Hedgehog spin texture and Berry’s phase tuning in a magnetic topological insulator

Abstract

Understanding and control of spin degrees of freedom on the surfaces of topological materials are the key to future applications as well as for realizing novel physics such as the axion electrodynamics associated with time-reversal 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 Bi 2 Se 3 thin films. The resulting electronic groundstate exhibits unique hedgehog-like spin textures at low energies which directly demonstrates the mechanics of timereversal symmetry breaking on the surface. We further show that an insulating gap induced by quantum tunneling between surfaces exhibits spin texture modulation at low energies but respects time-reversal invariance. These spin phenomena and the control of their Fermi surface geometrical phase first demonstrated in our experiments pave the way for future realization of many predicted exotic magnetic phenomena of topological origin.Since the discovery of three dimensional topological insulators [1][2][3][4][5], topological order proximity to ferromagnetism has been considered as one of the core interests of the field [6][7][8][9][10][11][12][13][14][15][16]. Such interest is strongly motivated by the proposed time-reversal (TR) breaking topological physics such as quantized anomalous chiral Hall current, spin current, axion electrodynamics, and inverse spin-galvanic effect [9][10][11][12], all of which critically rely on finding a way to break TR symmetry on the surface and utilize the unique TR broken spin texture for applications. Since quantum coherence is essential in many of these applications, devices need to be engineered into thin films in order to enhance or de-enhance surface-tosurface coupling or the quantum tunneling of the electrons. The experimental spin behavior of surface states under the two extreme limits, namely the doped magnetic groundstate and ultra-thin film quantum tunneling groundstate, is thus of central importance to the entire field. However, surprisingly, it is not known what happens to the spin configuration under these extreme conditions relevant for device fabrications. Fundamentally, TR symmetry is inherently connected to the Kramers' degeneracy theorem which states that when TR symmetry is preserved, the electronic states at the TR invariant momenta have to remain doubly spin degenerate. Therefore, the establishment of TR breaking effect fundamentally requires measurements of electronic groundstate with a spin-sensitive probe. Here we utilize spin-3 resolved angle-resolved photoemission spectroscopy to measure the momentum space spin configurations in systematically magnetically doped, non-magnetically doped, and ultra-thin quantum coherent topological insulator films [17], in order to understand the nature of electronic groundstates under two extreme limits vital for magnetic topological devices. These measurements allow us to make definitive conclusions regarding magnetism on to...

Breaking time-reversal symmetry at the topological insulator surface by metal-organic coordination networks

“…On the one hand, ab initio calculations performed for ordered arrays of Co [34] and Cr [35] adatoms at the Bi 2 Se 3 surface predict FM ordering, out-of-plane magnetization, and, therefore, the DP splitting. This is in agreement with angle-resolved photoemission spectroscopy (ARPES) measurements of magnetically doped bismuth chalcogenide surfaces exhibiting a feature near the DP which resembles a gap [36,37]. On the other hand, magnetic measurements found no evidence of the long-range FM order in the systems of Fe and Co adatoms at the Bi 2 Te 3 [38] and Bi 2 Se 3 [39][40][41] surfaces (the coverage in the latter case was reaching 0.9 monolayer).…”

Giant spin splitting of the two-dimensional electron gas at the surface of SrTiO3

“…1(c) and . Note also that our experiments were done in zero external magnetic field, to conserve the electron wave-vector, essential for momentum-resolved measurements.Thus, the combination of helical spin texture and magnetism with a sizeable component along the z-direction should lead to the occurrence of an out-of-plane spin-polarization P z of increasing amplitude close to the bottom of the band, as observed for topological insulators [22]. However, in the absence of an external magnetic field, both magnetic orientations along the z-axis are equally probable, and P z will average to zero in macroscopic measurements such as SARPES.…”

Momentum and photon energy dependence of the circular dichroic photoemission in the bulk Rashba semiconductorsBiTeX(X=I, Br, Cl)

“…Starting from the present knowledge on topological insulators (TIs) [1][2][3][4][5], the design of materials with spin-polarized bands requires the tailoring of the spin texture at the Fermi level (E F ), hence the synthesis of systems such as ternary TIs [6,7] or the bulk Rashba semiconductors BiTeX (X = I, Br, Cl) characterized by ambipolar surface states [8][9][10][11][12]. Nowadays one of the major challenges is to study the fully threedimensional spin properties of ternary TIs, and the bulk Rashba semiconductors, as is done for magnetic doped TIs [13].Spin-resolved angular resolved photoelectron spectroscopy (SR-ARPES) offers the unique possibility to directly address the spin polarization. Unfortunately, SR-ARPES, based on high-energy spin-dependent Mott scattering, is characterized by a low efficiency (1 × 10 −3 -1 × 10 −4 ) [14].…”