Session B27: Invited Session: Magneto-Electric and Magneto-Optical Properties of Topological Insulators

Magneto-Optical Properties of Electrically Gated Topological Insulators

Topological insulators (TI) are a predicted new quantum state of matter in which spin-orbit coupling gives rise to topologically protected surface states with unpaired spin-helical Dirac cones. TIs are predicted to have exotic properties including Majorana fermions induced by the proximity effect from a superconducting film, a intrinsic magnetoelectric effect and hybridized spin-plasmon modes. The magneto-electric effect leads to Faraday and Kerr rotations quantized in units of the fine structure constant. This effect corresponds to $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ integer quantum Hall step and is predicted both in field and \textbf{\textit{in the absence of a magnetic field when magnetic order gaps the Dirac spectrum}}. A key difference between this half integer QHE in TIs and the usual integer QHE is that the former cannot be measured by a dc transport experiment. I will describe experiments designed to measure the Kerr rotation in Bi2Se3, one example of a topological insulator. Gating the surface isolates the surface response from the bulk signals due to unavoidable bulk carriers from defects and impurities. Preliminary results will be presented on the surface state Kerr rotation for Bi2Se3 doped with Mg (ungapped) and Bi2Se3 doped with Sm (magnetically gapped).   

Magneto-Electric Effect in Three-Dimensional Topological Insulators from Surface Magnetic Disorder and Ferromagnetic Thin Film

Topologically nontrivial gapped phases can be characterized by the bulk topological indices and the surface gapless modes. The topological magneto-electric (ME) effect is a novel manifestation of the bulk-surface correspondence in which the bulk magnetization is generated by a circulating quantized Hall current flowing at the surface of topological insulators. To realize the topological ME effect, there are two difficulties: (a) one needs to attach an insulating ferromagnetic layer with the magnetization normal to the surface all pointing out or in. (b) The Fermi energy must be tuned accurately within the small gap of the surface Dirac spectrum opened by the exchange interaction. In this talk we discuss the anomalous quantized Hall current on the surface of a magnetically doped topological insulator, basing on the two-dimensional surface Dirac Hamiltonian with magnetic disorder. The scaling analysis indicates that, in sharp contrast to the time-reversal-invariant cases, the all surface states tend to be localized while the Hall conductivity is quantized no matter whether the Fermi level resides within or out of the surface gap. This resolves problem(b). Furthermore it is shown that this also resolves problem (a) with the simultaneous application of magnetic and electric fields parallel or antiparallel to each other. By this method, doped local spins can be controlled by the bulk energy which can overcome the magnetic anisotropy and Zeeman splitting at the surface. We also comment on the generalization of the topological responses to the case of topological superconductors and superfluids. This work was done in collaboration with Naoto Nagaosa, Shinsei Ryu, and Akira Furusaki. K. Nomura and N. Nagaosa, Phys. Rev. Lett. 106, 166802 (2011); K. Nomura, S. Ryu, A. Furusaki, N. Nagaosa, arXiv:1108.5054.   

Scotch Tape and Spectroscopy, Probing and Manipulating the surface of a Topological Insulator

Recently there has been a great deal of interest in studying the surfaces of materials with topologically non-trivial electronic states. In addition to probing the surfaces of topological insulators it is highly desirable to put them in proximity with other materials (ferromagnets and superconductors) to induce new particles such as Majoranna Fermions. I will discuss our groups efforts to study these materials using mechanical exfoliation and a variety of spectroscopic techniques (Raman, IR and tunneling). In addition I will detail a new method we have devised that enables us to produce high temperature superconductivity in a topological insulator via the proximity effect.   

Giant terahertz Faraday rotation in graphene

The Faraday rotation of the polarization of light in a medium, where the time-reversal symmetry is broken due to external magnetic field, is an optical analogue of the Hall effect. We recently demonstrated that graphene, the thinnest existing material, can turn the polarization of terahertz radiation by several degrees in modest magnetic fields, which is a spectacular manifestation of the cyclotron resonance. In this talk I will review our Faraday rotation spectroscopy studies of single-layer and twisted multilayer epitaxial graphene with an emphasis on the physical information that one can extract from these measurements and potential applications.\\[4pt] [1] I. Crassee et al. Nature Physics \textbf{7}, 48 (2011).\\[0pt] [2] I. Crassee et al. Phys. Rev. B, \textbf{84}, 035103 (2011).   

Ultrafast Probing of Dynamical Spin-Charge Coupling in Topological Insulators

The three-dimensional topological insulator (TI) is a new quantum phase of matter that exhibits quantum-Hall-like properties, even in the absence of an external magnetic field. Charge carriers on the surface of a TI behave like a two-dimensional gas of massless helical Dirac fermions for which the spin is ideally locked perpendicular to the momentum. In this talk, I will discuss recent experiments in which we used the angular momentum of circularly polarized ultrafast laser pulses to directly visualize and manipulate the spin-charge coupling in TIs. By using laser pulses in the UV region, we performed novel time of flight based angle-resolved photoemission spectroscopy that enabled simultaneously mapping all three components of spin over the entire Dirac cone of a TI. We find that an idealized description of helical Dirac fermions only applies within a small energy window about the Dirac point, beyond which strong textural deformations occur. Utilizing the pump-probe technique, we selectively obtained time-resolved dynamics of surface and bulk excitations. By using circularly polarized laser pulses in the optical region, we achieved preferential excitation of spin species on one side of the surface Dirac cone, resulting in a charge imbalance in momentum space and thus causing a current flow with a direction dependent on photon helicity.