Physical Review B

A very large Rashba-type spin splitting, which is a consequence of spin–orbit interaction, has been observed in the heavy-element semiconductor BiTeI. The results show the possibility, in principle, of using the material in spintronics devices in which the electron spin is controlled by electric currents.

Giant Rashba-type spin splitting in bulk BiTeI

The Rashba effect is spin degeneracy lift originated from spin-orbit coupling under inversion symmetry breaking and has been intensively studied for spintronics applications. However, easily implementable methods and corresponding materials for directional controls of Rashba splitting are still lacking. Here, we propose organic-inorganic hybrid metal halide perovskites as 3D Rashba systems driven by bulk ferroelectricity. In these materials, it is shown that the helical direction of the angular momentum texture in the Rashba band can be controlled by external electric fields via ferroelectric switching. Our tight-binding analysis and first-principles calculations indicate that S = 1/2 and J = 1/2 Rashba bands directly coupled to ferroelectric polarization emerge at the valence and conduction band edges, respectively. The coexistence of two contrasting Rashba bands having different compositions of the spin and orbital angular momentum is a distinctive feature of these materials. With recent experimental evidence for the ferroelectric response, the halide perovskites will be, to our knowledge, the first practical realization of the ferroelectric-coupled Rashba effect, suggesting novel applications to spintronic devices.

https://www.pnas.org/content/pnas/111/19/6900.full.pdf

Switchable S = 1/2 and J = 1/2 Rashba bands in ferroelectric halide perovskites

Hot electron lifetimes in metals probed by time-resolved two-photon photoemission

Topological insulators are a unique class of materials characterized by a Dirac cone state of helical Dirac fermions in the middle of a bulk gap. When the thickness of a three-dimensional topological insulator is reduced, however, the interaction between opposing surface states opens a gap that removes the helical Dirac cone, converting the material back to a normal system of ordinary fermions. Here we demonstrate, using density function theory calculations and experiments, that it is possible to create helical Dirac fermion state by interfacing two gapped films-a single bilayer Bi grown on a single quintuple layer Bi(2)Se(3) or Bi(2)Te(3). These extrinsic helical Dirac fermions emerge in predominantly Bi bilayer states, which are created by a giant Rashba effect with a coupling constant of ~4 eV·A due to interfacial charge transfer. Our results suggest that this approach is a promising means to engineer topological insulator states on non-metallic surfaces.

/pdf/creation-of-helical-dirac-fermions-by-interfacing-two-gapped-4swidgqwqt.pdf

Creation of helical Dirac fermions by interfacing two gapped systems of ordinary fermions

We report on the observation of a giant spin-orbit splitting of quantum-well states in the unoccupied electronic structure of a Bi monolayer on Cu(111). Up to now, Rashba-type splittings of this size have been reported exclusively for surface states in a partial band gap. With these quantum-well states we have experimentally identified a second class of states that show a huge spin-orbit splitting. First-principles electronic structure calculations show that the origin of the spin-orbit splitting is due to the perpendicular potential at the surface and interface of the ultrathin Bi film. This finding allows for the direct possibility to tailor spin-orbit splitting by means of thin-film nanofabrication.

/pdf/quantum-well-induced-giant-spin-orbit-splitting-2s6jn2jpd4.pdf

Quantum-well-induced giant spin-orbit splitting.

The band structure dependence of the inelastic lifetime of electrons is investigated in a Pb quantum-well system on a Cu(111) substrate with femtosecond time- and angle-resolved two-photon photoemission. For a single monolayer of Pb on Cu(111), we find an unoccupied quantum-well state with a free-electron-like parabolic dispersion around the $\overline{\ensuremath{\Gamma}}$ point and with negative dispersion for finite momentum. Our investigation of this state is a momentum-dependent study of hot-electron lifetimes in a quantum-well system. We demonstrate the importance of intrasubband-scattering processes for the decay of hot electrons at finite momentum. Furthermore, we find that the competition between intersubband- and intrasubband-scattering processes directly induces a momentum anisotropy in the hot-electron lifetimes. This momentum anisotropy is strongly dependent on the specific electronic band dispersion. We compare our findings with previous investigations of ultrafast electron dynamics in model-like surface-state systems and with ultrafast electron dynamics in Pb full-bulk material. Our findings suggest that the peculiar electronic structure of quantum-well systems can be used to tune ultrafast dynamical properties in metals.

Band structure dependence of hot-electron lifetimes in a Pb/Cu(111) quantum-well system

The angle resolved photoelectron spectroscopy (ARPES) has emerged as a leading technique in identifying static key properties of complex systems such as the electronic band structure of adsorbed molecules, ultrathin quantum-well films or high temperature superconductors. We efficiently combined ARPES by using a two-dimensional analyzer for parallel energy (E) and momentum (k||) detection with femtosecond time-resolved spectroscopies. Using time and angle resolved two photon photoemission (2PPE) with visible light pulses, the hot electron dynamics in complex electronic structures are directly accessible by means of angle resolved hot electron lifetime mapping. Furthermore, femtosecond ARPES spectra recorded with high harmonic generation (HHG) light pulses are presented, showing the potential of this technique for future investigations of surface dynamics and photo-induced phase transition processes.

http://iopscience.iop.org/article/10.1088/1742-6596/148/1/012042/pdf

F. Deicke

Papers

Quantum-well-induced giant spin-orbit splitting.

Band structure dependence of hot-electron lifetimes in a Pb/Cu(111) quantum-well system

Time and angle resolved photoemission spectroscopy using femtosecond visible and high-harmonic light