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  • 1. Ahola-Tuomi, M.
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Laukkanen, P.
    Kuzmin, M.
    Lang, J.
    Schulte, K.
    Pietzsch, A.
    Perala, R. E.
    Rasanen, N.
    Vayrynen, I. J.
    Properties of self-assembled Bi nanolines on InAs(100) studied by core-level and valence-band photoemission, and first-principles calculations2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 24, p. 245401-Article in journal (Refereed)
    Abstract [en]

    We have studied self-assembled bismuth (Bi) nanolines on the Bi-terminated InAs(100) surface by core-level and valence-band photoelectron spectroscopy, and ab initio first-principles calculations. A structural model for this intriguing surface is suggested based on the comparison of the measured and calculated core-level shifts. Also, the atomic origins for the core-level shifts are proposed based on the calculations. A clear peak related to this surface was observed in the valence band 0.34 eV below the Fermi level, which can be used as a "fingerprint" of a well-ordered Bi/InAs(100) nanoline surface.

  • 2. Airiskallio, E.
    et al.
    Nurmi, E.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, Marko P. J.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Tuning the surface chemistry of Fe-Cr by V doping2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 15Article in journal (Refereed)
    Abstract [en]

    The reversal of the magnitudes of the bulk and surface chemical-potential differences induces the outburst of Cr on the otherwise pure Fe surface of Fe-Cr alloys. This threshold value for the Cr content is about 10 at. %. It is found that vanadium addition to Fe-Cr shifts the Cr threshold to a substantially lower value suggesting V having a positive effect on the corrosion resistance of low Cr steels. The obtained shift in the Cr threshold is shown to be connected to the change in volume of the alloy.

  • 3.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Completeness of the exact muffin-tin orbitals: application to hydrogenated alloys2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 4Article in journal (Refereed)
    Abstract [en]

    We investigate the basis set convergence of the exact muffin-tin orbitals by monitoring the equation of state for Al, Cu, and Rh calculated in the conventional face-centered-cubic lattice (str-I) and in a face-centered-cubic lattice with one atomic and three empty sites per primitive cell (str-II). We demonstrate that three (spd) muffin-tin orbitals are sufficient to describe Al in both structures, but for str-II Cu and Rh at least five (spdfg) orbitals are needed to get converged equilibrium Wigner-Seitz radius (within <= 0.8%) and bulk modulus (<= 3.3%). We ascribe this slow convergence to the nearly spherical densities localized around the Cu and Rh atoms, which create strongly asymmetric charge distributions within the nearest cells around the empty sites. The potential sphere radius dependence of the theoretical results for structure str-II is discussed. It is shown that a properly optimized overlapping muffin-tin potential in combination with the spdfg basis yields acceptable errors in the equilibrium bulk properties. The basis set convergence is also shown on hydrogenated Sc and Sc-based alloys.

  • 4.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Influence of Magnesium on hydrogenated ScAl1-xMgx alloys: a theoretical study2011In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 50, no 10, p. 2848-2853Article in journal (Refereed)
    Abstract [en]

    Ab initio total energy calculations, based on the projector augmented wave method and the exact mu±n-tin orbitals method in combination with the coherent-potential approximation, are used to examine the effect of magnesium on hydrogen absorption/desorption temperature and phase stability of hydrogenated ScAl1-xMgx (0 ≤ x ≤ 0:3) alloys. According to the  experiments, ScAl1-xMgx adopts the CsCl structure, and upon hydrogen absorption it decomposes into ScH2 with CaF2 structure and Al-Mg with face centered cubic structure. Here we demonstrate that the stability field of the hydrogenated alloys depends sensitively on Mg content and on the microstructure of the decomposed system. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with Mg concentration.

  • 5.
    Delczeg-Czirjak, Erna Krisztina
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, O.
    Division for Materials Theory, Division of Physics and Materials Science, Uppsala University.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio study of structural and magnetic properties of Si-doped Fe(2)P2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 82, no 8, p. 085103-Article in journal (Refereed)
    Abstract [en]

    Ab initio electronic-structure methods are used to study the properties of Fe(2)P(1-x)Si(x) in ferromagnetic and paramagnetic states. The site preference and lattice relaxation are calculated with the projector augmented wave method as implemented in the Vienna ab initio simulation package. The paramagnetic state is modeled by the disordered local magnetic moment scheme, and the chemical and magnetic disorder is treated using the coherent potential approximation in combination with the exact muffin-tin orbital formalism. The calculated lattice parameters, atomic positions, and magnetic properties are in good agreement with the experimental and other theoretical results. In contrast to the observation, for the ferromagnetic state the body centered ortho-rhombic structure (bco, space group I (mm2) under bar) is predicted to have lower energy than the hexagonal structure (hex, space group P (6) over bar 2m). The zero-point spin fluctuation energy difference is found to be large enough to stabilize the hex phase. For the paramagnetic state, the hex structure is calculated to be the stable phase and the computed total energy versus composition indicates a hex to bco crystallographic phase transition with increasing Si content. The phonon vibrational free energy, estimated from the theoretical equation of state, turns out to stabilize the hexagonal phase, whereas the electronic and magnetic entropies favor the low symmetry orthorhombic structure.

  • 6. Heinonen, M. H.
    et al.
    Kokko, K.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Nurmi, E.
    Kollar, J.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Initial Oxidation of Fe-Al and Fe-Cr-Al Alloys: Cr as an Alumina Booster2011In: Oxidation of Metals, ISSN 0030-770X, E-ISSN 1573-4889, Vol. 76, no 3-4, p. 331-346Article in journal (Refereed)
    Abstract [en]

    The boosting effect of Cr on the growth of the protective alumina scale on Fe-Al alloys is investigated by X-ray photoelectron spectroscopy. Using low oxygen pressure the surface chemistry of the alloys is monitored starting from the first moments of oxidation. Chromium affects the Fe/Al surface-bulk exchange which is clearly detected by analyzing the measured surface concentrations within the atomic concentration models. Experimental results presented are in good agreement with the previous ones obtained by experiments at ambient conditions and ab initio calculations.

  • 7. Kuzmin, M.
    et al.
    Laukkanen, P.
    Perala, R. E.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ahola-Tuomi, M.
    Lang, J.
    Vayrynen, I. J.
    Stability, structural, and electronic properties of atomic chains on Yb/Ge(111)3X2 studied by STM and STS2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 15, p. 155312-Article in journal (Refereed)
    Abstract [en]

    By means of scanning tunneling microscopy and spectroscopy (STM/STS), we have investigated the stability and the structure of atomic chains on Yb/Ge (111) 3 X 2. STM allows the identification of different building blocks of this reconstruction, depending on the bias polarity and voltage, and validates the honeycomb chain-channel (HCC) structure with the Ge=Ge double bond and metal coverage of 1/6 ML for Yb/Ge (111)3 X 2, in agreement with the recent photoemission study [Kuzmin et al., Phys. Rev. B 75, 165305 (2007)]. The Yb atoms are found to be adsorbed on similar sites in the well-defined X 2 rows. Locally, such rows are distorted, leading to the X 4 periodicity, where the Yb atoms are adsorbed on two different sites that are well consistent with T4 and H3 sites. It is also assumed that Yb atoms can fluctuate rapidly between the neighboring T4 and H3 sites, leading to continuous rows observed together with the X 2 rows in STM images. The stability of Ge honeycomb chain is controlled by the presence of Yb atom per two (3 X 1) surface units in average, which results in the donation of one electron from Yb to the surface per (3 X 1) unit. When this density is locally changed, the Ge honeycomb chain is found to be broken. The inner structure of the Ge honeycomb chain is visualized in STM and shows dimerized features without any apparent buckling. The STM observations also account for why the double periodicity is missing in the low-energy electron diffraction pattern from Yb/Ge (111)3X2. The local electronic structure of this reconstruction, namely the Yb rows and Ge honeycomb chains, is studied by STS. The results support the HCC structure with the Ge=Ge double bond. It is believed that the present study elucidates the difference between the (3X2) reconstructions of Yb and Eu on Ge (111) and those of alkaline-earth and rare-earth metals on Si (111).

  • 8. Kuzmin, M.
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Laukkanen, P.
    Lang, J. J. K.
    Dahl, J.
    Tuominen, M.
    Tuominen, V.
    Adell, J.
    Balasubramanian, T.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kokko, K.
    Dimer-T(3) reconstruction of the Sm/Si(100)(2 x 3) surface studied by high-resolution photoelectron spectroscopy and density functional theory calculations2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 24, p. 245322-Article in journal (Refereed)
    Abstract [en]

    Silicon dimer-containing reconstructions on Si(100) can be induced by submonolayer amounts of rare earth (RE) metals. The tilt of dimer bonds in such reconstructions can be controlled by the coverage and electronic properties of RE adsorbates. In this study, we have utilized improved high-resolution photoelectron spectroscopy with the synchrotron radiation and density functional theory (DFT) calculations to exploit the structural and electronic properties of the Sm/Si(100)(2 x 3) system. A careful analysis of photoelectron spectra, in combination with DFT calculations of surface core-level shifts for silicon atoms in energetically plausible structural models, has allowed us to establish the favorable atomic configuration of Sm/Si(100)(2 x 3) with a buckled Si dimer and to explain characteristic features of Si 2p line shape in detail. It is shown that the dimer buckling leads to a significant core-level binding-energy splitting of the first-layer Si atoms, affecting the lower-binding-energy region of Si 2p spectra drastically. An interpretation of the Si 2p line shape for RE/Si(100)(2 x 3) that is based on combined initial state and complete screening data is suggested. The mechanism underlying the buckling and symmetrization of silicon dimers in RE/Si(100) reconstructions is discussed.

  • 9. Kuzmin, M.
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Laukkanen, P.
    Perälä, R. E.
    Lang, J. J. K.
    Dahl, J.
    Adell, J.
    Kokko, K.
    Photoemission and density functional theory study of Ge(100): Clean surface and Yb-induced (2x4) reconstruction2013In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 615, p. 88-96Article in journal (Refereed)
    Abstract [en]

    Clean and metal-adsorbed (100) surfaces of group-IV semiconductors, such as Si and Ge, often exhibit electronically and structurally similar reconstructions. However, the fundamental bulk properties of group-IV materials can have an impact on particular features of such systems, which are related, e.g., to final-state relaxation in photoemission and thus determine their spectral line shape. Here we have studied Yb/Ge(100)(2 x 4) reconstruction as well as clean Ge(100) surface by high-resolution photoelectron spectroscopy and ab initio calculations. An atomic geometry of both surfaces is thoroughly investigated. A detailed analysis of Ge 3d core-level photoemission, atomic origins of surface-shifted components, and final-state screening effects is presented. In particular, it is demonstrated that the core-hole screening plays an essential role in Ge 3d measurements, and that its amount in the complete screening model correlates well with the core-level binding energy of respective Ge atoms in the initial state. The results are discussed in the proper context of related reconstructions on Si(100).

  • 10. Kuzmin, Mikhail
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Laukkanen, Pekka
    Lang, Jouko J. K.
    Dahl, Johnny
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kokko, Kalevi
    Atomic-Level Understanding of Interfaces in the Synthesis of Crystalline Oxides on Semiconductors: Sr- and Ba/Si(100)(2 x 3) Reconstructions2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 4, p. 1894-1902Article in journal (Refereed)
    Abstract [en]

    The synthesis of novel functional crystalline films on semiconductor substrates calls for atomic-level knowledge and controlling of the initial stages of interface or junction formation. Technologically relevant epitaxial oxide films can be grown on Si(100) surfaces modified by submonolayer alkaline earth adsorbates, e.g., barium (Ba) and strontium (Sr). Nevertheless, the fundamental properties of such surfaces, that is, Ba/Si(100) and Sr/Si(100) reconstructions are still controversial, which hinders a deeper insight into the synthesis of crystalline oxide films on silicon. In this study, scanning tunneling microscopy (STM), low-energy electron diffraction, synchrotron-radiation photoemission, and ab initio calculations have been utilized to examine Sr- and Ba-induced Si(100)(2 x 3) reconstructions that form the first mediating step in the growth of various functional oxide films on Si(100). The presented results elucidate the atomic and electronic structures of the Si(100)(2 x 3)-Sr and -Ba interfaces, giving support to the so-called (2 x 3) dimer vacancy structure. In particular, using STM, we demonstrate an evidence for the Si dimer, one of the main structural elements of metal-induced reconstructions on semiconductor (100) surfaces. It is also shown that in contrast to the dimer vacancy geometry, the other models, proposed for the Sr- and Ba/Si(100)(2 x 3) earlier, cannot be adopted.

  • 11. Lang, J. J. K.
    et al.
    Laukkanen, P.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ahola-Tuomi, M.
    Kuzmin, M.
    Tuominen, V.
    Dahl, J.
    Tuominen, M.
    Perala, R. E.
    Schulte, K.
    Adell, J.
    Sadowski, J.
    Kanski, J.
    Guina, M.
    Pessa, M.
    Kokko, K.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vayrynen, I. J.
    Tin-stabilized (1 x 2) and (1 x 4) reconstructions on GaAs(100) and InAs(100) studied by scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations2011In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 605, no 9-10, p. 883-888Article in journal (Refereed)
    Abstract [en]

    Tin (Sn) induced (1 x 2) reconstructions on GaAs(100) and InAs(100) substrates have been studied by low energy electron diffraction (LEED), photoelectron spectroscopy, scanning tunneling microscopy/spectroscopy (STM/STS) and ab initio calculations. The comparison of measured and calculated STM images and surface core-level shifts shows that these surfaces can be well described with the energetically stable building blocks that consist of Sn-III dimers. Furthermore, a new Sn-induced (1 x 4) reconstruction was found. In this reconstruction the occupied dangling bonds are closer to each other than in the more symmetric (1 x 2) reconstruction, and it is shown that the (1 x 4) reconstruction is stabilized as the adatom size increases.

  • 12. Laukkanen, P.
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ahola-Tuomi, M.
    Lång, J.
    Schulte, K.
    Pietzsch, A.
    Kuzmin, M.
    Sadowski, J.
    Adell, J.
    Perälä, R. E.
    Ropo, M.
    Kokko, K.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Pessa, M.
    Väyrynen, I. J.
    Core-level shifts of the c(8 x 2)-reconstructed InAs(100) and InSb(100) surfaces2010In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 177, no 1, p. 52-57Article in journal (Refereed)
    Abstract [en]

    We have studied In-stabilized c(8 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) semiconductor surfaces, which play a key role in growing improved III-V interfaces for electronics devices, by core-level photoelectron spectroscopy and first-principles calculations. The calculated surface core-level shifts (SCLSs) for the zeta and zeta a models, which have been previously established to describe the atomic structures of the III-V(1 00)c(8 x 2) surfaces, yield hitherto not reported interpretation for the As 3d, In 4d, and Sb 4d core-level spectra of the III-V(1 00)c(8 x 2) surfaces, concerning the number and origins of SCLSs. The fitting analysis of the measured spectra with the calculated zeta and zeta a SCLS values shows that the InSb spectra are reproduced by the zeta SCLSs better than by the zeta a SCLSs. Interestingly, the zeta a fits agree better with the InAs spectra than the zeta fits do, indicating that the zeta a model describes the InAs surface better than the InSb surface. These results are in agreement with previous X-ray diffraction data. Furthermore, an introduction of the complete-screening model, which includes both the initial and final state effects, does not improve the fitting of the InSb spectra, proposing the suitability of the initial-state model for the SCLSs of the III-V(1 0 0)c(8 x 2) surfaces. The found SCLSs are discussed with the ab initio on-site charges.

  • 13. Laukkanen, P.
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Lång, J.
    Tuominen, M.
    Kuzmin, M.
    Tuominen, V.
    Dahl, J.
    Adell, J.
    Sadowski, J.
    Kanski, J.
    Polojärvi, V.
    Pakarinen, J.
    Kokko, K.
    Guina, M.
    Pessa, M.
    Väyrynen, I. J.
    Ultrathin (1x2)-Sn layer on GaAs(100) and InAs(100) substrates: A catalyst for removal of amorphous surface oxides2011In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 23, p. 231908-1-231908-3Article in journal (Refereed)
    Abstract [en]

    Amorphous surface oxides of III-V semiconductors are harmful in many contexts of device development. Using low-energy electron diffraction and photoelectron spectroscopy, we demonstrate that surface oxides formed at Sn-capped GaAs(100) and InAs(100) surfaces in air are effectively removed by heating. This Sn-mediated oxide desorption procedure results in the initial well-defined Sn-stabilized (1x2) surface even for samples exposed to air for a prolonged time. Based on ab initio calculations we propose that the phenomenon is due to indirect and direct effects of Sn. The Sn-induced surface composition weakens oxygen adsorption.

  • 14. Laukkanen, P.
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Puustinen, J.
    Levämäki, H.
    Tuominen, M.
    Schulte, K.
    Dahl, J.
    Lang, J.
    Zhang, Hualei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kuzmin, M.
    Palotas, K.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Guina, M.
    Kokko, K.
    Formation and destabilization of Ga interstitials in GaAsN: Experiment and theory2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 19, p. 195205-Article in journal (Refereed)
    Abstract [en]

    Using first-principles total energy calculations we have found complex defects induced by N incorporation in GaAsN. The formation energy of the Ga interstitial atom is very significantly decreased due to local effects within the defect complex. The stability of the Ga interstitials is further increased at surfaces. The present results suggest that the energetically favorable Ga interstitial atoms are much more abundant in GaAsN than the previously considered N defects, which have relatively large formation energies. Our synchrotron radiation core-level photoemission measurements support the computational results. The formation of harmful Ga interstitials should be reduced by incorporating large group IV B atoms in GaAsN.

  • 15. Laukkanen, P.
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Rasanen, N.
    Ahola-Tuomi, M.
    Kuzmin, M.
    Lang, J.
    Sadowski, J.
    Adell, J.
    Perala, R. E.
    Ropo, M.
    Kokko, K.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Pessa, M.
    Vayrynen, I. J.
    Bismuth-stabilized c(2X6) reconstruction on a InSb(100) substrate: Violation of the electron counting model2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 3Article in journal (Refereed)
    Abstract [en]

    By means of scanning tunneling microscopy/spectroscopy (STM/STS), photoelectron spectroscopy, and first-principles calculations, we have studied the bismuth (Bi) adsorbate-stabilized InSb(100) substrate surface which shows a c(2X6) low-energy electron diffraction pattern [thus labeled Bi/InSb(100)c(2X6) surface] and which includes areas with metallic STS curves as well as areas with semiconducting STS curves. The first-principles phase diagram of the Bi/InSb(100) surface demonstrates the presence of the Bi-stabilized metallic c(2X6) reconstruction and semiconducting (4X3) reconstruction depending on the chemical potentials, in good agreement with STS results. The existence of the metallic c(2X6) phase, which does not obey the electron counting model, is attributed to the partial prohibition of the relaxation in the direction perpendicular to dimer rows in the competing reconstructions and the peculiar stability of the Bi-stabilized dimer rows. Based on (i) first-principles phase diagram, (ii) STS results, and (iii) comparison of the measured and calculated STM and photoemission data, we show that the measured Bi/InSb(100)c(2X6) surface includes metallic areas with the stable c(2X6) atomic structure and semiconducting areas with the stable (4X3) atomic structure.

  • 16.
    Punkkinen, Marko P. J.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kuzmin, M.
    Laukkanen, P.
    Perala, R. E.
    Ahola-Tuomi, M.
    Lang, J.
    Ropo, M.
    Pessa, M.
    Vayrynen, I. J.
    Kokko, K.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Stability and structure of rare-earth metal and Ba-induced reconstructions on a Si(100) surface2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 23Article in journal (Refereed)
    Abstract [en]

    We have studied, by means of ab initio calculations, the energetics and the atomic and electronic structures of various reconstructions induced by rare-earth metals (RE=Eu, Nd, Sm, and Yb) and Ba on Si(100) in the coverage range up to 0.5 monolayer. It is shown that Si dimer buckling is an important structural element for such systems, leading frequently to oblique surface lattice symmetries. The strong metal atom-silicon binding favors the increased amount of metal atoms per unit surface area, i.e., the (2x3) reconstruction with two metal atoms per unit cell is found to be energetically unstable with respect to the (2x1) reconstruction with three metal atoms per the same surface area [Eu/Si(100) and Yb/Si(100)]. The influence of the atomic size and the valence of the adsorbates is also investigated. In particular, it is found that an increase in atomic size stimulates the metal-metal repulsion, stabilizing the (2x3) configuration [Ba/Si(100)]. In the case of trivalent metals, the stabilization of the (2x3) is mediated by the loss of semiconducting state in the competing phases [Sm/Si(100) and Nd/Si(100)]. Our results demonstrate the importance of many factors, which account for the abundance of RE/Si(100) reconstructions. Finally, prominent atomic models are proposed for (2x3) and (2x6) reconstructions, and the character of the wavy "(1x2)" reconstruction is discussed. The simulated scanning tunneling microscopy images for the proposed (2x6) reconstruction are in a particularly good agreement with the complex experimental images.

  • 17.
    Punkkinen, Marko P. J.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Laukkanen, P.
    Ahola-Tuomi, M.
    Pakarinen, J.
    Kuzmin, M.
    Tukiainen, A.
    Perala, R. E.
    Lang, J.
    Ropo, M.
    Kokko, K.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Pessa, M.
    Vayrynen, I. J.
    Core-level shifts of InP(100)(2 x 4) surface: Theory and experiment2009In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 603, no 16, p. 2664-2668Article in journal (Refereed)
    Abstract [en]

    Surface core-level shifts (SCLSs) of the (2 x 4)-reconstructed InP(100) surface with the established mixed In-P dimer structure have been investigated by first-principles calculations and photoelectron spectroscopy. Theoretical values were calculated using both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy functional. The obtained theoretical values are quite similar within both approximations. The found differences originate in the tiny structural differences. It is concluded that the expansion or contraction of the crystal lattice has smaller effect on the SCLSs than the geometrical details of the reconstruction, which suggests that the Madelung potential has the dominant effect on the SCLSs. The results support the presence of a P 2p peak at higher binding energy (BE) compared to bulk peak, as proposed with recent measurements [P. Laukkamen, J. Pakarinen, M. Ahola-Tuomi, M. Kuzmin, R. E. Perala, I. J. Vayrynen, A. Tukiainen, V. Rimpilainen, M. Pessa, M. Adell, J. Sadowski, Surf. Sci. 600 (2006) 3022], and reveal several hitherto not reported SCLSs. The calculated SCLSs reproduce the measured spectra within reasonable accuracy. Furthermore, the atomic origins of the InP(100)(2 x 4) SCLSs are solved. In particular, it is shown that the lowest SCLS of P 2p, of the InP(100)(2 x 4) arises from the topmost In-P dimers.

  • 18.
    Punkkinen, Marko P. J.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Laukkanen, P.
    Lång, J.
    Kuzmin, M.
    Dahl, J.
    Zhang, Hualei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Pessa, M.
    Guina, M.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kokko, K.
    Structure of ordered oxide on InAs(100) surface2012In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 606, no 23-24, p. 1837-1841Article in journal (Refereed)
    Abstract [en]

    It was recently found that oxygen induces ordered reconstructions on several III-V surfaces. The most oxygen-rich reconstruction shows (3x1) periodicity. Based on first-principles investigations, a detailed atomic model is presented for this reconstruction. The uncommon periodicity is attributed to the highly stable In - O - In trilayer below surface which also leads to stabilizing additional bonds within the surface layer. The strain induced by the trilayer is more effectively accommodated within the (3 x 1) reconstruction than within the competing (2 x 1) reconstruction due to smaller number of dimers. It is proposed that the experimentally found semiconductivity is reached by substitutional atoms within the surface layer. Suitable substitution preserves the magnitude of the bulk band gap.

  • 19.
    Punkkinen, Marko Patrick John
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kokko, K.
    Levämäki, H.
    Ropo, M.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhang, Hualei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg-Czirjak, Erna Krisztina
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Adhesion of the iron-chromium oxide interface from first-principles theory2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 49, p. 495501-Article in journal (Refereed)
    Abstract [en]

    We determine the interface energy and the work of separation of the Fe/Cr2O3 interface using first-principles density functional theory. Starting from different structures, we put forward a realistic interface model that is suitable to study the complex metal-oxide interaction. This model has the lowest formation energy and corresponds to an interface between Fe and oxygen terminated Cr2O3. The work of separation is calculated to be smaller than the intrinsic adhesion energy of pure Fe or Cr2O3, suggesting that stainless steel surfaces should preferentially break along the metal-oxide interface. The relative stabilities and magnetic interactions of the different interfaces are discussed. Next we introduce Cr atoms into the Fe matrix at different positions relative to the interface. We find that metallic Cr segregates very strongly to the (FeCr)/Cr2O3 interface, and increases the separation energy of the interface, making the adhesion of the oxide scale mechanically more stable. The Cr segregation is explained by the enthalpy of formation.

  • 20.
    Punkkinen, Marko Patrick John
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Kwon, S. K.
    Kollar, J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Compressive Surface Stress in Magnetic Transition Metals2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 106, no 5, p. 057202-Article in journal (Refereed)
    Abstract [en]

    Because of the increased electron density within the surface layer, metal surfaces are generally expected to have tensile surface stress. Here, using first-principles density functional calculations, we demonstrate that in magnetic 3d metals surface magnetism can alter this commonly accepted picture. We find that the thermodynamically stable surfaces of chromium and manganese possess compressive surface stress. The revealed negative surface stress is shown to be ascribed to the enhanced magnetic moments within the surface layer relative to the bulk values.

  • 21.
    Punkkinen, Marko Patrick John
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Laukkanen, P.
    Lang, J.
    Kuzmin, M.
    Tuominen, M.
    Tuominen, V.
    Dahl, J.
    Pessa, M.
    Guina, M.
    Kokko, K.
    Sadowski, J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vayrynen, I. J.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Oxidized In-containing III-V(100) surfaces: Formation of crystalline oxide films and semiconductor-oxide interfaces2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 19, p. 195329-Article in journal (Refereed)
    Abstract [en]

    Previously found oxidized III-V semiconductor surfaces have been generally structurally disordered and useless for applications. We disclose a family of well-ordered oxidized InAs, InGaAs, InP, and InSb surfaces found by experiments. The found epitaxial oxide-III-V interface is insulating and free of defects related to the harmful Fermi-level pinning, which opens up new possibilities to develop long-sought III-V metal-oxide-semiconductor transistors. Calculations reveal that the early stages in the oxidation process include only O-III bonds due to the geometry of the III-V(100)c(8 x 2) substrate, which is responsible for the formation of the ordered interface. The found surfaces provide a different platform to study the oxidation and properties of oxides, e. g., the origins of the photoemission shifts and electronic structures, using surface science methods.

  • 22. Ropo, M.
    et al.
    Kokko, K.
    Airiskallio, E.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hogmark, S.
    Kollár, J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles atomistic study of surfaces of Fe-rich Fe-Cr2011In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 26, p. 265004-Article in journal (Refereed)
    Abstract [en]

    The surface properties of Fe-rich ferromagnetic Fe-Cr alloys are investigated using a first-principles quantum-mechanical method. In dilute alloys, the surfaces are dominated by Fe, whereas the Cr-containing surfaces become favorable when the bulk Cr concentration exceeds the limit of similar to 10 atomic per cent. The abrupt change in the surface behavior is the consequence of complex competing magneto-chemical interactions between the alloying atoms. Considering the quantities of various features: equilibrium surface profiles, chemical potentials, segregation energies, surface energies, magnetic moments, mixing energies and pair interactions, within a wider range of bulk and surface concentrations enables us to build a comprehensive picture of the physics of Fe-Cr surfaces. Using the present achievements many previously controversial results can now be merged into a consistent model of Fe-rich Fe-Cr alloys.

  • 23.
    Zhang, Huaeli
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic parameters of paramagnetic iron-based alloys from first-principles calculations2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 5, p. 054107-Article in journal (Refereed)
    Abstract [en]

    The elastic properties of paramagnetic (PM) Fe1-xMx (M = Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) solid solutions in the body-centered-cubic (bcc) and face-centered-cubic (fcc) structures are investigated using the exact muffin-tin orbital density functional method in combination with the coherent-potential approximation and disordered local-magnetic-moment model. All impurities considered here enlarge or leave nearly constant the equilibrium volume of PM Fe but at the same time produce both positive and negative changes in the elastic parameters. Some of the alloying elements induce opposite effects on shear elastic parameters C' and C-44 of PM bcc and fcc Fe, which is discussed. With a few exceptions, we find that the alloying effects on PM bcc Fe are smaller than on PM fcc Fe. The trends in the tetragonal elastic constant C' show a general correlation with the trends obtained for the bcc-fcc lattice energy difference.

  • 24.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hu, Qing-Miao
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Static equation of state of bcc iron2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 13, p. 132409-Article in journal (Refereed)
    Abstract [en]

    Body-centered-cubic (bcc) iron is one of the most investigated solid-state systems. Using four different density-functional methods, we show that there is a magnetic transition close to the ground-state volume of bcc Fe, which originates from the particular magnetic band structure. The common equation of state functions, used to determine the basic ground-state physical quantities from the calculated total energies, cannot capture the physics of this magnetic transition leading to serious underestimation of the Fe bulk modulus. Ignorance of the magnetic transition found here is reflected by large scatter of the published theoretical bulk moduli of ferromagnetic bcc Fe. Due to the low performance of the exchange-correlation functionals, most of the erroneous results are accidentally in good agreement with the experimental values. The present finding is of fundamental importance, especially taking into account that bcc Fe is frequently used as a test system in assessing the performance of exchange-correlation approximations or total-energy methods.

  • 25.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Hertzman, Staffan
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Single-crystal elastic constants of ferromagnetic bcc Fe-based random alloys from first-principles theory2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 18, p. 184105-Article in journal (Refereed)
    Abstract [en]

    The elastic properties of ferromagnetic Fe1-xMx (M=Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys in the body-centered-cubic (bcc) crystallographic phase have been studied using the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The theoretical lattice parameters and the single-crystal elastic constants agree well with the available experimental data. The most significant alloying effects are found for Al, Si, and Ni additions. All elements enlarge the lattice parameter and decrease the C-11, C-12, and C' elastic constants and the bulk modulus of bcc Fe. At the same time, C-44 is found to increase with Al, Si, V, Cr, or Mn and remain nearly constant with Co, Ni, and Rh. Accordingly, the elastic anisotropy of bcc Fe increases with all alloying elements considered here. The calculated alloying effects on the single-crystal elastic constants are shown to originate from volume effects in combination with the peculiar electronic structure of bcc Fe.

  • 26.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Wang, Guisheng
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko P. J.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Hertzman, Staffan
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic anomalies in Fe-Cr alloys2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 19, p. 195501-Article in journal (Refereed)
    Abstract [en]

    Using ab initio alloy theory, we determine the elastic parameters of ferromagnetic and paramagnetic Fe1-cCrc (0 <= c <= 1) alloys in the body centered cubic crystallographic phase. Comparison with the experimental data demonstrates that the employed theoretical approach accurately describes the observed composition dependence of the polycrystalline elastic moduli. The predicted single-crystal elastic constants follow complex anomalous trends, which are shown to originate from the interplay between magnetic and chemical effects. The nonmonotonic composition dependence of the elastic parameters has marked implications on the micro-mechanical properties of ferrite stainless steels.

1 - 26 of 26
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