|
|
Yangkun
He
,
Gerhard H.
Fecher
,
Chenguang
Fu
,
Yu
Pan
,
Kaustuv
Manna
,
Johannes
Kroder
,
Ajay
Jha
,
Xiao
Wang
,
Zhiwei
Hu
,
Stefano
Agrestini
,
Javier
Herrero-martin
,
Manuel
Valvidares
,
Yurii
Skourski
,
Walter
Schnelle
,
Plamen
Stamenov
,
Horst
Borrmann
,
Liu Hao
Tjeng
,
Rudolf
Schaefer
,
Stuart S. P.
Parkin
,
John Michael D.
Coey
,
Claudia
Felser
Open Access
Abstract: The development of high‐density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat‐assisted magnetic recording was developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc, coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh2CoSb is introduced as a new hard magnet with potential for thin‐film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m−3 is combined with a saturation magnetization of μ0Ms = 0.52 T at 2 K (2.2 MJ m−3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare‐earth‐free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μB on Co, which is hybridized with neighboring Rh atoms with a large spin–orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its Tc of 450 K, together with a thermal conductivity of 20 W m−1 K−1, make Rh2CoSb a candidate for the development of heat‐assisted writing with a recording density in excess of 10 Tb in.−2.
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Oct 2020
|
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I11-High Resolution Powder Diffraction
|
Masoumeh
Keshavarz
,
Elke
Debroye
,
Martin
Ottesen
,
Cristina
Martin
,
Heng
Zhang
,
Eduard
Fron
,
Robert
Küchler
,
Julian A.
Steele
,
Martin
Bremholm
,
Joris
Van De Vondel
,
Hai I.
Wang
,
Mischa
Bonn
,
Maarten B. J.
Roeffaers
,
Steffen
Wiedmann
,
Johan
Hofkens
Diamond Proposal Number(s):
[22020]
Open Access
Abstract: Lead‐free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs2AgBiBr6 has emerged as a promising material, with suboptimal photon‐to‐charge carrier conversion efficiency, yet well suited for high‐energy photon‐detection applications. Here, the optoelectronic and structural properties of pure Cs2AgBiBr6 and alkali‐metal‐substituted (Cs1−xYx)2AgBiBr6 (Y: Rb+, K+, Na+; x = 0.02) single crystals are investigated. Strikingly, alkali‐substitution entails a tunability to the material system in its response to X‐rays and structural properties that is most strongly revealed in Rb‐substituted compounds whose X‐ray sensitivity outperforms other double‐perovskite‐based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali‐substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron–acoustic phonon scattering is found compared to Cs2AgBiBr6. The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.
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Aug 2020
|
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I10-Beamline for Advanced Dichroism
|
Yao
Guang
,
Yong
Peng
,
Zhengren
Yan
,
Yizhou
Liu
,
Junwei
Zhang
,
Xue
Zeng
,
Senfu
Zhang
,
Shilei
Zhang
,
David M.
Burn
,
Nicolas
Jaouen
,
Jinwu
Wei
,
Hongjun
Xu
,
Jiafeng
Feng
,
Chi
Fang
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Baoshan
Cui
,
Xixiang
Zhang
,
Guoqiang
Yu
,
Xiufeng
Han
Diamond Proposal Number(s):
[20183, 21868]
Abstract: The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high‐density and energy‐efficient magnetic memory devices for information technology, the manifestation of their nontrivial topology through single skyrmions and ordered and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is the key to advancing topological magnetism. Here, a new, yet general, approach to the “printing” of skyrmions with zero‐field stability in arbitrary patterns on a massive scale in exchange‐biased magnetic multilayers is presented. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, a focused electron beam with a graphic pattern generator to “print” skyrmions is used, which is referred to as skyrmion lithography. This work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.
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Aug 2020
|
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I05-ARPES
|
Alfred J. H.
Jones
,
Ryan
Muzzio
,
Paulina
Majchrzak
,
Sahar
Pakdel
,
Davide
Curcio
,
Klara
Volckaert
,
Deepnarayan
Biswas
,
Jacob
Gobbo
,
Simranjeet
Singh
,
Jeremy T.
Robinson
,
Kenji
Watanabe
,
Takashi
Taniguchi
,
Timur K.
Kim
,
Cephise
Cacho
,
Nicola
Lanata
,
Jill A.
Miwa
,
Philip
Hofmann
,
Jyoti
Katoch
,
Soeren
Ulstrup
Diamond Proposal Number(s):
[24072]
Abstract: The possibility of triggering correlated phenomena by placing a singularity
of the density of states near the Fermi energy remains an intriguing avenue toward engineering the properties of quantum materials. Twisted bilayer gra- phene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. Direct demonstration of electrostatic control of the superlattice bands over a wide energy range has, so far, been critically missing. This work examines the effect of electrical doping on the electronic band structure of twisted bilayer graphene using a back-gated device archi- tecture for angle-resolved photoemission measurements with a nano-focused light spot. A twist angle of 12.2° is selected such that the superlattice Brillouin zone is sufficiently large to enable identification of van Hove singularities and flat band segments in momentum space. The doping dependence of these fea- tures is extracted over an energy range of 0.4 eV, expanding the combinations of twist angle and doping where they can be placed at the Fermi energy and thereby induce new correlated electronic phases in twisted bilayer graphene.
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Jun 2020
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[23159]
Open Access
Abstract: The coupling of nickel‐rich LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes with high‐capacity silicon–graphite (Si–Gr) anodes is one promising route to further increase the energy density of lithium‐ion batteries. Practically, however, the cycle life of such cells is seriously hindered due to continuous electrolyte degradation on the surfaces of both electrodes. In this study, tris(trimethylsilyl) phosphite (TMSPi) is introduced as an electrolyte additive to improve the electrochemical performance of the NMC811/Si–Gr full cells through formation of protective surface layers at the electrode/electrolyte interfaces. This is thought to prevent the surface fluorination of the active materials and enhance interfacial stability. Notably, TMSPi is shown to significantly reduce the overpotential and operando X‐ray diffraction (XRD) confirms that an irreversible “two‐phase” transition reaction caused by the formed adventitious Li2CO3 layer on the surface of NMC811 can transfer to a solid‐solution reaction mechanism with TMSPi‐added electrolyte. Moreover, influences of TMSPi on the cathode electrolyte interphase (CEI) on the NMC811 and solid electrolyte interphase (SEI) on the Si–Gr are systematically investigated by electron microscopy and synchrotron‐based X‐ray photoelectron spectroscopy which allows for the nondestructive depth‐profiling analysis of chemical compositions and oxidation states close to the electrode surfaces.
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Jun 2020
|
|
I05-ARPES
|
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.
|
Feb 2020
|
|
I16-Materials and Magnetism
|
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.
|
Dec 2019
|
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B18-Core EXAFS
|
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.
|
Dec 2019
|
|
I07-Surface & interface diffraction
|
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.
|
Nov 2019
|
|
I07-Surface & interface diffraction
|
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.
|
Sep 2019
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