J. H. Dil

Observation of a topological crystalline insulator phase and topological phase transition in Pb 1− x Sn x Te

A topological insulator protected by time-reversal symmetry is realized via spinorbit interaction driven band inversion. The topological phase in the Bi1−xSbx system is due to an odd number of band inversions. A related spin-orbit system, the Pb1−xSnxTe, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry protected topological crystalline insulator phase in the Pb1−xSnxTe class of materials which has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite-Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi1−xSbx. Our observation of the spin-polarized Dirac surface states in the inverted Pb1−xSnxTe and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.A topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction-driven band inversion. The topological phase in the Bi(1-x)Sb(x) system is due to an odd number of band inversions. A related spin-orbit system, the Pb(1-x)Sn(x)Te, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb(1-x)Sn(x)Te class of materials that has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi(1-x)Sb(x). Our observation of the spin-polarized Dirac surface states in the inverted Pb(1-x)Sn(x)Te and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.

Direct observation of the spin texture in SmB6 as evidence of the topological Kondo insulator

Topological Kondo insulators have been proposed as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to strong spin-orbit coupling. In contrast to other three-dimensional topological insulators, a topological Kondo insulator is truly bulk insulating. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. By applying spin- and angle-resolved photoemission spectroscopy, here we show that the surface states of SmB6 are spin polarized. The spin is locked to the crystal momentum, fulfilling time reversal and crystal symmetries. Our results provide strong evidence that SmB6 can host topological surface states in a bulk insulating gap stemming from the Kondo effect, which can serve as an ideal platform for investigating of the interplay between novel topological quantum states with emergent effects and competing orders induced by strongly correlated electrons.

Observation of Fermi-Arc Spin Texture in TaAs

We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin polarized. The measured spin texture fulfills the requirement of mirror and time-reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first-principles calculations.

Measuring spin polarization vectors in angle-resolved photoemission spectroscopy

The quantitative analysis of spin-polarized photoemission data is discussed. An angle-resolving photoelectron spectrometer equipped with a three- dimensional (3D) spin polarimeter produces complete data sets consisting of photoemission intensities as well as spin asymmetry curves for three orthogonal vector components. In a two-step fitting routine, the photoemission spectrum is first dissected into individual peaks and background. Assigning trial spin polarization vectors to each of them, the asymmetry curves can be modeled until the best fit is reached. This procedure is crucial when analyzing strongly overlapping peaks or weak signals sitting on a large unpolarized background, especially in the presence of non-collinear spins. It is robust against strong intensity variations due to matrix element effects because it references the spin polarization contribution of each band to the measured peak intensity. The method is applied to 2D systems where spin-orbit effects lead to spin splittings and complex momentum-dependent spin structures. Presented case studies include surface alloys of Bi and Pb on Ag(111) that show a giant Rashba effect.

Exotic Kondo crossover in a wide temperature region in the topological Kondo insulator SmB6 revealed by high-resolution ARPES

Temperature dependence of the electronic structure of SmB6 is studied by high-resolution angle-resolved photoemission spectroscopy (ARPES) down to 1 K. We demonstrate that there is no essential difference for the dispersions of the surface states below and above the resistivity saturating anomaly (similar to 3.5 K). Quantitative analyses of the surface states indicate that the quasiparticle scattering rate increases linearly as a function of temperature and binding energy, which differs from Fermi-liquid behavior. Most intriguingly, we observe that the hybridization between the d and f states builds gradually over a wide temperature region (30 K < T < 110 K). The surface states appear when the hybridization starts to develop. Our detailed temperature-dependence results give a complete interpretation of the exotic resistivity result of SmB6, as well as the discrepancies among experimental results concerning the temperature regions in which the topological surface states emerge and the Kondo gap opens, and give insights into the exotic Kondo crossover and its relationship with the topological surface states in the topological Kondo insulator SmB6.

Disentangling bulk and surface Rashba effects in ferroelectric α -GeTe

1Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 2New Technologies-Research Center University of West Bohemia, Plzeň, Czech Republic 3Department of Chemistry, Ludwig Maximillian University, 81377 Munich, Germany 4Department of Condensed Matter Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Praha 2, Czech Republic 5Physik-Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland 6Institute of condensed matter physics, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland 7Institut für Halbleiter-und Festkörperphysik, Johannes Kepler Universität, A-4040 Linz, Austria 8SwissFEL, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Entanglement and manipulation of the magnetic and spin–orbit order in multiferroic Rashba semiconductors

Entanglement of the spin–orbit and magnetic order in multiferroic materials bears a strong potential for engineering novel electronic and spintronic devices. Here, we explore the electron and spin structure of ferroelectric α-GeTe thin films doped with ferromagnetic Mn impurities to achieve its multiferroic functionality. We use bulk-sensitive soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to follow hybridization of the GeTe valence band with the Mn dopants. We observe a gradual opening of the Zeeman gap in the bulk Rashba bands around the Dirac point with increase of the Mn concentration, indicative of the ferromagnetic order, at persistent Rashba splitting. Furthermore, subtle details regarding the spin–orbit and magnetic order entanglement are deduced from spin-resolved ARPES measurements. We identify antiparallel orientation of the ferroelectric and ferromagnetic polarization, and altering of the Rashba-type spin helicity by magnetic switching. Our experimental results are supported by first-principles calculations of the electron and spin structure.

Excitation of Coherent Phonons in the One-Dimensional Bi(114) Surface

We present time-resolved photoemission experiments from a peculiar bismuth surface, Bi(114). The strong one-dimensional character of this surface is reflected in the Fermi surface, which consists of spin-polarized straight lines. Our results show that the depletion of the surface state and the population of the bulk conduction band after the initial optical excitation persist for very long times. The disequilibrium within the hot electron gas along with strong electron-phonon coupling cause a displacive excitation of coherent phonons, which in turn are reflected in coherent modulations of the electronic states. Beside the well-known A(1g) bulk phonon mode at 2.76 THz, the time-resolved photoelectron spectra reveal a second mode at 0.72 THz which can be attributed to an optical surface phonon mode along the atomic rows of the Bi(114) surface.

Observation of correlated spin-orbit order in a strongly anisotropic quantum wire system.

Quantum wires with spin-orbit coupling provide a unique opportunity to simultaneously control the coupling strength and the screened Coulomb interactions where new exotic phases of matter can be explored. Here we report on the observation of an exotic spin-orbit density wave in Pb-atomic wires on Si(557) surfaces by mapping out the evolution of the modulated spin-texture at various conditions with spin- and angle-resolved photoelectron spectroscopy. The results are independently quantified by surface transport measurements. The spin polarization, coherence length, spin dephasing rate and the associated quasiparticle gap decrease simultaneously as the screened Coulomb interaction decreases with increasing excess coverage, providing a new mechanism for generating and manipulating a spin-orbit entanglement effect via electronic interaction. Despite clear evidence of spontaneous spin-rotation symmetry breaking and modulation of spin-momentum structure as a function of excess coverage, the average spin polarization over the Brillouin zone vanishes, indicating that time-reversal symmetry is intact as theoretically predicted.

Operando Imaging of All-Electric Spin Texture Manipulation in Ferroelectric and Multiferroic Rashba Semiconductors

The control of the electron spin by external means is a key issue for spintronic devices. Using spin- and angle-resolved photoemission spectroscopy (SARPES) with three-dimensional spin detection, we demonstrate operando electrostatic spin manipulation in ferroelectric GeTe and multiferroic Ge1-xMnxTe. We not only demonstrate for the first time electrostatic spin manipulation in Rashba semiconductors due to ferroelectric polarization reversal, but are also able to follow the switching pathway in detail, and show a gain of the Rashba-splitting strength under external fields. In multiferroic Ge1-xMnxTe operando SARPES reveals switching of the perpendicular spin component due to electric field induced magnetization reversal. This provides firm evidence of effective multiferroic coupling which opens up magnetoelectric functionality with a multitude of spin-switching paths in which the magnetic and electric order parameters are coupled through ferroelastic relaxation paths. This work thus provides a new type of magnetoelectric switching entangled with Rashba-Zeeman splitting in a multiferroic system.