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1.
Nano Lett ; 19(2): 716-721, 2019 02 13.
Article in English | MEDLINE | ID: mdl-30663307

ABSTRACT

The search for new topological materials and states of matter is presently at the forefront of quantum materials research. One powerful approach to novel topological phases beyond the thermodynamic space is to combine different topological/functional materials into a single materials platform in the form of superlattices. However, despite some previous efforts there has been a significant gap between theories and experiments in this direction. Here, we provide the first detailed set of experimentally verifiable phase diagrams of topological superlattices composed of archetypal topological insulator, Bi2Se3, and normal insulator, In2Se3, by combining molecular-level materials control, low-temperature magnetotransport measurements, and field theoretical calculations. We show how the electronic properties of topological superlattices evolve with unit-layer thicknesses and utilize the weak antilocalization effect as a tool to gain quantitative insights into the evolution of conducting channels within each set of heterostructures. This orchestrated study opens the door to the possibility of creating a variety of artificial-topological-phases by combining topological materials with various other functional building blocks such as superconductors and magnetic materials.

2.
Phys Rev Lett ; 121(17): 176803, 2018 Oct 26.
Article in English | MEDLINE | ID: mdl-30411918

ABSTRACT

The Berreman effect (BE) allows one to study the electrodynamics of ultrathin conducting films at the surface of dielectrics by use of grazing-angle infrared spectroscopy and polarized radiation. Here, we first apply the BE to the two-dimensional electron system (2DES) at the interface between a high-purity film of the topological insulator Bi_{2}Se_{3} and its sapphire substrate. We determine for the 2DES a charge density n_{s}=(8±1)×10^{12} cm^{-2}, a thickness d=0.6±0.2 nm, and a mobility µ^{IR}=290±30 cm^{2}/V s. Within errors, all of these parameters result in being independent of temperature between 300 and 10 K. These findings consistently indicate that the 2DES is formed by topological surface states, whose infrared response is then directly observed here.

3.
Sci Adv ; 1(10): e1501092, 2015 Nov.
Article in English | MEDLINE | ID: mdl-26702446

ABSTRACT

Weyl semimetals are expected to open up new horizons in physics and materials science because they provide the first realization of Weyl fermions and exhibit protected Fermi arc surface states. However, they had been found to be extremely rare in nature. Recently, a family of compounds, consisting of tantalum arsenide, tantalum phosphide (TaP), niobium arsenide, and niobium phosphide, was predicted as a Weyl semimetal candidates. We experimentally realize a Weyl semimetal state in TaP. Using photoemission spectroscopy, we directly observe the Weyl fermion cones and nodes in the bulk, and the Fermi arcs on the surface. Moreover, we find that the surface states show an unexpectedly rich structure, including both topological Fermi arcs and several topologically trivial closed contours in the vicinity of the Weyl points, which provides a promising platform to study the interplay between topological and trivial surface states on a Weyl semimetal's surface. We directly demonstrate the bulk-boundary correspondence and establish the topologically nontrivial nature of the Weyl semimetal state in TaP, by resolving the net number of chiral edge modes on a closed path that encloses the Weyl node. This also provides, for the first time, an experimentally practical approach to demonstrating a bulk Weyl fermion from a surface state dispersion measured in photoemission.

4.
Science ; 349(6248): 613-7, 2015 Aug 07.
Article in English | MEDLINE | ID: mdl-26184916

ABSTRACT

A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs). Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.

5.
Nat Commun ; 6: 6870, 2015 Apr 17.
Article in English | MEDLINE | ID: mdl-25882717

ABSTRACT

The topology of a topological material can be encoded in its surface states. These surface states can only be removed by a bulk topological quantum phase transition into a trivial phase. Here we use photoemission spectroscopy to image the formation of protected surface states in a topological insulator as we chemically tune the system through a topological transition. Surprisingly, we discover an exotic spin-momentum locked, gapped surface state in the trivial phase that shares many important properties with the actual topological surface state in anticipation of the change of topology. Using a spin-resolved measurement, we show that apart from a surface bandgap these states develop spin textures similar to the topological surface states well before the transition. Our results offer a general paradigm for understanding how surface states in topological phases arise from a quantum phase transition and are suggestive for the future realization of Weyl arcs, condensed matter supersymmetry and other fascinating phenomena in the vicinity of a quantum criticality.

6.
Science ; 347(6219): 294-8, 2015 Jan 16.
Article in English | MEDLINE | ID: mdl-25593189

ABSTRACT

The topology of the electronic structure of a crystal is manifested in its surface states. Recently, a distinct topological state has been proposed in metals or semimetals whose spin-orbit band structure features three-dimensional Dirac quasiparticles. We used angle-resolved photoemission spectroscopy to experimentally observe a pair of spin-polarized Fermi arc surface states on the surface of the Dirac semimetal Na3Bi at its native chemical potential. Our systematic results collectively identify a topological phase in a gapless material. The observed Fermi arc surface states open research frontiers in fundamental physics and possibly in spintronics.

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