BEAMS In the literature: contested

Is a mean transverse energy below the thermal value achievable and usable

In plain words

The sideways energy of emitted electrons was thought to bottom out at the temperature of the cathode. Some proposals and data suggest it can go lower, but this is not established.

Precise statement

For near-threshold photoemission the mean transverse energy is usually bounded below by $\mathrm{MTE} \sim k_B T$ of the electron distribution. Determine whether mechanisms such as refraction at an effective-mass discontinuity at the surface or band-structure selection give $\mathrm{MTE} < k_B T$ at useful quantum efficiency ($\mathrm{QE} > 1e-5$), and the minimum attainable MTE for a given material at $T = 300\,\mathrm{K}$.

What would settle it

Direct MTE measurements below $k_B T$ at room temperature with a calculation reproducing them from the material band structure.

Status in the literature

Unverified note

A 2026 Physical Review Letters report attributed subthermal MTE to electron refraction in multialkali cathodes; independent confirmation is pending.