Events: Interesting happenings in the next 4 weeks.

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July 2018

Mon., 02.

FriMat Day

Superconductor sandwiches: Emergent properties at the interface of thin film multilayers

Ben Mallett, The Photon Factory and Department of Physics, University of Auckland, New Zealand

Time: 14:00 - 15:00h

Location: UNI-Perolles, Physics Department, building 8, auditorium 1.58, Chemin du Musee 3, 1700 Fribourg, Switzerland

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Tue., 03.

Seminar Soft Matter and Photonics

Customizing Speckle Statistics

Hui CAO, Engineering and Applied Physics, Yale University, USA

Time: 15:00 - 16:00h

Location: UNI-Perolles, Physics Department, building 8, auditorium 2.73, Chemin du Musee 3, 1700 Fribourg, Switzerland

The Siegert relation between the first-order and the second-order correlation functions is a corner stone of statistical physics and has laid the foundation for seminal works such as the Hanbury-Brown Twiss experiment. Here we demonstrate a general method for breaking this fundamental relation, using the example of optical speckle patterns. Our work builds on theoretical insights from mesoscopic physics and recently developed optical wavefront shaping techniques. By judiciously modulating the phase-front of a monochromatic laser beam -with a spatial light modulator- we experimentally generate speckle patterns possessing either arbitrarily-tailored intensity probability density functions (PDFs) or non-local spatial intensity correlations. This approach provides a versatile framework for tailoring speckle patterns with varied applications in microscopy, imaging and optical manipulation.

Fri., 06.

Seminar FriMAT

Lattice and charge dynamics in thin films of SrIrO3: A Raman spectroscopy study

Kaushik Sen - Institute for Solid State Physics, Karlsruhe Institute of Technology, Germany

Time: 14:00h

Location: UNI-Perolles, Physics Departement, building 8, auditoire 1.58, Chemin du Musee 3, 1700 Fribourg Switzerland

 

SrIrO3 has been proposed as a candidate to realize the so-called ‘topological insulator’ phase due to its strong spin-orbit coupling (0.4 eV) and electron-electron correlations (0.5 eV) [1,2].  The narrow electron bands and their extreme sensitivity to the rotations of IrO6-octahedra place the compound close to a metal-insulator transition [3]. Such a strong correlation between lattice and electronic properties has motivated us to investigate the underlying lattice and charge dynamics in the system.

For the first time, we report lattice and charge dynamics in 50 nm thick SrIrO3 films using Raman scattering experiments as a function of temperature and light polarizations.

In bulk, SrIrO3 has a monoclinic crystal structure. However, under the influence of effective compressive strain of 0.25%, a stable perovskite structure is obtained in pulsed-laser-deposition grown SrIrO3 films on (101)-oriented DyScO3 substrates. Using confocal Raman spectroscopy, we measured temperature dependence of Ag and B2g phonon modes. Corresponding atomic displacements are assigned with the help of first-principles based lattice dynamics calculations. Finally, an electronic continuum could directly be evident. Moreover, such an electron continuum gives rise to a strong Fano-like asymmetry to a particular phonon line shape of Ag symmetry. Furthermore, the symmetry dependent electron scattering continuums agree well with the results of an angle resolved photoemission spectroscopy (ARPES) study of SrIrO3 thin films [3].  

 

References:

[1] D. Xiao et al. Nat. Commun., 2, 596 (2011)

[2] L. Zhang et al. Critical Reviews in Solid State and Materials Sciences, 0, 1-25 (2017)

[3] Y.F. Nie et al. Phys. Rev. Lett. 114, 016401 (2015)

 

Mon., 09.

Seminar FriMAT

Multiferroic properties of coherently grown orthorhombic RMnO3 (R = Gd-Lu) thin films

Christof Schneider, Thin Films and Interfaces Group, Paul Scherrer Institut (PSI), CH-5232 Villigen PSI

Time: 14:00 - 15:00h

Location: UNI-Perolles, Physics Department, building 8, auditorium 1.58, Chemin du Musee 3, 1700 Fribourg, Switzerland

In this presentation, I will give an overview of the magneto-electric properties of a series of orthorhombic RMnO3 (R = Gd-“Lu) thin films, coherently grown on (010) YAlO3 substrates. The combination of chemical pressure and epitaxial growth is changing the b-axis lattice parameter while fixing the in-plane lattice constants. As a result, all coherently grown films show electrical polarization values up to 1uC/cm2 along the a-axis and TFE is raised close to TN. The induced strain also modifies the magnetic ground state of these materials. As measured by neutron and x-ray diffraction, the entire series shows a commensurate Etype antiferromagnetic order as a common ground state consistent with the relatively large spontaneous polarization. Our results further indicate, that the asymmetric Dzyaloshinski-Moria interaction in o-RMnO3 (R = Gd-“Dy) is transformed by epitaxial strain into the symmetric exchange striction as the dominant mechanism for ferroelectricity.

Thu., 12.

Seminar

Phonons in superconductors

Frank Weber, Institute for Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe, Germany

Time: 11:00 - 12:00h

Location: UNI-Perolles, Physics Department, building 8, auditorium 0.58.5, Chemin du Musee 3, 1700 Fribourg, Switzerland

Momentum-resolved phonon spectroscopy, traditionally the domain of inelastic neutron scattering, has tremendously benefitted from the development of inelastic x-ray scattering with energy resolution down to 1 milli-electronvolt. Tiny single crystals weighing only few milligrams can now be routinely investigated. After a short comparison of state-of-the-art inelastic x-ray and neutron scattering experiments, I will review work using both neutrons and x-rays to investigate phonons in superconducting materials: (1) YNi2B2C is likely one of the best-investigated conventional superconductors with regard to lattice dynamics apart from elemental niobium or lead. We have investigated this material in various studies and are currently working on a combined study of phonons and the electronic band structure in order to disentangle the origin of pronounced anomalies in the temperature dependent lattice dynamical properties. (2) 2H-NbSe2 is one of the few compounds where CDW order and superconductivity co-exist without doping or other tuning parameters. We observe changes of the phonon line shape that are characteristic for systems with strong electron-phonon coupling in the presence of a superconducting energy gap 2Δc and from which we can demonstrate an l dependence of the superconducting gap. Reversely, our data imply that the CDW energy gap is strongly localized along the l direction. This conclusion is corroborated by recent soft x-ray angle-resolved photoemission spectroscopy revealing the 3D Fermi surface of 2H-NbSe2. The confinement of the CDW gap to a very small momentum region explains the rather low competition and easy coexistence of CDW order and superconductivity in 2H-NbSe2. (3) Nematicity is ubiquitous in electronic phases of high-𝑇c superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have either been restricted to uniform, i.e. q = 0 fluctuations, or measure momentum-averaged effects. Here, we investigate the behavior of finite-momentum nematic fluctuations near q ≈ 0 by utilizing the anomalous softening of transverse acoustic phonon modes in optimally doped Ba(Fe0.94Co0.06)2As2. We determine the nematic correlation length and find that it sharply changes its 𝑇-dependence at 𝑇c, revealing a strong connection between nematicity and superconductivity.

Sat., 23.06.2018 - Fri., 20.07.2018

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