Hans-Benjamin Braun, Jiri Kulda, Bertrand Roessli, Dirk Visser, Karl W. Krämer, Hans-Ulrich Güdel and Peter Böni
Emergence of soliton chirality in a quantum antiferromagnet
Nature Phys. 1, 159-163 (2005)                       

Abstract: Left- and right-handed chiral matter is present at every scale ranging from seashells to molecules to elementary particles. In magnetism, chirality may be inherited from the asymmetry of the underlying crystal structure, or it may emerge spontaneously. In particular, there has been a long-standing search for chiral spin states that emerge spontaneously with the disappearance of antiferromagnetic long-range order. Here we identify a generic system supporting such a behaviour and report on experimental evidence for chirality associated with the quantum dynamics of solitons in antiferromagnetic spin chains. The soliton chirality observed by polarized neutron scattering is in agreement with theoretical predictions and is a manifestation of a Berry phase. Our observations provide the first example of the emergence of spin currents and hidden chiral order that accompany the disappearance of antiferromagnetic order, a scheme believed to lie at the heart of the enigmatic normal state of cuprate superconductors.  
 

O. Waldmann, C. Dobe, H. Mutka, A. Furrer and H. U. Güdel
Néel-Vector Tunneling in Antiferromagnetic Molecular Clusters
Phys. Rev. Lett. 95, 057202/1-4 (2005)                 

Abstract: From inelastic neutron scattering experiments, we demonstrate quantum tunneling of the Néel vector in the antiferromagnetic molecular ferric wheel CsFe₈. Analysis of the linewidth of the tunneling transition evidences coherent tunneling.  
 

A. Shalav, B. S. Richards, T. Trupke, K. W. Krämer and H. U. Güdel
Application of NaYF₄:Er³⁺ up-converting phosphors for enhanced near-infrared silicon solar cell response
Appl. Phys. Lett. 86, 013505/1-3 (2005)                 

Abstract: Erbium-doped sodium yttrium fluoride (NaYF₄:Er³⁺) up-conversion phosphors were attached to the rear of a bifacial silicon solar cell to enhance its reponsivity in the near-infrared. The incident wavelength and light intensity were varied and the resulting short circuit current of the solar cell was measured. A close match between the spectral features of the external quantum efficiency and the phosphor absorption is consistent with the energy transfer up-conversion process. The peak external quantum efficiency of the silicon solar cell was measured to be (2.5±0.2)% under 5.1 mW laser excitation at 1523 nm, corresponding to an internal quantum efficiency of 3.8%.

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