Coherent control over three-dimensional spin polarization for the spin-orbit coupled surface state of Bi2Se3

Abstract

Interference of spin-up and spin-down eigenstates depicts spin rotation of electrons, which is a fundamental concept of quantum mechanics and accepts technological challenges for the electrical spin manipulation. Here, we visualize this coherent spin physics through laser spin-and angle-resolved photoemission spectroscopy on a spin-orbital entangled surface-state of a topological insulator. It is unambiguously revealed that the linearly polarized laser can simultaneously excite spin-up and spin-down states and these quantum spin-basis are coherently superposed in photoelectron states. The superposition and the resulting spin rotation is arbitrary manipulated by the direction of the laser field. Moreover, the full observation of the spin rotation displays the phase of the quantum states. This presents a new facet of laser-photoemission technique for investigation of quantum spin physics opening new possibilities in the field of quantum spintronic applications.PACS numbers: 73.20. At, Coherent manipulation of electron spin offers many potential applications in spintronic devices [1] and spinbased quantum information science [2,3]. The key is to take control over the superposition of spin-up and spindown states resulting in interference. Spin-orbit coupling (SOC) mediates electric field and electron spin; thus electric/optical control of spins may become possible. Up to now, the coherent spin manipulation through electric fields is demonstrated in a few systems, such as quantum qubits [3,4] and semiconductor-heterostructure interfaces [5]. Optical control of spins utilizing the superimposed states in diamond is also demonstrated [6]. In this Letter we introduce a great methodology which is capable of directly accessing this scheme in photoelectron spin, based on a combination of polarization-variable laser with spin-and angle-resolved photoemission spectroscopy (laser-SARPES).We use the technique for a well-understood model system, a spin-polarized surface state of a topological insulator (TI), Bi 2 Se 3 . The TI has been discovered as a new class of matter, which is characterized by a metallic topological surface-state (TSS) intersecting the bulk bandgap [7,8]. The TSS forms spin-polarized Dirac-cone-like energy dispersion [9][10][11][12][13][14]. In particular, as a consequence of the strong SOC, different spins and orbitals are mixed in the TSS wavefunction, which generates a spin-orbital entangled texture [15][16][17]. Since the electric field of light couples to the orbitals, polarized light can, in princi- * These two authors contributed equally. † kuroken224@issp.u-tokyo.ac.jp ‡ komori@issp.u-tokyo.ac.jp ple, selectively excite the fully spin-polarized electrons with either spin-up or spin-down from the spin-orbital entanglement. The spin-selective excitation in the TSS by polarized light was theoretically studied [18,19] and demonstrated in previous SARPES experiments [20][21][22][23]. The entangled spin-orbital texture of the TSS is therefore suitable for the coherent spin control as a novel source of spin...

Direct mapping of spin and orbital entangled wave functions under interband spin-orbit coupling of giant Rashba spin-split surface states

“…This explains not only that the even-and odd-parity orbitals couple with opposite spins but also that the SO entanglement is a general consequence of the SO coupling. Indeed the similar SO-coupled states have been recently confirmed in surface states of topological insulators [34][35][36][37][38][39] and Rashba states in BiTeI [40,41].…”

Circular-polarized-light-induced spin polarization characterized for the Dirac-cone surface state at W(110) with C2v symmetry

“…Recently, by utilizing the above-mentioned effect, a fascinating idea was proposed: the manipulation and control of spin polarization of the photoelectron signal by a proper selection of the light polarization, experimental geometry and photon energy 5 , 6 , 12 – 19 . These phenomena are promising in view of the material-light-spin relationship for potential applications in optospintronics devices with multiple functionalities.…”

Determination of the time scale of photoemission from the measurement of spin polarization

“…It is however very important to keep in mind that the measured spin polarization is not necessarily the one of the initial state, but modification of it can occur during the photoemission process. For example, matrix element effects can change or even reverse the direction of P as a function of photon energy or light polarization [63][64][65]; the diffraction through the surface can be spin-dependent, thus modifying P [66]; the coherent excitation of different spin states can result in spin interference effects [67]. All these possibilities make SARPES results difficult to interpret not only on a quantitative level, because of the requirement of sequential measurements with faster detectors or because of the required sample stability with slower ones, but also on a qualitative level.…”