We have used inelastic neutron scattering to measure the magnetic excitation spectrum along the high-symmetry directions of the first Brillouin zone of the magnetic skyrmion hosting compound Cu 2 OSeO 3 . The majority of our scattering data are consistent with the expectations of a recently proposed model for the magnetic excitations in Cu 2 OSeO 3 , and we report best-fit parameters for the dominant exchange interactions. Important differences exist, however, between our experimental findings and the model expectations. These include the identification of two energy scales that likely arise due to neglected anisotropic interactions. This feature of our work suggests that anisotropy should be considered in future theoretical work aimed at the full microscopic understanding of the emergence of the skyrmion state in this material. [8], and in the polar magnetic semiconductor GaV 4 S 8 [9]. To understand the formation and the microscopic origin of these skyrmion phases, one needs a multiscale approach that covers the macroscopic domain of the skyrmion as well as the quantum scale of the local spins. This, however, breaks down in the above-mentioned metals because the low-energy delocalized electrons and magnetic degrees of freedom are mixed, intrinsically involving multiple energy and spatial scales.Among cubic helimagnets, Cu 2 OSeO 3 is the only insulator with magnetoelectric properties in the ground state [8,[10][11][12][13][14]. It offers an ideal laboratory to explore the microscopic ingredients that lead to skyrmion formation in a quantitative manner, since its Bloch-type ground-state properties and low-energy excitations are fully governed by the magnetic interactions between localized spins and are not affected by the presence of itinerant carriers. Exchange pathway considerations, susceptibility measurements, and ab initio calculations reveal that two magnetic energy scales divide the system into weakly coupled Cu 4 tetrahedra [15]. These Cu 4 "molecules," with an effective spin of S = 1, are the elementary magnetic building blocks of Cu 2 OSeO 3 instead of the single Cu ions. The effective spins of the Cu 4 tetrahedra are ferromagnetically coupled and form a trillium lattice, just as the Mn and Fe ions do in the B20 structure of the metallic skyrmion compounds MnSi and FeGe.Prior to the undertaking of the present work, previous studies of the magnetic excitation spectra of Cu 2 OSeO 3 were conducted using Raman scattering [16] and microwave resonance absorption [17], i.e., techniques that are sensitive only to excitations in the center of the Brillouin zone. In contrast, inelastic neutron scattering (INS) is able to measure at finite momentum transfer and is therefore uniquely suited to probe the magnetic excitation spectra of Cu 2 OSeO 3 throughout reciprocal space. The additional information afforded by INS therefore provides more rigorous tests of theoretical models aimed at describing the excitation spectra of Cu 2 OSeO 3 .Single crystals of Cu 2 OSeO 3 (cubic P 2 1 3 space group, a = 8.82Å) were gro...
We determine the phase diagram of copper nitrate Cu(NO3)2·2.5D2O in the context of quantum phase transitions and novel states of matter. We establish this compound as an ideal candidate to study quasi-1D Luttinger liquids, 3D Bose-Einstein-Condensation of triplons, and the crossover between 1D and 3D physics. Magnetocaloric effect, magnetization, and neutron scattering data provide clear evidence for transitions into a Luttinger liquid regime and a 3D long-range ordered phase as function of field and temperature. Theoretical simulations of this model material allow us to fully establish the phase diagram and to discuss it in the context of dimerized spin systems.PACS numbers: 75.10. Jm, 75.30.Sg, 75.40.Cx There has been a flourish of interest in quantum antiferromagnets of late, due to a fascinating range of novel ground states as well as a multitude of exotic fieldinduced phases. A current focus of these studies involves materials with a reduced dimensionality. In particular, one-dimensional (1D) systems [1] have been shown to exhibit remarkable properties such as Luttinger liquid (LL) behavior, a concept relevant to a wide range of systems including quantum wires or nanotubes [2,3]. In this context, magnetic insulators such as the gapless uniform spin chain KCuF 3 [4] have been used as model systems allowing extensive studies of LLs.Presently, of particular interest are spin S = 1 2 alternating antiferromagnetic chain systems. Here, an antiferromagnetic coupling J 1 leads to a formation of spin pairs (dimers) while a weaker antiferromagnetic interdimer exchange J 2 couples the dimers along one dimension. Thus, the system is described in an external field h by the HamiltonianBecause of the dimer formation, such materials exhibit a singlet ground state separated from a low lying triplet of finite width by an energy gap, ∆. The gap is closed by the application of a magnetic field which Zeemansplits the triplet into its three constituents. At the critical field H c1 , the lower S z = 1 mode starts to collapse into the ground state, while at a second critical field H c2 the S z = 1 triplet state has fully shifted below the singlet and a gap reopens. Between the two critical fields a LL of interacting triplets develops.At very low temperatures, and with a weak interchain interaction J ′ present in real materials, the triplet states (triplons) condense into a long-range ordered (LRO) ground state between the two critical fields, a phase that is described as Bose-Einstein-condensation (BEC) of triplons [5][6][7]. The concept of a BEC of triplons was first introduced for the 3D interacting dimer system TlCuCl 3 [8], and later extended to other 2D or 3D coupled dimer systems [9,10]. Quasi-1D materials involving alternating spin chains or ladders, in addition, may show evidence of both LL and BEC phases [11]. Therefore they would allow the unique opportunity to study crossover effects between 1D and 3D physics. Only the ladder series (Hpip) 2 Cu(Br,Cl) 4 is discussed in terms of such a dimensional crossover from a LL ...
The upgrade of the cold neutron triple-axis spectrometer FLEXX is described. We discuss the characterisation of the gains from the new primary spectrometer, including a larger guide and double focussing monochromator, and present measurements of the energy and momentum resolution and of the neutron flux of the instrument. We found an order of magnitude gain in intensity (at the cost of coarser momentum resolution), and that the incoherent elastic energy widths are measurably narrower than before the upgrade. The much improved count rate should allow the use of smaller single crystals samples and thus enable the upgraded FLEXX spectrometer to continue making leading edge measurements.
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