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 OPTIMIZATION OF PLASMA ASSISTED MOLECULAR BEAM EPITAXY GROWN NbxTi1-xN FOR EPITAXIAL JOSEPHSON JUNCTIONS(2023) Thomas, Austin Michael; Richardson, Christopher; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis is an investigation into the growth and characterization of NbNx and TiN transition metal nitrides, along with the alloy NbxTi1-xN. These materials are commonly used in many applications ranging from superconducting quantum computing, superconducting conventional computing, high kinetic inductance devices such as single photon detectors, and hard coatings for industrial applications. This thesis will begin with an overview of superconducting quantum computing and superconducting materials, then review the fabrication of Josephson junctions and highlight the need for material improvement. The goal of this work is to grow a superconducting nitride material which can be engineered to lattice match with AlN, the barrier layer in a hypothetical all-nitride, epitaxially grown superconducting quantum computing structure. The alloy NbxTi1-xN is chosen as the superconducting alloy of choice due to the range of lattice constants available, the high critical temperature of these nitrides, and the high quality of material able to be grown using PAMBE. The first aim of this thesis studies the binary transition metal nitrides NbNx and TiN to generate endpoints for various properties of the alloy NbxTi1-xN. This thesis is one of the first investigations of multi-phase growth of ε-NbN and γ-Nb4N3, and demonstrates control over the phase, crystal orientation, superconducting properties, and surface morphology by changing PAMBE growth parameters. The second aim of this thesis demonstrates the growth of NbxTi1-xN and is the first investigation of tunable material properties for this alloy by adjusting the composition. The last aim of this work is the development of a novel annealing scheme used to prepare NbxTi1-xN thin films for Josephson junction integration. The novel annealing scheme ensures excellent surface roughness of NbxTi1-xN thin films, increases the superconducting critical temperature of this alloy from approximately 14 K to 16.8 K, and improves the crystal quality by way of nitrogen incorporation and improvement of the crystal quality. The results from this work will be crucial in developing NbxTi1-xN / AlN / NbxTi1-xN Josephson junctions with smooth, uniform interfaces and low-loss, defect free nitride materials. Additionally, this thesis represents an investigation into the relationship between phases of NbNx and TiN, the role of nitrogen incorporation caused by in-situ annealing, and a useful record of control over this material using PAMBE growth conditions and alloy composition.Item Supercurrent and Andreev bound states in multi-terminal Josephson junctions(2022) Lee, Hanho; Manucharyan, Vladimir; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A Josephson junction (JJ) is known as a weak link connecting two superconductors, in which the non-dissipative supercurrent flows. More than two superconductors also can form a single composite JJ, called a “multi-terminal JJ”, by being connected through a common weak link. The supercurrent in multi-terminal JJs may depend on multiple superconducting phase differences defined across the junction. The multi-phase-dependence of the supercurrent is attributed to the sub-gap quasiparticle bound states, called Andreev bound states (ABSs), which carry the supercurrent across the junction. First, we investigate the supercurrent of three- and four-terminal JJs fabricated on hybrid two-dimensional Al/InAs (superconductor/semiconductor) heterostructures. The critical current of an N-terminaljunction is given as a (N-1)-dimensional hypersurface of the DC bias currents, which can be reduced to a set of critical current contours (CCCs) in low dimensional space. Non-trivial modifications of the geometry of the CCCs in response to magnetic field, electrical gating and phase biasing can be understood in the presence of the multi-phase-dependent ABSs. Second, we demonstrate the multi-phase-dependent ABSs in three-terminal JJs by tunneling spectroscopy measurements. Multi-loop superconducting quantum interference devices (SQUIDs) are realized to detect the multi-phase-dependence. The ABS energy spectrum mimics electronic band structure in solid, which makes multi-terminal JJs provide a new platform to study band topology in higher dimensional parameter space. Moreover, spin-splitting of ABS energies induced by the multi-phase and gapless energy spectrum facilitated by the presence of a discrete vortex, a nonzero winding of the superconducting phases, are investigated.Item On the nature of the Josephson effect in topologically nontrivial materials(2021) Trimble, Christie Jordan; Williams, James R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A Josephson junction (JJ) couples the supercurrent flowing between two weakly linked superconductors to the phase difference between them via a tunnel barrier, giving rise to a current-phase relation (CPR). While a sinusoidal CPR is expected for conventional junctions with insulating weak links, devices made from some exotic materials may give rise to unconventional CPRs and unusual Josephson effects. Here, I experimentally investigate three such cases. In the first part of the thesis, I fabricate JJs with weak links made of the topological crystalline insulator Pb$_{0.5}$Sn$_{0.5}$Te and compare them with JJs made from its topologically trivial cousin, PbTe. I find that measurements of the AC Josephson effect reveal a stark difference between the two: while the PbTe JJs exhibit Shapiro steps at the expected values of $V=nhf/2e$, Pb$_{0.5}$Sn$_{0.5}$Te JJs show more complicated subharmonic structure. I present the skewed sinusoidal CPR necessary to reproduce these measurements and discuss a potential origin for this alteration. Next, I investigate the proximity-induced superconductivity in SnTe nanowires by incorporating them as weak links in Josephson junctions. I report indications of an unexpected breaking of time-reversal symmetry in these devices, including observations of an asymmetric critical current in the DC Josephson effect, a prominent second harmonic in the AC Josephson effect, and a magnetic diffraction pattern with a minimum in critical current at zero magnetic field. I analyze how multiband effects and the experimentally visualized ferroelectric domain walls may give rise to a nonstandard CPR in the junction. Finally, I measure JJs with weak links made of the topological insulator (BiSb)$_2$Te$_3$. Under low frequency RF radiation, I observe suppression of the first and third Shapiro steps, consistent with the fractional AC Josephson effect. This could indicate a 4$\pi$ periodic component in the junction's CPR, potentially implying the presence of Majorana bound states. However, not all of the devices showed this behavior; some devices show suppression of only the first step, while others show distortions to the AC Josephson effect which differ upon repeated measurements, possibly indicating other nonequilibrium effects at play. I discuss this behavior and possible topologically trivial sources of step suppression found in the literature.Item Modelling and Measurement of Reciprocal Quantum Logic Circuits(2021) Luo, Henry; Wellstood, Frederick; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents work on three aspects of Reciprocal Quantum Logic (RQL) circuits: devising accurate inductance models of Nb wires, analyzing, and measuring RQL logic gates, and mitigating the effects of parasitic coupling in RQL circuits. These separate aspects are all important to verify or improve the operating margins of RQL circuits. An inductance model for Nb wire routes based on electromagnetic simulation and measurement of inductance test circuits is described. The inductance test circuits are fabricated in the D-Wave Systems process. The inductance of straight segments, corners, and vias, and the mutual inductance between straight segments are modelled and measured. Measured data from 6 different releases and 8 wafers is presented. Proximity effects and wafer radial effects are discussed. Finally, a 2-material model for Nb wires is introduced and yields a good fit to the measured inductances. An RQL gates testbed chip was designed, fabricated, and tested. This chip was also fabricated in the D-wave Systems process. The measured gates included the buffer Josephson Transmission Line (JTL), delay JTL, and2, and or2 gates. Fifteen different design variations of each of these gates, which varied three inductors in the gate, are analyzed and shown to match well to simulation models. The variation in the upper operating limit or upper margin per design variation was roughly 3.5%, 5%, 7%, and 5% for the buffer JTL, delay JTL, and2, and or2, respectively. Tools were developed to account for and mitigate the effect of parasitic mutual coupling in RQL circuits. A back-annotation RQL tool (BART), to back-annotate parasitic mutual inductance, was developed for the extraction and simulation of small RQL designs. Using this tool on a 2x2 bit memory cell, the back-annotated simulation was able to explain measurements of the different operating regions for different test vectors. A methodology, bias point delta (BPD), for improving signal integrity of RQL circuits is also introduced. This methodology was applied to a 3,274 JJ design and the amount of coupled flux before and after routing improvements are compared. The work in this dissertation describes the development of models and tools that aid in the design of large RQL circuits, and it will serve as the groundwork for future research in RQL and potentially other Single Flux Quantum (SFQ) technologies.Item Superconducting Enhancement and Electronic Nematicity in Substituted BaNi2As2(2019) Eckberg, Christopher; Paglione, Johnpierre; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Inspired by the frequent presence of nematicity in the high Tc superconducting systems, this thesis is focused on the interplay of nematic and superconducting order in a system that does not possess long range magnetism. Here I describe my measurements of the physical properties of Ba(Ni[1-x]Cox)2As2 and Ba[1-x]SrxNi2As2 intermetallic compounds, characterizing both superconductivity and nematicity in these series. Thermodynamic, transport, and magnetic properties of single crystals synthesized using a flux growth technique are reported. Using the results of these physical property measurements, I construct the electronic phase diagrams of the Ba(Ni[1-x]Cox)2As2 and Ba[1-x]SrxNi2As2 series. In both substitution series, increasing x smoothly suppresses a tetragonal-triclinic structural phase transition. At the low temperature structural phase boundary, a large enhancement in superconducting Tc is also observed in both systems. The Ba[1-x]SrxNi2As2 series was further characterized through measurements of symmetry isolated components of the fourth-rank elastoresistivity tensor. I observe a divergence in elastoresistivity over a wide range of temperatures and x values in Ba[1-x]SrxNi2As2 crystals, indicative of electronically driven rotational symmetry breaking in this series. The low temperature elastoresistivity is peaked in the vicinity of optimal Tc, suggesting the enhanced superconducting pairing observed in this region is born from strong nematic fluctuations in the system.Item SUPERCONDUCTORS THAT BREAK TIME-REVERSAL SYMMETRY(2019) Boyer, Lance L.; Yakovenko, Victor M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since 2006 it has been discovered experimentally that the superconducting state spontaneously breaks time-reversal symmetry (TRS) in several materials, such as Sr2RuO4, UPt3, URu2Si2, PrOs4Sb12, and Bi/Ni bilayers. This dissertation studies three physical phenomena related to time-reversal symmetry breaking (TRSB) in these superconductors. The experimental evidence for TRSB comes from the magneto-optical polar Kerr effect, which is determined by the high frequency ac Hall conductivity. However, these superconductors are also expected to exhibit a spontaneous dc Hall effect in the absence of an applied magnetic field. In the first part of this dissertation we propose a method for measuring the low frequency Hall conductivity in superconductors with TRSB. The method is based on a Corbino disk geometry where an oscillating co-axial magnetic field induces circular electric field, which, in turn, induces radial charge oscillations due to the Hall conductivity. In the second part, we propose an explanation for the polar Kerr effect observed in the Hidden-Order phase of the heavy-fermion superconductor URu2Si2. Using a Ginzburg-Landau model for a complex order parameter, we show that the system can have a metastable ferromagnetic state, which produces the Kerr signal, even if the Hidden-Order state respects TRS. We predict that applying a reversed magnetic field should reset the system to the non-magnetic ground state, resulting in zero Kerr signal. In the third part of the dissertation, we investigate the conditions for the existence of a Majorana bound state on a vortex in a 2D d+id superconductor with strong spin-orbit coupling. This TRSB pairing was proposed earlier for the Ni/Bi bilayer. We find that the Majorana bound state can exist for a d+id pairing under conditions similar to those for s-wave pairing.Item INVESTIGATION OF TUNNELING IN SUPERCONDUCTORS USING A MILLIKELVIN SCANNING TUNNELING MICROSCOPE(2019) Liao, Wan-Ting; Lobb, Christopher J.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, I discuss my use of a millikelvin scanning tunneling microscope (STM) to investigate tunneling phenomena in superconductors. As part of an effort to construct an STM to measure the superconducting phase difference, I first describe how I modified a dual-tip scanning tunneling microscope by electrically connecting the two tips together with a short (3 mm) strip of flexible 25 µm thick Nb foil. I also discuss the technique I developed for keeping each tip in feedback when only the total tunnel current through both tips can be measured. I then describe simultaneous room-temperature imaging with both tips on samples of Au/mica and highly oriented pyrolytic graphite (HOPG). Next, I report single-tip results from scanning tunneling microscopy of 25 nm and 50 nm thick films of superconducting TiN at 0.5 K. I found large variations in the tip-sample conductance-voltage characteristics in these samples. At some locations the characteristics showed a clear superconducting gap, as expected for superconductor-normal (S-I-N) tunneling through a high barrier height. At other locations there was a distinct zero-voltage conductance peak, as expected for S-N Andreev tunneling through a low barrier height. I compare the data to the BlonderTinkham-Klapwijk (BTK) theory and the Dynes model of tunneling into a superconductor with broadened density of states. I find that the BTK model provides better fits and reveals a remarkable correlation between the superconducting gap ∆, the temperature T and the barrier height Z. Possible causes for this correlation, including local heating and surface contamination, are discussed. Finally, I describe measurements of I(V) characteristics of a Josephson junction formed by a scanning tunneling microscope with a Nb sample and a Nb tip at 50 mK and 1.5 K. To better understand the physics of this system, I generalized the multiple Andreev reflection (MAR) theory of Averin and Bardas to describe junctions having electrodes with different superconducting gaps. For tunneling resistance Rn between 10 MΩ and 100 kΩ, there was no observable supercurrent at 50 mK or 1.5 K. For Rn between 100 kΩ and about 10 kΩ, the junctions showed hysteretic behavior, with the forward-sweep switching current Is larger than the reverse-sweep retrapping current Ir. In this regime, the critical current I0 was suppressed and the current-voltage characteristics showed a relatively small non-zero resistance R0 at V = 0 that scaled with . For Rn less than the quantum resistance (∼ 12 kΩ), the I-V characteristics deviate from single channel MAR theory. In this limit, the tip makes contact with the sample, as revealed by the dependence of the junction conductance curves on the tip-sample separation. By fitting my two-gap MAR theory to the I(V) data, I obtain superconducting gaps of the tip and sample as a function of the tunnel resistance Rn. I find the sample has nearly the full gap of bulk Nb (∆∼ 1.5 meV), but the tip gap is only about 0.67 meV, and decreases for Rn ≤ 10 kΩ.Item Classical Analogies in the Solution of Quantum Many-Body Problems(2017) Keser, Aydin Cem; Galitski, Victor M.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We consider three quantum many-body systems motivated by recent developments in condensed matter physics, namely topological superconductivity, strongly interacting Bose-Einstein condensates and many-body localization with periodically driven systems. In each of the three problems, an analogy with classical mechanics is employed in the solution of the problem and the interpretation of results. These analogies, in addition to facilitating the solution, illustrate how unique features of classical mechanics or macroscopic phenomena such as macroscopic order parameter and observables, hydrodynamics, spacetime curvature, noise and dissipation, chaos and delocalization emerge out of quantum mechanics. The three problems we study are as follows. In the 1st problem, we use quasiclassical methods of superconductivity to study the superconducting proximity effect from a topological p-wave superconductor into a disordered quasi-one-dimensional metallic wire. We demonstrate that the corresponding Eilenberger equations with disorder reduce to a closed nonlinear equation for the superconducting component of the matrix Green's function. Remarkably, this equation is formally equivalent to a classical mechanical system (i.e., Newton's equations), with the Green's function corresponding to a coordinate of a fictitious particle and the coordinate along the wire corresponding to time. This mapping allows us to obtain exact solutions in the disordered nanowire in terms of elliptic functions. A surprising result that comes out of this solution is that the p-wave superconductivity proximity induced into the disordered metal remains long range, decaying as slowly as the conventional s-wave superconductivity. It is also shown that impurity scattering leads to the appearance of a zero-energy peak. In the second problem, we consider a system of bosons in the superfluid phase. Collective modes propagating in a moving superfluid are known to satisfy wave equations in a curved spacetime, with a metric determined by the underlying superflow. We use the Keldysh technique in a curved spacetime to develop a quantum geometric theory of fluctuations in superfluid hydrodynamics. This theory relies on a ``quantized" generalization of the two-fluid description of Landau and Khalatnikov, where the superfluid component is viewed as a quasi-classical field coupled to a normal component { the collective modes/phonons representing a quantum bath. This relates the problem in the hydrodynamic limit to the \quantum friction" problem of Caldeira-Leggett type. By integrating out the phonons, we derive stochastic Langevin equations describing a coupling between the superfluid component and phonons. These equations have the form of Euler equations with additional source terms expressed through a fluctuating stress-energy tensor of phonons. Conceptually, this result is similar to stochastic Einstein equations that arise in the theory of stochastic gravity. We formulate the fluctuation-dissipation theorem in this geometric language and discuss possible physical consequences of this theory. In the third problem, we investigate dynamical many-body localization and delocalization in an integrable system of periodically-kicked, interacting linear rotors. The linear-in-momentum Hamiltonian makes the Floquet evolution operator analytically tractable for arbitrary interactions. One of the hallmarks of this model is that depending on certain parameters, it manifests both localization and delocalization in momentum space. We present a set of \emergent" integrals of motion, which can serve as a fundamental diagnostic of dynamical localization in the interacting case. We also propose an experimental scheme, involving voltage-biased Josephson junctions, to realize such many-body kicked models.Item Investigation of Iron-Based and Topological Superconductors via Point-Contact Spectroscopy(2015) Ziemak, Steven Joseph; Paglione, Johnpierre; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I report results of point-contact spectroscopy (PCS) measurements preformed on a variety of superconductors which are predicted to exhibit unconventional Cooper pairing mechanisms. Point-contact spectra of the iron pnictide BaFe_{1-x}$Pt$_x$As$_2$ are consistent with a two-gap isotropic s-wave model. This conclusion is supported by previously published results from thermal conductivity, angle-resolved photoemission spectroscopy (ARPES), and Raman spectroscopy, which confirm a lack of nodes in the order parameter and the presence of a gap of magnitude 3 meV. Conductivity spectra were also measured for the half-Heusler materials YPtBi and LuPdBi using the soft point contact method. I argue that the repeated observation of a single peak in $dI/dV$ at zero bias is not consistent with a conventional $s$-wave model. Based on attempts to fit my data to the Blonder-Tinkham-Klapwijk theory and comparison to previous experimental and theoretical work, I conclude that my results are most consistent with a model of triplet Cooper pairs and an order parameter with an odd-parity component.