The broad goal of this proposal is to explore the interplay between localized high-spin states of an individual molecule and conduction electrons in order to develop molecular electronic devices for local magnetic field sensing, ultra-high-density information storage, and quantum information processing. Single-molecule magnets are characterized by a large total spin and a strong intrinsic anisotropy. They present some unique characteristics, such as quantum tunneling of the magnetization and Berry phase interference. Although extensively studied in crystalline form, some of their key properties remain elusive. For instance, it is unclear how quantum tunneling of the magnetization influences electronic conduction through these molecules. Understanding this property is crucial for any electronic device development. The proposed research program addresses these issues by combining chemical synthesis with experimental and theoretical physics to probe quantum properties of isolated single-molecule magnets. The molecules will be attached to nanometer-gapped metal electrodes and gated electrically to form a single-electron transistor. Device fabrication will make use of lithographic and electromigration techniques. The molecule?s electric conduction will be studied both statically and dynamically to reveal excited molecular states, the effect of different ligands, the Kondo effect, spin-polarized transport, the Berry-phase blockade, quantum oscillations of the magnetization, and decoherence,. The proposed study emphasizes exploring these phenomena toward practical devices. In particular: (i) to employ the intense magnetic field tunability of the Berry phase to obtain high-sensitivity local magnetic field nanosensors; (ii) to develop reading and writing procedures for molecular bits in high-density magnetic memories; and (iii) to demonstrate quantum logic gate operations in a molecular qubit. The team has extensive experience with single-molecule magnets and in quantum electronic transport. Preliminary results have demonstrated the team?s ability to fabricate suitable devices and to measure the IV characteristics of isolated molecules in the Coulomb blockade regime. Available facilities permit efficient device fabrication with a short turnover time. The facilities available to the team include low temperatures, high magnetic fields oriented in arbitrary directions, continuous-wave and pulsed high-frequency microwave excitations, and ultra-fast pulsed voltage gating.
Intellectual Merit: Molecular electronics is rapidly becoming a separate research field within Applied Sciences and Engineering. The main effort so far has been on carbon-based systems or isotropic molecules containing a small net spin. This proposal focuses on molecules that are intrinsically magnetic due to their large spin and strong axial anisotropy. The research encompasses chemistry, physics, device fabrication and development, as well as fundamental studies at low temperatures and high magnetic fields. The proposed studies will lead to a better understanding of the quantum properties of isolated single-molecule magnets and how magnetism can be combined with electronic transport in a single-electron transistor setup.
Broader Impact: The proposal will advance our knowledge of single molecule-based electronic devices. These devices have great potential for ultra-high density integration and quantum information processing, which may lead to new and revolutionary technologies. Several graduate and undergraduate students will be trained in the interface between inorganic chemistry and fundamental and applied physics within an environment that constantly crosses the boundaries of these disciplines.
PROJECT OBJECTIVES: This project aimed at investigating On the experimental side, the objectives for this period were: 1) To develop a room temperature fabrication method of nano-gapped single-electron transistors (SETs) 2) To perform single-elecron transport studies of single-molecule magnets 3) To develop graphene-based SETs for studies with spin-polarized currents. 4) To study mononuclear molecular magnets in order to unveil their potential for SET studies On the theory side, the objectives for this period were: 1) To study the Kondo effect in SIM molecules. 2) To study the co-tunneling regime in SMM-based SETs. SIGNIFICANT RESULTS Experiment: We have shown that the capacitive coupling between the active component of a molecule (i.e., ferrocene) and the electrodes in a molecular junction allows for the molecular frontier orbitals to follow changes in the Fermi-level of the electrode they are interacting with. This, together with the fact that non-covalent interactions (especially van der Waals interactions) are, by definition, not associated with hybridization of orbitals, enables efficient rectification in our molecular diodes. In particular, we demonstrate that a proper control of the couplings in our molecules allows turning around a molecular diode (an ideal platform to study several coupling parameters; see below) with optimal device performance. Our results also made it possible to estimate the non-linear shape of the electrostatic potential profile across our junctions experimentally. Our findings are extendable to other types of (bio)molecular- and organic-electronic devices. The results are under review at Nature Communications. In addition, we have disclosed the low temperature magnetic behavior of two closely related mononuclear cobalt(II) complexes in a trigonal–bipyramidal ligand environment, as delineated by single-crystal X-ray diffraction studies. Magnetization measurements in the temperature range 0.25–4 K show hysteretic behavior for temperatures below 2 K, with a coercive field of 600 G. The hysteresis may be associated to the intrinsic magneto-anisotropy of the Co complex, although intermolecular interactions could also be behind the observed hysteretic behavior. The results have been published in Polyhedron (see reference in products section). Theory: We have obtained a clear indication that the Kondo effect in a SIM can be modulated by an applied transverse magnetic field. Our results indicate that, much in the same way that such a field induces Berry-phase oscillations in the tunneling magnetization of SMMs in crystal form, it also induces periodicity in the linear conductance of a SIM at low temperatures. In the absence of transverse anisotropy, the Kondo effect is completely absent for an S = 3/2 SIM. However, when the anisotropy is present, a clear enhancement in the linear conductance at low temperatures is obtained at zero magnetic field, which amounts to the Kondo screening of the pseudospin formed by the Sz = ± 3/2 components of the total spin S. Upon increasing the field, the conductance is reduced significantly, but then a peak emerges as one approaches a value of the magnetic field that corresponds to a diabolical point in the molecule’s energy spectrum (namely, a level crossing of two states with opposite spin orientations). We have found that the temperature regime where this modulation takes place is rather accessible to experiments: we estimated that the Kondo temperature should be in the range of a few kelvin. The magnetic fields required to observe the modulation are under 1 tesla, thus also very easy to achieve in our experimental setup. A letter paper with these results has been submitted to Physical Review B Rapid Communications. BROADER IMPACTS Three graduate, one undergraduate and two high-school students have benefited from the knowledge acquired during this project. Experimental students have been trained in microscopy and microfabrication techniques, as well as low temperature and high sensitivity magnetization and transport spectroscopic characterization techniques. The theory student has been trained on numerical renormalization group techniques for electronic transport calculations. Students have also benefited from active interdisciplinary collaborations with other groups in USA and abroad and have gained experience from attendance to local, national and international conferences where they have presented their research results. In addition, the PIs have been actively involved with outreach activities in different fronts to involve the youngest students in the research covered under this project. Under-represented groups of population in science are the main targets of our approach. For this, both Hispanic and female high-school students have been involved on the research carried out in the PIs' groups.
