Session E01: Electronic Structures of Weyl Semimetals and Topological Insulators

Evolution of the Electronic Structure of MoTe2 under Pressure

Transition metal dichalcogenides have recently been heavily studied owing to their intriguing physical properties such as extremely large magnetoresistance (MR) in WTe₂ and MoTe₂. Further interests are attracted when they are predicted as candidates of type-II Weyl semimetals. Moreover, as both WTe₂ and MoTe₂ host superconductivity, this opens up the possibility of topological superconductivity.
In this work, we present the magneto-electronic transport study of MoTe₂ under pressure. We did the magneto-resistance measurement with magnetic field up to 14 T and temperature down to 30 mK. The large MR and the Hall coefficient decrease rapidly with increasing pressure. Beyond 10 kbar, where the T_d phase is completely suppressed and the superconducting transition temperature T_c is significantly enhanced, the Hall coefficient becomes very small and saturated. We will discuss the evolution of Fermi surface across the temperature-pressure phase diagram. The electronic structure study would help us investigate the ground state and superconductivity of MoTe₂.   

Crystal structure and electronic structure of non-centrosymmetrical trigonal PtBi2

Electronic structure of single crystalline γ-PtBi2 is studied by angle-resolved photoemission spectroscopy(ARPES). It is proposed by theoretical calculation that the stable structure of γ-PtBi2 should be non-centrosymmetric. ARPES measurement gives powerful evidences in this paper that different band structures between two types of cleaved surface are clearly observed. Bulk and surface bands are characterized through systematically photon dependent measurement. The bulk band dispersions agree well with the first-principle calculation except Rashba shaped surface states. The topography studied by STM also confirms the existence of two different cleaved surfaces. Besides, some efforts has been made to demonstrate the existence of triply degenerate point fermions.   

Spin-Resolved Electronic Response to the Phase Transition in MoTe2

The Weyl semimetal MoTe₂ is studied by spin- and angle-resolved photoemission spectroscopy across the centrosymmetry-breaking structural transition temperature of the bulk. A three-dimensional spin-texture is observed in the bulk Fermi surface in the low temperature, noncentrosymmetric phase that is consistent with first-principles calculations. The spin texture and two types of surface Fermi arc are not completely suppressed above the bulk transition temperature. The lifetimes of quasiparticles forming the Fermi arcs depend on thermal history and lengthen considerably upon cooling toward the bulk structural transition. The results indicate that a new form of polar instability exists near the surface when the bulk is largely in a centrosymmetric phase.   

Investigation of the magnetic Weyl semimetal candidate Co3Sn2S2 by ARPES.

Topological phases in magnetic materials are a topic of current interest in the community. Recently, the kagome-lattice ferromagnet Co₃Sn₂S₂ was predicted to be a Weyl semimetal, while transport showed an exceptionally large anomalous Hall angle. Motivated by these exciting results, we use synchrotron ARPES to study single crystal Co₃Sn₂S₂. We observe a strong photon energy dependence consistent with out-of-plane k_z dispersion, even at moderate VUV photon energies. We compare our ARPES results with DFT and find a general match, with typical disagreement of ~ 0.1 eV. To address this disagreement, we perform muon/neutron scattering on Co₃Sn₂S₂ to better understand the magnetic state experimentally. We comment on progress using our muon/neutron results to match DFT to ARPES and evaluating possible changes in the predicted Weyl phase and other topological indices. Our results suggest that the search for topological magnets might benefit from an iterative approach building on muon/neutron measurements of the magnetic state, DFT and ARPES.   

Surface resonances of topological origin in bcc iron

Chiral band degeneracies are ubiquitous in the band structures of systems wtih broken inversion or time-reversal symmetries. While in Weyl semimetals such degeneracies are isolated near the Fermi level, the broadly defined notion of topological metal implies the presence of disconnected Fermi surface sheets characterized by non-zero Chern numbers. Bcc iron is an archetypal example of ferromagnetic metal with two non-trivial Fermi surface electron pockets along the direction parallel to the magnetization [1]. In this work, we investigate the surface-state signatures of the topological nature of these Fermi surface pockets. We find that the (110) surface presents arc-like resonances attached to the non-trivial electron pockets. These Fermi arc resonances are due to two chiral degeneracies, a type-I elementary Weyl point and a type-II composite (Chern number ±2) Weyl point, located at slightly different energies. We further show that such surface resonances can be controlled by the orientation of magnetization, and can eventually be eliminated following a topological phase transition.
[1] D. Gosálbez-Martínez, I. Souza, and D. Vanderbilt, Phys. Rev. B 92, 085138 (2015).   

Sharp Orientation-dependent Quantum Oscillations in HoPtBi

Rare-earth platinum and palladium-bismuthides have been host to rich phenomenon in the search for topological states and Weyl semimetals. YPtBi in particular is host to a spin-split fermi surface which hosts a spin-3/2 superconducting state. In this study we expand this investigation to HoPtBi, a magnetic analog to YPtBi with an AFM ground state and a paramagnetic state above 1.2K. We present orientation-dependent measurements of quantum oscillations in HoPtBi and find a strong orientation dependence near the crystallographic [110] direction very similar to, but phenomenologically distinct from the behavior of YPtBi.   

Observation of surface Dirac dispersion in transition metal dipnictides

The experimental discovery of Dirac and Weyl Fermions, quasi-particles existing in low energy excitation in condensed matter physics, has paved a new avenue of research interests in condensed matter physics. Recently, the Lorentz-invariance breaking Weyl semimetallic state has been reported in MoP₂ and WP₂. Arsenic counterparts of these compounds, which also harbor high magnetoresistance, demand the detailed study to investigate the possible semimetallic state possessed by them. Here, we discuss our angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations results on the electronic structures of MoAs₂ and WAs₂, which reveal the presence of a surface Dirac cone. Our systematic electronic structures analysis reveals the termination dependent electronic structure in these compounds. Our study offers the opportunity to study the various quantum phase transitions within this family.   

ARPES measurements on Dirac nodal-line semimetal candidate TiRhAs

In the family of topological materials, a new Dirac nodal-line (DNL) semimetal TiRhAs was recently predicted by first-principles calculations. Unlike most of DNL semimetals, TiRhAs has a single nodal ring residing near the Fermi level with no interference from other bands, which makes that the carriers in TiRhAs are entirely contributed by nodal-line fermions. Moreover, the calculations show that the hybridization gap induced by SOC is negligible in TiRhAs. Therefore, TiRhAs can be regarded as a hydrogen-atom-like DNL semimetal. By the soft X-ray ARPES, we systemetically studied the band structure of TiRhAs and observed a single nodal ring at kx=0 plane in BZ.

  

Observation of Yamaji Magic Angles in Bismuth Nanowires

Experimental studies that probe the surface of bulk bismuth as a composite of topological insulator edges and hinges on the surface of bulk Bi are lacking. We present angle-dependent transverse magnetoresistance (TMR) oscillation measurements of small diameter ( 50 nm ) bismuth nanowires where electronic transport is dominated by the surface, rather than the bulk in the core of the nanowire, because of quantum confinement. We find that the TMR of the surface states in our nanowires exhibits peaks for a sequence of nanowire rotation angles that strongly suggest an interpretation in terms of Yamaji magic angles. Magic angles are observed in layered and other low-dimensional conductors with weak interplanar coupling that are amenable to be described by an open, corrugated, Fermi surface. In contrast to surface states, bulk bismuth does not display magic angles since the Fermi surface is closed. We will discuss our interpretation of surface states of bismuth and the main Fermi surface parameters that we observe: orientation of the Fermi surface, Fermi wavevector, and coupling strength.   

Topological Line-Like Bound States in the Continuum

Bound states in the continuum (BIC) have been studied mainly in optics. Recently, electronic BIC have been proposed. They appear as points in the momentum space and are protected topologically by the Chern number. In this study, we propose a new type of BIC protected by the winding number, which is one of the topological invariants. These BIC appear as lines in the momentum space, and are realized in a multilayer model consisting of honeycomb-lattice layers. We show band structure and spatial localization of the BIC in this model. The wavenumbers at which the BIC appear can be explained in terms of topology in the momentum space.   

Scanning tunneling spectroscopy studies of the topological states in the Dirac metal Hf2Te2P

Recent angle-resolved photoemission spectroscopy experiments and theoretical calculations indicate the existence of multiple Dirac states in the topological material Hf₂Te₂P [1]. In this study we use scanning tunneling spectroscopy (STS) to measure the electronic band structure of the material. Using STS as a direct probe of the real-space local density of states, we identify quasiparticle interference patterns that allow us to measure the topological surface states. The results will be discussed in the context of density functional theory calculations that predict multiple topological states below and above the Fermi energy.

[1]: Hosen et al. Distinct multiple fermionic states in a single topological metal Hf₂Te₂P. Nat. Commun. 9-3002, 2018.   

A Consistent View of the Samarium Hexaboride Surface Terminations

Since the establishment of topological insulators, a lot of interest has turned to finding systems that combine topology with strong electron correlations. This led Dzero et al. to propose that Kondo insulators, materials that open a band gap by mixing local and itinerant states of opposite parity, should fall into this category [1]. The effort spurred by this proposal has advanced the state of knowledge on SmB₆, the leading candidate for a topological Kondo insulator [2]. The question of its topology, however, remains unanswered.

The apparently contradictory results from different experimental techniques form major obstacle to resolving this issue. In this paper we tackle one of these apparent contradictions: Researchers using scanning tunneling microscopy [3,4] have reached different conclusions on the surface terminations than those using photoemission spectroscopy [5]. By combining both techniques we find the results from these methods can be reconciled, a conclusion that will help settle this topical issue.

[1] M. Dzero et al., Phys. Rev. Lett. 104 (2010) 106408.
[2] J. W. Allen, Phil. Mag. 96 (2016) 3227.
[3] M. M. Yee et al., arxiv.org 1308.1085 (2013).
[4] S. Rößler et al., Proc. Natl. Acad. Sci. 111 (2014) 4798.
[5] P. Hlawenka et al., Nat. Commun. 9 (2018) 517.   

Disentangling orbital and spin characteristics of surface-derived states in Dirac nodal line semimetals

In this work, we explore surface states of nonsymmorphic semimetals with lifted band degeneracy at the symmetry points. HfSiS, a typical Dirac nodal-line semimetal with the nonsymmorphic symmetry exhibits a Rashba-type spin-split surface state. In this regard, we explore the Rashaba surface states which draw an analogy to the topological materials, where the spin and orbital entanglement takes place. We have performed spin- and angle-resolved photoemission spectroscopy experiment on HfSiS and ZrSiS single crystals with p-polarized and s-polarized light. The observed variation in the intensity distribution upon changing the light polarization tells us that the surface states along the M-X-M line are mainly composed of dz² orbitals, and change into d_yz along the G-X-G line. Combining the observed spin polarizations with the result of the slab calculation, we have concluded that the surface bands surrounding the X point exhibit a quite anisotropic spin and orbital texture. Our findings provide a new way of light manipulation by controlling the spin structure in materials which is important for future applications.   

Controlling Spin-Polarized Currents on the Surface of Topological Insulators

The appearance of topologically protected spin-momentum locked surface states with forbidden backward scattering on the surface of topological insulators gives rise to robust room temperature spin currents making them ideal candidates for the realization of spintronic devices. So far however, despite a decade of research in this field, very little is known on how to access and manipulate such currents in a manner that is accessible in chip-based devices. Here we reveal a technique to generate long-lived spin-currents of arbitrary geometry with tunable magnitude and duration on the surface of Bi₂Se₃ topological insulators. Time-resolved angle-resolved photoemission spectroscopy, together with a quantitative model, uncovers a remarkable contrast with respect to doping thanks to the intrinsic correlation between the spin-locked surface state and an insulating bulk. This work demonstrates that localized spin-currents are accessible with long lifetimes, easily tuned in both magnitude and duration, merging photonics and spintronics based devices on the same material.

  

Interaction of coherent phonons with spin-orbit coupled surface states

Spin-polarized electronic states form at the surface of materials due to the combination of spin-orbit coupling and broken inversion symmetry. This phenomenon was first established for Shockley states at the surface of noble metals such as gold, and more recently, has been recognized as a central mechanism responsible for the formation of topologically non-trivial states. Given the fundamental significance of these states, as well as their potential role in spintronics applications, it is important to understand their interaction with collective excitations of the crystal lattice.

Here we use time- and angle-resolved photoemission spectroscopy (trARPES) to investigate electron-phonon coupling in a model system: the Rashba-split surface state of the semimetal Antimony (Sb). The simplicity of the structure permits us to coherently excite all zero-momentum optical modes of the crystal, and study their coupling to both bulk and surface electronic states. In addition, this material provides an ideal platform for testing coherent control schemes, as well as exploring excitation mechanisms for finite-momentum phonon modes. We will discuss our results in the context of similar measurements on topological insulators.