I05ARPES

Suyang
Xu
,
Nasser
Alidoust
,
Guoqing
Chang
,
Hong
Lu
,
Bahadur
Singh
,
Ilya
Belopolski
,
Daniel S.
Sanchez
,
Xiao
Zhang
,
Guang
Bian
,
Hao
Zheng
,
Mariousadrian
Husanu
,
Yi
Bian
,
Shinming
Huang
,
Chuanghan
Hsu
,
Tayrong
Chang
,
Horngtay
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 lowenergy 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 socalled 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.

Jun 2017


I05ARPES

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.

Aug 2015


I05ARPES

Suyang
Xu
,
Nasser
Alidoust
,
Ilya
Belopolski
,
Zhujun
Yuan
,
Guang
Bian
,
Tayrong
Chang
,
Hao
Zheng
,
Vladimir N.
Strocov
,
Daniel
Sanchez
,
Guoqing
Chang
,
Chenglong
Zhang
,
Daixiang
Mou
,
Yun
Wu
,
Lunan
Huang
,
Chicheng
Lee
,
Shinming
Huang
,
Baokai
Wang
,
Arun
Bansil
,
Horngtay
Jeng
,
Titus
Neupert
,
Adam
Kaminski
,
Hsin
Lin
,
Shuang
Jia
,
M.
Zahid Hasan
Diamond Proposal Number(s):
[10074]
Abstract: Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. Whereas Dirac fermions have been known for decades, the latter two have not been observed as any fundamental particle in highenergy physics, and have emerged as a muchsoughtout treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose lowenergy electronic excitations behave as Weyl fermions. It has received worldwide interest and is believed to open the next era of condensed matter physics after graphene and threedimensional topological insulators. However, experimental research has been held back because Weyl semimetals are extremely rare in nature. Here, we present the experimental discovery of the Weyl semimetal state in an inversionsymmetrybreaking singlecrystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft Xray 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 band structure calculations, identify the Weyl semimetal state in NbAs, which provides a real platform to test the potential of Weyltronics.

Aug 2015


I05ARPES

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 angleresolved 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 highdegeneracy 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 fourunit 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 particlehole symmetry without superconductivity. Lastly, I present the discovery of a roomtemperature 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


I05ARPES
I19Small Molecule Single Crystal Diffraction

Diamond Proposal Number(s):
[8776, 11039, 13797, 17065]
Abstract: The charge density wave (CDW) in ZrTe3 is quenched in samples with a small amount of Te isoelectronically substituted by Se. Using angleresolved photoemission spectroscopy we observe subtle changes in the electronic band dispersions and Fermi surfaces upon Se substitution. The scattering rates are substantially increased, in particular for the large threedimensional Fermi surface sheet. The quasionedimensional band is unaffected by the substitution and still shows a gap at low temperature, which starts to open from room temperature. Longrange order is, however, absent in the electronic states as in the periodic lattice distortion. The competition between superconductivity and the CDW is thus linked to the suppression of longrange order of the CDW.

Jan 2019


I05ARPES

Diamond Proposal Number(s):
[13438, 16262, 18705]
Abstract: We investigate the electronic structure of a twodimensional electron gas created at the surface of the multivalley semimetal 1T−PtSe2. Using angleresolved photoemission and firstprinciplesbased surface spacecharge calculations, we show how the induced quantum well subband states form multiple Fermi surfaces, which exhibit highly anisotropic Rashbalike spin splittings. We further show how the presence of both electronlike and holelike bulk carriers causes the nearsurface band bending potential to develop an unusual nonmonotonic form, with spatially segregated electron accumulation and hole accumulation regions, which in turn amplifies the induced spin splitting. Our results thus demonstrate the novel environment that semimetals provide for tailoring electrostatically induced potential profiles and their corresponding quantum subband states.

Jan 2019


I05ARPES

Diamond Proposal Number(s):
[11543]
Open Access
Abstract: Electronically driven nematic order is often considered as an essential ingredient of hightemperature superconductivity. Its elusive nature in ironbased superconductors resulted in a controversy not only as regards its origin but also as to the degree of its influence on the electronic structure even in the simplest representative material FeSe. Here we utilized angleresolved photoemission spectroscopy and density functional theory calculations to study the influence of the nematic order on the electronic structure of FeSe and determine its exact energy and momentum scales. Our results strongly suggest that the nematicity in FeSe is electronically driven, we resolve the recent controversy and provide the necessary quantitative experimental basis for a successful theory of superconductivity in ironbased materials which takes into account both, spinorbit interaction and electronic nematicity.

Nov 2016


I05ARPES

Diamond Proposal Number(s):
[12153, 15663]
Open Access
Abstract: We report high resolution ARPES measurements of detwinned FeSe single crystals. The application of a mechanical strain is used to promote the volume fraction of one of the orthorhombic domains in the sample, which we estimate to be 80% detwinned. While the full structure of the electron pockets consisting of two crossed ellipses may be observed in the tetragonal phase at temperatures above 90 K, we find that remarkably, only one peanutshaped electron pocket oriented along the longer <i>a</i> axis contributes to the ARPES measurement at low temperatures in the nematic phase, with the expected pocket along <i>b</i> being not observed. Thus the low temperature Fermi surface of FeSe as experimentally determined by ARPES consists of one elliptical hole pocket and one orthogonallyoriented peanutshaped electron pocket. Our measurements clarify the longstanding controversies over the interpretation of ARPES measurements of FeSe.

Sep 2017


I05ARPES

Q.
Yao
,
D.
Kaczorowski
,
P.
Swatek
,
D.
Gnida
,
C. H. P.
Wen
,
X. H.
Niu
,
R.
Peng
,
H. C.
Xu
,
P.
Dudin
,
S.
Kirchner
,
Q. Y.
Chen
,
D. W.
Shen
,
D. L.
Feng
Diamond Proposal Number(s):
[16345]
Abstract: The localizedtoitinerant transition of f electrons lies at the heart of heavyfermion physics, but has only been directly observed in singlelayer Cebased materials. Here, we report a comprehensive study on the electronic structure and nature of the Ce 4f electrons in the heavyfermion superconductor Ce2PdIn8, a typical n=2 CenMmIn3n+2m compound, using highresolution and 4d−4f resonant photoemission spectroscopies. The electronic structure of this material has been studied over a wide temperature range, and hybridization between f and conduction electrons can be clearly observed to form a Kondo resonance near the Fermi level at low temperatures. The characteristic temperature of the localizedtoitinerant transition is around 120 K, which is much higher than its coherence temperature Tcoh∼30K.

Feb 2019


I05ARPES

Jiagui
Feng
,
Deepnarayan
Biswas
,
Akhil
Rajan
,
Matthew D.
Watson
,
Federico
Mazzola
,
Oliver J.
Clark
,
Kaycee
Underwood
,
I.
Markovic
,
Martin
Mclaren
,
Andrew
Hunter
,
David M.
Burn
,
Liam B.
Duffy
,
Sourabh
Barua
,
Geetha
Balakrishnan
,
Francois
Bertran
,
Patrick
Le Fevre
,
Timur
Kim
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Peter
Wahl
,
Phil D. C.
King
Diamond Proposal Number(s):
[19771]
Abstract: How the interacting electronic states and phases of layered transitionmetal dichalcogenides
evolve when thinned to the singlelayer limit is a key open question in the study of twodimensional
materials. Here, we use angleresolved photoemission to investigate the electronic structure of monolayer VSe2 grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe2, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below Tc = 140 5 K, concomitant with the emergence
of chargeorder superstructures evident in lowenergy electron diffraction. These observations point
to a chargedensity wave instability in the monolayer which is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of xray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from densityfunctional theory. Our study thus points
to a delicate balance that can be realised between competing interacting states and phases in
monolayer transitionmetal dichalcogenides.

Jun 2018

