Topological Surface States with Persistent High Spin Polarization across the Dirac Point in Bi2Te2Se and Bi2Se2Te (original) (raw)
Related papers
2010
We report high-resolution spin-resolved photoemission spectroscopy (Spin-ARPES) measurements on the parent compound Sb of the first discovered 3D topological insulator Bi1−xSbx [D. Hsieh et al., Nature 452, 970 (2008) Submitted 2007. By modulating the incident photon energy, we are able to map both the bulk and (111) surface band structure, from which we directly demonstrate that the surface bands are spin polarized by the spin-orbit interaction and connect the bulk valence and conduction bands in a topologically non-trivial way. A unique asymmetric Dirac surface state gives rise to a k-splitting of its spin polarized electronic channels. These results complement our previously published works on this materials class and re-confirm our discovery of first bulk (3D) topological insulator -topological order in bulk solids.
New Journal of Physics, 2010
The spin-polarized surface band structure of the three-dimensional (3D) quantum spin Hall phase of Bi 1−x Sb x (x = 0.12-0.13) was studied by spinand angle-resolved photoemission spectroscopy (SARPES) using a high-yield spin polarimeter equipped with a high-resolution electron spectrometer. The spin-integrated spectra were also measured and compared to those of Bi 1−x Sb x with x = 0.04. Band dispersions of the edge states were fully elucidated between the two time-reversal-invariant points,¯ andM, of the (111) surface Brillouin zone. The observed spin-polarized band dispersions at x = 0.12-0.13 indicate an odd number of the band crossing at the Fermi energy, giving unambiguous evidence that this system is a 3D strong topological insulator, and determine the 'mirror chirality' to be −1, which excludes the existence of a Dirac point in the middle of the¯-M line. The present research demonstrates that the SARPES
Widespread spin polarization effects in photoemission from topological insulators
Physical Review B, 2011
High resolution spin-and angle-resolved photoemission spectroscopy (spin-ARPES) was performed on the three-dimensional topological insulator Bi2Se3 using a recently developed highefficiency spectrometer. The topological surface state's helical spin structure is observed, in agreement with theoretical prediction. Spin textures of both chiralities, at energies above and below the Dirac point, are observed, and the spin structure is found to persist at room temperature. The measurements reveal additional unexpected spin polarization effects, which also originate from the spin-orbit interaction, but are well differentiated from topological physics by contrasting momentum and photon energy and polarization dependencies. These observations demonstrate significant deviations of photoelectron and quasiparticle spin polarizations. Our findings illustrate the inherent complexity of spin-resolved ARPES and demonstrate key considerations for interpreting experimental results.
The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi 2 Se 3. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic ab initio spin-resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin-polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.
Photoemission Spectroscopy Studies of New Topological Insulator Materials
As the size of a solid shrinks, the ratio of surface area to bulk volume grows and surface effects become more important. In a world where technologies advance with the shrinking size of electronic devices, one phase of matter has emerged which is fit for the near future of surface-dominated performance. Moreover, it has brought a new set of ideas to solid-state physics and chemistry, especially the understanding that the discipline of topology can be applied to classify the electron band structures. The topological insulator phase yields an exotic metal surface state in which the orientation of the electron’s spin is locked perpendicular to its momentum. This property suppresses backscattering (making it possible to pass spin-polarized currents through the material without loss), offers a crucial ingredient for innovative approaches to quantum computation, and provides the basis for observing unique magnetoelectric effects. However, the surface states of materials in the topological insulator phase can wildly differ, so it is of interest to systematically characterize new materials to understand how the structure in position-space is related to the spin-resolved structure of electrons in energy- and momentum-space. We will discuss this relationship as it is probed through spin- and angle-resolved photoemission spectroscopy experiments on three topological (Bi2)m(Bi2Se3)n superlattices: (a) Bi2Se3 (m = 0, n = 1), (b) Bi4Se3 (m = 1, n = 1), and (c) BiSe (m = 1, n = 2). Our studies have not only proven the topological nature of these materials, but also demonstrate how bulk band structure and polar chemical bonding control the surface metal’s concentration, dispersion, and spin-orbital character. Case (a) is considered to provide an ideal model of the topological surface metal. Case (b) provides the three important findings: (1) the chemical identity of the surface-termination controls the orbital composition and energy distribution of the surface states, (2) there are two topological states in sequential bulk band gaps, (3) of these, one of topological state undergoes a hybridization effect that yields a momentum-dependent gap in the band structure as large as 85 meV. Case (c) has a practical significance in that the surface metal has a potentially record-breaking carrier density of ~10^13 cm^−2 (estimated from the Fermi surface area), more than an order of magnitude higher than in Bi2Se3. This occurs as a result of charge transfer from the Bi2 layers to the Bi2Se3 layers.
Photoelectron Spin-Polarization Control in the Topological Insulator Bi2Se3
Physical Review Letters, 2014
We study the manipulation of the spin polarization of photoemitted electrons in Bi 2 Se 3 by spin-and angleresolved photoemission spectroscopy. General rules are established that enable controlling the photoelectron spin-polarization. We demonstrate the AE 100% reversal of a single component of the measured spinpolarization vector upon the rotation of light polarization, as well as full three-dimensional manipulation by varying experimental configuration and photon energy. While a material-specific density-functional theory analysis is needed for the quantitative description, a minimal yet fully generalized two-atomic-layer model qualitatively accounts for the spin response based on the interplay of optical selection rules, photoelectron interference, and topological surface-state complex structure. It follows that photoelectron spin-polarization control is generically achievable in systems with a layer-dependent, entangled spin-orbital texture.
Complex Spin Texture in the Pure and Mn-Doped Topological Insulator Bi_{2}Te_{3}
Physical Review Letters, 2012
Topological insulators are characterized by the presence of spin-momentum-locked surface states with Dirac points that span the fundamental bulk band gap. We show by first-principles calculations that the surface state of the insulator Bi 2 Te 3 survives upon moderate Mn doping of the surface layers. The spin texture of both undoped and Mn-doped Bi 2 Te 3 is much more complicated than commonly believed, showing layer-dependent spin reversal and spin vortices.
Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface
Nature Physics, 2009
Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap of such systems are protected by time-reversal symmetry. The study of such states was originally inspired by the robustness to scattering of conducting edge states in quantum Hall systems. Recently, such analogies have resulted in the discovery of topologically protected states in two-dimensional and three-dimensional band insulators with large spin-orbit coupling. So far, the only known three-dimensional topological insulator is Bi x Sb 1−x , which is an alloy with complex surface states. Here, we present the results of first-principles electronic structure calculations of the layered, stoichiometric crystals Sb 2 Te 3 , Sb 2 Se 3 , Bi 2 Te 3 and Bi 2 Se 3 . Our calculations predict that Sb 2 Te 3 , Bi 2 Te 3 and Bi 2 Se 3 are topological insulators, whereas Sb 2 Se 3 is not. These topological insulators have robust and simple surface states consisting of a single Dirac cone at the point. In addition, we predict that Bi 2 Se 3 has a topologically non-trivial energy gap of 0.3 eV, which is larger than the energy scale of room temperature. We further present a simple and unified continuum model that captures the salient topological features of this class of materials.
Nature Communications, 2016
Three-dimensional topological insulators (TIs) exhibit time-reversal symmetry protected, linearly dispersing Dirac surface states with spin–momentum locking. Band bending at the TI surface may also lead to coexisting trivial two-dimensional electron gas (2DEG) states with parabolic energy dispersion. A bias current is expected to generate spin polarization in both systems, although with different magnitude and sign. Here we compare spin potentiometric measurements of bias current-generated spin polarization in Bi2Se3(111) where Dirac surface states coexist with trivial 2DEG states, and in InAs(001) where only trivial 2DEG states are present. We observe spin polarization arising from spin–momentum locking in both cases, with opposite signs of the measured spin voltage. We present a model based on spin dependent electrochemical potentials to directly derive the sign expected for the Dirac surface states, and show that the dominant contribution to the current-generated spin polarizatio...