Physics (Internal)

Field induced phases in quantum magnets - theory and experiments

This event is part of the Biophysics/Condensed Matter Seminar Series.

 Abstract: Quantum magnets that remain invariant under continuous rotations (along at least one axis) can be described as gases of interacting bosons whose number is conserved. An external field that acts like a chemical potential controls the particle concentration, while the single-ion anisotropy and exchange terms determine the kinetic energy and boson-boson interactions. We will see how these simple ingredients lead to unusual phases and magnetic field induced quantum phase transitions. The region near the field induced transition between a quantum paramagnet and a magnetically ordered state can be universally described as a dilute gas of bosons. Depending on the sign of the boson-boson interactions and the geometric frustration of the underlying lattice, the applied field can induce simple Bose-Einstein condensates (BECs), BECs of pairs of bosons that are similar to the condensation of diatomic molecules, complex crystals or even the coexistence of BEC and crystalline states that is also known as supersolid. We will see what is the magnetic interpretation of this spectrum of phases and what are the basic principles for finding them in simple materials. In particular, I will present a few examples of compounds that exhibit some of these states, together with comparisons between experiments and theory. Finally, I will propose a few ideas for finding experimental realizations of the exotic magnetic phases that remain to be discovered.