| 1. |
- Akerman, J. J., et al.
(författare)
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Dislocation-mediated creep of highly separated vortices in a-axis-oriented HgBa2CaCu2O6+delta thin films
- 2001
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 64:2
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Tidskriftsartikel (refereegranskat)abstract
- Using ac susceptibility, we determine the critical current density J(c) and the flux creep activation energy U of an a-axis-oriented HgBa2CaCu2O6+delta thin film. The critical current density at helium temperatures is found to be 4.6 x 10(4) A/cm(2), i.e., about two orders of magnitude smaller than for corresponding films with c-axis orientation. The temperature and ac field dependent activation energy is consistent with dislocation-mediated flux creep and well described by U(T,H-ac)=U-o(1-t(4))H-ac(-1/2) with t=T/T-c, T-c=120K, and U-o = 0.77 eV Oe(1/2) for temperatures T>45 K and in the field range studied. The activation energy is of the same order as that found in c-axis-oriented films. Below T = 45 K the activation energy is observed to decrease as thermally assisted quantum creep becomes increasingly important.
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| 2. |
- Akerman, J. J., et al.
(författare)
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Two-stage crossover from thermal to quantum flux creep of dilute vortex ensembles in various high-T-c superconducting thin films
- 2001
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 64:9
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Tidskriftsartikel (refereegranskat)abstract
- The thermal-to-quantum flux creep crossover at low vortex densities has been studied in YBa2Cu3O7, TlBa2CaCu2O7-delta, and HgBa2CaCu2O6+delta thin films using ac susceptibility. The crossover temperatures T-cr are 10-11, 17, and 30 K, respectively. Both thermal and quantum flux creep is suppressed as the vortex density is decreased. We observe a two-stage nature in the crossover behavior which appears to be a general property of all the three materials studied.
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| 3. |
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| 4. |
- Göthelid, M., et al.
(författare)
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Adatom and rest-atom contributions in Ge(111)c(2×8) and Ge(111)-Sn(7×7) core-level spectra
- 1993
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Ingår i: Physical Review B. - 0163-1829. ; 48:3, s. 2012-2015
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Tidskriftsartikel (refereegranskat)abstract
- We have identified the adatom contribution in the Ge 3d core-level spectra from the clean Ge(111)c(2×8) surface, shifted 0.17 eV to higher binding energy compared to the bulk. This adatom component vanishes in the Ge(111)-Sn(7×7) surface core-level spectra where Sn occupies the adatom site. Moreover we report the observation of an earlier proposed difference between the rest atoms in the c(2×8) structure, and also a shift to lower binding energy for the rest atoms in both structures studied. © 1993 The American Physical Society.
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| 5. |
- Göthelid, M., et al.
(författare)
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Sn-induced surface reconstructions on the Ge(111) surface studied with scanning tunneling microscopy
- 1992
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Ingår i: Surface Science. - 0039-6028. ; 271:3, s. L357-L361
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Tidskriftsartikel (refereegranskat)abstract
- Scanning tunneling microscopy (STM) has been used to study different Sn induced reconstructions on the Ge(111) surface; namely the (7 × 7), (5 × 5) and ( 3 × 3) R30° structures. The first two have been confirmed to be of the dimer adatom stacking faults (DAS) type with adatoms mainly being Sn. The ( 3 × 3)R30° superstructure was found at different Sn depositions. At 0.4 monolayer (ML) Sn coverage a homogeneous Sn adatom layer is adsorbed on the(1 × 1) surface in threefold sites directly over second-layer atoms (T4), while at low coverage, 0.1 ML, the top layer is a mixture of Sn and Ge atoms. We also propose the chemical identities of the different atoms seen in the STM images as related to their apparent height. © 1992.
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| 6. |
- Hammar, M., et al.
(författare)
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Initial growth of silver on Ge(111) studied by scanning tunneling microscopy
- 1993
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Ingår i: Physical Review B. - 0163-1829. ; 47:23, s. 15669-15674
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Tidskriftsartikel (refereegranskat)abstract
- Scanning tunneling microscopy (STM) has been used to study the Ge(111) surface after submonolayer depositions of Ag. At the lowest coverages a (4×4) reconstruction, with small insets of a (3×1) edge structure, is observed. Based on the STM images we propose a mixed Ag and Ge adatom structure for the (4×4) reconstruction, in which the adatoms have local (1×1) and (2×2) arrangements corresponding to a Ag coverage of 6/16 monolayers. The (3×1) structure appears to have a configuration similar to the Si(111)/Ag-(3×1) reconstruction, although in this case it has a much more limited extension, restricted to a few unit cells in the near vicinity of step edges and phase boundaries. At higher coverages, after the completion of a full (4×4)/(3×1) surface, a (3 × 3) R30°structure appears showing STM images similar to those previously published for the Si(111)/Ag system. © 1993 The American Physical Society.
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| 7. |
- Hammar, M., et al.
(författare)
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Morphology and atomic structure of the sputtered and annealed Mo3Si and Cr3Si (110) surfaces
- 1994
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Ingår i: Physical Review B. - 0163-1829. ; 50:23, s. 17639-17642
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Tidskriftsartikel (refereegranskat)abstract
- Scanning tunneling microscopy was used to study the sputtered and annealed (110) surfaces of Mo3Si and Cr3Si. Both surfaces show extended and atomically flat terraces, but in the case of Mo3Si there is also a uniform distribution of Mo crystallites. This difference in morphology is discussed in terms of different preferential sputtering effects. In both cases, measured step heights show that the ideally bulk-truncated surfaces are either purely Si or metal terminated. Atomically resolved images suggest that the Mo3Si surface is Si terminated, but although no atomic resolution could be obtained for the Cr3Si surface, there are indications that this surface is instead metal terminated. © 1994 The American Physical Society.
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| 8. |
- Hammar, Mattias, 1961-, et al.
(författare)
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Scanning tunnelling microscopy studies of Pt80Fe 20(110)
- 1993
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 5:18, s. 2837-2842
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Tidskriftsartikel (refereegranskat)abstract
- Results of scanning tunnelling microscopy (STM) show that two different kinds of surface structures coexist on the (110) surface of Pt 80Fe20. The predominant structure corresponds well to the missing row type (1*2) reconstruction previously observed on, for example Pt(110), whereas the minority domains show a faintly buckled structure with approximately (1*1) geometry. Based on the STM images, the authors propose that the two domains have different surface chemical compositions and correspond to two different metallurgical phases known for the PtFe system at this composition.
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| 9. |
- Hirschauer, B., et al.
(författare)
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Oxidation of Ce on Si(111) studied by high-resolution photoelectron spectroscopy
- 2000
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 464:03-feb, s. 117-122
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Tidskriftsartikel (refereegranskat)abstract
- The Si(lll)-Ce (2x2) surface was studied by photoelectron spectroscopy during oxidation and annealing. Detailed analysis of the Si 2p core-level spectra and the Ce valence band levels shows that Ce is first oxidised and then promotes oxidation of Si at room temperature by improving the oxygen uptake of the surface. Initially, no oxidation of Si can be recorded, but at exposures of 3 L O-2 or more, SiOx and higher silicon oxides are formed. After annealing to 750 degreesC, a temperature that is generally used to oxidise Si, almost all O leaves the surface. At 1045 degreesC, the Si 2p and the Ce valence band spectra of the sample show almost the same shape as for the original Si(lll)-Ce 2x2 surface. This means that oxidation/reduction of the Si(lll)-Ce 2x2 surface is reversible.
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| 10. |
- Hirschauer, B, et al.
(författare)
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Studies of highly oriented CeO2 films grown on Si(111) by pulsed laser deposition
- 1999
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Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 348:1-2, s. 3-7
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Tidskriftsartikel (refereegranskat)abstract
- CeO2 is an interesting buffer layer material for the growth of YBa(2)Cu(3)O(7-)delta overlayers on Si in devices, with the aim of preventing heat-diffusion due to its excellent lattice matching with Si and YBa(2)Cu(3)O(7-)delta. Epitaxial CeO2-films have been synthesised on Si(lll) by pulsed laser deposition. Stoichiometric changes of the CexOy-film depending on the ambient oxygen pressure during the deposition were studied by X-ray photoelectron spectroscopy. A method is presented for growing a sharp interface between CeO2 and Si(111). The dependence of the inplane orientation of the CeO2 film on the substrate temperature was investigated by X-ray diffraction. The best films, grown at 700 degrees C, showed full width at half maximum of the rocking curve close to 0.1 degrees, but already at room temperature very highly oriented films with less than 0.2 degrees were synthesised. (C) 1999 Elsevier Science S,A. All rights reserved.
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| 11. |
- Janin, E., et al.
(författare)
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Bridge-bonded atomic oxygen on Pt(110)
- 2000
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 61:19, s. 13144-13149
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Tidskriftsartikel (refereegranskat)abstract
- We use hybrid density-functional theory (DFT) and scanning tunneling microscopy (STM) to study oxygen adsorption on the Pt(110) surface. In the STM images oxygen appear as bright protrusions on the ridges of the missing row reconstructed surface. DFT calculations of binding energies for atomic oxygen on a bridge site shows a clear preference over hollow sites.
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| 12. |
- Janin, E., et al.
(författare)
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Formation of two-dimensional graphite islands on the Pt(110)(1x2) surface
- 2000
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Ingår i: Applied Surface Science. - 0169-4332 .- 1873-5584. ; 162, s. 184-189
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Tidskriftsartikel (refereegranskat)abstract
- The formation of thin carbon overlayers at temperatures above 600 degrees C has been studied on the Pt(110)(1 X 2) surface by Scanning Tunneling Microscopy (STM), PES and Low-Energy Electron Diffraction (LEED). Their graphitic character is evidenced through the binding energy of the Cls core-level, the LEED pattern observed and the atomic structure resolved on the islands. The orientation of the graphite layers is determined to be C < 1010 > parallel to the Pt[110] direction. Their thickness and the nature of the substrate are discussed below.
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| 13. |
- Le Lay, G., et al.
(författare)
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STM and synchrotron radiation studies of "prototypical" metal{plus 45 degree rule}semiconductor systems
- 1994
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Ingår i: Surface Science. - 0039-6028. ; 307-309:PART A, s. 280-294
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Tidskriftsartikel (refereegranskat)abstract
- Since the origin of surface science noble metal{plus 45 degree rule}elemental semiconductor couples have been considered as "prototypical" systems. After three decades of research their structural and electronic properties remain an intriguing maze despite recent advances made, especially thanks to the development of the near-field microscopies and the extensive use of synchrotron radiation in surface crystallography and in high-resolution photoelectron spectroscopy. In the last few years, lead, as a replacement inert metal, has nearly gained the pole position in the display of exotic behaviour. This paper gives a flavour of this mystery story and highlights some puzzling questions. © 1994.
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| 14. |
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| 15. |
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| 16. |
- Nyholm, R., et al.
(författare)
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SYNCHROTRON-RADIATION SOFT-X-RAY PHOTOEMISSION-STUDY OF LEAD ON BI2CASR2CU2O8
- 1992
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 46:10, s. 6488-6494
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a study of the interaction of Pb with a clean single-crystal Bi2CaSr2Cu2O8 superconductor surface based on photoemission and low-energy electron diffraction (LEED). Deposition of Pb on a Bi2CaSr2Cu2O8 Crystal kept at room temperature gives rise to the formation of metallic Bi and oxidized Pb at the interface. This behavior could not be observed when the crystal was kept at 100 K during Pb deposition. For all investigated Pb overlayers on a cold crystal (100 K), surface-sensitive photoelectron spectroscopy revealed the growth of a covering metallic Pb overlayer film. The growth at 100 K, contrary to the growth at room temperature, preserved the original LEED 5 X 1 pattern even for Pb depositions corresponding to a 24-angstrom thick overlayer indicating epitaxial growth. Furthermore, a rigid 0.4-eV shift of the valence band and the Bi 5d core levels is observed upon initial Pb deposition and is tentatively attributed to electron doping.
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| 17. |
- Nylén, H., et al.
(författare)
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O KVV Auger emission versus resonant photoemission at the O K edge of high-Tc superconductors
- 1998
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Ingår i: Physica C: Superconductivity and its Applications. - 0921-4534. ; 300:3-4, s. 161-170
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Tidskriftsartikel (refereegranskat)abstract
- Photoelectron spectroscopy results on single crystals of the superconductors Bi2Sr2CaCu2O8,Bi2Sr 2CuO6, Ba0.6K0.4BiO3 and the semiconductor Ba0.9K0.1BiO3 are reported for the photon energy region around the O K absorption threshold. The development of the O-KVV Auger structure has been carefully monitored as a function of photon energy. A non-monotonic behavior displaying a feature at a constant binding energy of about 14 eV was found for Bi2Sr2CaCu2O8 and Bi2Sr2CuO6 in a narrow photon energy region of 1 eV at the main edge of the O K absorption spectrum around 530 eV. The corresponding enhancement, connected with the autoionization of O 2 p states, is absent in Ba1-xKxBiO3 in contrast to Bi2Sr2CaCu2O8 and Bi2Sr2CuO6. The resonant enhancement is more pronounced for Bi2Sr2CuO6 as compared to Bi2Sr2CaCu2O8, which can be explained by a lower charge carrier concentration in the former case, leading to a more localized nature of intermediate O 2 p states. The model parameters Cu d-d and O p-p Coulomb interactions and the charge transfer energy Δ are estimated from the experiments.
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| 18. |
- Qvarford, M., et al.
(författare)
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X-ray absorption study of oxygen in the high-Tc superconductor Bi2Sr2CaCu2O8 near the interfaces to Cu, Ag and Au
- 1996
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Ingår i: Physica C: Superconductivity and its Applications. - : Elsevier BV. - 0921-4534 .- 1873-2143. ; 265:1-2, s. 113-120
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Tidskriftsartikel (refereegranskat)abstract
- The influence on O 2p holes in single crystalline Bi2Sr2CaCu2O8 upon the interface formation to Cu, Ag and Au has been studied by O K edge X-ray absorption measurements. It was found that Cu reduces the amount of doping induced O 2p holes significantly in the vicinity of the interface, whereas Ag and Au gave a much smaller reduction of these states. Photoemission spectra confirmed previous findings that Cu causes a strong chemical reaction at the Bi-O surface of Bi2Sr2CaCu2O8, in contrast to Ag and Au which induced only a minimal reaction. The results support the opinion that the Bi-O layers are essential for the doping of the Cu-O2 layers in Bi2Sr2CaCu2O8.
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| 19. |
- Rad, M. G., et al.
(författare)
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Influence of charged impurities on the surface phases of Sn/Ge(111)
- 2001
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 477:03-feb, s. 227-234
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Tidskriftsartikel (refereegranskat)abstract
- The root3 alpha and 3 x 3 reconstructions on the Ge(111)-Sn surface have been perturbed by small amounts of iodine and potassium to test the spectral response in the course of a synchrotron radiation photoemission study. We demonstrate the inadequacy of the static model of the root3 alpha phase at room temperature and of the surface charge density wave model of the 3 x 3 phase at low temperature. Instead, the influence of the charged impurities is consistently explained when considering that dynamical fluctuations in the tin adatom positions are frozen in upon passing the phase transition temperature.
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| 20. |
- Rad, M. G., et al.
(författare)
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Surface structure and local bonding on the Si(111)-Ce surface
- 2000
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Ingår i: Applied Surface Science. - 0169-4332 .- 1873-5584. ; 166:04-jan, s. 209-213
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Tidskriftsartikel (refereegranskat)abstract
- We used scanning tunnelling microscopy (STM) and photoelectron spectroscopy (PES) to study the cerium-induced (3 x 2) surface reconstruction of cerium (Ce) on Sn(111). Comparison with similar rare-earth-metal-induced low-coverage reconstructions on the Sn(111) surface reveals a similar nature of the chemical bond independent of the valence of the rare earth metal.
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| 21. |
- Sandell, A., et al.
(författare)
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THE INNER VALENCE REGION OF CO ADSORBED ON PD(100)
- 1994
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 26:19, s. 10659-10668
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Tidskriftsartikel (refereegranskat)abstract
- The inner valence region of CO/Pd(100) p(2square-root2 x square-root2)R45-degrees has been studied by angular resolved photoemission at the Pd 4d Cooper minimum, and with resonant Auger spectroscopy at photon energies corresponding to the C 1s and O 1s x-ray absorption (XA) maxima of the unoccupied parts of the 2pi*-Pd 4d hybrid (2pi(un)). Previously unobserved inner valence states are revealed in the direct photoemission and are compared with resonant Auger results. The interpretation and assignment of the different spectral features to different main final state configurations are based on energy, symmetry and intensity arguments, as well as comparisons with previous results.
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| 22. |
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| 23. |
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| 24. |
- Söderholm, S, et al.
(författare)
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A photoemission spectroscopy and X-ray absorption study of Bi(2)Sr(2)CaCu(2)O(8) single crystal with adsorbed Cs: On the origin of states affected by electron doping and evidence for spatially resolved electron doping
- 1996
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 8, s. 1307-1320
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Tidskriftsartikel (refereegranskat)abstract
- The influence of electron doping, via deposition of small amounts of Cs, on the electronic structure of has been studied by high-resolution photoelectron spectroscopy (PES) and x-ray absorption spectroscopy (XAS), utilizing synchrotron radiation. The changes in the electronic structure were monitored by PES of the valence band and of the O 1s, Bi 4f, Bi 5d, Ca 2p and Sr 3d core levels, and by XAS at the O 1s, Cu 2p and Ca 2p edges. The experimental data suggest that the loss of the Fermi edge and the loss of spectral intensity down to about 2 eV below the Fermi level, and the substantial loss of spectral intensity of the pre-edge structure in the O 1s XAS spectrum are mainly due to annihilation of states with O 2p character in the Cu - O layer. It is evident from bulk- and surface-sensitive XAS spectra that the electron doping by Cs affects the electronic structure more strongly close to the surface. This implies that the doping occurs locally and that the charge transfer between the different layers in the unit cell is not uniform. Thus it seems possible to alter the electronic properties of locally through spatially resolved electron doping. When larger amounts of Cs are deposited, a chemical reaction occurs which causes a disruption of the Bi - O layer. This deposition regime is characterized by the presence of caesium oxide and reduced (metallic) Bi.
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| 25. |
- Tjernberg, O, et al.
(författare)
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Evidence of pseudogap related core level shifts in Bi2Sr2Ca1-xYxCu2O8+delta
- 1997
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 79:3, s. 499-502
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Tidskriftsartikel (refereegranskat)abstract
- Photoelectron spectroscopy data from Bi2Sr2Ca1-xYxCu2O8+delta Single crystals, for x = 0, 0.16, and 0.55, are presented. It is shown that there are core level shifts related to the opening of a pseudogap and that similar shifts are observed at the opening of the superconducting gap in optimally doped samples. This result is in agreement with pair formation above T-c as suggested by V.J. Emery and S.A. Kivelson [Nature (London) 374, 434 (1995)].
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| 26. |
- Törnevik, C., et al.
(författare)
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Adsorption of Sn onSi(111)7 × 7 : reconstructions in the monolayer regime
- 1994
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Ingår i: Surface Science. - 0039-6028. ; 314:2, s. 179-187
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Tidskriftsartikel (refereegranskat)abstract
- Different monolayer phases of Sn on Si(111)7 × 7 have been studied by means of scanning tunneling microscopy (STM), core-level photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). The STM results show that 3 × 3 reconstructions are obtained for room-temperature deposition of 1 3 ML of Sn followed by sample annealing in a broad temperature range. A T4 Sn adatom 3 ×3 phase is formed for temperatures between 500 and 800°C, with a concentration of defects that is strongly dependent on the temperature and which is as high as 25% for the lowest temperatures. Above 825°C a second 3×3 adatom reconstruction is formed, a mosaic-like phase with a 1:1 mixture of Si and Sn atoms in T4 positions. The results from investigations of the higher coverage 2 3 × 2 3 reconstruction by XPS and RBS support the theory that this phase is a two-layer epitaxial Sn structure with all Si(111) dangling bonds saturated. The Sn coverage for this phase was determined to be between 1 and 1.2 ML. © 1994.
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| 27. |
- Wigren, C., et al.
(författare)
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Sm- and Yb-induced reconstructions of the Si(111) surface
- 1993
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Ingår i: Physical Review B. - 0163-1829. ; 48:15, s. 11014-11019
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Tidskriftsartikel (refereegranskat)abstract
- Low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy results from the submonolayer Sm- and Yb-induced surface structures are presented. Several similar metal-induced surface reconstructions are found to exist for Yb and Sm on Si(111) for low submonolayer coverages: 3×2, 5×1, and 7×1. At higher submonolayer coverage, Yb induces a 2×1 reconstruction while Sm induces a (3 × 3) R30°-like reconstruction. Yb is found to be divalent in all structures, whereas the Sm valence increases with increasing coverage. In the 3×2 structure only divalent Sm is present, in the 5×1 and 7×1 structures a small amount of trivalent Sm appears, and, finally, in the (3 × 3) R30°structure approximately half of the Sm atoms are trivalent. The surface Fermi-level position in the band gap for the different Sm and Yb reconstructions has been measured. The difference in valence stability between Sm and Yb is suggested to be the cause of the difference in the high-coverage structures found and the differences in pinning level for the two elements observed for the 5×1 and 7×1 structures. © 1993 The American Physical Society.
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| 28. |
- Åkerman, Johan, et al.
(författare)
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Re-entrant behavior of low-field flux creep in c-axis-oriented HgBa2CaCu2O6+delta thin films
- 2001
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 6418:18
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Tidskriftsartikel (refereegranskat)abstract
- The temperature, ac and dc field, and current dependent activation energy U(T,H)[(J(c0)/J(c))(mu)-1]/mu governing low-field flux creep in epitaxial c-axis-oriented HgBa2CaCu2O6+delta thin films has been determined from measurements of the frequency-dependent in-phase ac susceptibility. Above 35 K three different thermally activated flux creep regimes can be identified: (i) dislocation-mediated plastic flux creep, described by U(T,H) = U-0(1 - t(4))H-1/2 and mu = 0, (ii) elastic collective flux creep which decreases with temperature and has a weaker field dependence of H-0.22 above a field-dependent temperature Tc.(H) where A acquires finite values, and (iii) reappearance of dislocation-mediated plastic flux creep which rapidly increases as T-c is approached. It is argued that the re-entrant plastic-elastic-plastic vortex creep behavior is driven by the underlying temperature and field dependence of the shear modulus c(66). T-cm(H) marks a line in the H-T plane where the increasing c(66) promotes long-range correlations in the dilute vortex phase and creep becomes collective. At high H and T, c(66) again decreases and plastic creep reappears as the ordered phase starts to melt. Evidence for thermally assisted quantum creep is observed up to temperatures as high as T-0 = 35 K.
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