L. O. Manuel

SCHWINGER-BOSON APPROACH TO QUANTUM SPIN SYSTEMS : GAUSSIAN FLUCTUATIONS IN THE NATURAL GAUGE

We compute the Gaussian-fluctuation corrections to the saddle-point Schwinger-boson results using collective coordinate methods. Concrete application to investigate the frustrated J1-J2 antiferromagnet on the square lattice shows that, unlike the saddle-point predictions, there is a quantum nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by considering the corrections to the spin stiffness on large lattices and extrapolating to the thermodynamic limit, which avoids the infinite-lattice infrared divergencies associated to Bose condensation. The very good agreement of our results with exact numerical values on finite clusters lends support to the calculational scheme employed.

ROTATIONAL INVARIANCE AND ORDER-PARAMETER STIFFNESS IN FRUSTRATED QUANTUM SPIN SYSTEMS

We compute, within the Schwinger-boson scheme, the Gaussian-fluctuation corrections to the order-parameter stiffness of two frustrated quantum spin systems: the triangular-lattice Heisenberg antiferromagnet and the J1-J2 model on the square lattice. For the triangular-lattice Heisenberg antiferromagnet we found that the corrections weaken the stiffness, but the ground state of the system remains ordered in the classical 120 spiral pattern. In the case of the J1-J2 model, with increasing frustration the stiffness is reduced until it vanishes, leaving a small window 0.53 < J2/J1 < 0.64 where the system has no long-range magnetic order. In addition, we discuss several methodological questions related to the Schwinger-boson approach. In particular, we show that the consideration of finite clusters which require twisted boundary conditions to fit the infinite-lattice magnetic order avoids the use of ad hoc factors to correct the Schwinger-boson predictions.

Classical antiferromagnetism in kinetically frustrated electronic models.

We study, by means of the density matrix renormalization group, the infinite U Hubbard model--with one hole doped away from half filling--in triangular and square lattices with frustrated hoppings, which invalidate Nagaokas theorem. We find that these kinetically frustrated models have antiferromagnetic ground states with classical local magnetization in the thermodynamic limit. We identify the mechanism of this kinetic antiferromagnetism with the release of the kinetic energy frustration, as the hole moves in the established antiferromagnetic background. This release can occur in two different ways: by a nontrivial spin Berry phase acquired by the hole, or by the effective vanishing of the hopping amplitude along the frustrating loops.

Evolution of Nagaoka phase with kinetic energy frustrating hopping

Fil: Lisandrini, Franco Thomas. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Rosario. Instituto de Fisica de Rosario. Universidad Nacional de Rosario. Instituto de Fisica de Rosario; Argentina

Magnons and Excitation Continuum in XXZ triangular antiferromagnetic model: Application to Ba3CoSb2O9

We investigate the excitation spectrum of the triangular-lattice antiferromagnetic XXZ model using series expansion and mean field Schwinger boson approaches. The single-magnon spectrum computed with series expansion exhibits rotonic minima at the middle points of the edges of the Brillouin zone, for all values of the anisotropy parameter in the range 0≤Jz/J≤1. Based on the good agreement with series expansion for the single-magnon spectrum, we compute the full dynamical magnetic structure factor within the mean field Schwinger boson approach to investigate the relevance of the XXZ model for the description of the unusual spectrum found recently in Ba3CoSb2O9. In particular, we obtain an extended continuum above the spin wave excitations, which is further enhanced and brought closer to those observed in Ba3CoSb2O9 with the addition of a second neighbor exchange interaction approximately 15% of the nearest-neighbor value. Our results support the idea that excitation continuum with substantial spectral-weight are generically present in two-dimensional frustrated spin systems and fractionalization in terms of bosonic spinons presents an efficient way to describe them.

Quantum Magnons of the Intermediate Phase of Half-Doped Manganite Oxides

At half doping, the ground state of three-dimensional manganite perovskite oxides like R1-xCaxMnO3, where R is a trivalent ion such as La, Pr, etc., is still unclear. Many experimental findings agree better with the combined magnetic, charge, and orbital order characteristic of the “intermediate phase”, introduced by Efremov in 2004 [Nat. Mater., 3, 853]. This phase consists of spin dimers (thus incorporating aspects of the Zener polaron phase (ZP) proposed in 2002 by Daoud-Aladine [Phys. Rev. Lett., 89, 097205]), though formed by a pair of parallel Mn spins of different magnitude, in principle (thereby allowing for a degree of Mn charge disproportionation: not necessarily as large as that of Mn3+-Mn4+ in Goodenoughs original CE phase [Phys. Rev. 100, 564 (1955)]). In the intermediate phase, consecutive spin dimers localed along the planar zig-zag chains are oriented at a constant relative angle φ between them. Varying Mn-charge disproportionation and φ, the intermediate phase should allow to continuously interpolate between the two limiting cases of the CE phase and the dimer phase denoted as “orthogonal intermediate π/2-phase”. It is not easy to find a microscopic model able to describe the phenomenological intermediate phase adequately for the spin, charge, and orbital degrees of freedom simultaneously. Here, we study the quantum spin excitations of a planar model of interacting localized spins, which we found can stabilize the intermediate phase classically. We compare the quantum magnons of the intermediate phase with those of the CE and orthogonal π/2 phases, in the context of recent experimental results.

1/T1 nuclear relaxation time of κ-(BEDT–TTF)2Cu[N(CN)2]Cl : effects of magnetic frustration

We study the role played by the magnetic frustration in the antiferromagnetic phase of the organic salt κ-(BEDT–TTF)2Cu[N(CN)2]Cl. Using the spatially anisotropic triangular Heisenberg model we analyse previous and newly performed NMR experiments. We compute the 1/T1 relaxation time by means of the modified spin wave theory. The strong suppression of the nuclear relaxation time observed experimentally under varying pressure and magnetic field is qualitatively well reproduced by the model. Our results suggest the existence of a close relation between the effects of pressure and magnetic frustration.

The spectral function for Mott insulating surfaces

We show theoretically the fingerprints of short-range spiral magnetic correlations in the photoemission spectra of the Mott insulating ground states realized in the ?3 ?? 3 triangular silicon surfaces K/Si(111)?B and SiC(0001). The calculated spectra present low-energy features of magnetic origin with a reduced dispersion ~10?40?meV compared with the centre-of-mass spectra bandwidth ~0.2?0.3?eV. Remarkably, we find that the quasiparticle (QP) signal survives only around the magnetic Goldstone modes. Our findings position these silicon surfaces as new candidates for investigation in the search for non-conventional QP excitations.

HEISENBERG MODEL ON THE 1/5-DEPLETED SQUARE LATTICE AND THE CAV4O9 COMPOUND

We investigate the ground state structure of the Heisenberg model on the 1/5-depleted square lattice for arbitrary values of the first- and second-neighbor exchange couplings. By using a mean-field Schwinger-boson approach we present a unified description of the rich ground-state diagram, which include the plaquette and dimer resonant-valence-bond phases, an incommensurate phase and other magnetic orders with complex magnetic unit cells. We also discuss some implications of ours results for the experimental realization of this model in the CaV4O9 compound.

One dimensionalization in the spin-1 Heisenberg model on the anisotropic triangular lattice

We investigate the effect of dimensional crossover in the ground state of the antiferromagnetic spin-