Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe₂

Nature Physics 12, 1105 (2016)

Weyl semimetal is a new quantum state of matter hosting the condensed matter physics counterpart of the relativistic Weyl fermions originally introduced in high-energy physics. The Weyl semimetal phase realized in the TaAs class of materials features multiple Fermi arcs arising from topological surface states and exhibits novel quantum phenomena, such as a chiral anomaly-induced negative magnetoresistance and possibly emergent supersymmetry. It was proposed theoretically that a new type (type-II) of Weyl fermion, that arises due to the breaking of Lorentz invariance, which does not have a counterpart in high-energy physics, can emerge as topologically protected touching between electron and hole pockets. We report direct experimental evidence of topological Fermi arcs in the predicted type-II Weyl semimetal MoTe₂. The topological surface states are confirmed by directly observing the surface states using bulk- and surface-sensitive angle-resolved photoemission spectroscopy, and the quasi-particle interference pattern between the putative topological Fermi arcs in scanning tunneling microscopy. By establishing MoTe₂ as an experimental realization of a type-II Weyl semimetal, the work opens up opportunities for probing the physical properties of this exciting new state.

Experimental observation of Dirac-like surface states and topological phase transition in Pb_1-xSn_xTe(111) films 

Physical Review Letters 112, 186801 (2014)

The surface of a topological crystalline insulator (TCI) carries an even number of Dirac cones protected by crystalline symmetry. We epitaxially grew high-quality Pb_1−xSn_xTe(111) films and investigated the TCI phase by in situ angle-resolved photoemission spectroscopy. Pb_1−xSn_xTe(111) films undergo a topological phase transition from a trivial insulator to TCI via increasing the Sn/Pb ratio, accompanied by a crossover from n-type to p-type doping. In addition, a hybridization gap is opened in the surface states when the thickness of the film is reduced to the two-dimensional limit. The work demonstrates an approach to manipulating the topological properties of TCI, which is of importance for future fundamental research and applications based on TCI.

Fully gapped topological surface states in Bi₂Se₃ films induced by a d-wave high-temperature superconductor

Nature Physics 9, 621 (2013)

Topological insulators are a class of material that exhibit robust gapless surface states protected by time-reversal symmetry. The interplay of such symmetry-protected topological surface states and symmetry-broken states provides a platform for exploring new quantum phenomena and functionalities, such as one-dimensional chiral or helical gapless Majorana fermions, and Majorana zero modes that may find application in fault tolerant quantum computation. Inducing superconductivity on the topological surface states is a prerequisite for their experimental realization. Here, by growing high-quality topological insulator Bi₂Se₃ films on a d-wave superconductor Bi₂Sr₂CaCu₂O_8+d using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi₂Se₃ films with a large pairing gap up to 15 meV. Interestingly, distinct from the d-wave pairing of Bi₂Sr₂CaCu₂O_8+d, the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant s-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes.

Power-law decay of standing waves on the surface of topological insulators

Physical Review B 84, 235447 (2011)

We propose a general theory on the standing waves (quasiparticle interference pattern) caused by the scattering of surface states off step edges in topological insulators in which the extremal points on the constant energy contour of surface band play a dominant role. Experimentally, we image the interference patterns on both Bi₂Te₃ and Bi₂Se₃ films by measuring the local density of states with a scanning tunneling microscope. The observed decay indices of the standing waves agree excellently with the theoretical prediction: In Bi₂Se₃, only a single decay index of −3/2 exists, while in Bi₂Te₃ with strongly warped surface band, it varies from −3/2 to −1/2 and finally to −1 as the energy increases. The −1/2 decay indicates that the suppression of backscattering due to time-reversal symmetry does not necessarily lead to a spatial decay rate faster than that in the conventional two-dimensional electron system. Our formalism can also better explain the characteristic scattering wave vectors of the standing wave caused by nonmagnetic impurities on Bi₂Te₃.

Landau quantization of topological surface states in Bi₂Se₃ 

Physical Review Letters 105, 076801 (2010)

We report the direct observation of Landau quantization in Bi₂Se₃ thin films by using a low-temperature scanning tunneling microscope. In particular, we discovered the zeroth Landau level, which is predicted to give rise to the half-quantized Hall effect for the topological surface states. The existence of the discrete Landau levels (LLs) and the suppression of LLs by surface impurities strongly support the 2D nature of the topological states. These observations may eventually lead to the realization of quantum Hall effect in topological insulators.

Experimental demonstration of topological surface states protected by time-reversal symmetry
Physical Review Letters 103, 266803 (2009)

We report direct imaging of standing waves of the nontrivial surface states of topological insulator Bi2Te3 using a scanning tunneling microscope. The interference fringes are caused by the scattering of the topological states off Ag impurities and step edges on the Bi2Te3 (111) surface. By studying the voltage dependent standing wave patterns, we determine the energy dispersion E(k), which confirms the Dirac cone structure of the topological states. We further show that, very different from the conventional surface states, backscattering of the topological states by nonmagnetic impurities is completely suppressed. The absence of backscattering is a spectacular manifestation of the time-reversal symmetry, which offers a direct proof of the topological nature of the surface states.