Session S28: Advances in Topological Materials II

Recent results on materials aspects of the investigation of new topological states of matter

Investigation of the electronic states of matter that are determined by topological physics has exploded in recent years through parallel progress in theory, experimental characterization, device fabrication and new materials development. In our group working in this area, the speaker has been responsible for the development of new materials to allow the experimentalists to probe the emergence of new topological properties, and to help embody the concepts of our theorists in real materials. The field is fast-moving, with particular thrusts at the present time toward Weyl and Dirac semimetals, and in this talk I will describe the materials aspects of our work in the past year in these areas, specifically as they are related to our strong collaborations with the groups of N.P. Ong, A. Yazdani, and B.A. Bernevig at Princeton, T. Valla and J. Tao at Brookhaven, and A. Vishwanath at Berkeley.   

Observation of Weyl nodes and Fermi arcs in TaP

A Weyl semimetal possesses topologically unavoidable crossings of spin-polarized bands dispersing linearly along all momentum directions, called Weyl nodes, connected by topological surface arcs. The crossing points have fixed chirality and behave the same as massless Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport and other novel properties dominated by Weyl fermions, it is required that Weyl nodes (1) locate nearly at the chemical potential and (2) are well separated in momentum space and enclosed by pairs of individual Fermi surfaces with nonzero Fermi Chern numbers. By investigating the electronic structure of TaP using angle-resolved photoemission spectroscopy and first-principles calculation, we establish that TaP is a Weyl semimetal with only single type of Weyl fermions with well-separated Weyl nodes locating at the chemical potential, distinguished from TaAs where there are two types of Weyl fermions contributing to the low-energy physical properties. We have also observed Fermi arcs on the Ta-terminated surface, which appear in a different pattern from that on the As-terminated surface observed in TaAs and NbAs.   

Fermiology in the Weyl semimetals (Nb,Ta)(As,P)

We present a detailed angular and temperature dependent study of the quantum oscillatory phenomena in the Weyl semimetals, (Ta,Nb)(As,P) under high magnetic fields. In general we find that the P compounds exhibit larger Fermi surfaces with lighter effective masses when compared to the As ones. We show that for (Nb,Ta)As we can reach the quantum limit with the currently available magnetic fields, which seemingly leads to an electronic phase transition.   

Discovery of a Weyl fermion state with Fermi arcs in niobium arsenide

Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. Here, we present the experimental discovery of the Weyl semimetal state in an inversion-symmetry-breaking single-crystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft X-ray and ultraviolet photoemission spectroscopy, we systematically study both the surface and bulk electronic structure of NbAs. We experimentally observe both the Weyl cones in the bulk and the Fermi arcs on the surface of this system. Our ARPES data, in agreement with our theoretical calculations, identify the Weyl semimetal state in NbAs, which provides a platform to test the potential of Weyltronics.   

Discovery of the first Weyl fermion semimetal and topological Fermi arcs in TaAs

Weyl semimetals have opened a new era in condensed matter physics and materials science. They host Weyl fermions as emergent quasiparticles and admit a topological classification that protects Fermi arc surface states on the boundary. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of the first Weyl semimetal, TaAs. 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.   

Electronic structures and evolution in a transition metal pnictide Weyl semimetal family

Topological Weyl semimetals (TWSs) represent a novel state of quantum matter which not only possesses Weyl fermions (massless chiral particles that can be viewed as magnetic monopoles in momentum space) in the bulk and unique Fermi arcs generated by topological surface states, but also exhibits appealing physical properties such as extremely large magnetoresistance and ultra-high carrier mobility. By performing angle-resolved photoemission spectroscopy (ARPES) on compounds from a transition metal pnictide family (NbP, TaP and TaAs), we systematically investigated their electronic structures and discovered the unique surface ``Fermi-arcs'' and linear bulk band dispersion across the Weyl points. Furthermore, we also illustrated their Fermiology evolution with the spin--orbit coupling (SOC) strength. Our experimental findings not only reveal the mechanism to realize and fine-tune the electronic structures of TWSs, but also provide a rich material base for exploring many exotic physical phenomena (for example, chiral magnetic effects, negative magnetoresistance, and the quantum anomalous Hall effect) and novel future applications.   

Experimental discovery of a topological Weyl semimetal state in TaP

We observed Weyl fermion cones and nodes in the bulk and the Fermi arcs on the surface of Weyl semimetal TaP. Also, we found that the surface states show an unexpectedly rich structure, including both topological Fermi arcs and topologically trivial closed contours in the vicinity of the Weyl points. A rigorous scheme for directly demonstrating the bulk-boundary correspondence and, hence, establishing the Weyl semimetal state in TaP is discussed.   

Spin polarization of the Fermi arcs in the Weyl semimetal TaAs

In a Weyl semimetal, Weyl fermion quasiparticle excitations correspond to points of bulk band degeneracy separated in $k-$space and connected on the surface by Fermi arcs. Our spin-resolved ARPES measurements on the experimentally discovered Weyl semimetal TaAs have revealed that the Fermi arcs have spin polarization larger than 80{\%} and that the spin texture is in-plane and does not match with that of the bulk Weyl cones where they meet.   

Observation of surface states derived from topological Fermi arcs in the Weyl semimetal NbP

The recent experimental discovery of the first Weyl semimetal (WSM) provides the first observation of a Weyl fermion in nature and demonstrates a novel type of anomalous surface state band structure, consisting of Fermi arcs. NbP may realize the first WSM in the limit of weak spin-orbit coupling. Here we study the surface states of NbP by ARPES and we find that we cannot show Fermi arcs based on our experimental data alone. However, the excellent agreement between our data and calculations suggests that NbP is a WSM and that we observe trivial surface states which evolve continuously from topological Fermi arcs above the Fermi level.   

Observation of Fermi Arcs in non-CentrosymmetricWeyl Semi-metal Candidate NbP

We report the surface electronic structure of niobium phosphide NbP single crystal on (001) surface by vacuum ultraviolet angle-resolved photoemission spectroscopy. Combining with our first principle calculations, we identify the existence of the Fermi arcs originated from topological surface states. Furthermore, the surface states exhibit circular dichroism pattern, which may correlated with spin texture. Our results not only provide critical evidence for the existence of the Weyl Fermions in NbP, but also lay foundations for further investigations.   

Observation of Fermi arc spin texture in TaAs

We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin-polarized. The measured spin texture fulfills the requirement of mirror and time reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first-principles calculations.   

Complete Fermi Surface and Surface State in WTe2 Revealed by High-Resolution Laser-Based Angle-Resolved Photoemission Spectroscopy

WTe2, an unique transition metal dichalcogenide, attracts considerable attention recently, which shows an extremely large magnetoresistance (MR) with no saturation under very high field. In this talk, we will present our high resolution laser-ARPES study on WTe2. Our distinctive ARPES system is equipped with the VUV laser and the time-of-flight (TOF) electron energy analyzer, being featured by super-high energy resolution, simultaneous data acquisition for two-dimensional momentum space and much reduced nonlinearity effect. With this advanced apparatus, the very high quality of electronic structure data are obtained for WTe2 which gives a full picture of the Fermi surface. Meanwhile, the obtained systematic temperature dependence of its electronic state leads us to a better understanding on the origin of large magnetoresistance in WTe2.   

First-principles study of temperature effects in topological insulator phase diagrams

Recent studies have identified several tunable three-dimensional topological insulators. Upon varying experimental parameters such as pressure or doping, these materials exhibit a transition between a trivial and a topological insulating phase. We present a first-principles study of temperature effects in the family of alloyed BiTlS$_2$ / BiTlSe$_2$ topological phase transition materials. Through the electron-phonon coupling, the electronic bands being renormalized at finite temperature allow for a topological phase transition at some critical temperature. We find a temperature-doping phase diagram having a confined topological phase region, with the topological phase suppressed at high temperature. We also discuss the converse scenario in which phonons might favour the topological phase, as previously anticipated.