Highlights

Exchange interactions between electrons have been experimentally measured in a spiral magnet close to a metal-insulator transition. The neutron scattering data indicate that spiral magnetic state arises from a competition between short-range ferromagnetic exchange and longer-distance antiferromagnetic exchange, confirming predictions by de Gennes in 1960.

Spiral spin correlations at the metal-insulator transition

Exchange interactions between electrons have been experimentally measured in a spiral magnet close to a metal-insulator transition. The neutron scattering data indicate that spiral magnetic state arises from a competition between short-range ferromagnetic exchange and longer-distance antiferromagnetic exchange, confirming predictions by de Gennes in 1960.
The heavy-fermion metal CeB6 is known to exhibit antiferromagnetic order at low temperatures. Neutron scattering experiments have now shown that the dominant magnetic excitations are actually ferromagnetic, suggesting that the antiferromagnetism is highly unconventional.

Ferromagnetic correlations in a heavy-fermion antiferromagnet

The heavy-fermion metal CeB6 is known to exhibit antiferromagnetic order at low temperatures. Neutron scattering experiments have now shown that the dominant magnetic excitations are actually ferromagnetic, suggesting that the antiferromagnetism is highly unconventional.
Infrared ellipsometry reveals pronounced Fano resonances indicating unusual electron-phonon interactions in a geometrically frustrated iridium-oxide compound. D. Pröpper et al., Phys.Rev. Lett. 2014

Fano resonances in iridates

Infrared ellipsometry reveals pronounced Fano resonances indicating unusual electron-phonon interactions in a geometrically frustrated iridium-oxide compound. D. Pröpper et al., Phys.Rev. Lett. 2014
Using inelastic neutron scattering, we have found a magnetic exciton mode in the non-superconducting heavy-fermion metal CeB6, resembling resonant magnetic excitations in unconventional superconductors. G. Friemel et al., Nature Communications, 2012

Magnetic resonant mode in CeB6

Using inelastic neutron scattering, we have found a magnetic exciton mode in the non-superconducting heavy-fermion metal CeB6, resembling resonant magnetic excitations in unconventional superconductors. G. Friemel et al., Nature Communications, 2012
An unusual form of magnetic order is identified as the ground state of the Kitaev-Heidenberg model (which has deep relations to quantum computation), and found to explain experimental observations on iridium oxide compounds. J.Chaloupka et al Phys. Rev. Lett. 110 (2013)

Zigzag magnetic order in iridates

An unusual form of magnetic order is identified as the ground state of the Kitaev-Heidenberg model (which has deep relations to quantum computation), and found to explain experimental observations on iridium oxide compounds. J.Chaloupka et al Phys. Rev. Lett. 110 (2013)

Spin orbital and charge order

The exceptionally rich phase behavior observed in transition metal oxides and other correlated-electron materials originates in a competition between many-body states with different spin, orbital and charge ordering patterns. Work in our group seeks to unravel the microscopic mechanisms underlying this competition. To this end, we combine various advanced spectroscopic methods, and we analyse and interpret the resulting data in close collaboration between experimentalists and theorists.

The exceptionally rich phase behavior observed in transition metal oxides and other correlated-electron materials originates in a competition between many-body states with different spin, orbital and charge ordering patterns. Work in our group seeks to unravel the microscopic mechanisms underlying this competition. To this end, we combine various advanced spectroscopic methods, and we analyse and interpret the resulting data in close collaboration between experimentalists and theorists.

Recent focus areas and key publications

Novel spin-orbit coupling physics in iridates

Excitonic quasiparticles in a spin–orbit Mott insulator
Jungho Kim, M. Daghofer, A.H. Said, T. Gog, J. van den Brink, G. Khaliullin, B.J. Kim
Nature Comm. 5, 4453, 17 July 2014

Fermi arcs in a doped pseudospin-1/2 Heisenberg antiferromagnet
Y. K. Kim1, O. Krupin, J. D. Denlinger, A. Bostwick, E. Rotenberg, Q. Zhao, J. F. Mitchell, J. W. Allen, B. J. Kim.
Science Vol. 345 no. 6193 pp. 187-190 (2014); published 12 June 2014.

Fano Resonances in the Infrared Spectra of Phonons in Hyperkagome Na3Ir3O8
D. Pröpper, A. N. Yaresko, T. I. Larkin, T. N. Stanislavchuk, A. A. Sirenko, T. Takayama, A. Matsumoto, H. Takagi, B. Keimer, and A. V. Boris.
Phys. Rev. Lett. 112 (2014) 087401; published 26 February 2014.

Zigzag Magnetic Order in the Iridium Oxide Na2IrO3
J. Chaloupka, G. Jackeli, G. Khaliullin.
Phys. Rev. Lett. 110 (2013) 097204; published 28 February 2013.

Magnetic couplings, optical spectra, and spin-orbit exciton in 5d electron Mott insulator Sr2IrO4
B. H. Kim, G. Khaliullin, B. I. Min.
Phys. Rev. Lett. 109 (2012) 167205; published 17 October 2012.

Large spin-wave energy gap for the bilayer iridate Sr3Ir2O7: Evidence for enhanced dipolar interactions near the Mott metal-insulator transition
J. Kim, A. H. Said, D. Casa, M. H. Upton, T. Gog, M. Daghofer, G. Jackeli, J. van den Brink, G. Khaliullin, B. J. Kim.
Phys. Rev. Lett. 109 (2012), 157402; published 10 October 2012.

Dimensionality driven spin-flop transition in layered iridates
J. W. Kim, Y. Choi, J. Kim, J. F. Mitchell, G. Jackeli, M. Daghofer, J. van den Brink, G. Khaliullin, and B. J. Kim.
Phys. Rev. Lett. 109 (2012), 037204; published 17 July 2012.

Interplay between spin and charge order in transition metal oxides

Competing Exchange Interactions on the Verge of a Metal-Insulator Transition in the Two-Dimensional Spiral Magnet Sr3Fe2O7

J.-H. Kim, Anil Jain, M. Reehuis, G. Khaliullin, D. C. Peets, C. Ulrich, J. T. Park, E. Faulhaber, A. Hoser, H. C. Walker, D. T. Adroja, A. C. Walters, D. S. Inosov, A. Maljuk, and B. Keimer.
Phys. Rev. Lett. 113, 147206; published 03. October 2014

Magnetic neutron diffraction study of the charge-ordered chain compounds Rb11Mn8O16 and Cs3Mn2O4
M. Reehuis, M. A. Señaris-Rodriguez, A. Hoser, B. Keimer, M. Jansen.
Phys. Rev. B 87, 014426 – Published 24 January 2013

Magnetic phase diagram of Sr3Fe2O7-x
D. C. Peets J.-H. Kim, P. Dosanjh, M. Reehuis, A. Maljuk, N. Aliouane, C. Ulrich, B. Keimer.
Phys. Rev. Lett 87 (2013) 214410; published 10 June 2013.

Neutron diffraction study of spin and charge ordering in SrFeO3−δ
M. Reehuis, C. Ulrich, A. Maljuk, Ch. Niedermayer, B. Ouladdiaf, A. Hoser, T. Hofmann, B. Keimer.
Phys. Rev. Lett. B 85 (2012), 184109; published 22 May 2012.

Magnetic and optical properties of spin chains

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.

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.

Anisotropic optical response of the mixed-valent Mott-Hubbard insulator NaCu2O2
Y. Matiks, A. N. Yaresko, K. Myung-Whun, A. Maljuk, P. Horsch, B. Keimer, A. V. Boris.
Phys. Rev. B 84 (2011), 245116; published 13 December 2011.

Skyrmion lattices

Formation and rotation of skyrmion crystal in the chiral-lattice insulator Cu2OSeO3
S. Seki, J.-H. Kim, D. S. Inosov, R. Georgii, B. Keimer, S. Ishiwata, and Y. Tokura.
Phys. Rev. B 85 (2012), 220406(R); published 22 June 2012.

Possible skyrmion-lattice ground state in the B20 chiral-lattice magnet MnGe as seen via small-angle neutron scattering
N. Kanazawa, J.-H. Kim, D. S. Inosov, J. S. White, N. Egetenmeyer, J. L. Gavilano, S. Ishiwata, Y. Onose, T. Arima, B. Keimer, Y. Tokura.
Phys. Rev. B 86 (2012) 134425; published 25 October 2012.

Hidden order in heavy-fermion compounds

Intense low-energy ferromagnetic fluctuations in the antiferromagnetic heavy-fermion metal CeB6
Hoyoung Jang, G. Friemel, J. Ollivier, A. V. Dukhnenko, N. Yu. Shitsevalova, V. B. Filipov, B. Keimer
Nature Materials published 11 May 2014.

Resonant magnetic exciton mode in the heavy-fermion antiferromagnet CeB6
G. Friemel, Yuan Li, A. V. Dukhnenko, N. Yu. Shitsevalova, N. E. Sluchanko, A. Ivanov, V. B. Filipov, B. Keimer, D. S. Inosov.
Nature Communications 3 (2012), 830; published 15 May 2012.

 
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