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Molecular nanomagnets are among the first examples of finite-size spin systems and have been test beds for addressing several phenomena in quantum dynamics. In fact, for short-enough timescales the spin wavefunctions evolve coherently according to an appropriate spin Hamiltonian, which can be engineered to meet specific requirements. Unfortunately, so far it has been impossible to determine these spin dynamics directly. Here we show that recently developed instrumentation yields the four-dimensional inelastic-neutron scattering function in vast portions of reciprocal space and enables the spin dynamics to be determined directly. We use the Cr 8 antiferromagnetic ring as a benchmark to demonstrate the potential of this approach which allows us, for example, to examine how quantum fluctuations propagate along the ring or to test the degree of validity of the Néel-vector-tunnelling framework. © 2012 Macmillan Publishers Limited. All rights reserved.

Molecular nanomagnets are attractive candidate qubits because of their wide inter- and intra-molecular tunability. Uniform magnetic pulses could be exploited to implement one- and two-qubit gates in presence of a properly engineered pattern of interactions, but the synthesis of suitable and potentially scalable supramolecular complexes has proven a very hard task. Indeed, no quantum algorithms have ever been implemented, not even a proof-of-principle two-qubit gate. Here we show that the magnetic couplings in two supramolecular {Cr7Ni}-Ni-{Cr7Ni} assemblies can be chemically engineered to fit the above requisites for conditional gates with no need of local control. Microscopic parameters are determined by a recently developed many-body ab-initio approach and used to simulate quantum gates. We find that these systems are optimal for proof-of-principle two-qubit experiments and can be exploited as building blocks of scalable architectures for quantum simulation.

Molecular nanomagnets, besides promising to open new frontiers in technology, have attracted huge interest in the scientific community because they can exhibit the phenomenon known as quantum tunnelling of the magnetization, i.e. coherent fluctuations of the direction of the total spin vector. In this paper we study a different quantum phenomenon involving fluctuations of the magnitude of the total spin vector. These fluctuations are related to the mixing between states with different spin quantum number, and imply new macroscopic effects, which we theoretically investigated in the Mn-[3x3] grid.

The experimental values of the energy levels of Er3+, Dy3+, and Nd3+ in BaY2F8 were fitted to a single-ion Hamiltonian containing free-ion and crystal-field interactions. The crystal-field parameters so evaluated were then analyzed by using Newman's Superposition Model. The agreement between the two sets of parameters is good, provided a possible distortion of the F- polyhedron around the rare-earth site is taken into account. The effects of a possible displacement of the rare-earth ion substituting for Y3+ are also evaluated.

Neutron powder diffraction has been used to investigate the magnetic order in a series of RMn4Al8 (space group I4/mmm) compounds with R = (Nd, Dy, Ho and Er). For compounds with Mn, no magnetic order was detected down to 1.6 K, whilst for the compound ErCr4Al8, two very weak magnetic reflections were observed at 1.6 K. The observed magnetic peaks cannot be indexed on the tetragonal unit cell, indicating a possible incommensurate magnetic structure. For ErCu4Al8, a type 1 antiferromagnetic structure is observed. The magnitude and temperature dependence of the Er3+ magnetic moment can be adequately modelled by a combined isotropic exchange and tetragonal crystal field Hamiltonian within the Mean Field Approximation.

Several dense Kondo compounds have a low-temperature ordered phase in which the magnetic moments are reduced with respect to the values expected for the crystal-field (CEF) ground state. In the present work the phenomenon of moment reduction is studied within a molecular-field theory combined with a variational solution of the one-impurity Anderson model with CEF effects. The calculated zero-temperature magnetization and susceptibility agree well with available exact results; the present method is easily applied to systems of any symmetry. We first study the f1 configuration in cubic symmetry, for small values of the ratio TK/Δ between Kondo temperature and CEF splitting. With a Γ∞ ground state and a field along a 〈100〉 direction, an inflection point occurs in the magnetization curve, which gives rise to a first order transition in the zero-temperature phase diagram. This feature is not found for a field along 〈110〉 or 〈111〉, for which the transition is second order. For a Γ7 ground state and small values of TK/Δ, the magnetic-nonmagnetic transition is second order for all field directions. On increasing TK/Δ an inflection point in the magnetization curve appears first for a field along 〈111〉. The theory is applied to a study of cubic CeAg, CeAl2, CePb3, CeIn3, CeTe, and hexagonal CePd2Ga3. The bare value of the moment is found to be strongly increased by exchange coupling to excited CEF states, and the amount of Kondo reduction is found to be substantial, confirming the importance of the Kondo effect in these compounds.

We report the results of neutron spectroscopy for a tetranuclear iron cluster, Fe4(OCH3)6(dpm)6 (Hdpm = dipivaloylmethane), having an S=5 spin ground state. The experiments were carried out on a deuterated powder sample at ILL, Grenoble. The transitions within the ground multiplet, below 0.3 meV, were measured with a high-resolution time-of-flight spectrometer. Taking into account the presence of three different isomers, the parameters of the zero-field-splitting Hamiltonian have been determined. The results are discussed with reference to high-field electron paramagnetic resonance data. A non-dispersive peak at 7.8 meV, detected at T=1.4 K using a three-axis spectrometer with polarization analysis, is interpreted as the transition between the S=5 ground state and the two degenerate S=4 excited multiplets. The Q dependence of the cross section is discussed in comparison with the theoretical model.

Various samples of single-phase YBa2Cu3O7−y have been examined by X-band ESR in theg≃2 region forT≳T c .The observed spectra are attributed to oxygen coordinated Cu2+ ions with a large delocalization of the unpaired spins. These are probably due, in part, to the inclusion of some impurity phase. However, an analysis of the intensity as a function of the temperature in different samples indicates that a possible intrinsic contribution to the ESR signal of Cu2+ ions from the superconducting phase is due, at most, to ∼1% of the total copper in the formula. This small quantity of isolated Cu2+ ions may be due to the presence of low symmetry Cu3+ ions or, equivalently, non-magnetic centers such as Cu2+−O− pairs in the chains.