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Prof. Albert Furrer, Dr. Joel Mesot
Prof. Albert Furrer
Dr. Joel Mesot
Groupe Furrer
Team spring 2003
SANS diffraction patterns of the vortex lattice in La1.83Sr0.17CuO4. As the applied field is increased, the vortex lattice changes from hexagonal (left) to square (right) coordination
Neutron Scattering in Cuprates and Quantum Spin Systems

The experimental activities are primarily based on the spallation neutron source SINQ at PSI Villigen. The diverse instrumentation for neutron scattering experiments is available to the partners of MaNEP on a collaborative basis. Our research within MaNEP concentrates on investigations of the statics and dynamics of cuprates and quantum spin systems.
At present our research is focused on studies of
  1. Doping, pressure and isotope effects on the pseudogap in rare-earth based high-temperature superconductors by neutron spectroscopic measurements of the relaxation rates of crystal-field excitations. The observation of large upward shifts of the pseudogap temperature upon copper and oxygen isotope substitution [1] as well as pressure application points to the importance of lattice effects for the understanding of the pseudogap phenomenon.
  2. Vortex structure and spin dynamics in high-temperature superconductors. By means of small-angle neutron scattering (SANS) we have been able to observe vortex lattices in all doping regimes of La2-xSrxCuO4. In the overdoped regime a field-induced transition from hexagonal to square coordination at around 0.4 Tesla was observed [2] which is indicative of in-plane anisotropies such as those provided by a d-wave order parameter or the presence of stripes.
  3. Field-induced quantum phase transitions in the Cu2+ dimer compounds ACuCl3 (A=K, Tl) which exhibit a singlet-triplet energy gap in zero field. At a critical field Hc the lowest components of the Zeeman split triplet (which can be regarded as diluted bosons) intersect the ground-state singlet, thus Bose-Einstein condensation is expected to occur. The magnetic excitation spectrum associated with the condensate has been theoretically predicted to be a gapless Goldstone mode which we verified by inelastic neutron scattering experiments [3].

[1] D. Rubio et al., Phys. Rev. B 66, 184506 (2002).
[2] R. Gilardi et al., Phys. Rev. Lett. 88, 217003 (2002).
[3] Ch. Rüegg et al., Nature 423, 62 (2003).

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