Finite size Spin Wave theory of the triangular Heisenberg model

We present a finite size spin wave calculation on the Heisenberg antiferromagnet on the triangular lattice focusing in particular on the low-energy part of the excitation spectrum. For s=1/2 the good agreement with the exact diagonalization and quantum Monte Carlo results supports the reliability of the spin wave expansion to describe the low-energy spin excitations of the Heisenberg model even in presence of frustration. This indicates that the spin susceptibility of the triangular antiferromagnet is very close to the linear spin wave result.Comment: 6 pages (LateX), 2 ps-figure

Quasiparticle excitations in frustrated antiferromagnets

We have computed the quasiparticle wave function corresponding to a hole injected in a triangular antiferromagnet. We have taken into account multi-magnon contributions within the self consistent Born approximation. We have found qualitative differences, under sign reversal of the integral transfer t, regarding the multi-magnon components and the own existence of the quasiparticle excitations. Such differences are due to the subtle interplay between magnon-assisted and free hopping mechanisms. We conclude that the conventional quasiparticle picture can be broken by geometrical frustration without invoking spin liquid phases.Comment: 5 pages, 4 figures, presented at " At the Frontiers of the condensed Matter II, Buenos Aires. June, 2004 ". To be published in Physica

Spin wave analysis to the spatially-anisotropic Heisenberg antiferromagnet on triangular lattice

We study the phase diagram at T=0 of the antiferromagnetic Heisenberg model on the triangular lattice with spatially-anisotropic interactions. For values of the anisotropy very close to J_alpha/J_beta=0.50, conventional spin wave theory predicts that quantum fluctuations melt the classical structures, for S=1/2. For the regime J_beta<J_alpha, it is shown that the incommensurate spiral phases survive until J_beta/J_alpha=0.27, leaving a wide region where the ground state is disordered. The existence of such nonmagnetic states suggests the possibility of spin liquid behavior for intermediate values of the anisotropy.Comment: Revised version, 4 pages, Latex (twocolumn), 4 figures as eps files. To appear in PR

Antiferromagnetically coupled alternating spin chains

The effect of antiferromagnetic interchain coupling in alternating spin (1,1/2) chains is studied by mean of a spin wave theory and density matrix renormalization group (DMRG). In particular, two limiting cases are investigated, the two-leg ladder and its two dimensional (2D) generalization. Results of the ground state properties like energy, spin gap, magnetizations, and correlation functions are reported for the whole range of the interchain coupling $J_{\perp}$. For the 2D case the spin wave results predict a smooth dimensional crossover from 1D to 2D keeping the ground state always ordered. For the ladder system, the DMRG results show that any $J_{\perp}>0$ drives the system to a gapped ground state. Furthermore the behaviour of the correlation functions closely resemble the uniform spin-1/2 ladder. For $J_{\perp}$ lower than 0.3, however, the gap behaves quadratically as $\Delta\sim0.6 J^2_{\perp}$. Finally, it is argued that the behaviour of the spin gap for an arbitrary number of mixed coupled spin chains is analogous to that of the uniform spin-1/2 chains.Comment: 5 pages, 7 ps-figure

Long range Neel order in the triangular Heisenberg model

We have studied the Heisenberg model on the triangular lattice using several Quantum Monte Carlo (QMC) techniques (up to 144 sites), and exact diagonalization (ED) (up to 36 sites). By studying the spin gap as a function of the system size we have obtained a robust evidence for a gapless spectrum, confirming the existence of long range Neel order. Our best estimate is that in the thermodynamic limit the order parameter m= 0.41 +/- 0.02 is reduced by about 59% from its classical value and the ground state energy per site is e0=-0.5458 +/- 0.0001 in unit of the exchange coupling. We have identified the important ground state correlations at short distance.Comment: 4 pages, RevTeX + 4 encapsulated postscript figure

Hole dynamics in canted antiferromagnets : coexistence of many-body and free-like excitations

We have analyzed the dynamics of a single hole doped in a canted antiferromagnet using the t-J model. Within the self-consistent Born approximation we have found that the hole propagates at two different energy scales along the antiferromagnetic and the ferromagnetic components of the canted order, respectively. While the many body quasiparticle excitation has its origin in the coherent coupling of the hole with the magnon excitations of the antiferromagnetic component, the ferromagnetic component gives rise to a free-like hole motion at higher energies. We have found a nontrivial behavior of the hole spectral function with the canting angle θ. In particular, in the strong coupling regime, the quasiparticle weight strongly depends on the momenta, vanishing inside the magnetic Brillouin zone for θ >~60°

Hole dynamics in canted antiferromagnets : coexistence of many-body and free-like excitations

We have analyzed the dynamics of a single hole doped in a canted antiferromagnet using the t-J model. Within the self-consistent Born approximation we have found that the hole propagates at two different energy scales along the antiferromagnetic and the ferromagnetic components of the canted order, respectively. While the many body quasiparticle excitation has its origin in the coherent coupling of the hole with the magnon excitations of the antiferromagnetic component, the ferromagnetic component gives rise to a free-like hole motion at higher energies. We have found a nontrivial behavior of the hole spectral function with the canting angle θ. In particular, in the strong coupling regime, the quasiparticle weight strongly depends on the momenta, vanishing inside the magnetic Brillouin zone for θ >~60°