Conductance of atomic size contacts of Ag and Au at high magnetic fields

Electronic conduction at the atomic scale can be described by Landauer’s formalism. In single atom point contacts of noble metals like Au and Ag, there is just one channel open between both electrodes and the conductance is very close to the quantum of conductance $G \approx G_0=\frac{2e^2}{h}$, with the factor of two coming from spin degeneracy. The magnetoconductivity of atomic size contacts has been studied for numerous systems, unveiling local Kondo screening, magnetic order and spin-polarized currents. However, these have been mostly performed in elements with multiple open conduction channels where $G$ differs from $G_0$. The realization of a magnetically active conductor with a single open channel remains difficult to achieve. Here we present measurements of the electronic conductance of single channel Au and Ag atomic-size contacts in magnetic fields up to 20 Tesla. We observe a decrease in $G$ which goes up to about 15% in many Au contacts at 20 T. We perform calculations and find that pure Ag and Au do not present a strong field dependence of $G$, in agreement with previous results at smaller magnetic fields. We also find, however, that residual O$_2$ molecules attached close to the contact produce an an induced spin-polarized current, which leads to a decrease in $G$. We discuss the role of the magnetic response of the electrodes in the jump to contact. Our results suggest that single channel atomic size conductors with a sizeable response to a magnetic field can be built by combining noble metals and magnetically active molecular systems.

💡 Research Summary

The paper investigates how high magnetic fields affect the electrical conductance of atomic‑scale single‑channel contacts made of gold (Au) and silver (Ag). Using a cryogenic scanning tunneling microscope (STM) operated at 4.2 K and integrated with a 20 Tesla superconducting magnet, the authors repeatedly form and break atom‑size junctions by indenting and retracting the tip. For each junction they record conductance versus tip‑sample distance, defining G_a as the conductance just before the sudden “jump‑to‑contact” and G_b as the conductance immediately after the jump (the single‑atom contact plateau). Thousands of such traces are collected at 0 T and 20 T, and two‑dimensional histograms of G_b versus G_a are constructed.

Experimentally, Au contacts show a clear magnetic‑field‑induced reduction of G_b: at 20 T the most probable conductance drops from ≈ 1 G₀ (the quantum of conductance, G₀ = 2e²/h) to as low as 0.85 G₀, corresponding to a ≈ 15 % decrease. A smaller but similar trend is observed for Ag. In contrast, G_a is essentially unchanged for Au, while for Ag it modestly increases with field. The fraction of contacts with G_b < 0.85 G₀ grows monotonically with magnetic field, confirming a genuine magnetoresistive effect in the single‑atom regime.

To understand the origin of this effect, the authors perform first‑principles calculations. Density‑functional theory (DFT) with the PBE functional, Grimme D3BJ dispersion correction, and scalar relativistic Douglas‑Kroll‑Hess treatment is used to compute binding curves for Au and Ag clusters. These calculations show that pure Au and Ag have negligible magnetic susceptibility; their electronic structure and transmission are essentially field‑independent, in agreement with earlier studies.

The key insight comes from modeling the presence of an O₂ molecule near the contact. O₂ possesses a high spin‑orbit coupling and a triplet ground state, making it magnetically active. By adding an O₂ molecule to the DFT‑optimized Au or Ag junction and performing non‑equilibrium Green’s function (NEGF) transport calculations (using the ANT.Gaussian code with a self‑consistent magnetic field in the z‑direction), the authors find that the molecule introduces spin‑dependent scattering. Under a strong external field the O₂ spin polarizes, reducing the transmission of one spin channel while the other remains largely unchanged. The net effect is a reduction of the total conductance that matches the experimentally observed 10–15 % drop.

The paper also discusses the mechanical aspects of contact formation. Au has a larger binding energy than Ag, so the jump‑to‑contact occurs at a larger tip‑sample separation, leading to a lower G_a for Au. Ag’s weaker bonding results in a higher G_a and a more pronounced field‑dependence of G_a. The authors argue that surface Shockley states, which could in principle provide spin splitting, are absent in the disordered, non‑ideal surfaces of the broken junctions, further supporting the O₂‑mediated mechanism.

In summary, the study demonstrates that while pristine Au and Ag atomic contacts are essentially non‑magnetic, the adsorption of a magnetically active molecule such as O₂ can endow a single‑channel junction with a sizable magnetoresistance (up to ~15 % at 20 T). This finding opens a route to engineer magnetic‑field‑responsive nano‑electronics by combining noble‑metal electrodes with spin‑active molecular species, offering potential applications in spintronic devices, magnetic sensors, and molecular spin filters.