In: Physical Review B, 2014, vol. 88, no. 18, p. 180508
We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe1−xCox)2As2 single crystals. We show that a weak band around 1000 cm−1, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low-energy interband transitions. Specifically, we show that this band exhibits a...
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In: Physical Review B, 2013, vol. 88, no. 10, p. 104110
We have measured the temperature dependence of the direct band gap Eg in SrTiO₃ and BaTiO₃ and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that Eg exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be...
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In: EPJ Web of Conferences, 2013, vol. 41, p. 03012
We trace the ultrafast dynamics of the spin density wave gap of the pnictide system BaFe2As2 probing resonantly with broadband multi-terahertz pulses. The photoexcitation in the low-temperature ground state leads to a fast suppression of the spin order followed by a slower recovery process. Surprisingly, in the normal state, we observe periodic oscillations of the spin density wave feature at a...
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In: Physical Review Letters, 2013, vol. 110, no. 13, p. 136805
With ellipsometry, x-ray diffraction, and resistance measurements we investigated the electric-field effect on the confined electrons at the LaAlO₃/SrTiO₃ interface. We obtained evidence that the localization of the electrons at negative gate voltage is induced, or at least enhanced, by a polar phase transition in SrTiO₃ which strongly reduces the lattice polarizability and the subsequent...
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In: Nature Materials, 2012, vol. 11, p. 497–501
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In: Physical Review Letters, 2011, vol. 106, no. 4, p. 047006
We show that a multilayer analysis of the infrared c-axis response of RBa₂Cu₃O7-δ (R=Y,Gd,Eu) provides important new information about the anomalous normal-state properties of underdoped cuprate high temperature superconductors. In addition to competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below...
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In: Physical Review Letters, 2010, vol. 105, no. 6, p. 067001
We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa₂Cu₃O7-δ after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs, demonstrating that the lattice absorbs a major portion of the...
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In: Physical Review B, 2010, vol. 81, p. 214508
We performed combined infrared reflection and ellipsometry measurements of the in-plane optical response of single crystals of the pnictide high-temperature superconductor BaFe1.87Co0.13As₂ with Tc=24.5 K. Our experimental data provide evidence for multiple superconducting energy gaps and can be well described in terms of three isotropic gaps with...
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In: Physical Review Letters, 2010, vol. 104, p. 156807
With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO₃ and SrTiO₃. We obtained a sheet carrier concentration of Ns≈5–9×10¹³ cm⁻², an effective mass of m*=3.2±0.4me, and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi1-xNbxO₃ and...
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In: Physical Review Letters, 2010, vol. 105, no. 5, p. 057001
We report muon spin rotation (μSR) and infrared spectroscopy experiments on underdoped BaFe1.89Co0.11As₂ which show that bulk magnetism and superconductivity (SC) coexist and compete on the nanometer length scale. Our combined data reveal a bulk magnetic order, likely due to an incommensurate spin density wave (SDW), which develops below Tmag≈32 K and...
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