Tuning of the Rashba effect in Pb quantum well states via a variable Schottky barrier

Spin-orbit interaction (SOI) in low-dimensional systems results in the fascinating property of spin-momentum locking. In a Rashba system the inversion symmetry normal to the plane of a two-dimensional (2D) electron gas is broken, generating a Fermi surface spin texture reminiscent of spin vortices of different radii. This can be exploited in a spin-based field-effect transistor (spin- FET), where the Rashba system forms a 2D channel between ferromagnetic (FM) source and drain electrodes. The electron spin precesses when propagating through the Rashba channel and spin orientations (anti)parallel to the drain give (low) high conductivity. Crucial is the possibility to tune the momentum splitting, and consequently the precession angle, through an external parameter. Here we show that this can be achieved in Pb quantum well states through the doping dependence of the Schottky barrier, opening up the possibility of a terahertz spin-FET.Comment: 8 pages, 7 figure

Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering

The in-plane effective mass of quantum well states in thin Pb films on a Bi reconstructed Si(111) surface is studied by angle-resolved photoemission spectroscopy. It is found that this effective mass is a factor of three lower than the unusually high values reported for Pb films grown on a Pb reconstructed Si(111) surface. Through a quantitative low-energy electron diffraction analysis the change in effective mass as a function of coverage and for the different interfaces is linked to a change of around 2% in the in-plane lattice constant. To corroborate this correlation, density functional theory calculations were performed on freestanding Pb slabs with different in-plane lattice constants. These calculations show an anomalous dependence of the effective mass on the lattice constant including a change of sign for values close to the lattice constant of Si(111). This unexpected relation is due to a combination of reduced orbital overlap of the 6p_z states and altered hybridization between the 6p_z and 6p_xy derived quantum well states. Furthermore it is shown by core level spectroscopy that the Pb films are structurally and temporally stable at temperatures below 100 K.Comment: 7 pages, 6 figure

Response of the topological surface state to surface disorder in TlBiSe2_2

Through a combination of experimental techniques we show that the topmost layer of the topo- logical insulator TlBiSe$_2$ as prepared by cleavage is formed by irregularly shaped Tl islands at cryogenic temperatures and by mobile Tl atoms at room temperature. No trivial surface states are observed in photoemission at low temperatures, which suggests that these islands can not be re- garded as a clear surface termination. The topological surface state is, however, clearly resolved in photoemission experiments. This is interpreted as a direct evidence of its topological self-protection and shows the robust nature of the Dirac cone like surface state. Our results can also help explain the apparent mass acquisition in S-doped TlBiSe$_2$.Comment: 16 pages, 5 figure

Unconventional transformation of spin Dirac phase across a topological quantum phase transition

The topology of a topological material can be encoded in its surface states. These surface states can only be removed by a bulk topological quantum phase transition into a trivial phase. Here we use photoemission spectroscopy to image the formation of protected surface states in a topological insulator as we chemically tune the system through a topological transition. Surprisingly, we discover an exotic spin-momentum locked, gapped surface state in the trivial phase that shares many important properties with the actual topological surface state in anticipation of the change of topology. Using a spin-resolved measurement, we show that apart from a surface band-gap these states develop spin textures similar to the topological surface states well-before the transition. Our results offer a general paradigm for understanding how surface states in topological phases arise and are suggestive for future realizing Weyl arcs, condensed matter supersymmetry and other fascinating phenomena in the vicinity of topological quantum criticality.Comment: 20 pages, 5 Figures, Related papers at http://physics.princeton.edu/zahidhasangroup/index.html, Accepted for publication in Nature Commun.(2015

Sputtering-induced reemergence of the topological surface state in Bi2Se3

We study the fate of the surface states of Bi2Se3 under disorder with strength larger than the bulk gap, caused by neon sputtering and nonmagnetic adsorbates. We find that neon sputtering introduces strong but dilute defects, which can be modeled by a unitary impurity distribution, whereas adsorbates, such as water vapor or carbon monoxide, are best described by Gaussian disorder. Remarkably, these two disorder types have a dramatically different effect on the surface states. Our soft x-ray angle-resolved photoemission spectroscopy (ARPES) measurements combined with numerical simulations show that unitary surface disorder pushes the Dirac state to inward quintuplet layers, burying it below an insulating surface layer. As a consequence, the surface spectral function becomes weaker but retains its quasiparticle peak. This is in contrast to Gaussian disorder, which smears out the quasiparticle peak completely. At the surface of Bi2Se3, neon sputtering adds additional unitary scatterers to the Gaussian disorder of the adsorbates. Since the introduced unitary disorder pushes the surface state to inward layers, the effects of Gaussian disorder are reduced. As a result the ARPES signal becomes sharper upon sputtering

Hedgehog Spin-texture and Berry's Phase tuning in a Magnetic Topological Insulator

Understanding and control of spin degrees of freedom on the surfaces of topological materials are key to future applications as well as for realizing novel physics such as the axion electrodynamics associated with time-reversal (TR) symmetry breaking on the surface. We experimentally demonstrate magnetically induced spin reorientation phenomena simultaneous with a Dirac-metal to gapped-insulator transition on the surfaces of manganese-doped Bi2Se3 thin films. The resulting electronic groundstate exhibits unique hedgehog-like spin textures at low energies, which directly demonstrate the mechanics of TR symmetry breaking on the surface. We further show that an insulating gap induced by quantum tunnelling between surfaces exhibits spin texture modulation at low energies but respects TR invariance. These spin phenomena and the control of their Fermi surface geometrical phase first demonstrated in our experiments pave the way for the future realization of many predicted exotic magnetic phenomena of topological origin.Comment: 38 pages, 18 Figures, Includes new text, additional datasets and interpretation beyond arXiv:1206.2090, for the final published version see Nature Physics (2012

A spin- and angle-resolved photoemission study of the Rashba-Bychkov effect in lead quantum well states

In the rapidly developing field of spintronics whose central issue is the utilization of the spin instead of the charge of the electron, the active control of spin-polarized carriers (electrons or holes) utilizing electric rather than magnetic fields has highest priority. Researchers world wide are searching for a realization of a spin-based field-effect transistor (spin-FET) that has the potential to revolutionize electronic devices and carries new prospects of data manipulation. The present thesis deals with the investigation of the Rashba-Bychkov effect as a possible can- didate for a spin-FET formed by an ultra-thin Pb film on a silicon substrate, using spin- and angle-resolved photoemission spectroscopy. The reduced dimensionality of such an epitaxially grown metallic film results in a two-dimensional (2D) electron gas associated with quantum well states (QWS). These states are free to move within the sample plane, while they are confined perpendicularly by the energy gap of the Si substrate on one side and by the repulsive image potential toward the vacuum on the other side, reminiscent of the particle-in-the-box model. Due to the broken inversion symmetry along the confinement direction and the high nuclear charge of Pb the QWS show a Rashba-type spin splitting which builds up throughout the whole metal layer along the growth direction as a result of competing effects between the metal-substrate and metal-vacuum interfaces. In this thesis the focus lies on altering the interface region between the film and the substrate to provide more insight into the origin of the Rashba effect and other phenomena such as the effective mass of the bands and Schottky barrier formation. In the first approach we have varied the chemistry of the interface by studying various interfactants √ √ such as Pb, Bi and Ag, which form highly regular structures of ( 3 × 3)R30◦ symmetry on Si. It is found that the size of the Rashba effect changes dramatically among these interfaces: replacing the Pb interface by a Bi layer reduced the Rashba parameter by 60%, whereas QWS in Pb films grown on a Ag reconstructed Si substrate showed no measurable spin splitting. Interestingly, the Schottky barrier of these systems, and the effective mass of the states varies in the same manner. For future device application it is necessary to control the Rashba parameter by external means. The results of our second approach, the study of the influence of the substrate doping concentration on the Rashba effect in Pb QWS, revealed a very promising pathway: by increasing the donor concentration by a factor of 20, we could tune the Rashba parameter by a factor of two. A simulation of the relevant parameter indeed showed that a gate voltage of only 12 V is enough to switch a 1 nm lateral spin-FET from the insulating to the conducting state. These findings are discussed in the framework of the interface dipole model and a doping dependent Schottky barrier. In order to gain more insight into the interplay of band dispersion, orbital character, and spin, we have focused on a wave vector region in Pb QWS where avoided crossing hybridization induced by spin-orbit coupling alters the band structure significantly