I05-ARPES
|
Songyuan
Geng
,
Xin
Wang
,
Risi
Guo
,
Chen
Qiu
,
Fangjie
Chen
,
Qun
Wang
,
Kangjie
Li
,
Peipei
Hao
,
Hanpu
Liang
,
Yang
Huang
,
Yunbo
Wu
,
Shengtao
Cui
,
Zhe
Sun
,
Timur K.
Kim
,
Cephise
Cacho
,
Daniel S.
Dessau
,
Benjamin T.
Zhou
,
Haoxiang
Li
Diamond Proposal Number(s):
[38254]
Open Access
Abstract: Flat electronic bands, where interactions among electrons overwhelm their kinetic energies, hold the promise for exotic correlation physics. The dice lattice has long been theorized as a host of flat bands with intriguing band topology. However, to date, no material has ever been found to host the characteristic flat bands of a dice lattice. Here, using angle-resolved photoemission spectroscopy (ARPES), we discover a dice-lattice flat band at EF in the van der Waals (vdW) electride [YCl]2+: 2e-. In this system, excess valence electrons from Y deconfine from the cation framework to form an interstitial anionic electron lattice that constitutes the dice lattice. Our ARPES measurements unambiguously identify two sets of dice-lattice bands in YCl, including a nearly dispersionless band at the Fermi level. The near-EF electronic structure observed in ARPES, which consists of the flat bands and other dispersive band features, find excellent agreement with first-principles calculations and is well captured by a simple dice-lattice model. Our findings thus end the long quest of a real dice flat band material and establish vdW electride YCl as a prototype of dice metals. Our results further demonstrate the anionic electron lattice as a novel scheme for realizing lattice geometries and electronic structures rare to find in conventional crystalline systems.
|
Jan 2026
|
|
I05-ARPES
|
I.
Biało
,
Qisi
Wang
,
J.
Küspert
,
X.
Hong
,
L.
Martinelli
,
O.
Gerguri
,
Y.
Chan
,
K.
Von Arx
,
O. K.
Forslund
,
W. R.
Pudełko
,
C.
Lin
,
N. C.
Plumb
,
Y.
Sassa
,
D.
Betto
,
N. B.
Brookes
,
M.
Rosmus
,
N.
Olszowska
,
Ma. D.
Watson
,
T. K.
Kim
,
C.
Cacho
,
M.
Horio
,
M.
Ishikado
,
H. M.
Rønnow
,
J.
Chang
Diamond Proposal Number(s):
[32147]
Open Access
Abstract: Strong electron correlations drive Mott insulator transitions. Yet, there exists no framework to classify Mott insulators by their degree of correlation. Cuprate superconductors, with their tunable doping and rich phase diagrams, offer a unique platform to investigate the evolution of these interactions. However, spectroscopic access to a clean half-filled Mott-insulating state is lacking in compounds with the highest superconducting onset temperature. To fill this gap, we introduce a pristine, half-filled thallium-based cuprate system, Tl2Ba5Cu4Ox. Using high-resolution resonant inelastic x-ray scattering, we probe long-lived magnon excitations and uncover a pronounced kink in the magnon dispersion, marked by a simultaneous change in group velocity and lifetime broadening. Modeling the dispersion within a Hubbard-Heisenberg approach, we extract the interaction strength and compare it with other cuprate systems. Our results establish a cuprate universal relation between electron-electron interaction and magnon zone-boundary dispersion. Superconductivity seems to be optimal at intermediate correlation strength, suggesting an optimal balance between localization and itinerancy.
|
Dec 2025
|
|
I05-ARPES
|
Cong
Li
,
Yang
Wang
,
Jianfeng
Zhang
,
Hongxiong
Liu
,
Wanyu
Chen
,
Guowei
Liu
,
Hanbin
Deng
,
Timur K.
Kim
,
Craig
Polley
,
Balasubramanian
Thiagarajan
,
Jiaxin
Yin
,
Youguo
Shi
,
Tao
Xiang
,
Oscar
Tjernberg
Diamond Proposal Number(s):
[34265, 39652]
Open Access
Abstract: For several decades, it was widely believed that a noninteracting disordered electronic system could only undergo an Anderson metal–insulator transition due to Anderson localization. However, numerous recent theoretical works have predicted the existence of a disorder-driven non-Anderson phase transition that differs from Anderson localization. The frustration lies in the fact that this non-Anderson disorder-driven transition has not yet been experimentally demonstrated in any system. Here, using angle-resolved photoemission spectroscopy, we present a case study of observing the non-Anderson disorder-driven transition by visualizing the electronic structure of the Weyl semimetal NdAlSi on surfaces with varying amounts of disorder. Our observations reveal that strong disorder can effectively suppress all surface states in the Weyl semimetal NdAlSi, including the topological surface Fermi arcs. This disappearance of surface Fermi arcs is associated with the vanishing of the topological invariant, indicating a quantum phase transition from a Weyl semimetal to a diffusive metal. These observations provide direct experimental evidence of the non-Anderson disorder-driven transition occurring in real quantum systems, a finding long anticipated by theoretical physicists.
|
Oct 2025
|
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I05-ARPES
|
Y.
Alexanian
,
A.
De La Torre
,
S.
Mckeown Walker
,
M.
Straub
,
G.
Gatti
,
A.
Hunter
,
S.
Mandloi
,
E.
Cappelli
,
S.
Riccò
,
F. Y.
Bruno
,
M.
Radovic
,
N. C.
Plumb
,
M.
Shi
,
J.
Osiecki
,
C.
Polley
,
T. K.
Kim
,
P.
Dudin
,
M.
Hoesch
,
R. S.
Perry
,
A.
Tamai
,
F.
Baumberger
Diamond Proposal Number(s):
[10348, 12404, 17381]
Open Access
Abstract: The fate of the Fermi surface in bulk electron-doped Sr2IrO4 remains elusive, as does the origin and extension of its pseudogap phase. Here, we use high-resolution angle-resolved photoelectron spectroscopy (ARPES) to investigate the electronic structure of Sr2−xLaxIrO4 up to x = 0.2, a factor of two higher than in previous work. We find that the antinodal pseudogap persists up to the highest doping level, and thus beyond the sharp increase in Hall carrier density to ≃ 1 + x recently observed above x* ≃ 0.161. This suggests that doped iridates host a unique phase of matter in which a large Hall density coexists with an anisotropic pseudogap, breaking up the Fermi surface into disconnected arcs. The temperature boundary of the pseudogap is T* ≃ 200 K for x = 0.2, comparable to cuprates and to the energy scale of short range antiferromagnetic correlations in cuprates and iridates.
|
Oct 2025
|
|
I05-ARPES
|
Cong
Li
,
Mengli
Hu
,
Zhilin
Li
,
Yang
Wang
,
Wanyu
Chen
,
Balasubramanian
Thiagarajan
,
Mats
Leandersson
,
Craig
Polley
,
Timur
Kim
,
Hui
Liu
,
Cosma
Fulga
,
Maia G.
Vergniory
,
Oleg
Janson
,
Oscar
Tjernberg
,
Jeroen
Van Den Brink
Diamond Proposal Number(s):
[36464]
Open Access
Abstract: Altermagnets constitute a novel, third fundamental class of collinear magnetic ordered materials, alongside with ferro- and antiferromagnets. They share with conventional antiferromagnets the feature of a vanishing net magnetization. At the same time they show a spin-splitting of electronic bands, just as in ferromagnets, caused by the atomic exchange interaction. On the other hand, topology has recently revolutionized our understanding of condensed matter physics, introducing new phases of matter classified by intrinsic topological order. Here we connect the worlds of altermagnetism and topology, showing that the electronic structure of the altermagnet CrSb is topological. Using high-resolution angle-resolved photoemission spectroscopy, we observe the large momentum-dependent spin-splitting in CrSb that induces altermagnetic Weyl nodes. We observe the related topological Fermi-arcs, which in electronic structure calculations are spin polarized. This indicates that in altermagnets the large energy scale intrinsic to their spin-splitting creates its own realm of robust electronic topology.
|
Jul 2025
|
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I05-ARPES
|
Zhisheng
Zhao
,
Tongrui
Li
,
Peng
Li
,
Xueliang
Wu
,
Jianghao
Yao
,
Ziyuan
Chen
,
Yajun
Yan
,
Shengtao
Cui
,
Zhe
Sun
,
Yichen
Yang
,
Zhicheng
Jiang
,
Zhengtai
Liu
,
Alex
Louat
,
Timur
Kim
,
Cephise
Cacho
,
Aifeng
Wang
,
Yilin
Wang
,
Dawei
Shen
,
Juan
Jiang
,
Donglai
Feng
Diamond Proposal Number(s):
[32274]
Abstract: The kagome metal FeGe provides a rich platform for understanding the mechanisms behind competing orders, as it exhibits charge order (CO) emerging deep within the antiferromagnetic phase. To investigate the intrinsic origin of this behavior, we examine the evolution of the low-energy electronic structure across the phase transition in annealed FeGe samples using angle-resolved photoemission spectroscopy. We find no evidence supporting a conventional nesting mechanism, such as Fermi surface nesting or van Hove singularities. However, we observe two notable changes in the band structure: an electron-like band around the K point and another around the A point, both shifting upward in energy when CO forms. These findings are consistent with our density-functional theory calculations, which suggest that the charge order in FeGe is primarily driven by magnetic energy savings due to a lattice distortion involving Ge1-dimerization. Our results provide photoemission evidence supporting this novel mechanism for CO formation in FeGe, in contrast to the conventional nesting-driven mechanisms.
|
Jun 2025
|
|
I05-ARPES
|
Steef
Smit
,
Kourosh L.
Shirkoohi
,
Saumya
Mukherjee
,
Sergio Barquero
Pierantoni
,
Lewis
Bawden
,
Erik
Van Heumen
,
Arnaud Pastel Nono
Tchiomo
,
Jans
Henke
,
Jasper
Van Wezel
,
Yingkai
Huang
,
Takeshi
Kondo
,
Tsunehiro
Takeuchi
,
Timur K.
Kim
,
Cephise
Cacho
,
Marta
Zonno
,
Sergey
Gorovikov
,
Stephen B.
Dugdale
,
Jorge I.
Facio
,
Mariia
Roslova
,
Laura
Folkers
,
Anna
Isaeva
,
Nigel
Hussey
,
Mark
Golden
Diamond Proposal Number(s):
[19403, 22464]
Open Access
Abstract: High-resolution angle-resolved photoemission spectroscopy (ARPES) performed on the single-layered cuprate (Pb
1
−
y
,Bi
y
)
2
Sr
2
−
x
La
x
CuO
6
+
δ
(Bi2201) reveals a 6-10% difference in the nodal
k
F
vectors along the
Γ
Y and
Γ
X directions. This asymmetry is notably larger than the 2% orthorhombic distortion in the CuO
2
plane lattice constants determined using X-ray crystallography from the same samples. First principles calculations indicate that crystal-field splitting of the bands lies at the root of the
k
F
asymmetry. Concomitantly, the nodal Fermi velocities for the
Γ
Y quadrant exceed those for
Γ
X by 4%. Momentum distribution curve widths for the two nodal dispersions are also anisotropic, showing identical energy dependencies, bar a scaling factor of
∼
1.17
±
0.05
between
Γ
Y and
Γ
X. Consequently, the imaginary part of the self-energy is found to be 10-20% greater along
Γ
Y than
Γ
X. These results emphasize the need to account for Fermi surface asymmetry in the analysis of ARPES data on Bi-based cuprate high temperature superconductors such as Bi2201. To illustrate this point, an orthorhombic tight-binding model (with twofold in-plane symmetry) was used to fit ARPES Fermi surface maps spanning all four quadrants of the Brillouin zone, and the ARPES-derived hole-doping (Luttinger count) was extracted. Comparison of the Luttinger count with one assuming four-fold in-plane symmetry strongly suggests the marked spread in previously-reported Fermi surface areas from ARPES on Bi2201 results from the differences in
k
F
along
Γ
Y and
Γ
X. Using this analysis, a new, linear relationship emerges between the hole-doping derived from ARPES (
p
ARPES
) and that derived using the Presland (
p
Presland
) relation such that
p
ARPES
=
p
Presland
+
0.11
. The implications for this difference between the ARPES- and Presland-derived estimates for
p
are discussed and possible future directions to elucidate the origin of this discrepancy are presented.
|
Jun 2025
|
|
I05-ARPES
|
Ola Kenji
Forslund
,
Xiaoxiong
Liu
,
Soohyeon
Shin
,
Chun
Lin
,
Masafumi
Horio
,
Qisi
Wang
,
Kevin
Kramer
,
Saumya
Mukherjee
,
Timur
Kim
,
Cephise
Cacho
,
Chennan
Wang
,
Tian
Shang
,
Victor
Ukleev
,
Jonathan S.
White
,
Pascal
Puphal
,
Yasmine
Sassa
,
Ekaterina
Pomjakushina
,
Titus
Neupert
,
Johan
Chang
Diamond Proposal Number(s):
[22091]
Abstract: The anomalous Hall effect (AHE) has emerged as a key indicator of time-reversal symmetry breaking (TRSB) and topological features in electronic band structures. Absent of a magnetic field, the AHE requires spontaneous TRSB but has proven hard to probe due to averaging over domains. The anomalous component of the Hall effect is thus frequently derived from extrapolating the magnetic field dependence of the Hall response. We show that discerning whether the AHE is an intrinsic property of the field-free system becomes intricate in the presence of strong magnetic fluctuations. As a study case, we use the Weyl semimetal PrAlGe, where TRSB can be toggled via a ferromagnetic transition, providing a transparent view of the AHE’s topological origin. Through a combination of thermodynamic, transport, and muon spin relaxation measurements, we contrast the behavior below the ferromagnetic transition temperature to that of strong magnetic fluctuations above. Our results on PrAlGe provide general insights into the interpretation of anomalous Hall signals in systems where TRSB is debated, such as families of kagome metals or certain transition metal dichalcogenides.
|
Mar 2025
|
|
I05-ARPES
|
Cong
Li
,
Yang
Wang
,
Jianfeng
Zhang
,
Guowei
Liu
,
Hongxiong
Liu
,
Wanyu
Chen
,
Hanbin
Deng
,
Wenbo
Ma
,
Craig
Polley
,
Balasubramanian
Thiagarajan
,
Timur K.
Kim
,
Jiaxin
Yin
,
Youguo
Shi
,
Tao
Xiang
,
Oscar
Tjernberg
Diamond Proposal Number(s):
[34265]
Open Access
Abstract: Non-Hermitian physics, studying systems described by non-Hermitian Hamiltonians, reveals unique phenomena not present in Hermitian systems. Unlike Hermitian systems, non-Hermitian systems have complex eigenvalues, making their effects less directly observable. Recently, significant efforts have been devoted to incorporating the non-Hermitian effects into condensed matter physics. However, progress is hindered by the absence of a viable experimental approach. Here, the discovery of the surface-selectively spontaneous reconstructed Weyl semimetal NdAlSi provides a feasible experimental platform for studying non-Hermitian physics. Utilizing angle-resolved photoemission spectroscopy (ARPES) measurements, surface-projected density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements, it is demonstrated that surface reconstruction in NdAlSi alters surface Fermi arc (SFA) connectivity and generates new isolated non-topological SFAs (NTSFAs) by introducing non-Hermitian terms. The surface-selective spontaneous reconstructed Weyl semimetal NdAlSi can be viewed as a Hermitian bulk – non-Hermitian boundary system. The isolated non-topological SFAs on the reconstructed surface act as a loss mechanism and open boundary condition (OBC) for the topological electrons and bulk states, serving as non-Hermitian boundary states. This discovery provides a good experimental platform for exploring new physical phenomena and potential applications based on boundary non-Hermitian effects, extending beyond purely mathematical concepts. Furthermore, it provides important enlightenment for constructing topological photonic crystals with surface reconstruction and studying their topological properties.
|
Feb 2025
|
|
I05-ARPES
|
Chun
Lin
,
Armando
Consiglio
,
Ola Kenji
Forslund
,
Julia
Kuespert
,
M. Michael
Denner
,
Hechang
Lei
,
Alex
Louat
,
Matthew D.
Watson
,
Timur K.
Kim
,
Cephise
Cacho
,
Dina
Carbone
,
Mats
Leandersson
,
Craig
Polley
,
Thiagarajan
Balasubramanian
,
Domenico
Di Sante
,
Ronny
Thomale
,
Zurab
Guguchia
,
Giorgio
Sangiovanni
,
Titus
Neupert
,
Johan
Chang
Diamond Proposal Number(s):
[30650, 33528]
Open Access
Abstract: Tunable quantum materials hold great potential for applications. Of special interest are materials in which small lattice strain induces giant electronic responses. The kagome compounds AV3Sb5 (A = K, Rb, Cs) provide a testbed for electronic tunable states. In this study, through angle-resolved photoemission spectroscopy, we provide comprehensive spectroscopic measurements of the electronic responses induced by compressive and tensile strains on the charge-density-wave (CDW) and van Hove singularity (VHS) in CsV3Sb5. We observe a tripling of the CDW gap magnitudes with ~ 1% strain. Simultaneously, changes of both energy and mass of the VHS are observed. Combined, this reveals an anticorrelation between the unconventional CDW order parameter and the mass of the VHS, and highlight the role of the latter in the superconducting pairing. The substantial electronic responses uncover a rich strain tunability of the versatile kagome system in studying quantum interplays under lattice variations.
|
Dec 2024
|
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