Z.-H. Pan

Electronic Structure of the Topological Insulator Bi2Se3 Using Angle-Resolved Photoemission Spectroscopy: Evidence for a Nearly Full Surface Spin Polarization

We performed high-resolution spin- and angle-resolved photoemission spectroscopy studies of the electronic structure and the spin texture on the surface of Bi2Se3, a model TI. By tuning the photon energy, we found that the topological surface state is well separated from the bulk states in the vicinity of kz = Z plane of the bulk Brillouin zone. The spin-resolved measurements in that region indicate a very high degree of spin polarization of the surface state, ~0.75, much higher than previously reported. Our results demonstrate that the topological surface state on Bi2Se3 is highly spin polarized and that the dominant factors limiting the polarization are mainly extrinsic.

Photoemission spectroscopy of magnetic and nonmagnetic impurities on the surface of the Bi2Se3 topological insulator.

Dirac-like surface states on surfaces of topological insulators have a chiral spin structure that suppresses backscattering and protects the coherence of these states in the presence of nonmagnetic scatterers. In contrast, magnetic scatterers should open the backscattering channel via the spin-flip processes and degrade the states coherence. We present angle-resolved photoemission spectroscopy studies of the electronic structure and the scattering rates upon the adsorption of various magnetic and nonmagnetic impurities on the surface of Bi2Se3, a model topological insulator. We reveal a remarkable insensitivity of the topological surface state to both nonmagnetic and magnetic impurities in the low impurity concentration regime. Scattering channels open up with the emergence of hexagonal warping in the high-doping regime, irrespective of the impuritys magnetic moment.

Measurement of an exceptionally weak electron-phonon coupling on the surface of the topological insulator Bi2Se3 using angle-resolved photoemission spectroscopy.

Gapless surface states on topological insulators are protected from elastic scattering on nonmagnetic impurities which makes them promising candidates for low-power electronic applications. However, for widespread applications, these states should have to remain coherent at ambient temperatures. Here, we studied temperature dependence of the electronic structure and the scattering rates on the surface of a model topological insulator, Bi2Se3, by high-resolution angle-resolved photoemission spectroscopy. We found an extremely weak broadening of the topological surface state with temperature and no anomalies in the states dispersion, indicating exceptionally weak electron-phonon coupling. Our results demonstrate that the topological surface state is protected not only from elastic scattering on impurities, but also from scattering on low-energy phonons, suggesting that topological insulators could serve as a basis for room-temperature electronic devices.

Coupling of spin and orbital excitations in the iron-based superconductor FeSe0.5Te0.5

We present a combined analysis of neutron scattering and photoemission measurements on superconducting FeSe{sub 0.5}Te{sub 0.5}. The low-energy magnetic excitations disperse only in the direction transverse to the characteristic wave vector (1/2,0,0) whereas the electronic Fermi surface near (1/2,0,0) appears to consist of four incommensurate pockets. While the spin resonance occurs at an incommensurate wave vector compatible with nesting, neither spin-wave nor Fermi-surface-nesting models can describe the magnetic dispersion. We propose that a coupling of spin and orbital correlations is key to explaining this behavior. If correct, it follows that these nematic fluctuations are involved in the resonance and could be relevant to the pairing mechanism.

Spin-Orbit Interactions and the Nematicity Observed in the Fe-Based Superconductors

High-resolution angle-resolved photoelectron spectroscopy is used to examine the electronic band structure of FeTe_{0.5}Se_{0.5} near the Brillouin zone center. A consistent separation of the α_{1} and α_{2} bands is observed with little k_{z} dependence of the α_{1} band. First-principles calculations for bulk and thin films demonstrate that the antiferromagnetic coupling between the Fe atoms and hybridization-induced spin-orbit effects lifts the degeneracy of the Fe d_{xz} and d_{yz} orbitals at the zone center leading to orbital ordering. These experimental and computational results provide a natural microscopic basis for the nematicity observed in the Fe-based superconductors.

Persistent Coherence and Spin-Polarization of Topological Surface States on Topological Insulators

Gapless surface states on topological insulators are protected from elastic scattering on non-magnetic impurities which makes them promising candidates for low-power electronic applications. However, for wide-spread applications, these states should remain coherent and significantly spin polarized at ambient temperatures. Here, we studied the coherence and spin-structure of the topological states on the surface of a model topological insulator, Bi2Se3, at elevated temperatures in spin and angle-resolved photoemission spectroscopy. We found an extremely weak broadening and essentially no decay of spin polarization of the topological surface state up to room temperature. Our results demonstrate that the topological states on surfaces of topological insulators could serve as a basis for room temperature electronic devices.

Panet al.Reply

Reply to Comment by Calandra et al on “Electronic Structure of Superconducting KC8 and Nonsuperconducting LiC6 Graphite Intercalation Compounds: Evidence for a Graphene-SheetDriven Superconducting State” In their comment Calandra et al [1], assert two points: (1) the estimate of charge transfer from Li to graphene layers in LiC6 in our letter [2] is incorrect because of the three dimensional (3D) character of the electronic structure in bulk LiC6; (2) our main claim that the superconductivity in graphite intercalation compounds (GICs) is graphene-sheet-driven is therefore invalid. First, we point out that our claim on graphene driven superconductivity in GICs is based on the experimental results from a whole series of different materials (graphite, KC24, LiC6, KC8 and CaC6) and that it is valid regardless of the charge transfer estimate. In these different GICs, we observe a strong electron phonon coupling (EPC) between the graphene derived electrons and graphene derived phonons [2, 3]. When put in the McMillan’s formula, the measured coupling constants give the superconducting transition temperatures, Tc, that are very close to the measured ones in LiC6, KC8 and CaC6, demonstrating that the graphene sheets are indeed crucial for superconductivity in GICs. The side observation that the filling of the π states follows the same trend is in accord with a simple picture where the EPC strengthens as the phase space for the scattering grows with the size of the Fermi surface. However, this observation is not essential for the main conclusion of our letter. Second, we note that the validity of the calculations and the estimate for the charge transfer in Calandra et al is heavily based on comparison with the data from another material, lithium intercalated graphene bi-layer [4], irrelevant for the studies of bulk GICs. The third and the most important point is that the calculations for LiC6 show essentially a 3D electronic structure, virtually unchanged from the early work by Holzwarth et al [5], whereas our photoemission experiments show no out-of -plane dispersion. Fig. 1 shows the π-derived Fermi surface (FS) of LiC6 recorded at different photon energies from samples with larger crystallites and a higher degree of intercalant order than those from Pan et al [2]. The three contours, originating from the AαAβAγ stacking in LiC6 below 220 K [6], are now clearly visible, indicating perfect stacking. The relative intensity of these three contours varies, but their areas do not change with kz. As the FS contours are sharper than in ref. [2], the charge transfer could be more precisely determined: the FS area is somewhat larger than in ref. [2], corresponding to the charge transfer of 0.052 e− per graphene unit cell (GUC), still significantly smaller than in KC8 (0.11 e −/GUC). The momentum averaged EPC is the same, within the error bars, to the value reported in Pan et al [2]. The lack of kz dispersion in the experiments clearly demonstrates inability of DFT calculations FIG. 1: Fermi surface of LiC6 measured at T = 15 K at four different photon energies: a) 40 eV, b) 50 eV, c) 55 eV and d) 65 eV. Corresponding kz values are indicated.