26
|
Hu C, Ding L, Gordon KN, Ghosh B, Tien HJ, Li H, Linn AG, Lien SW, Huang CY, Mackey S, Liu J, Reddy PVS, Singh B, Agarwal A, Bansil A, Song M, Li D, Xu SY, Lin H, Cao H, Chang TR, Dessau D, Ni N. Realization of an intrinsic ferromagnetic topological state in MnBi 8Te 13. SCIENCE ADVANCES 2020; 6:eaba4275. [PMID: 32743072 PMCID: PMC7375807 DOI: 10.1126/sciadv.aba4275] [Citation(s) in RCA: 42] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/04/2019] [Accepted: 06/09/2020] [Indexed: 05/13/2023]
Abstract
Novel magnetic topological materials pave the way for studying the interplay between band topology and magnetism. However, an intrinsically ferromagnetic topological material with only topological bands at the charge neutrality energy has so far remained elusive. Using rational design, we synthesized MnBi8Te13, a natural heterostructure with [MnBi2Te4] and [Bi2Te3] layers. Thermodynamic, transport, and neutron diffraction measurements show that despite the adjacent [MnBi2Te4] being 44.1 Å apart, MnBi8Te13 manifests long-range ferromagnetism below 10.5 K with strong coupling between magnetism and charge carriers. First-principles calculations and angle-resolved photoemission spectroscopy measurements reveal it is an axion insulator with sizable surface hybridization gaps. Our calculations further demonstrate the hybridization gap persists in the two-dimensional limit with a nontrivial Chern number. Therefore, as an intrinsic ferromagnetic axion insulator with clean low-energy band structures, MnBi8Te13 serves as an ideal system to investigate rich emergent phenomena, including the quantized anomalous Hall effect and quantized magnetoelectric effect.
Collapse
|
27
|
Chang TR, Šuta D, Chiu TW. Responses of midbrain auditory neurons to two different environmental sounds-A new approach on cross-sound modeling. Biosystems 2019; 187:104021. [PMID: 31574292 DOI: 10.1016/j.biosystems.2019.104021] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2019] [Revised: 07/07/2019] [Accepted: 08/19/2019] [Indexed: 11/29/2022]
Abstract
When modeling auditory responses to environmental sounds, results are satisfactory if both training and testing are restricted to datasets of one type of sound. To predict 'cross-sound' responses (i.e., to predict the response to one type of sound e.g., rat Eating sound, after training with another type of sound e.g., rat Drinking sound), performance is typically poor. Here we implemented a novel approach to improve such cross-sound modeling (single unit datasets were collected at the auditory midbrain of anesthetized rats). The method had two key features: (a) population responses (e.g., average of 32 units) instead of responses of individual units were analyzed; and (b) the long sound segment was first divided into short segments (single sound-bouts), their similarity was then computed over a new metric involving the response (called Stimulus Response Model map or SRM map), and finally similar sound-bouts (regardless of sound type) and their associated responses (peri-stimulus time histograms, PSTHs) were modelled. Specifically, a committee machine model (artificial neural networks with 20 stratified spectral inputs) was trained with datasets from one sound type before predicting PSTH responses to another sound type. Model performance was markedly improved up to 92%. Results also suggested the involvement of different neural mechanisms in generating the early and late responses to amplitude transients in the broad-band environmental sounds. We concluded that it is possible to perform rather satisfactory cross-sound modeling on datasets grouped together based on their similarities in terms of the new metric of SRM map.
Collapse
|
28
|
Belopolski I, Manna K, Sanchez DS, Chang G, Ernst B, Yin J, Zhang SS, Cochran T, Shumiya N, Zheng H, Singh B, Bian G, Multer D, Litskevich M, Zhou X, Huang SM, Wang B, Chang TR, Xu SY, Bansil A, Felser C, Lin H, Hasan MZ. Discovery of topological Weyl fermion lines and drumhead surface states in a room temperature magnet. Science 2019; 365:1278-1281. [DOI: 10.1126/science.aav2327] [Citation(s) in RCA: 252] [Impact Index Per Article: 50.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2018] [Accepted: 08/14/2019] [Indexed: 01/18/2023]
|
29
|
Hsu CH, Zhou X, Chang TR, Ma Q, Gedik N, Bansil A, Xu SY, Lin H, Fu L. Topology on a new facet of bismuth. Proc Natl Acad Sci U S A 2019; 116:13255-13259. [PMID: 31196954 PMCID: PMC6613101 DOI: 10.1073/pnas.1900527116] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Bismuth-based materials have been instrumental in the development of topological physics, even though bulk bismuth itself has been long thought to be topologically trivial. A recent study has, however, shown that bismuth is in fact a higher-order topological insulator featuring one-dimensional (1D) topological hinge states protected by threefold rotational and inversion symmetries. In this paper, we uncover another hidden facet of the band topology of bismuth by showing that bismuth is also a first-order topological crystalline insulator protected by a twofold rotational symmetry. As a result, its [Formula: see text] surface exhibits a pair of gapless Dirac surface states. Remarkably, these surface Dirac cones are "unpinned" in the sense that they are not restricted to locate at specific k points in the [Formula: see text] surface Brillouin zone. These unpinned 2D Dirac surface states could be probed directly via various spectroscopic techniques. Our analysis also reveals the presence of a distinct, previously uncharacterized set of 1D topological hinge states protected by the twofold rotational symmetry. Our study thus provides a comprehensive understanding of the topological band structure of bismuth.
Collapse
|
30
|
Gui X, Pletikosic I, Cao H, Tien HJ, Xu X, Zhong R, Wang G, Chang TR, Jia S, Valla T, Xie W, Cava RJ. A New Magnetic Topological Quantum Material Candidate by Design. ACS CENTRAL SCIENCE 2019; 5:900-910. [PMID: 31139726 PMCID: PMC6535778 DOI: 10.1021/acscentsci.9b00202] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/28/2019] [Indexed: 05/31/2023]
Abstract
Magnetism, when combined with an unconventional electronic band structure, can give rise to forefront electronic properties such as the quantum anomalous Hall effect, axion electrodynamics, and Majorana fermions. Here we report the characterization of high-quality crystals of EuSn2P2, a new quantum material specifically designed to engender unconventional electronic states plus magnetism. EuSn2P2 has a layered, Bi2Te3-type structure. Ferromagnetic interactions dominate the Curie-Weiss susceptibility, but a transition to antiferromagnetic ordering occurs near 30 K. Neutron diffraction reveals that this is due to two-dimensional ferromagnetic spin alignment within individual Eu layers and antiferromagnetic alignment between layers-this magnetic state surrounds the Sn-P layers at low temperatures. The bulk electrical resistivity is sensitive to the magnetism. Electronic structure calculations reveal that EuSn2P2 might be a strong topological insulator, which can be a new magnetic topological quantum material (MTQM) candidate. The calculations show that surface states should be present, and they are indeed observed by angle-resolved photoelectron spectroscopy (ARPES) measurements.
Collapse
|
31
|
Sanchez DS, Belopolski I, Cochran TA, Xu X, Yin JX, Chang G, Xie W, Manna K, Süß V, Huang CY, Alidoust N, Multer D, Zhang SS, Shumiya N, Wang X, Wang GQ, Chang TR, Felser C, Xu SY, Jia S, Lin H, Hasan MZ. Topological chiral crystals with helicoid-arc quantum states. Nature 2019; 567:500-505. [DOI: 10.1038/s41586-019-1037-2] [Citation(s) in RCA: 171] [Impact Index Per Article: 34.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2018] [Accepted: 01/10/2019] [Indexed: 11/09/2022]
|
32
|
Ma Q, Xu SY, Shen H, MacNeill D, Fatemi V, Chang TR, Mier Valdivia AM, Wu S, Du Z, Hsu CH, Fang S, Gibson QD, Watanabe K, Taniguchi T, Cava RJ, Kaxiras E, Lu HZ, Lin H, Fu L, Gedik N, Jarillo-Herrero P. Observation of the nonlinear Hall effect under time-reversal-symmetric conditions. Nature 2018; 565:337-342. [DOI: 10.1038/s41586-018-0807-6] [Citation(s) in RCA: 224] [Impact Index Per Article: 37.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2018] [Accepted: 11/14/2018] [Indexed: 11/09/2022]
|
33
|
Chang G, Wieder BJ, Schindler F, Sanchez DS, Belopolski I, Huang SM, Singh B, Wu D, Chang TR, Neupert T, Xu SY, Lin H, Hasan MZ. Topological quantum properties of chiral crystals. NATURE MATERIALS 2018; 17:978-985. [PMID: 30275564 DOI: 10.1038/s41563-018-0169-3] [Citation(s) in RCA: 92] [Impact Index Per Article: 15.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/12/2017] [Accepted: 08/15/2018] [Indexed: 05/02/2023]
Abstract
Chiral crystals are materials with a lattice structure that has a well-defined handedness due to the lack of inversion, mirror or other roto-inversion symmetries. Although it has been shown that the presence of crystalline symmetries can protect topological band crossings, the topological electronic properties of chiral crystals remain largely uncharacterized. Here we show that Kramers-Weyl fermions are a universal topological electronic property of all non-magnetic chiral crystals with spin-orbit coupling and are guaranteed by structural chirality, lattice translation and time-reversal symmetry. Unlike conventional Weyl fermions, they appear at time-reversal-invariant momenta. We identify representative chiral materials in 33 of the 65 chiral space groups in which Kramers-Weyl fermions are relevant to the low-energy physics. We determine that all point-like nodal degeneracies in non-magnetic chiral crystals with relevant spin-orbit coupling carry non-trivial Chern numbers. Kramers-Weyl materials can exhibit a monopole-like electron spin texture and topologically non-trivial bulk Fermi surfaces over an unusually large energy window.
Collapse
|
34
|
Zhu Z, Chang TR, Huang CY, Pan H, Nie XA, Wang XZ, Jin ZT, Xu SY, Huang SM, Guan DD, Wang S, Li YY, Liu C, Qian D, Ku W, Song F, Lin H, Zheng H, Jia JF. Quasiparticle interference and nonsymmorphic effect on a floating band surface state of ZrSiSe. Nat Commun 2018; 9:4153. [PMID: 30297777 PMCID: PMC6175950 DOI: 10.1038/s41467-018-06661-9] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2018] [Accepted: 09/18/2018] [Indexed: 11/09/2022] Open
Abstract
Non-symmorphic crystals are generating great interest as they are commonly found in quantum materials, like iron-based superconductors, heavy-fermion compounds, and topological semimetals. A new type of surface state, a floating band, was recently discovered in the nodal-line semimetal ZrSiSe, but also exists in many non-symmorphic crystals. Little is known about its physical properties. Here, we employ scanning tunneling microscopy to measure the quasiparticle interference of the floating band state on ZrSiSe (001) surface and discover rotational symmetry breaking interference, healing effect and half-missing-type anomalous Umklapp scattering. Using simulation and theoretical analysis we establish that the phenomena are characteristic properties of a floating band surface state. Moreover, we uncover that the half-missing Umklapp process is derived from the glide mirror symmetry, thus identify a non-symmorphic effect on quasiparticle interferences. Our results may pave a way towards potential new applications of nanoelectronics.
Collapse
|
35
|
Yin JX, Zhang SS, Li H, Jiang K, Chang G, Zhang B, Lian B, Xiang C, Belopolski I, Zheng H, Cochran TA, Xu SY, Bian G, Liu K, Chang TR, Lin H, Lu ZY, Wang Z, Jia S, Wang W, Hasan MZ. Giant and anisotropic many-body spin–orbit tunability in a strongly correlated kagome magnet. Nature 2018; 562:91-95. [DOI: 10.1038/s41586-018-0502-7] [Citation(s) in RCA: 176] [Impact Index Per Article: 29.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2018] [Accepted: 07/04/2018] [Indexed: 11/09/2022]
|
36
|
Singh B, Chang G, Chang TR, Huang SM, Su C, Lin MC, Lin H, Bansil A. Tunable double-Weyl Fermion semimetal state in the SrSi 2 materials class. Sci Rep 2018; 8:10540. [PMID: 30002388 PMCID: PMC6043586 DOI: 10.1038/s41598-018-28644-y] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2018] [Accepted: 06/20/2018] [Indexed: 11/13/2022] Open
Abstract
We discuss first-principles topological electronic structure of noncentrosymmetric SrSi2 materials class based on the hybrid exchange-correlation functional. Topological phase diagram of SrSi2 is mapped out as a function of the lattice constant with focus on the semimetal order. A tunable double-Weyl Fermion state in Sr1-xCaxSi2 and Sr1-xBaxSi2 alloys is identified. Ca doping in SrSi2 is shown to yield a double-Weyl semimetal with a large Fermi arc length, while Ba doping leads to a transition from the topological semimetal to a gapped insulator state. Our study indicates that SrSi2 materials family could provide an interesting platform for accessing the unique topological properties of Weyl semimetals.
Collapse
|
37
|
Chang G, Xu SY, Wieder BJ, Sanchez DS, Huang SM, Belopolski I, Chang TR, Zhang S, Bansil A, Lin H, Hasan MZ. Unconventional Chiral Fermions and Large Topological Fermi Arcs in RhSi. PHYSICAL REVIEW LETTERS 2017; 119:206401. [PMID: 29219365 DOI: 10.1103/physrevlett.119.206401] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/11/2017] [Indexed: 05/02/2023]
Abstract
The theoretical proposal of chiral fermions in topological semimetals has led to a significant effort towards their experimental realization. In particular, the Fermi surfaces of chiral semimetals carry quantized Chern numbers, making them an attractive platform for the observation of exotic transport and optical phenomena. While the simplest example of a chiral fermion in condensed matter is a conventional |C|=1 Weyl fermion, recent theoretical works have proposed a number of unconventional chiral fermions beyond the standard model which are protected by unique combinations of topology and crystalline symmetries. However, materials candidates for experimentally probing the transport and response signatures of these unconventional fermions have thus far remained elusive. In this Letter, we propose the RhSi family in space group No. 198 as the ideal platform for the experimental examination of unconventional chiral fermions. We find that RhSi is a filling-enforced semimetal that features near its Fermi surface a chiral double sixfold-degenerate spin-1 Weyl node at R and a previously uncharacterized fourfold-degenerate chiral fermion at Γ. Each unconventional fermion displays Chern number ±4 at the Fermi level. We also show that RhSi displays the largest possible momentum separation of compensative chiral fermions, the largest proposed topologically nontrivial energy window, and the longest possible Fermi arcs on its surface. We conclude by proposing signatures of an exotic bulk photogalvanic response in RhSi.
Collapse
|
38
|
Zheng H, Chang G, Huang SM, Guo C, Zhang X, Zhang S, Yin J, Xu SY, Belopolski I, Alidoust N, Sanchez DS, Bian G, Chang TR, Neupert T, Jeng HT, Jia S, Lin H, Hasan MZ. Mirror Protected Dirac Fermions on a Weyl Semimetal NbP Surface. PHYSICAL REVIEW LETTERS 2017; 119:196403. [PMID: 29219493 DOI: 10.1103/physrevlett.119.196403] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/20/2016] [Indexed: 06/07/2023]
Abstract
The first Weyl semimetal was recently discovered in the NbP class of compounds. Although the topology of these novel materials has been identified, the surface properties are not yet fully understood. By means of scanning tunneling spectroscopy, we find that NbP's (001) surface hosts a pair of Dirac cones protected by mirror symmetry. Through our high-resolution spectroscopic measurements, we resolve the quantum interference patterns arising from these novel Dirac fermions and reveal their electronic structure, including the linear dispersions. Our data, in agreement with our theoretical calculations, uncover further interesting features of the Weyl semimetal NbP's already exotic surface. Moreover, we discuss the similarities and distinctions between the Dirac fermions here and those in topological crystalline insulators in terms of symmetry protection and topology.
Collapse
|
39
|
Belopolski I, Yu P, Sanchez DS, Ishida Y, Chang TR, Zhang SS, Xu SY, Zheng H, Chang G, Bian G, Jeng HT, Kondo T, Lin H, Liu Z, Shin S, Hasan MZ. Signatures of a time-reversal symmetric Weyl semimetal with only four Weyl points. Nat Commun 2017; 8:942. [PMID: 29038436 PMCID: PMC5752680 DOI: 10.1038/s41467-017-00938-1] [Citation(s) in RCA: 68] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2017] [Accepted: 08/04/2017] [Indexed: 11/09/2022] Open
Abstract
Through intense research on Weyl semimetals during the past few years, we have come to appreciate that typical Weyl semimetals host many Weyl points. Nonetheless, the minimum nonzero number of Weyl points allowed in a time-reversal invariant Weyl semimetal is four. Realizing such a system is of fundamental interest and may simplify transport experiments. Recently, it was predicted that TaIrTe4 realizes a minimal Weyl semimetal. However, the Weyl points and Fermi arcs live entirely above the Fermi level, making them inaccessible to conventional angle-resolved photoemission spectroscopy (ARPES). Here, we use pump-probe ARPES to directly access the band structure above the Fermi level in TaIrTe4. We observe signatures of Weyl points and topological Fermi arcs. Combined with ab initio calculation, our results show that TaIrTe4 is a Weyl semimetal with the minimum number of four Weyl points. Our work provides a simpler platform for accessing exotic transport phenomena arising in Weyl semimetals.Weyl semimetals are interesting because they are characterized by topological invariants, but specific examples discovered to date tend to have complicated band structures with many Weyl points. Here, the authors show that TaIrTe4 has only four Weyl points, the minimal number required by time-reversal symmetry.
Collapse
|
40
|
Hsu WT, Lu LS, Wang D, Huang JK, Li MY, Chang TR, Chou YC, Juang ZY, Jeng HT, Li LJ, Chang WH. Evidence of indirect gap in monolayer WSe 2. Nat Commun 2017; 8:929. [PMID: 29030548 PMCID: PMC5640683 DOI: 10.1038/s41467-017-01012-6] [Citation(s) in RCA: 43] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2017] [Accepted: 08/11/2017] [Indexed: 11/18/2022] Open
Abstract
Monolayer transition metal dichalcogenides, such as MoS2 and WSe2, have been known as direct gap semiconductors and emerged as new optically active materials for novel device applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS2 and WSe2. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe2-MoS2 and MoSe2-WSe2 lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe2 is actually an indirect gap material, as manifested in the observed photoluminescence intensity-energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe2 exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.Monolayer transition metal dichalcogenides have so far been thought to be direct bandgap semiconductors. Here, the authors revisit this assumption and find that unstrained monolayer WSe2 is an indirect-gap material, as evidenced by the observed photoluminescence intensity-energy correlation.
Collapse
|
41
|
Kim H, Yoshida Y, Lee CC, Chang TR, Jeng HT, Lin H, Haga Y, Fisk Z, Hasegawa Y. Atomic-scale visualization of surface-assisted orbital order. SCIENCE ADVANCES 2017; 3:eaao0362. [PMID: 28948229 PMCID: PMC5609848 DOI: 10.1126/sciadv.aao0362] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/07/2017] [Accepted: 09/01/2017] [Indexed: 05/31/2023]
Abstract
Orbital-related physics attracts growing interest in condensed matter research, but direct real-space access of the orbital degree of freedom is challenging. We report a first, real-space, imaging of a surface-assisted orbital ordered structure on a cobalt-terminated surface of the well-studied heavy fermion compound CeCoIn5. Within small tip-sample distances, the cobalt atoms on a cleaved (001) surface take on dumbbell shapes alternatingly aligned in the [100] and [010] directions in scanning tunneling microscopy topographies. First-principles calculations reveal that this structure is a consequence of the staggered d xz -d yz orbital order triggered by enhanced on-site Coulomb interaction at the surface. This so far overlooked surface-assisted orbital ordering may prevail in transition metal oxides, heavy fermion superconductors, and other materials.
Collapse
|
42
|
Okada Y, Shiau SY, Chang TR, Chang G, Kobayashi M, Shimizu R, Jeng HT, Shiraki S, Kumigashira H, Bansil A, Lin H, Hitosugi T. Quasiparticle Interference on Cubic Perovskite Oxide Surfaces. PHYSICAL REVIEW LETTERS 2017; 119:086801. [PMID: 28952762 DOI: 10.1103/physrevlett.119.086801] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/25/2016] [Indexed: 06/07/2023]
Abstract
We report the observation of coherent surface states on cubic perovskite oxide SrVO_{3}(001) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a d_{xy}-derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO_{2} plane play a critical role in controlling the coherent surface state via modulating orbital state.
Collapse
|
43
|
Gui X, Chang TR, Kong T, Pan MT, Cava RJ, Xie W. Monoclinic 122-Type BaIr₂Ge₂ with a Channel Framework: A Structural Connection between Clathrate and Layered Compounds. MATERIALS 2017; 10:ma10070818. [PMID: 28773175 PMCID: PMC5551861 DOI: 10.3390/ma10070818] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/26/2017] [Revised: 07/08/2017] [Accepted: 07/10/2017] [Indexed: 11/16/2022]
Abstract
A new 122-type phase, monoclinic BaIr₂Ge₂ is successfully synthesized by arc melting; X-ray diffraction and scanning electron microscopy are used to purify the phase and determine its crystal structure. BaIr₂Ge₂ adopts a clathrate-like channel framework structure of the monoclinic BaRh₂Si₂-type, with space group P2₁/c. Structural comparisons of clathrate, ThCr₂Si₂, CaBe₂Ge₂, and BaRh₂Si2 structure types indicate that BaIr₂Ge₂ can be considered as an intermediate between clathrate and layered compounds. Magnetic measurements show it to be diamagnetic and non-superconducting down to 1.8 K. Different from many layered or clathrate compounds, monoclinic BaIr₂Ge₂ displays a metallic resistivity. Electronic structure calculations performed for BaIr₂Ge₂ support its observed structural stability and physical properties.
Collapse
|
44
|
Chang TR, Xu SY, Sanchez DS, Tsai WF, Huang SM, Chang G, Hsu CH, Bian G, Belopolski I, Yu ZM, Yang SA, Neupert T, Jeng HT, Lin H, Hasan MZ. Type-II Symmetry-Protected Topological Dirac Semimetals. PHYSICAL REVIEW LETTERS 2017; 119:026404. [PMID: 28753359 DOI: 10.1103/physrevlett.119.026404] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/20/2016] [Indexed: 06/07/2023]
Abstract
The recent proposal of the type-II Weyl semimetal state has attracted significant interest. In this Letter, we propose the concept of the three-dimensional type-II Dirac fermion and theoretically identify this new symmetry-protected topological state in the large family of transition-metal icosagenides, MA_{3} (M=V, Nb, Ta; A=Al, Ga, In). We show that the VAl_{3} family features a pair of strongly Lorentz-violating type-II Dirac nodes and that each Dirac node can be split into four type-II Weyl nodes with chiral charge ±1 via symmetry breaking. Furthermore, we predict that the Landau level spectrum arising from the type-II Dirac fermions in VAl_{3} is distinct from that of known Dirac or Weyl semimetals. We also demonstrate a topological phase transition from a type-II Dirac semimetal to a quadratic Weyl semimetal or a topological crystalline insulator via crystalline distortions.
Collapse
|
45
|
Xu SY, Alidoust N, Chang G, Lu H, Singh B, Belopolski I, Sanchez DS, Zhang X, Bian G, Zheng H, Husanu MA, Bian Y, Huang SM, Hsu CH, Chang TR, Jeng HT, Bansil A, Neupert T, Strocov VN, Lin H, Jia S, Hasan MZ. Discovery of Lorentz-violating type II Weyl fermions in LaAlGe. SCIENCE ADVANCES 2017; 3:e1603266. [PMID: 28630919 PMCID: PMC5457030 DOI: 10.1126/sciadv.1603266] [Citation(s) in RCA: 46] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2017] [Accepted: 04/07/2017] [Indexed: 05/17/2023]
Abstract
In quantum field theory, Weyl fermions are relativistic particles that travel at the speed of light and strictly obey the celebrated Lorentz symmetry. Their low-energy condensed matter analogs are Weyl semimetals, which are conductors whose electronic excitations mimic the Weyl fermion equation of motion. Although the traditional (type I) emergent Weyl fermions observed in TaAs still approximately respect Lorentz symmetry, recently, the so-called type II Weyl semimetal has been proposed, where the emergent Weyl quasiparticles break the Lorentz symmetry so strongly that they cannot be smoothly connected to Lorentz symmetric Weyl particles. Despite some evidence of nontrivial surface states, the direct observation of the type II bulk Weyl fermions remains elusive. We present the direct observation of the type II Weyl fermions in crystalline solid lanthanum aluminum germanide (LaAlGe) based on our photoemission data alone, without reliance on band structure calculations. Moreover, our systematic data agree with the theoretical calculations, providing further support on our experimental results.
Collapse
|
46
|
Chang G, Xu SY, Huang SM, Sanchez DS, Hsu CH, Bian G, Yu ZM, Belopolski I, Alidoust N, Zheng H, Chang TR, Jeng HT, Yang SA, Neupert T, Lin H, Hasan MZ. Nexus fermions in topological symmorphic crystalline metals. Sci Rep 2017; 7:1688. [PMID: 28490762 PMCID: PMC5431971 DOI: 10.1038/s41598-017-01523-8] [Citation(s) in RCA: 104] [Impact Index Per Article: 14.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2016] [Accepted: 02/17/2017] [Indexed: 11/09/2022] Open
Abstract
Topological metals and semimetals (TMs) have recently drawn significant interest. These materials give rise to condensed matter realizations of many important concepts in high-energy physics, leading to wide-ranging protected properties in transport and spectroscopic experiments. It has been well-established that the known TMs can be classified by the dimensionality of the topologically protected band degeneracies. While Weyl and Dirac semimetals feature zero-dimensional points, the band crossing of nodal-line semimetals forms a one-dimensional closed loop. In this paper, we identify a TM that goes beyond the above paradigms. It shows an exotic configuration of degeneracies without a well-defined dimensionality. Specifically, it consists of 0D nexus with triple-degeneracy that interconnects 1D lines with double-degeneracy. We show that, because of the novel form of band crossing, the new TM cannot be described by the established results that characterize the topology of the Dirac and Weyl nodes. Moreover, triply-degenerate nodes realize emergent fermionic quasiparticles not present in relativistic quantum field theory. We present materials candidates. Our results open the door for realizing new topological phenomena and fermions including transport anomalies and spectroscopic responses in metallic crystals with nontrivial topology beyond the Weyl/Dirac paradigm.
Collapse
|
47
|
Trainer DJ, Putilov AV, Di Giorgio C, Saari T, Wang B, Wolak M, Chandrasena RU, Lane C, Chang TR, Jeng HT, Lin H, Kronast F, Gray AX, Xi X, Nieminen J, Bansil A, Iavarone M. Erratum: Inter-Layer Coupling Induced Valence Band Edge Shift in Mono- to Few-Layer MoS 2. Sci Rep 2017; 7:42619. [PMID: 28290442 PMCID: PMC5349580 DOI: 10.1038/srep42619] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
|
48
|
Sankar R, Rao GN, Muthuselvam IP, Chang TR, Jeng HT, Murugan GS, Lee WL, Chou FC. Anisotropic superconducting property studies of single crystal PbTaSe 2. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2017; 29:095601. [PMID: 28098075 DOI: 10.1088/1361-648x/aa4edb] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
The anisotropic superconducting properties of PbTaSe2 single crystal is reported. Superconductivity with T c = 3.83 ± 0.02 K has been characterized fully with electrical resistivity ρ(T), magnetic susceptibility χ(T), and specific heat C p (T) measurements using single crystal samples. The superconductivity is type-II with lower critical field H c1 and upper critical field H c2 of 65 and 450 Oe (H⊥ to the ab-plane), 140 and 1500 Oe (H|| to the ab-plane), respectively. These results indicate that the superconductivity of PbTaSe2 is anisotropic. The superconducting anisotropy, electron-phonon coupling λ ep, superconducting energy gap Δ0, and the specific heat jump ΔC/γT c at T c confirms that PbTaSe2 can be categorized as a bulk superconductor.
Collapse
|
49
|
Trainer DJ, Putilov AV, Di Giorgio C, Saari T, Wang B, Wolak M, Chandrasena RU, Lane C, Chang TR, Jeng HT, Lin H, Kronast F, Gray AX, Xi XX, Nieminen J, Bansil A, Iavarone M. Inter-Layer Coupling Induced Valence Band Edge Shift in Mono- to Few-Layer MoS 2. Sci Rep 2017; 7:40559. [PMID: 28084465 PMCID: PMC5233980 DOI: 10.1038/srep40559] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2016] [Accepted: 12/08/2016] [Indexed: 12/04/2022] Open
Abstract
Recent progress in the synthesis of monolayer MoS2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here, we report a study of highly crystalline islands of MoS2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green's function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS2.
Collapse
|
50
|
Yu P, Lin J, Sun L, Le QL, Yu X, Gao G, Hsu CH, Wu D, Chang TR, Zeng Q, Liu F, Wang QJ, Jeng HT, Lin H, Trampert A, Shen Z, Suenaga K, Liu Z. Metal-Semiconductor Phase-Transition in WSe 2(1-x) Te 2x Monolayer. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2017; 29:1603991. [PMID: 27874223 DOI: 10.1002/adma.201603991] [Citation(s) in RCA: 60] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2016] [Revised: 09/12/2016] [Indexed: 06/06/2023]
Abstract
A metal-semiconductor phase transition in a ternary transition metal dichalcogenide (TMD) monolayer is achieved by alloying Te into WSe2 (WSe2(1-x) Te2x , where x = 0%-100%). The optical bandgaps of the WSe2(1-x) Te2x monolayer can be tuned from 1.67 to 1.44 eV (2H semiconductor) and drops to 0 eV (1Td metal), which opens up an exciting opportunity in functional electronic/optoelectronic devices.
Collapse
|