A physical system’s otherwise hidden nature can reveal itself in the system’s response to a short, sharp, excitatory shock—which is why seismologists set off explosions in Earth’s crust and why biophysicists zap chlorophyll and its attendant proteins with laser pulses.
It’s also why MIT’s Ray Ashoori and Oliver Dial force electrons to tunnel into a thin layer of gallium arsenide millions of times a second. The electrons already in the GaAs layer adjust to each batch of newcomers. In doing so, they endow the emerging current with information about their density of states (DOS), a key quantity that experimenters measure and theorists predict.
Ashoori’s technique is a variant of two-dimensional tunneling spectroscopy. Electrons are sent tunneling at low temperature from a three-dimensional metal to a two-dimensional electron system (2DES). If the tunneling barrier preserves the electrons’ energy, the differential current dI/dV is proportional to the 2D system’s density of states....
