Bounding and approximating parabolas for the spectrum of Heisenberg spin systems

We prove that for a wide class of quantum spin systems with isotropic Heisenberg coupling the energy eigenvalues which belong to a total spin quantum number S have upper and lower bounds depending at most quadratically on S. The only assumption adopted is that the mean coupling strength of any spin w.r.t. its neighbours is constant for all N spins. The coefficients of the bounding parabolas are given in terms of special eigenvalues of the N times N coupling matrix which are usually easily evaluated. In addition we show that the bounding parabolas, if properly shifted, provide very good approximations of the true boundaries of the spectrum. We present numerical examples of frustrated rings, a cube, and an icosahedron.Comment: 8 pages, 3 figures. Submitted to Europhysics Letter

Low temperature magnetization and the excitation spectrum of antiferromagnetic Heisenberg spin rings

Accurate results are obtained for the low temperature magnetization versus magnetic field of Heisenberg spin rings consisting of an even number N of intrinsic spins s = 1/2, 1, 3/2, 2, 5/2, 3, 7/2 with nearest-neighbor antiferromagnetic (AF) exchange by employing a numerically exact quantum Monte Carlo method. A straightforward analysis of this data, in particular the values of the level-crossing fields, provides accurate results for the lowest energy eigenvalue E(N,S,s) for each value of the total spin quantum number S. In particular, the results are substantially more accurate than those provided by the rotational band approximation. For s <= 5/2, data are presented for all even N <= 20, which are particularly relevant for experiments on finite magnetic rings. Furthermore, we find that for s > 1 the dependence of E(N,S,s) on s can be described by a scaling relation, and this relation is shown to hold well for ring sizes up to N = 80 for all intrinsic spins in the range 3/2 <= s <= 7/2. Considering ring sizes in the interval 8 <= N <= 50, we find that the energy gap between the ground state and the first excited state approaches zero proportional to 1/N^a, where a = 0.76 for s = 3/2 and a = 0.84 for s = 5/2. Finally, we demonstrate the usefulness of our present results for E(N,S,s) by examining the Fe12 ring-type magnetic molecule, leading to a new, more accurate estimate of the exchange constant for this system than has been obtained heretofore.Comment: Submitted to Physical Review B, 10 pages, 10 figure

g-engineering in hybrid rotaxanes to create AB and AB2 electron spin systems: EPR spectroscopic studies of weak interactions between dissimilar electron spin qubits

Hybrid [2]rotaxanes and pseudorotaxanes are reported where the magnetic interaction between dissimilar spins is controlled to create AB and AB2 electron spin systems,allowing independent control of weakly interacting S =1=2 centers

Self Assembly of Copper(I) and Silver(I) Butterfly Clusters with 2-Mercaptothiazoline

X-ray data obtained from poor crystals which formed from the reaction of copper(II) acetate with 2-mercaptothiazoline reveal the formation of a product that is a polymer formed of tetranuclear, butterfly shaped Cu4(MT)4, 1, clusters. Preparation, isolation and structural characterization of a series of isostructural butterfly complexes was accomplished by addition of a Lewis base (pyridine, PPh3, or ASPI13) to the precipitate obtained from the reaction of copper(II) and/or silver(I) acetate with the appropriate stoichiometric amount of 2-mercaptothiazoline. The general formula of these clusters is L2M4(MT)4; 2, L = PPI13 and M = Cu; 3, L = AsPh3 and M = Cu; 6, L = PPI13 and M = Ag; MT = C3H4NS2_, known as 2-mer- captothiazolinate. The polymer [pyCu4(MT)4]„, 4, formed by the addition of pyridine to 1, was also characterized crystallographically. A mixed metal butterfly complex, (PPh3)2Ag2Cu2(MT)4, 8, is formed by addition of PPI13 to a suspension of the precipitate formed upon reaction of the free HMT ligand with a 1:1 mixture of copper(II) and silver(I) acetates in CH2CI2. FD-MS results of each of the precipitates obtained from the metal acetates and the free ligand indicate that the monomeric unit is M4(MT)4. 1H-NMR and 31P{1H}-NMR, both in solution and in the solid state are presented and interpreted

Five-Spin Supramolecule for Simulating Quantum Decoherence of Bell States

We report a supramolecule that contains five spins of two different types and with, crucially, two different and predictable interaction energies between the spins. The supramolecule is characterized, and the interaction energies are demonstrated by electron paramagnetic resonance (EPR) spectroscopy. Based on the measured parameters, we propose experiments that would allow this designed supramolecule to be used to simulate quantum decoherence in maximally entangled Bell states that could be used in quantum teleportation

Observation of the influence of dipolar and spin frustration effects on the magnetocaloric properties of a trigonal prismatic {Gd-7} molecular nanomagnet

Pineda EM, Lorusso G, Zangana KH, et al. Observation of the influence of dipolar and spin frustration effects on the magnetocaloric properties of a trigonal prismatic {Gd-7} molecular nanomagnet. CHEMICAL SCIENCE. 2016;7(8):4891-4895.We report the synthesis and structure of a molecular {Gd-7} cage of the formula ((Pr2NH2)-Pr-i)(6)[Gd-7(mu(3)-OH)(3)(CO3)(6)((O2CBu)-Bu-t)(12)] which has crystallographic C-3h symmetry. Low temperature specific heat and adiabatic demagnetization experiments (the latter achieving temperatures below 100 mK), lead to the observation of the effects of both intramolecular dipolar interactions and geometric spin frustration. The dipolar interaction leads to a massive rearrangement of energy levels such that specific heat and entropy below 2 K are strongly modified while magnetic susceptibility and magnetization above 2 K are not affected. The consequences of these phenomena for low temperature magnetocaloric applications are discussed