17. October 2017

Coulomb spin liquid in anion-disordered pyrochlore Tb₂Hf₂O₇

R. Sibille et al., Nature Communications 8, 892 (2017) (full article). The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. Here we demonstrate in the pyrochlore Tb₂Hf₂O₇, that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.

28. July 2017

4-spin plaquette singlet state in the Shastry–Sutherland compound SrCu₂(BO₃)₂

M.E. Zayed et al., Nature Physics, adv. online publication (July 2017). The study of interacting spin systems is of fundamental importance for modern condensed-matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states. The frustrated two-dimensional Shastry–Sutherland lattice realized by SrCu₂(BO₃)₂ is an important test to our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated. We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu₂(BO₃)₂. It is observed by inelastic neutron scattering after pressure tuning at 21.5kbar. This gapped singlet state leads to a transition to an ordered Neel state above 40 kbar, which can realize a deconfined quantum critical point.

1. July 2017

Michel Kenzelmann new head of the LNS

Prof. Dr. Michel Kenzelmann has been appointed as new head of the Laboratory for Neutron Scattering and Imaging LNS starting on July 1, 2017. Michel received his Ph.D. from the University of Oxford before he moved to NIST and John Hopkins University as postdoctoral fellow. In 2004 he joined PSI as an SNF Professor and became head of the NUM laboratory LDM in 2008. Since 2014 Michel holds a titular professorship at the University of Basel. The NUM department also thanks Christof Niedermayer cordially for leading the laboratory ad interim for the last six months.

6. June 2017

Field-induced magnetic instability within a superconducting condensate

D. Mazzone et al., Science Advances 3, e1602055 (2017). The application of magnetic fields, chemical substitution, or hydrostatic pressure to strongly correlated electron materials can stabilize electronic phases with different organizational principles. We present evidence for a field-induced quantum phase transition, in superconducting Nd_0.05Ce_0.95CoIn₅, that separates two antiferromagnetic phases with identical magnetic symmetry. At zero field, we find a spin-density wave that is suppressed at the critical field μ₀H* = 8 T. For H > H*, a spin-density phase emerges and shares many properties with the Q phase in CeCoIn₅. These results suggest that the magnetic instability is not magnetically driven, and we propose that it is driven by a modification of superconducting condensate at H*.

18. April 2017

20 years of SINQ

In 1997 the Swiss spallation neutron source SINQ started its user operation. PSI has celebrated the 20th anniversary of SINQ with a scientific symposium on April 18, 2017 together with many colleagues from Switzerland and abroad. At the symposium it was not only looked back at past achievements, also recent scientific highlights were presented as well as the SINQ neutron guide and instrument upgrade program that will make SINQ fit for the next 20 years. Finally, the symposium also marked the change of the NUM Division Head from Kurt N. Clausen to Christian Rüegg.