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• 1. Aboelfotoh, M O
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media. Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
Schottky-barrier behavior of metals on n- and p-type 6H-SiC2003In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 67, no 7, p. 075312-Article in journal (Refereed)
• 2. Abrashev, M. V.
Raman spectroscopy of CaMnO(3): Mode assignment and relationship between Raman line intensities and structural distortions2002In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 65, no 18, p. 1-9Article in journal (Refereed)

Polarized Raman spectra of oriented finely twinned CaMnO(3) thin films and nonpolarized Raman spectra of CaMnO(3) ceramics were studied at room temperature using several excitation laser wavelengths. The selection rules for the polarized Raman spectra, obtained from samples consisting of finely twinned orthorhombic domains, were calculated and the symmetry of all observed Raman lines was determined. The relationship between the intensity of the Raman lines and the distortions in the ABO(3) perovskites with GdFeO(3)-type structure is discussed. These distortions can be described as superposition of four simple basic distortions: two MnO(6) octahedral tilts, Jahn-Teller deformation of Mn(3+) O(6) octahedra, and shift of the A ions from their sites in the ideal perovskite. Twenty of the 24 Raman-allowed modes in the real GdFeO(3)-type structure have counterparts in only one of the four simpler structures, obtained by a single basic distortion. The assignment of the Raman lines of CaMnO(3) to definite atomic vibrations, most of them activated by a single basic distortion, was made in close comparison with the results of lattice dynamical calculations and the Raman spectra of isostructural LaMnO(3) and CaGeO(3).

• 3. Andreasson, Jakob
Franck-Condon higher order lattice excitations in the LaFe1-xCrxO3 (x=0, 0.1, 0.5, 0.9, 1.0) perovskites due to Fe-Cr charge transfer effects2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75Article in journal (Refereed)

First and higher order lattice excitations in the B-site disordered perovskites LaFe1-xCrxO3 (x=0, 0.1, 0.5, 0.9, and 1) and La0.835Sr0.165Fe0.5Cr0.5O3-delta are investigated using temperature dependent and polarized inelastic light scattering [lambda=515 nm (2.41 eV) and 676 nm (1.83 eV)] on oriented crystallites. A peak at approximately 2.4 eV in the imaginary part of the dielectric function of LaFe0.5Cr0.5O3 is assigned to a charge transfer from Fe3+ (d(5)) to Cr3+ (d(3)) ions, coupled with the appearance of an intense A(g)-like mode at approximately 700 cm(-1) in the Raman data. This excitation is identified as a symmetric oxygen breathing mode activated by the Fe-Cr charge transfer through an orbital coupling mechanism. Higher order scattering (up to seventh order) of the intrinsic Raman active symmetric breathing mode is also explained by an orbital-mediated electron-phonon coupling, similar to the Franck-Condon effect observed in the Jahn-Teller active-perovskite-structured manganite LaMaO(3). These results show that the Franck-Condon mechanism is a more common mechanism for resonant higher order scattering in solids than previously believed and propose the LaFe1-xCrxO3 system as a model system for electron-phonon coupling and higher order Raman scattering in solids.

• 4. Bjornsson, P.
Lattice and charge excitations in La1-xSrxMnO32000In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 61, p. 1193-1197Article in journal (Refereed)

We employ inelastic Light scattering to study the dependence of the lattice and charge excitations in La1-xSrxMnO3 compounds on doping and temperature. The phonons reveal strong local lattice distortions in the paramagnetic state, which gradually vanish below the ferromagnetic transition. We identify charge excitations in the metallic state. They exhibit a dependence on the doping level and symmetry selection rules typical for a plasmonlike excitation. Their energy scale of 100 meV requires a low-carrier-density component of the plasma outlining the importance of electronic interactions in La1-xSrxMnO3.

• 5. Brauer, H.E.
Na intercalation of VSe2 studied by photoemission and scanning tunneling microscopy1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, no 15, p. 10022-10026Article in journal (Refereed)

In situ Na intercalation of the layered compound VSe2 has been studied with photoemission and scanning tunneling microscopy. Core-level spectroscopy proves that Na deposited in UHV onto the VSe2 surface rapidly intercalates, leaving only small amounts at the surface. The scanning tunneling microscopy measurements show that the intercalated Na is not uniformly distributed between the VSe2 layers, but preferentially in two-dimensional islands. Thus the surface region is divided into intercalated and nonintercalated areas. Hole-like features in the intercalated areas are interpreted as locally missing Na.

• 6. Budelmann, D.
Antiferromagnetic and superconducting proximity effects in YBa2Cu3O7-delta/PrBa2Cu3O7-delta superlattices2003In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 67Article in journal (Refereed)

We investigate the interplay between the antiferromagnetic and superconducting order parameters in YBa2Cu3O7-delta/PrBa2Cu3O7-delta superlattices by inelastic light scattering. The ratio of superconducting to antiferromagnetic order is varied through different modulations 4/6, 4/9, and 4/12 of (Y/Pr)Ba2Cu3O7-delta layers. This allows us to identify the proximity effect of the superconducting order parameter into the antiferromagnetic barrier as signified, e.g., by the sharpening of the two-magnon excitation. This proximity effect as well as gap feature and phonon anomalies reveals the delicate interplay between superconducting and antiferromagnetic order parameters.

• 7. Bäckström, Joakim
Optical properties of YBa$_2$Cu$_3$O$_7 - \delta$ and PrBa$_2$Cu$_3$O$_7 - \delta$ films: High-energy correlations and metallicity2004In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 70, no 17Article in journal (Refereed)

We have investigated the temperature dependence of the dielectric functions of a high-Tc superconducting YBa2Cu3O7-- (Y-123) and a nonsuperconducting PrBa2Cu3O7-- (Pr-123) thin film using spectral ellipsometry. We evaluate the data by means of thermal-difference and sum-rule analysis techniques. We find that a spectral-weight transfer into the bands between 4 and 5 eV takes place before Y-123 becomes superconducting. We identify an anomaly around 1 eV that can be explained by a sudden plasma-frequency drop of the order of an meV around Tc. The absence of any sudden effects in the dielectric properties of Pr-123 suggests that both observations are intimately related to the superconducting state. Our findings point out that high-energy degrees of freedom must be considered for the understanding of high-temperature superconductivity.

• 8. Bäckström, Joakim
Raman scattering in \RmYBa$_2$Cu$_4$O$_8$ and PrBa$_2$Cu$_4$O$_8$: Indications of pseudogap effects in non-superconducting \RmPrBa$_2$Cu$_4$O$_8$2000In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 61, p. 7049-7054Article in journal (Refereed)

By analysis of the Raman spectra of superconducting YBa2Cu4O8 (Y-124), we observe a suppression of low-energy electronic density of states that we link to changes in the linewidth of the Ba-phonon mode. These effects occur at a temperature below 150 K, where also anomalies in resistivity and other physical properties take place. We argue that these changes in the Raman spectra of Y-124 are connected to the opening of a pseudogap. Analogous anomalies in resistivity and the Ba-phonon width in the nonsuperconducting homologue PrBa2Cu4O8 (Pr-124) below 150 K suggest that similar pseudogap effects can occur without a following transition to a superconducting state at a lower temperature.

• 9. Costa-Kramer, J.L.
Department of Physics, Chalmers University of Technology and Göteborg University.
Conductance quantization histograms of gold nanowires at 4 K1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, no 19, p. 12910-12913Article in journal (Refereed)

Driving a scanning tunneling microscope (STM) tip into a metallic surface and pulling it out afterwards results in the formation of a nanometer-sized wire (nanowire) between the electrodes. The electrical conduction measured during this process shows signs of quantized conductance in units of G0=2e2/h. Due to the inherent irreproducibility of the measured conductance curves, the standard technique has been to construct histograms with a few hundred selected curves. These histograms, for gold nanowires at room temperature, have shown three to four peaks at integer values of 2e2/h, while in a low-temperature mechanically controlled break junction study, with statistics using only 65 curves, only the first peak has been reported. A proposed explanation for this apparent experimental discrepancy has been a higher nanowire temperature arising from the higher retraction speed used in scanning tunneling microscopy measurements. However, our simple estimation using macroscopic heat transport theory produces a very low temperature rise, less than 1 μK. In this work, an improved statistical study is presented, where histograms built with thousands of consecutive gold contact breakage experiments at 4 K show up to seven conductance peaks. Thus, no significant differences with our previous room-temperature (RT) studies are observed, pointing to a conductance quantization behavior that is the same at low, intermediate, and high (RT) temperatures.

• 10.
Mid Sweden University.
Resonant excitation x-ray-fluorescence from C-601995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 52, p. 10681-Article in journal (Refereed)

X-ray fluorescence of condensed C-60 has been recorded in high resolution using monochromatic synchrotron radiation excitation. Strong intensity modulation of constituent spectral features is observed with varying excitation energy up to 10 eV above threshold. The energy dependence is interpreted as due to resonant inelastic x-ray scattering, leading to symmetry selection rules governing the two-photon process in the fully symmetric molecule.

Symmetry of phonon, magnetic, and spin-phonon excitations in GdSr2RuCu2O8 single crystals2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 64Article in journal (Refereed)

We present a polarized Raman scattering study of GdSr2RuCu2O8 single crystals. In this compound, the RuO6 octahedra are rotated around the c axis that in turn gives some Raman activity of the oxygen (O-Ru) vibrations at 265 cm(-1) (A(1g)), 411 cm(-1) (B-2g) and 607 cm(-1) (B-1g). These vibrations, as well as those of the apical oxygen of RuO6, at 654 cm(-1) (A(1g)) and the Cu-plane oxygen at 318 cm(-1) (B-2g), clearly respond to the onset of magnetic ordering at T-m approximate to 140 K. In particular, the 265 cm(-1) mode hardens anomalously with decreasing temperature below T-m. Notably, the corresponding phonon line is absent in the Raman spectra of GdSr2NbCu2O8 (Nb5+:4d(0)), a compound isomorphic to GdSr2RuCu2O8 (Ru5+:4d(3)). We argue that the eigenvector of the 265 cm(-1) phonon (rotational mode) facilitates efficient modulation of the Ru(t(1g))-O-Ru (p) bands. This modulation gives both Raman scattering strength to the 265 cm(-1) mode and strong spin-phonon coupling.

• 12. Iliev, M. N.
Comparative Raman studies of Sr$_2$RuO$_4$, Sr$_3$Ru$_2$O$_7$ and Sr$_4$Ru$_3$O$_10$2005In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 358, no 1--4Article in journal (Refereed)

The polarized Raman spectra of layered ruthenates of the Srn+1RunO3n+1 (n=1,2,3) Ruddlesden-Popper series were measured between 10 and 300 K. The phonon spectra of Sr3Ru2O7 and Sr4Ru3O10 confirmed earlier reports for correlated rotations of neighboring RuO6 octahedra within double or triple perovskite blocks. The observed Raman lines of Ag or B1g symmetry were assigned to particular atomic vibrations by considering the Raman modes in simplified structures with only one double or triple RuO6 layer per unit cell and by comparison to the predictions of lattice dynamical calculations for the real Pban and Pbam structures. Along with discrete phonon lines, a continuum scattering, presumably of electronic origin, is present in the zz, xx and xy, but not in the x'y' and zx spectra. Its interference with phonons results in Fano shape for some of the lines in the xx and xy spectra. The temperature dependences of phonon parameters of Sr3Ru2O7 exhibit no anomaly between 10 and 300 K where no magnetic transition occur. In contrast, two B1g lines in the spectra of Sr4Ru3O10, corresponding to oxygen vibrations modulating the Ru-O-Ru bond angle, show noticeable hardening with ferromagnetic ordering at 105 K, thus indicating strong spin-phonon interaction.

• 13. Kim, J J
Department of Physics, Chalmers University of Technology and Göteborg University.
Temperature-dependent scanning tunneling spectroscopy of 1T-TaS21996In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 54, p. 2244-2248Article in journal (Refereed)

We have studied temperature-dependent tunneling spectroscopy of 1T-TaS2 from room temperature down to about 40 K. The spectra indicated an abrupt transition from the charge-density-wave-induced broad depletion to an opening of a deep pseudogap within 1 K of the nearly commensurate to commensurate transition temperature. The measured pseudogap was deep enough to form localized states at the Fermi level. In contrast to an inverse photoelectron spectroscopy study, our tunneling results indicate gap structures of two Hubbard subbands resulting from the band splitting of the Ta 5d band due to the electron correlation effect. We have also found an abrupt increase of thermoelectric voltage developing between the tip and sample at the transition temperature.

• 14. Kunze, J.
Optical study of competition between ordering and metallicity in La(2-2x)Sr(1+2x)Mn(2)O(7)2003In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 67Article in journal (Refereed)

We study by spectroscopic ellipsometry the optical in-plane anisotropies in La(2-2x)Sr(1+2x)Mn(2)O(7) with 0.32less than or equal toxless than or equal to0.40 above and below the metal to insulator (MI) transition. Spectral-weight changes in the optical conductivity occur at a cross-over temperature T(’)=280 K for all dopings. Local ordering of orbital and charge degrees of freedom sets in doping dependently at temperatures of 145 Kless than or equal toT(*)less than or equal to310 K. Below the MI transition we observe for all dopings except x=0.36 a vanishing of the charge ordering and an isotropic and metallic state at low frequencies, whereas the local orbital ordering persists. For x=0.36 we observe two different characteristic developments of the ordering processes, one being compatible with the parabolic doping dependence on T(*) established by the other doping levels and the second showing ordering at a temperature slightly above the MI transition. We argue that our observations are compatible with a phase separation scenario.

• 15.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
Resonances and rotation symmetries in the conductance of armchair carbon nanotubes with extended defect pairs2005In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 72, no 15, p. 153405-1Article in journal (Refereed)

The influence on the conductance of armchair carbon nanotubes due to the presence of extended vacancy pairs has been investigated using a conventional pi-electron tight-binding model. It is found that the conductance in the linear band region around the Fermi energy contains resonance peaks and dips, which can be understood as double-barrier scattering resonances and Fano antiresonances and whose number depends on the longitudinal intervacancy separation. Furthermore, for a given longitudinal intervacancy separation, only two distinctly different conductance spectra appear in this energy region as the second vacancy in the pair assumes all different lateral positions around the tube circumference. It is also found that the conductance can be very sensitive to minimal changes in the relative intervacancy positions.

• 16. Lindefelt, Ulf
First- and second-nearest-neighbor divacancies in silicon: Origin and ordering of gap levels1988In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 38, no 6, p. 4107-4112Article in journal (Refereed)
• 17.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics. Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media. Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
Nonharmonic oscillations of nanosized cantilevers due to quantum-size effects2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, no 5, p. 054304-Article in journal (Refereed)

Using a one-dimensional jellium model and standard beam theory we calculate the spring constant of a vibrating nanowire cantilever. By using the asymptotic energy eigenvalues of the standing electron waves over the nanometer-sized cross-section area, the change in the grand canonical potential is calculated and hence the force and the spring constant. As the wire is bent more electron states fits in its cross section. This has an impact on the spring "constant" which oscillates slightly with the bending of the wire. In this way we obtain an amplitude-dependent resonans frequency that should be detectable.

In situ resonant Raman scattering and reversible photoinduced structural change in YBa2Cu3O6+x2005In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 71Article in journal (Refereed)

We report on bidirectional photoswitching associated with the CuO chains in oxygen-deficient YBa2Cu3O6+x single crystals. By varying the wavelength of light polarized along the CuO chains, the material can be reversibly switched between two metastable states characterized by the existence or absence of a specific Raman scattering resonance. A comparison of the spectral efficiencies for this photoswitching with analogous data for the persistent photoconductivity and photoconductivity quenching effects suggests that the two phenomena have the same microscopic origin. We argue that the effects are due to photoinduced Cu-O charge-transfer excitations, which destabilize chains of different length depending on wavelength, and promote the growth of thermally inaccessible oxygen ordering configurations.

• 19.
University of Aizu, Japan.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
Excitation of two-dimensional plasmon polaritons by an incident electromagnetic wave at a contact2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75, no 4, article id 045328Article in journal (Refereed)

We consider the scattering of an incident electromagnetic radiation on a two-dimensional (2D) electron layer with an imbedded metallic contact. We show that the incident wave excites in the system the 2D plasmon polaritons running along the 2D layer and localized near it, and electromagnetic waves reflected back from the system. The ratio of the energy transformed to the 2D plasmon polaritons and reflected back from the layer depends on the frequency and the value of a retardation parameter, which characterizes the importance of retardation effects. When the retardation parameter is large, the energy of the incident radiation is mainly reflected from the 2D electron system and the excitation of the 2D plasmon polaritons is less effective. The results obtained are discussed in connection with recent experiments on the microwave response of the 2D electron systems.

• 20.
Department ofPhysics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden.
Department ofPhysics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden. Department ofPhysics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden. Department ofPhysics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden. Department ofPhysics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden. Department ofPhysics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden. Physics Department, University of Washington, Seattle, Washington 98195 and Molecular Science Research Centre, Pacijic Northwest Laboratories, Richland, Washington 99352.
Angle-resolved soft-x-ray fluorescence and absorption study of graphite: Physical Review B (Condensed Matter)1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, no 15Article in journal (Refereed)

The x-ray fluorescence and absorption of highly oriented pyrolytic graphite have been measured using monochromatic synchrotron radiation. The spectra can be separated into contributions from pi- and sigma-band components by measuring at different angles of incidence and at different emission angles. The shape of the x-ray fluorescence spectra varies dramatically with excitation energy near the C K edge. This dependence on excitation energy can be interpreted within a resonant-inelastic-scattering formalism. The results are compared with previously published band-structure calculations and photoemission results, and demonstrate the potential for using x-ray fluorescence to obtain symmetry-resolved band information.

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