The intriguing (and potentially usable) properties occasionally found in crystals, such as, e.g., optical frequency conversion, electro-optics, (multi-)ferroicity or complex magnetic states, motivate and challenge simultaneously the search and development of new crystal materials. In Materials Crystallography at the University of Cologne we focus on (large) single crystals, which possess one or more functionalities corresponding inter alia to the above mentioned areas, however, without aiming to the step of device development.
In general, development of crystalline materials, from the here used point of view of Materials Crystallography, encompasses both, approaches of solid state chemistry as well as of solid state physics, and the translation of the physical background theory of the property in question into terms of crystal chemistry is one of the fundamental tasks in materials development. In addition, symmetry (or the absence of symmetry) and the resulting anisotropy of physical properties (or even their presence at all) in crystals plays a key role for the targeted realization of physical effects in these materials. A third central issue of materials development is the question of availability of single crystals of required size and quality, a crucial aspect which in many cases turns out to be a bottleneck. Consequently, development of crystalline materials includes also the development of strategies of synthesis and growth of large single crystals.
Some selected specific examples illustrate our research activities in Materials Crystallography:
New magnetoelectric multiferroics
Ackermann, M., Anderson, L., Lorenz, T., Bohatý, L., Becker, P.: Anisotropy study of multiferroicity in the pyroxene NaFeGe2O6. New J. Phys. 17 (2015) 013045 (10 pp).
Linear magnetoelectric crystals
Ackermann, M., Lorenz, T., Becker, P., Bohatý, L.: Magnetoelectric properties of A2[FeCl5(H2O)] with A = K, Rb, Cs. J. Phys.: Condens. Matter 26 (2014) 506002 (11pp).
Tolédano, P., Ackermann, M., Bohatý, L., Becker, P., Lorenz, T., Leo, N. & Fiebig, M.: Primary ferrotoroidicity in antiferromagnets. Phys. Rev. B 92 (2015) 094431-1/7.
Systems with magnetic frustration or topological defects
Spin-liquid crystals (spin-1/2 XXZ system)
Breunig, O., Garst, M., Rosch, A., Sela, E., Buldmann, B., Becker, P., Bohatý, L., Müller, R. & Lorenz, T.: Low –temperature ordered phases of the spin-1/2 XXZ chain system Cs2CoCl4 . Phys. Rev. B 91 (2015) 024423-1/10.
Crystals with strong impact of spin orbit coupling
Revelli, A., Moretti Sala, M., Monaco, G., Becker, P., Bohatý, L., Hermanns, M., Koethe, T., Fröhlich, T., Warzanowski, P., Lorenz, T., Streltsov, S.V., Van Loosdrecht, P.H.M., Khomskii, D.I., van den Brink, J. & Grüninger M.: Resonant inelastic x-ray incarnation of Young's double-slit experiment. Science Advances 5 (2019) eaav4020.
Crystals for frequency conversion via x² processes (SHG, SFG)
Bohatý, L., Bayarjargal, L. & Becker, P.: Linear and nonlinear optical properties of yttrium formate dihydrate, Y(HCOO)3∙2H2O. Appl. Phys. B 86 (2007) 523-527.
Crystals for generation of entangled photons via optical parametric down-conversion
Halevy, A., Megidish, E., Dovrat, L., Eisenberg, H. S., Becker, P. & Bohatý, L.: The biaxial nonlinear crystal BiB3O6 as a polarization entangled photon source using non-linear type-II parametric down-conversion. Opt. Express 19 (2011) 20420-20434.
Crystals for frequency conversion via cascaded x² - x³ processes
Kaminskii, A. A., Bohatý, L., Becker, P., Liebertz, J., Held, P., Eichler, H. J., Rhee, H. & Hanuza, J.: Tetragonal Ba2MgGe2O7 – a novel multifunctional optical crystal with numerous manifestations of nonlinear-laser effects: almost sesqui-octave Stokes and anti-Stokes combs and cascaded χ(3)- χ(2) lasing with involved second and third harmonic generation. Laser Phys. Lett. 5 (2008) 845-868.
Crystals for generation of frequency combs via SRS
Kaminskii, A. A., Lux, O., Rhee, H., Eichler, H. J., Yoneda, H., Shirakawa, A., Ueda, K., Rückamp, R., Bohatý, L. & Becker, P.: Manifestations of nonlinear optical effects in a novel SRS-active crystal – natural topaz, Al2(F1-x(OH)x)2SiO4: many-phonon χ(3 )-lasing, more than sesqui-octave Stokes and anti-Stokes multi-wavelength comb lasing, cascaded and cross-cascaded χ(3)« χ(3 )Raman induced interactions under single- and dual-wavelength picosecond collinear coherent pumping, THG and combined SRS-promoting phonon modes. Laser Phys. Lett. 10 (2013) 073001.
New laser crystals
Kaminskii, A. A., Nakao, H., Shirakawa, A., Ueda, K., Liebertz, J., , Becker, P. & Bohatý, L.: CW one-micron (4F3/2 ®4I11/2) laser oscillation of Nd3+ ions in the melilite-type crystal Ca2MgSi2O7:Nd3+(Na+) at its incommensurate-commensurate phase transition. Appl. Phys. B 103 (2011) 629-635.
Pyroelectric (non-ferroelectric) crystals
Ackermann, M. Andersen, L., Becker, P., Bohatý, L.: Pyroelectric properties of the monoclinic rare earth nitrates A2Ln(NO3)5·4H2O (A = NH4, Rb; Ln = La, Ce). Z. Kristallogr. 230 (2015) 633-638.