Condensed Matter Seminars

Strongly correlated systems where the degrees of freedom cannot order despite their strong interactions have constituted an important issue in modern condensed matter physics. Such systems usually have many competing states as ground states that can lead to qualitatively new states of matter. An example is geometric frustration, a magnetic phenomenon in which the topology of the lattice induces a macroscopic ground state degeneracy and prohibits the spin system from ordering. The important issues in this field are: (1) what the nature of the spin liquid phase is and (2) how the system responds to the ground state degeneracy. In this talk, I will address these issues by discussing a spinel antiferromagnet ZnCr2O4. In ZnCr2O4, the magnetic Cr3+ ions form a lattice of corner-sharing tetrahedra with uniform nearest neighbor antiferromagnetic couplings and makes the system as the most frustrated magnet so far. Recently we found by inelastic neutron scattering that a composite spin degree of freedom emerges in the cubic spinel. In the gapless spin liquid phase, spins self-organize into weakly interacting antiferromagnetic hexagonal loops rather than fluctuating individually. The emergence of the composite spin degree of freedom suggests an organizing principle for frustrated systems such that if macroscopic condensation is not possible, interacting degrees of freedom combine to form rigid clusters. We have also shown that the system can lift the degeneracy via a spin-Peierls-like phase transition from the cubic spin liquid to a tetragonal Neel state. Finally, if time allows, I will also discuss how the spin liquid state and the phase transition change in the presence of bond/site disorders or further neighbor interactions.