-temperature measurements of the electrical transport properties of atomic contacts of the superconducting metal zinc, arranged with lithographically fabricated mechanically controllable breakjunctions (MCB) as well as with MCBs made from whiskers. The conductance histogram shows several peaks—not regularly observed for multivalent metals. The first peak—corresponding to the single-atom contact—is located slightly below one conductance quantum and is split into two subpeaks, indicating two preferred configurations of the single-atom contact. The existence of two configurations can also be traced by analyzing individual conductance vs distance curves of the MCB. The current-voltage characteristics in the superconducting state show nonlinearities due to multiple Andreev reflections (MAR), which we use for determining the transport channels. With the help of a tight-binding model for the transport channels we establish a correlation between the preferred conductance values and preferred atomic configurations.

Figure 1

Upper panel: Electron micrograph of sample No. 1 under an inclination angle of $70°$, taken after measurement. The polycrystalline structure of the Zn film is clearly visible. The distance between the two wide electrodes is $2\mathrm{\mu }\mathrm{m}$. Lower panel: Magnification of the central part of the sample. A remaining spacing of the bridge arms of several nm and a fine nanowire between them is observed.

Figure 2

(a), (b), and (c): Conductance histograms for opening and closing the contacts, recorded on sample No. 1 (2) at $T=1.5\mathrm{K}$. The histograms have been calculated from $\approx 600$ individual opening and closing curves, respectively.

Figure 3

Opening [panels (a) and (b)] and closing [(c) and (d)] traces of sample No. 1 recorded at $1.4\mathrm{K}$. Panels (a) and (c) [(b) and (d)] show examples without (with) the plateau at $1.4G_0$.

Figure 4

Histogram of step heights deduced from 600 individual opening and closing curves recorded on sample No. 1, that have been used to calculate the conductance histogram in Fig. 2a.

Figure 5

Current-voltage characteristics of two atomic-size contacts of Zn arranged with a whisker MCB at $T=0.27\mathrm{K}$ (symbols) and best fits according to the theory of MAR (Refs. 21, 22) (lines) with a superconducting gap value $\Delta =165\mathrm{\mu }\mathrm{eV}$. The transmission coefficients are for the lower curve (triangles): ${\tau }_1=0.82$, ${\tau }_2=0.25$, ${\tau }_3=0.19$, ${\tau }_4=0.09$; and for the upper curve (circles) ${\tau }_1=0.96$, ${\tau }_2=0.47$, ${\tau }_3=0.08$. The lower (upper) curve has been measured at position $0.35\mathrm{nm}$ $\left(1.37\mathrm{nm}\right)$ of the upper panel of Fig. 6.

Figure 6

Upper panel: Opening traces with decomposition into individual channels for a whisker breakjunction showing plateaus at $1.4G_0$, $0.5G_0$, $1.5G_0$, and $0.6G_0$. Transport direction is presumably close to [001]. Lower panel: Opening traces with decomposition into individual channels for a whisker breakjunction showing plateaus at 2.5 and $1G_0$. Transport direction presumably close to [110]. The origins of the electrode distance axes are at arbitrary values.

Figure 7

(Color online) Left-hand panels: The upper one displays a contact grown along the [110] direction with a single central atom. The lower one shows the channel transmissions $\left\{{\tau }_{\mathrm{i}}\right\}$ as a function of the energy for this contact. The Fermi energy is set to zero and is indicated with a vertical line. The total conductance is $3.6G_0$ and it is dominated by five conduction channels. Right-hand panels: The same as in the left-hand panels for a contact grown in the [001] direction with a minimum cross section of three atoms. The total conductance is $2.1G_0$ with the contribution of four channels. Notice that the second and third channels are degenerate at the Fermi energy.