S. G. Altendorf

Publications (11)11.55 Total impact

• Article: Local correlations, non-local screening, multiplets, and band formation in NiO

  T. Haupricht, J. Weinen, A. Tanaka, R. Gierth, S. G. Altendorf, Y. -Y. Chin, T. Willers, J. Gegner, H. Fujiwara, F. Strigari, A. Hendricks, D. Regesch, Z. Hu, Hua Wu, K. -D. Tsuei, Y. F. Liao, H. H. Hsieh, H. -J. Lin, C. T. Chen, L. H. Tjeng
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  ABSTRACT: We report on a comparative study of the valence band electronic structure of NiO as bulk material and of NiO as impurity in MgO. From the impurity we have been able to determine reliably the parameters which describe the local correlations, thereby establishing the compensated-spin character of the first ionization state or the state created by hole doping. Using bulk-sensitive x-ray photoemission we identify pronounced satellite features in the valence band of bulk NiO which cannot be explained by single-site many body approaches nor by mean field calculations. We infer the presence of screening processes involving local quasi-core states in the valence band and non-local coherent many body states. These processes are strong and the propagation of an extra hole in the valence band of NiO will therefore be accompanied by a range of high energy excitations. This in turn will make the observation of the dispersion relations in the Ni 3d bands difficult, also because the effective band width is no more than 0.25 eV as estimated from multi-site calculations.
  10/2012;
• Article: NO-assisted molecular-beam epitaxial growth of nitrogen substituted EuO

  R. Wicks, S. G. Altendorf, C. Caspers, H. Kierspel, R. Sutarto, L. H. Tjeng, A. Damascelli
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  ABSTRACT: We have investigated a method for substituting oxygen with nitrogen in EuO thin films, which is based on molecular beam epitaxy distillation with NO gas as the oxidizer. By varying the NO gas pressure, we produce crystalline, epitaxial EuO_(1-x)N_x films with good control over the films' nitrogen concentration. In-situ x-ray photoemission spectroscopy reveals that nitrogen substitution is connected to the formation Eu3+ 4f6 and a corresponding decrease in the number of Eu2+ 4f7, indicating that nitrogen is being incorporated in its 3- oxidation state. While small amounts of Eu3+ in over-oxidized Eu_(1-delta)O thin films lead to a drastic suppression of the ferromagnetism, the formation of Eu3+ in EuO_(1-x)N_x still allows the ferromagnetic phase to exist with an unaffected Tc, thus providing an ideal model system to study the interplay between the magnetic f7 (J=7/2) and the non-magnetic f6 (J=0) states close to the Fermi level.
  04/2012;
• Source

  Available from: J. W. Freeland

  Article: Asymmetric orbital-lattice interactions in ultrathin correlated oxide films.

  J Chakhalian, J M Rondinelli, Jian Liu, B A Gray, M Kareev, E J Moon, N Prasai, J L Cohn, M Varela, I C Tung, M J Bedzyk, S G Altendorf, F Strigari, B Dabrowski, L H Tjeng, P J Ryan, J W Freeland
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  ABSTRACT: Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.
  Physical Review Letters 09/2011; 107(11):116805. · 7.37 Impact Factor
• Article: Strain-dependent transport properties of the ultra-thin correlated metal, LaNiO3

  E J Moon, B A Gray, M Kareev, J Liu, S G Altendorf, F Strigari, L H Tjeng, J W Freeland, J Chakhalian
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  ABSTRACT: We explore the electrical transport and magneto-conductance (MC) in quasi-two-dimensional strongly correlated ultra-thin films of LaNiO3 (LNO) to investigate the effect of hetero-epitaxial strain on electron–electron and electron–lattice interactions from the low to intermediate temperature range (2–170 K). The fully epitaxial 10 unit cell thick films spanning tensile strain up to ~4% are used to investigate the effects of enhanced carrier localization driven by a combination of weak localization (WL) and electron–electron interactions at low temperatures. The MC data show the importance of the increased contribution of WL to low-temperature quantum corrections. The obtained results demonstrate that with increasing tensile strain and reduced temperature, the quantum-confined LNO system gradually evolves from the Mott into the Mott–Anderson regime.
  New Journal of Physics 07/2011; 13(7):073037. · 4.18 Impact Factor
• Source

  Available from: ArXiv

  Article: Strain dependent transport properties of the quasi two-dimensional correlated metal, LaNiO$_{3}$

  E. J. Moon, B. A. Gray, M. Kareev, J Liu, S. G. Altendorf, F. Strigari, L. H. Tjeng, J. W. Freeland, J. Chakhalian
  [show abstract] [hide abstract]
  ABSTRACT: We explore the electrical transport and magneto-conductance in quasi two-dimensional strongly correlated ultrathin films of LaNiO$_{3}$ (LNO) to investigate the effect of hetero-epitaxial strain on electron-electron and electron-lattice interactions from the low to intermediate temperature range (2K$\sim$170K). The fully epitaxial 10 unit cell thick films spanning tensile strain up to $\sim4%$ are used to investigate effects of enhanced carrier localization driven by a combination of weak localization and electron-electron interactions at low temperatures. The magneto-conductance data shows the importance of the increased contribution of weak localization to low temperature quantum corrections. The obtained results demonstrate that with increasing tensile strain and reduced temperature the quantum confined LNO system gradually evolves from the Mott into the Mott-Anderson regime.
  05/2011;
• Source

  Available from: ArXiv

  Article: Asymmetric orbital-lattice interactions in ultra-thin correlated oxide films

  J. Chakhalian, J. M. Rondinelli, Jian Liu, B. A. Gray, M. Kareev, E. J. Moon, N. Prasai, J. L. Cohn, M. Varela, I. C. Tung, M. J. Bedzyk, S. G. Altendorf, F. Strigari, B. Dabrowski, L. H. Tjeng, P. J. Ryan, J. W. Freeland
  [show abstract] [hide abstract]
  ABSTRACT: Using resonant X-ray spectroscopies combined with density functional calculations, we find an asymmetric bi-axial strain-induced $d$-orbital response in ultra-thin films of the correlated metal LaNiO$_3$ which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.
  08/2010;
• Source

  Available from: M. Moretti Sala

  Article: Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)

  R. Sutarto, S. G. Altendorf, B. Coloru, M. Moretti Sala, T. Haupricht, C. F. Chang, Z. Hu, C. Schüßler-Langeheine, N. Hollmann, H. Kierspel, J. A. Mydosh, H. H. Hsieh, H.-J. Lin, C. T. Chen, L. H. Tjeng
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  ABSTRACT: We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm the absence of Eu3+ contaminants and to determine the actual Gd concentration. The distillation process ensures the absence of oxygen vacancies in the films. From magnetization measurements we found the Curie temperature to increase smoothly as a function of doping from 70 K up to a maximum of 125 K. A threshold behavior was not observed for concentrations as low as 0.2%.
  Phys. Rev. B. 03/2009; 80(8).
• Source

  Available from: M. Moretti Sala

  Article: Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation

  R. Sutarto, S. G. Altendorf, B. Coloru, M. Moretti Sala, T. Haupricht, C F Chang, Z Hu, C. Schüßler-Langeheine, N. Hollmann, H. Kierspel, H. H. Hsieh, H J Lin, C T Chen, L. H. Tjeng
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  ABSTRACT: We have succeeded in growing epitaxial and highly stoichiometric films of EuO on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation process during the molecular beam epitaxy assisted growth enables the consistent achievement of stoichiometry. We have also succeeded in growing the films in a layer-by-layer fashion by fine tuning the Eu vs. oxygen deposition rates. The initial stages of growth involve the limited supply of oxygen from the YSZ substrate, but the EuO stoichiometry can still be well maintained. The films grown were sufficiently smooth so that the capping with a thin layer of aluminum was leak tight and enabled ex situ experiments free from trivalent Eu species. The findings were used to obtain recipes for better epitaxial growth of EuO on MgO (001). Comment: 10 pages, 15 figures
  02/2009;
• Article: Oxygen off-stoichiometry and phase separation in EuO thin films

  S. G. Altendorf, A. Efimenko, V. Oliana, H. Kierspel, A. D. Rata, L. H. Tjeng
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  ABSTRACT: We report on our study on the influence of the growth conditions on the europium/oxygen stoichiometry, morphology, magnetic properties, and electrical conductivity of EuO thin films. SQUID magnetometry and x-ray photoelectron spectroscopy were utilized as complementary techniques to determine the oxygen content of EuO1±x thin films grown by molecular beam epitaxy with and without the employment of the so-called Eu distillation process. We found indications for phase separation to occur in Eu-rich as well as in over-oxidized EuO for films grown at substrate temperatures below the Eu distillation temperature. Only a fraction of the excess Eu contributes to the metal-insulator transition in Eu-rich films grown under these conditions. We also observed that the surfaces of these films were ill defined and may even contain more Eu excess than the film average. Only EuO films grown under distillation conditions are guaranteed to have the same magnetic and electrical properties as stoichiometric bulk EuO, and to have surfaces with the proper Eu/O stoichiometry and electronic structure.
  Phys. Rev. B. 84(15).
• Article: Spectroscopic observation of strain-assisted T_ {C} enhancement in EuO upon Gd doping

  S. G. Altendorf, N. Hollmann, R. Sutarto, C. Caspers, R. C. Wicks, Y.-Y. Chin, Z. Hu, H. Kierspel, I. S. Elfimov, H. H. Hsieh, H.-J. Lin, C. T. Chen, L. H. Tjeng
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  ABSTRACT: The origin of the Curie temperature enhancement of EuO upon Gd doping is studied by using soft x-ray absorption spectroscopy on epitaxial pure and Gd-doped EuO thin films. Temperature- and doping-dependent changes in the oxygen K edge spectra provide information about the correlation of magnetism and lattice. Band-structure calculations reveal that these spectral changes as well as the increase of the Curie temperature to 125 K for Gd-doped EuO cannot be explained by electron doping alone. The compression of the crystal lattice due to the incorporation of the smaller Gd3+ ions plays also an important role.
  Phys. Rev. B. 85(8).
• Article: Epitaxial europium oxide on Ni(100) with single-crystal quality

  D. F. Förster, J. Klinkhammer, C. Busse, S. G. Altendorf, T. Michely, Z Hu, Y.-Y. Chin, L. H. Tjeng, J. Coraux, D. Bougault
  Physical Review B, v.83, 045424-1-045424-11 (2011).