Well-defined Magnetic Excitations Research Articles

Magnetic Excitations and Continuum of a Possibly Field-Induced Quantum Spin Liquid in α-RuCl_{3}.

We report on terahertz spectroscopy of quantum spin dynamics in α-RuCl_{3}, a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. We follow the evolution of an extended magnetic continuum below the structural phase transition at T_{s2}=62 K. With the onset of a long-range magnetic order at T_{N}=6.5 K, spectral weight is transferred to a well-defined magnetic excitation at ℏω_{1}=2.48 meV, which is accompanied by a higher-energy band at ℏω_{2}=6.48 meV. Both excitations soften in a magnetic field, signaling a quantum phase transition close to B_{c}=7 T, where a broad continuum dominates the dynamical response. Above B_{c}, the long-range order is suppressed, and on top of the continuum, emergent magnetic excitations evolve. These excitations follow clear selection rules and exhibit distinct field dependencies, characterizing the dynamical properties of a possibly field-induced quantum spin liquid.

Magnetic correlations in the intermetallic antiferromagnet Nd3Co4Sn13

Specific heat, magnetic susceptibility, and neutron scattering have been used to investigate the nature of the spin system in the antiferromagnet Nd3Co4Sn13. At room temperature Nd3Co4Sn13 has a cubic, Pm-3n structure similar to Yb3Rh4Sn13. Antiferromagnetic interactions between, Nd3+ ions dominate the magnetic character of this sample and at 2.4 K the Nd spins enter a long range order state with a magnetic propagation vector q = (0 0 0) with an ordered moment of 1.78(2) µB at 1.5 K. The magnetic Bragg intensity grows very slowly below 1 K, reaching ~2.4 µB at 350 mK. The average magnetic Nd3+ configuration corresponds to the 3D irreducible representation Γ7. This magnetic structure can be viewed as three sublattices of antiferromagnetic spin chains coupled with each other in the 120°-configuration. A well-defined magnetic excitation was measured around the 1 1 1 zone centre and the resulting dispersion curve is appropriate for an antiferromagnet with a gap of 0.20(1) meV.

Neutron scattering andμSR investigations of quasi-one-dimensional magnetism in the spin=3/2compound Li3RuO4

The $S$ = 3/2, quasi-one-dimensional (1D) zig-zag chain Heisenberg antiferromagnet Li${}_{3}$RuO${}_{4}$ has been investigated using heat capacity, inelastic neutron scattering, neutron diffraction, and \ensuremath{\mu}SR measurements on a powder sample. Our neutron diffraction and \ensuremath{\mu}SR studies confirm a long-range ordering of the magnetic moments on the Ru${}^{5+}$ cations below 40 K. The magnetic excitations were measured at various temperatures above and below the three-dimensional (3D) ordering temperature in order to understand the broad peak observed in the temperature dependence of the magnetic susceptibility. At 5 K we have observed two well-defined magnetic excitations at 5.5 meV and 8.5 meV and a weak low-energy peak near \ensuremath{\sim}2 meV. We interpret the 5.5 meV energy peak as a 1D zone-boundary mode and that at 8.5 meV as arising from a maximum away from the zone boundary in the dispersion curve for spin-wave modes along the chain of Ru${}^{5+}$ ions. The weaker peak near 2 meV is thought to arise from a weak interchain coupling. Our data are best reproduced using a model with three intrachain interactions and one weak interchain interaction. The experimental spin-exchange interactions are in good agreement with those calculated for a 1D model by density functional theory (DFT) methods. Furthermore, above ${T}_{N}$ we observe strong diffuse scattering at the same $Q$-position as the 5.5 meV mode, which suggests the presence of short-range magnetic correlations above ${T}_{N}$. We have estimated the correlation length \ensuremath{\xi} \ensuremath{\sim} 2.9 \AA{} at 50 K, which is close to 2.99 \AA{}, the shortest distance between the Ru${}^{5+}$ cations along the zig-zag chain.

Neutron scattering study on stripe correlations in La 2− xBa xCuO 4

We have performed neutron scattering experiment on the single crystals of La 2− x Ba x CuO 4 ( x = 0.10 and 0.125) in order to study the doping dependence of stripe correlations. At low temperatures, clear incommensurate (IC) peaks from spin-density-wave orders (SDW) were observed in both samples. The incommensurability ( δ) of the SDW in the x = 0.10 sample was found to be 0.110, which slightly deviates from the simple relation of δ = x seen in La 2− x Sr x CuO 4, possibly due to the large stripe pinning effect in the low-temperature tetragonal (LTT) structure. The inelastic magnetic signal at the low-energy region below 6 meV drastically changes around the structural transition temperature between LTT and low-temperature orthorhombic (LTO) phases T d2. Temperature dependences of the peak-width and the incommensurability also show an anomalous behavior at T d2 rather than the spin ordering temperature, T sp. These results suggest that spin correlations are well stabilized in the LTT phase as expected from the stripe model. With further increasing temperature in LTO phase, the incommensurability decreases and the peak-width grows due to the degradation of stripe correlations. However, the stiffness of stripe correlation seen in the low-energy IC spin fluctuations depends on the doping. In the x = 0.10 sample no well-defined magnetic excitation peak was observed below 6 meV at 200 K, while a flat top shaped single peak remains at 6 meV in the x = 0.125 sample. Deviation from the 1/8 doping more easily lose the spatial coherence of spin correlations above T d2.

Reinvestigation of magnetic excitations in the spin density wave of chromium

Low-energy magnetic excitations of chromium have been reinvestigated with a single-Q crystal using neutron scattering technique. In the transverse spin-density-wave phase a new type of well-defined magnetic excitation is found around (0,0,1) with a weak dispersion perpendicular to the wavevector of the incommensurate structure. The magnetic excitation has an energy gap of E ~ 4 meV and at (0,0,1) exactly corresponds to the Fincher mode previously studied only along the incommensurate wavevector.

Magnetic excitations and quadrupolar ordering in UNiSn

UNiSn transforms from a paramagnetic semiconductor to an antiferromagnetic metal at T N=43 K. We have made a detailed neutron scattering study of UNiSn and find well-defined magnetic excitations in the metallic phase, at 10 and 35 meV, with no evidence of excitations at higher energies. The excitations persist in the semiconducting phase, but their lifetime changes rapidly above T N. High-resolution studies reveal a temperature-dependent splitting of the 10 meV peak, consistent with a tetragonal strain induced by quadrupolar ordering.

Magnetic Response Functions in Actinide Compounds

A brief review is given of neutron inelastic scattering experiments on actinide (5f) systems. Only a few compounds exhibit sharp crystal-field levels so that analogies with 4f compounds are rarely straight forward. In the systems that order magnetically, we do find well-defined magnetic excitations, although in many cases the strong interaction with the conduction electrons gives rise to overdamped modes. Finally in the strongly enhanced paramagnets and heavy-fermion systems the response is essentially featureless in Q space and can be characterized by a single Lorentzian with full-width at half-maximum of Γ. Attemps to relate this quantity Γ to other physical properties, particularly the coefficient of the electronic specific heat γ, have not been too successful.

A high resolution neutron scattering study of single crystal neodymium

It is well known that neodymium develops a modulated antiferromagnetic structure below a first-order transition at T N = 19.9 K. In marked contrast to the other lanthanide metals, no well-defined magnetic excitations have previously been observed in Nd, either in the paramagnetic or magnetically ordered phases. A careful study of the inelastic scattering of neutrons from a new large single crystal of Nd (grown by the solid state electrotransport technique) has been carried out at the Institut Laue-Langevin. Quasielastic scattering which varies markedly with both wavevector and temperature has been found. This scattering has a maximum intensity around T N and exhibits a characteristic linewidth of some 0.4 meV. These results may be interpreted in terms of heavily-damped excitations within low-lying crystal field levels. At lower temperatures, and further from the satellite wavevectors, two weak but well-defined magnetic excitations have been observed.

Experimental studies of the magnetic and phonon excitations in cobalt fluoride

The excitations of cobalt fluoride have been studied with the technique of slow-neutron inelastic scattering. A triple-axis crystal spectrometer was used throughout in its constant Q mode of operation. Several of the branches of the phonon spectra were measured and are in reasonable agreement with calculations based on a very simple rigid ion model. The magnetic excitations are of interest because the Co2+ ion has unpaired orbital angular momentum which gives rise to several low-lying energy levels. Excitations have been observed from the ground state to the next three excited states in the antiferromagnetic phase and their dispersion and temperature dependence obtained below, near, and above the Néel temperature. The temperature dependences of the excitations to the different excited states are not the same. The measurements on the magnetic excitation branch of lowest frequency show that, contrary to current theories of magnetism, fairly well-defined magnetic excitations occur to at least 1.5TN Another excitation, which gave a weak scattering intensity, was also observed and it is suggested that this may be associated with a distortion of the crystal structure.