Quantum phase transition of the near-Ising antiferromagnet CoCl2 - 2D2O – Niels Bohr Institute - University of Copenhagen

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Quantum phase transition of the near-Ising antiferromagnet CoCl2 - 2D2O

The one dimensional Ising model in a transverse applied magnetic field is widely recognized as the simplest textbook-example of a quantum phase transition. The mechanism of a quantum phase transition is not thermal fluctuations, but rather fluctuations which are quantum mechanical of nature and driven only by the Heisenberg uncertainty relation.

The monoclinic salt CoCl2.2D2O is considered by many experimentalists to be a close realization of the one dimensional Ising ferromagnet and has an energy scale well suited for detailed neutron scattering investigations. The ordered phase consists of ferromagnetic ordered chains with antiferromagnetic interchain coupling.

Cooling the crystal to 1.5 K and applying a magnetic field in a transverse direction with respect to the spin orientation, a new ground state is obtained, with Bc = 15.6T, where the spins are parallel with the field. Using neutron scattering I have measured the antiferromagnetic order parameter and the spin wave energy gap below Bc, and I also present a numerical model which can give insight into the nature of this quantum phase transition.

Master thesis by Jacob Larsen