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665 items found (3 items not approved), displaying 71 to 80.[First/Prev] 4, 5, 6, 7, 8, 9, 10, 11 [Next/Last]

Instruments / Technical Area	Publication Details	Published
I09-Surface and Interface Structural Analysis	Ni3+-induced semiconductor-to-metal transition in spinel nickel cobaltite thin films X. C. Huang , W.-W. Li , S. Zhang , F. E. Oropeza , G. Gorni , V. A. De La Pena-O'Shea , T.-L. Lee , M. Wu , L.-S. Wang , D.-C. Qi , L. Qiao , J. Cheng , K. H. L. Zhang Physical Review B 104 DOI: 10.1103/PhysRevB.104.125136 Diamond Proposal Number(s): [24219] Show Abstract ▼ Abstract: In this paper, we report insights into the local atomic and electronic structure of NiCo 2 O 4 epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined NiCo 2 O 4 epitaxial thin films with controlled properties on Mg Al 2 O 4 ( 001 ) substrates using pulsed laser deposition. Films grown at low temperatures ( < 400 ∘ C ) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the Ni 3 + valence state promoted at low growth temperature introduces delocalized Ni 3 d -derived states at the Fermi level ( E F ), responsible for the metallic state in NiCo 2 O 4 , while the Co 3 d -related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and ∼ + 2 . Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the Ni 2 + valence state in NiCo 2 O 4 . The Ni 3 d -related state becomes localized away from E F , opening a band gap for a semiconducting state. The band gap of the semiconducting NiCo 2 O 4 is estimated to be < 0.8 eV , which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is d − d dipole forbidden, and therefore, the semiconducting NiCo 2 O 4 still shows reasonable transparency in the infrared-visible light region. The present insights into the role of Ni 3 + in determining the electronic structure and defect chemistry of NiCo 2 O 4 provide important guidance for use of NiCo 2 O 4 in electrocatalysis and opto-electronics.	Sep 2021
I10-Beamline for Advanced Dichroism	Ferromagnetism in ultrathin surface-free La0.7Sr0.3MnO3 layers in electrostatically defined heterostructures A. Galdi , N. Coppola , C. Sacco , L. Maritato , P. Bencok , P. Steadman , P. Orgiani , C. Aruta Physical Review Materials 5 DOI: 10.1103/PhysRevMaterials.5.084409 Diamond Proposal Number(s): [14915] Show Abstract ▼ Abstract: Electrostatically defined perovskite oxide heterostructures, in which carriers are confined by the modulation of the A-site ion charge, offer new possibilities of tuning the magnetic properties of manganite oxides. We investigate the preferential orientation of ferromagnetic and antiferromagnetic moments in ultrathin La 0.7 Sr 0.3 MnO 3 layers embedded in antiferromagnetic Sr MnO 3 as they undergo a metal-to-insulator transition with decreasing thickness. Our results evince the role of orbital occupation, metallicity, and competition of different magnetic phases, in absence of spurious effects occurring in thin films as a result of symmetry breaking at La 0.7 Sr 0.3 MnO 3 interfaces and of incorporation of oxygen vacancies.	Aug 2021
	Hidden charge order in an iron oxide square-lattice compound Jung-Hwa Kim , Darren C. Peets , Manfred Reehuis , Peter Adler , Andrey Maljuk , Tobias Ritschel , Morgan C. Allison , Jochen Geck , Jose R. L. Mardegan , Pablo J. Bereciartua Perez , Sonia Francoual , Andrew Walters , Thomas Keller , Paula M. Abdala , Philip Pattison , Pinder Dosanjh , Bernhard Keimer Physical Review Letters 127 DOI: 10.1103/PhysRevLett.127.097203 Open Access Show Abstract ▼ Abstract: Since the discovery of charge disproportionation in the FeO 2 square-lattice compound Sr 3 Fe 2 O 7 by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained “hidden” to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K -edge resonant x-ray scattering to demonstrate checkerboard charge order in the FeO 2 planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty of detecting them by conventional probes. We discuss the implications of these findings for research on “hidden order” in other materials.	Aug 2021
I09-Surface and Interface Structural Analysis	Vertical position of Sr dopants in the SrxBi2Se3 superconductor You-Ron Lin , Mahasweta Bagchi , Serguei Soubatch , Tien-Lin Lee , Jens Brede , Francois C. Bocquet , Christian Kumpf , Yoichi Ando , F. Stefan Tautz Physical Review B 104 DOI: 10.1103/PhysRevB.104.054506 Diamond Proposal Number(s): [23317] Show Abstract ▼ Abstract: The discovery of topological superconductivity in doped Bi 2 Se 3 made this class of materials highly important for the field of condensed matter physics. However, the structural origin of the superconducting state remained elusive, despite being investigated intensively in recent years. We use scanning tunneling microscopy and the normal incidence x-ray standing wave (NIXSW) technique in order to determine the vertical position of the dopants—one of the key parameters for understanding topological superconductivity in this material— for the case of Sr x Bi 2 Se 3 . In particular, we analyze the NIXSW data in consideration of the inelastic mean free path of the photoemitted electrons, which allows us to distinguish between symmetry-equivalent sites. We find that Sr atoms are not situated inside the van der Waals gap between the Bi 2 Se 3 quintuple layers but rather in the quintuple layer close to the outer Se planes.	Aug 2021
I16-Materials and Magnetism	Measurements of polarization dependencies in parametric down-conversion of x rays into ultraviolet radiation S. Sofer , O. Sefi , A. G. A. Nisbet , S. Shwartz Physical Review B 104 DOI: 10.1103/PhysRevB.104.085207 Diamond Proposal Number(s): [20485] Show Abstract ▼ Abstract: We present measurements of the polarization dependencies of the x-ray signal photons generated by the effect of parametric down-conversion of x rays into ultraviolet radiation. The results exhibit pronounced discrepancies with the classical model for the nonlinearity but qualitatively agree with a recently developed quantum mechanical theory for the nonlinear interaction. Our work shows that the reconstruction of the atomic scale charge distribution of valence electrons in crystals by using nonlinear interaction between x rays and longer wavelength radiation, as was suggested in previous works, requires the knowledge of polarization of the generated x-ray signal beam. The results presented in this work indicate a methodology for the study of properties of the Wannier functions in crystals.	Aug 2021
B16-Test Beamline	Controlling core-hole lifetime through an x-ray planar cavity Xin-Chao Huang , Xiang-Jin Kong , Tian-Jun Li , Zi-Ru Ma , Hong-Chang Wang , Gen-Chang Liu , Zhan-Shan Wang , Wen-Bin Li , Lin-Fan Zhu Physical Review Research 3 DOI: 10.1103/PhysRevResearch.3.033063 Diamond Proposal Number(s): [21446] Open Access Show Abstract ▼ Abstract: It has long been believed that the core-hole lifetime (CHL) of an atom is an intrinsic physical property, and controlling it is significant yet is very hard. Here, the CHL of the 2 p state of a W atom is manipulated experimentally by adjusting the emission rate of a resonant fluorescence channel with the assistance of an x-ray thin-film planar cavity. The enhanced emission rate is linearly accelerated by the photonic density of states inside the cavity, which can be directly controlled by adjusting the cavity field amplitude through choosing different cavity modes or changing the angle offset in experiment. This experimental observation is in good agreement with the prediction of a developed theoretical model. It is found that the manipulated resonant fluorescence channel can even dominate the CHL. The controllable CHL realized here will facilitate the nonlinear investigations and modern x-ray scattering techniques in the hard x-ray region.	Jul 2021
	Ultrawide temperature range super-invar behavior of R2(Fe,Co)17 materials (R = rare earth) Yili Cao , Kun Lin , Sergii Khmelevskyi , Maxim Avdeev , Keith M. Taddei , Qiang Zhang , Qingzhen Huang , Qiang Li , Kenichi Kato , Chiu Chung Tang , Alexandra Gibbs , Chin-Wei Wang , Jinxia Deng , Jun Chen , Hongjie Zhang , Xianran Xing Physical Review Letters 127 DOI: 10.1103/PhysRevLett.127.055501 Show Abstract ▼ Abstract: Super Invar (SIV), i.e., zero thermal expansion of metallic materials underpinned by magnetic ordering, is of great practical merit for a wide range of high precision engineering. However, the relatively narrow temperature window of SIV in most materials restricts its potential applications in many critical fields. Here, we demonstrate the controlled design of thermal expansion in a family of R 2 ( Fe , Co ) 17 materials ( R = rare Earth). We find that adjusting the Fe-Co content tunes the thermal expansion behavior and its optimization leads to a record-wide SIV with good cyclic stability from 3–461 K, almost twice the range of currently known SIV. In situ neutron diffraction, Mössbauer spectra and first-principles calculations reveal the 3 d bonding state transition of the Fe-sublattice favors extra lattice stress upon magnetic ordering. On the other hand, Co content induces a dramatic enhancement of the internal molecular field, which can be manipulated to achieve “ultrawide” SIV over broad temperature, composition and magnetic field windows. These findings pave the way for exploiting thermal-expansion-control engineering and related functional materials.	Jul 2021
I16-Materials and Magnetism	Assessing temperature effects on multipole contributions and angular dependence in core-level spectroscopies S. Delhommaye , G. Radtke , C. Brouder , S. P. Collins , S. Huotari , C. Sahle , M. Lazzeri , L. Paulatto , D. Cabaret Physical Review B 104 DOI: 10.1103/PhysRevB.104.024302 Show Abstract ▼ Abstract: This study aims at assessing the thermal nuclei motion effects on the multipole transition channels involved in two core-level spectroscopies, x-ray absorption spectroscopy (XAS) and x-ray Raman scattering (XRS). Temperature effects on the 1 s → s monopole, 1 s → p dipole, and 1 s → d quadrupole transitions are investigated using two reference systems for which we present original experimental data: α − Al 2 O 3 at the Al K edge probed by XRS at room temperature and rutile Ti O 2 at the Ti K pre-edge probed by XAS at temperatures ranging from 6 to 700 K. Through the latter, this work enlightens the part of the pre-edge peak enhancement due to temperature in the K pre-edge region of 3 d transition metal, which is known to be routinely used to determine the concentration, valence or symmetry of the probed element in a given sample. Nuclear thermal fluctuations are taken into account using a method based on density functional theory that consists in averaging spectra over atomic configurations, generated within the harmonic approximation and obeying quantum statistics at finite temperature. Since only a finite number of such configurations are used, the numerically averaged spectra generally lose the symmetry of the equilibrium crystal positions. In this paper, we demonstrate that the physical average has to be symmetric and propose a method to restore the physical angular dependence of the spectra. The approach is successfully applied to investigate the angular dependent XAS spectra in rutile as a function of temperature. The two systems under study allow to draw general conclusions regarding the effect of nuclear quantum fluctuations on the different transition channels available to both core-level spectroscopies.	Jul 2021
I16-Materials and Magnetism	Stable humplike Hall effect and noncoplanar spin textures in SrRuO 3 ultrathin films Byungmin Sohn , Bongju Kim , Se Young Park , Hwan Young Choi , Jae Young Moon , Taeyang Choi , Young Jai Choi , Hua Zhou , Jun Woo Choi , Alessandro Bombardi , Dan. G. Porter , Seo Hyoung Chang , Jung Hoon Han , Changyoung Kim Physical Review Research 3 DOI: 10.1103/PhysRevResearch.3.023232 Diamond Proposal Number(s): [22181] Open Access Show Abstract ▼ Abstract: We observed a humplike feature in Hall effects of SrRuO 3 ultrathin films, and systematically investigated it by controlling thicknesses, temperatures and magnetic fields. The humplike feature is extremely stable, even surviving as a magnetic field is tilted by as much as 85 ∘ . Based on the atomic-level structural analysis of a SrRuO 3 ultrathin film with a theoretical calculation, we reveal that atomic rumplings at the thin-film surface enhance Dzyaloshinskii-Moriya interaction, which can generate stable chiral spin textures and a humplike Hall effect. Moreover, temperature dependent resonant x-ray measurements at the Ru L edge under a magnetic field showed that the intensity modulation of unexpected peaks was correlated with the hump region in the Hall effect. We verify that the two-dimensional property of ultrathin films generates stable noncoplanar spin textures having a magnetic order in a ferromagnetic oxide material.	Jun 2021
I15-Extreme Conditions	Behavior of rubidium at over eightfold static compression C. V. Storm , J. D. Mchardy , S. E. Finnegan , E. J. Pace , M. G. Stevenson , M. J. Duff , S. G. Macleod , M. I. Mcmahon Physical Review B 103 DOI: 10.1103/PhysRevB.103.224103 Diamond Proposal Number(s): [22350] Show Abstract ▼ Abstract: The high pressure phases of Rb have previously been investigated to 101 GPa, above which Rb is predicted to adopt a double-hexagonal close-packed (dhcp, Pearson hP4) structure similar to that already observed in cesium at 72 GPa. Previous ab initio structure searches have indicated that the hP4 phase should become stable in rubidium at 143 GPa . We present data from static compression experiments on Rb up to 264 ( 8 ) GPa , showing the onset of the hP4 phase at 207 ( 6 ) GPa . The V / V 0 of ∼ 0.121 measured at 264 GPa constitutes the highest compression ratio (more than eightfold) at which structural information has been obtained from a metal using x-ray diffraction methods and is second only to x-ray measurements performed on hydrogen at V / V 0 ∼ 0.094 at 190 GPa . At these extreme compression ratios, the compressive behavior of rubidium shifts from that of a free electron metal to that of a regular d -block metal.	Jun 2021

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