I05-ARPES
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Junzhang
Ma
,
Han
Wang
,
Simin
Nie
,
Changjiang
Yi
,
Yuanfeng
Xu
,
Hang
Li
,
Jasmin
Jandke
,
Wulf
Wulfhekel
,
Yaobo
Huang
,
Damien
West
,
Pierre
Richard
,
Alla
Chikina
,
Vladimir N.
Strocov
,
Joël
Mesot
,
Hongming
Weng
,
Shengbai
Zhang
,
Youguo
Shi
,
Tian
Qian
,
Ming
Shi
,
Hong
Ding
Diamond Proposal Number(s):
[17080]
Abstract: Parity‐time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle‐resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd2As2. As a result, ideal magnetic Dirac fermions with simplest double cone structure near the Fermi level emerge in the antiferromagnetic (AFM) phase. The magnetic order breaks time reversal symmetry, but preserves inversion symmetry. The double degeneracy of the Dirac bands is protected by a combination of inversion, time‐reversal, and an additional translation operation. Moreover, the calculations show that a deviation of the magnetic moments from the c‐axis leads to the breaking of C3 rotation symmetry, and thus, a small bandgap opens at the Dirac point in the bulk. In this case, the system hosts a novel state containing three different types of topological insulator: axion insulator, AFM topological crystalline insulator (TCI), and higher order topological insulator. The results provide an enlarged platform for the quest of topological Dirac fermions in a magnetic system.
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Feb 2020
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I16-Materials and Magnetism
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Han Gyeol
Lee
,
Lingfei
Wang
,
Liang
Si
,
Xiaoyue
He
,
Daniel G.
Porter
,
Jeong Rae
Kim
,
Eun Kyo
Ko
,
Jinkwon
Kim
,
Sung Min
Park
,
Bongju
Kim
,
Andrew Thye Shen
Wee
,
Alessandro
Bombardi
,
Zhicheng
Zhong
,
Tae Won
Noh
Diamond Proposal Number(s):
[22181]
Abstract: The metal–insulator transition (MIT) in transition‐metal‐oxide is fertile ground for exploring intriguing physics and potential device applications. Here, an atomic‐scale MIT triggered by surface termination conversion in SrRuO3 ultrathin films is reported. Uniform and effective termination engineering at the SrRuO3(001) surface can be realized via a self‐limiting water‐leaching process. As the surface termination converts from SrO to RuO2, a highly insulating and nonferromagnetic phase emerges within the topmost SrRuO3 monolayer. Such a spatially confined MIT is corroborated by systematic characterizations on electrical transport, magnetism, and scanning tunneling spectroscopy. Density functional theory calculations and X‐ray linear dichroism further suggest that the surface termination conversion breaks the local octahedral symmetry of the crystal field. The resultant modulation in 4d orbital occupancy stabilizes a nonferromagnetic insulating surface state. This work introduces a new paradigm to stimulate and tune exotic functionalities of oxide heterostructures with atomic precision.
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Dec 2019
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B18-Core EXAFS
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Celeste A. M.
Van Den Bosch
,
Andrea
Cavallaro
,
Roberto
Moreno
,
Giannantonio
Cibin
,
Gwilherm
Kerherve
,
José M.
Caicedo
,
Thomas K.
Lippert
,
Max
Doebeli
,
José
Santiso
,
Stephen J.
Skinner
,
Ainara
Aguadero
Diamond Proposal Number(s):
[13530, 16839]
Abstract: Understanding the effects of lattice strain on oxygen surface and diffusion kinetics in oxides is a controversial subject that is critical for developing efficient energy storage and conversion materials. In this work, high‐quality epitaxial thin films of the model perovskite La0.5Sr0.5Mn0.5Co0.5O3−δ (LSMC), under compressive or tensile strain, are characterized with a combination of in situ and ex situ bulk and surface‐sensitive techniques. The results demonstrate a nonlinear correlation of mechanical and chemical properties as a function of the operation conditions. It is observed that the effect of strain on reducibility is dependent on the “effective strain” induced on the chemical bonds. In‐plain strain, and in particular the relative BO length bond, is the key factor controlling which of the B‐site cation can be reduced preferentially. Furthermore, the need to use a set of complimentary techniques to isolate different chemically induced strain effects is proven. With this, it is confirmed that tensile strain favors the stabilization of a more reduced lattice, accompanied by greater segregation of strontium secondary phases and a decrease of oxygen exchange kinetics on LSMC thin films.
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Dec 2019
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[17223]
Open Access
Abstract: Halide perovskites are emerging as valid alternatives to conventional photovoltaic active materials owing to their low cost and high device performances. This material family also shows exceptional tunability of properties by varying chemical components, crystal structure, and dimensionality, providing a unique set of building blocks for new structures. Here, highly stable self‐assembled lead–tin perovskite heterostructures formed between low‐bandgap 3D and higher‐bandgap 2D components are demonstrated. A combination of surface‐sensitive X‐ray diffraction, spatially resolved photoluminescence, and electron microscopy measurements is used to reveal that microstructural heterojunctions form between high‐bandgap 2D surface crystallites and lower‐bandgap 3D domains. Furthermore, in situ X‐ray diffraction measurements are used during film formation to show that an ammonium thiocyanate additive delays formation of the 3D component and thus provides a tunable lever to substantially increase the fraction of 2D surface crystallites. These novel heterostructures will find use in bottom cells for stable tandem photovoltaics with a surface 2D layer passivating the 3D material, or in energy‐transfer devices requiring controlled energy flow from localized surface crystallites to the bulk.
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Nov 2019
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I07-Surface & interface diffraction
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Zahra
Andaji-garmaroudi
,
Mojtaba
Abdi-jalebi
,
Dengyang
Guo
,
Stuart
Macpherson
,
Aditya
Sadhanala
,
Elizabeth M.
Tennyson
,
Edoardo
Ruggeri
,
Miguel
Anaya
,
Krzysztof
Galkowski
,
Ravichandran
Shivanna
,
Kilian
Lohmann
,
Kyle
Frohna
,
Sebastian
Mackowski
,
Tom J.
Savenije
,
Richard H.
Friend
,
Samuel D.
Stranks
Diamond Proposal Number(s):
[17223]
Open Access
Abstract: Mixed‐halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of solution‐processed triple‐cation mixed‐halide (Cs0.06MA0.15FA0.79)Pb(Br0.4I0.6)3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar‐equivalent illumination. It is found that the illumination leads to localized surface sites of iodide‐rich perovskite intermixed with passivating PbI2 material. Time‐ and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide‐rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed‐halide mixed‐cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.
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Sep 2019
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E01-JEM ARM 200CF
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Diamond Proposal Number(s):
[16854]
Abstract: 2D crystals are typically uniform and periodic in‐plane with stacked sheet‐like structure in the out‐of‐plane direction. Breaking the in‐plane 2D symmetry by creating unique lattice structures offers anisotropic electronic and optical responses that have potential in nanoelectronics. However, creating nanoscale‐modulated anisotropic 2D lattices is challenging and is mostly done using top‐down lithographic methods with ≈10 nm resolution. A phase transformation mechanism for creating 2D striated lattice systems is revealed, where controlled thermal annealing induces Se loss in few‐layered PdSe2 and leads to 1D sub‐nm etched channels in Pd2Se3 bilayers. These striated 2D crystals cannot be described by a typical unit cells of 1–2 Å for crystals, but rather long range nanoscale periodicity in each three directions. The 1D channels give rise to localized conduction states, which have no bulk layered counterpart or monolayer form. These results show how the known family of 2D crystals can be extended beyond those that exist as bulk layered van der Waals crystals by exploiting phase transformations by elemental depletion in binary systems.
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Sep 2019
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Xiyang
Li
,
Shilei
Zhang
,
Hang
Li
,
Diego
Alba Venero
,
Jonathan S
White
,
Robert
Cubitt
,
Qingzhen
Huang
,
Jie
Chen
,
Lunhua
He
,
Gerrit
Van Der Laan
,
Wenhong
Wang
,
Thorsten
Hesjedal
,
Fangwei
Wang
Open Access
Abstract: A biskyrmion consists of two bound, topologically stable, skyrmion spin textures. These coffee‐bean‐shaped objects are observed in real space in thin plates using Lorentz transmission electron microscopy (LTEM). From LTEM imaging alone, it is not clear whether biskyrmions are surface‐confined objects, or, analogous to skyrmions in non-centrosymmetric helimagnets, 3D tube‐like structures in a bulk sample. Here, the biskyrmion form factor is investigated in single‐ and polycrystalline‐MnNiGa samples using small‐angle neutron scattering. It is found that biskyrmions are not long‐range ordered, not even in single crystals. Surprisingly all of the disordered biskyrmions have their in‐plane symmetry axis aligned along certain directions, governed by the magneto-crystalline anisotropy. This anisotropic nature of biskyrmions may be further exploited to encode information.
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Mar 2019
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I10-Beamline for Advanced Dichroism
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Wenjing
Li
,
Iuliia
Bykova
,
Shilei
Zhang
,
Guoqiang
Yu
,
Riccardo
Tomasello
,
Mario
Carpentieri
,
Yizhou
Liu
,
Yao
Guang
,
Joachim
Gräfe
,
Markus
Weigand
,
David M.
Burn
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Zhengren
Yan
,
Jiafeng
Feng
,
Caihua
Wan
,
Jinwu
Wei
,
Xiao
Wang
,
Xiaomin
Zhang
,
Hongjun
Xu
,
Chenyang
Guo
,
Hongxiang
Wei
,
Giovanni
Finocchio
,
Xiufeng
Han
,
Gisela
Schütz
Diamond Proposal Number(s):
[18898]
Abstract: Room temperature magnetic skyrmions in magnetic multilayers are considered as information carriers for future spintronic applications. Currently, a detailed understanding of the skyrmion stabilization mechanisms is still lacking in these systems. To gain more insight, it is first and foremost essential to determine the full real-space spin configuration. Here, two advanced X-ray techniques are applied, based on magnetic circular dichroism, to investigate the spin textures of skyrmions in [Ta/CoFeB/MgO]n multilayers. First, by using ptychography, a high-resolution diffraction imaging technique, the 2D out-of-plane spin profile of skyrmions with a spatial resolution of 10 nm is determined. Second, by performing circular dichroism in resonant elastic X-ray scattering, it is demonstrated that the chirality of the magnetic structure undergoes a depthdependent evolution. This suggests that the skyrmion structure is a complex 3D structure rather than an identical planar texture throughout the layer stack. The analyses of the spin textures confirm the theoretical predictions that the dipole–dipole interactions together with the external magnetic field play an important role in stabilizing sub-100 nm diameter skyrmions and the hybrid structure of the skyrmion domain wall. Our combined X-ray-based approach opens the door for in-depth studies of magnetic skyrmion systems, which allows for precise engineering of optimized skyrmion heterostructures.
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Feb 2019
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[7223]
Abstract: Efficient vacuum‐processed organic light‐emitting diodes are fabricated using a carbene–metal–amide material, CMA1. An electroluminescence (EL) external quantum efficiency of 23% is achieved in a host‐free emissive layer comprising pure CMA1. Furthermore external quantum efficiencies of up to 26.9% are achieved in host–guest emissive layers. EL spectra are found to depend on both the emissive‐layer doping concentration and the choice of host material, enabling tuning of emission color from mid‐green (Commission Internationale de l'Éclairage co‐ordinates [0.24, 0.46]) to sky blue ([0.22 0.35]) without changing dopant. This tuning is achieved without compromising luminescence efficiency (>80%) while maintaining a short radiative lifetime of triplets (<1 μs).
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Jul 2018
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I09-Surface and Interface Structural Analysis
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Philipp
Scheiderer
,
Matthias
Schmitt
,
Judith
Gabel
,
Michael
Zapf
,
Martin
Stuebinger
,
Philipp
Schütz
,
Lenart
Dudy
,
Christoph
Schlueter
,
Tien-lin
Lee
,
Michael
Sing
,
Ralph
Claessen
Diamond Proposal Number(s):
[14106, 15200, 15856, 18372]
Abstract: The Mott transistor is a paradigm for a new class of electronic devices—often referred to by the term Mottronics—which are based on charge correlations between the electrons. Since correlation‐induced insulating phases of most oxide compounds are usually very robust, new methods have to be developed to push such materials right to the boundary to the metallic phase in order to enable the metal–insulator transition to be switched by electric gating. Here, it is demonstrated that thin films of the prototypical Mott insulator LaTiO3 grown by pulsed laser deposition under oxygen atmosphere are readily tuned by excess oxygen doping across the line of the band‐filling controlled Mott transition in the electronic phase diagram. The detected insulator to metal transition is characterized by a strong change in resistivity of several orders of magnitude. The use of suitable substrates and capping layers to inhibit oxygen diffusion facilitates full control of the oxygen content and renders the films stable against exposure to ambient conditions. These achievements represent a significant advancement in control and tuning of the electronic properties of LaTiO3+x thin films making it a promising channel material in future Mottronic devices.
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May 2018
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