Adolfo Trumper - Academia.edu (original) (raw)
Papers by Adolfo Trumper
Spin wave analysis to the spatially-anisotropic Heisenberg
Physical Review Letters, 1999
Physical Review B
The Schwinger boson theory (SBT) provides a natural path for treating quantum spin systems with l... more The Schwinger boson theory (SBT) provides a natural path for treating quantum spin systems with large quantum fluctuations. In contrast to semi-classical treatments, this theory allows us to describe a continuous transition between magnetically ordered and spin liquid states, as well as the continuous evolution of the corresponding excitation spectrum. The square lattice Heisenberg antiferromagnet is one of the first models that was approached with the Schwinger boson theory. Here we revisit this problem to reveal several subtle points that were omitted in previous treatments and that are crucial to further develop this formalism. These points include the freedom for the choice of the saddle point (Hubbard-Stratonovich decoupling and choice of the condensate) and the 1/N expansion in the presence of a condensate. A key observation is that the spinon condensate leads to Feynman diagrams that include contributions of different order in 1/N, which must be accounted to get a qualitatively correct excitation spectrum. We demonstrate that a proper treatment of these contributions leads to an exact cancellation of the single-spinon poles of the dynamical spin structure factor, as expected for a magnetically ordered state. The only surviving poles are the ones arising from the magnons (two-spinon bound states), which are the true collective modes of an ordered magnet.
The spin-1/2 triangular lattice Heisenberg Hamiltonian is a prototypical model to study the enhan... more The spin-1/2 triangular lattice Heisenberg Hamiltonian is a prototypical model to study the enhancement of quantum fluctuations induced by geometric frustration. Nearly fifty years ago, P. W. Anderson invoked this model to propose the celebrated resonance valence bond ground state that sparked the modern interest in quantum spin liquids. While later studies demonstrated that the ground state exhibits long-range magnetic ordering when the spin exchange interaction is restricted to nearest-neighbors, recent inelastic neutron scattering experiments on the triangular antiferromagnet Ba$_3$CoSb$_2$O$_9$ have revealed strong deviations from the dynamical spin structure factor predicted by semi-classical approaches. This work demonstrates that key qualitative and quantitative features of the data are reproduced by a parton Schwinger-boson theory, suggesting that the observed anomalies originate from the proximity of this material to a critical quantum melting point.
arXiv (Cornell University), Sep 23, 2021
The Heisenberg triangular lattice quantum spin liquid and the phase transitions to nearby magneti... more The Heisenberg triangular lattice quantum spin liquid and the phase transitions to nearby magnetic orders have received much theoretical attention, but clear experimental manifestations of these states are rare. This work investigates a new spin-half Yb 3+ delafossite material, KYbSe 2 , whose inelastic neutron scattering spectra reveal a diffuse continuum with a sharp lower bound. Applying entanglement witnesses to the data reveals significant multipartite entanglement spread between its neighbors, and analysis of its magnetic exchange couplings shows close proximity to the triangular lattice Heisenberg quantum spin liquid. Key features of the data are reproduced by Schwinger-boson theory and tensor network calculations with a significant second-neighbor coupling 2. The strength of the dynamical structure factor at the point shows a scaling collapse in / B down to 0.3 K, indicating a second-order quantum phase transition. Comparing this to previous theoretical work suggests that the proximate phase at larger 2 is a gapped Z 2 spin liquid, resolving a long-debated issue. We thus show that KYbSe 2 is close to a spin liquid phase, which in turn sheds light on the theoretical phase diagram itself.
The effect of antiferromagnetic interchain coupling in alternating spin (1,1/2) chains is studied... more The effect of antiferromagnetic interchain coupling in alternating spin (1,1/2) chains is studied by mean of spin wave theory and density matrix renormalization group(DMRG). Two limiting cases are investigated, the two-leg ladder and its two dimensional (2D) generalization. For the 2D case, spin wave approximation predicts a smooth dimensional crossover keeping the ground state ordered, whereas in the ladder case the DMRG results show a gapped ground state for any J⊥ > 0. Furthermore, the behavior of the correlation functions closely resemble the uniform spin-1/2 ladder. However, for small J⊥, the gap behaves quadratically as ∆ ∼ 0.6J 2 ⊥. Similarly to uniform spin chains, it is conjectured an analogous spin gap behavior for an arbitrary number of mixed spin chains.
Physical Review Letters, 2019
Physical Review B, 2018
We compute the zero temperature dynamical structure factor S(q, ω) of the triangular lattice Heis... more We compute the zero temperature dynamical structure factor S(q, ω) of the triangular lattice Heisenberg model (TLHM) using a Schwinger boson approach that includes the Gaussian fluctuations (1/N corrections) of the saddle point solution. While the ground state of this model exhibits a well-known 120 • magnetic ordering, experimental observations have revealed a strong quantum character of the excitation spectrum. We conjecture that this phenomenon arises from the proximity of the ground state of the TLHM to the quantum melting point separating the magnetically ordered and spin liquid states. Within this scenario, magnons are described as collective modes (two spinonbound states) of a spinon condensate (Higgs phase) that spontaneously breaks the SU(2) symmetry of the TLHM. Crucial to our results is the proper account of this spontaneous symmetry breaking. The main qualitative difference relative to semi-classical treatments (1/S expansion) is the presence of a high-energy spinon continuum extending up to about three times the single-magnon bandwidth. In addition, the magnitude of the ordered moment (m = 0.224) agrees very well with numerical results and the low energy part of the single-magnon dispersion is in very good agreement with series expansions. Our results indicate that the Schwinger boson approach is an adequate starting point for describing the excitation spectrum of some magnetically ordered compounds that are near the quantum melting point separating this Higgs phase from the deconfined spin liquid state.
Physical Review B, 2017
We investigate the effect of dimensional crossover in the ground state of the antiferromagnetic s... more We investigate the effect of dimensional crossover in the ground state of the antiferromagnetic spin-1 Heisenberg model on the anisotropic triangular lattice that interpolates between the regime of weakly coupled Haldane chains (J J) and the isotropic triangular lattice (J = J). We use the density-matrix renormalization group (DMRG) and Schwinger boson theory performed at the Gaussian correction level above the saddle-point solution. Our DMRG results show an abrupt transition between decoupled spin chains and the spirally ordered regime at (J /J)c ∼ 0.42, signaled by the sudden closing of the spin gap. Coming from the magnetically ordered side, the computation of the spin stiffness within Schwinger boson theory predicts the instability of the spiral magnetic order toward a magnetically disordered phase with one-dimensional features at (J /J)c ∼ 0.43. The agreement of these complementary methods, along with the strong difference found between the intra-and the interchain DMRG short spin-spin correlations; for sufficiently large values of the interchain coupling, suggests that the interplay between the quantum fluctuations and the dimensional crossover effects gives rise to the one-dimensionalization phenomenon in this frustrated spin-1 Hamiltonian.
EPL (Europhysics Letters), 2016
We investigate the spin dynamics of the square-lattice spin-1 2 Heisenberg antiferromagnet by mea... more We investigate the spin dynamics of the square-lattice spin-1 2 Heisenberg antiferromagnet by means of an improved mean field Schwinger boson calculation. By identifying both, the long range Néel and the RVB-like components of the ground state, we propose an educated guess for the mean field magnetic excitation consisting on a linear combination of local and bond spin flips to compute the dynamical structure factor. Our main result is that when this magnetic excitation is optimized in such a way that the corresponding sum rule is fulfilled, we recover the low and high energy spectral weight features of the experimental spectrum. In particular, the anomalous spectral weight depletion at (π, 0) found in recent inelastic neutron scattering experiments can be attributed to the interference of the triplet bond excitations of the RVB component of the ground state. We conclude that the Schwinger boson theory seems to be a good candidate to adequately interpret the dynamic properties of the square-lattice Heisenberg antiferromagnet.
Physical Review Letters, 1997
We compute the Gaussian-fluctuation corrections to the saddle-point Schwinger-boson results using... more 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.
Physical Review B, 2004
We investigate the single hole dynamics in the triangular t − J model. We study the structure of ... more 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 Néel 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 magnonassisted 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.
New Journal of Physics, 2012
Physical Review B
Recent inelastic neutron scattering (INS) experiments of the triangular antiferromagnet Ba3CoSb2O... more Recent inelastic neutron scattering (INS) experiments of the triangular antiferromagnet Ba3CoSb2O9 revealed strong deviations from semiclassical theories. We demonstrate that key features of the INS data are well reproduced by a parton Schwinger boson theory beyond the saddlepoint approximation. The measured magnon dispersion is well reproduced by the dispersion of two-spinon bound states (poles of the emergent gauge fields propagator), while the low-energy continuum scattering is reproduced by a quasifree two-spinon continuum, suggesting that a free spinon gas is a good initial framework to study magnetically ordered states near a quantum melting point. II. MATERIAL AND MODEL Ba 3 CoSb 2 O 9 comprises vertically stacked triangular layers of effective spin-1/2 moments arising from the J = 1/2 Kramers doublet of Co 2+ in a trigonally
We have computed the quasiparticle wave function corresponding to a hole injected in a triangular... more 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.
Physical Review B
We study the phase diagram at T = 0 of the antiferromagnetic Heisenberg model on the triangular l... more 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 Jα/J β = 0.5, conventional spin wave theory predicts that quantum fluctuations melt the classical structures, for S = 1/2. For the regime J β < Jα, it is shown that the incommensurate spiral phases survive until J β /Jα = 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.
Physical Review B, 2008
We have studied the validity of the spin polaron picture in the frustrated J1 − J2 Heisenberg mod... more We have studied the validity of the spin polaron picture in the frustrated J1 − J2 Heisenberg model. For this purpose, we have computed the hole spectral functions for the Néel, collinear, and disordered phases of this model, by means of the self-consistent Born approximation and Lanczos exact diagonalization on finite-size clusters. We have found that the spin polaron quasiparticle excitation is always well defined for the magnetically ordered Néel and collinear phases, even in the vicinity of the magnetic quantum critical points, where the local magnetization vanishes. As a general feature, the effect of frustration is to increase the amplitude of the multimagnon states that build up the spin polaron wave function, leading to the reduction of the quasiparticle coherence. Based on Lanczos results, we discuss the validity of the spin polaron picture in the disordered phase.
Physical Review B, 2000
We present a finite size spin wave calculation on the Heisenberg antiferromagnet on the triangula... more 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.
Physical Review B, 2015
We investigate the excitation spectrum of the triangular-lattice antiferromagnetic XXZ model usin... more We investigate the excitation spectrum of the triangular-lattice antiferromagnetic XXZ model using series expansions and mean field Schwinger bosons approaches. The single-magnon spectrum computed with series expansions 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 ≤ J z /J ≤ 1. Based on the good agreement with series expansions 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.
Spin wave analysis to the spatially-anisotropic Heisenberg
Physical Review Letters, 1999
Physical Review B
The Schwinger boson theory (SBT) provides a natural path for treating quantum spin systems with l... more The Schwinger boson theory (SBT) provides a natural path for treating quantum spin systems with large quantum fluctuations. In contrast to semi-classical treatments, this theory allows us to describe a continuous transition between magnetically ordered and spin liquid states, as well as the continuous evolution of the corresponding excitation spectrum. The square lattice Heisenberg antiferromagnet is one of the first models that was approached with the Schwinger boson theory. Here we revisit this problem to reveal several subtle points that were omitted in previous treatments and that are crucial to further develop this formalism. These points include the freedom for the choice of the saddle point (Hubbard-Stratonovich decoupling and choice of the condensate) and the 1/N expansion in the presence of a condensate. A key observation is that the spinon condensate leads to Feynman diagrams that include contributions of different order in 1/N, which must be accounted to get a qualitatively correct excitation spectrum. We demonstrate that a proper treatment of these contributions leads to an exact cancellation of the single-spinon poles of the dynamical spin structure factor, as expected for a magnetically ordered state. The only surviving poles are the ones arising from the magnons (two-spinon bound states), which are the true collective modes of an ordered magnet.
The spin-1/2 triangular lattice Heisenberg Hamiltonian is a prototypical model to study the enhan... more The spin-1/2 triangular lattice Heisenberg Hamiltonian is a prototypical model to study the enhancement of quantum fluctuations induced by geometric frustration. Nearly fifty years ago, P. W. Anderson invoked this model to propose the celebrated resonance valence bond ground state that sparked the modern interest in quantum spin liquids. While later studies demonstrated that the ground state exhibits long-range magnetic ordering when the spin exchange interaction is restricted to nearest-neighbors, recent inelastic neutron scattering experiments on the triangular antiferromagnet Ba$_3$CoSb$_2$O$_9$ have revealed strong deviations from the dynamical spin structure factor predicted by semi-classical approaches. This work demonstrates that key qualitative and quantitative features of the data are reproduced by a parton Schwinger-boson theory, suggesting that the observed anomalies originate from the proximity of this material to a critical quantum melting point.
arXiv (Cornell University), Sep 23, 2021
The Heisenberg triangular lattice quantum spin liquid and the phase transitions to nearby magneti... more The Heisenberg triangular lattice quantum spin liquid and the phase transitions to nearby magnetic orders have received much theoretical attention, but clear experimental manifestations of these states are rare. This work investigates a new spin-half Yb 3+ delafossite material, KYbSe 2 , whose inelastic neutron scattering spectra reveal a diffuse continuum with a sharp lower bound. Applying entanglement witnesses to the data reveals significant multipartite entanglement spread between its neighbors, and analysis of its magnetic exchange couplings shows close proximity to the triangular lattice Heisenberg quantum spin liquid. Key features of the data are reproduced by Schwinger-boson theory and tensor network calculations with a significant second-neighbor coupling 2. The strength of the dynamical structure factor at the point shows a scaling collapse in / B down to 0.3 K, indicating a second-order quantum phase transition. Comparing this to previous theoretical work suggests that the proximate phase at larger 2 is a gapped Z 2 spin liquid, resolving a long-debated issue. We thus show that KYbSe 2 is close to a spin liquid phase, which in turn sheds light on the theoretical phase diagram itself.
The effect of antiferromagnetic interchain coupling in alternating spin (1,1/2) chains is studied... more The effect of antiferromagnetic interchain coupling in alternating spin (1,1/2) chains is studied by mean of spin wave theory and density matrix renormalization group(DMRG). Two limiting cases are investigated, the two-leg ladder and its two dimensional (2D) generalization. For the 2D case, spin wave approximation predicts a smooth dimensional crossover keeping the ground state ordered, whereas in the ladder case the DMRG results show a gapped ground state for any J⊥ > 0. Furthermore, the behavior of the correlation functions closely resemble the uniform spin-1/2 ladder. However, for small J⊥, the gap behaves quadratically as ∆ ∼ 0.6J 2 ⊥. Similarly to uniform spin chains, it is conjectured an analogous spin gap behavior for an arbitrary number of mixed spin chains.
Physical Review Letters, 2019
Physical Review B, 2018
We compute the zero temperature dynamical structure factor S(q, ω) of the triangular lattice Heis... more We compute the zero temperature dynamical structure factor S(q, ω) of the triangular lattice Heisenberg model (TLHM) using a Schwinger boson approach that includes the Gaussian fluctuations (1/N corrections) of the saddle point solution. While the ground state of this model exhibits a well-known 120 • magnetic ordering, experimental observations have revealed a strong quantum character of the excitation spectrum. We conjecture that this phenomenon arises from the proximity of the ground state of the TLHM to the quantum melting point separating the magnetically ordered and spin liquid states. Within this scenario, magnons are described as collective modes (two spinonbound states) of a spinon condensate (Higgs phase) that spontaneously breaks the SU(2) symmetry of the TLHM. Crucial to our results is the proper account of this spontaneous symmetry breaking. The main qualitative difference relative to semi-classical treatments (1/S expansion) is the presence of a high-energy spinon continuum extending up to about three times the single-magnon bandwidth. In addition, the magnitude of the ordered moment (m = 0.224) agrees very well with numerical results and the low energy part of the single-magnon dispersion is in very good agreement with series expansions. Our results indicate that the Schwinger boson approach is an adequate starting point for describing the excitation spectrum of some magnetically ordered compounds that are near the quantum melting point separating this Higgs phase from the deconfined spin liquid state.
Physical Review B, 2017
We investigate the effect of dimensional crossover in the ground state of the antiferromagnetic s... more We investigate the effect of dimensional crossover in the ground state of the antiferromagnetic spin-1 Heisenberg model on the anisotropic triangular lattice that interpolates between the regime of weakly coupled Haldane chains (J J) and the isotropic triangular lattice (J = J). We use the density-matrix renormalization group (DMRG) and Schwinger boson theory performed at the Gaussian correction level above the saddle-point solution. Our DMRG results show an abrupt transition between decoupled spin chains and the spirally ordered regime at (J /J)c ∼ 0.42, signaled by the sudden closing of the spin gap. Coming from the magnetically ordered side, the computation of the spin stiffness within Schwinger boson theory predicts the instability of the spiral magnetic order toward a magnetically disordered phase with one-dimensional features at (J /J)c ∼ 0.43. The agreement of these complementary methods, along with the strong difference found between the intra-and the interchain DMRG short spin-spin correlations; for sufficiently large values of the interchain coupling, suggests that the interplay between the quantum fluctuations and the dimensional crossover effects gives rise to the one-dimensionalization phenomenon in this frustrated spin-1 Hamiltonian.
EPL (Europhysics Letters), 2016
We investigate the spin dynamics of the square-lattice spin-1 2 Heisenberg antiferromagnet by mea... more We investigate the spin dynamics of the square-lattice spin-1 2 Heisenberg antiferromagnet by means of an improved mean field Schwinger boson calculation. By identifying both, the long range Néel and the RVB-like components of the ground state, we propose an educated guess for the mean field magnetic excitation consisting on a linear combination of local and bond spin flips to compute the dynamical structure factor. Our main result is that when this magnetic excitation is optimized in such a way that the corresponding sum rule is fulfilled, we recover the low and high energy spectral weight features of the experimental spectrum. In particular, the anomalous spectral weight depletion at (π, 0) found in recent inelastic neutron scattering experiments can be attributed to the interference of the triplet bond excitations of the RVB component of the ground state. We conclude that the Schwinger boson theory seems to be a good candidate to adequately interpret the dynamic properties of the square-lattice Heisenberg antiferromagnet.
Physical Review Letters, 1997
We compute the Gaussian-fluctuation corrections to the saddle-point Schwinger-boson results using... more 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.
Physical Review B, 2004
We investigate the single hole dynamics in the triangular t − J model. We study the structure of ... more 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 Néel 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 magnonassisted 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.
New Journal of Physics, 2012
Physical Review B
Recent inelastic neutron scattering (INS) experiments of the triangular antiferromagnet Ba3CoSb2O... more Recent inelastic neutron scattering (INS) experiments of the triangular antiferromagnet Ba3CoSb2O9 revealed strong deviations from semiclassical theories. We demonstrate that key features of the INS data are well reproduced by a parton Schwinger boson theory beyond the saddlepoint approximation. The measured magnon dispersion is well reproduced by the dispersion of two-spinon bound states (poles of the emergent gauge fields propagator), while the low-energy continuum scattering is reproduced by a quasifree two-spinon continuum, suggesting that a free spinon gas is a good initial framework to study magnetically ordered states near a quantum melting point. II. MATERIAL AND MODEL Ba 3 CoSb 2 O 9 comprises vertically stacked triangular layers of effective spin-1/2 moments arising from the J = 1/2 Kramers doublet of Co 2+ in a trigonally
We have computed the quasiparticle wave function corresponding to a hole injected in a triangular... more 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.
Physical Review B
We study the phase diagram at T = 0 of the antiferromagnetic Heisenberg model on the triangular l... more 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 Jα/J β = 0.5, conventional spin wave theory predicts that quantum fluctuations melt the classical structures, for S = 1/2. For the regime J β < Jα, it is shown that the incommensurate spiral phases survive until J β /Jα = 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.
Physical Review B, 2008
We have studied the validity of the spin polaron picture in the frustrated J1 − J2 Heisenberg mod... more We have studied the validity of the spin polaron picture in the frustrated J1 − J2 Heisenberg model. For this purpose, we have computed the hole spectral functions for the Néel, collinear, and disordered phases of this model, by means of the self-consistent Born approximation and Lanczos exact diagonalization on finite-size clusters. We have found that the spin polaron quasiparticle excitation is always well defined for the magnetically ordered Néel and collinear phases, even in the vicinity of the magnetic quantum critical points, where the local magnetization vanishes. As a general feature, the effect of frustration is to increase the amplitude of the multimagnon states that build up the spin polaron wave function, leading to the reduction of the quasiparticle coherence. Based on Lanczos results, we discuss the validity of the spin polaron picture in the disordered phase.
Physical Review B, 2000
We present a finite size spin wave calculation on the Heisenberg antiferromagnet on the triangula... more 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.
Physical Review B, 2015
We investigate the excitation spectrum of the triangular-lattice antiferromagnetic XXZ model usin... more We investigate the excitation spectrum of the triangular-lattice antiferromagnetic XXZ model using series expansions and mean field Schwinger bosons approaches. The single-magnon spectrum computed with series expansions 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 ≤ J z /J ≤ 1. Based on the good agreement with series expansions 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.