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Register a new userThermal control of spin excitations in the coupled Ising-chain material RbCoCl3
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Added by
Christian R\\\"uegg
Publication date
2018
fields
Physics
and research's language is
English
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We have used neutron spectroscopy to investigate the spin dynamics of the quantum (S = 1/2) antiferromagnetic Ising chains in RbCoCl3. The structure and magnetic interactions in this material conspire to produce two magnetic phase transitions at low temperatures, presenting an ideal opportunity for thermal control of the chain environment. The high-resolution spectra we measure of two-domain-wall excitations therefore characterize precisely both the continuum response of isolated chains and the Zeeman-ladder bound states of chains in three different effective staggered fields in one and the same material. We apply an extended Matsubara formalism to obtain a quantitative description of the entire dataset, Monte Carlo simulations to interpret the magnetic order, and finite-temperature DMRG calculations to fit the spectral features of all three phases.
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We discuss the recent progress and the current status of experimental investigations of spin-mediated energy transport in spin-chain and spin-ladder materials with antiferromagnetic coupling. We briefly outline the central results of theoretical studies on the subject but focus mainly on recent experimental results that were obtained on materials which may be regarded as adequate physical realizations of the idealized theoretical model systems. Some open questions and unsettled issues are also addressed.
Magnetic excitations in a weakly coupled spin dimers and chains compound Cu2Fe2Ge4O13 are measured by inelastic neutron scattering. Both structure factors and dispersion of low energy excitations up to 10 meV energy transfer are well described by a semiclassical spin wave theory involving interacting Fe$^{3+}$ ($S = 5/2$) chains. Additional dispersionless excitations are observed at higher energies, at $hbar omega = 24$ meV, and associated with singlet-triplet transitions within Cu$^{2+}$-dimers. Both types of excitations can be understood by treating weak interactions between the Cu$^{2+}$ and Fe$^{3+}$ subsystems at the level of the Mean Field/ Random Phase Approximation. However, this simple model fails to account for the measured temperature dependence of the 24 meV mode.
We report a combination of physical property and neutron scattering measurements for polycrystalline samples of the one-dimensional spin chain compound CoV2O6. Heat capacity measurements show that an effective S = 1/2 state is found at low temperatures and that magnetic fluctuations persist up to 6.Tn. Above Tn = 6.3 K, measurements of the magnetic susceptibility as a function of T and H show that the nearest neighbour exchange is ferromagnetic. In the ordered state, we have discovered a crossover from a metamagnet with strong fluctuations between 5 K and Tn to a state with a 1/3 magnetisation plateau at 2 < T < 5 K. We use neutron powder diffraction measurements to show that the AFM state has incommensurate long range order and inelastic time of flight neutron scattering to examine the magnetic fluctuations as a function of temperature. Above Tn, we find two broad bands between 3.5 and 5 meV and thermally activated low energy features which correspond to transitions within these bands. These features show that the excitations are deconfined solitons rather than the static spin reversals predicted for a uniform FM Ising spin chain. Below Tn, we find a ladder of states due to the confining effect of the internal field. A region of weak confinement below Tn, but above 5 K, is identified which may correspond to a crossover between 2D and 3D magnetic ordering.
We present an inelastic neutron scattering investigation of Li2CuO2 detecting the long sought quasi-1D magnetic excitations with a large dispersion along the CuO2-chains studied up to 25 meV. The total dispersion is governed by a surprisingly large ferromagnetic (FM) nearest-neighbor exchange integral J1=-228 K. An anomalous quartic dispersion near the zone center and a pronounced minimum near (0,0.11,0.5) r.l.u. (corresponding to a spiral excitation with a pitch angle about 41 degree point to the vicinity of a 3D FM-spiral critical point. The leading exchange couplings are obtained applying standard linear spin-wave theory. The 2nd neighbor inter-chain interaction suppresses a spiral state and drives the FM in-chain ordering below the Neel temperature. The obtained exchange parameters are in agreement with the results for a realistic five-band extended Hubbard Cu 3d O 2p model and L(S)DA+U predictions.
We study the dynamics of the quasi-one-dimensional Ising-Heisenberg antiferromagnet BaCo2V2O8 under a transverse magnetic field. Combining inelastic neutron scattering experiments and theoretical analyses by field theories and numerical simulations, we mainly elucidate the structure of the spin excitation spectrum in the high field phase, appearing above the quantum phase transition point mu0Hc ~ 10 T. We find that it is characterized by collective solitonic excitations superimposed on a continuum. These solitons are strongly bound in pairs due to the effective staggered field induced by the nondiagonal g tensor of the compound, and are topologically different from the fractionalized spinons in the weak field region. The dynamical susceptibility numerically calculated with the infinite time-evolving block decimation method shows an excellent agreement with the measured spectra, which enables us to identify the dispersion branches with elementary excitations. The lowest energy dispersion has an incommensurate nature and has a local minimum at an irrational wave number due to the applied transverse field.
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