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Université de Fribourg

X-ray study of structural domains in the near-surface region of SrTiO₃ substrates with Y0.6Pr0.4Ba₂Cu₃O₇/La2/3Ca1/3MnO₃ superlattices grown on top

Hoppler, Justin ; Stahn, J. ; Bouyanfif, Houssny ; Malik, Vivek Kumar ; Patterson, B. D. ; Willmott, P. R. ; Cristiani, G. ; Habermeier, H.-U. ; Bernhard, Christian

In: Physical Review B, 2008, vol. 78, no. 13, p. 134111

We investigated with synchrotron x-ray diffraction and reflectometry the formation of structural domains in the near-surface region of single crystalline SrTiO₃ (001) substrates with Y0.6Pr0.4Ba₂Cu₃O₇/La2/3Ca1/3MnO₃ superlattices grown on top. We find that the antiferrodistortive cubic to tetragonal transition, which occurs at...

Université de Fribourg

Coexistence of magnetic fluctuations and superconductivity in the pnictide high temperature superconductor SmFeAsO1-xFx measured by muon spin rotation

Drew, Alan J. ; Pratt, F. L. ; Lancaster, T. ; Blundell, S. J. ; Baker, P. J. ; Liu, R. H. ; Wu, G. ; Chen, X. H. ; Watanabe, I. ; Malik, Vivek Kumar ; Dubroka, Adam ; Kim, Kyung Wan ; Rössle, Matthias ; Bernhard, Christian

In: Physical Review Letters, 2008, vol. 101, no. 9, p. 097010

Muon spin rotation experiments were performed on the pnictide high temperature superconductor SmFeAsO1-xFx with x=0.18 and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting transition which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An...

Université de Fribourg

Superconducting energy gap and c-axis plasma frequency of (Nd,Sm)FeAsO0.82F0.18 superconductors from infrared ellipsometry

Dubroka, Adam ; Kim, Kyung Wan ; Rössle, Matthias ; Malik, Vivek Kumar ; Drew, Alan J. ; Liu, R. H. ; Wu, G. ; Chen, X. H. ; Bernhard, Christian

In: Physical Review Letters, 2008, vol. 101, no. 9, p. 097011

We present far-infrared ellipsometric measurements of polycrystalline samples of the pnictide superconductor RFeAsO0.82F0.18 (R=Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar to the cuprate high-temperature superconductors....

Université de Fribourg

Evidence for two separate energy gaps in underdoped high-temperature cuprate superconductors from broadband infrared ellipsometry

Yu, Li ; Munzar, D. ; Boris, A. V. ; Yordanov, P. ; Chaloupka, J. ; Wolf, Th. ; Lin, C. T. ; Keimer, B. ; Bernhard, Christian

In: Physical Review Letters, 2008, vol. 100, p. 177004

We present broadband infrared ellipsometry measurements of the c-axis conductivity of underdoped RBa₂Cu₃O₇₋δ (R=Y, Nd, and La) single crystals. Our data show that separate energy scales are underlying the redistributions of spectral weight due to the normal state pseudogap and the superconducting gap. Furthermore, they provide evidence that these gaps do not...

Université de Fribourg

Charge dynamics of doped holes in high Tc cuprate superconductors: A clue from optical conductivity

Mishchenko, A. S. ; Nagaosa, N. ; Shen, Z.-X. ; Filippis, G. De ; Cataudella, V. ; Devereaux, T. P. ; Bernhard, Christian ; Kim, Kyung Wan ; Zaanen, J.

In: Physical Review Letters, 2008, vol. 100, no. 16, p. 166401

The charge dynamics in weakly hole doped high temperature superconductors is studied in terms of the accurate numerical solution to a model of a single hole interacting with a quantum lattice in an antiferromagnetic background, and accurate far-infrared ellipsometry measurements. The experimentally observed two electronic bands in the infrared spectrum can be identified in terms of the interplay...

Université de Fribourg

Optical Study of the Free-Carrier Response of ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ Superlattices

Seo, S. S. A. ; Choi, W. S. ; Lee, H. N. ; Yu, Li ; Kim, Kyung Wan ; Bernhard, Christian ; Noh, T. W.

In: Physical Review Letters, 2007, vol. 99, no. 26, p. 266801

We used infrared spectroscopic ellipsometry to investigate the electronic properties of LaTiO₃/SrTiO₃ superlattices (SLs). Our results indicated that, independent of the SL periodicity and individual layer thickness, the SLs exhibited a Drude metallic response with sheet carrier density per interface ≈3×10¹⁴ cm⁻². This is probably due to the leakage of d electrons at...

Université de Fribourg

Electronic Liquid Crystal State in the High-Temperature Superconductor YBa₂Cu₃O6.45

Hinkov, V. ; Haug, D. ; Fauqué, B. ; Bourges, P. ; Sidis, Y. ; Ivanov, A. ; Bernhard, Christian ; Lin, C. T. ; Keimer, B.

In: Science, 2008, vol. 319, p. 597-600

Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at millikelvin temperatures. We report the spontaneous onset of a one-dimensional, incommensurate modulation of the spin system in the high-transition-temperature superconductor...

Université de Fribourg

Optical response of ferromagnetic YTiO₃ studied by spectral ellipsometry

Kovaleva, N. N. ; Boris, A. V. ; Yordanov, P. ; Maljuk, A. ; Brücher, E. ; Strempfer, J. ; Konuma, M. ; Zegkinoglou, I. ; Bernhard, Christian ; Stoneham, A. M. ; Keimer, B.

In: Physical Review B: Condensend Matter and Materials Physics, 2007, vol. 76, no. 15, p. 155125

We have studied the temperature dependence of spectroscopic ellipsometry spectra of an electrically insulating, nearly stoichiometric YTiO₃ single crystal with ferromagnetic Curie temperature TC=30 K. The optical response exhibits a weak but noticeable anisotropy. Using a classical dispersion analysis, we identify three low-energy optical bands at 2.0, 2.9, and 3.7 eV....

Université de Fribourg

Muon-spin rotation study of magnetism in NaxCoO₂ single crystals with 0.78 ≼ x ≼ 0.97

Bernhard, Christian ; Niedermayer, Ch. ; Drew, Alan J. ; Khaliullin, G. ; Bayrakci, S. ; Strempfer, J. ; Kremer, R. K. ; Chen, D. P. ; Lin, T. ; Keimer, B.

In: Europhysics Letters, 2007, vol. 80, no. 2, p. 27005

By muon spin rotation we investigated the magnetic properties of a series of highly Na-doped NaxCoO₂ single crystals with 0.78(1) ≼ x ≼ 0.97(1). Our data provide evidence for an intrinsically inhomogeneous magnetic state which can be described in terms of hole-doping (Na vacancy)-induced magnetic clusters that percolate at 1−x≳0.04 until they yield a bulk...

Université de Fribourg

Isotope effect on the optical phonons of YBa₂Cu₄O₈ studied by far-infrared ellipsometry and Raman scattering

Trajnerowicz, A. ; Golnik, A. ; Bernhard, Christian ; Machtoub, L. ; Ulrich, C. ; Tallon, J. L. ; Cardona, M.

In: Physical Review B, 2006, vol. 74, p. 104513

The phonon spectra of YBa₂Cu₄O₈ polycrystalline samples with different isotope substitution for Ba, Cu, and O have been studied using far-infrared ellipsometry and Raman scattering. The observed isotope shifts of Raman- and infrared-active phonons provide insight into their relative Ba, Cu, and O normal mode content. This allows us to examine mode eigenvectors obtained from lattice...