The group develops new spectroscopic methods, especially spetral ellipsometry with synchroton radiation and neutron resonance spin-echo spectroscopy. To this end, we operate experimental facilities at the ANKA synchroton in Karlsruhe and at the FRM-II research reactor in Garching, in addition to our in-house laboratories. The picture shows the TRISP spectrometer at the FRM-II, which allows the determination of the lifetimes of collective excitations in solids with unprecedented accuracy.

Recent focus areas and key publications

Lifetimes of elementary excitations

Lifetimes of antiferromagnetic magnons in two and three dimensions: experiment, theory, and numerics
S. P. Bayrakci, D. A. Tennant, Ph. Leininger, T. Keller, M. C. R. Gibson, S. D. Wilson, R. J. Birgeneau, B. Keimer.
Phys. Rev. Let. 111, 017204; published 03 July 2013

Bond randomness induced magnon decoherence in a spin-½ ladder compound
B. Náfrádi, T. Keller, H. Manaka, U. Stuhr, A. Zheludev, B. Keimer.
Phys. Rev. B 87 (2013) 020408; published 28 January 2013.

Roton-phonon interactions in superfluid ⁴He
B. Fåk, T. Keller, M. E. Zhitomirsky, A. L. Chernyshev.
Phys. Rev. Lett. 109 (2012), 155305; published 10 October 2012.

Low-Temperature Dynamics of Magnons in a Spin-1/2 Ladder Compound
B. Náfrádi, T. Keller, H. Manaka, A. Zheludev, B. Keimer.
Phys. Rev. Lett. 106 (2011), 177202; published 25 April 2011.

Energy Gaps and Kohn Anomalies in Elemental Superconductors
P. Aynajian, T. Keller, L. Boeri, S. M. Shapiro, K. Habicht, B. Keimer.
Science 319 (2008), 1509

Larmor diffraction

Possible realization of an antiferromagnetic Griffiths phase in Ba[Fe_1-xMn_x]₂As₂
D. S. Inosov, G. Friemel, J. T. Park, A. C. Walters, Y. Texier, Y. Laplace, J. Bobroff, V. Hinkov, D. L. Sun, Y. Liu, R. Khasanov, K. Sedlak, Ph. Bourges, Y. Sidis, A. Ivanov, C. T. Lin, T. Keller, B.Keimer.
Phys. Rev. B 87 (2013), 224425, published 26 June 2013

Parasitic Small-Moment Antiferromagnetism and Nonlinear Coupling of Hidden Order and Antiferromagnetism in URu₂Si₂ Observed by Larmor Diffraction
P. G. Niklowitz, C. Pfleiderer, T. Keller, M. Vojta, Y.-K. Huang, J. A. Mydosh.
Phys. Rev. Lett. 104 (2010), 106406; published 12 March 2010.

Neutron Larmor diffraction measurements for materials science
J. Repper, T. Keller, M. Hofmann, C. Krempaszky, W. Petry, E. Werner.
Acta Materialia 58 (2010), 3459; published 02 March 2010.