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The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in reports of topological surface superconductivity and Majorana zero modes (MZMs) in FeTe0.55Se0.45. However, the topological features and superconducting properties are not observed uniformly across the sample surface. The understanding and practical control of these electronic inhomogeneities present a prominent challenge for potential applications. Here, we combine neutron scattering, scanning angle-resolved photoemission spectroscopy, and microprobe composition and resistivity measurements to characterize the electronic state of Fe1+yTe1-xSex. We establish a phase diagram in which the superconductivity is observed only at sufficiently low Fe concentration, in association with distinct antiferromagnetic correlations, whereas the coexisting topological surface state occurs only at sufficiently high Te concentration. We find that FeTe0.55Se0.45 is located very close to both phase boundaries, which explains the inhomogeneity of superconducting and topological states. Our results demonstrate the compositional control required for use of topological MZMs in practical applications.

In quantum magnets, magnetic moments fluctuate heavily and are strongly entangled with each other, a fundamental distinction from classical magnetism. Here, with inelastic neutron scattering measurements, we probe the spin correlations of the honeycomb lattice quantum magnet YbCl3. A linear spin wave theory with a single Heisenberg interaction on the honeycomb lattice, including both transverse and longitudinal channels of the neutron response, reproduces all of the key features in the spectrum. In particular, we identify a Van Hove singularity, a clearly observable sharp feature within a continuum response. The demonstration of such a Van Hove singularity in a two-magnon continuum is important as a confirmation of broadly held notions of continua in quantum magnetism and additionally because analogous features in two-spinon continua could be used to distinguish quantum spin liquids from merely disordered systems. These results establish YbCl3 as a benchmark material for quantum magnetism on the honeycomb lattice., Honeycomb lattices with interacting spins can host rich magnetic behaviour; however, typically features are complicated by additional interactions. Here, the authors perform neutron scattering on YbCl3, which exhibits near perfect two-dimensional magnetism, providing a benchmark for other materials.

New surprises continue to be revealed about ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the ${\mathrm{CuO}}_{2}$ planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in ${\mathrm{CuO}}_{6}$ octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the ``chiral'' phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.

Single crystals of the underscreened Kondo ferromagnet Yb14MnSb11 were investigated using polarised neutron diffraction and magnetisation measurements. The magnitude and direction of the ma...

We investigate the physical properties of the Shastry-Sutherland lattice material BaNd$_2$ZnO$_5$. Neutron diffraction, magnetic susceptibility, and specific heat measurements reveal antiferromagnetic order below 1.65 K. The magnetic order is found to be a 2-$\boldsymbol{Q}$ magnetic structure with the magnetic moments lying in the Shastry-Sutherland lattice planes comprising the tetragonal crystal structure of BaNd$_2$ZnO$_5$. The ordered moment for this structure is 1.9(1) $\mu_B$ per Nd ion. Inelastic neutron scattering measurements reveal that the crystal field ground state doublet is well separated from the first excited state at 8 meV. The crystal field Hamiltonian is determined through simultaneous refinement of models with both the LS coupling and intermediate coupling approximations to the inelastic neutron scattering and magnetic susceptibility data. The ground state doublet indicates that the magnetic moments lie primarily in the basal plane with magnitude consistent with the size of the determined ordered moment., Comment: 13 pages, 8 figures

In conventional quasi-one-dimensional antiferromagnets with quantum spins, magnetic excitations are carried by either magnons or spinons in different energy regimes: they do not coexist independently, nor could they interact with each other. In this Letter, by combining inelastic neutron scattering, quantum Monte Carlo simulations and Random Phase Approximation calculations, we report the discovery and discuss the physics of the coexistence of magnons and spinons and their interactions in Botallackite-Cu2(OH)3Br. This is a unique quantum antiferromagnet consisting of alternating ferromagnetic and antiferromagnetic Spin-1/2 chains with weak inter-chain couplings. Our study presents a new paradigm where one can study the interaction between two different types of magnetic quasiparticles, magnons and spinons., Comment: Accepted by PRL

We use neutron scattering and bulk property measurements to determine the single-ion crystal-field Hamiltonians of delafossites ${\mathrm{KErSe}}_{2}$ and ${\mathrm{CsErSe}}_{2}$. These two systems contain planar equilateral triangular Er lattices arranged in two stacking variants: rhombohedral (for K) or hexagonal (Cs). Our analysis shows that regardless of the stacking order both compounds exhibit an easy-plane ground-state doublet with large ${J}_{z}=1/2$ terms and the potential for significant quantum effects, making them candidates for quantum-spin-liquid or other exotic ground states.

We present inelastic neutron scattering (INS) measurements of magnetic excitations in YbMnBi$_2$, which reveal features consistent with a direct coupling of magnetic excitations to Dirac fermions. In contrast with the large broadening of magnetic spectra observed in antiferromagnetic metals such as the iron pnictides, here the spin waves exhibit a small but resolvable intrinsic width, consistent with our theoretical analysis. The subtle manifestation of spin-fermion coupling is a consequence of the Dirac nature of the conduction electrons, including the vanishing density of states near the Dirac points. Accounting for the Dirac fermion dispersion specific to \ymb\ leads to particular signatures, such as the nearly wave-vector independent damping observed in the experiment., Comment: 8 pages, 11 figures including both main text and supplemental material

${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_{4}$ develops charge and spin stripe orders at temperatures of roughly 200 K, with modulation wave vectors that are temperature independent. Various probes of spin and charge response have provided independent evidence for some sort of change below $\ensuremath{\sim}50$ K. In combination with a new set of neutron scattering measurements, we propose a unified interpretation of all of these observations in terms of a freezing of Ni-centered charges stripes, together with a glassy ordering of the spin stripes that shows up in neutron scattering as a slight rotation of the average spin direction.

The structural and magnetic properties of a glaserite-type Na2BaFe(VO4)2 compound, featuring a triangular magnetic lattice of Fe2+ (S = 2), are reported. Temperature dependent X-ray single crystal studies indicate that at room temperature the system adopts a trigonal P3m1 structure and undergoes a structural phase transition to a C2/c monoclinic phase slightly below room temperature (Ts = 288 K). This structural transition involves a tilting of Fe-O-V bond angles and strongly influences the magnetic correlation within the Fe triangular lattice. The magnetic susceptibility measurements reveal a ferromagnetic transition near 7 K. Single crystal neutron diffraction confirms the structural distortion and the ferromagnetic spin ordering in Na2BaFe(VO4)2. The magnetic structure of the ordered state is modeled in the magnetic space group C2'/c' that implies a ferromagnetic order of the a and c moment components and antiferromagnetic arrangement for the b components. Overall, the Fe magnetic moments form ferromagnetic layers that are stacked along the c-axis, where the spins point along one of the (111) facets of the FeO6 octahedron.

We report inelastic neutron scattering experiments in Ca2Y2Cu5O10 and map out the full one magnon dispersion which extends up to a record value of 53 meV for frustrated ferromagnetic (FM) edge-sharing CuO2 chain (FFESC) cuprates. A homogeneous spin-1/2 chain model with a FM nearest-neighbor (NN), an antiferromagnetic (AFM) next-nearest-neighbor (NNN) inchain, and two diagonal AFM interchain couplings (ICs) analyzed within linear spin-wave theory (LSWT) reproduces well the observed strong dispersion along the chains and a weak one perpendicularly. The ratio R=|J_{a2}/J_{a1}| of the FM NN and the AFM NNN couplings is found as ~0.23, close to the critical point Rc=1/4 which separates ferromagnetically and antiferromagnetically correlated spiral magnetic ground states in single chains, whereas Rc>0.25 for coupled chains is considerably upshifted even for relatively weak IC. Although the measured dispersion can be described by homogeneous LSWT, the scattering intensity appears to be considerably reduced at ~11.5 and ~28 meV. The gap-like feature at 11.5 meV is attributed to magnon-phonon coupling whereas based on DMRG simulations of the dynamical structure factor the gap at 28 meV is considered to stem partly from quantum effects due to the AFM IC. Another contribution is ascribed to the intrinsic superstructure from the distorting incommensurate pattern of CaY cationic chains adjacent to the CuO2 ones. It gives rise to non-equivalent CuO4 units and Cu-O-Cu bond angles Phi and a resulting distribution of all exchange integrals. The J's fitted by homogeneous LSWT are regarded as average values. The record value of the FM NN integral J1=24 meV among FFESC cuprates can be explained by a non-universal Phi (not 90 deg.) and Cu-O bond length dependent anisotropic mean direct FM Cu-O exchange K_{pd}~120 meV. Enhanced K_{pd} values are also needed to compensate a significant AFM J_{dd} > ~6 meV.

Lead chalcogenides have exceptional thermoelectric properties and intriguing anharmonic lattice dynamics underlying their low thermal conductivities. An ideal material for thermoelectric efficiency is the phonon glass–electron crystal, which drives research on strategies to scatter or localize phonons while minimally disrupting electronic-transport. Anharmonicity can potentially do both, even in perfect crystals, and simulations suggest that PbSe is anharmonic enough to support intrinsic localized modes that halt transport. Here, we experimentally observe high-temperature localization in PbSe using neutron scattering but find that localization is not limited to isolated modes – zero group velocity develops for a significant section of the transverse optic phonon on heating above a transition in the anharmonic dynamics. Arrest of the optic phonon propagation coincides with unusual sharpening of the longitudinal acoustic mode due to a loss of phase space for scattering. Our study shows how nonlinear physics beyond conventional anharmonic perturbations can fundamentally alter vibrational transport properties., To optimize the performance of lead chalcogenides for thermoelectric applications, strategies to further reduce the crystal’s thermal conductivity is required. Here, the authors discover anharmonic localized vibrations in PbSe crystals for optimizing the crystal’s vibrational transport properties.

We study the correlated quantum magnet, YbCl$_3$, with neutron scattering, magnetic susceptibility, and heat capacity measurements. The crystal field Hamiltonian is determined through simultaneous refinements of the inelastic neutron scattering and magnetization data. The ground state doublet is well isolated from the other crystal field levels and results in an effective spin-1/2 system with local easy plane anisotropy at low temperature. Cold neutron spectroscopy shows low energy excitations that are consistent with nearest neighbor antiferromagnetic correlations of reduced dimensionality., Comment: main paper plus supplemental information

Exotic magnetic phases often have frustration as a key ingredient. Chemically and structurally simple materials are particularly prized as testbeds for concepts found in more complex ones. Using a combination of neutron diffraction, inelastic neutron scattering, and laboratory-based characterization methods, the authors find long-range antiferromagnetic order in Nd${}_{2}$O${}_{3}$ only below 0.55 K, despite indications that the magnetic energy scales are significantly higher. The ground state possesses alternating stripes of local moments in the plane of the triangular lattice and is characterized by strong XY anisotropy that originates from the local crystal field. These results suggest that Nd${}_{2}$O${}_{3}$ may be a model system for studying frustration originating from competing interactions between magnetic moments subject to strong spin-orbit coupling on a centrosymmetric lattice.

In spinels ACr2O4 (A=Mg, Zn), realization of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN = 12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et al. [Pow. Diff. 17, 230 (2002)], while other features depend on sample or cooling protocol. A complex, partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1 = (1/2 1/2 0), k2 = (1 0 1/2 ) appear. The ordered moment reaches 1.94(3) μB/Cr3+ for k1 and 2.08(3) μB/Cr3+ for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1−10] diagonals and a smaller c component.We use inelastic neutron scattering to investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV.We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of clusterlike excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

The electronic ground state of ${\mathrm{Ca}}_{3}{\mathrm{LiOsO}}_{6}$ was recently considered within an intermediate coupling regime that revealed $J=3/2$ spin-orbit entangled magnetic moments. Through inelastic neutron scattering and density functional theory we investigate the magnetic interactions and probe how the magnetism is influenced by the change in hierarchy of interactions as we move from ${\mathrm{Ca}}_{3}{\mathrm{LiOsO}}_{6}$ ($5{d}^{3}$) to ${\mathrm{Ca}}_{3}{\mathrm{LiRuO}}_{6}$ ($4{d}^{3}$). An alteration of the spin gap and ordered local moment is observed, however the magnetic structure, N\'eel temperature, and exchange interactions are unaltered. To explain this behavior it is necessary to include both spin-orbit coupling and hybridization, indicating the importance of an intermediate coupling approach when describing $5d$ oxides.

In this study, Heusler alloys of Ni-Mn-Ga compositions demonstrate ferromagnetic shape memory effect in the martensitic state. The transformation temperature and the chemical order depend strongly on the composition. In the current work, the structure and chemical order of the martensitic phase of Ni1.91Mn1.29Ga0.8 were studied using neutron diffraction; the diffraction pattern was refined using the FullProf software. It was determined that the structural transition occurs around 330 K. At room temperature, 300 K, which is below the martensite transformation temperature, all the Bragg reflections can be described by a monoclinic lattice with a symmetry of space group P 1 2/m 1 and lattice constants of a = 4.23047(7) [A], b = 5.58333(6) [A], c = 21.0179(2) [A], beta = 90.328(1). The chemical order is of critical importance in these alloys, and it was previously studied at 363 K. Analysis of the neutron diffraction in the monoclinic phase shows that the chemical order is maintained during the martensitic transformation.

Tb 6 FeBi 2 adopts a noncentrosymmetric crystal structure and orders ferromagnetically at T C1 = 250 K with an additional magnetic transition at T C2 = 60 K. The low temperature magnetoelastic response in this material is strong, and is enhanced by cobalt substitution. Here, the temperature dependence of the atomic and magnetic structure of Tb 6 Fe 1 − x Co x Bi 2 ( x = 0, 0.125, 0.25, and 0.375) is reported from powder X-ray diffraction (XRD) and powder neutron diffraction (PND) measurements. Below the Neel temperature a ferrimagnetic ordering between the terbium and iron moments exists in all compounds studied. Related to the enhanced magnetostructural response, the Co-doped compounds undergo a crystallographic phase transition below about 60 K. This transition also involves a canting of the magnetic moments away from the c -axis. The structural transition is sluggish and not fully completed in the parent Tb 6 FeBi 2 compound, where a mixture of monoclinic and hexagonal phases is identified below 60 K. The spin reorientation transition is discussed in terms of competing exchange interactions and magnetocrystalline anisotropies of the two Tb sites and Fe/Co sublattices.

$\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$, an unconventional superconductor, is known to possess an incommensurate spin-density wave instability driven by Fermi surface nesting. Here we report a static spin-density wave ordering with a commensurate propagation vector ${\mathbf{q}}_{\mathrm{c}}=(0.25\phantom{\rule{0.16em}{0ex}}0.25\phantom{\rule{0.16em}{0ex}}0)$ in Fe-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$, despite the magnetic fluctuations persisting at the incommensurate wave vectors ${\mathbf{q}}_{\mathrm{ic}}=(0.3\phantom{\rule{0.16em}{0ex}}0.3\phantom{\rule{0.16em}{0ex}}L)$ as in the parent compound. The latter feature is corroborated by the first-principles calculations, which show that Fe substitution barely changes the nesting vector of the Fermi surface. These results suggest that in addition to the known incommensurate magnetic instability, $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ is also in proximity to a commensurate magnetic tendency that can be stabilized via Fe doping.

While it is mostly well-known for its excellent thermoelectric properties at high temperatures, Yb${}_{14}$MnSb${}_{11}$ has also come to be characterized as a ferromagnetic semiconductor. Such systems support ferromagnetic correlations similar to metallic compounds, although with semiconducting electronic configurations. As such, they are promising materials for use in spintronic devices. The authors here use neutron scattering techniques to examine the distribution of the magnetization density and the energy scale of the magnetic exchange interactions in the Yb${}_{14}$MnSb${}_{11}$ compound. They find that there is a large ferromagnetic moment localized to the Mn site in the crystal structure with a `Kondo cloud' of negative moments distributed over the other atomic sites. The results provide long sought answers to questions regarding the valence, magnetization distribution, and energy scales of the magnetic interactions in this compound. The ferromagnetic spin-wave spectrum supports this premise and is well characterized by inclusion of nearest and next-nearest neighbor exchange interactions.

We report a neutron diffraction study of the multiferroic mechanism in ${({\mathrm{ND}}_{4})}_{2}{\mathrm{FeCl}}_{5}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{D}}_{2}\mathrm{O}$, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at ${T}_{N}=7.3$ K, followed by a second transition to an IC cycloidal spin state at ${T}_{FE}=6.8$ K, the latter of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling, as evidenced by the observations of both odd and even higher-order harmonics associated with the cycloid wave vector, and a weak commensurate phase that coexists with the IC phase. The second-order harmonic appears at ${T}_{FE}$, thereby providing unambiguous evidence that the onset of the electric polarization is accompanied by a lattice modulation due to spin-lattice interaction. The neutron results, in conjunction with the negative thermal expansion and large magnetostriction observed in Ref. [19], indicate that spin-lattice coupling plays a critical role in the ferroelectric mechanism of ${({\mathrm{ND}}_{4})}_{2}{\mathrm{FeCl}}_{5}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{D}}_{2}\mathrm{O}$.

$\mathrm{GeC}{\mathrm{u}}_{2}{\mathrm{O}}_{4}$ spinel exhibits a tetragonal structure due to the strong Jahn-Teller distortion associated with $\mathrm{C}{\mathrm{u}}^{2+}$ ions. We show that its magnetic structure can be described as slabs composed of a pair of layers with orthogonally oriented spin-$\frac{1}{2}$ Cu chains in the basal $ab$ plane. The spins between the two layers within a slab are collinearly aligned while the spin directions of neighboring slabs are perpendicular to each other. Interestingly, we find that spins along each chain form an unusual up-up-down-down (UUDD) pattern, suggesting a non-negligible nearest-neighbor biquadratic exchange interaction in the effective classical spin Hamiltonian. We hypothesize that spin-orbit coupling and orbital mixing of $\mathrm{C}{\mathrm{u}}^{2+}$ ions in this system are non-negligible, which calls for future calculations using perturbation theory with extended Hilbert (spin and orbital) space and calculations based on density functional theory including spin-orbit coupling and looking at the global stability of the UUDD state.

We report quantum transport and Dirac fermions in YbMnBi2 single crystals. YbMnBi2 is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, and small cyclotron mass indicate the presence of Dirac fermions. Lastly, angular-dependent magnetoresistance indicates a possible quasi-two-dimensional Fermi surface, whereas the deviation from the nontrivial Berry phase expected for Dirac states suggests the contribution of parabolic bands at the Fermi level or spin-orbit coupling.

The low energy spin excitation spectrum of the breathing pyrochlore ${\mathrm{Ba}}_{3}{\mathrm{Yb}}_{2}{\mathrm{Zn}}_{5}{\mathrm{O}}_{11}$ has been investigated with inelastic neutron scattering. Several nearly resolution limited modes with no observable dispersion are observed at 250 mK while, at elevated temperatures, transitions between excited levels become visible. To gain deeper insight, a theoretical model of isolated ${\mathrm{Yb}}^{3+}$ tetrahedra parametrized by four anisotropic exchange constants is constructed. The model reproduces the inelastic neutron scattering data, specific heat, and magnetic susceptibility with high fidelity. The fitted exchange parameters reveal a Heisenberg antiferromagnet with a very large Dzyaloshinskii-Moriya interaction. Using this model, we predict the appearance of an unusual octupolar paramagnet at low temperatures and speculate on the development of intertetrahedron correlations.

The descloizite-type compound, SrMn(VO4)(OH), was synthesized as large single crystals (1-2mm) using a high-temperature high-pressure hydrothermal technique. X-ray single crystal structure analysis reveals that the material crystallizes in the acentric orthorhombic space group of P212121 (no. 19), Z = 4. The structure exhibits a one-dimensional feature, with [MnO4] chains propagating along the a-axis which are interconnected by VO4 tetrahedra. Raman and infrared spectra were obtained to identify the fundamental vanadate and hydroxide vibrational modes. Magnetization data reveal a broad maximum at approximately 80 K, arising from one-dimensional magnetic correlations with intrachain exchange constant of J/kB = 9.97(3) K between nearest Mn neighbors and a canted antiferromagnetic behavior below TN = 30 K. Single crystal neutron diffraction at 4 K yielded a magnetic structure solution in the lower symmetry of the magnetic space group P21 with two unique chains displaying antiferromagnetically ordered Mn moments oriented nearly perpendicular to the chain axis. Lastly, the presence of the Dzyaloshinskii Moriya antisymmetric exchange interaction leads to a slight canting of the spins and gives rise to a weak ferromagnetic component along the chain direction.

Polycrystalline samples of the quarter-doped manganites $R_{0.75}$Ca$_{0.25}$MnO$_3$ ($R$ = Y, Tb, Dy, Ho, and Er) were studied by X-ray diffraction and AC/DC susceptibility measurements. All five samples are orthorhombic and exhibit similar magnetic properties: enhanced ferromagnetism below $T_1$ ($\sim80$ K) and a spin glass (SG) state below $T_{SG}$ ($\sim30$ K). With increasing $R^{3+}$ ionic size, both $T_1$ and $T_{SG}$ generally increase. The single crystal neutron diffraction results on Tb$_{0.75}$Ca$_{0.25}$MnO$_3$ revealed that the SG state is mainly composed of a short-range ordered version of a novel canted (i.e. noncollinear) antiferromagnetic spin state. Furthermore, calculations based on the double exchange model for quarter-doped manganites reveal that this new magnetic phase provides a transition state between the ferromagnetic state and the theoretically predicted spin-orthogonal stripe phase., Comment: 10 pages, 10 figures. Submitted to Physical Review B

The structural and the magnetic properties of ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$ $(0\ensuremath{\le}x\ensuremath{\le}0.85)$ and ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$ $(0\ensuremath{\le}x\ensuremath{\le}0.4)$ have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), x-ray diffraction measurements, and first principles calculations. The structural and magnetic phase diagrams of ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$ and ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$ as a function of Ag/Pd composition are reported. The end member, ${\mathrm{CeCu}}_{6}$, undergoes a structural phase transition from an orthorhombic $(Pnma)$ to a monoclinic $(P{2}_{1}/c)$ phase at 240 K. In ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$, the structural phase transition temperature $({T}_{s})$ decreases linearly with Ag concentration and extrapolates to zero at ${x}_{S}$ $\ensuremath{\approx}0.1$. The structural transition in ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$ remains unperturbed with Pd substitution within the range of our study. The lattice constant $b$ slightly decreases with Ag/Pd doping, whereas $a$ and $c$ increase with an overall increase in the unit cell volume. Both systems, ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$ and ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$, exhibit a magnetic quantum critical point (QCP), at $x\ensuremath{\approx}0.2$ and $x\ensuremath{\approx}0.05$, respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector $({\ensuremath{\delta}}_{1}0$ ${\ensuremath{\delta}}_{2})$, where ${\ensuremath{\delta}}_{1}\ensuremath{\sim}0.62,{\ensuremath{\delta}}_{2}\ensuremath{\sim}0.25,x=0.125$ for ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$ and ${\ensuremath{\delta}}_{1}\ensuremath{\sim}0.64,{\ensuremath{\delta}}_{2}\ensuremath{\sim}0.3,x=0.3$ for ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$. The magnetic structure consists of an amplitude modulation of the Ce moments which are aligned along the $c$ axis of the orthorhombic unit cell.

We have studied the lattice dynamics of a Ni 54 Mn 23 Al 23 (at.%) Heusler single-crystalline alloy by means of neutron scattering and ultrasonic techniques. Results show the existence of a number of precursor phenomena. We have found an anomaly (dip) in the low TA 2 phonon branch at the wave number ξ 0 ≈ 0.33 (in reciprocal lattice units) that becomes more pronounced (phonon softening) with decreasing temperature. We have also observed softening of the associated shear elastic constant ( C ′ ) with decreasing temperature. Ultrasonic measurements under applied magnetic field, both isothermally and varying the temperature show that the values of elastic constants depend on magnetic order thus evidencing magnetoelastic coupling.

Neutron incoherent scattering measurements were conducted on Zircaloy-4 round bars. The specimens were charged in a tube furnace at 430 °C, using a 12.5 vol. % hydrogen in an argon mixture for 30, 60, and 90 minutes at 13.8 kPa pressure. The volume-average neutron diffraction measurements showed the presence of the face-centered-cubic delta zirconium hydride (δ-ZrH2) phase in the hydrogenated specimens. The assessment of the background in the diffraction profiles due to the incoherent scattering from the hydrogen atoms was carried out by performing inelastic scans around zero energy transfer and at a fixed two-theta value for which there was only flat background and no coherent scattering. To estimate the relative amount of hydrogen in the Zircaloy-4 samples, the increase in incoherent scattering intensities with hydrogen content was calibrated using samples for which the hydrogen content was known. Measurement of the background scattering from locations within the round bar was also performed to map the distribution of hydrogen content.

In this study, we describe why Ising spin chains with competing interactions in SrHo2O4 segregate into ordered and disordered ensembles at low temperatures (T). Using elastic neutron scattering, magnetization, and specific heat measurements, the two distinct spin chains are inferred to have Neel (↑↓↑↓) and double-Neel (↑↑↓↓) ground states, respectively. Below TN = 0.68(2)K, the Neel chains develop three-dimensional long range order (LRO), which arrests further thermal equilibration of the double-Neel chains so they remain in a disordered incommensurate state for T below TS = 0.52(2)K. SrHo2O4 distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a quasi–d–dimensional spin system can preclude order in d + 1 dimensions.

We determine the low-field ordered magnetic phases of the $S=1$ dimerized antiferromagnet ${\mathrm{Ba}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{8}$ using single-crystal neutron diffraction. We find that for magnetic fields between ${\ensuremath{\mu}}_{0}H=8.80\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ and $10.56\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ applied along the $[1\overline{1}0]$ direction the system exhibits spin density wave order with incommensurate wave vectors of type $(\ensuremath{\eta},\ensuremath{\eta},\ensuremath{\epsilon})$. For ${\ensuremath{\mu}}_{0}Hg10.56\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the $[hh0]$ direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order.

The ferrimagnetic spinel $\mathrm{CoV_2O_4}$ has been a topic of intense recent interest, both as a frustrated insulator with unquenched orbital degeneracy and as a near-itinerant magnet which can be driven metallic with moderate applied pressure. Here, we report on our recent neutron diffraction and inelastic scattering measurements on powders with minimal cation site disorder. Our main new result is the identification of a weak ($\frac{\Delta a}{a} \sim 10^{-4}$), first order structural phase transition at $T^*$ = 90 K, the same temperature where spin canting was seen in recent single crystal measurements. This transition is characterized by a short-range distortion of oxygen octahedral positions, and inelastic data further establish a weak $\Delta\sim 1.25 meV$ spin gap at low temperature. Together, these findings provide strong support for the local orbital picture and the existence of an orbital glass state at temperatures below $T^*$., Comment: 8 pages, 5 figures, To be published in Physical Review B

We report a study of the evolution of the structure of the delafossite-derived compounds YCuO2+δ as a function of oxygen stoichiometry. The structural details of the oxygenated material YCuO2.5 were examined by means of high-resolution neutron powder diffraction. We confirmed that YCuO2.5 adopts an orthorhombic superstructure ( a = 3 a H , b = c H , c = 2 a H ) in which the anions are located at the center of corner-sharing triangles to form undulating chains of Cu2+ ( s = 1 / 2 ), running along a-axis direction.

The $A{R}_{2}{\text{O}}_{4}$ family ($R=\text{rare}$ earth) has recently been attracting interest as a new series of frustrated magnets, with the magnetic $R$ atoms forming zigzag chains running along the $c$ axis. We have investigated polycrystalline ${\mathrm{BaNd}}_{2}{\mathrm{O}}_{4}$ with a combination of magnetization, heat-capacity, and neutron powder diffraction measurements. Magnetic Bragg peaks are observed below ${T}_{N}=1.7\phantom{\rule{4pt}{0ex}}\mathrm{K}$, and they can be indexed with a propagation vector of $\stackrel{P\vec}{k}=(0,1/2,1/2)$. The signal from magnetic diffraction is well described by long-range ordering of only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations (double N\'eel state). Furthermore, low-temperature magnetization and heat-capacity measurements reveal two magnetic-field-induced spin transitions at 2.75 and 4 T for $T=0.46\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The high-field phase is paramagnetic, while the intermediate-field state may arise from a spin transition of the long-range ordered Nd chains. One possible candidate for the field-induced ordered state corresponds to an up-up-down intrachain spin configuration, as predicted for a classical ${J}_{1}\ensuremath{-}{J}_{2}$ Ising chain with a double N\'eel ground state in zero field.

We present a study of the crystal structure and physical properties of single crystals of a new Fe-based ternary compound, Zr2-xFe4Si16-y(x = 0.81, y = 6.06). Zr1.19Fe4Si9.94 is a layered compound, where stoichiometric β-FeSi2-derived slabs are separated by Zr-Si planes with substantial numbers of vacancies. High resolution transmission electron microscopy (HRTEM) experiments show that these Zr-Si layers consist of 3.5 nm domains where the Zr and Si vacancies are ordered within a supercell sixteen times the volume of the stoichiometric cell. Within these domains, the occupancies of the Zr and Si sites obey symmetry rules that permit only certain compositions, none of which by themselves reproduce the average composition found in x-ray diffraction experiments. Magnetic susceptibility and magnetization measurements reveal a small but appreciable number of magnetic moments that remain freely fluctuating to 1.8 K, while neutron diffraction confirms the absence of bulk magnetic order with a moment of 0.2μB or larger down to 1.5 K. Electrical resistivity measurements find that Zr1.19Fe4Si9.94 is metallic, and the modest value of the Sommerfeld coefficient of the specific heat γ = C/T suggests that quasi-particle masses are not particularly strongly enhanced. The onset of superconductivity at Tc ≃ 6 K results in a partial resistive transition and a small Meissner signal, although a bulk-like transition is found in the specific heat. Sharp peaks in the ac susceptibility signal the interplay of the normal skin depth and the London penetration depth, typical of a system in which nano-sized superconducting grains are separated by a non-superconducting host. Ultra low field differential magnetic susceptibility measurements reveal the presence of a surprisingly large number of trace magnetic and superconducting phases, suggesting that the Zr-Fe-Si ternary system could be a potentially rich source of new bulk superconductors.

A layered perovskite ${\mathrm{LaSrVO}}_{4}$ was studied by neutron diffraction, pair distribution function measurement using synchrotron x-ray, susceptibility, and specific heat measurements, and first-principles calculation. The results show (i) a weak structural distortion around 100 K with the existence of orbital fluctuations both above and below it; (ii) the absence of the long range magnetic ordering down to 0.35 K but the appearance of a short range magnetic ordering around 11 K with a ${T}^{2}$ behavior of the specific heat below it. Meanwhile, the calculation based on the density functional theory predicts a magnetic ordered ground state. All facts indicate a melting of the magnetic ordering due to the orbital fluctuations in ${\mathrm{LaSrVO}}_{4}$, which makes it a rare candidate for the spin-orbital liquid state related to ${t}_{2g}$ orbitals.

The spinel vanadates have become a model family for exploring orbital order on the frustrated pyrochlore lattice, and recent debate has focused on the symmetry of local crystal fields at the cation sites. Here, we present neutron scattering measurements of the magnetic excitation spectrum in $\mathrm{FeV_2O_4}$, a recent example of a ferrimagnetic spinel vanadate which is available in single crystal form. We report the existence of two emergent magnon modes at low temperatures, which draw strong parallels with the closely related material, $\mathrm{MnV_2O_4}$. We were able to reproduce the essential elements of both the magnetic ordering pattern and the dispersion of the inelastic modes with semi- classical spin wave calculations, using a minimal model that implies a sizeable single-ion anisotropy on the vanadium sublattice. Taking into account the direction of ordered spins, we associate this anisotropy with the large trigonal distortion of $\mathrm{VO_6}$ octahedra, previously observed via neutron powder diffraction measurements. We further report on the spin gap, which is an order-of-magnitude larger than that observed in $\mathrm{MnV_2O_4}$. By looking at the overall temperature dependence, we were able to show that the gap magnitude is largely associated with the ferro-orbital order known to exist on the iron sublattice, but the contribution to the gap from the vanadium sublattice is in fact comparable to what is reported in the Mn compound. This reinforces the conclusion that the spin canting transition is associated with the ordering of vanadium orbitals in this system, and closer analysis indicates closely related physics underlying orbital transitions in $\mathrm{FeV_2O_4}$ and $\mathrm{MnV_2O_4}$., Comment: 8 pages, 6 figures, includes supplementary figure; accepted for publication in Physical Review B

Neutron diffraction, X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magnetic and structural phase transitions of the spinel system Fe1+xCr2 xO4 (0.0 x 1.0). The temperature versus Fe concentration (x) phase diagram contains two magnetically ordered phases and four structural phases below 420 K. The complicated transitions are closely related to the change in the spin and orbital degrees of freedom induced by substitution of Fe ions for Cr ions. The systematic change in the crystal structure is explained by the combined effects of Jahn-Teller distortion, spin-lattice interaction, Fe2+-Fe3+ hopping, and disorder among Fe2+, Fe3+, and Cr3+ ions.

We report magnetic, transport, and neutron diffraction measurements as well as a doping study of the V-phase compound Zr${}_{4}$Fe${}_{4}$Si${}_{7}$. This compound exhibits collinear antiferromagnetic order below ${T}_{N}=98\ifmmode\pm\else\textpm\fi{}1$ K with a staggered moment of 0.57(3)${\ensuremath{\mu}}_{B}/$Fe as $T\ensuremath{\rightarrow}$ 0. The magnetic order can be quenched with Co substitution to the Fe site, but even then a 1.5${\ensuremath{\mu}}_{B}/$Fe paramagnetic moment remains. The resistivity and heat capacity of Zr${}_{4}$Fe${}_{4}$Si${}_{7}$ are Fermi-liquid-like below 16 and 7 K, respectively, and reveal correlations on the scale of those observed in superconducting Fe pnictides and chalcogenides. Electronic structure calculations overestimate the ordered moment, suggesting the importance of dynamical effects. The existence of magnetic order, electronic correlations, and spin fluctuations make Zr${}_{4}$Fe${}_{4}$Si${}_{7}$ distinct from the majority of Fe-Si compounds, fostering comparison instead with the parent compounds of Fe-based superconductors.

The hole-doped kagome lattice of Cu${}^{2+}$ ions in LaCuO${}_{2.66}$ was investigated by nuclear quadrupole resonance, electron spin resonance, electrical resistivity, bulk magnetization, and specific-heat measurements. For temperatures above $\ensuremath{\sim}180$ K, the spin and charge properties show an activated behavior suggestive of a narrow-gap semiconductor. At lower temperatures, the results indicate an insulating ground state which may or may not be charge ordered. While the frustrated spins in remaining patches of the original kagome lattice might not be directly detected here, the observation of coexisting nonmagnetic sites, free spins, and frozen moments reveals an intrinsically inhomogeneous magnetism. Numerical simulations of a $\frac{1}{3}$-diluted kagome lattice rationalize this magnetic state in terms of a heterogeneous distribution of cluster sizes and morphologies near the site-percolation threshold.

We report a comprehensive study of dc susceptibility, specific-heat, neutron-diffraction, and inelastic neutron-scattering measurements on polycrystalline Ba${}_{3}$(Cr${}_{1\ensuremath{-}x}$V${}_{x}$)${}_{2}$O${}_{8}$ samples, where $x=0$, 0.06, 0.15, and 0.53. A Jahn-Teller structure transition occurs for $x=0$, 0.06, and 0.15 samples, and the transition temperature is reduced upon vanadium substitution from 70(2) K at $x=0$ to 60(2) K at $x=0.06$ and 0.15. The structure becomes less distorted as $x$ increases, and such transition disappears at $x=0.53$. The observed magnetic excitation spectrum indicates that the singlet ground state remains unaltered and spin-gap energy $\ensuremath{\Delta}=1.3(1)$ meV is identical within the instrument resolution for all $x$. In addition, the dispersion bandwidth $W$ decreases with increase of $x$. At $x=0.53$, $W$ is reduced to 1.4(1) meV from 2.0(1) meV at $x=0$.

We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)${}_{4\ensuremath{-}x}$La${}_{2x}$Co${}_{4}$O${}_{15}$ ($x=0$, 0.5, and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co${}_{4}$O${}_{15}$ clusters, which are packed together into a honeycomblike network. Partial substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing, but all compositions remain subject to a random distribution of charge. Magnetic susceptibility together with neutron scattering measurements reveal that all studied specimens are characterized by competing ferro- and antiferro-magnetic exchange interactions that give rise to a three-dimensional Heisenberg spin-glass state. Neutron spectroscopy shows a clear trend of slowing down of spin-dynamics upon increasing La concentration, suggesting a reduction in charge randomness in the doped samples.

We present a neutron-diffraction study of FeV${}_{2}$O${}_{4}$, which is rare in exhibiting spin and orbital degrees of freedom on both cation sublattices of the spinel structure. Our data confirm the existence of three structural phase transitions previously identified with x-ray powder diffraction and reveal that the lower two transitions are associated with sequential collinear and canted ferrimagnetic transitions involving both cation sites. Through consideration of local crystal and spin symmetry, we further conclude that Fe${}^{2+}$ cations are ferro-orbitally ordered below 135 K and V${}^{3+}$ orbitals order at 60 K, in accordance with predictions for vanadium spinels with large trigonal distortions and strong spin-orbit coupling. Intriguingly, the direction of ordered vanadium spins at low temperature obey ``ice rules'' more commonly associated with the frustrated rare-earth pyrochlore systems.

We present a neutron and x-ray scattering investigation of Ca${}_{3}$LiOsO${}_{6}$, a 5d material predicted to host magnetic ordering solely through an extended superexchange pathway involving two anions. This contrasts with investigations of extended superexchange interactions that have been largely limited to low-dimensional 3d systems involving both superexchange and extended superexchange. Despite the apparent one-dimensional nature and triangular units of magnetic osmium ions in Ca${}_{3}$LiOsO${}_{6}$, the onset of magnetic correlations has been observed at a high temperature of 117 K in bulk measurements. We experimentally determine the magnetically ordered structure and show it to be long range and three dimensional. Our results support the model of extended superexchange interaction.

We report a sequence of continuous phase transformations in iron telluride, Fe${}_{1+y}$Te with $y=0.10(1)$, which is observed by combining neutron diffraction, magnetic susceptibility, and specific-heat measurements on single-crystal samples. While a gradual increase of magnetic scattering near the wave vector $(0.5,0,0.5)$ is seen below $T\ensuremath{\approx}70$ K, a temperature where the discontinuous first-order magnetostructural phase transition is found in systems with small $y$ ($\ensuremath{\lesssim}0.06$), the reduction of the lattice symmetry in Fe${}_{1.1}$Te only occurs at ${T}_{s}\ensuremath{\approx}63$ K. Below ${T}_{N}\ensuremath{\approx}57.5$ K, the long-range magnetic order develops, the incommensurate wave vector ${\text{Q}}_{m}$ of which varies with temperature. Finally, at ${T}_{m}\ensuremath{\lesssim}45$ K, the system enters the low-$T$ phase, where ${\text{Q}}_{m}$ is locked at $\ensuremath{\approx}(0.48,0,0.5)$. We conclude that these instabilities are weak compared to the strength of the underlying interactions, and we suggest that the impact of the Fe interstitials on the transitions can be treated with random-field models.

The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder) and Peierls (localization via distortion of a periodic 1D lattice). One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and X-ray scattering we show that the MIT in NaOsO3 is coincident with the onset of long-range commensurate three dimensional magnetic order. Whilst candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT. We discuss our results in the light of recent reports of a Mott spin-orbit insulating state in other 5d oxides.

Layered ruthenates are prototype materials for the study of structure-property correlations. We report the structural and physical properties of double-layered perovskite Sr${}_{3}$(Ru${}_{1\ensuremath{-}x}$Mn${}_{x}$)${}_{2}$O${}_{7}$ single crystals with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.7$. Single-crystal x-ray diffraction refinements reveal that Mn doping on the Ru site leads to the shrinkage of unit-cell volume and the disappearance of (Ru/Mn)O${}_{6}$ octahedron rotation when $xg0.16$, but the crystal structure remains tetragonal. Upon doping, the electrical resistivity reveals a metallic character $(d\ensuremath{\rho}/dTg0)$ at high temperatures but insulating behavior $(d\ensuremath{\rho}/dTl0)$ below a characteristic temperature ${T}_{\mathrm{MIT}}$. Interestingly, ${T}_{\mathrm{MIT}}$ is different from ${T}_{\mathrm{M}}$, at which magnetic susceptibility reaches maximum. While ${T}_{\mathrm{MIT}}$ increases monotonically with increasing $x,{T}_{\mathrm{M}}$ displays a nonmonotonic dependence with $x$ even though the effective spin increases from $S\ensuremath{\sim}1$ $(x=0)$ to $\ensuremath{\sim}3/2$ $(x=0.7)$. The phase diagram consists of three distinct magnetic ground states due to local structure change.

We review a strategy for targeted synthesis of large single crystal samples of prototype quantum magnets for inelastic neutron scattering experiments. Four case studies of organic copper halogenide S=1/2 systems are presented. They are meant to illustrate that exciting experimental results pertaining to forefront many-body quantum physics can be obtained on samples grown using very simple techniques, standard laboratory equipment, and almost no experience in in advanced crystal growth techniques., Comment: 16 pages, 10 figures

Polycrystalline samples of ${\text{Sr}}_{2}{\text{Mn}}_{2}{\text{CuAs}}_{2}{\text{O}}_{2}$, ${\text{Sr}}_{2}{\text{Mn}}_{3}{\text{As}}_{2}{\text{O}}_{2}$, and ${\text{Sr}}_{2}{\text{Zn}}_{2}{\text{MnAs}}_{2}{\text{O}}_{2}$ were synthesized. Their temperature- and applied magnetic field-dependent structural, transport, thermal, and magnetic properties were characterized by means of x-ray and neutron diffraction, electrical resistivity $\ensuremath{\rho}$, heat capacity, magnetization, and magnetic susceptibility measurements. These compounds have a body-centered-tetragonal crystal structure (space group $I4/mmm$) that consists of $M{\text{O}}_{2}$ ($M=\text{Zn}$ and/or Mn) oxide layers similar to the ${\text{CuO}}_{2}$ layers in high superconducting transition temperature ${T}_{c}$ cuprate superconductors, and intermetallic $M\text{As}$ ($M=\text{Cu}$ and/or Mn) layers similar to the FeAs layers in high-${T}_{c}$ pnictides. These two types of layers alternate along the crystallographic $c$ axis and are separated by Sr atoms. The site occupancies of Mn, Cu, and Zn were studied using Rietveld refinements of x-ray and neutron powder diffraction data. The temperature dependences of $\ensuremath{\rho}$ suggest metallic character for ${\text{Sr}}_{2}{\text{Mn}}_{2}{\text{CuAs}}_{2}{\text{O}}_{2}$ and semiconducting character for ${\text{Sr}}_{2}{\text{Mn}}_{3}{\text{As}}_{2}{\text{O}}_{2}$ and ${\text{Sr}}_{2}{\text{Zn}}_{2}{\text{MnAs}}_{2}{\text{O}}_{2}$. ${\text{Sr}}_{2}{\text{Mn}}_{2}{\text{CuAs}}_{2}{\text{O}}_{2}$ is inferred to be a ferrimagnet with a Curie temperature ${T}_{\text{C}}=95(1)\text{ }\text{K}$. Remarkably, we find that the magnetic ground-state structure changes from a $\text{G}$-type antiferromagnetic structure in ${\text{Sr}}_{2}{\text{Mn}}_{3}{\text{As}}_{2}{\text{O}}_{2}$ to an $\text{A}$-type ferrimagnetic structure in ${\text{Sr}}_{2}{\text{Mn}}_{2}{\text{CuAs}}_{2}{\text{O}}_{2}$ in which the Mn ions in each layer are ferromagnetically aligned but are antiferromagnetically aligned between layers.

We study excitations in weakly interacting pairs of quantum spin ladders coupled through geometrically frustrated bonds. The ground state is a disordered spin liquid, that at high fields is replaced by an ordered chiral helimagnetic phase. The spectra observed by high-field inelastic neutron scattering experiments on the prototype compound Sul-Cu2Cl4 are qualitatively different from those in the previously studied frustration-free spin liquids. Beyond the critical field Hc=3.7 T, the soft mode that drives the quantum phase transition spawns two separate excitations: a gapless Goldstone mode and a massive magnon. Additional massive quasiparticles are clearly visible below Hc, but are destroyed in the ordered phase. In their place one observes a sharply bound excitation continuum., Comment: 6 pages, 4 figures