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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...

Université de Fribourg

Clocking the onset of bilayer coherence in a high-${T}_{c}$ cuprate

Baldini, Edoardo ; Mann, Andreas ; Mallett, Benjamin P. P. ; Arrell, Christopher ; Mourik, Frank van ; Wolf, Thomas ; Mihailovic, Dragan ; Tallon, Jeffrey L. ; Bernhard, Christian ; Lorenzana, José ; Carbone, Fabrizio

In: Physical Review B, 2017, vol. 95, no. 2, p. 024501

In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature TC. Here we show via a combination of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that signatures of such a state can be obtained via three independent observables in an underdoped sample of NdBa2Cu3O6+δ. The pseudogap correlations were disentangled from the...

Université de Fribourg

Anisotropy of infrared-active phonon modes in the monodomain state of tetragonal ${\mathrm{SrTiO}}_{3}$ (110)

Yazdi-Rizi, Meghdad ; Maršík, Premysl ; Mallett, Benjamin P. P. ; Bernhard, Christian

In: Physical Review B, 2017, vol. 95, no. 2, p. 024105

With infrared (IR) and terahertz (THz) ellipsometry we investigated the anisotropy of the IR-active phonon modes in ${\mathrm{SrTiO}}_{3}$ (110) single crystals in the tetragonal state below the so-called antiferrodistortive transition at T∗=105 K. In particular, we show that the anisotropy of the oscillator strength of the so-called R mode, which becomes weakly IR active below T∗, is a...

Université de Fribourg

Optical constants, band gap, and infrared-active phonons of (LaAlO₃)0.3(Sr₂AlTaO₆)0.35 (LSAT) from spectroscopic ellipsometry

Nunley, Timothy Nathan ; Willett-Gies, Travis I. ; Cooke, Jacqueline A. ; Manciu, Felicia S. ; Marsik, Premysl ; Bernhard, Christian ; Zollner, Stefan

In: Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2016, vol. 34, no. 5, p. 051507

Using spectroscopic ellipsometry, the authors determined the optical constants (complex dielectric function) for (LaAlO₃)0.3(Sr₂AlTaO₆) 0.35 (LSAT) from 0.01 to 6.5 eV. Above 0.5 eV, the data were described with a sum of two Tauc-Lorentz oscillators and two poles. A direct gap of 5.8 ± 0.1 eV was found. An Urbach tail extends to even lower photon energies...

Université de Fribourg

Granular superconductivity and magnetic-field-driven recovery of macroscopic coherence in a cuprate/manganite multilayer

Mallett, Benjamin P. P. ; Khmaladze, J. ; Marsik, Premysl ; Perret, Edith ; Cerreta, A. ; Orlita, M. ; Biškup, N. ; Varela, M. ; Bernhard, Christian

In: Physical Review B, 2016, vol. 94, no. 18, p. 180503

We show that in Pr0.5La0.2Ca0.3MnO3/YBa2Cu3O7 (PLCMO/YBCO) multilayers the low temperature state of YBCO is very resistive and resembles that of a granular superconductor or a frustrated Josephson-junction network. Notably, a coherent superconducting response can be restored with a large magnetic field which also suppresses the charge-orbital order in PLCMO. This coincidence suggests that the...