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When many quantum particles act in unison novel states of matter can occur and we aim to experimentally explore such macroscopic quantum behaviour in frustrated quantum magnets and superconductors. Recent work includes: (1) discovery of a new type of quantum spin liquid state in two dimensions where quasiparticles with fractional spin quantum number (S=1/2 spinons) emerge due to strong correlations and fluctuations, and (2) observation of Bose-Einstein condensation of magnons via applied magnetic field.

Much progress in understanding can be made through neutron scattering experiments, which probe directly the spin fluctuations and thus give microscopic information about order and fluctuations. Applied magnetic fields can be used to externally control the quantum fluctuations and drive transitions between distinct quantum phases and one of the aims of the experiments is to investigate the critical fluctuations near such zero-temperature quantum phase transitions. Experiments are performed at the ISIS pulsed neutron source near Oxford and at neutron sources in Grenoble (ILL) and Berlin (HMI).

**Experimental Realization of a 2D Fractional Quantum Spin Liquid**

*R. Coldea, D.A. Tennant, A.M. Tsvelik and Z. Tylczynsli*

Phys. Rev. Lett. **86**,
1335 (2001).

We report observation of a new type of quantum spin liquid phase in a
two-dimensional frustrated magnet where quasi-particles are spin-1/2 spinons.

**Direct Measurement of the Spin Hamiltonian and Observation of Condensation
of Magnons in the 2D Frustrated Quantum Magnet Cs _{2}CuCl_{4}**

Phys. Rev. Lett.

We propose an experimental method to measure spin Hamiltonians combining neutron scattering and high magnetic fields. Using the field to control the excitations we observe Bose-Einstein condensation of magnons into an ordered ground state.

**Spin waves and electronic interactions in La _{2}CuO_{4}**

Phys. Rev. Lett.

We observe evidence for a 4-spin ring exchange in the parent compound of the high-temperature superconductors by mapping the dispersion of the magnetic excitations throughout the 2D Brillouin zone.

**Order to disorder transition in the XY-like quantum magnet Cs _{2}CoCl_{4}
induced by noncommuting applied fields**

Phys. Rev. B.

Applied magnetic fields that do not commute with the exchange Hamiltonian introduce quantum fluctuations in the ground state and here we observe how such fluctuations introduced via transverse field melt the antiferromagnetic order in an XY magnet and stabilize instead a quantum paramagnetic state.