In: Physical Review B, 2020, vol. 101, no. 21, p. 214512
We studied the infrared response of polycrystalline samples of the iron arsenide superconductor (Rb,Cs)Ca2Fe4As4F2 (Rb,Cs-12442), which has a bilayer structure similar to the high-Tc cuprates YBa2Cu3O7 (YBCO) and Bi2Sr2CaCu2O8. The c-axis reflectivity spectra Rc have been derived from the reflectivity spectra of the polycrystalline samples Rpoly and the in-plane spectrum of a corresponding...
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In: Physical Review B, 2019, vol. 99, no. 5, p. 054513
We report on results of our analysis of the c-axis infrared conductivity, σc(ω), of bilayer LnBa2Cu3O7−δ (Ln=La, Nd, Y) and trilayer Bi2Sr2Ca2Cu3O10+δ high-Tc superconductors. The analysis employs the multilayer model involving the conductivity of the bilayer or trilayer unit, σbl(ω), and that of the spacing layers separating the latter units, σint(ω). For the YBa2Cu3O7−δ sample...
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In: Physical Review B, 2017, vol. 95, no. 19, p. 195107
With infrared (IR) ellipsometry and dc resistance measurements, we investigated the photodoping at the (001) and (110) surfaces of SrTiO3 (STO) single crystals and at the corresponding interfaces of LaAlO3/SrTiO3 (LAO/STO) heterostructures. In the bare STO crystals, we find that the photogenerated charge carriers, which accumulate near the (001) surface, have a similar depth profile and sheet...
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In: EPL (Europhysics Letters), 2016, vol. 113, no. 4, p. 47005
With infrared ellipsometry we studied the response of the confined electrons in γ-Al₂O₃/SrTiO₃ (GAO/STO) heterostructures in which they originate predominantly from oxygen vacancies. From the analysis of a so-called Berreman mode, that develops near the highest longitudinal optical phonon mode of SrTiO₃, we derive the sheet carrier density, N s , the...
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In: New Journal of Physics, 2015, vol. 17, no. 5, p. 053022
We performed calculations of the in-plane infrared response of underdoped cuprate superconductors to clarify the origin of a characteristic dip feature which occurs in the published experimental spectra of the real part of the in-plane conductivity below an onset temperature ${{T}^{{\rm ons}}}$ considerably higher than ${{T}_{{\rm c}}}$. We provide several arguments, based on a detailed...
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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: 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: Physical Review B - Condensed Matter and Materials Physics, 2012, vol. 86, no. 14, p. 144112
Far-infrared spectra of ferrimagnetic Dy₃Fe₅O₁₂ single crystals have been studied in the spectral range between 12 and 700 cm⁻¹ and in a wide temperature range between 5 K and 300 K using transmission spectroscopy and rotating analyzer ellipsometry. In the temperature range below TC = 16 K for the magnetic ordering of Dy³⁺ spins, a number of ligand field and...
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In: Physical Review B - Condensed matter and materials physics, 2012, vol. 85, no. 21, p. 214503
Using muon spin rotation and infrared spectroscopy, we investigated the recently discovered superconductor K0.73Fe1.67Se₂ with Tc≈32 K. We show that the combined data can be consistently described in terms of a macroscopically phase-segregated state with a matrix of ∼88% volume fraction that is insulating and strongly magnetic and inclusions with an ∼12%...
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