Mixed-spin systems composed of quantum spin chains with gapped magnetic excitations interacting through `auxiliary' magnetic ions show an effective separation between low-frequency classical and high-frequency quantum spin correlations. This phenomenon is realized in a family of rare-earth nickelates. Studies of these materials enable experimental measurements of some previously inaccessible fundamental properties of quantum spin chains to be made. The non-trivial intrinsic dynamics of the auxiliary spins gives rise to peculiar excitations of a mixed nature, and in certain cases may play a key role in long-range magnetic ordering.
Mixed-spin systems composed of quantum spin chains with gapped magnetic excitations interacting through `auxiliary' magnetic ions show an effective separation between low-frequency classical and high-frequency quantum spin correlations. This phenomenon is realized in a family of rare-earth nickelates. Studies of these materials enable experimental measurements of some previously inaccessible fundamental properties of quantum spin chains to be made. The non-trivial intrinsic dynamics of the auxiliary spins gives rise to peculiar excitations of a mixed nature, and in certain cases may play a key role in long-range magnetic ordering.
Recent experiments on the quasi-one-dimensional (1D) antiferromagnets R2BaNiO5(R=rare earth) have shown the existence of purely 1D Haldane gap excitations propagating on the Ni chains. Below an ordering temperature, the gap excitations survive and coexist with the conventional spin waves in the ordered phase. We construct a model mixed-spin system in 2D for which the ground state can be exactly specified. Using the matrix product method, we show the existence of Haldane gap excitations in the ordered phase. We consider different cases of ordering to study the effect of ordering on the degeneracy of the Haldane gap excitations.
Magnetic systems composed of weakly coupled spin-1/2 chains are fertile ground for hosting the fractional magnetic excitations that are intrinsic to interacting fermions in one-dimension (1D). However, the exotic physics arising from the quantum many-body interactions beyond 1D are poorly understood in materials of this class. Spinons and psinons are two mutually exclusive low-energy magnetic quasiparticles; the excitation seen depends on the ground state of the spin chain. Here, we present inelastic neutron scattering and neutron diffraction evidence for their coexistence in SrCo2V2O8 at milli-Kelvin temperatures in part of the N\'eel phase (2.4 T ≤μ0H < 3.9 T) and possibly also the field-induced spin density wave phase up to the highest field probed (μ0H ≥ 3.9 T, μ0Hmax = 5.5 T). These results unveil a novel spatial phase inhomogeneity for the weakly coupled spin chains in this compound. This quantum dynamical phase separation is a new phenomenon in quasi-1D quantum magnets, highlighting the non-trivial consequences of inter-chain coupling.
Bulk magnetism in solids is fundamentally quantum mechanical in nature. Yet in many situations, including our everyday encounters with magnetic materials, quantum effects are masked, and it often suffices to think of magnetism in terms of the interaction between classical dipole moments. Whereas this intuition generally holds for ferromagnets, even as the size of the magnetic moment is reduced to that of a single electron spin (the quantum limit), it breaks down spectacularly for antiferromagnets, particularly in low dimensions. Considerable theoretical and experimental progress has been made in understanding quantum effects in one-dimensional quantum antiferromagnets, but a complete experimental description of even simple two-dimensional antiferromagnets is lacking. Here we describe a comprehensive set of neutron scattering measurements that reveal a non-spin-wave continuum and strong quantum effects, suggesting entanglement of spins at short distances in the simplest of all two-dimensional quantum antiferromagnets, the square lattice Heisenberg system.
Decades after the discovery of high-temperature superconductivity, understanding the effects of doping a Mott insulator remains a topic of great relevance, since the fundamental excitations in the normal state are not known, nor the precise origin of the pairing mechanism. In particular, the nature of the pseudogap phase at finite temperatures, above the superconducting dome, or the redistribution of spectral weight observed in experiments indicate that our knowledge is incomplete and theories may be missing some key ingredients.Due to the two-dimensionality of the problem and the limitations imposed by the Mermin-Wagner theorem, the numerical study of these questions is remarkably difficult: most treatments are either limited to small or quasi one-dimensional systems at zero temperature, or simulations may be carried out at finite temperature, completely missing the possibility for long-range order. In this dissertation, we tackle paradigmatic problems in condensed matter from a fundamentally new perspective. We numerically study interacting chains and ladders introducing RKKY-like SU(2)-symmetric long-range interactions that allow one to realize actual spontaneous symmetry breaking and true antiferromagnetic order. We shed new light on problems such as the role of magnetic order on quasi-particle formation and fractionalization, and pairing instabilities in generalized Hubbard-like models. In particular, we address questions of a very fundamental nature: (i) How do spinons evolve into magnons and what are their signatures in the excitation spectrum? (ii) how do coherent quasi-particles form in the presence of long range order? Will we still observe signatures of spin-charge separation in the spectrum? (iii) what is the interplay between long-range antiferromagnetic order and pairing? Our work offers new insight onto the role played by long-range antiferromagnetism in quasi-particle formation and pairing in the context of new toy-models that realize much of the phenomenology of their higher dimensional counterparts.--Author's abstract
Many phenomena in magnetism can be described using classical models, yet magnetism is a quantum mechanical effect. In addition to thermal fluctuations, quantum fluctuations arise due to the quantum uncertainty of the ground state. These fluctuations are enhanced by low dimensionality and low spin. Spin chains show interesting excitations associated with quasiparticles. Model materials for spin chains can be realized in compounds where the interchain interactions are weak enough. This thesis presents neutron scattering studies of two model materials for spin chains with different spin symmetry. The spin2 Heisenberg antiferromagnetic chain CuSO4*5D2O has a main exchange of J ∼ 3 K, which permits to study its excitation spectrum up to temperatures of ten times the exchange. The spin2 Heisenberg magnet has been studied widely both theoretically and experimentally. The excitation spectrum of the spin2 Heisenberg antiferromagnet consists of quasiparticles called spinons, and calculations for it exist for T = 0 and as a function of temperature. A quantitative comparison between experimental data and theoretical models across a range of temperatures is presented. The excitation spectrum of CuSO4*5D2O is excellently described by the excitation spectrum of a spin2 Heisenberg antiferromagnetic chain at finite temperatures. Energy-temperature scaling is observed at temperatures up to 2J . For these temperatures the Luttinger liquid description of the spin2 Heisenberg antiferromagnetic chain is valid. A spinon continuum is confirmed at all measured temperatures. Therefore, even at ten times the main exchange (T ≈ 10 J), CuSO4*5D2O shows the behavior of a quantum magnet, and not of a classical system at infinite temperatures, for which one would expect single spin flips. The Ising chain is one of the most studied models in magnetism, one model compound for it is RbCoCl3. Optical spectroscopy measurements suggest a lower gap than for similar compounds, which makes it better suited for high resolution neutron spectroscopy. Magnetic correlations along the chains arise in RbCoCl3 below 80 K. A gapped continuum is observed, which arises from isolated antiferromagnetic Ising chains. Below the first magnetic ordering temperature TN1 ≈ 28 K bound states are observed, which arise due to correlations which form between the chains. Strong diffuse scattering leads to the conclusion that disorder is still present. At the second magnetic ordering transition, at TN2 ≈ 14 K, the bound states split as the correlations between the chains change. Fits indicate that the correlations change continuously as a function of temperature. Above TN2 the excitation spectrum of RbCoCl3 can be described by a spin1 2 Ising chain with an XY-component which is treated as a perturbation. Below TN2 the main features are reproduced correctly, but deviations from the details indicate that additional terms in the Hamiltonian may be necessary. This thesis was carried out at the Institut Laue-Langevin, the Laboratory for Neutron Scattering and Imaging at the Paul Scherrer Institute and ETH Zurich.
We investigate the quantum dynamics of the spins on different Heisenberg antiferromagnetic spin lattice systems. Firstly, we applied the coupled-cluster method to the spin-1/2 antiferromagnetic \emph{XXZ} model on a square lattice by employing an approximation which contains two-body long-range correlations and high-order four-body local correlations. Improvement is found for the ground-state energy, sublattice magnetization, and the critical anisotropy when comparing with the approximation including the two-body correlations alone. We also obtain the full excitation spectrum which is in good agreement with the quantum Monte Carlo results and the high-order spin-wave theory.Secondly, we study the longitudinal excitations of quantum antiferromagnets on a triangular lattice by a recently proposed microscopic many-body approach based on magnon-density waves. We calculate the full longitudinal excitation spectra of the antiferromagnetic Heisenberg model for a general spin quantum number in the isotropic limit. Similar to the square lattice model, we find that, at the center of the first hexagonal Brillouin zone Γ(q=0) and at the magnetic ordering wavevectors ±[Q=(4π/3,0)], the excitation spectra become gapless in the thermodynamic limit, due to the slow, logarithmic divergence of the structure factor. However, these longitudinal modes on two-dimensional models may be considered as quasi-gapped, as any finite-size effect or small anisotropy will induce a large energy gap, when compared with the counterpart of the transverse spin-wave excitations. We have also investigated the excited states of the quasi-one-dimensional quantum antiferromagnets on hexagonal lattices, including the longitudinal modes based on the magnon-density waves. A model Hamiltonian with a uniaxial single-ion anisotropy is first studied by a spin-wave theory based on the one-boson method; the ground state thus obtained is employed for the study of the longitudinal modes. The full energy spectra of both the transverse modes (i.e., magnons) and the longitudinal modes are obtained as functions of the nearest-neighbor coupling and the anisotropy constants. We have found two longitudinal modes due to the non-collinear nature of the triangular antiferromagnetic order, similar to that of the phenomenological field theory approach by Affleck. The excitation energy gaps due to the anisotropy and the energy gaps of the longitudinal modes without anisotropy are then investigated. We then compares our results for the longitudinal energy gaps at the magnetic wavevectors with the experimental results for several antiferromagnetic compounds with both integer and non-integer spin quantum numbers, and we find good agreements after the higher-order contributions are included in our calculations.