B. Lake

Linear spin wave theory for single-Q incommensurate magnetic structures.

Linear spin wave theory provides the leading term in the calculation of the excitation spectra of long-range ordered magnetic systems as a function of 1/√S. This term is acquired using the Holstein-Primakoff approximation of the spin operator and valid for small δS fluctuations of the ordered moment. We propose an algorithm that allows magnetic ground states with general moment directions and single-Q incommensurate ordering wave vector using a local coordinate transformation for every spin and a rotating coordinate transformation for the incommensurability. Finally we show, how our model can determine the spin wave spectrum of the magnetic C-site langasites with incommensurate order.

Multispinon Continua at Zero and Finite Temperature in a Near-Ideal Heisenberg Chain

The space-and time-dependent response of many-body quantum systems is the most informative aspect of their emergent behavior. The dynamical structure factor, experimentally measurable using neutron scattering, can map this response in wave vector and energy with great detail, allowing theories to be quantitatively tested to high accuracy. Here, we present a comparison between neutron scattering measurements on the one-dimensional spin-1/2 Heisenberg antiferromagnet KCuF3, and recent state-of-the-art theoretical methods based on integrability and density matrix renormalization group simulations. The unprecedented quantitative agreement shows that precise descriptions of strongly correlated states at all distance, time, and temperature scales are now possible, and highlights the need to apply these novel techniques to other problems in low-dimensional magnetism.

One- and two-triplon spectra of a cuprate ladder.

We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was the separation of one-triplon and two-triplon scattering. Two-triplon scattering is observed quantitatively for the first time and so access is realized to the important strong magnetic quantum fluctuations. The spin gap is found to be 26.4+/-0.3 meV. The data are successfully modeled using the continuous unitary transformation method, and the exchange constants are determined by fitting to be Jleg=186 meV and Jrung=124 meV along the leg and rung, respectively; a substantial cyclic exchange of Jcyc=31 meV is confirmed.

Magnetic excitations in the ordered phase of the antiferromagnetic alternating chain compound

In this paper we present the results of a detailed experimental investigation of the magnetic excitations in the spin- Heisenberg antiferromagnet . Inelastic neutron scattering measurements made using a triple-axis spectrometer were performed in the low-temperature ordered phase and also briefly in the high-temperature phase at Brookhaven National Laboratory. The excitations in the ordered phase are compared to a spin-wave model and two possible sets of exchange paths are deduced, both of which provide good fits to the data. In both models consists of weakly coupled alternating one-dimensional chains running in the [2,-1,0] direction. A calculation of the spin-wave intensities was also performed using the fitted exchange constants and agreement was found with the observed intensities. In the ordered phase, has an energy gap at the zone centre and the excitations are well defined. Above the transition temperature constant-wavevector scans at the antiferromagnetic lattice points suggest the existence of a continuum of excitations.

Inelastic neutron scattering and frequency domain magnetic resonance studies of S=4 and S=12 Mn6 single-molecule magnets

We investigate the magnetic properties of three Mn6 single-molecule magnets by means of inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The experimental data reveal that small structural distortions of the molecular geometry produce a significant effect on the energy-level diagram and therefore on the magnetic properties of the molecule. We show that the giant spin model completely fails to describe the spin-level structure of the ground spin multiplets. We analyze theoretically the spin Hamiltonian for the low-spin Mn6 molecule S =4 and we show that the excited S multiplets play a key role in determining the effective energy barrier for the magnetization reversal, in analogy to what was previously found for the two high spin Mn6 S =1 2 molecules S. Carretta et al., Phys. Rev. Lett. 100, 157203 2008.

A dimer theory of the magnetic excitations in the ordered phase of the alternating-chain compound

A theory is developed to model the excitations in a dimerized, spin-1/2 system with a magnetically ordered ground state and where the dimer exchange constant is antiferromagnetic. This method starts by considering the energy levels of a single dimer in the effective, staggered magnetic field due to the mean-field ordering of the surrounding dimers. Pseudo-boson operators are introduced which create and annihilate these excitations, and the Hamiltonian of the magnetic system can be rewritten in terms of these operators and then diagonalized to yield one doubly degenerate transverse mode and a longitudinal singlet mode for each non-equivalent dimer in the magnetic unit cell. The dimer theory has been used to model the measured dispersion relations in the antiferromagnetically ordered phase of the alternating-chain compound . It provides a good fit to the data and is as successful as spin-wave theory in accounting for the transverse excitations although with different values of the exchange constants. In addition the transition temperature and the size of the reduced moment at T = 0 K calculated in the dimer theory are closer to the experimental values of than those calculated by spin-wave theory. An important difference between these two models lies in their predictions of the longitudinal excitations: whereas in spin-wave theory these are regarded as two-magnon events resulting in a continuum of scattering, in the dimer theory one well defined mode is expected. An experimental measurement of the longitudinal excitations should distinguish between these models.

Magnetic exchange interactions in BaMn 2 As 2 : A case study of the J 1 - J 2 - J c Heisenberg model

BaMn2As2 is unique among BaT2As2 compounds crystallizing in the body-centered-tetragonal ThCr2Si2 structure, which contain stacked square lattices of 3d transition metal T atoms, since it has an insulating large-moment (3.9 muB/Mn) G-type (checkerboard) antiferromagnetic AF ground state. We report measurements of the anisotropic magnetic susceptibility chi versus temperature T from 300 to 1000 K of single crystals of BaMn2As2, and magnetic inelastic neutron scattering measurements at 8 K and 75As NMR measurements from 4 to 300 K of polycrystalline samples. The Neel temperature determined from the chi(T) measurements is TN = 618(3) K. The measurements are analyzed using the J1-J2-Jc Heisenberg model. Linear spin wave theory for G-type AF ordering and classical and quantum Monte Carlo simulations and molecular field theory calculations of chi(T) and of the magnetic heat capacity Cmag(T) are presented versus J1, J2 and Jc. We also obtain band theoretical estimates of the exchange couplings in BaMn2As2. From analyses of our chi(T), NMR, neutron scattering, and previously published heat capacity data for BaMn2As2 on the basis of the above theories for the J1-J2-Jc Heisenberg model and our band-theoretical results, our best estimates of the exchange constants in BaMn2As2 are J1 = 13 meV, J2/J1 = 0.3 and Jc/J1 = 0.1, which are all antiferromagnetic. From our classical Monte Carlo simulations of the G-type AF ordering transition, these exchange parameters predict TN = 640 K for spin S = 5/2, in close agreement with experiment. Using spin wave theory, we also utilize these exchange constants to estimate the suppression of the ordered moment due to quantum fluctuations for comparison with the observed value and again obtain S = 5/2 for the Mn spin.

Magnetic structure and interactions in the quasi-one-dimensional antiferromagnet CaV 2 O 4

CaV2O4 is a spin-1 antiferromagnet, where the magnetic vanadium ions have an orbital degree of freedom and are arranged on quasi-one-dimensional zig-zag chains. The first- and second-neighbor vanadium separations are approximately equal suggesting frustrated antiferromagnetic exchange interactions. High-temperature susceptibility and single-crystal neutron diffraction measurements are used to deduce the dominant exchange paths and orbital configurations. The results suggest that at high temperatures CaV2O4 behaves as a Haldane chain, but at low temperatures, it is a spin-1 ladder. These two magnetic structures are explained by different orbital configurations and show how orbital ordering can drive a system from one exotic spin Hamiltonian to another.

Longitudinal magnetic dynamics and dimensional crossover in the quasi-one-dimensional spin-1/2 heisenberg antiferromagnet KCuF3

The spin dynamics of coupled spin-1/2, antiferromagnetic Heisenberg chains is predicted to exhibit a novel longitudinal mode at low energies and temperatures below the Neel temperature. This mode is a dimensional crossover effect and reveals the presence of a limited amount of long-range antiferromagnetic order co-existing with quantum fluctuations. In this paper the existence of such a mode is confirmed in the model material KCuF3 using polarized and unpolarized inelastic neutron scattering and the longitudinal polarization of the mode is definitively established. The lineshape is broadened suggesting a reduced lifetime due to decay into spin-waves. In addition the data shows evidence of continuum scattering with a lower edge greater than the longitudinal mode energy. A detailed comparison is made with theoretical predictions and experimental work on other model materials.

Spinon confinement in a quasi-one-dimensional anisotropic Heisenberg magnet

Confinement is a process by which particles with fractional quantum numbers bind together to form quasiparticles with integer quantum numbers. The constituent particles are confined by an attractive interaction whose strength increases with increasing particle separation and, as a consequence, individual particles are not found in isolation. This phenomenon is well known in particle physics where quarks are confined in baryons and mesons. An analogous phenomenon occurs in certain spatially anisotropic magnetic insulators. These can be thought of in terms of weakly coupled chains of spins S = 1/2, and a spin flip thus carries integer spin S = 1. The collective excitations in these systems, called spinons, turn out to carry fractional spin quantum number S = 1/2. Interestingly, at sufficiently low temperatures the weak coupling between chains can induce an attractive interaction between pairs of spinons that increases with their separation and thus leads to confinement. In this paper, we employ inelastic neutron scattering to investigate the spinon-confinement process in the quasi-one-dimensional, spin-1/2 antiferromagnet with Heisenberg-Ising (XXZ) anisotropy SrCo2V2O8. A wide temperature range both above and below the long-range ordering temperature T-N = 5.2 K is explored. Spinon excitations are observed above TN in quantitative agreement with established theory. Below T-N pairs of spinons are confined and two sequences of meson-like bound states with longitudinal and transverse polarizations are observed. Several theoretical approaches are used to explain the data. These are based on a description in terms of a one-dimensional, S = 1/2 XXZ antiferromagnetic spin chain, where the interchain couplings are modeled by an effective staggered magnetic mean field. A wide range of exchange anisotropies are investigated and the parameters specific to SrCo2V2O8 are identified. Recently developed theoretical technique based on tangent-space matrix product states gives a very complete description of the data and provides good agreement not only with the energies of the bound modes but also with their intensities. We also successfully explain the effect of temperature on the excitations including the experimentally observed thermally induced resonance between longitudinal modes below T-N and the transitions between thermally excited spinon states above T-N. In summary, our work establishes SrCo2V2O8 as a beautiful paradigm for spinon confinement in a quasi-one-dimensional quantum magnet and provides a comprehensive picture of this process.