I09-Surface and Interface Structural Analysis
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H. J.
Elmers
,
O.
Tkach
,
Y.
Lytvynenko
,
P.
Yogi
,
M.
Schmitt
,
D.
Biswas
,
J.
Liu
,
S. V.
Chernov
,
Quynh
Nguyen
,
M.
Hoesch
,
D.
Kutnyakhov
,
N.
Wind
,
L.
Wenthaus
,
M.
Scholz
,
K.
Rossnagel
,
A.
Gloskovskii
,
C.
Schlueter
,
A.
Winkelmann
,
A. A.
Haghighirad
,
T.-L.
Lee
,
M.
Sing
,
R.
Claessen
,
M.
Le Tacon
,
J.
Demsar
,
G.
Schönhense
,
O.
Fedchenko
Diamond Proposal Number(s):
[33765]
Abstract: Using x-ray photoelectron diffraction (XPD) and angle-resolved photoemission spectroscopy, we study photoemission intensity changes related to changes in the geometric and electronic structure in the kagome metal CsV3Sb5 upon transition to an unconventional charge density wave (CDW) state. The XPD patterns reveal the presence of a chiral atomic structure in the CDW phase. Furthermore, using circularly polarized x-rays, we have found a pronounced nontrivial circular dichroism in the angular distribution of the valence band photoemission in the CDW phase, indicating a chirality of the electronic structure. This observation is consistent with the proposed orbital loop current order. In view of a negligible spontaneous Kerr signal in recent magneto-optical studies, the results suggest an antiferromagnetic coupling of the orbital magnetic moments along the 𝑐 axis. While the inherent structural chirality may also induce circular dichroism, the observed asymmetry values seem to be too large in the case of the weak structural distortions caused by the CDW.
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Mar 2025
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I05-ARPES
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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.
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Feb 2025
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I05-ARPES
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Diamond Proposal Number(s):
[32858]
Open Access
Abstract: Rare-earth tellurides present unique opportunities to explore the interplay between charge density waves (CDWs) and electronic structure in quasi-two-dimensional systems. Unlike their van der Waals counterparts, the rare-earth tritellurides, ditellurides such as LaTe2 feature distinct polar surfaces and Fermi-surface nesting properties. In this paper, we investigate the electronic band structure of LaTe2, distinguishing the surface states from the bulk bands, and enabling the study of the impact of band filling on CDW stability. We uncover the intricate dependence of CDW formation on the electronic environment and highlight the robustness of the CDW order against variations in band filling. The results provide insights into the complex relationship between electronic structure and CDW formation in this class of materials.
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Feb 2025
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I05-ARPES
I10-Beamline for Advanced Dichroism - scattering
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E. L.
Arnold
,
J. M.
Riley
,
L. B.
Duffy
,
A. I.
Figueroa
,
R.
Held
,
K. M.
Shen
,
D. G.
Schlom
,
P. D. C.
King
,
M.
Hoesch
,
G.
Van Der Laan
,
T.
Hesjedal
Diamond Proposal Number(s):
[16162, 15481]
Open Access
Abstract: We present a detailed x-ray magnetic circular dichroism (XMCD) study of the magnetic properties of Gd-doped EuO thin films, synthesized via molecular-beam epitaxy with Gd doping levels up to over 12%. The impact of Gd doping on the electronic and magnetic behavior of EuO is studied using XMCD and magnetometry. Gd doping significantly enhances the Curie temperature (𝑇C) from 69 K in undoped EuO to over 120 K, driven by increased carrier density, while preserving the high quality of the single-crystalline films. At higher doping levels, a plateau in 𝑇C is observed, which is attributed to the formation of Eu-Gd nearest-neighbor pairs that limit dopant activation. We also observe a distinctive “double-dome” structure in the temperature-dependent magnetization, which we attribute to both the ferromagnetic ordering of Eu 4𝑓 moments at lower temperatures and the influence of conduction electrons via 4𝑓−5𝑑 exchange interactions at higher temperatures. These findings provide key insights into the mechanisms of carrier-induced magnetic transitions.
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Feb 2025
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I05-ARPES
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Diamond Proposal Number(s):
[33694, 34335]
Open Access
Abstract: Stacking of strongly-correlated 2D materials is opening the possibility to demonstrate novel electronic or magnetic ordering phenomena. In this regard the intrinsic polytypism of tantalum dichalcogenides has emerged as a platform to generate clean and controllable material interfaces. Here, we report on the Fermi surface of 4Hb-TaSe2, a polytype which consists of alternately stacked layers with octahedral (T) and trigonal prismatic (H) coordination of tantalum in the Se-Ta-Se layers. The material is known to host a charge density wave (CDW) phase with star clusters in the T-layers, intercalated by metallic H-layers, but its momentum resolved electronic structure remains undetermined. Using selective area angle resolved photoemission spectroscopy on the T termination combined with ab initio calculations, we unveil a finely structured Fermi surface arising from band folding in the reconstructed Brillouin zone caused by the CDW star clusters. The star-shaped Fermi surface is rotated away from the high-symmetry directions of the normal phase, and exhibits pseudochirality. Theoretical analysis supports the metallic nature of the system and interlayer interactions leading to hybridization. The work provides a detailed overview on the impact of band hybridization with the CDW on the Fermi surface of a material for new phases of quantum matter.
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Feb 2025
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I05-ARPES
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Abstract: In this master's thesis a novel doped van der Waals ferromagnet (Fe1-xCox)3GeTe2 was investigated, with potential magnetic and spintronic applications, addressing challenges related to scalability, integration, and performance in current technologies. The material was systematically characterized using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), magnetic measurements (VSM), and comprehensive X-ray studies. X-Ray Magnetic Circular Dichroism (XMCD) was employed to determine the material's magnetic properties and compare them to its undoped counterpart, Fe3GeTe2. Additionally, Angle-Resolved Photoemission Spectroscopy (ARPES) was used to probe the electronic structure and assess the impact of doping on its band structure. The XMCD results show a total moment of 0.120 μB for Co and 1.034 μB for Fe. ARPES results show a shift in the band structure, compared to the undoped Fe3GeTe2, which indicates hole-doping. The findings provide a foundation for future work, including theoretical calculations, temperature-dependent studies, and investigations into spin dynamics, which will further clarify the material's potential for next-generation spintronic applications.
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Jan 2025
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I05-ARPES
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Liam
Trzaska
,
Lei
Qiao
,
Matthew D.
Watson
,
Monica
Ciomaga Hatnean
,
Igor
Markovic
,
Edgar
Abarca Morales
,
Tommaso
Antonelli
,
Cephise
Cacho
,
Geetha
Balakrishnan
,
Wei
Ren
,
Silvia
Picozzi
,
Phil D. C.
King
Diamond Proposal Number(s):
[21986, 25564]
Open Access
Abstract: The recent discovery of the persistence of long-range magnetic order when van der Waals magnets are thinned towards monolayers provides a tunable platform for engineering of novel magnetic structures and devices. Here, we study the evolution of the electronic structure of CrGeTe3 as a function of surface electron doping. From angle-resolved photoemission, we observe spectroscopic fingerprints that this electron doping drives a marked increase in TC, reaching values more than double that of the undoped material, in agreement with recent studies using electrostatic gating. Together with density functional theory calculations and Monte Carlo simulations, we show that, surprisingly, the increased TC is mediated by the population of spin-minority Cr t2g states, forming a half-metallic 2D electron gas. This promotes a novel variant of double exchange, and unlocks a significant influence of Ge – which was previously thought to be electronically inert in this system – in mediating Cr-Cr exchange.
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Jan 2025
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I05-ARPES
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Abstract: Unconventional superconductors hold promise for high-temperature superconductivity, but their pairing mechanism remains uncertain. A key approach to understanding this is studying the normal state phases from which superconductivity emerges. In iron chalcogenide superconductors, this phase is the nematic electronic phase, and this thesis investigates its influence on the electronic structure using angle-resolved photoemission spectroscopy (ARPES). Using low-energy models I can quantify the experimental band dispersion and identify the orbital character. I further compliment these measurements with density functional theory calculations.
Initially, I examine the electronic structure of nematic FeSe1−xTex. From increasing the Te concentration, I find the effective masses of each hole band to reduce for x < 0.3. However, beyond this I observe significant variations in the hole band of dxy orbital character. As Te concentration increases, this band shifts towards the Fermi level, correlating with an increase in its effective mass and an enhanced superconductivity. To explore how strain couples to the nematic order, I apply uniaxial strain to FeSe1−xSx and track the electronic spectral function. I quantify momentum- dependent band splittings, orbital transmutation and an orbital dependent spectral weight transfer at hole pockets. These findings align well with other nematic systems, offering a comprehensive description of how the nematic phase distorts the electronic structure. Furthermore, I conduct temperature-dependent ARPES on three FeSe1−xSx systems under fixed symmetry-breaking strain. I observe significant temperature dependence within the spectral weight and band splittings at the hole pockets. In contrast, the band splittings at the electron pockets show a weaker temperature dependence, indicating they are driven predominantly by pure lattice distortion, rather than via the nematic order. Lastly, transport studies reveal no clear coupling between symmetry-breaking strain and superconductivity. Instead, I observe a linear strain dependence of the superconducting transition, where the trends agree with symmetry preserving pressure studies.
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Dec 2024
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I05-ARPES
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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.
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Dec 2024
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I05-ARPES
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Diamond Proposal Number(s):
[25906]
Open Access
Abstract: Competing interactions in low-dimensional materials can produce nearly degenerate electronic and structural phases. We investigate structural phase transitions in layered IrTe2 for which a number of potential transition mechanisms have been postulated. The spatial coexistence of multiple phases on the micron scale has prevented a detailed analysis of the electronic structure. By exploiting micro-angle-resolved photoemission spectroscopy obtained with synchrotron radiation we extract the electronic structure of the multiple structural phases in IrTe2 in order to address the mechanism underlying the phase transitions. We find direct evidence of lowered energy states that appear in the low-temperature phases, states previously predicted by ab initio calculations and extended here. Our results validate a proposed scenario of bonding and antibonding states as the driver of the phase transitions.
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Nov 2024
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