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Université de Fribourg

Infrared study of the spin reorientation transition and its reversal in the superconducting state in underdoped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$

Mallett, B. P. P. ; Maršík, Premysl ; Yazdi-Rizi, M. ; Wolf, Th. ; Böhmer, A. E. ; Hardy, F. ; Meingast, C. ; Munzar, Dominik ; Bernhard, Christian

In: Physical Review Letters, 2015, vol. 115, no. 2, p. 027003

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba1−xKxFe2As2. In agreement with the predicted transition from a single-Q to a double-Q AF structure, we found that a distinct spin density wave develops in the...

Université de Fribourg

Evidence for precursor superconducting pairing above T c in underdoped cuprates from an analysis of the in-plane infrared response

Šopík, B. ; Chaloupka, Jiří ; Dubroka, Adam ; Bernhard, Christian ; Munzar, Dominik

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...

Université de Fribourg

Atomic-resolution studies of epitaxial strain release mechanisms in $\mathrm{L}{\mathrm{a}}_{1.85}\mathrm{S}{\mathrm{r}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}/\mathrm{L}{\mathrm{a}}_{0.67}\mathrm{C}{\mathrm{a}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ superlattices

Biškup, N. ; Das, Saikat ; Gonzalez-Calbet, J. M. ; Bernhard, Christian ; Varela, M.

In: Physical Review B, 2015, vol. 91, no. 20, p. 205132

In this paper we present an atomic-resolution electron microscopy study of superlattices (SLs) where the colossal magnetoresistant manganite La0.67Ca0.33MnO3 (LCMO) and the high critical temperature superconducting cuprate La1.85Sr0.15CuO4 (LSCO) are combined. Although good quality epitaxial growth can be achieved, both the choice of substrate and the relatively large lattice mismatch between...

Université de Fribourg

Enhanced superconducting properties of rare-earth oxides and graphene oxide added MgB₂

Sudesh ; Das, S. ; Bernhard, Christian ; Varma, G.D.

In: Physica C: Superconductivity, 2014, vol. 505, p. 32–38

In this paper, the effects of addition of (i) graphene oxide (GO), (ii) a series of rare-earth (RE, RE = La, Sm, Eu, Gd, Tb and Ho) oxides (REO) and (iii) a mixture of GO and rare-earth oxides (GO + REO) on the superconducting properties of MgB₂, have been studied with the help of electrical transport and magnetic measurements. All the samples have been prepared following the standard...

Université de Fribourg

Controlling the near-surface superfluid density in underdoped YBa₂Cu₃O6+x by photo-illumination

Stilp, E. ; Suter, A. ; Prokscha, T. ; Salman, Z. ; Morenzoni, E. ; Keller, H. ; Pahlke, P. ; Hühne, R. ; Bernhard, Christian ; Liang, Ruixing ; Hardy, W. N. ; Bonn, D. A. ; Baglo, J. C. ; Kiefl, R. F.

In: Scientific Reports, 2014, vol. 4, p. -

The interaction with light weakens the superconducting ground state in classical superconductors. The situation in cuprate superconductors is more complicated: illumination increases the charge carrier density, a photo-induced effect that persists below room temperature. Furthermore, systematic investigations in underdoped YBa₂Cu₃O6+x (YBCO) have shown an enhanced critical...

Université de Fribourg

Optical probe of ferroelectric order in bulk and thin-film perovskite titanates

Rössle, Matthias ; Wang, Chen Nan ; Maršík, Premysl ; Yazdi-Rizi, M. ; Kim, Kyung Wan ; Dubroka, Adam ; Marozau, Ivan ; Schneider, C. W. ; Humlíček, J. ; Baeriswyl, Dionys ; Bernhard, Christian

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...

Université de Fribourg

Effect of graphene oxide doping on superconducting properties of bulk MgB2

Sudesh, Sharma ; Kumar, N. ; Das, Saikat ; Bernhard, Christian ; Varma, G. D.

In: Superconductor Science and Technology, 2013, vol. 26, no. 9, p. 095008

In the present paper we report the effect of graphene oxide (GO) doping on the structural and superconducting properties of MgB₂. Bulk polycrystalline samples have been synthesized via a solid state reaction route with compositions MgB₂ + x wt% of GO (x = 0, 1, 2, 3, 5, 7 and 10) by sintering at ~850 ° C in a reducing atmosphere of Ar/H₂ (9:1). The x-ray diffraction...

Université de Fribourg

Electric-Field-Induced Polar Order and Localization of the Confined Electrons in LaAlO₃/SrTiO₃ Heterostructures

Rössle, Matthias ; Kim, Kyung Wan ; Dubroka, Adam ; Maršík, Premysl ; Wang, Chen Nan ; Jany, R. ; Richter, C. ; Mannhart, J. ; Schneider, C. W. ; Frano, A. ; Wochner, P. ; Lu, Y. ; Keimer, B. ; Shukla, D. K. ; Strempfer, J. ; Bernhard, Christian

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...

Université de Fribourg

Magnetic Proximity Effect in YBa₂Cu₃O₇/La2/3Ca1/3MnO₃ and YBa₂Cu₃O₇/LaMnO₃₊ Superlattices

Satapathy, Dillip Kumar ; Uribe-Laverde, M. A. ; Marozau, I. ; Malik, Vivek K. ; Das, S. ; Wagner, Th. ; Marcelot, C. ; Stahn, J. ; Brück, S. ; Rühm, A. ; Macke, S. ; Tietze, T. ; Goering, E. ; Frañó, A. ; Kim, J. -H. ; Wu, M. ; Benckiser, E. ; Keimer, B. ; Devishvili, A. ; Toperverg, B. P. ; Merz, M. ; Nagel, P. ; Schuppler, S. ; Bernhard, Christian

In: Physical Review Letters, 2012, vol. 108, no. 19, p. 197201

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa₂Cu₃O₇ and ferromagnetic-metallic La0.67Ca0.33MnO₃ or ferromagnetic-insulating LaMnO₃₊. We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the...

Université de Fribourg

Macroscopic phase segregation in superconducting K0.73Fe1.67Se₂ as seen by muon spin rotation and infrared spectroscopy

Wang, Chen Nan ; Maršik, Premysl ; Schuster, Roman ; Dubroka, Adam ; Rössle, Matthias ; Niedermayer, Ch. ; Varma, G. D. ; Wang, A. F. ; Chen, X. H. ; Wolf, T. ; Bernhard, Christian

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%...