January 31 , 2007

Where the Spectrum Ends: Spectrum Termination and Reentrance in a Two-Dimensional Organometallic Magnet

 

Matthew B. Stone

Oak Ridge National Laboratory

Although magnetism is fundamentally a quantum mechanical effect, it has clear macroscopic consequences.  For example, the interactions occurring on the length scale of several Angstroms lead to materials with properties that allow one to attach messages to a refrigerator door, or produce 100 GB hard drives.  The technological implications and diverse behavior have made magnetic materials a consistent venue of study for condensed matter physics.  I will be presenting research on an organometallic antiferromagnet, piperazinium hexachlorodicuprate (PHCC).  The frustrated interactions in this material lead to a magnetic ground state which only has very short range correlations such that PHCC is best described as a quantum spin liquid.  The excitations associated with the quantum spin liquid ground state can easily be modified by the application of external magnetic fields.  I will discuss the magnetic field versus temperature phase diagram of this system as determined by pulsed field magnetic susceptibility measurements, specific heat measurements and both elastic and inelastic neutron scattering experiments.  In addition to two magnetic field-driven quantum critical points which can be described as a Bose-Einstein condensation of excitations, there also exists a reentrant phase transition between the disordered ground state and a long range ordered phase.  Along with the ability to dramatically alter the excitations in PHCC using an applied magnetic field, I will also discuss a peculiar point in the zero-field excitation spectrum where higher energy excitations cause the lower energy excitations to abruptly decay.