Seminar: “Spin nematics in an S=1/2 frustrated ferromagnet: a mean-field theory and beyond" Tsutomu Momoi, RIKEN

Seminar Information:

• Date: Tue, November 27th, 2012
• Time: 14:00 -15:00
• Venue : Meeting Room D014, Lab 1

“Spin nematics in an S=1/2 frustrated ferromagnet: a mean-field theory and beyond”

Tsutomu Momoi, Senior Research Scientist in Condensed Matter Theory Lab, RIKEN

Abstract:
Appearance of spin nematic phase has been discovered in the S=1/2 square lattice J1-J2 model with ferromagnetic J1 and competing antiferromagnetic J2 interactions. Since this spin nematic state has a quadrupolar order on bond degrees of freedom, the ground state wave-function cannot be described with any site-product state and theoretical method for studying this state is very limited.

To study bond spin nematics in S=1/2 spin systems, we developed an SU(2) slave-fermion formulation of spin nematic orders in frustrated ferromagnets. Using this fermion representation, we constructed a mean-field theory for bond spin nematic states. (Here, the d vectors of spin-triplet pairing ansatz play the role of the directors of the quadrupole moments.) This formulation demonstrates that the spin nematic state can be characterized as a spin-triplet resonating-valencebond (RVB) state. Starting with this mean-field theory, we further treated fluctuations through 1/N
expansion in a generalized N-flavor spin model, and thereby we obtained dynamical spin structure factors in the spin nematic state.