80 research outputs found
Molecular engineering of antiferromagnetic rings for quantum computation
The substitution of one metal ion in a Cr-based molecular ring with dominant
antiferromagnetic couplings allows to engineer its level structure and
ground-state degeneracy. Here we characterize a Cr7Ni molecular ring by means
of low-temperature specific-heat and torque-magnetometry measurements, thus
determining the microscopic parameters of the corresponding spin Hamiltonian.
The energy spectrum and the suppression of the leakage-inducing S-mixing render
the Cr7Ni molecule a suitable candidate for the qubit implementation, as
further substantiated by our quantum-gate simulations.Comment: To appear in Physical Review Letter
On the possibility of magneto-structural correlations: detailed studies of di-nickel carboxylate complexes
A series of water-bridged dinickel complexes of the general formula [Ni<sub>2</sub>(μ<sub>2</sub>-OH<sub>2</sub>)(μ2-
O<sub>2</sub>C<sup>t</sup>Bu)<sub>2</sub>(O<sub>2</sub>C<sup>t</sup>Bu)2(L)(L0)] (L = HO<sub>2</sub>C<sup>t</sup>Bu, L0 = HO<sub>2</sub>C<sup>t</sup>Bu (1), pyridine (2),
3-methylpyridine (4); L = L0 = pyridine (3), 3-methylpyridine (5)) has been synthesized
and structurally characterized by X-ray crystallography. The magnetic properties
have been probed by magnetometry and EPR spectroscopy, and detailed measurements
show that the axial zero-field splitting, D, of the nickel(ii) ions is on the same order as
the isotropic exchange interaction, J, between the nickel sites. The isotropic exchange
interaction can be related to the angle between the nickel centers and the bridging
water molecule, while the magnitude of D can be related to the coordination sphere at
the nickel sites
Topology and spin dynamics in magnetic molecules
We investigate the role of topology and distortions in the quantum dynamics
of magnetic molecules, using a cyclic spin system as reference. We consider
three variants of antiferromagnetic molecular ring, i.e. Cr, CrZn and
CrNi, characterized by low lying states with different total spin . We
theoretically and experimentally study the low-temperature behavior of the
magnetic torque as a function of the applied magnetic field. Near level
crossings, this observable selectively probes quantum fluctuations of the total
spin ('' mixing") induced by lowering of the ideal ring symmetry. We show
that while a typical distortion of a model molecular structure is very
ineffective in opening new -mixing channels, the spin topology is a major
ingredient to control the degree of mixing. This conclusion is further
substantiated by low-temperature heat capacity measurements.Comment: 5 pages, 4 figure
A simple methodology for constructing ferromagnetically coupled Cr(iii) compounds
A large family of chromium(iii) dimers has been synthesised and magneto-structurally characterised using a combination of carboxylate and diethanolamine type ligands. The compounds have the general formula [Cr-2(R-1-deaH)(2)(O2CR2)Cl-2]Cl where R-1 = Me and R-2 = H (1), Me (2), CMe3 (3), Ph (4), 3,5-(Cl)(2)Ph (5), (Me)(5)Ph (6), R-1 = Et and R-2 = H (7), Ph (8). The compound [Cr-2(Me-deaH)(2)Cl-4] (9) was synthesised in order to study the effect of removing/adding the carboxylate bridge on the observed magnetic behaviour. Direct current (DC) magnetic susceptibility measurements showed ferromagnetic (FM) exchange interactions between the Cr(iii) centres in the carboxylate bridged family with coupling constants in the range +0.37 < J < +8.02 cm(-1). Removal of the carboxylate to produce the dialkoxide-bridged compound 9 resulted in antiferromagnetic (AFM) exchange between the Cr(iii) ions. DFT calculations reveal the ferromagnetic exchange is the result of an orbital counter-complementarity effect occuring upon introduction of the bridging carboxylate
From antiferromagnetic to ferromagnetic exchange in a family of oxime-based Mn(III) dimers:a magneto-structural study
The reaction of Mn(ClO4)(2)center dot 6H(2)O, a derivatised phenolic oxime (R-saoH(2)) and the ligand tris(2-pyridylmethyl)amine (tpa) in a basic alcoholic solution leads to the formation of a family of cluster compounds of general formula [(Mn2O)-O-III(R-sao)(tpa)(2)](ClO4)(2) (1, R = H; 2, R = Me; 3, R = Et; 4, R = Ph). The structure is that of a simple, albeit asymmetric, dimer of two Mn-III ions bridged through one mu-O2- ion and the -N-O- moiety of the phenolic oxime. Magnetometry reveals that the exchange interaction between the two MnIII ions in complexes 1, 3 and 4 is antiferromagnetic, but that for complex 2 is ferromagnetic. A theoretically developed magneto-structural correlation reveals that the dominant structural parameter influencing the sign and magnitude of the pairwise interaction is the dihedral Mn-O-N-Mn (torsion) angle. A linear correlation is found, with the magnitude of J varying significantly as the dihedral angle is altered. As the torsion angle increases the AF exchange decreases, matching the experimentally determined data. DFT calculations reveal that the (dyz)vertical bar pi vertical bar d(yz) interaction decreases as the dihedral angle increases leading to ferromagnetic coupling at larger angles
Recipes for spin-based quantum computing
Technological growth in the electronics industry has historically been
measured by the number of transistors that can be crammed onto a single
microchip. Unfortunately, all good things must come to an end; spectacular
growth in the number of transistors on a chip requires spectacular reduction of
the transistor size. For electrons in semiconductors, the laws of quantum
mechanics take over at the nanometre scale, and the conventional wisdom for
progress (transistor cramming) must be abandoned. This realization has
stimulated extensive research on ways to exploit the spin (in addition to the
orbital) degree of freedom of the electron, giving birth to the field of
spintronics. Perhaps the most ambitious goal of spintronics is to realize
complete control over the quantum mechanical nature of the relevant spins. This
prospect has motivated a race to design and build a spintronic device capable
of complete control over its quantum mechanical state, and ultimately,
performing computations: a quantum computer.
In this tutorial we summarize past and very recent developments which point
the way to spin-based quantum computing in the solid-state. After introducing a
set of basic requirements for any quantum computer proposal, we offer a brief
summary of some of the many theoretical proposals for solid-state quantum
computers. We then focus on the Loss-DiVincenzo proposal for quantum computing
with the spins of electrons confined to quantum dots. There are many obstacles
to building such a quantum device. We address these, and survey recent
theoretical, and then experimental progress in the field. To conclude the
tutorial, we list some as-yet unrealized experiments, which would be crucial
for the development of a quantum-dot quantum computer.Comment: 45 pages, 12 figures (low-res in preprint, high-res in journal)
tutorial review for Nanotechnology; v2: references added and updated, final
version to appear in journa
[CrIII8MII6]12+ Coordination Cubes (MII=Cu, Co)
Four [CrIII8MII6]n+ (MII = Cu, Co) coordination cubes of formulae [Cr8Co6L24Cl12] (1), [Cr8Co6L24(SCN)12] (2), [Cr8Cu6L24(H2O)12](SO4)6 (3), and [Cr8Cu6L24Cl12] (4) (where HL is 1-(4-pyridyl)butane-1,3-dione), were synthesised using the [CrIIIL3] metalloligand in combination with a variety of MII salts. The metallic skeleton of each cage describes a cube in which the [CrIIIL3] moieties occupy the eight vertices and the MII ions lie at the centre of the six faces. The axial coordination sites of the MII cations are occupied by either H2O molecules or Cl?/SCN? anions originating from the MII salt used in the synthesis, resulting in neutral 1, 2 and 4 and the cage in 3 being a 12+ cation; the charge-balancing SO42? anions accommodated both inside and outside the cube. Magnetic susceptibility and magnetisation measurements reveal weak exchange between nearest neighbour metal ions, mediated via the L? ligands. The modular assembly of the cubes suggests that any combination of [MIIIL3] metalloligand and MII salt will work, potentially resulting in an enormous family of supramolecular assemblies. The charge of the cubes is controlled by the nature of the ligand occupying the axial sites on the MII ions, suggesting trivial ligand exchange may offer control over, amongst others, solubility, reactivity, post-synthetic modification and substrate specificity. The large internal cavities of the cubes also suggest host–guest chemistry may be a fruiful route to encapsulating magnetic and/or redox active guests which could be employed to control magnetic behaviour, and the construction of multifunctional materials
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