Schwinger-boson Approach Research Articles

On Slave-Fermion Mean Field Theory of Spiral Magnetic State in t-J Model

The analysis of the present paper shows that the current mean field theory of the spiral magnetic state in the Schwinger-boson slave-fermion approach is incorrect.

Nonclassical disordered phase in the strong quantum limit of frustrated antiferromagnets.

The rotationally invariant Schwinger-boson approach to quantum helimagnets is discussed. It is shown that in order to get quantitative agreement with exact results on finite lattices, parity-breaking pairing of bosons must be allowed. For the ${\mathit{J}}_{1}$-${\mathit{J}}_{2}$-${\mathit{J}}_{3}$ model on the special line ${\mathit{J}}_{2}$=2${\mathit{J}}_{3}$, a quantum disordered phase is found, though notably only in the strong quantum limit S=1/2. The theory predicts the direct melting of biaxial helimagnetic order into an isotropic spin fluid, without the formation of an intermediate spin-nematic phase as recently suggested.

Magnetization and spin-wave velocities in La2CuO4.

By means of a Schwinger-boson approach we show that the staggered magnetization M in the quasi-two-dimensional quantum Heisenberg antiferromagnet is for temperatures T close to the N\'eel temperature ${\mathit{T}}_{\mathit{N}}$ proportional to (1-T/${\mathit{T}}_{\mathit{N}}$${)}^{1/2}$, in agreement with experiments on ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. We calculate M for all T and show that the recently proposed lnT law breaks down as soon as M becomes small. We also show that the interplane spin-wave velocity vanishes slightly above ${\mathit{T}}_{\mathit{N}}$, while spin waves propagating within the planes extrapolate smoothly across ${\mathit{T}}_{\mathit{N}}$.

Schwinger-boson studies of the t-J model: A self-consistent systematic expansion.

A self-consistent systematic expansion is developed for the t-J model in the Schwinger-boson approach. We treat the fermion part with use of a systematic-expansion technique and study the effects of magnetic fluctuations on the normal-state properties. It is found that the hole motion is strongly renormalized and incoherent bands are produced by magnetic fluctuations. The conductivity and the electron density of states are calculated and shown to be anomalous. The antiferromagnetic nature of fluctuations is found to be essential for these unusual properties. The relevance of the obtained results to oxide superconductors is also discussed.

Schwinger bosons and hydrodynamics of two-dimensional magnets.

Motivated by the strong experimental evidence in favor of the crucial role of antiferromagnetic fluctuations in copper oxide superconductors, I considered low-temperature properties of the disordered two-dimensional ferromagnets and antiferromagnets by means of the now-familiar Schwinger boson approach. The main aim of the paper was to go beyond the mean-field approximation and to derive an expression for the dynamical susceptibility \ensuremath{\chi}(q,\ensuremath{\Omega}) in the hydrodynamic region where the energies of physical excitations, which are collective modes in Schwinger boson approach, are small in comparison with the characteristic damping rate of single-particle excitations. In ferromagnets, the order parameter (magnetization) is a conserved quantity and the pole of \ensuremath{\chi}(q,\ensuremath{\Omega}) near q=0 describes a diffusion mode with \ensuremath{\Omega}=-${\mathit{iDq}}^{2}$. In antiferromagnets, the low-energy part of the spectrum involves the fluctuations of the (conserved) uniform magnetization and (nonconserved) antiferromagnetic order parameter. Correspondingly, a dynamical susceptibility near q=0 also has a pole for a diffusion mode, \ensuremath{\Omega}=-${\mathit{iDq}}^{2}$, while that near q=(\ensuremath{\pi},\ensuremath{\pi}) descibes a relaxation mode with a finite gap ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{\ensuremath{\pi}}}$. The temperature variations of D and ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{\ensuremath{\pi}}}$ as well as of the antiferromagnetic part of spin-lattice relaxation rate are calculated and a comparison is made with the experimental data and with the works of other authors.

MAGNETIC FLUCTUATIONS AND NORMAL STATE PROPERTIES OF THE t − J MODEL AT FINITE DOPINGS

A loop expansion beyond the mean field approximation is developed to study the effects of magnetic fluctuations on normal state properties of the t – J model at finite dopings, using the Schwinger boson approach. The calculated conductivity consists of a Drude peak and a broad band, while the electron density of states is linear in |ω| at the scale of J, in consistency with experiments on oxide superconductors. The antiferromagnetic nature of fluctuations is found to be essential for these properties.

Short-range order above TN in quasi-two-dimensional Heisenberg antiferromagnets

Using the Schwinger-boson approach the temperature dependences of long-range and short-range order parameters in 3D and quasi-2D Heisenberg antiferromagnets are calculated. The existence of a temperature region above T N where a well-pronounced short-range order persists is demonstrated. The width of this region increases with decreasing exchange interaction among the layers.

A quantum fluids approach to frustrated Heisenberg models

Emphasising the close analogies between antiferromagnetism and neutral superfluidity, the authors develop a gauge-invariant quantum fluids description of non-bipartite Heisenberg systems. The antiferromagnet is treated as a spin superfluid with a rotational gauge invariance associated with the continuity of spin flow. They show how an extended Schwinger boson approach naturally incorporates the Onsager reaction fields generated by spin fluctuations, and correctly reproduces the semiclassical behaviour of spin wave theory in the large-S limit. The important short-wavelength physics of fluctuation-stabilised order is also captured by this description. For two-dimensional helimagnets at small S, the method predicts that the twist will survive the loss of sublattice magnetisation, closely analogous to the biaxial-uniaxial transition of nematic liquid crystals.