DQMP Seminar – Prof. Markus Braden, Physikalisches Institut, Universität zu Köln, Germany

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Date(s) - 01/03/2016
13 h 00 min - 14 h 00 min

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Mardi 1er mars 2016 à 13.00
Auditoire Stuckelberg
Ecole de physique

 

Anisotropy of magnetic correlations in FeAs based superconductors: an orbital finger print

 

Neutron scattering gives direct insight to the spin-space anisotropy of magnetic correlations driven through spin-orbit coupling. There is clear evidence that magnetic excitations in FeAs-based materials – in antiferromagnetic and in superconducting compounds – exhibit strong anisotropy resulting even in split resonance modes. In the antiferromagnetic BaFe2As2 parent compound [1] it costs more energy to rotate the spins within the FeAs planes than perpendicular to them in contrast to a simple easy-plane model of magnetic anisotropy.  Qualitatively the same anisotropy persists in the superconducting materials [2-4]. All measurements on doped superconducting 122 samples yield significant anisotropies reflecting those in the SDW phase of the parent compound [4]. Spin-orbit coupling remains thus a relevant parameter in the superconducting part of the phase diagrams. For 6% Co doping, there is evidence for a well-defined resonance excitation [2] sitting in energy below the broader isotropic mode. Superconducting materials that are close to the SDW order and near optimum doping, exhibit thus two characteristics reminiscent of the AFM ordering: a finite l-dispersion [3] and strong spin-space anisotropy [2].

In slightly Na-underdoped BaFe2As2 polarized and unpolarized neutron diffraction experiments reveal a spin reorientation from the usual alignment along the in-plane component of the propagation vector towards vertical orientation [5]. This spin reorientation reflects the general spin-space anisotropy of FeAs-based materials which is characterized by an in-plane hard axis. An orbital explanation of these effects will be discussed.

[1] N. Qureshi, P. Steffens, S. Wurmehl, S. Aswartham, B. Büchner, and M. Braden, Phys. Rev. B 86, 060410 (2012).

[2] P. Steffens, C. H. Lee, N. Qureshi, K. Kihou, A. Iyo, H. Eisaki, and M. Braden, Phys. Rev. Lett. 110, 137001 (2013).

[3] C. H. Lee, P. Steffens, N. Qureshi, M. Nakajima, K. Kihou, A. Iyo, H. Eisaki, and M. Braden,  Phys. Rev. Lett. 111, 167002 (2013).

[4] N. Qureshi, C. H. Lee, K. Kihou, K. Schmalzl, P. Steffens, and M. Braden, Phys. Rev. B 90, 100502(R) (2014).

[5] F. Waßer, A. Schneidewind, Y. Sidis, S. Wurmehl, S. Aswartham, B. Büchner, and M. Braden Phys. Rev. B 91, 060505(R) (2015)

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