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Instruments / Technical Area	Publication Details	Published
I10-Beamline for Advanced Dichroism - scattering	Incipient antiferromagnetism in the Eu-doped topological insulator Bi2Te3 A. Tcakaev , V. B. Zabolotnyy , C. I. Fornari , P. Rüßmann , T. R. F. Peixoto , F. Stier , M. Dettbarn , P. Kagerer , E. Weschke , E. Schierle , P. Bencok , P. H. O. Rappl , E. Abramof , H. Bentmann , E. Goering , F. Reinert , V. Hinkov Physical Review B 102 DOI: 10.1103/PhysRevB.102.184401 Diamond Proposal Number(s): [19994] Show Abstract ▼ Abstract: Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet in a recent photoemission study of Eu-doped Bi 2 Te 3 films, the spectra remained gapless down to T = 20 K. Here we scrutinize whether the conditions for a substantial gap formation in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the M 4 , 5 x-ray absorption and magnetic circular dichroism spectra reveals a Eu 2 + valence and confirms a large magnetic moment, consistent with a 4 f 7 8 S 7 / 2 ground state. At temperatures below 10 K, bulk magnetometry indicates the onset of antiferromagnetic (AFM) ordering. This is in good agreement with density functional theory, which predicts AFM interactions between the Eu impurities. Our results support the notion that antiferromagnetism can coexist with topological surface states in rare-earth-doped Bi 2 Te 3 and call for spectroscopic studies in the Kelvin range to look for novel quantum phenomena such as the quantum anomalous Hall effect.	Nov 2020
I09-Surface and Interface Structural Analysis	Incorporation of europium in Bi2Te3 topological insulator epitaxial films Celso I. Fornari , Hendrik Bentmann , Sergio L. Morelhao , Thiago R. F. Peixoto , Paulo Rappl , Abdul Tcakaev , Volodymyr Zabolotnyy , Martin Kamp , Tien-Lin Lee , Chul-Hee Min , Philipp Kagerer , Raphael Vidal , Anna Isaeva , Michael Ruck , Vladimir Hinkov , Friedrich Reinert , Eduardo Abramof The Journal Of Physical Chemistry C DOI: 10.1021/acs.jpcc.0c05077 Show Abstract ▼ Abstract: In the field of topological materials, the interaction between band topology and magnetism remains a current frontier for the advance of new topological states and spintronic functionalities. Doping with rare-earth elements with large magnetic moment is a current approach to exploit the phenomenology of such interaction. However, dopant solubility into the main matrix plays a major role. In this sense, the present work is focused on elucidating how Eu incorporates into Bi2Te3 lattice as a function of doping. This work reports a systematic investigation of the structural and electronic properties of bismuth telluride epitaxial layers doped with Eu. Bi2Te3 films were grown by molecular beam epitaxy (MBE) on (111) BaF2 substrates with nominal Eu doping ranging from 0 % up to 9 %. X-ray diffraction (XRD) analysis and scanning transmission electron microscopy (TEM) reveal that Eu atoms enter substitutionally on Bi sites up to 4 % of Eu doping. In contrast, the 9 % Eu-doped sample contains epitaxially oriented nanoclusters of EuTe. X ray photoelectron (XPS) and absorption (XAS) spectroscopies show that Eu atoms enter into the Bi2Te3 crystal matrix in the divalent Eu2+ state for all Eu concentrations. Angle resolved photoemission (ARPES) experiments indicate that the topological surface state is preserved in the presence of the local magnetic moments introduced by the Eu impurities.	Jun 2020
I09-Surface and Interface Structural Analysis	Chemical and valence reconstruction at the surface of SmB 6 revealed by means of resonant soft x-ray reflectometry V. B. Zabolotnyy , K. Fürsich , R. J. Green , P. Lutz , K. Treiber , Chul-Hee Min , A. V. Dukhnenko , N. Y. Shitsevalova , V. B. Filipov , B. Y. Kang , B. K. Cho , R. Sutarto , Feizhou He , Friedrich Reinert , D. S. Inosov , V. Hinkov Physical Review B 97 DOI: 10.1103/PhysRevB.97.205416 Diamond Proposal Number(s): [14732] Show Abstract ▼ Abstract: Samarium hexaboride (SmB6), a Kondo insulator with mixed valence, has recently attracted much attention as a possible host for correlated topological surface states. Here, we use a combination of x-ray absorption and reflectometry techniques, backed up with a theoretical model for the resonant M4,5 absorption edge of Sm and photoemission data, to establish laterally averaged chemical and valence depth profiles at the surface of SmB6. We show that upon cleaving, the highly polar (001) surface of SmB6 undergoes substantial chemical and valence reconstruction, resulting in boron termination and a Sm3+ dominated subsurface region. Whereas at room temperature, the reconstruction occurs on a timescale of less than 2 h, it takes about 24 h below 50 K. The boron termination is eventually established, irrespective of the initial termination. Our findings reconcile earlier depth resolved photoemission and scanning tunneling spectroscopy studies performed at different temperatures and are important for better control of surface states in this system.	May 2018
	High-energy electronic interaction in the 3d band of high-temperature iron-based superconductors D. V. Evtushinsky , A. N. Yaresko , V. B. Zabolotnyy , J. Maletz , T. K. Kim , A. A. Kordyuk , M. S. Viazovska , M. Roslova , I. Morozov , R. Beck , S. Aswartham , L. Harnagea , S. Wurmehl , H. Berger , V. A. Rogalev , V. N. Strocov , T. Wolf , N. D. Zhigadlo , B. Büchner , S. V. Borisenko Physical Review B 96 DOI: 10.1103/PhysRevB.96.060501 Show Abstract ▼ Abstract: One of the most unique and robust experimental facts about iron-based superconductors is the renormalization of the electronic band dispersion by factor of 3 and more near the Fermi level. Obviously related to the electron pairing, this prominent deviation from the band theory lacks understanding. Experimentally studying the entire spectrum of the valence electrons in iron arsenides, we have found an unexpected depletion of the spectral weight in the middle of the iron-derived band, which is accompanied by a drastic increase of the scattering rate. At the same time, the measured arsenic-derived band exhibits very good agreement with theoretical calculations. We show that the low-energy Fermi velocity renormalization should be viewed as a part of the modification of the spectral function by a strong electronic interaction. Such an interaction with an energy scale of the whole d band appears to be a hallmark of many families of unconventional superconductors.	Aug 2017
	Strong electron pairing at the iron 3dxz,yz orbitals in hole-doped BaFe2As2 superconductors revealed by angle-resolved photoemission spectroscopy D. V. Evtushinsky , V. B. Zabolotnyy , T. K. Kim , A. A. Kordyuk , A. N. Yaresko , J. Maletz , S. Aswartham , S. Wurmehl , A. V. Boris , D. L. Sun , C. T. Lin , B. Shen , H. H. Wen , A. Varykhalov , R. Follath , B. Büchner , S. V. Borisenko Physical Review B 89 (6) DOI: 10.1103/PhysRevB.89.064514 Show Abstract ▼ Abstract: Using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we explicate the electronic structure of hole-dopedBaFe2As2, and show that widely discussed nesting and dimensionality of Fermi surface (FS) sheets have no immediate relation to the superconducting pairing in iron-based superconductors. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: The magnitude of the superconducting gap maximizes at 10 meV exclusively for iron 3dxz,yz orbitals, while for others drops to 3 meV. Presented results imply that the relation between superconducting and magnetostructural transitions goes beyond simple competition for FS, and demonstrate importance of orbital physics in iron superconductors.	Feb 2014
	Renormalized band structure of Sr2RuO4: A quasiparticle tight-binding approach V. B. Zabolotnyy , D. V. Evtushinsky , A. A. Kordyuk , T. K. Kim , E. Carleschi , B. P. Doyle , R. Fittipaldi , M. Cuoco , A. Vecchione , S. V. Borisenko Journal Of Electron Spectroscopy And Related Phenomena DOI: 10.1016/j.elspec.2013.10.003 Show Abstract ▼ Abstract: We derive an effective quasiparticle tight-binding model which is able to describe with high accuracy the low-energy electronic structure of Sr2RuO4 obtained by means of low temperature angle resolved photoemission spectroscopy. Such an approach is applied to determine the momentum and orbital dependent effective masses and velocities of the electron quasiparticles close to the Fermi level. We demonstrate that the model can provide, among the various computable physical quantities, a very good agreement with the experimentally measured specific heat coefficient and compares well with the plasma frequency estimated from local density calculations. Its use is underlined as a realistic input in the analysis of the possible electronic mechanisms related to the superconducting state of Sr2RuO4.	Oct 2013
	Crystal growth and physical properties of Na-doped BaFe2As2 superconducting single crystals S. Aswartham , M. Abdel-Hafiez , D. Bombor , A. U. B. Wolter , M. Kumar , C. Hess , D. V. Evtushinsky , V. B. Zabolotnyy , A. A. Kordyuk , S. V. Borisenko , S. Wurmehl , B. Buechner , T. K. Kim DPG Spring meeting of the condensed matter section Show Abstract ▼ Abstract: Single crystals of Ba1-xNaxFe2As2 with x = 0, 0.25, 0.35, 0.4 were grown using a self-flux high temperature solution growth technique. The superconducting and normal state properties were studied by temperature dependent magnetic susceptibility, electrical resistivity and specific heat revealing that the magnetic and structural transition is rapidly suppressed upon Na-substitution at the Ba-site in BaFe2As2, giving rise to superconductivity. A superconducting transition as high as 34 K is reached for a Na-content of x = 0.4. The positive Hall coefficient confirms that the substitution of Ba by Na results in hole-doping similarly to the substitution of Ba by K. Angle resolved photoemission spectroscopy was performed on all Ba1-xNaxFe2As2 crystals. The Fermi surface of hole-doped Ba1-xNaxFe2As2 is to high extent the same as the Fermi surface found for the K-doped sister compounds, suggesting a similar impact of the substitution of Ba by either K or Na on the electronic band dispersion at the Fermi level.	Mar 2013
	Weak-coupling superconductivity in electron-doped NaFe0.95Co0.05As revealed by ARPES S. Thirupathaiah , D. V. Evtushinsky , J. Maletz , V. B. Zabolotnyy , A. A. Kordyuk , T. K. Kim , S. Wurmehl , M. Roslova , I. Morozov , B. Buechner , S. V. Borisenko Physical Review B 86 (21) DOI: 10.1103/PhysRevB.86.214508 Show Abstract ▼ Abstract: We report a systematic study on the electronic structure and superconducting (SC) gaps in electron-doped NaFe0.95Co0.05As superconductor using angle-resolved photoemission spectroscopy. Holelike Fermi sheets are at the zone center and electronlike Fermi sheets are at the zone corner, and are mainly contributed by xz and yz orbital characters. Our results reveal a ?KBTc in the range of 1.8–2.1, suggesting a weak-coupling superconductivity in these compounds. Gap closing above the transition temperature (Tc) shows the absence of pseudogaps. Gap evolution with temperature follows the BCS gap equation near the ?, Z, and M high symmetry points. Furthermore, an almost isotropic superconductivity along the kz direction in the momentum space is observed by varying the excitation energies.	Dec 2012
	Incommensurate magnetic fluctuations and Fermi surface topology in LiFeAs J. Knolle , V. B. Zabolotnyy , I. Eremin , S. V. Borisenko , N. Qureshi , M. Braden , D. V. Evtushinsky , T. K. Kim , A. A. Kordyuk , S. Sykora , Ch. Hess , I. V. Morozov , S. Wurmehl , R. Moessner , B. Buechner Physical Review B 86 (17) DOI: 10.1103/PhysRevB.86.174519 Show Abstract ▼ Abstract: Using the angle-resolved photoemission spectroscopy data accumulated over the whole Brillouin zone (BZ) in LiFeAs, we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the (?,?) point of the BZ, and the large two-dimensional hole pocket, centered around the ? point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign-changing symmetry of the superconducting gap between electron and hole pockets and the sign-preserving gap, depending on the assumptions made for the fermionic damping.	Nov 2012
	Angle-resolved photoemission spectroscopy at ultra-low temperatures Sergey Borisenko , Volodymyr B. Zabolotnyy , Alexander A. Kordyuk , Danil V. Evtushinsky , Timur K. Kim , Emanuela Carleschi , Bryan P. Doyle , Rosalba Fittipaldi , Mario Cuoco , Antonio Vecchione , Helmut Berger Journal Of Visualized Experiments 68 DOI: 10.3791/50129 PMID: 23093178 Show Abstract ▼ Abstract: The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (ω) and momentum (k) and the probability to find them in a particular state with given ω and k is described by the spectral function A(k, ω). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.	Oct 2012

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