Direct Optical Coupling to an Unoccupied Dirac Surface State in the Topological InsulatorBi2Se3

Abstract

We characterize the occupied and unoccupied electronic structure of the topological insulator Bi2Se3 by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:Sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state.PACS numbers: 79.60.Bm, 78.47.JThree dimensional topological insulators (TIs) are materials characterized by an insulating bulk and a conductive surface electronic structure. The hallmark of a TI is its linearly dispersing surface state (SS) which is guaranteed to cross the band gap separating the bulk valence band (VB) and conduction band (CB) [1][2][3][4]. In addition to this so-called topological protection, the SS has a chiral spin texture [5,6] which offers the electrons protection against backscattering and has great appeal for spintronics applications [7,8]. A number of interesting phenomena are associated with optical coupling to TIs, such as colossal Kerr rotation [9], divergent photon absorption [10], spin transport [11,12], and a long-lived SS population [13]. To exploit these phenomena, a detailed understanding of the response to optical excitation is required. A number of studies have investigated electron and phonon dynamics initiated by ultrafast optical excitation either by optical reflectivity [14,15], second harmonic generation [16], or time-and angle-resolved photoemission spectroscopy (trARPES) [13,[17][18][19]. However, these studies have not explicitly addressed the electronic transition driven by the excitation. A detailed understanding of the unoccupied electronic structure is required to understand precisely how photons couple to electrons in these materials. Inverse photoemission has previously been applied to Bi 2 Se 3 , but the energy resolution of 0.56 eV and lack of momentum information precluded the ability to get a detailed picture of the unoccupied states [20]. More recently, significant progress was made by Niesner et. al., who used two-photon photoemission spectroscopy to investigate the material family Bi 2 Te x Se 3−x [21]. Remarkably, they identified signatures of a second SS in the unoccupied states which is expected to have the same topological protection and chiral spin texture [21,22].In this work, we use a combination of one-photon photoemission (1PPE) and two-photon photoemission (2PPE) to resolve the occupied and unoccupied electronic structure of p− and n−type Bi 2 Se 3 , which is substantiated with theoretical band structure calculations. The energy and momentum resolution is drastically improved over previous work [21], allowing an unambiguous identification of the second SS. Moreover, we show that in n-type samples, 1.5 eV photo...

Bulk and surface electron dynamics in ap-type topological insulatorSnSb2Te4

“…The calculations reveal the existence of another spin-polarized surface state at 0.96 eV. Such states have recently been found in other topological insulators [24,26,27]. The second spin-polarized surface state could not be confirmed by the present experiments on SnSb 2 Te 4 due to overlap with the IPS emission.…”

Acoustic phonon dynamics in thin-films of the topological insulator Bi2Se3

“…1,2 However, because surface effects can be easily masked by three-dimensional (3D) free carriers of the insulating medium (bandgap of bulk Bi 2 Se 3 ~0.3 eV), [3][4][5][6][7][8][9][10] the control of purely Dirac-SS-governed properties of TIs remains a topic of interest. The problem can be partially solved by using the compensation doping method or a backgate voltage technique, 11,12 which efficiently deplete 3D carriers and hence switch electronic properties of the material to those related to 2D Dirac SS.…”

Optical Response ofSr2RuO4Reveals Universal Fermi-Liquid Scaling and Quasiparticles Beyond Landau Theory

“…Recently, two-photon ARPES has been shown to provide energy and momentumresolved information on unoccupied states [37], and we propose that our theoretical results ( [26], Fig. SM14) could be put to the test in the future by using this technique for Sr 2 RuO 4 .…”