Condensed Matter Seminar

Oleg Tchernyshyov
Johns Hopkins University

Tuesday, February 10, 2009; 4:00 pm in JFB 334

Spin excitations with Fermi statistics in the S=1/2 Heisenberg model on kagome

Heisenberg antiferromagnets typically have ground states with long-range magnetic order, in which spins point in a particular direction, spontaneously breaking the global SU(2) symmetry of spin. Elementary excitations are magnons, which can be viewed as quantized spin waves or as bosonic particles with integer spin S_z=1. Notable exceptions to this rule are antiferromagnets in one spatial dimension, where quantum fluctuations destroy magnetic order and make the ground state rotationally symmetric. In some of these magnets elementary excitations are spinons, domain walls carrying half-integer spin S=1/2; these fractional spin excitations have been observed experimentally in real spin chains.

Can we have a quantum-disordered ground state and fractional spin excitations in higher dimensions (d=2 and 3)? Several candidate magnetic models have been identified, among them the Heisenberg antiferromagnet on kagome, a 2-dimensional lattice of corner-sharing triangles. Our recent work shows that its ground state contains a finite concentration of spinons bound into small bosonic pairs with spin S=0 by exchange-mediated attraction. The spinons are fermions with spin S=1/2 interacting with an emergent U(1) gauge field. Low-energy spin excitations correspond to breaking a pair into two nearly free spinons.