arXiv subjects

L. O. Manuel

Publications and source records attributed to L. O. Manuel.

46 records · Page 3Linked to original sources

arXiv2004

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.

cond-mat.str-el↗

arXiv2004

Quasiparticle vanishing driven by geometrical frustration

We investigate the single hole dynamics in the triangular t-J model. We study the structure of the hole spectral function, assuming the existence of a 120 magnetic Neel order. Within the self-consistent Born approximation (SCBA) there is a strong momentum and t sign dependence of the spectra, related to the underlying magnetic structure and the particle-hole asymmetry of the model. For positive t, and in the strong coupling regime, we find that the low energy quasiparticle excitations vanish outside the neighbourhood of the magnetic Goldstone modes; while for negative t the quasiparticle excitations are always well defined. In the latter, we also find resonances of magnetic origin whose energies scale as (J/t)^2/3 and can be identified with string excitations. We argue that this complex structure of the spectra is due to the subtle interplay between magnon-assisted and free hopping mechanisms. Our predictions are supported by an excellent agreement between the SCBA and the exact results on finite size clusters. We conclude that the conventional quasiparticle picture can be broken by the effect of geometrical magnetic frustration.

cond-mat.str-el↗

arXiv2003

Spectral Function in 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 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 center-of-mass spectra bandwidth ~0.2-0:3 eV. Remarkably, we find that the quasiparticle signal survives only around the magnetic Goldstone modes. Our findings would position these silicon surfaces as new candidates to investigate non-conventional quasiparticle excitations.

cond-mat.str-el↗

arXiv1999

Stability of homogeneous magnetic phases in a generalized t-J model

We study the stability of homogeneous magnetic phases in a generalized t-J model including a same-sublattice hopping t' and nearest-neighbor repulsion V by means of the slave fermion-Schwinger boson representation of spin operators. At mean-field order we find, in agreement with other authors, that the inclusion of further-neighbor hopping and Coulomb repulsion makes the compressibility positive, thereby stabilizing at this level the spiral and Neel orders against phase separation. However, the consideration of Gaussian fluctuation of order parameters around these mean-field solutions produces unstable modes in the dynamical matrix for all relevant parameter values, leaving only reduced stability regions for the Neel phase. We have computed the one-loop corrections to the energy in these regions, and have also briefly considered the effects of the correlated hopping term that is obtained in the reduction from the Hubbard to the t-J model.

cond-mat.str-el↗

arXiv1999

Magnetic and quantum disordered phases in triangular-lattice Heisenberg antiferromagnets

We study, within the Schwinger-boson approach, the ground-state structure of two Heisenberg antiferromagnets on the triangular lattice: the J1-J2 model, which includes a next-nearest-neighbor coupling J2, and the spatially-anisotropic J1-J'1 model, in which the nearest-neighbor coupling takes a different value J'1 along one of the bond directions. The motivations for the study of these systems range from general theoretical questions concerning frustrated quantum spin models to the concrete description of the insulating phase of some layered molecular crystals. For both models, the inclusion of one-loop corrections to saddle-point results leads to the prediction of nonmagnetic phases for particular values of the parameters J1/J2 and J'1/J1. In the case of the J1-J2 model we shed light on the existence of such disordered quantum state, a question which is controversial in the literature. For the J1-J'1 model our results for the ground-state energy, quantum renormalization of the pitch in the spiral phase, and the location of the nonmagnetic phases, nicely agree with series expansions predictions.

cond-mat.stat-mech↗

arXiv1998

The 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.

cond-mat.stat-mech↗

arXiv1997

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.

cond-mat.stat-mech↗

arXiv1997

Schwinger-boson approach to quantum spin systems: Gaussian fluctuactions 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.

cond-mat.stat-mech↗

arXiv1997

Superconductivity and incommensurate spin fluctuations in a generalized t-J model for the cuprates

We consider the slave-fermion Schwinger-boson decomposition of an effective model obtained through a systematic low-energy reduction of the three-band Hubbard Hamiltonian. The model includes a three-site term t'' similar to that obtained in the large-U limit of the Hubbard model but of opposite sign for realistic or large O-O hopping. For parameters close to the most realistic ones for the cuprates, the mean-field solution exhibits d+s superconductivity (predominantly d_{x^2-y^2}) with a dependence on doping x very similar to the experimentally observed. We also obtained incommensurate peaks at wave vectors near $π(1,1 +(-) 2x)$ in the spin structure factor, which also agree with experiment.

cond-mat.str-el↗

arXiv1996

Heisenberg model with Dzyaloshinskii-Moriya interaction: A Schwinger boson study

We present a Schwinger-boson approach to the Heisenberg model with Dzyaloshinskii-Moriya interaction. We write the anisotropic interactions in terms of Schwinger bosons keeping the correct symmetries present in the spin representation, which allows us to perform a conserving mean-field approximation. Unlike previous studies of this model by linear spin-wave theory, our approach takes into account magnon-magnon interactions and includes the effects of three-boson terms characteristic of noncollinear phases. The results reproduce the linear spin-wave predictions in the semiclassical large-S limit, and show a small renormalization in the strong quantum limit S=1/2. For the sake of definiteness, we specialize the calculations for the pattern of Moriya vectors corresponding to the orthorhombic phase in La_2CuO_4, and give a fairly detailed account of the behavior of ground-state energy, anisotropy gap, and net ferromagnetic moment. In the last part of this work we generalize our approach to describe the geometry of the intermediate phase in La_{2-x}Nd_xCuO_4, and discuss the effects of including nondegenerate 2p_z oxygen orbitals in the calculations.

cond-mat.stat-mech↗