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Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Atomic, Molecular and Optical Physics | Condensed Matter Physics | Quantum Physics
This dissertation presents work in exploring novel quantum phenomena in singlemolecule magnets (SMMs) and superconducting circuits. The degree of the freedom studied is the magnetic moment of a single molecule and the flux quantum trapped in a superconducting ring. These phenomena provide us with new insights into some basic questions of physics and may also find their application in quantum computing.
The molecule we studied is Ni4 ([Ni4(hmp)(dmp)Cl]4) which can be treated as a spin-4 magnet. The large magnetic anisotropy of the molecule leads to bistability of the magnetic moment at low temperatures, with spin-up and spin-down states separated by a barrier. We applied electron spin resonance (ESR) measurements to study the forbidden transitions between spin-projection states. These transitions are usually not allowed due to the symmetry of the molecule but become possible under certain circumstance by symmetry breaking. In the first experiment, we attempted to couple the SMMs to a microstrip resonator hoping to v observe highly forbidden transitions between the states jm = �2i and jm = 2i. We found that the resonator traps magnetic flux at high fields so that it fails to provide reliable results. To address this issue, we developed a mechanism that in-situ orients the resonator surface with the magnetic field to minimize flux trapping. In the second ESR experiment, we coupled the molecule to a 3D-cavity resonator and observed highly forbidden transitions when absorbing photons where the angular momentum changes by several times ~. These transitions are observed at low applied fields, where tunneling is dominated by the molecule’s intrinsic anisotropy and the field acts as a perturbation.
In another experiment associated with superconducting circuits, we studied a single Cooper pair transistor (SCPT) driven by a microwave field, hoping to observe the Aharonov- Casher effect where flux tunneling paths can interfere and lead to a gate-charge modulation of the I-V behavior of the SCPT.We simulated the process and demonstrated that by choosing the parameters carefully, we should be able to fully suppress the flux-tunneling rate.
Chen, Yiming, "Tunneling Assisted Forbidden Transitions in the Single Molecule Magnet Ni4" (2017). Doctoral Dissertations. 869.