Samvel Badalyan

Anisotropic plasmons in a two-dimensional electron gas with spin-orbit interaction

Spin-orbit coupling-induced anisotropies of plasmon dynamics are investigated in two-dimensional semiconductor structures. The interplay of the linear Bychkov-Rashba and Dresselhaus spin-orbit interactions drastically affects the plasmon spectrum: the dynamical structure factor exhibits variations over several decades, prohibiting plasmon propagation in specific directions. While this plasmon filtering makes the presence of spin-orbit coupling in plasmon dynamics observable, it also offers a control tool for plasmonic devices. Remarkably, if the strengths of the two interactions are equal, not only the anisotropy but all the traces of the linear spin-orbit coupling in the collective response disappear.

Beating of Friedel oscillations induced by spin-orbit interaction

By exploiting our recently derived exact formula for the Lindhard polarization function in the presence of Bychkov-Rashba (BR) and Dresselhaus (D) spin-orbit interaction (SOI), we show that the interplay of different SOI mechanisms induces highly anisotropic modifications of the static dielectric function. We find that under certain circumstances the polarization function exhibits doubly singular behavior. It leads to an intriguing phenomenon, beating of Friedel oscillations, which can be controlled by external fields. This effect is a general feature of systems with BR+D SOI and should be observed in structures with a sufficiently strong SOI.

Electron polarization function and plasmons in metallic armchair graphene nanoribbons

We calculate the polarization function of Dirac fermions in metallic armchair graphene nanoribbons for an arbitrary temperature and doping. We find that at finite temperatures due to the phase space redistribution among inter-band and intra-band electronic transitions in the conduction and valence bands, the full polarization function becomes independent of the temperature and the position of the chemical potential. As a result, for a given width of nanoribbons there exists a single plasmon mode, with the energy dispersion determined by the graphenes fine structure constant. In Coulomb-coupled nanoribbons, this plasmon splits into the basic in-phase and out-of-phase plasmon modes, with the splitting energy determined additionally by the inter-ribbon spacing.

Spin Hall Drag in Electronic Bilayers

We predict a new effect in electronic bilayers: spin Hall drag. The effect consists of the generation of spin accumulation across one layer by an electric current along the other layer. It arises from the combined action of spin-orbit and Coulomb interactions. Our theoretical analysis, based on the Boltzmann equation formalism, identifies two main contributions to the spin Hall drag resistivity: the side-jump contribution, which dominates at low temperature, going as T2, and the skew-scattering contribution, which is proportional to T3. The induced spin accumulation, while generally quite small, should be observable in optical rotation experiments.

Frictional drag between spatially separated two-dimensional electron gases mediated by virtual-phonon exchange

We have calculated the frictional drag between spatially separated quantum wells with parallel two-dimensional electron gases due to interlayer electron-electron interaction mediated by virtual exchange of acoustic phonons due to piezoelectric and deformation potential interaction. It is shown that the frictional drag is dominated by the piezoelectric coupling. According to our calculations temperature dependence of the drag scattering rate divided by squared temperature T exhibits a pronounced peak which for the experimental situation and in agreement with the finding of T. J. Gramila, et al., Phys. Rev. B 47, (1993) 12957 is obtained at about T=2.1 K. We ascribe the appearance of this peak to a change from small to large angle scattering in the virtual phonon exchange.

Spin-current generation from Coulomb-Rashba interaction in semiconductor bilayers

Electrons in double-layer semiconductor heterostructures experience a special type of spin-orbit interaction that arises in each layer from the perpendicular component of the Coulomb electric field created by electron-density fluctuations in the other layer. We show that this interaction, acting in combination with the usual spin-orbit interaction, can generate a spin current in one layer when a charge current is driven in the other. This effect is distinct symmetrywise from the spin-Hall drag. The spin current is not, in general, perpendicular to the drive current.

Spin Edge Helices in a Perpendicular Magnetic Field

We present an exact solution to the problem of the spin edge states in the presence of equal Bychkov-Rashba and Dresselhaus spin-orbit fields in a two-dimensional electron system, restricted by a hard-wall confining potential and exposed to a perpendicular magnetic field. We find that the spectrum of the spin edge states depends critically on the orientation of the sample edges with respect to the crystallographic axes. Such a strikingly different spectral behavior generates new modes of the persistent spin helix-spin edge helices with novel properties, which can be tuned by the applied electric and magnetic fields.

Exchange and correlation effects on drag in low density electron bilayers: Coulomb and virtual-optical-phonon-mediated electron–electron interaction

Abstract We investigate the effect of exchange and correlation (xc) in low-density electron bilayers. Along with the direct Coulomb interaction, the effective electron–electron interaction mediated by the exchange of virtual polar optical (PO) phonons is considered. We find that the introduction of xc corrections results in a significant enhancement of the transresistivity and qualitative changes in its temperature dependence. The virtual PO-phonon contribution behaves similarly to the Coulomb drag and reduces noticeably the total drag thereby resulting in a better agreement with the recent experimental findings.

Spin-orbit-interaction induced singularity of the charge density relaxation propagator

The charge density relaxation propagator of a two dimensional electron system, which is the slope of the imaginary part of the polarization function, exhibits singularities for bosonic momenta having the order of the spin-orbit momentum and depending on the momentum orientation. We have provided an intuitive understanding for this non-analytic behavior in terms of the inter chirality subband electronic transitions, induced by the combined action of Bychkov-Rashba (BR) and Dresselhaus (D) spin-orbit coupling. It is shown that the regular behavior of the relaxation propagator is recovered in the presence of only one BR or D spin-orbit field or for spin-orbit interaction with equal BR and D coupling strengths. This creates a new possibility to influence carrier relaxation properties by means of an applied electric field.

Intra-Landau-level collective excitations in a bilayer disordered electronic system

We investigate intra-Landau-level collective excitations in a bilayer disordered two-dimensional electron system exposed to a perpendicular magnetic field. The energy spectrum is calculated within the random phase approximation by taking into account the electron-impurity scattering in the self-consistent Born approximation which includes consistent vertex corrections. It is shown that the whole spectrum lies in the finite-wave-vector domain and that there exist multiple wave-vector gaps in the weak Landau-level coupling regime. The signatures of the obtained bilayer excitations in drag and collective excitation measurements are identified.