UMD Theses and Dissertations
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Item NEXT-GENERATION SUPERCONDUCTING METAMATERIALS: CHARACTERIZATION OF SUPERCONDUCTING RESONATORS AND STUDY OF STRONGLY COUPLED SUPERCONDUCTING QUANTUM INTERFERENCE META-ATOMS(2024) Cai, Jingnan; Anlage, Steven SMA; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metamaterials are artificial structures consisting of sub-wavelength ‘atoms’ with engineered electromagnetic properties that create exotic light-matter interactions through the effective medium approximation. Since the early 2000s, superconductors have been incorporated into a variety of structures to achieve tunable, low-loss, and nonlinear metamaterials, and have enabled applications such as negative index of refraction, near zero permittivity, and parametric amplification. We have designed, fabricated and characterized two types of superconducting metamaterials based on the quantum three-junction flux qubits and classical radio frequency superconducting quantum interference devices (rf SQUIDs). The coplanar waveguide resonators hosting the qubit meta-atoms exhibit anomalous reduction in loss in microwave transmission measurements at low rf excitation levels upon decreasing temperature below 40 mK. In contrast, the well-known standard tunneling model (STM) of the two-level system (TLS), believed to be the dominant source of loss at low temperatures, predicts a loss increasing then saturating with lowering temperatures. This anomalous loss reduction is attributed to the discrete nature of an ensemble of TLSs in the resonator. As temperature decreases, the individual TLS response bandwidth reduces with their coherence rate Γ2 ∼ T, creating less overlap between neighboring TLSs in the energy spectrum. This effective reduction in the density of states around the probe frequency is responsible for the observed lower loss at low rf excitation levels and low temperatures as compared to the STM prediction. We also incorporate the discrete TLS ansatz with the generalized tunneling model proposed by Faoro and Ioffe [PRL 2012, 109, 157005 and PRB 2015, 91, 014201] to fit the experimental data over a wide range of temperatures and rf excitation powers. The resulting goodness of fit is better than all common alternative explanations for the observed phenomenon. Metamaterials made of large arrays of hysteretic (βrf= Lgeo/LJJ > 1) classical rf SQUIDs are also designed and characterized in microwave transmission measurements, where we observed the SQUID self-resonances tuning with applied dc and rf magnetic flux, as well as temperature. The resonance features are tuned with dc flux in integers of the flux quantum, as expected. Due to the phenomenon of multistability present in the large system, the resonance bands can cross those from adjacent dc flux periodicities resulting in hysteresis in dc flux sweeps, which is observed in the experiment. Furthermore, we developed a new three-dimensional architecture of rf SQUID metamaterials where the nearest-neighbor SQUID loops overlap. The resulting capacitive coupling dramatically changes the response by introducing many more resonance bands that spread over a broad range of frequencies, the upper limit of which is much higher than the single-layer counterparts. A resistively and capacitively shunted junction (RCSJ) model with additional capacitive coupling between SQUIDs is proposed and successfully attributes the high frequency bands to the displacement current loops formed between the overlapping wiring of neighboring SQUIDs. The capacitively-coupled rf SQUID metamaterial is relevant to the design of single-flux-quantum-based superconducting digital electronic circuits, which has adopted three-dimensional wiring to reduce the circuit footprint.Item 3D Magnetic Imaging using SQUIDs and Spin-valve Sensors(2016) Jeffers, Alex; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We have used 2 µm by 4 µm thin-film Cu-Mn-Ir spin-valve sensors and high Tc YBa2Cu3O7-x dc SQUIDs to take magnetic images of test samples with current paths that meander between 1 and 5 metallization layers separated by 1 µm to 10 µm vertically. I describe the development and performance of a 3D magnetic inverse for reconstructing current paths from a magnetic image. I present results from this inverse technique that demonstrate the reconstruction of the 3D current paths from magnetic images of samples. This technique not only maps active current paths in the sample but also extracts key parameters such as the layer-to-layer separations. When imaging with 2 µm by 4 µm spin-valve sensors I typically applied currents of 1 mA at 95 kHz and achieved system noise of about 200 nT for a 3 ms averaging time per pixel. This enabled a vertical resolution of 1 µm and a lateral resolution of 1 µm in the top layers and 3 µm in the bottom layer. For our roughly 30 µm square SQUID sensors, I typically applied currents of 1 mA at 5.3 kHz, and achieved system noise of about 200 pT for a 3 ms averaging time per pixel. The higher sensitivity compared to the spin-valve sensor allowed me to resolve more deeply buried current paths.Item Multi-junction effects in dc SQUID phase qubits(2013) Cooper, Benjamin Kevin; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I discuss experimental and theoretical results on an LC filtered dc SQUID phase qubit. This qubit is an asymmetric aluminum dc SQUID, with junction critical currents 1.5 and 26.8 μA, on a sapphire substrate. The layout differs from earlier designs by incorporating a superconducting ground plane and weakly coupled coplanar waveguide microwave drive line to control microwave-qubit coupling. I begin with a discussion of quantizing lumped element circuit models. I use nodal analysis to construct a 2d model for the dc SQUID phase qubit that goes beyond a single junction approximation. I then discuss an extension of this ``normal modes'' SQUID model to include the on-chip LC filter with design frequency ∼ 180 MHz. I show that the filter plus SQUID model yields an effective Jaynes-Cummings Hamiltonian for the filter-SQUID system with coupling g / 2 π ∼ 32 MHz. I present the qubit design, including a noise model predicting a lifetime T1 = 1.2 μs for the qubit based on the design parameters. I characterized the qubit with measurements of the current-flux characteristic, spectroscopy, and Rabi oscillations. I measured T1 = 230 ns, close to the value 320 ns given by the noise model using the measured parameters. Rabi oscillations show a pure dephasing time Tφ = 1100 ns. The spectroscopic and Rabi data suggest two-level qubit dynamics are inadequate for describing the system. I show that the effective Jaynes-Cummings model reproduces some of the unusual features.Item Quantum Coherent Dynamics in a dc SQUID Phase Qubit Using an LC Filter(2010) Kwon, Hyeokshin; Wellstood, Frederick C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A dc SQUID phase qubit consists of two Josephson junctions in a loop. One junction acts as a qubit with two lowest energy levels forming the |0> and |1> status. The second junction and the loop inductance act to isolate the qubit junction from noise. In this thesis, I report on the improvement of the relaxation time and the coherence time in a dc SQUID phase qubit that used an LC filter. I also report the measurement of anomalous switching curves. In order to improve the relaxation and coherence times, I used two isolation networks, an LC isolation network and an inductive isolation network, to decouple the device from the current bias lines. This produced a very large total effective resistance of the input leads that increases the relaxation time of the qubit. In addition, I connected a low-loss SiNx shunting capacitor across the qubit junction to reduce dielectric losses. I measured two dc SQUID phase qubits. Device DS6 had a 4 (μm)2 Al/AlOx/Al qubit junction with a critical current of 0.5 μA and a 1 pF shunting capacitor. It used an LC filter made from a 10 nH inductor and a 145 pF capacitor. The capacitors contained N-H rich SiNx which produced a loss tangent of about 7×10-4. Device DS8 had a 2 (μm)2 Al/AlOx/Al qubit junction with a critical current of 77 nA and an LC filter similar to the first one. The shunting capacitor contained Si-H rich SiNx. Using a pulse readout technique, I measured the characteristics of the qubits, including the transition spectrum, Rabi oscillations, relaxation, Ramsey fringes and state tomography. The best relaxation time T1 for device DS6 was 32 ns and 280 ns for device DS8. The best Rabi decay time T' for DS6 was 42 ns while for device DS8 it was 120 ns. From these and other data I obtained estimates for the best coherence time T2 in device DS6 of 61 ns and 76 ns in device DS8. In DS8, I observed anomalous switching curves; i.e. switching curves which were qualitatively different from conventional switching curves. In the conventional case, the switching curve for the superposition state is the weighted sum of the |0> and |1> curves, but it was not in device DS8. Instead, the switching curve shifted along the current axis as the exited state probability increased. I present a model for understanding the behavior and use this model to extract the probability to be in the excited state.Item A SCANNING SQUID MICROSCOPE FOR IMAGING HIGH-FREQUENCY MAGNETIC FIELDS(2009) Vlahacos, Constantine Peter; Wellstood, Frederick C.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis examines the design and operation of a large-bandwidth scanning SQUID microscope for spatially imaging high frequency magnetic fields. Towards this end, I present results on a cryo-cooled 4.2 K scanning SQUID microscope with a bandwidth of dc to 2 GHz and a sensitivity of about 52.4 nT per sample. By using a thin-film hysteretic Nb dc-SQUID and a pulsed sampling technique, rather than a non-hysteretic SQUID and a flux-locked loop, the bandwidth limitation of existing scanning SQUID microscopes is overcome. The microscope allows for non-contact images of time-varying magnetic field to be taken of room-temperature samples with time steps down to 50 ps and spatial resolution ultimately limited by the size of the SQUID to about 10 micrometers. The new readout scheme involves repeatedly pulsing the bias current to the dc SQUID while the voltage across the SQUID is monitored. Using a fixed pulse amplitude and applying a fixed dc magnetic flux allows the SQUID to measure the applied magnetic flux with a sampling time set by the pulse length of about 400 ps. To demonstrate the capabilities of the microscope, I imaged magnetic fields from 0 Hz (static fields) up to 4 GHz. Samples included a magnetic loop, microstrip transmission lines, and microstrip lines with a break in order to identify and isolate electrical opens in circuits. Finally, I discuss the operation and modeling of the SQUID and how to further increase the bandwidth of the microscope to allow bandwidth of upwards of 10 GHz.Item Coherence in dc SQUID phase qubits(2007-09-17) Paik, Hanhee; Lobb, Christopher J; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I report measurements of energy relaxation and quantum coherence times in an aluminum dc SQUID phase qubit and a niobium dc SQUID phase qubit at 80 mK. In a dc SQUID phase qubit, the energy levels of one Josephson junction are used as qubit states and the rest of the SQUID forms an inductive network to isolate the qubit junction. Noise current from the SQUID's current bias leads is filtered by the network, with the amount of filtering depending on the ratio of the loop inductance to the Josephson inductance of the isolation junction. The isolation unction inductance can be tuned by adjusting the current, and this allows the isolation to be varied in situ. I quantify the isolation by the isolation factor rI which is the ratio of the current noise power in the qubit junction to the total noise current power on its bias leads. I measured the energy relaxation time T1, the spectroscopic coherence time T2* and the decay time constant T' of Rabi oscillations in the Al dc SQUID phase qubit AL1 and the Nb dc SQUID phase qubit NBG, which had a gradiometer loop. In particular, I investigated the dependence of T1 on the isolation rI . T1 from the relaxation measurements did not reveal any dependance on the isolation factor rI. For comparison, I found T1 by fitting to the thermally induced background escape rate and found that it depended on rI . However, further investigation suggests that this apparent dependence may be due to a small-noise induced population in j2i so I cannot draw any firrm conclusion. I also measured the spectroscopic coherence time T2* , Rabi oscillations and the decay constant T' at significantly different isolation factors. Again, I did not observe any dependence of T2* and T' on rI , suggesting that the main decoherence source in the qubit AL1 was not the noise from the bias current. Similar results were found previously in our group's Nb devices. I compared T1, T2* and T0 for the qubit AL1 with those for NBG and a niobium dc SQUID phase qubit NB1 and found significant differences in T2* and T' among the devices but similar T1 values. If flux noise was dominant, NBG which has a gradiometer loop would have the longest Rabi decay time T'. However, T' for NBG was similar to NB1, a Nb dc SQUID phase qubit without a gradiometer. I found that T' = 28 ns for AL1, the Al dc SQUID phase qubit, and this was more than twice as long as in NBG (T' ~ 15 ns) or NB1 (T' ~ 15 ns). This suggests that materials played an important role in determining the coherence times of the different devices. Finally, I discuss the possibility of using a Cooper pair box to produce variable coupling between phase qubits. I calculated the effective capacitance of a Cooper pair box as a function of gate voltage. I also calculated the energy levels of a Josephson phase qubit coupled to a Cooper pair box and showed that the energy levels of the phase qubit can be tuned with the coupled Cooper pair box.Item Combinatorial Investigation of Ferromagnetic Shape Memory Materials(2005-09-15) Famodu, Olugbenga Olawale; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Combinatorial synthesis is research methodology which allows one to systemically study a large number of compositionally varying samples simultaneously. We apply this technique to the investigation of multifunctional materials. Different designs of combinatorial libraries and various characterization tools are implemented in order to rapidly map composition-structure-property relationships in a variety of materials systems. In this thesis, I will discuss combinatorial investigation of various shape memory alloys. We have utilized the combinatorial magnetron co-sputtering deposition technique for fabricating composition spreads of ternary alloy systems containing ferromagnetic shape memory alloys (FSMAs) and thermoelastic shape memory alloys (SMAs). Magnetic properties of the composition spreads were rapidly characterized using a room temperature scanning semiconducting quantum interference device (SQUID) microscope which provides mapping of the magnetic field emanating from different parts of the composition spreads. By applying the inversion technique to the mapping of the magnetic field distribution, we have mapped the magnetic phase diagram of the Ni-Mn-Ga and Ni-Mn-Al systems whose Heusler compositions Ni2MnGa and Ni2MnAl are well known ferromagnetic shape memory alloys (FSMAs). In addition, a rapid visual inspection technique was developed for detection of reversible martensites using arrays of micromachined cantilevers. A large, previously unexplored compositional region of FSMAs outside the Heusler composition was found. In search of novel FSMAs, we have also investigated a number of other ternary alloys systems. These systems included Ni-Mn-In, Gd-Ge-Si, Co-Mn-Ga, Ni-Fe-Al, and Co-Ni-Ga. A summary of the results from the investigation of these systems is presented. We have used the combinatorial technique to search for "ideal" SMAs with minimal hysteresis. For pursuing this, we had first set out to verify the geometric non-linear theory of martensites which predicts the conditions under which the "ideal" SMA can occur. This was facilitated by the composition spread investigation of the Ni-Ti-Cu system and the use of synchrotron x-ray microdiffraction. We found that one of the criteria prescribed by the theory for achieving minimal hysteresis is closely obeyed. We have demonstrated that we can indeed use the technique we have developed here together with the theory to explore SMAs with minimal hysteresis.