I know the HN protocol is to use the headline, but the sciencebulletin article headline is pretty broken. Negative differential resistance (NDR) in tunneling diodes has been understood for several decades, and is pretty far from a mystery.

The original article is about an I-V curve from a structure involving a single-atom + a scanning-tunneling microscope (STM). That such a system would also exhibit NDR is not particularly surprising. Tunneling to a structure with discrete energy levels will have current flow when energies line up, and less current flow when energies don't line up. So even the "mystery" isn't much of a mystery.

The interesting results imho are:

1. The team got reproducible single-atom tunneling with an STM tip, and

2. They measured the time-response of about 10 microseconds.

fwiw.

Edit: Resonant tunneling diodes used to be a hobby of mine: https://dspace.mit.edu/handle/1721.1/38419