Two-dimensional Quantum Magnets
With the discovery of high-temperature superconductivity in doped layered Mott-insulating cupric oxides, spin excitations in two-dimensional quantum antiferromagnets became one of the main foci of condensed matter research. Magnons are the most important low-energy modes in cuprates and therefore have naturally been considered as a possible "glue" for Cooper pairs. Another interesting proposal is that superconductivity emerges from a resonating-valence-bond (RVB) type spin-liquid state, which is not realized in the un-doped parent materials possessing an ordinary Neel antiferromagnetic order, but which is nearby in energy. Hence, studying behavior of spin excitations and emerging novel ground states in two-dimensional quantum magnets is an important focus of our research.
One of our interesting recent findings is an instability of magnon quasiparticles in a quantum spin-liquid (QSL) state of a 2D S=1/2 antiferromagnet towards their pair decays when the latter become cinematically allowed. In fact, such behavior is a generic feature of quantum Bose liquids, first predicted by Pitaevskii for liquid helium-4. Pitaevskii's prediction is also applicable to quantum magnets with a gap in the spin excitation spectrum - QSL. An example of one-dimensional quantum spin liquid is a Haldane-gap state found in S=1 spin chains (one of the best known model compounds is CsNiCl3) and of two-dimensional QSL - system of strongly interacting S=1/2 spin dimers on a 2D lattice, such as found in the Cu-organic material PHCC. In our neutron scattering measurements we have found strong evidence that a spectrum termination, similar to that in 4He, also exists in these QSL's. Important difference between helium and quantum spin liquids is that in addition to energy and momentum conservation, decays in spin system also require conservation of spin angular momentum. This conservation comes into play in magnetic field, and can be probed by studying spin excitations in magnetic field.
Last Modified: Friday, 21-Mar-2008 18:44:35 EDT