LiCu_{3}O_{3} is an antiferromagnetic mixed valence cuprate where trilayers of edge-sharing Cu(II)O (3d^{9}) are sandwiched in between planes of Cu(I) (3d^{10}) ions, with Li stochastically substituting Cu(II). Angle-resolved photoemission spectroscopy (ARPES) and density functional theory reveal two insulating electronic subsystems that are segregated in spite of sharing common oxygen atoms: a Cu d_{z^{2}}/O p_{z} derived valence band (VB) dispersing on the Cu(I) plane, and a Cu 3d_{x^{2}-y^{2}}/O 2p_{x,y} derived Zhang-Rice singlet (ZRS) band dispersing on the Cu(II)O planes. First-principle analysis shows the Li substitution to stabilize the insulating ground state, but only if antiferromagnetic correlations are present. Li further induces substitutional disorder and a 2D electron glass behavior in charge transport, reflected in a large 530 meV Coulomb gap and a linear suppression of VB spectral weight at E_{F} that is observed by ARPES. Surprisingly, the disorder leaves the Cu(II)-derived ZRS largely unaffected. This indicates a local segregation of Li and Cu atoms onto the two separate corner-sharing Cu(II)O_{2} sub-lattices of the edge-sharing Cu(II)O planes, and highlights the ubiquitous resilience of the entangled two hole ZRS entity against impurity scattering.

Quantum spin systems exhibit an enormous range of collective excitations, but their spin waves, gapped triplons, fractional spinons, or yet other modes are generally held to be mutually exclusive. Here we show by neutron spectroscopy on SeCuO_3 that magnons, triplons, and spinons are present simultaneously. We demonstrate that this is a consequence of a structure consisting of two coupled subsystems and identify all the interactions of a minimal magnetic model. Our results serve qualitatively to open the field of multi-excitation spin systems and quantitatively to constrain the complete theoretical description of one member of this class of materials.

In the field of molecular nanoclusters, cubane and defect-dicubane, or butterfly structures, are typical examples of tetranuclear metal core architectures. In this work, a halogenated and anionic Schiff-base ligand (L2–) is utilized as it is predisposed to chelate within a cluster core to both 3d and 4f metal ions, in different binding configurations (H2L = 4-chloro-2-(2-hydroxy-3-methoxybenzyliden amino)phenol). The phenolate oxygen atoms of the deprotonated ligand can act in μ-O and μ3-O bridging binding modes for the intramolecular assembly of metal ions. Based on that, two tetranuclear and isostructural compounds [Ni2Tb2(L)4(NO3)2(DMF)2]·2CH3CN (1) and [Ni2Er2(L)4(NO3)2(DMF)2]·0.5CH3CN (2) were synthesized and structurally characterized. Magnetic susceptibility and magnetization data indicate the occurrence of dominant intramolecular ferromagnetic interactions between the spin centers. Particular emphasis is given to the theoretical description of the magnetic behavior, taking into account the Ln–Ni and Ni–Ni coupling paths and the magnetic anisotropy of the LnIII and NiII ions. The study is distinguished for its discussion of two distinct models, whereby model A relies on the uniaxial B20 Stevens term describing the lanthanide anisotropy and model B is based on point-charge model calculations. Importantly, the physical meaning of the obtained parameters for both models was critically scrutinized.

Single crystal growth by chemical vapour transport has resulted samples with Fe1.35Ge stoichiometry. Structural study has shown that the large number of vacancies introduces an incommensuately modulated structure. The electrical resistivity is in the 200 mu Omega cm range at room temperature, and although the ferromagnetic transition temperature at 425 K is clearly visible, it hardly varies down to 4.2 K. It is suggested that the large number of vacancies (and the incommensurate modulation) introduce a strong backscattering, and the system is at the brink of a Mooij correlation. The thermal conductivity and Seebeck coefficient carry also the consequences of the high concentration of vacancies. (c) 2019 Elsevier B.V. All rights reserved.

The Cover Feature shows the main results of a theoretical study of the magnetic excitations in ferromagnetic molecules, ferromagnetic cluster spin waves. The theoretical approach may be used to describe four-dimensional inelastic neutron spectra of these systems at low temperatures. The key result is that the experimentally observable interference pattern can be written in terms of functions dependent only on the geometry of the molecule, with weight factors directly related to the magnetic wave functions. The authors acknowledge J. Mutschler for designing the cover. More information can be found in the Full Paper by K. Prša and O. Waldmann.

The magnetism of the mixed-valence high-spin cluster [Mn18SrO8(N-3)(7)Cl(MedhmpH)(12)(MeCN)(6)]Cl-2 (1) exhibiting intramolecular ferromagnetic interactions was studied using inelastic neutron scattering (INS), and reliable values for the exchange coupling constants were determined based on the quality of simultaneous fits to the INS and magnetic data. The challenge of the huge size of the Hilbert space (3 375 000) and many exchange coupling constants (7 assuming a C-3 symmetry) generally encountered in large spin clusters was resolved as follows: (a) The results of the restricted Hilbert space ferromagnetic cluster spin wave theory were compared to the experimental spectroscopic data. The observed INS transitions were thus assigned to spin wave excitations in a bounded ferromagnetic spin cluster and moreover could be visualized in a straightforward way based on this theory. (b) Simultaneously, Quantum Monte Carlo (QMC) calculations of the temperature-dependent magnetic susceptibility with the same parameter set were compared to the experimental data. Application of state-of-the-art QMC algorithms, as available in the open source ALPS package, in ferromagnetic clusters avoids the full Hamiltonian diagonalization without sacrificing calculation accuracy of the magnetic susceptibility down to the lowest temperatures, which was crucial for the successful analysis. The combined fits revealed two exchange-coupling models with equally good overall agreement to the data. Our preferred model was inspired by magnetostructural correlations and is consistent with them. The model involves three different exchange interactions, one describing the interaction between the core Mn-III spins J(a) = 14.3(1.0) K and two interactions linking the core and the peripheral Mn-II spins: J(b) = 8.3(4) K and J(6) = 3.6(4) K. The use of open-source QMC software and our systematic approach to fitting multiple sets of data obtained by different experimental techniques are described in detail and are generally applicable for understanding large ferromagnetically coupled clusters.

Inelastic neutron scattering (INS) is the key method for studying magnetic spectra. For ferromagnetic molecular nanomagnets the ferromagnetic cluster spin wave theory (FCSWT) exactly describes the low-energy excitations from the ferromagnetic ground state. We provide a procedure starting from these molecular spin wave modes and ending with the experimentally observable INS spectra. The formulas for powder and single-crystal spectra of isotropic and anisotropic ferromagnetic molecules are given. The INS interference pattern can be written in terms of “basis functions” fixed by the molecular geometry. We argue that this decomposition explicates the INS data when the spin wave modes are known and, vice versa, suggests a method to directly determine the quantum wavefunctions of the spin wave excitations based on the INS interference pattern. The calculation of INS observables is exemplified on three molecules: the Mn7 disc, the Mn10 supertetrahedron, and the Mn6 single molecule magnet.

M. Månsson1,∗, J. Sugiyama2, B. Roessli3, B. Hitti4, Y. Ikedo5, I. Zivkovic6, H. Nozaki2, M. Harada2, Y. Sassa7, D. Andreica8, T. Goko9, A. Amato9 O. Ofer4, E.J. Ansaldo4, J.H. Brewer4, K.H. Chow10, H.T. Yi11, S.-W. Cheong11, and K. Prsa12 1Materials Physics, KTH Royal Institute of Technology, SE-16440 Stockholm Kista, Sweden 2Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan 3Lab. for Neutron Scattering & Imaging, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 4TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2A3 5Muon Science Lab., KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan 6Institute of Physics, Bijenika 46, HR-10000 Zagreb, Croatia 7Dep. of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden 8Faculty of Physics, Babes-Bolyai University, 3400 Cluj-Napoca, Romania 9Lab. for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 10Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2G7 11Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA 12Phys. Inst., Universitt Freiburg, Hermann-Herder-Strasse 3, D-79104 Freiburg, Germany E-mail: ∗ condmat@kth.se

We present an experimental investigation of the magnetic structure in a tetramer system SeCuO3 using neutron powder-diffraction and nuclear resonance techniques. We establish a commensurate antiferromagnetic ordering with a propagation vector k = (0, 0, 1). The order parameter follows a critical behavior near T-N = 8 K with a critical exponent beta = 0.32 in agreement with a three-dimensional universality class. Evidence is presented that a singlet state starts to form on tetramers at temperatures as high as 200 K, and its signature is preserved within the ordered state through a strong renormalization of the ordered magnetic moment on two nonequivalent copper sites m(Cu1) approximate to 0.35(mu B) and m(Cu2) < 0.8(mu B) at 1.5 K.

A pentanuclear CuII5-hydroxo cluster possessing an unusual linear-shaped configuration was formed and crystallized under hydrothermal conditions as a result of the unique cooperation of bridging 1,2,4-triazole ligand ( trans-1,4-cyclohexanediyl-4,4'-bi(1,2,4-triazole) ( tr2 cy)), MoVI-oxide, and CuSO4. This structural motif can be rationalized by assuming in situ generation of {Cu2Mo6O22}4- anions, which represent heteroleptic derivatives of γ-type [Mo8O26]4- further interlinked by [Cu3(OH)2]4+ cations through [ N- N] bridges. The framework structure of the resulting compound [Cu5(OH)2( tr2 cy)2Mo6O22]·6H2O (1) is thus built up from neutral heterometallic {Cu5(OH)2Mo6O22} n layers pillared with tetradentate tr2 cy. Quantum-chemical calculations demonstrate that the exclusive site of the parent γ-[Mo8O26]4- cluster into which CuII inserts corresponds with the site that has the lowest defect ("MoO2 vacancy") formation energy, demonstrating how the local metal-polyoxomolybdate chemistry can express itself in the final crystal structure. Magnetic susceptibility measurements of 1 show strong antiferromagnetic coupling within the Cu5 chain with exchange parameters J1 = -500(40) K (-348(28) cm-1), J2 = -350(10) K (-243(7) cm-1) and g = 2.32(2), χ2 = 6.5 × 10-4. Periodic quantum-chemical calculations reproduce the antiferromagnetic character of 1 and connect it with an effective ligand-mediated spin coupling mechanism that comes about from the favorable structural arrangement between the Cu centers and the OH-, O2-, and tr2 cy bridging ligands.

Ferromagnetic cluster spin wave theory (FCSWT) provides an exact and concise description of the low-energy excitations from the ferromagnetic ground state in finite magnetic systems, such as bounded magnetic molecules. In particular, this theory is applicable to the description of experimental inelastic neutron scattering (INS) spectra at low temperatures. We provide a detailed conceptual overview of the FCSWT. Additionally, we introduce a pictorial representation of calculated wavefunctions, similar to the usual depiction of vibrational normal modes in molecules. We argue that this representation leads to a better intuitive understanding of the excitations, their symmetry properties, and has links to the energy and wavevector dependence of intensity in the neutron scattering experiments. We apply FCSWT and illustrate the results on a series of examples with available low-temperature INS data, ranging from the Mn-10 supertetrahedron, the Mn-7 disk to the Mn-6 single molecule magnet.

We have investigated the magnetic ground state of a novel one-dimensional compound, NaCr2O4, in which Cr2O4 double chains, i.e. zig-zag chains are aligned parallel to the b-axis, by means of both muon-spin rotation and relaxation (μ+SR) and neutron diffraction (ND) measurements. The μ+SR results reveal the formation of static antiferromagnetic order below Néel temperature (TN=124K). The ND measurements also demonstrate the appearance of magnetic Bragg peaks with the propagation vector k→=(1, 0, 1) below TN. Combined analyses of the μ+SR and ND data clarify that the Cr moments in each zig-zag chain are aligned ferromagnetically along the c-axis, whereas antiferromagnetically along the a-axis between the adjacent zig-zag chains.

In this paper, we tackle the complexity of coexisting disorder and Coulomb electron-electron interactions (CEEIs) in solids by addressing a strongly disordered system with intricate CEEIs and a screening that changes both with charge carrier doping level Q and temperature T. We report on an experimental comparative study of the T dependencies of the electrical conductivity sigma and magnetic susceptibility x of polyaniline pellets doped with dodecylbenzenesulfonic acid over a wide range. This material is special within the class of doped polyaniline by exhibiting in the electronic transport a crossover between a low-T variable range hopping (VRH) and a high-T nearest-neighbor hopping (NNH) well below room temperature. Moreover, there is evidence of a soft Coulomb gap Delta(C) in the disorder band, which implies the existence of a long-range CEEI. Simultaneously, there is an onsite CEEI manifested as a Hubbard gap U and originating in the electronic structure of doped polyaniline, which consists of localized electron states with dynamically varying occupancy. Therefore, our samples represent an Anderson-Mott insulator in which long-range and short-range CEEIs coexist. The main result of the study is the presence of a crossover between low- and high-T regimes not only in sigma(T) but also in x(T), the crossover temperature T* being essentially the same for both observables over the entire doping range. The relatively large electron localization length along the polymer chains results in U being small, between 12 and 20 meV for the high and low Q, respectively. Therefore, the thermal energy at T* is sufficiently large to lead to an effective closing of the Hubbard gap and the consequent appearance of NNH in the electronic transport within the disorder band. Delta(C) is considerably larger than U, decreasing from 190 to 30 meV as Q increases, and plays the role of an activation energy in the NNH.

Single-molecule magnets (SMMs) based on lanthanide ions display the largest known blocking temperatures and are the best candidates for molecular magnetic devices. Understanding their physical properties is a paramount task for the further development of the field. In particular, for the poly-nuclear variety of lanthanide SMMs, a proper understanding of the magnetic exchange interaction is crucial. We discuss the strengths and weaknesses of the neutron scattering technique in the study of these materials and particularly for the determination of exchange. We illustrate these points by presenting the results of a comprehensive inelastic neutron scattering study aimed at a radical-bridged diterbium(III) cluster, Tb2(μ-N23−), which exhibits the largest blocking temperature for a poly-nuclear SMM. Results on the YIII analogue Y2(μ-N23−) and the parent compound Tb2(μ-N22−) (showing no SMM features) are also reported. The results on the parent compound include the first direct determination of the lanthanide-lanthanide exchange interaction in a molecular cluster based on inelastic neutron scattering. In the SMM compound, the resulting physical picture remains incomplete due to the difficulties inherent to the problem.

A novel one–dimensional single azido–bridged coordination polymer of formula [Cu2(μ–NAPPR)(μ1,1–N3)2]n, where NAPPR is the 1,3–Bis (naphthylideneimino) propane dianion, has been synthesized and characterized by elemental analyses and spectroscopic methods. This compound consists of 1D uniform chains in which the two types of Cu(II) ions, one with a distorted square pyramidal (Cu1) and the other with a distorted trigonal bipyramidal (Cu2) geometry, are interlinked by single end–on azido bridges. The magnetic properties of the complex have also been studied. Temperature– and field–dependent magnetic analyses reveal that antiferromagnetic interactions between Cu2 centers are mediated by the single EO bridges, while Cu1 moments remain paramagnetic. The cyclic voltammetric study reveals the existence of two structurally different copper(II) centers and the resulting data conform to the crystal structure of this complex determined by single crystal X-ray crystallography.

The novel iridate Ba8Al2IrO14 was prepared as single crystals by self-flux method, thereby providing a rare example of an all-Ir(VI) compound that can be synthesized under ambient pressure conditions. The preparation of all-Ir(6+) iridate without using traditional high-pressure techniques has to our knowledge previously only been reported in Nd2K2IrO7 and Sm2K2IrO7. The monoclinic crystal structure (space group C2/m, No.12) is stable down to 90 K and contains layers of IrO6 octahedra separated by Ba and AlO4 tetrahedra. The material exhibits insulating behavior with a narrow band gap of ∼0.6 eV. The positive Seebeck coefficient indicates hole-like dominant charge carriers. Susceptibility measurement shows antiferromagnetic coupling with no order down to 2 K.

A 1D, end-on azido- and phenoxo-bridged, coordination polymer, [Cu2(μ-NAPPR)(μ1,1-N3)2]n (1), (NAPPR2− = 1,3-bis(naphthylideneimino)propane dianion), was synthesized and characterized.

Frustrated systems exhibit remarkable properties due to the high degeneracy of their ground states. Stabilised by competing interactions, a rich diversity of typically nanometre-sized phase structures appear in polymer and colloidal systems, while the surface of ice pre-melts due to geometrically frustrated interactions. Atomic spin systems where magnetic interactions are frustrated by lattice geometry provide a fruitful source of emergent phenomena, such as fractionalised excitations analogous to magnetic monopoles. The degeneracy inherent in frustrated systems may prevail all the way down to absolute zero temperature, or it may be lifted by small perturbations or entropic effects. In the geometrically frustrated Ising--like magnet Ca3Co2O6, we follow the temporal and spatial evolution of nanoscale magnetic fluctuations firmly embedded inside the spin--density--wave magnetic structure. These fluctuations are a signature of a competing ferrimagnetic phase with an incommensurability that is different from, but determined by the host. As the temperature is lowered, the fluctuations slow down into a super-paramagnetic regime of stable spatiotemporal nano-structures.