Plasmons in the presence of Tamm-Shockley states with Rashba splitting at noble metal surfaces

TL;DR

This paper investigates how Rashba-split Tamm-Shockley surface states on noble metal surfaces influence plasmon behavior, revealing resonances and enabling plasmon-polariton propagation in otherwise forbidden frequency ranges.

Contribution

It demonstrates the impact of spin-orbit split surface states on plasmon spectra and proposes a method to observe intersubband resonances via surface impedance measurements.

Findings

Resonances occur near the spin-orbit splitting energy Δ.

Plasmons are significantly modified when intersecting the Δ line.

Plasmon-polaritons can propagate in forbidden frequency ranges due to surface states.

Abstract

Au(111) or similar noble metal surfaces feature Tamm-Shockley surface states that are known to possess considerable spin-orbit splitting of the Rashba type of order $\Delta=0.1$ eV. When interacting with an electromagnetic field such states are expected to have resonances when the frequency of the field is near the energy of the spin-orbit splitting $\Delta$. These resonances originate in the intersubband transitions between spin-split subbands. Such resonances can be observed in the frequency dependence of the surface impedance. Plasmons in thin metal films are gapless and can be strongly affected by these spin resonances, acquiring significant modification of the spectrum when it intersects the $\omega=\Delta$ line. Finally, an interesting demonstration of the intersubband resonances can be obtained when metal films are coated with ionic dielectrics that have a frequency of longitudinal/transverse optical phonons above/below $\Delta$. The dielectric function between the two optical phonon frequencies is negative which forbids propagation of conventional plasmon-polaritons. However, the presence of spin-orbit-split surface states allows plasmon-polaritons to exist in this otherwise forbidden range of frequencies.