Chirality and intrinsic dissipation of spin modes in two-dimensional electron liquids

Abstract

We review recent theoretical and experimental developments concerning collective spin excitations in two-dimensional electron liquid (2DEL) systems, with particular emphasis on the interplay between many-body and spin-orbit effects, as well as the intrinsic dissipation due to the spin-Coulomb drag. Historically, the experimental realization of 2DELs in silicon inversion layers in the 60s and 70s created unprecedented opportunities to probe subtle quantum effects, culminating in the discovery of the quantum Hall effect. In the following years, high quality 2DELs were obtained in doped quantum wells made in typical semiconductors like GaAs or CdTe. These systems became important test beds for quantum many-body effects due to Coulomb interaction, spin dynamics, spin-orbit coupling, effects of applied magnetic fields, as well as dissipation mechanisms. Here we focus on recent results involving chiral effects and intrinsic dissipation of collective spin modes: these are not only of fundamental interest but also important towards demonstrating new concepts in spintronics. Moreover, new realizations of 2DELs are emerging beyond traditional semiconductors, for instance in multilayer graphene, oxide interfaces, dichalcogenide monolayers, and many more. The concepts discussed in this review will be relevant also for these emerging systems.

Spin waves in doped graphene: a time-dependent spin-density-functional approach to collective excitations in paramagnetic two-dimensional Dirac fermion gases

“…In practice, achieving a spin-polarized Dirac fermion gas most likely involves interaction with a magnetic substrate, which will also introduce spin-orbit coupling. To account for these effects, our model can be generalized to include Rashba-type spin-orbit coupling; if the Rashba terms are not too strong, this will preserve the essential features of the spin waves (as is the case in the 2DEG [17][18][19][20][21]). On the other hand, if the spin waves in graphene are coupled with spin excitations in the magnetic substrate, more complex hybrid modes may occur.…”

“…(31). where µ B is the Bohr magneton and the effective magnetic field B eff is the sum of the externally applied magnetic field B ext and an additional magnetic field B xc due to exchange-correlation many-body effects [21]. Using the experimental g-factor of graphene, g = 1.952 [76], we can calculate the B eff that produces a given value of Z * .…”

“…In our model, Z * is treated as an adjustable parameter. Notice that Z * denotes the Zeeman energy renormalized by electronic many-body effects, which in general differs from the bare Zeeman energy Z [21]. To determine Z * from first principles would require a self-consistent calculation of the band structure in the presence of an externally applied uniform in-plane magnetic field, or a magnetic field induced by proximity to a ferromagnetic substrate; how this could be realized will be further discussed in Sec.…”

“…, which is smaller than Z * since f xc 0 < 0. From electronic many-body theory one would have expected that ω L = Z (Larmor's theorem), where Z is the bare Zeeman energy, i.e., all many-body effects cancel out exactly in the Larmor precessional state [21]. However, Larmor's theorem does not apply here since the band structure is obtained from a tight-binding Dirac fermion Hamiltonian and not from first principles; in other words, Z * is given but Z remains unknown.…”

“…The corresponding spin waves in magnetic 2DEGs have been well studied theoretically and experimentally [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. On the other hand, apart from a recent study based on Fermi-liquid theory [27], spin waves in doped graphene have not been investigated to our knowledge.…”