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  1. Free, publicly-accessible full text available May 1, 2024
  2. A combination of spin–orbit coupling and electron–electron interaction gives rise to a new type of collective spin modes, which correspond to oscillations of magnetization even in the absence of the external magnetic field. We review recent progress in theoretical understanding and experimental observation of such modes, focusing on three examples of real-life systems: a two-dimensional electron gas with Rashba and/or Dresselhaus spin–orbit coupling, graphene with proximity-induced spin–orbit coupling, and the Dirac state on the surface of a three-dimensional topological insulator. This paper is dedicated to the 95th birthday of Professor Emmanuel I. Rashba. 
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    Abstract The longstanding view of the zero sound mode in a Fermi liquid is that for repulsive interaction it resides outside the particle-hole continuum and gives rise to a sharp peak in the corresponding susceptibility, while for attractive interaction it is a resonance inside the particle-hole continuum. We argue that in a two-dimensional Fermi liquid there exist two additional types of zero sound: “hidden” and “mirage” modes. A hidden mode resides outside the particle-hole continuum already for attractive interaction. It does not appear as a sharp peak in the susceptibility, but determines the long-time transient response of a Fermi liquid and can be identified in pump-probe experiments. A mirage mode emerges for strong enough repulsion. Unlike the conventional zero sound, it does not correspond to a true pole, yet it gives rise to a peak in the particle-hole susceptibility. It can be detected by measuring the width of the peak, which for a mirage mode is larger than the single-particle scattering rate. 
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  5. We observe novel composite particles -- chiral excitons -- residing on the surface of a topological insulator (TI), Bi_2Se_3. Unlike other known excitons composed of massive quasiparticles, chiral excitons are the bound states of surface massless electrons and surface massive holes, both subject to strong spin-orbit coupling which locks their spins and momenta into chiral textures. Due to this unusual feature, chiral excitons emit circularly polarized secondary light (photoluminescence) that conserves the polarization of incident light. This means that the out-of-plane angular momentum of a chiral excitonis preserved against scattering events during thermalization, thus enabling optical orientation of carriers even at room temperature. The discovery of chiral excitons adds to the potential of TIs as a platform for photonics and optoelectronics devices. 
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