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.
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May 2019
|
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I05-ARPES
|
Bin-Bin
Fu
,
Chang-Jiang
Yi
,
Zhi-Jun
Wang
,
Meng
Yang
,
Bai-Qing
Lv
,
Xin
Gao
,
Man
Li
,
Yao-Bo
Huang
,
Hong-Ming
Weng
,
You-Guo
Shi
,
Tian
Qian
,
Hong
Ding
Abstract: Topological Dirac semimetals (DSMs) present a kind of topologically nontrivial quantum state of matter, which has massless Dirac fermions in the bulk and topologically protected states on certain surfaces. In superconducting DSMs, the effects of their nontrivial topology on superconducting pairing could realize topological superconductivity in the bulk or on the surface. As superconducting pairing takes place at the Fermi level E F, to make the effects possible, the Dirac points should lie in the vicinity of E F so that the topological electronic states can participate in the superconducting paring. Here, we show using angle-resolved photoelectron spectroscopy that in a series of (Ir1−x Pt x )Te2 compounds, the type-II Dirac points reside around E F in the superconducting region, in which the bulk superconductivity has a maximum T c of ~ 3 K. The realization of the coexistence of bulk superconductivity and low-energy Dirac fermions in (Ir1−x Pt x )Te2 paves the way for studying the effects of the nontrivial topology in DSMs on the superconducting state.
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Mar 2019
|
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I05-ARPES
|
M. M.
Otrokov
,
I. I.
Klimovskikh
,
H.
Bentmann
,
D.
Estyunin
,
A.
Zeugner
,
Z. S.
Aliev
,
S.
Gaß
,
A. U. B.
Wolter
,
A. V.
Koroleva
,
A. M.
Shikin
,
M.
Blanco-Rey
,
M.
Hoffmann
,
I. P.
Rusinov
,
A. Yu.
Vyazovskaya
,
S. V.
Eremeev
,
Yu. M.
Koroteev
,
V. M.
Kuznetsov
,
F.
Freyse
,
J.
Sánchez-Barriga
,
I. R.
Amiraslanov
,
M. B.
Babanly
,
N. T.
Mamedov
,
N. A.
Abdullayev
,
V. N.
Zverev
,
A.
Alfonsov
,
V.
Kataev
,
B.
Büchner
,
E. F.
Schwier
,
S.
Kumar
,
A.
Kimura
,
L.
Petaccia
,
G.
Di Santo
,
R. C.
Vidal
,
S.
Schatz
,
K.
Kißner
,
M.
Unzelmann
,
C. H.
Min
,
Simon
Moser
,
T. R. F.
Peixoto
,
F.
Reinert
,
A.
Ernst
,
P. M.
Echenique
,
A.
Isaeva
,
E. V.
Chulkov
Abstract: Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, magnetic topological insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics, such as the quantum anomalous Hall effect and chiral Majorana fermions. So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. An intrinsic magnetic topological insulator—a stoichiometric well ordered magnetic compound—could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi2Te4. The antiferromagnetic ordering that MnBi2Te4 shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ2 topological classification; ℤ2 = 1 for MnBi2Te4, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi2Te4 exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling and axion electrodynamics. Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect and chiral Majorana fermions.
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Dec 2019
|
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I05-ARPES
|
Diamond Proposal Number(s):
[15822]
Open Access
Abstract: Van der Waals materials offer unprecedented control of electronic properties via stacking of different types of two-dimensional materials. A fascinating frontier, largely unexplored, is the stacking of strongly correlated phases of matter. We study 4Hb-TaS2, which naturally realizes an alternating stacking of 1T-TaS2 and 1H-TaS2 structures. The former is a well-known Mott insulator, which has recently been proposed to host a gapless spin-liquid ground state. The latter is a superconductor known to also host a competing charge density wave state. This raises the question of how these two components affect each other when stacked together. We find a superconductor with a Tc of 2.7 Kelvin and anomalous properties, of which the most notable one is a signature of time-reversal symmetry breaking, abruptly appearing at the superconducting transition. This observation is consistent with a chiral superconducting state.
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Mar 2020
|
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I05-ARPES
|
Saumya
Mukherjee
,
Sung Won
Jung
,
Sophie F.
Weber
,
Chunqiang
Xu
,
Dong
Qian
,
Xiaofeng
Xu
,
Pabitra K.
Biswas
,
Timur K.
Kim
,
Laurent C.
Chapon
,
Matthew D.
Watson
,
Jeffrey B.
Neaton
,
Cephise
Cacho
Diamond Proposal Number(s):
[21591]
Open Access
Abstract: Transition-metal dichalcogenides (TMDs) offer an ideal platform to experimentally realize Dirac fermions. However, typically these exotic quasiparticles are located far away from the Fermi level, limiting the contribution of Dirac-like carriers to the transport properties. Here we show that NiTe2 hosts both bulk Type-II Dirac points and topological surface states. The underlying mechanism is shared with other TMDs and based on the generic topological character of the Te p-orbital manifold. However, unique to NiTe2, a significant contribution of Ni d orbital states shifts the energy of the Type-II Dirac point close to the Fermi level. In addition, one of the topological surface states intersects the Fermi energy and exhibits a remarkably large spin splitting of 120 meV. Our results establish NiTe2 as an exciting candidate for next-generation spintronics devices.
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Jul 2020
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I05-ARPES
|
Takafumi
Sato
,
Zhiwei
Wang
,
Daichi
Takane
,
Seigo
Souma
,
Chaoxi
Cui
,
Yongkai
Li
,
Kosuke
Nakayama
,
Tappei
Kawakami
,
Yuya
Kubota
,
Cephise
Cacho
,
Timur
Kim
,
Arian
Arab
,
Vladimir N.
Strocov
,
Yugui
Yao
,
Takashi
Takahashi
Diamond Proposal Number(s):
[23799]
Open Access
Abstract: We have performed angle-resolved photoemission spectroscopy on
EuIn
2
As
2
which is predicted to be an axion insulator in the antiferromagnetic state. By utilizing soft-x-ray and vacuum-ultraviolet photons, we revealed a three-dimensional hole pocket centered at the
Γ
point of the bulk Brillouin zone together with a heavily hole-doped surface state in the paramagnetic phase. Upon entering the antiferromagnetic phase, the band structure exhibits a marked reconstruction characterized by the emergence of an “M”-shaped bulk band near the Fermi level. The qualitative agreement with first-principles band-structure calculations suggests the occurrence of bulk-band inversion at the
Γ
point in the antiferromagnetic phase. We suggest that
EuIn
2
As
2
provides a good opportunity to study the exotic quantum phases associated with a possible axion-insulator phase.
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Sep 2020
|
|
I05-ARPES
|
Niels B. M.
Schroeter
,
Iñigo
Robredo
,
Sebastian
Klemenz
,
Robert J.
Kirby
,
Jonas A.
Krieger
,
Ding
Pei
,
Tianlun
Yu
,
Samuel
Stolz
,
Thorsten
Schmitt
,
Pavel
Dudin
,
Timur K.
Kim
,
Cephise
Cacho
,
Andreas
Schnyder
,
Aitor
Bergara
,
Vladimir N.
Strocov
,
Fernando
De Juan
,
Maia G.
Vergniory
,
Leslie M.
Schoop
Diamond Proposal Number(s):
[26098, 20617]
Open Access
Abstract: Magnetic Weyl semimetals are a newly discovered class of topological materials that may serve as a platform for exotic phenomena, such as axion insulators or the quantum anomalous Hall effect. Here, we use angle-resolved photoelectron spectroscopy and ab initio calculations to discover Weyl cones in CoS2, a ferromagnet with pyrite structure that has been long studied as a candidate for half-metallicity, which makes it an attractive material for spintronic devices. We directly observe the topological Fermi arc surface states that link the Weyl nodes, which will influence the performance of CoS2 as a spin injector by modifying its spin polarization at interfaces. In addition, we directly observe a minority-spin bulk electron pocket in the corner of the Brillouin zone, which proves that CoS2 cannot be a true half-metal.
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Dec 2020
|
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I05-ARPES
|
Wujun
Shi
,
Benjamin J.
Wieder
,
Holger L.
Meyerheim
,
Yan
Sun
,
Yang
Zhang
,
Yiwei
Li
,
Lei
Shen
,
Yanpeng
Qi
,
Lexian
Yang
,
Jagannath
Jena
,
Peter
Werner
,
Klaus
Koepernik
,
Stuart
Parkin
,
Yulin
Chen
,
Claudia
Felser
,
B. Andrei
Bernevig
,
Zhijun
Wang
Abstract: Topological physics and strong electron–electron correlations in quantum materials are typically studied independently. However, there have been rapid recent developments in quantum materials in which topological phase transitions emerge when the single-particle band structure is modified by strong interactions. Here we demonstrate that the room-temperature phase of (TaSe4)2I is a Weyl semimetal with 24 pairs of Weyl nodes. Owing to its quasi-one-dimensional structure, (TaSe4)2I also hosts an established charge-density wave instability just below room temperature. We show that the charge-density wave in (TaSe4)2I couples the bulk Weyl points and opens a bandgap. The correlation-driven topological phase transition in (TaSe4)2I provides a route towards observing condensed-matter realizations of axion electrodynamics in the gapped regime, topological chiral response effects in the semimetallic phase, and represents an avenue for exploring the interplay of correlations and topology in a solid-state material.
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Jan 2021
|
|
I05-ARPES
|
A. Garrison
Linn
,
Peipei
Hao
,
Kyle N.
Gordon
,
Dushyant
Narayan
,
Bryan S.
Berggren
,
Nathaniel
Speiser
,
Sonka
Reimers
,
Richard P.
Campion
,
Vít
Novák
,
Sarnjeet S.
Dhesi
,
Timur K.
Kim
,
Cephise
Cacho
,
Libor
Šmejkal
,
Tomáš
Jungwirth
,
Jonathan D.
Denlinger
,
Peter
Wadley
,
Daniel S.
Dessau
Diamond Proposal Number(s):
[24224]
Open Access
Abstract: Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically reorientable Néel vector and a Dirac semimetal candidate. Direct measurements of the electronic structure of single-crystalline thin films of tetragonal CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported, including Fermi surfaces (FS) and energy-wavevector dispersions. After correcting for a chemical potential shift of ≈− 390 meV (hole doping), there is excellent agreement of FS, orbital character of bands, and Fermi velocities between the experiment and density functional theory calculations. In addition, 2×1 surface reconstructions are found in the low energy electron diffraction (LEED) and ARPES. This work underscores the need to control the chemical potential in tetragonal CuMnAs to enable the exploration and exploitation of the Dirac fermions with tunable masses, which are predicted to be above the chemical potential in the present samples.
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May 2023
|
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I05-ARPES
|
Kifu
Kurokawa
,
Shunsuke
Isono
,
Yoshimitsu
Kohama
,
So
Kunisada
,
Shiro
Sakai
,
Ryotaro
Sekine
,
Makoto
Okubo
,
Matthew D.
Watson
,
Timur K.
Kim
,
Cephise
Cacho
,
Shik
Shin
,
Takami
Tohyama
,
Kazuyasu
Tokiwa
,
Takeshi
Kondo
Diamond Proposal Number(s):
[30646, 28930]
Open Access
Abstract: he currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO2 planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba2Ca5Cu6O12(F,O)2 with inner CuO2 layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates.
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Jul 2023
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