I05-ARPES
|
Xian P.
Yang
,
Harrison
Labollita
,
Zi-Jia
Cheng
,
Hari
Bhandari
,
Tyler A.
Cochran
,
Jia-Xin
Yin
,
Md. Shafayat
Hossain
,
Ilya
Belopolski
,
Qi
Zhang
,
Yuxiao
Jiang
,
Nana
Shumiya
,
Daniel
Multer
,
Maksim
Liskevich
,
Dmitry A.
Usanov
,
Yanliu
Dang
,
Vladimir N.
Strocov
,
Albert V.
Davydov
,
Nirmal J.
Ghimire
,
Antia S.
Botana
,
M. Zahid
Hasan
Diamond Proposal Number(s):
[29230]
Abstract: Layered transition metal dichalcogenides have a rich phase diagram and they feature two-dimensionality in numerous physical properties.
Co
1
/
3
NbS
2
is one of the newest members of this family where Co atoms are intercalated into the van der Waals gaps between
NbS
2
layers. We study the three-dimensional electronic band structure of
Co
1
/
3
NbS
2
using both surface and bulk sensitive angle-resolved photoemission spectroscopy. We show that the electronic bands do not fit into the rigid band shift picture after the Co intercalation. Instead,
Co
1
/
3
NbS
2
displays a different orbital character near the Fermi level compared to the pristine
NbS
2
compound and has a clear band dispersion in the
k
z
direction despite its layered structure. Our photoemission study demonstrates the out-of-plane electronic correlations introduced by the Co intercalation, thus offering a different perspective on this compound. Finally, we propose how Fermi level tuning could lead to exotic phases such as spin density wave instability.
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Mar 2022
|
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I05-ARPES
|
Ilya
Belopolski
,
Tyler A.
Cochran
,
Xiaoxiong
Liu
,
Zi-Jia
Cheng
,
Xian P.
Yang
,
Zurab
Guguchia
,
Stepan S.
Tsirkin
,
Jia-Xin
Yin
,
Praveen
Vir
,
Gohil S.
Thakur
,
Songtian S.
Zhang
,
Junyi
Zhang
,
Konstantine
Kaznatcheev
,
Guangming
Cheng
,
Guoqing
Chang
,
Daniel
Multer
,
Nana
Shumiya
,
Maksim
Litskevich
,
Elio
Vescovo
,
Timur K.
Kim
,
Cephise
Cacho
,
Nan
Yao
,
Claudia
Felser
,
Titus
Neupert
,
M. Zahid
Hasan
Diamond Proposal Number(s):
[17924, 19313]
Abstract: The manipulation of topological states in quantum matter is an essential pursuit of fundamental physics and next-generation quantum technology. Here we report the magnetic manipulation of Weyl fermions in the kagome spin-orbit semimetal
Co
3
Sn
2
S
2
, observed by high-resolution photoemission spectroscopy. We demonstrate the exchange collapse of spin-orbit-gapped ferromagnetic Weyl loops into paramagnetic Dirac loops under suppression of the magnetic order. We further observe that topological Fermi arcs disappear in the paramagnetic phase, suggesting the annihilation of exchange-split Weyl points. Our findings indicate that magnetic exchange collapse naturally drives Weyl fermion annihilation, opening new opportunities for engineering topology under correlated order parameters.
|
Dec 2021
|
|
I05-ARPES
|
Jia-Xin
Yin
,
Wenlong
Ma
,
Tyler A.
Cochran
,
Xitong
Xu
,
Songtian S.
Zhang
,
Hung-Ju
Tien
,
Nana
Shumiya
,
Guangming
Cheng
,
Kun
Jiang
,
Biao
Lian
,
Zhida
Song
,
Guoqing
Chang
,
Ilya
Belopolski
,
Daniel
Multer
,
Maksim
Litskevich
,
Zi-Jia
Cheng
,
Xian P.
Yang
,
Bianca
Swidler
,
Huibin
Zhou
,
Hsin
Lin
,
Titus
Neupert
,
Ziqiang
Wang
,
Nan
Yao
,
Tay-Rong
Chang
,
Shuang
Jia
,
M.
Zahid Hasan
Diamond Proposal Number(s):
[22332]
Abstract: The quantum-level interplay between geometry, topology and correlation is at the forefront of fundamental physics. Kagome magnets are predicted to support intrinsic Chern quantum phases owing to their unusual lattice geometry and breaking of time-reversal symmetry. However, quantum materials hosting ideal spin–orbit-coupled kagome lattices with strong out-of-plane magnetization are lacking. Here, using scanning tunnelling microscopy, we identify a new topological kagome magnet, TbMn6Sn6, that is close to satisfying these criteria. We visualize its effectively defect-free, purely manganese-based ferromagnetic kagome lattice with atomic resolution. Remarkably, its electronic state shows distinct Landau quantization on application of a magnetic field, and the quantized Landau fan structure features spin-polarized Dirac dispersion with a large Chern gap. We further demonstrate the bulk–boundary correspondence between the Chern gap and the topological edge state, as well as the Berry curvature field correspondence of Chern gapped Dirac fermions. Our results point to the realization of a quantum-limit Chern phase in TbMn6Sn6, and may enable the observation of topological quantum phenomena in the RMn6Sn6 (where R is a rare earth element) family with a variety of magnetic structures. Our visualization of the magnetic bulk–boundary–Berry correspondence covering real space and momentum space demonstrates a proof-of-principle method for revealing topological magnets.
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Jul 2020
|
|
I05-ARPES
|
Open Access
Abstract: Topological phases of matter have established a new paradigm in physics, bringing quantum phenomena to the macroscopic scale and hosting exotic emergent quasiparticles. In this thesis, I demonstrate with my collaborators the first Weyl semimetal, TaAs, using angle-resolved photoemission spectroscopy (ARPES), directly observing its emergent Weyl fermions and topological Fermi arc surface states [Science 349, 6248 (2015); Physical Review Letters 116, 066802 (2016)]. Next, I consider structurally chiral crystals, which I argue are guaranteed to host exotic chiral fermions leading to giant topological Fermi arcs. I study the chiral crystals RhSi and CoSi and I discover high-degeneracy chiral fermions with wide topological energy window, maximal separation in momentum space and giant Fermi arcs [Nature 567, 500 (2019); Nature Materials 17, 978 (2018)]. I establish a natural relationship between structural and topological chirality, producing a robust topological state which we predict supports a four-unit quantized photogalvanic effect [Physical Review Letters 119, 206401 (2017)]. Next, I discuss the first quantum topological superlattice [Science Advances 3, e1501692 (2017)]. I study multilayer heterostructures of alternating topological and trivial insulators. The Dirac cones at each interface tunnel across layers, realizing a new kind of emergent superlattice, where the interfaces act as lattice sites and the Dirac cones act as atomic orbitals. Adjusting the stacking pattern offers unprecedented control of individual hopping parameters in the atomic chain. I realize a novel topological phase transition and I predict that this platform may allow particle-hole symmetry without superconductivity. Lastly, I present the discovery of a room-temperature topological magnet [arXiv:1712.09992]. I study crystals of Co2MnGa and I observe a topological invariant supported by the material's intrinsic magnetic order [Physical Review Letters 119, 156401 (2017)]. In particular I observe topological Weyl lines and drumhead surface states by ARPES and, through a scaling analysis of the anomalous Hall transport response, I find that the large anomalous Hall effect in Co2MnGa arises from the Weyl lines. I hope that my discovery of Co2MnGa establishes topological magnetism as a new research frontier in condensed matter physics.
|
Jun 2019
|
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I05-ARPES
|
Su-Yang
Xu
,
Nasser
Alidoust
,
Guoqing
Chang
,
Hong
Lu
,
Bahadur
Singh
,
Ilya
Belopolski
,
Daniel S.
Sanchez
,
Xiao
Zhang
,
Guang
Bian
,
Hao
Zheng
,
Marious-Adrian
Husanu
,
Yi
Bian
,
Shin-Ming
Huang
,
Chuang-Han
Hsu
,
Tay-Rong
Chang
,
Horng-Tay
Jeng
,
Arun
Bansil
,
Titus
Neupert
,
Vladimir N.
Strocov
,
Hsin
Lin
,
Shuang
Jia
,
M. Zahid
Hasan
Open Access
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.
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Jun 2017
|
|
I05-ARPES
|
Ilya
Belopolski
,
Su-Yang
Xu
,
Nikesh
Koirala
,
Chang
Liu
,
Guang
Bian
,
Vladimir N.
Strocov
,
Guoqing
Chang
,
Madhab
Neupane
,
Nasser
Alidoust
,
Daniel
Sanchez
,
Hao
Zheng
,
Matthew
Brahlek
,
Victor
Rogalev
,
Timur
Kim
,
Nicholas C.
Plumb
,
Chaoyu
Chen
,
François
Bertran
,
Patrick
Le Fèvre
,
Amina
Taleb-Ibrahimi
,
Maria-Carmen
Asensio
,
Ming
Shi
,
Hsin
Lin
,
Moritz
Hoesch
,
Seongshik
Oh
,
M. Zahid
Hasan
Diamond Proposal Number(s):
[11742]
Open Access
Abstract: Engineered lattices in condensed matter physics, such as cold-atom optical lattices or photonic crystals, can have properties that are fundamentally different from those of naturally occurring electronic crystals. We report a novel type of artificial quantum matter lattice. Our lattice is a multilayer heterostructure built from alternating thin films of topological and trivial insulators. Each interface within the heterostructure hosts a set of topologically protected interface states, and by making the layers sufficiently thin, we demonstrate for the first time a hybridization of interface states across layers. In this way, our heterostructure forms an emergent atomic chain, where the interfaces act as lattice sites and the interface states act as atomic orbitals, as seen from our measurements by angle-resolved photoemission spectroscopy. By changing the composition of the heterostructure, we can directly control hopping between lattice sites. We realize a topological and a trivial phase in our superlattice band structure. We argue that the superlattice may be characterized in a significant way by a one-dimensional topological invariant, closely related to the invariant of the Su-Schrieffer-Heeger model. Our topological insulator heterostructure demonstrates a novel experimental platform where we can engineer band structures by directly controlling how electrons hop between lattice sites.
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Mar 2017
|
|
I05-ARPES
|
Ilya
Belopolski
,
Su-Yang
Xu
,
Yukiaki
Ishida
,
Xingchen
Pan
,
Peng
Yu
,
Daniel S.
Sanchez
,
Hao
Zheng
,
Madhab
Neupane
,
Nasser
Alidoust
,
Guoqing
Chang
,
Tay-Rong
Chang
,
Yun
Wu
,
Guang
Bian
,
Shin-Ming
Huang
,
Chi-Cheng
Lee
,
Daixiang
Mou
,
Lunan
Huang
,
You
Song
,
Baigeng
Wang
,
Guanghou
Wang
,
Yao-Wen
Yeh
,
Nan
Yao
,
Julien E.
Rault
,
Patrick
Le Fèvre
,
François
Bertran
,
Horng-Tay
Jeng
,
Takeshi
Kondo
,
Adam
Kaminski
,
Hsin
Lin
,
Zheng
Liu
,
Fengqi
Song
,
Shik
Shin
,
M. Zahid
Hasan
Diamond Proposal Number(s):
[13653]
Abstract: It has recently been proposed that electronic band structures in crystals can give rise to a previously overlooked type of Weyl fermion, which violates Lorentz invariance and, consequently, is forbidden in particle physics. It was further predicted that MoxW1−xTe2 may realize such a type-II Weyl fermion. Here, we first show theoretically that it is crucial to access the band structure above the Fermi level ɛF to show a Weyl semimetal in MoxW1−xTe2. Then, we study MoxW1−xTe2 by pump-probe ARPES and we directly access the band structure >0.2 eV above ɛF in experiment. By comparing our results with ab initio calculations, we conclude that we directly observe the surface state containing the topological Fermi arc. We propose that a future study of MoxW1−xTe2 by pump-probe ARPES may directly pinpoint the Fermi arc. Our work sets the stage for the experimental discovery of the first type-II Weyl semimetal in MoxW1−xTe2.
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Aug 2016
|
|
I05-ARPES
|
Cheng-Long
Zhang
,
Su-Yang
Xu
,
Ilya
Belopolski
,
Zhujun
Yuan
,
Ziquan
Lin
,
Bingbing
Tong
,
Guang
Bian
,
Nasser
Alidoust
,
Chi-Cheng
Lee
,
Shin-Ming
Huang
,
Tay-Rong
Chang
,
Guoqing
Chang
,
Chuang-Han
Hsu
,
Horng-Tay
Jeng
,
Madhab
Neupane
,
Daniel
Sanchez
,
Hao
Zheng
,
Junfeng
Wang
,
Hsin
Lin
,
Chi
Zhang
,
Hai-Zhou
Lu
,
Shun-Qing
Shen
,
Titus
Neupert
,
M.
Zahid Hasan
,
Shuang
Jia
Open Access
Abstract: Weyl semimetals provide the realization of Weyl fermions in solid-state physics. Among all the physical phenomena that are enabled by Weyl semimetals, the chiral anomaly is the most unusual one. Here, we report signatures of the chiral anomaly in the magneto-transport measurements on the first Weyl semimetal TaAs. We show negative magnetoresistance under parallel electric and magnetic fields, that is, unlike most metals whose resistivity increases under an external magnetic field, we observe that our high mobility TaAs samples become more conductive as a magnetic field is applied along the direction of the current for certain ranges of the field strength. We present systematically detailed data and careful analyses, which allow us to exclude other possible origins of the observed negative magnetoresistance. Our transport data, corroborated by photoemission measurements, first-principles calculations and theoretical analyses, collectively demonstrate signatures of the Weyl fermion chiral anomaly in the magneto-transport of TaAs.
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Feb 2016
|
|
I05-ARPES
|
S. Y.
Xu
,
I.
Belopolski
,
D.
Sanchez
,
C.
Zhang
,
G.
Chang
,
C.
Guo
,
G.
Bian
,
Z.
Yuan
,
H.
Lu
,
T. R
Chang
,
P. P
Shibayev
,
Mykhaylo
Prokopovych
,
N.
Alidoust
,
H.
Zheng
,
C. C
Lee
,
S. M.
Huang
,
R.
Sankar
,
F.
Chou
,
C. H.
Hsu
,
H. T
Jeng
,
A.
Bansil
,
T.
Neupert
,
V. N.
Strocov
,
H.
Lin
,
S.
Jia
,
M. Z.
Hasan
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.
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Nov 2015
|
|
I05-ARPES
|
S. Y.
Xu
,
Ilya
Belopolski
,
N.
Alidoust
,
M.
Neupane
,
G.
Bian
,
C.
Zhang
,
R.
Sankar
,
G.
Chang
,
Z.
Yuan
,
C. C.
Lee
,
S. M.
Huang
,
H.
Zheng
,
J.
Ma
,
D. S.
Sanchez
,
B.
Wang
,
A.
Bansil
,
F.
Chou
,
Pavel
Shibayev
,
H.
Lin
,
M. Zahid
Hasan
Diamond Proposal Number(s):
[10074]
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.
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Aug 2015
|
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