I05-ARPES
|
H. F.
Yang
,
L. X.
Yang
,
Z. K.
Liu
,
Y.
Sun
,
C.
Chen
,
H.
Peng
,
M.
Schmidt
,
D.
Prabhakaran
,
B. A.
Bernevig
,
C.
Felser
,
B. H.
Yan
,
Y. L.
Chen
Diamond Proposal Number(s):
[19310, 18005]
Open Access
Abstract: Surface Fermi arcs (SFAs), the unique open Fermi-surfaces (FSs) discovered recently in topological Weyl semimetals (TWSs), are unlike closed FSs in conventional materials and can give rise to many exotic phenomena, such as anomalous SFA-mediated quantum oscillations, chiral magnetic effects, three-dimensional quantum Hall effect, non-local voltage generation and anomalous electromagnetic wave transmission. Here, by using in-situ surface decoration, we demonstrate successful manipulation of the shape, size and even the connections of SFAs in a model TWS, NbAs, and observe their evolution that leads to an unusual topological Lifshitz transition not caused by the change of the carrier concentration. The phase transition teleports the SFAs between different parts of the surface Brillouin zone. Despite the dramatic surface evolution, the existence of SFAs is robust and each SFA remains tied to a pair of Weyl points of opposite chirality, as dictated by the bulk topology.
|
Aug 2019
|
|
I05-ARPES
|
Niels B. M.
Schröter
,
Ding
Pei
,
Maia G.
Vergniory
,
Yan
Sun
,
Kaustuv
Manna
,
Fernando
De Juan
,
Jonas A.
Krieger
,
Vicky
Süss
,
Marcus
Schmidt
,
Pavel
Dudin
,
Barry
Bradlyn
,
Timur K.
Kim
,
Thorsten
Schmitt
,
Cephise
Cacho
,
Claudia
Felser
,
Vladimir N.
Strocov
,
Yulin
Chen
Diamond Proposal Number(s):
[19883, 21400]
Abstract: Topological semimetals in crystals with a chiral structure (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, including fermionic excitations with large topological charge1, long Fermi arc surface states2,3, unusual magnetotransport4 and lattice dynamics5, as well as a quantized response to circularly polarized light6. So far, all experimentally confirmed topological semimetals exist in crystals that contain mirror operations, meaning that these properties do not appear. Here, we show that AlPt is a structurally chiral topological semimetal that hosts new four-fold and six-fold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. These multifold fermions are located at high symmetry points and have Chern numbers larger than those in Weyl semimetals, thus resulting in multiple Fermi arcs that span the full diagonal of the surface Brillouin zone. By imaging these long Fermi arcs, we experimentally determine the magnitude and sign of their Chern number, allowing us to relate their dispersion to the handedness of their host crystal.
|
May 2019
|
|
I05-ARPES
|
Cheng
Chen
,
Meixiao
Wang
,
Jinxiong
Wu
,
Huixia
Fu
,
Haifeng
Yang
,
Zhen
Tian
,
Teng
Tu
,
Han
Peng
,
Yan
Sun
,
Xiang
Xu
,
Juan
Jiang
,
Niels B. M.
Schroeter
,
Yiwei
Li
,
Ding
Pei
,
Shuai
Liu
,
Sandy A.
Ekahana
,
Hongtao
Yuan
,
Jiamin
Xue
,
Gang
Li
,
Jinfeng
Jia
,
Zhongkai
Liu
,
Binghai
Yan
,
Hailin
Peng
,
Yulin
Chen
Diamond Proposal Number(s):
[18005]
Open Access
Abstract: Semiconductors are essential materials that affect our everyday life in the modern world. Two-dimensional semiconductors with high mobility and moderate bandgap are particularly attractive today because of their potential application in fast, low-power, and ultrasmall/thin electronic devices. We investigate the electronic structures of a new layered air-stable oxide semiconductor, Bi2O2Se, with ultrahigh mobility (~2.8 × 105 cm2/V⋅s at 2.0 K) and moderate bandgap (~0.8 eV). Combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we mapped out the complete band structures of Bi2O2Se with key parameters (for example, effective mass, Fermi velocity, and bandgap). The unusual spatial uniformity of the bandgap without undesired in-gap states on the sample surface with up to ~50% defects makes Bi2O2Se an ideal semiconductor for future electronic applications. In addition, the structural compatibility between Bi2O2Se and interesting perovskite oxides (for example, cuprate high–transition temperature superconductors and commonly used substrate material SrTiO3) further makes heterostructures between Bi2O2Se and these oxides possible platforms for realizing novel physical phenomena, such as topological superconductivity, Josephson junction field-effect transistor, new superconducting optoelectronics, and novel lasers.
|
Sep 2018
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[15364]
Abstract: Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we have mapped out the reconstructed electronic structure in the commensurate charge-density-wave (CDW) state of quasi-two-dimensional transition metal dichalcogenide
2
H
−
TaSe
2
. The observation of the fine structure near Brillouin zone (BZ) center supplements the picture of Fermi surface folding in the
3
×
3
CDW state. In addition to the anisotropic CDW band gaps that energetically stabilize the system at the Fermi level in the first-order lock-in transition, we found band reconstruction at high binding energy, which can be well explained by the hybridization between main bands (MBs) and folded bands (FBs). Furthermore, in contrast to the perfectly nested quasi-one-dimensional system, triple-nesting-vector-induced CDW FBs increase the degeneracy of the band crossing and thus further enlarge the magnitude of band gap at certain momentum-energy positions. The visualization and modeling of CDW gaps in momentum-energy space reconciles the long-lasting controversy on the gap magnitude and suggests a weak-coupling Peierls physics in this system.
|
Mar 2018
|
|
I05-ARPES
|
Yiwei
Li
,
Yunyouyou
Xia
,
Sandy Adhitia
Ekahana
,
Nitesh
Kumar
,
Juan
Jiang
,
Lexian
Yang
,
Cheng
Chen
,
Chaoxing
Liu
,
Binghai
Yan
,
Claudia
Felser
,
Gang
Li
,
Zhongkai
Liu
,
Yulin
Chen
Diamond Proposal Number(s):
[15364]
Abstract: Group VIII transition-metal dichalcogenides have recently been proposed to host type-II Dirac fermions. They are Lorentz-violating quasiparticles marked by a strongly tilted conic dispersion along a certain momentum direction and therefore have no analogs in the standard model. Using high-resolution angle-resolved photoemission spectroscopy, we systematically studied the electronic structure of PtSe2 in the full three-dimensional Brillouin zone. As predicted, a pair of type-II Dirac crossings is experimentally confirmed along the kz axis. Interestingly, we observed conic surface states around time-reversal-invariant momenta
¯¯¯
Γ and
¯¯¯¯
M points. The signatures of nontrivial topology are confirmed by the first-principles calculation, which shows an intricate parity inversion of bulk states. Our discoveries not only contribute to a better understanding of topological band structure in PtSe2 but also help further explore the exotic properties, as well as potential application, of group VIII transition-metal dichalcogenides.
|
Dec 2017
|
|
I05-ARPES
|
Qihang
Zhang
,
Zhongkai
Liu
,
Yan
Sun
,
Haifeng
Yang
,
Juan
Jiang
,
Sung-kwan
Mo
,
Zahid
Hussain
,
Xiaofeng
Qian
,
Liang
Fu
,
Shuhua
Yao
,
Minghui
Lu
,
Claudia
Felser
,
Binghai
Yan
,
Yulin
Chen
,
Lexian
Yang
Diamond Proposal Number(s):
[16846]
Abstract: Using high resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structure of Td-WTe2, which has attracted substantial research attention due to its diverse and fascinating properties, especially the predicted type-II topological Weyl semimetal (TWS) phase. The observed significant lattice contraction and the fact that our ARPES measurements are well reproduced by our ab initio calculations under reduced lattice constants support the theoretical prediction of a type-II TWS phase in Td-WTe2 at temperatures below 10 K. We also investigate the evolution of the electronic structure of Td-WTe2 and realize two-stage Lifshitz transitions induced by temperature regulation and surface modification, respectively. Our results not only shed light on the understanding of the electronic structure of Td-WTe2, but also provide a promising method to manipulate the electronic structures and physical properties of the type-II TWS Td-XTe2.
|
Oct 2017
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[12799]
Open Access
Abstract: After the discovery of Dirac fermions in graphene, it has become a natural question to ask whether it
is possible to realize Dirac fermions in other two-dimensional (2D) materials as well. In this work, we
report the discovery of multiple Dirac-like electronic bands in ultrathin Ge films grown on Au(1 1 1)
by angle-resolved photoelectron spectroscopy. By tuning the thickness of the films, we are able to
observe the evolution of their electronic structure when passing through the monolayer limit. Our
discovery may signify the synthesis of germanene, a 2D honeycomb structure made of Ge, which is a
promising platform for exploring exotic topological phenomena and enabling potential applications.
|
Jul 2017
|
|
I05-ARPES
|
Sandy Adhitia
Ekahana
,
Shu-chun
Wu
,
Juan
Jiang
,
Kenjiro
Okawa
,
Dharmalingam
Prabhakaran
,
Chan-cuk
Hwang
,
Sung-kwan
Mo
,
Takao
Sasagawa
,
Claudia
Felser
,
Binghai
Yan
,
Zhongkai
Liu
,
Yulin
Chen
Open Access
Abstract: Topological Nodal Semimetal (TNS), characterised by its touching conduction and valence bands, is a newly discovered state of quantum matter which exhibits various exotic physical phenomena. Recently, a new type of TNS called Topological Nodal Line Semimetal (TNLS) is predicted where its conduction and valence band form a degenerate 1D line which is further protected by its crystal symmetry. In this work, we systematically investigated the bulk and surface electronic structure of the non-symmorphic, TNLS in InBi with strong spin-orbit coupling by using Angle Resolved Photoemission Spectroscopy (ARPES). By tracking the crossing points of the bulk bands at the Brillouin zone boundary, we discovered the nodal-line feature along the XRX line, in agreement with the ab-initio calculations and confirmed it to be a new compound in the TNLS family. Our discovery provides new material platform for the study of these exotic topological quantum phases and paves the way for possible future applications.
|
Jun 2017
|
|
I05-ARPES
|
Jinxiong
Wu
,
Hongtao
Yuan
,
Mengmeng
Meng
,
Cheng
Chen
,
Yan
Sun
,
Zhuoyu
Chen
,
Wenhui
Dang
,
Congwei
Tan
,
Yujing
Liu
,
Jianbo
Yin
,
Yubing
Zhou
,
Shaoyun
Huang
,
H. Q.
Xu
,
Yi
Cui
,
Harold Y.
Hwang
,
Zhongfan
Liu
,
Yulin
Chen
,
Binghai
Yan
,
Hailin
Peng
Diamond Proposal Number(s):
[14132]
Abstract: High-mobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new two-dimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal goal of fundamental research1, 2, 3, 4, 5, 6, 7, 8, 9. However, air-stable ultrathin semiconducting materials with superior performances remain elusive at present10. Here, we report ultrathin films of non-encapsulated layered Bi2O2Se, grown by chemical vapour deposition, which demonstrate excellent air stability and high-mobility semiconducting behaviour. We observe bandgap values of ∼0.8 eV, which are strongly dependent on the film thickness due to quantum-confinement effects. An ultrahigh Hall mobility value of >20,000 cm2 V−1 s−1 is measured in as-grown Bi2O2Se nanoflakes at low temperatures. This value is comparable to what is observed in graphene grown by chemical vapour deposition11 and at the LaAlO3–SrTiO3 interface12, making the detection of Shubnikov–de Haas quantum oscillations possible. Top-gated field-effect transistors based on Bi2O2Se crystals down to the bilayer limit exhibit high Hall mobility values (up to 450 cm2 V−1 s−1), large current on/off ratios (>106) and near-ideal subthreshold swing values (∼65 mV dec–1) at room temperature. Our results make Bi2O2Se a promising candidate for future high-speed and low-power electronic applications.
|
Apr 2017
|
|
|
|
Hongtao
Yuan
,
Zhongkai
Liu
,
Gang
Xu
,
Bo
Zhou
,
Sanfeng
Wu
,
Dumitru
Dumcenco
,
Kai
Yan
,
Yi
Zhang
,
Sung-kwan
Mo
,
Pavel
Dudin
,
Victor
Kandyba
,
Mikhail
Yablonskikh
,
Alexei
Barinov
,
Zhixun
Shen
,
Shoucheng
Zhang
,
Yingsheng
Huang
,
Xiaodong
Xu
,
Zahid
Hussain
,
Harold Y.
Hwang
,
Yi
Cui
,
Yulin
Chen
Abstract: Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.
|
Aug 2016
|
|
