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Study and characterization of New SMMs

Molecular engineering allows the design of the structural properties of SMMs during the synthesis process. As stated above, the structural properties of SMMs modulate the quantum tunneling phenomena observed in these materials. Among the characteristics of SMMs that can be designed during the synthesis of these materials are the spin, symmetry and anisotropy of the molecules and inter- and intra-molecule interactions as well as the relative orientation between molecules in the crystal lattice. Currently, SMMs with spin values from S = 0 up to S > 30, molecular size from 1 to 5 nm (containing up to eighty-four magnetic ions and hundreds of non-magnetic atoms) have been obtained and studied. Moreover, a wide range of SMMs with different magnitudes and types of molecular interactions are available for study. For example, SMMs without hyperfine interactions (magnetic atoms with no nuclear spin) or without inter-molecule dipolar interactions (SMMs with S = 0) are easily obtainable. Inter-molecule exchange interactions (which are very important for quantum information applications) can also be designed and, in fact, there are several SMMs showing magnetic responses associated with exchange interaction between neighboring molecules. The nature of the exchange interactions can also be designed by manipulating the disposition of the SMMs along the crystal. For example, in the SMM Mn4 (S = 9/2) the molecules are found in pairs placed at the nodes of the crystallographic lattice. In this case, the exchange interaction is restricted to the two molecules of the pair, generating a 3D-array of two-body magnetic systems, where the spin of each molecule of the pair can be entangled. However, in the SMM Ni4 (S = 4) there is only one molecule per unit cell and the exchange interaction couples neighbor molecules indistinctively throughout the crystal.

Antiferromagnetic SMMs

The ability to control exchange interactions between SMMs is of crucial importance for the application of these materials in quantum information and computation processes (i.e. allowing the creation of quantum logic gates). For example, recent work by Loss et al. suggests the possibility to implementing two-qubit quantum-logic operations in a sample of single-molecule magnetic rings with an odd number of ions, resulting in a non-compensated spin value. They propose that the exchange interaction between neighboring ions at different rings (which will affect to the spin of the whole molecule due to spin spatial-delocalization) could be activated and deactivated by the action of an external optical source (see figure). However, focusing a laser beam between two neighboring molecules (as shown in the cartoon) is not possible because the exposed area would be much bigger than the separation between molecules. One possibility is to engineer inter-molecule bounds with different structural composition for different molecules which would react differently for different light frequencies. Other possibility is the use of a strongly inhomogeneous magnetic field that will shift the Zeeman energy of two neighboring molecules, generating a barrier between them which could be tuned with the use of a laser beam through the Stark effect. These two possibilities will be studied during the period of this project
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2D arrays and individual SMMs

The ability to manipulate the nature and spatial structure of exchange interactions will allow fundamental studies of the physics of interacting systems. For example, the PI, in collaboration with chemistry groups in USA, wants to make an effort in the synthesis of single crystals of SMMs coupled through exchange interaction in a 2D-plane. Note that a SMM at low temperature (much lower than the separation between the lowest and the excited spin-projections, T << 5K) is a real Ising system with two well-defined states (up and down). The synthesis of a single crystal in which the exchange interactions are restricted to different (not interacting) 2D-planes (see figure) will provide an excellent prototype for the study of fundamental phenomena of low-dimension interacting systems such as, for example, the Onseger phase transition (order-disorder) which has been predicted more than twenty years ago.

We also want to explore the possibilities of engineer 2D-arrays of SMMs by the use of self-organizing organic nanostructures, which form regular 2D-patterns over an extended surface. The idea is to use these organic nano-composites as templates for building SMMs arrays. High-sensitivity micro-Hall magnetometry will be used to study the magnetic properties of these particular SMM-arrays.

We also want to make efforts in the creation of a SMM-transistor by placing a single-molecule-magnet in between the leads of nono-scale electrodes that will be fabricated by e-beam lithography capabilities at UCF. Transport measurements to study the coulomb blockage are planned to be carried out in the presence of high magnetic fields and high frequency microwave excitations. The objective is the study of the effects of MQT in the transport properties of single-molecule-magnets.

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