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

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

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...

Université de Fribourg

Terahertz ellipsometry study of the soft mode behavior in ultrathin SrTiO films

Marsik, Premysl ; Sen, Kaushik ; Khmaladze, Jarji ; Yazdi-Rizi, Meghdad ; Mallett, Benjamin P. P. ; Bernhard, Christian

In: Applied Physics Letters, 2016, vol. 108, no. 5, p. 052901

We present a combined study with time-domain terahertz and conventional far-infrared ellipsometry of the temperature dependent optical response of SrTiO₃thin films (82 and 8.5 nm) that are grown by pulsed-laser deposition on (La0.3Sr0.7) (Al0.65Ta0.35)O₃ (LSAT) substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical...

Université de Fribourg

Infrared ellipsometry study of the confined electrons in a high-mobility γ-Al₂O₃/SrTiO₃ heterostructure

Yazdi-Rizi, Meghdad ; Maršík, Premysl ; Mallett, Benjamin P. P. ; Dubroka, Adam ; Christensen, D. V. ; Chen, Y. Z. ; Pryds, N. ; Bernhard, Christian

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...