Charging effects in niobium nanostructures

Three types of metallic nanostructures comprising niobium were investigated experimentally; in all three types, electric transport at very low temperatures was governed by Coulomb blockade effects. 1. Thin film strips of niobium could be tuned into resistor strips by an electrochemical anodisation process, using microfabricated masks and in situ resistance monitoring. These resistors showed a transition from superconducting to insulating behaviour with increasing sheet resistance, occurring at a value approximately equal to the quantum resistance for Cooper pairs, h/(4e^2). 2. Combining the anodisation technique with lateral size minimisation by shadow evaporation, devices in a single electron transistor-like configuration with two weak links and a small island between these were made. Direct evidence for the Coulomb blockade in the anodisation thinned niobium films was found when the transport characteristics could be modulated periodically by sweeping the voltage applied to a gate electrode placed on top of the structure. 3. Conventional single electron transistors with Al base electrodes, AlO_x barriers formed in situ by oxidation, and Nb top electrodes were made by angular evaporation. The output current noise of such a transistor was measured as a function of bias voltage, gate voltage, and temperature. The low frequency noise was found to be dominated by charge input noise. The dependence of the noise on the bias voltage is consistent with self-heating of the transistor activating the noise sources.Comment: PhD thesis, 177 pages, 42 figures (images downsampled

Tunneling Assisted Forbidden Transitions in the Single Molecule Magnet Ni4

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

Experimental study of the quantum phase-slip effect in NbN nanowires

Coherent quantum phase-slip (QPS) in a superconducting nanowire is the dual phenomenon to the well-known Josephson effect. Josephson junctions form the basis of superconducting electronic circuits with a wide range of applications, and each of those circuits has a corresponding dual quantum phase-slip device with a dual purpose. Examples that draw particular attention are a new quantum standard of electric current, and a quantum phase-slip qubit. The aim of this project is to develop methods of design, fabrication, and measurement of quantum phase-slip nanowires, and to demonstrate the potential of these devices for technological application. In our experiments we incorporate NbN nanowires into a superconducting loop and bias the loop with a magnetic flux. The state of the nanowire-embedded loop is then read out by coupling to a high quality coplanar waveguide resonator. In this thesis we present the results of two such experiments. First, we fabricated NbN nanowires using a neon focused-ion-beam, and measured their properties at T=300 mK. Periodic tuning of the resonant frequency of the readout resonator revealed that magnetic flux is transferred to the interior of the loop with flux-quantum-periodicity. Our measurements confirm that the flux-quantum transfer is mediated by incoherent quantum phase-slips occurring in the nanowires, and that these incoherent QPS can be fully controlled with an external bias. In the second experiment, nanowire-embedded NbN loops were fabricated by electron-beam lithography and cooled to T=10 mK. The resonant frequency tuning exhibited avoided crossings, which is evidence of coherent coupling between the resonator and a coherent quantum two-level system. We numerically fit these avoided crossings to the Jaynes-Cummings model to extract the properties of the two-level system, and find a good fit with the design parameters of our nanowire qubit. Finally we discuss whether the observation of coherent dynamics is evidence of coherent QPS in the EBL-fabricated nanowire

Disorder in Superconductors in Reduced Dimensions

Superconducting nanowires have been identified as dual elements to the Josephson junction. This duality is attributed to the existence of quantum phase slip (QPS) phenomena, where the magnitude of the superconducting order parameter fl uctuates to zero. This has provided scope for equivalents to well-established applications of Josephson physics, such as a quantum current standard. Existing literature indicates an in uence of disorder on the rate of QPS events in superconductors, but few studies have looked specifically at quantifying this disorder as dimensions are reduced for QPS materials. We have investigated disorder in two superconductors of particular significance for QPS phenomena, NbSi and NbN. We have engineered compositions of these materials and grown thin-films using magnetron sputter deposition. We developed a novel top-down nanowire fabrication technique to overcome factors limiting the minimum widths achievable using standard lift-off techniques and used this method to reduce our films to nanowires for the investigation of dimensional effects. We present low-temperature transport behaviour in our superconducting thin- films and demonstrate the dependence of the critical temperature on both the sheet resistance and film thickness. These illustrate a trend in disorder from which we extract the Finkel'stein disorder parameter γ, the mean free path, and the BCS and Ginzburg-Landau coherence lengths in our films. As films are reduced to nanowires, we show the infl uence of noise on measurements of superconductivity in nanowires and demonstrate the importance of filtering. We demonstrate that the relationship between the critical temperature and dimension is also observable for a reduction in width of the superconductor when reduced to nanowire dimensions. Finally, using characteristics we have extracted from our investigations, we present a feasibility study on the integration of these nanowires into a voltage-biased QPS junction circuit, dual to the current-biased Josephson junction. Using QPS theory, we predict how our materials are expected to behave in such a circuit and present measurements of a prototype device

Landau-Zener-Stuckelberg interferometry

A transition between energy levels at an avoided crossing is known as a Landau-Zener transition. When a two-level system (TLS) is subject to periodic driving with sufficiently large amplitude, a sequence of transitions occurs. The phase accumulated between transitions (commonly known as the Stuckelberg phase) may result in constructive or destructive interference. Accordingly, the physical observables of the system exhibit periodic dependence on the various system parameters. This phenomenon is often referred to as Landau-Zener-Stuckelberg (LZS) interferometry. Phenomena related to LZS interferometry occur in a variety of physical systems. In particular, recent experiments on LZS interferometry in superconducting TLSs (qubits) have demonstrated the potential for using this kind of interferometry as an effective tool for obtaining the parameters characterizing the TLS as well as its interaction with the control fields and with the environment. Furthermore, strong driving could allow for fast and reliable control of the quantum system. Here we review recent experimental results on LZS interferometry, and we present related theory.Comment: 34 single-column pages, 11 figure