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The crystal structure and magnetic correlations in triangular antiferromagnet $\mathrm{Fe}\mathrm{Ga}{}_{2}\mathrm{S}{}_{4}$ are studied by x-ray diffraction, magnetic susceptibility, neutron diffraction, and neutron inelastic scattering. We report significant mixing at the cation sites and disentangle magnetic properties dominated by major and minor magnetic sites. The magnetic short-range correlations at $0.77\phantom{\rule{0.28em}{0ex}}{\AA{}}^{\ensuremath{-}1}$ correspond to the major sites and being static at base temperature they evolve into dynamic correlations around 30--50 K. The minor sites contribute to the magnetic peak at $0.6\phantom{\rule{0.28em}{0ex}}{\AA{}}^{\ensuremath{-}1}$, which vanishes at 5.5 K. Our analytical studies of triangular lattice models with bilinear and biquadratic terms provide the ratios between exchanges for the proposed ordering vectors. The modeling of the inelastic neutron spectrum within linear spin-wave theory results in the set of exchange couplings ${J}_{1}=1.7,\phantom{\rule{0.28em}{0ex}}{J}_{2}=0.9,\phantom{\rule{0.28em}{0ex}}{J}_{3}=0.8\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ for the bilinear Heisenberg Hamiltonian. However, not all features of the excitation spectrum are explained with this model.

Inelastic neutron scattering is used to investigate the temperature dependence of spin correlations in the 3-dimensional XY antiferromagnet $\mathrm{Ni}{({\mathrm{Cl}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}4\mathrm{SC}{({\mathrm{NH}}_{2})}_{2}, x=0.14(1)$, tuned close to the chemical-composition-induced soft-mode transition. The local dynamic structure factor shows $\ensuremath{\hbar}\ensuremath{\omega}/T$ scaling behavior characteristic of a quantum-critical point. The deviation of the measured critical exponent from spin wave theoretical expectations is attributed to disorder. Another effect of disorder is local excitations above the magnon band. Their energy, structure factor, and temperature dependence are well explained by simple strong-bond dimers associated with Br-impurity sites.

CeCoSi is an intermetallic antiferromagnet with a very unusual temperature-pressure phase diagram: at ambient pressure it orders below $T_{\mathrm{N}} = 8.8$ K, while application of hydrostatic pressure induces a new magnetically ordered phase with exceptionally high transition temperature of $\sim40$ K at 1.5 GPa. We studied the magnetic properties and the pressure-induced magnetic phase of CeCoSi by means of elastic and inelastic neutron scattering (INS) and heat capacity measurements. At ambient pressure CeCoSi orders into a simple commensurate AFM structure with a reduced ordered moment of only $m_{\mathrm{Ce}} = 0.37(6)$ $\mu_{\mathrm{B}}$. Specific heat and low-energy INS indicate a significant gap in the low-energy magnon excitation spectrum in the antiferromagnetic phase, with the CEF excitations located above 10 meV. Hydrostatic pressure gradually shifts the energy of the magnon band towards higher energies, and the temperature dependence of the magnons measured at 1.5 GPa is consistent with the phase diagram. Moreover, the CEF excitations are also drastically modified under pressure., Comment: 8 pages, 11 figures

The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modelling of the dynamical response demonstrates our complete quantitative understanding of these states., Comment: 6 pages, 3 figures plus supplementary material 7 pages 5 figures

The compound GdPtBi is known as a material where the non-trivial topology of electronic bands interplays with an antiferromagnetic order, which leads to the emergence of many interesting magnetotransport phenomena. Although the magnetic structure of the compound has previously been reliably determined, the magnetic interactions responsible for this type of order remained controversial. In the present study, we employed time-of-flight inelastic neutron scattering to map out the low-temperature spectrum of spin excitations in single-crystalline GdPtBi. The observed spectra reveal two spectrally sharp dispersive spin-wave modes, which reflects the multi-domain state of the $\mathbf{k} = (\frac{1}{2}\frac{1}{2}\frac{1}{2})$ face-centred cubic antiferromagnet in the absence of a symmetry-breaking magnetic field. The magnon dispersion reaches an energy of $\sim 1.1$~meV and features a gap of $\sim 0.15$~meV. Using linear spin-wave theory, we determined the main magnetic microscopic parameters of the compound that provide good agreement between the simulated spectra and the experimental data. We show that GdPtBi is well within the ($\frac{1}{2}\frac{1}{2}\frac{1}{2}$) phase and is dominated by second-neighbor interactions, thus featuring low frustration., Comment: 8 pages, 5 figures

Inelastic neutron-scattering and finite-temperature density matrix renormalization group (DMRG) calculations are used to investigate the spin excitation spectrum of the $S=1/2$ Heisenberg spin chain compound ${\mathrm{K}}_{2}{\mathrm{CuSO}}_{4}{\mathrm{Cl}}_{2}$ at several temperatures in a magnetic field near saturation. Critical correlations characteristic of the predicted $z=2,\phantom{\rule{0.16em}{0ex}}d=1$ quantum phase transition occurring at saturation are shown to be consistent with the observed neutron spectra. The data is well described with a scaling function computed using a free fermion description of the spins, valid close to saturation, and the corresponding scaling limits. One of the most prominent nonuniversal spectral features of the data is a novel thermally activated longitudinal mode that remains underdamped across most of the Brillouin zone.

There is great interest in finding materials possessing quasiparticles with topological properties. Such materials may have novel excitations that exist on their boundaries which are protected against disorder. We report experimental evidence that magnons in an insulating kagome ferromagnet can have a topological band structure. Our neutron scattering measurements further reveal that one of the bands is flat due to the unique geometry of the kagome lattice. Spin wave calculations show that the measured band structure follows from a simple Heisenberg Hamiltonian with a Dzyaloshinkii-Moriya interaction. This serves as the first realization of an effectively two-dimensional topological magnon insulator--a new class of magnetic material that should display both a magnon Hall effect and protected chiral edge modes.

We report temperature-dependent neutron scattering measurements of the local dynamic structure factor in the quantum spin ladder (C$_{7}$H$_{10}$N)$_{2}$CuBr$_{4}$ in a magnetic field $H=9$~T, in its gapless quantum-critical phase. We show that the measured quantity has a scaling form consistent with expectations for a Tomonaga-Luttinger liquid with attraction. The measured Luttinger parameter $K\approx1.25$ and scaling function are in excellent agreement with density matrix renormalization group numerical calculations for the underlying spin Hamiltonian., Comment: 5 pages, 3 figures

Zero field muon spin relaxation (I¼SR) has been used to study the magnetic properties of n=2 Ruddlesden-Popper phases Sr2RMn2O7, where R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho. The results show that the size of the lanthanide ion is crucial in determining the magnetic state and dynamics of the system. Because muons are implanted throughout the bulk of the sample, impurity phases contribute only according to their volume fraction. Hence in the case of biphasic samples the data are dominated by the majority phase. Although none of our samples has a ferromagnetic ground state, colossal magnetoresistance (CMR) is observed over a wide temperature range, 4 K

We present the results of a combined magnetization, muon-spin rotation, transport and magnetotransport study of the n = 3 Ruddlesden - Popper (RP) compound . This compound adopts a layered structure in which groups of three perovskite layers alternate with single rock-salt layers. The muon-spin rotation data show that there is a sharp magnetic phase transition at 115 K. The resistance and magnetoresistance of the sample show no particular features at this temperature, but the transition affects the energy barriers associated with hopping transport. The magnetoresistance is proportional to the square of the magnetization, and is largest at low temperatures; a 40% drop in resistivity is observed in a magnetic field of 14 T at 61 K, much smaller than that measured in the related RP (perovskite) manganites which exhibit colossal magnetoresistance (CMR).

Inelastic neutron scattering measurements have been carried out to investigate the crystal field (CF) excitations of DyB 2 C 2 in order to understand the mechanisms of the quadrupolar ordering ( T Q ) observed at 24.7 K. Our analysis shows that the ground state is made up of nearly degenerate two Kramer doublets forming a ‘quasi-quartet’ ground state. This is consistent with heat capacity measurements, which have shown an estimated entropy of ∼ R ln 2 at T N (15.3 K) and an additional ∼ R ln 2 at T Q . A considerable shift and change in the width of the CF excitations and the splitting of the quasi-quartet ground state into two well-resolved doublets below T Q as well as a Zeeman splitting below T N have also been observed.

High degeneracy in ground states leads to the generation of exotic zero-energy modes, a representative example of which is the formation of molecular spin-liquid-like fluctuations in a frustrated magnet. Here we present single-crystal inelastic neutron scattering results for the frustrated magnet ${\mathrm{MgCr}}_{2}{\mathrm{O}}_{4}$, which show that a common set of finite-energy molecular spin excitation modes is sustained in both the liquid-like phase above magnetic ordering temperature ${T}_{\mathrm{N}}$ and an ordered phase with an extremely complex magnetic structure below ${T}_{\mathrm{N}}$. Based on this finding, we propose the concept of high degeneracy in excited states, which promotes local resonant elementary excitations. This concept is expected to have ramifications on our understanding of excitations in many complex systems, including not only spin but also atomic liquids, complex order systems, and amorphous systems.

High degeneracy in ground states leads to the generation of exotic zero-energy modes, a representative example of which is the formation of molecular spin-liquid-like fluctuations in a frustrated magnet. Here we present single-crystal inelastic neutron scattering results for the frustrated magnet MgCr(2)O(4), which show that a common set of finite-energy molecular spin excitation modes is sustained in both the liquid-like phase above magnetic ordering temperature T(N) and an ordered phase with an extremely complex magnetic structure below T(N). Based on this finding, we propose the concept of high degeneracy in excited states, which promotes local resonant elementary excitations. This concept is expected to have ramifications on our understanding of excitations in many complex systems, including not only spin but also atomic liquids, complex order systems, and amorphous systems.

The influence of the coherence breakdown in the Yb sublattice on the excitation spectra of YbB12-based systems and the characteristics of the crystal field potential have been studied by inelastic neutron scattering on Yb0.75Lu0.25B12 and Yb0.9Er0.1B12 samples. The crystal field potential has been determined by use of the Er3+ ions as a sensor. The spectral shape of the Yb magnetic response is considerably changed in the near-gap region due to Lu substitution with respect to the pure YbB12. The possible splitting of the Yb3+ ground-state multiplet due to the crystal field appears to be comparable with the gap energy as well as with the Kondo temperature and energy of the only inelastic peak in the YbB12 spectra at T>TKondo.

We have studied the evolution of the magnetic spectral response over a wide energy range across the γ→α transition in YbInCu 4 . Above T v , the magnetic response is that of a heavy fermion compound showing a central quasi-elastic distribution of half-width Γ ∼ 1.2 meV together with a broader peak (Γ ∼ 1.7 meV) centred around ∼ 3.2 meV representing unresolved (close-lying) crystal field excitations. As the compound transits to the α phase the magnetic response broadens to form a single lorentzian spectrum of half-width ∼ 21 meV centered on the characteristic energy of ∼ 32 meV. The Q-dependence of the magnetic intensity around the peak follows the normal Yb 3+ form factor, consistent with the persistence of the localised-ion character in the α-phase. High-energy measurements show spin-orbit excitations in both phases, but shifted upward in energy by ∼ 50±30 meV in the α-phase.

The magnetic response function and the valence of 154 Sm 1− x Y x S (0⩽ x ⩽0.33) have been determined by powder and single-crystal inelastic neutron scattering and by L III -edge spectroscopy, in the temperature range 10–300 K. Considerable influence of the Sm–Sm interactions has been observed on the excitation spectrum of the mixed-valence f-electrons.

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.

We report neutron-scattering measurements of cooperative spin excitations in antiferromagnetically ordered ${\text{BaFe}}_{2}{\text{As}}_{2}$, the parent phase of an iron pnictide superconductor. The data extend up to $\ensuremath{\sim}100\text{ }\text{meV}$ and show that the spin excitation spectrum is sharp and highly dispersive. By fitting the spectrum to a linear spin-wave model we estimate the magnon bandwidth to be in the region of 0.17 eV. The large characteristic spin-fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state.

A new family of superconductors containing layers of iron arsenide has attracted considerable interest because of their high transition temperatures (T{sub c}), some of which are >50 K, and because of similarities with the high-{sub c} copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase. This resonance, which has also been observed in several heavy-fermion superconductors is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface. Here we report inelastic neutron scattering observations of a magnetic resonance below T{sub c} in Ba{sub 0.6}K{sub 0.4}Fe{sub 2}As{sub 2}, a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry inmore » the iron arsenide superconductors.« less

The magnetic properties of the skutterudite compound $\mathrm{Ce}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ have been investigated by heat capacity and inelastic neutron scattering measurements. Heat capacity measurements reveal a broad peak centered at $125\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, whose magnitude is much larger than that expected from a Schottky anomaly due to a cubic crystal electric field. At $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, inelastic neutron scattering experiments clearly show the existence of a broad magnetic peak at $40(3)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The absence of quasielastic scattering at this temperature, together with the almost total account of the magnetic signal in the inelastic peak, shows that the excitation has a different origin than a splitting of the electronic levels due to crystal field potential. Instead, we propose a model in which the signal originates from inelastic excitations across two hybridization bands near the Fermi energy, usually referred to as a spin gap. A simple phenomenological two-level model can account for the peak in the specific heat, with a spin-gap energy of $36(2)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, which is in very good agreement with the inelastic scattering data. Further, at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the inelastic response becomes purely quasielastic, which is in agreement with the theoretical calculations. Interestingly, the spin-gap energy in $\mathrm{Ce}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ exhibits a universal scaling behavior with the Kondo temperature ${T}_{K}$. The relation between the spin-gap energy and the associated anomalies in the heat capacity or thermal expansion is discussed for a series of Ce- and Yb-based compounds.

A synchrotron x-ray diffraction study of the crystallographic structure of $\mathrm{Pr}{\mathrm{O}}_{2}$ in the Jahn-Teller distorted phase is reported. The distortion of the oxygen sublattice, which was previously ambiguous, is shown to be a chiral structure in which neighboring oxygen chains have opposite chiralities. A temperature dependent study of the magnetic excitation spectrum, probed by neutron inelastic scattering, is also reported. Changes in the energies and relative intensities of the crystal field transitions provide an insight into the interplay between the static and dynamic Jahn-Teller effects.

Mixed-valence phenomena occurring in the "black" (B) and "gold" (G) phases of Sm1-x Yx S have been studied by x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering. Lattice-constant and phonon-dispersion results confirm that the valence instability occurs already inside the B phase. On the other hand, pronounced temperature anomalies in the thermal expansion \alpha(T ), as well as in the Sm mean-square displacements denote the onset of the B-G transition for the compositions x = 0.33 and 0.45. It is argued that these anomalies primarily denote an effect of electron-phonon coupling. The magnetic spectral response, measured on both powder and single crystals, is dominated by the Sm2+ spin-orbit component close to 36 meV. A strongly overdamped Sm3+ contribution appears only for x >= 0.33 near room-temperature. The quasielastic signal is strongly suppressed below 70 K, reflecting the formation of the singlet mixed-valence ground state. Quite remarkably, the signal around 36 meV is found, from the single-crystal spectra, to arise from two distinct, dispersive, interacting branches. The lower peak, confirmed to exist from x = 0.17 to x = 0.33 at least, is tentatively ascribed to an excitation specific to the mixed-valence regime, reminiscent of the "exciton" peak reported previously for SmB6 .

Neutron inelastic scattering investigations have been performed on the spin tetrahedral system ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{X}_{2}$ $(X=\mathrm{Cl},\mathrm{Br})$. We report the observation of magnetic excitations with a dispersive component in both compounds, associated with the three-dimensional incommensurate magnetic order that develops below ${T}_{N}^{\mathit{Cl}}=18.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{N}^{\mathit{Br}}=11.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The excitation in ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}$ softens as the temperature approaches ${T}_{N}^{\mathit{Cl}}$, leaving diffuse quasi-elastic scattering above the transition temperature. In the bromide, the excitations are present well above ${T}_{N}^{\mathit{Br}}$, which might be attributed to the presence of a degree of low dimensional correlations above ${T}_{N}^{\mathit{Br}}$ in this compound.

A peak is detected in the high-energy inelastic neutron scattering spectra of ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ and ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}{\mathrm{N}}_{3},$ which is associated with the intermultiplet transition in the $4f$ shell of samarium. The peak in the nitride ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}{\mathrm{N}}_{3}$ is situated at a lower energy (163 meV) as compared with the parent compound ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ (177 meV). The peak position provides direct information on the strength of the exchange field on Sm in both compounds: ${B}_{\mathrm{ex}}=380$ and 270 T in ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ and ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}{\mathrm{N}}_{3},$ respectively. The 30% reduction in ${B}_{\mathrm{ex}}$ as a result of nitrogenating ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ is consistent with the earlier discovered similar effect in ${\mathrm{Gd}}_{2}{\mathrm{Fe}}_{17}.$ High-field $(Bl~50\mathrm{T})$ magnetization curves have been measured on an oriented powder sample of ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}{\mathrm{N}}_{3}$ and analyzed using the newly determined ${B}_{\mathrm{ex}}.$ The leading crystal field parameter is thus found to be ${A}_{2}^{0}〈{r}^{2}〉=\ensuremath{-}28\mathrm{meV}.$

Inelastic neutron scattering measurements of the magnetic excitations in YbNi2B2C are presented. Measurements on a polycrystalline sample of YbNi2B2C reveal an energy spectrum that is broad and extends above 150 meV. Three inelastic peaks are observed in the data, centred at energies of 3 meV, 17 meV and 43 meV. Measurements on single crystals show that the intensity of the scattering is Q dependent. A possible interpretation of the data in terms of broadened crystal field transitions is proposed.

Neutron spectroscopic measurements of the magnetic excitations in PrO2 reveal (1) sharp peaks characteristic of transitions between levels of the 4f(1) configuration of Pr4+ split by the cubic crystal field, and (2) broad bands of scattering centered near 30 and 160 meV. We present a simple model based on a vibronic Hamiltonian that accounts for these contrasting features of the data. The analysis shows that 90%+/-10% of the Pr ions have a localized 4f(1) configuration and provides strong evidence for a dynamic Jahn-Teller effect in the gamma(8) electronic ground state.

The magnetic response of CeNi 0.9 Cu 0.1 has been measured over a wide range of temperatures using both inelastic neutron scattering and detailed DC and AC magnetic susceptibility measurements. The purpose of these measurements was to clarify the role of Ce–Ni interactions in the formation of the CeNi intermediate-valence ground state. The results indicate that substitution of Ni by Cu results in a strong decrease of hybridization between the 4f and conduction band electron states, with the Ce ions changing character from an intermediate valence state in CeNi to a heavy-fermion short-range-correlated state in CeNi 0.9 Cu 0.1 .

Disrupting coherence on the Yb sublattice produces significant changes on the magnetic spectral response of the Kondo insulator YbB12, as evidenced by inelastic neutron scattering measurements on Yb1−xLuxB12 solid solutions. However, the spin gap existing in pure YbB12 is not suppressed by substitution of Lu for Yb.

Recent advances in superconducting technology and cryocooler refrigeration have resulted in a new generation of advanced superconducting magnets for neutron beam applications. These magnets have outstanding parameters such as high homogeneity and stability at highest magnetic fields possible, a reasonably small stray field, low neutron scattering background and larger exposure to neutron detectors. At the same time the pulse tube refrigeration technology provides a complete re-condensing regime which allows to minimise the requirements for cryogens without introducing additional noise and mechanical vibrations. The magnets can be used with dilution refrigerator insert which expands the temperature range from 20mK to 300K. Here we are going to present design, test results and the operational data of the 14T magnet for neutron diffraction and the 9T wide angle chopper magnet for neutron spectroscopy developed by Oxford Instruments in collaboration with ISIS neutron source. First scientific results obtained from the neutron scattering experiments with these magnets are also going to be discussed.

The structure of the (SrxCa1−x)14Cu24O41 series contains both Cu2O3 spin-ladder layers and CuO2 spin chains. Previous measurements on the undoped sample (x=1) showed all the holes were located on the chains, with the magnetic Cu2+ ions on the chain forming a well-defined dimer configuration. We have performed inelastic neutron scattering on a single crystal of the Ca-doped sample Sr2.5Ca11.5Cu24O41. The results show that two magnetic states now coexist on the chain, with one hole per formula unit migrating to the ladders.

Lattice dynamics and magneto-elastic coupling effects have been studied in the Kondo insulator YbB12 by means of inelastic neutron scattering. The analysis of the phonon density of states, dispersion, and symmetry properties is presented in connection with a possible magneto-elastic coupling. Manifestation of such effects was found for the phonons corresponding to the vibrations of Yb atoms.

Inelastic neutron scattering is used to measure the spin excitation spectrum of the Heisenberg S = 1/2 ladder material (C7H10N)(2)CuBr4 in its entirety, both in the gapped spin liquid and the magnetic field-induced Tomonaga-Luttinger spin liquid regimes. A fundamental change of the spin dynamics is observed between these two regimes. Density matrix renormalization group calculations quantitatively reproduce and help understand the observed commensurate and incommensurate excitations. The results validate long-standing quantum field-theoretical predictions but also test the limits of that approach.