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106 items found (0 items not approved), displaying 1 to 10.[First/Prev] 1, 2, 3, 4, 5, 6, 7, 8 [Next/Last]

Instruments / Technical Area	Publication Details	Published
I05-ARPES	Pseudogap in a crystalline insulator doped by disordered metals Sae Hee Ryu , Minjae Huh , Do Yun Park , Chris Jozwiak , Eli Rotenberg , Aaron Bostwick , Keun Su Kim Nature 596, 68 - 73 DOI: 10.1038/s41586-021-03683-0 Diamond Proposal Number(s): [25869] Show Abstract ▼ Abstract: Key to our understanding of how electrons behave in crystalline solids is the band structure that connects the energy of electron waves to their wavenumber. Even in phases of matter with only short-range order (liquid or amorphous solid), the coherent part of electron waves still has a band structure. Theoretical models for the band structure of liquid metals were formulated more than five decades ago, but, so far, band-structure renormalization and the pseudogap induced by resonance scattering have remained unobserved. Here we report the observation of the unusual band structure at the interface of a crystalline insulator (black phosphorus) and disordered dopants (alkali metals). We find that a conventional parabolic band structure of free electrons bends back towards zero wavenumber with a pseudogap of 30–240 millielectronvolts from the Fermi level. This is wavenumber renormalization caused by resonance scattering, leading to the formation of quasi-bound states in the scattering potential of alkali-metal ions. The depth of this potential tuned by different kinds of disordered alkali metal (sodium, potassium, rubidium and caesium) allows the classification of the pseudogap of p-wave and d-wave resonance. Our results may provide a clue to the puzzling spectrum of various crystalline insulators doped by disordered dopants, such as the waterfall dispersion observed in copper oxides.	Aug 2021
I10-Beamline for Advanced Dichroism	Periodically modulated skyrmion strings in Cu2OSeO3 D. M. Burn , R. Brearton , Kejing Ran , S. L. Zhang , Gerrit Van Der Laan , T. Hesjedal Npj Quantum Materials 6 DOI: 10.1038/s41535-021-00373-y Diamond Proposal Number(s): [17402, 21868] Open Access Show Abstract ▼ Abstract: Magnetic skyrmions are vortex-like spin textures, which are usually treated as two-dimensional objects. In their lattice state, they form well-ordered, hexagonal structures, which have been studied in great detail. To obtain a three-dimensional (3D) skyrmion crystal, these planes can be envisaged to be stacked up forming skyrmion strings in the third dimension. Here, we report the observation of a 3D skyrmion phase in Cu2OSeO3 by carrying out reciprocal space mapping in resonant elastic x-ray scattering. We observe regions in the magnetic field-cooling phase diagram in which the skyrmion phase apparently coexists with the conical phase. However, such a coexistence is forbidden due to symmetry arguments. Instead, the skyrmion strings themselves are periodically modulated along their axes, as confirmed by micromagnetic simulations. The periodic modulation is in fact a necessary consequence of the evolution of the skyrmion phase out of the conical state and should therefore be a universal property of skyrmion strings in chiral helimagnets.	Aug 2021
I10-Beamline for Advanced Dichroism	Electronic and spin control in functional oxide heterostructures Ronja Anika Hinz DOI: 10.17877/DE290R-22350 Show Abstract ▼ Abstract: Transition metal oxides exhibit a variety of physical properties due to their high tunability of spin, orbital, and charge degrees of freedom. This thesis studies two complex, transition metal oxide thin film/substrate systems, the NiFe2O4/Nb:SrTiO3 (NFO/STO) and the La0.3Sr0.7MnO3/Nb:SrTiO3 (LSMO/ STO). NFO is a ferrimagnetic insulator. LSMO is a ferromagnetic half metal and below a critical thickness transitions to an insulator. However, the physical properties of those complex oxides heterostructures are often directly linked to the quality of the thin oxide film and the interactions between the oxide thin film and its substrate at the interface. In this thesis the ultra thin NFO film growth via pulsed laser deposition is optimized. First those optimized, smooth and single crystalline NFO films are studied with lab measurement methods to ensure a high film quality. In a next step their stoichiometry and crystalline occupancy is studied via hard X-ray photoemission (HAXPES) and X-ray absorption spectroscopy, which were recorded at the large scale synchrotron BESSY II, PETRA III and the DIAMOND Light source. The stoichiometry and occupancy is found to match the theoretical expectations of the NFO oxide in the inverse spinel structure. The magnetic response of the NFO/STO heterostructure is researched with the element selective X-ray magnetic circular dichroism (XMCD). Due to the proximity effect the Ti signal mimics the ferrimagnetic response of the NFO thin film. The electronic properties of Au/NFO/STO and Pt/LSMO/STO devices were investigated via in-operando HAXPES measurements. In-operando HAXPES is recording HAXPES spectra during a voltage is applied to the sample. The band alignment and band bending are investigated by applying Kraut' s method to the HAXPES spectra.	Aug 2021
	Spectroscopic view of ultrafast charge carrier dynamics in single- and bilayer transition metal dichalcogenide semiconductors Paulina Majchrzak , Klara Volckaert , Antonija Grubišić Čabo , Deepnarayan Biswas , Marco Bianchi , Sanjoy K. Mahatha , Maciej Dendzik , Federico Andreatta , Signe S. Grønborg , Igor Markovic , Jonathon M. Riley , Jens C. Johannsen , Daniel Lizzit , Luca Bignardi , Silvano Lizzit , Cephise Cacho , Oliver Alexander , Dan Matselyukh , Adam S. Wyatt , Richard T. Chapman , Emma Springate , Jeppe V. Lauritsen , Phil D. C. King , Charlotte Sanders , Jill A. Miwa , Philip Hofmann , Soeren Ulstrup Journal Of Electron Spectroscopy And Related Phenomena 250 DOI: 10.1016/j.elspec.2021.147093 Open Access Show Abstract ▼ Abstract: The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS2 and WS2 on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9–2.5 eV between our systems. The transient conduction band signals decay on a sub-50 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe a fast timescale on the order of 170 fs, followed by a slow dynamics for the conduction band decay in MoS . These timescales are explained by Auger recombination involving MoS and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.	Jul 2021
I15-Extreme Conditions	Melting line of calcium characterized by in situ LH-DAC XRD and first-principles calculations Simone Anzellini , Dario Alfé , Monica Pozzo , Daniel Errandonea Scientific Reports 11 DOI: 10.1038/s41598-021-94349-4 Diamond Proposal Number(s): [21191] Open Access Show Abstract ▼ Abstract: In this work, the melting line of calcium has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using first-principles calculations. In the investigated pressure and temperature range (pressure between 10 and 40 GPa and temperature between 300 and 3000 K) it was possible to observe the face-centred phase of calcium and to confirm (and characterize for the first time at these conditions) the presence of the body-centred cubic and the simple cubic phase of calcium. The melting points obtained with the two techniques are in excellent agreement. Furthermore, the present results agree with the only existing melting line of calcium obtained in laser-heated diamond anvil cells, using the speckle method as melting detection technique. They also confirm a flat slope of the melting line in the pressure range between 10 and 30 GPa. The flat melting curve is associated with the presence of the solid high-temperature body-centered cubic phase of calcium and to a small volume change between this phase and the liquid at melting. Reasons for the stabilization of the body-centered face at high-temperature conditions will be discussed.	Jul 2021
I05-ARPES	Electronic structure of a thermoelectric material: BiCuSO D. Pei , Y.-Y. Lv , Y. Y. Y. Xia , Y. W. Li , J. Y. Liu , N. B. M. Schröter , C. Cacho , Z. K. Liu , L. X. Yang , G. Li , Y. B. Chen , Y. Chen Physical Review B 103 DOI: 10.1103/PhysRevB.103.245121 Diamond Proposal Number(s): [20617, 25135] Show Abstract ▼ Abstract: We investigate transport properties and the electronic structure of BiCuSO, a thermoelectric material which was predicted as a possible parent compound for unconventional superconductivity. For the p -type BiCuSO samples studied in this paper, our transport measurements show metallic behavior down to 2 K, and we observe an increase of saturated magnetic moment around 4 K, which could be due to the reorientation of the magnetic easy axis. Using angle-resolved photoemission spectroscopy measurements, we acquired the comprehensive electronic structure of BiCuSO including the predicted flat band, and by carrying out photon polarization dependent measurements we further investigated the orbital characters of bands near the Fermi level. Compared with its sister compound, BiCuSeO, we find the flat band could be the origin of high figure of merit ( Z T ) value in the BiCu C h O ( C h = S , Se ) family.	Jun 2021
I10-Beamline for Advanced Dichroism	Deriving the skyrmion Hall angle from skyrmion lattice dynamics Richard Brearton , L. A. Turnbull , J. A. T. Verezhak , G. Balakrishnan , P. D. Hatton , G. Van Der Laan , T. Hesjedal Nature Communications 12 DOI: 10.1038/s41467-021-22857-y Diamond Proposal Number(s): [23784, 23451] Open Access Show Abstract ▼ Abstract: Magnetic skyrmions are topologically non-trivial, swirling magnetization textures that form lattices in helimagnetic materials. These magnetic nanoparticles show promise as high efficiency next-generation information carriers, with dynamics that are governed by their topology. Among the many unusual properties of skyrmions is the tendency of their direction of motion to deviate from that of a driving force; the angle by which they diverge is a materials constant, known as the skyrmion Hall angle. In magnetic multilayer systems, where skyrmions often appear individually, not arranging themselves in a lattice, this deflection angle can be easily measured by tracing the real space motion of individual skyrmions. Here we describe a reciprocal space technique which can be used to determine the skyrmion Hall angle in the skyrmion lattice state, leveraging the properties of the skyrmion lattice under a shear drive. We demonstrate this procedure to yield a quantitative measurement of the skyrmion Hall angle in the room-temperature skyrmion system FeGe, shearing the skyrmion lattice with the magnetic field gradient generated by a single turn Oersted wire.	May 2021
I15-Extreme Conditions	High-pressure structural systematics in neodymium up to 302 GPa S. E. Finnegan , C. V. Storm , E. J. Pace , M. I. Mcmahon , S. G. Macleod , E. Plekhanov , N. Bonini , C. Weber Physical Review B 103 DOI: 10.1103/PhysRevB.103.134117 Show Abstract ▼ Abstract: Angle-dispersive x-ray powder diffraction experiments have been performed on neodymium metal to a pressure of 302 GPa. Up to 70 GPa we observe the h P 4 → c F 4 → h R 24 → o I 16 → h P 3 transition sequence reported previously. At 71(2) GPa we find a transition to a phase which has an orthorhombic structure ( o F 8 ) with eight atoms in the unit cell, space group F d d d . This structure is the same as that recently observed in samarium above 93 GPa, and is isostructural with high-pressure structures found in the actinides Am, Cf, and Cm. We see a further phase transition at 98(1) GPa to a phase with the orthorhombic α -U ( o C 4 ) structure, which remains stable up to 302 GPa, the highest pressure reached in this study. Electronic structure calculations find the same structural sequence, with calculated transition pressures of 66 and 88 GPa, respectively, for the h P 3 → F 8 and o F 8 → o C 4 transitions. The calculations further predict that o C 4 -Nd loses its magnetism at 100 GPa, in agreement with previous experimental results, and it is the accompanying decrease in enthalpy and volume that results in the transition to this phase. Comparison calculations on the o F 8 and o C 4 phases of Sm show that they both retain their magnetism to at least 240 GPa, with the result that o C 4 -Sm is calculated to have the lowest enthalpy over a narrow pressure region near 200 GPa at 0 K.	Apr 2021
B18-Core EXAFS	Monitoring the process of formation of ZnO from ZnO2 using in situ combined XRD/XAS technique Thomas Daley , Kwasi Opuni , Edwin Raj , Andrew J. Dent , Giannantonio Cibin , Timothy I Hyde , Gopinathan Sankar Journal Of Physics: Condensed Matter DOI: 10.1088/1361-648X/abfb91 Diamond Proposal Number(s): [6966] Open Access Show Abstract ▼ Abstract: Use of in situ combined X-ray diffraction and X-ray absorption spectroscopy for the study of the thermal decomposition of zinc peroxide to zinc oxide is reported here. Comparison of data extracted from both X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) with thermo gravimetric analysis (TGA) enabled us to follow the nature of the conversion of ZnO2 to ZnO. A temperature range between 230 and 350oC appears to show a very poorly crystalline ZnO is formed prior to the formation of ordered ZnO material. Both the decrease in white line intensity in the Zn K-edge XANES and resulting lower coordination numbers estimated from analysis of Zn K-edge data of ZnO heated at 500oC, in comparison to bulk ZnO, suggest that that the ZnO produced by this method has significant defects in the system.	Apr 2021
	The nuts and bolts of core-hole constrained ab initio simulation for K-shell x-ray photoemission and absorption spectra Benedikt P. Klein , Samuel J Hall , Reinhard J Maurer Journal Of Physics: Condensed Matter 33 DOI: 10.1088/1361-648X/abdf00 Open Access Show Abstract ▼ Abstract: X-ray photoemission (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy play an important role in investigating the structure and electronic structure of materials and surfaces. Ab initio simulations provide crucial support for the interpretation of complex spectra containing overlapping signatures. Approximate core-hole simulation methods based on density functional theory (DFT) such as the delta-self-consistent-field (ΔSCF) method or the transition potential (TP) method are widely used to predict K-shell XPS and NEXAFS signatures of organic molecules, inorganic materials and metal–organic interfaces at reliable accuracy and affordable computational cost. We present the numerical and technical details of our variants of the ΔSCF and TP method (coined ΔIP-TP) to simulate XPS and NEXAFS transitions. Using exemplary molecules in gas-phase, in bulk crystals, and at metal–organic interfaces, we systematically assess how practical simulation choices affect the stability and accuracy of simulations. These include the choice of exchange–correlation functional, basis set, the method of core-hole localization, and the use of periodic boundary conditions (PBC). We particularly focus on the choice of aperiodic or periodic description of systems and how spurious charge effects in periodic calculations affect the simulation outcomes. For the benefit of practitioners in the field, we discuss sensible default choices, limitations of the methods, and future prospects.	Feb 2021

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