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Université de Fribourg

Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

Xu, B. ; Xiao, H. ; Gao, B. ; Ma, Y. H. ; Mu, G. ; Mar?ík, Premysl ; Sheveleva, Evgeniia ; Lyzwa, F. ; Dai, Y. M. ; Lobo, R. P. S. M. ; Bernhard, Christian

In: Physical Review B, 2018, vol. 97, no. 19, p. 195110

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at Ts=121 K and a magnetic one to a spin density wave (SDW) state at TN=110 K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a...

Université de Fribourg

Lattice-mediated magnetic order melting in ${\mathrm{TbMnO}}_{3}$

Baldini, Edoardo ; Kubacka, Teresa ; Mallett, Benjamin P. P. ; Ma, Chao ; Koohpayeh, Seyed M. ; Zhu, Yimei ; Bernhard, Christian ; Johnson, Steven L. ; Carbone, Fabrizio

In: Physical Review B, 2018, vol. 97, no. 12, p. 125149

Recent ultrafast magnetic-sensitive measurements [Johnson et al., Phys. Rev. B 92, 184429 (2015); Bothschafter et al., Phys. Rev. B 96, 184414 (2017)] have revealed a delayed melting of the long-range cycloid spin order in TbMnO3 following photoexcitation across the fundamental Mott-Hubbard gap. The microscopic mechanism behind this slow transfer of energy from the photoexcited carriers to...

Université de Fribourg

In situ monitoring of atomic layer epitaxy via optical ellipsometry

Lyzwa, F. ; Marsik, Premysl ; Roddatis, V. ; Bernhard, Christian ; Jungbauer, M. ; Moshnyaga, V.

In: Journal of Physics D: Applied Physics, 2018, vol. 51, no. 12, p. 125306

We report on the use of time-resolved optical ellipsometry to monitor the deposition of single atomic layers with subatomic sensitivity. Ruddlesden–Popper thin films of SrO(SrTiO3) n=4 were grown by means of metalorganic aerosol deposition in the atomic layer epitaxy mode on SrTiO3(1 0 0), LSAT(1 0 0) and DyScO3(1 1 0) substrates. The measured time dependences of ellipsometric...

Université de Fribourg

Structural, magnetic and electronic properties of pulsed-laser-deposition grown SrFeO 3− δ thin films and SrFeO 3− δ /La 2/3 Ca 1/3 MnO 3 multilayers

Perret, Edith ; Sen, Kaushik ; Khmaladze, Jarji ; Mallett, Benjamin P. P. ; Yazdi-Rizi, Meghdad ; Maršík, Premysl ; Das, Saikat ; Marozau, Ivan ; Uribe-Laverde, Miguel Angel ; Andrés Prada, R. de ; Strempfer, J. ; Döbeli, Max ; Biškup, N. ; Varela, M. ; Mathis, Y.-L. ; Bernhard, Christian

In: Journal of Physics: Condensed Matter, 2017, vol. 29, no. 49, p. 495601

We studied the structural, magnetic and electronic properties of ${\rm SrFeO}_{3- \delta}$ (SFO) thin films and ${\rm SrFeO}_{3-\delta}$ /${\rm La}_{2/3}$ ${\rm Ca}_{1/3}$ MnO3 (LCMO) superlattices that have been grown with pulsed laser deposition (PLD) on ${\rm La}_{0.3}$ ${\rm Sr}_{0.7}$ ${\rm Al}_{0.65}$ ${\rm Ta}_{0.35}$ ${\rm O}_{3}$ (LSAT) substrates. X-ray reflectometry and scanning ...

Université de Fribourg

Superconductivity and charge-carrier localization in ultrathin ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}/{\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ bilayers

Sen, Kaushik ; Maršík, Premysl ; Das, Saikat ; Perret, E. ; Andres Prada, R. de ; Alberca, Aurora ; Biškup, N. ; Varela, M. ; Bernhard, Christian

In: Physical Review B, 2017, vol. 95, no. 21, p. 214506

La1.85Sr0.15CuO4/La2CuO4 (LSCO15/LCO) bilayers with a precisely controlled thickness of N unit cells (UCs) of the former and M UCs of the latter ([LSCO15_N/LCO_M]) were grown on (001)-oriented SrLaAlO4 (SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and reciprocal space map (RSM) studies confirmed the epitaxial growth of the bilayers and showed that a [LSCO15_2/LCO_2]...

Université de Fribourg

Anomalous anisotropic exciton temperature dependence in rutile ${\mathrm{TiO}}_{2}$

Baldini, Edoardo ; Dominguez, Adriel ; Chiodo, Letizia ; Sheveleva, Evgeniia ; Yazdi-Rizi, Meghdad ; Bernhard, Christian ; Rubio, Angel ; Chergui, and Majed

In: Physical Review B, 2017, vol. 96, no. 4, p. 041204

Elucidating the details of electron-phonon coupling in semiconductors and insulators is a topic of pivotal interest, as it governs the transport mechanisms and is responsible for various phenomena such as spectral-weight transfers to phonon sidebands and self-trapping. Here, we investigate the influence of the electron-phonon interaction on the excitonic peaks of rutile TiO2, revealing a strong...

Université de Fribourg

Infrared ellipsometry study of photogenerated charge carriers at the (001) and (110) surfaces of $\mathrm{SrTi}{\mathrm{O}}_{3}$ crystals and at the interface of the corresponding $\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ heterostructures

Yazdi-Rizi, Meghdad ; Maršík, Premysl ; Mallett, Benjamin P. P. ; Sen, Kaushik ; Cerreta, A. ; Dubroka, Adam ; Scigaj, M. ; Sánchez, F. ; Herranz, G. ; Bernhard, Christian

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...

Université de Fribourg

Strongly bound excitons in anatase TiO₂ single crystals and nanoparticles

Baldini, Eduardo ; Chiodo, Letizia ; Dominguez, A. ; Palummo, M. ; Moser, S. ; Yazdi-Rizi, Meghdad ; Auböck, G. ; Mallett, Benjamin P. P. ; Berger, Helmuth ; Magrez, Arnaud ; Bernhard, Christian ; Grioni, M. ; Rubio, Angel ; Chergui, Majed

In: Nature Communications, 2017, vol. 8, no. 1, p. 13

Anatase TiO₂ is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron–hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved...

Université de Fribourg

Muon spin rotation and infrared spectroscopy study of magnetism and superconductivity in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$

Mallett, Benjamin P. P. ; Wang, Chen Nan ; Marsik, Premysl ; Sheveleva, E. ; Yazdi-Rizi, Meghdad ; Tallon, Jeffery L. ; Adelmann, P. ; Wolf, Th. ; Bernhard, Christian

In: Physical Review B, 2017, vol. 95, no. 5, p. 054512

Using muon spin rotation and infrared spectroscopy, we study the relation between magnetism and superconductivity in Ba1−xKxFe2As2 single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the Néel temperature TN. This applies for both the total...

Université de Fribourg

Local terahertz field enhancement for time-resolved x-ray diffraction

Kozina, M. ; Pancaldi, Matteo ; Bernhard, Christian ; Driel, T. van ; Glownia, J. M. ; Maršík, Premysl ; Radovic, M. ; Vaz, Carlos A. F. ; Zhu, D. ; Bonetti, Stephano ; Staub, U. ; Hoffmann, Matthias C.

In: Applied Physics Letters, 2017, vol. 110, no. 8, p. 081106

We report local field strength enhancement of single-cycle terahertz (THz) pulses in an ultrafast time-resolved x-ray diffraction experiment. We show that patterning the sample with gold microstructures increases the THz field without changing the THz pulse shape or drastically affecting the quality of the x-ray diffraction pattern. We find a five-fold increase in THz-induced x-ray...