I05-ARPES
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Q. Y.
Chen
,
D. F.
Xu
,
X. H.
Niu
,
R.
Peng
,
H. C.
Xu
,
C. H. P.
Wen
,
X.
Liu
,
L.
Shu
,
S. Y.
Tan
,
X. C.
Lai
,
Y. J.
Zhang
,
H.
Lee
,
V. N.
Strocov
,
F.
Bisti
,
P.
Dudin
,
J.-x.
Zhu
,
H. Q.
Yuan
,
S.
Kirchner
,
D. L.
Feng
Diamond Proposal Number(s):
[11914]
Abstract: A key issue in heavy fermion research is how subtle changes in the hybridization between the 4f (5f) and conduction electrons can result in fundamentally different ground states. CeRhIn5 stands out as a particularly notable example: when replacing Rh with either Co or Ir, antiferromagnetism gives way to superconductivity. In this photoemission study of CeRhIn5, we demonstrate that the use of resonant angle-resolved photoemission spectroscopy with polarized light allows us to extract detailed information on the 4f crystal field states and details on the 4f and conduction electron hybridization, which together determine the ground state. We directly observe weakly dispersive Kondo resonances of f electrons and identify two of the three Ce 4f
1
5/2 crystal-electric-field levels and band-dependent hybridization, which signals that the hybridization occurs primarily between the Ce 4f states in the CeIn3 layer and two more three-dimensional bands composed of the Rh 4d and In 5p orbitals in the RhIn2 layer. Our results allow us to connect the properties observed at elevated temperatures with the unusual low-temperature properties of this enigmatic heavy fermion compound.
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Feb 2018
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I09-Surface and Interface Structural Analysis
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P.
Schütz
,
D. V.
Christensen
,
V.
Borisov
,
F.
Pfaff
,
P.
Scheiderer
,
L.
Dudy
,
M.
Zapf
,
J.
Gabel
,
Y. Z.
Chen
,
N.
Pryds
,
Victor
Rogalev
,
V. N.
Strocov
,
C.
Schlueter
,
T.-l.
Lee
,
H. O.
Jeschke
,
R.
Valentí
,
M.
Sing
,
R.
Claessen
Abstract: The spinel/perovskite heterointerface γ−Al2O3/SrTiO3 hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO3/SrTiO3 by more than an order of magnitude, despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations, we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO3 layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.
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Oct 2017
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D. V.
Evtushinsky
,
A. N.
Yaresko
,
V. B.
Zabolotnyy
,
J.
Maletz
,
T. K.
Kim
,
A. A.
Kordyuk
,
M. S.
Viazovska
,
M.
Roslova
,
I.
Morozov
,
R.
Beck
,
S.
Aswartham
,
L.
Harnagea
,
S.
Wurmehl
,
H.
Berger
,
V. A.
Rogalev
,
V. N.
Strocov
,
T.
Wolf
,
N. D.
Zhigadlo
,
B.
Büchner
,
S. V.
Borisenko
Abstract: One of the most unique and robust experimental facts about iron-based superconductors is the renormalization of the electronic band dispersion by factor of 3 and more near the Fermi level. Obviously related to the electron pairing, this prominent deviation from the band theory lacks understanding. Experimentally studying the entire spectrum of the valence electrons in iron arsenides, we have found an unexpected depletion of the spectral weight in the middle of the iron-derived band, which is accompanied by a drastic increase of the scattering rate. At the same time, the measured arsenic-derived band exhibits very good agreement with theoretical calculations. We show that the low-energy Fermi velocity renormalization should be viewed as a part of the modification of the spectral function by a strong electronic interaction. Such an interaction with an energy scale of the whole d band appears to be a hallmark of many families of unconventional superconductors.
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Aug 2017
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I05-ARPES
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Q. Y.
Chen
,
D. F.
Xu
,
X. H.
Niu
,
J.
Jiang
,
R.
Peng
,
H. C.
Xu
,
C. H. P.
Wen
,
Z. F.
Ding
,
K.
Huang
,
L.
Shu
,
Y. J.
Zhang
,
H.
Lee
,
V. N.
Strocov
,
M.
Shi
,
F.
Bisti
,
T.
Schmitt
,
Y. B.
Huang
,
P.
Dudin
,
X. C.
Lai
,
S.
Kirchner
,
H. Q.
Yuan
,
D. L.
Feng
Diamond Proposal Number(s):
[11914]
Abstract: Heavy-fermion systems share some of the strange metal phenomenology seen in other unconventional superconductors, providing a unique opportunity to set strange metals in a broader context. Central to understanding heavy-fermion systems is the interplay of localization and itinerancy. These materials acquire high electronic masses and a concomitant Fermi volume increase as the f electrons delocalize at low temperatures. However, despite the wide-spread acceptance of this view, a direct microscopic verification has been lacking. Here we report high-resolution angle-resolved photoemission measurements on CeCoIn5, a prototypical heavy-fermion compound, which spectroscopically resolve the development of band hybridization and the Fermi surface expansion over a wide temperature region. Unexpectedly, the localized-to-itinerant transition occurs at surprisingly high temperatures, yet f electrons are still largely localized even at the lowest temperature. These findings point to an unanticipated role played by crystal-field excitations in the strange metal behavior of CeCoIn5. Our results offer a comprehensive experimental picture of the heavy-fermion formation, setting the stage for understanding the emergent properties, including unconventional superconductivity, in this and related materials.
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Jul 2017
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I05-ARPES
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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
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I05-ARPES
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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]
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
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I05-ARPES
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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
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I05-ARPES
|
Su-yang
Xu
,
Nasser
Alidoust
,
Ilya
Belopolski
,
Zhujun
Yuan
,
Guang
Bian
,
Tay-rong
Chang
,
Hao
Zheng
,
Vladimir N.
Strocov
,
Daniel
Sanchez
,
Guoqing
Chang
,
Chenglong
Zhang
,
Daixiang
Mou
,
Yun
Wu
,
Lunan
Huang
,
Chi-cheng
Lee
,
Shin-ming
Huang
,
Baokai
Wang
,
Arun
Bansil
,
Horng-tay
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 high-energy physics, and have emerged as a much-sought-out treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose low-energy 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 three-dimensional 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 inversion-symmetry-breaking single-crystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft X-ray 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.
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Aug 2015
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Ke-jin
Zhou
,
Yao-bo
Huang
,
Claude
Monney
,
Xi
Dai
,
Vladimir N.
Strocov
,
Nan-lin
Wang
,
Zhi-guo
Chen
,
Chenglin
Zhang
,
Pengcheng
Dai
,
Luc
Patthey
,
Jeroen
Van Den Brink
,
Hong
Ding
,
Thorsten
Schmitt
Abstract: Motivated by the premise that superconductivity in iron-based superconductors is unconventional and mediated by spin fluctuations, an intense research effort has been focused on characterizing the spin-excitation spectrum in the magnetically ordered parent phases of the Fe pnictides and chalcogenides. For these undoped materials, it is well established that the spin-excitation spectrum consists of sharp, highly dispersive magnons. The fate of these high-energy magnetic modes upon sizable doping with holes is hitherto unresolved. Here we demonstrate, using resonant inelastic X-ray scattering, that optimally hole-doped superconducting Ba0.6K0.4Fe2As2 retains well-defined, dispersive high-energy modes of magnetic origin. These paramagnon modes are softer than, though as intense as, the magnons of undoped antiferromagnetic BaFe2As2. The persistence of spin excitations well into the superconducting phase suggests that the spin fluctuations in Fe-pnictide superconductors originate from a distinctly correlated spin state. This connects Fe pnictides to cuprates, for which, in spite of fundamental electronic structure differences, similar paramagnons are present.
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Feb 2013
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C.
Monney
,
K. J.
Zhou
,
H.
Cercellier
,
Z.
Vydrova
,
M. G.
Garnier
,
G.
Monney
,
V. N.
Strocov
,
H.
Berger
,
H.
Beck
,
T.
Schmitt
,
P.
Aebi
Abstract: In high-resolution resonant inelastic x-ray scattering at the Ti L edge of the charge-density-wave system 1T-TiSe2, we observe sharp low energy loss peaks from electron-hole pair excitations developing at low temperature. These excitations are strongly dispersing as a function of the transferred momentum of light. We show that the unoccupied bands close to the Fermi level can effectively be probed in this broadband material. Furthermore, we extract the order parameter of the charge-density-wave phase from temperature-dependent measurements.
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Jul 2012
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