Spiral magnetism and collective excitations in doped Hubbard models

Avinash Singh, Zlatko Tešanović, Ju H Kim

Michael Dzierzawa

Andrey V. Chubukov, David M. Frenkel

We consider magnetic instabilities in the two-dimensional Hubbard model at small doping. We find that the renormalization of the effective interaction prevents an immediate instability of a commensurate antiferromagnetic state upon doping. At increased doping levels, our calculations indicate the occurrence of an instability in the channel of transverse spin fluctuations. This instability is known to lead to a spiral magnetic phase. We also consider a dynamical spin susceptibility near the instability and find that conventional spin waves play no role in the transition. Instead, the incommensurate instability is governed by collective fermionic excitations coupled to the spin background.

Alexander A. Ovchinnikov, M. Ya. Ovchinnikova

The properties of spin excitations superposed on a uniform ground state with antiferromagnetic (or spiral) spin structure are studied in a 2 D Hubbard model. Expressions are derived for the spin susceptibility in the random phase approximation (RPA) using split Hubbard bands as a zeroth approximation. The calculated collective modes with dispersion ω ( Q )= c | Q −(π, π)| near Q ∼(π, π) reproduce well the characteristics of the spin excitations observed in undoped cuprates. For doped systems with an antiferromagnetic structure of the ground state, calculating X ″( Q,ω →0) gives the same mode with a peak at Q ∼(π, π), regardless of the type of Fermi surface. It is shown that in doped systems with a spiral ground state spin structure, X ″( Q,ω →0) peaks occur with incommensurate quasimomenta Q that are coupled to the spirality vector.

M. Yu. Nikolaev, Natalia Ryzhanova, A. Vedyayev +1 more

Zheng-Yu Weng, C. S. Ting