Previously employed wet-chemistry approaches to DNA metallization from granular wires that become highly resistive at low temperatures. We have developed a process to make superconducting nanowires templated by DNA molecules that are homogeneous, are less than 10 nm in diameter, make seamless contacts the leads, and become superconducting at low temperatures. Our method involves isolating single DNA strands stretched across a narrow but deep trench and sputter-coating MoGe on the DNA scaffold. We have used these nanowires as the basis for a quantum interference device in which two wires are connected in parallel by superconducting films. We have discovered resistance oscillations for the two-wire device as a function of phase gradients in the leads caused by magnetic screening currents or applied currents, and we have developed a theory to explain our observations based on an extension of the Langer- Ambegaokar-McCumber-Halperin theory of thermally activated phase slips. In addition, we also measure the effect of vortex motions on these oscillations and confirm the field dependence of the Campbell penetration depth.
Previously employed wet-chemistry approaches to DNA metallization from granular wires that become highly resistive at low temperatures. We have developed a process to make superconducting nanowires templated by DNA molecules that are homogeneous, are less than 10 nm in diameter, make seamless contacts the leads, and become superconducting at low temperatures. Our method involves isolating single DNA strands stretched across a narrow but deep trench and sputter-coating MoGe on the DNA scaffold. We have used these nanowires as the basis for a quantum interference device in which two wires are connected in parallel by superconducting films. We have discovered resistance oscillations for the two-wire device as a function of phase gradients in the leads caused by magnetic screening currents or applied currents, and we have developed a theory to explain our observations based on an extension of the Langer- Ambegaokar-McCumber-Halperin theory of thermally activated phase slips. In addition, we also measure the effect of vortex motions on these oscillations and confirm the field dependence of the Campbell penetration depth.
Quantum phase slippage (QPS) in a superconducting nanowire is a new candidate for developing a quantum bit [Mooij et al. New J. Phys. 2005, 7, 219; Mooij et al. Nat. Phys. 2006, 2, 169; Khlebnikov http://arxiv.org/abs/quant-ph/0210019 2007]. It has also been theoretically predicted that the occurrence of QPS significantly changes the current-phase relationship (CPR) of the wire due to the tunneling between topologically different metastable states [Khlebnikov Phys. Rev. B 2008, 78, 014512]. We present studies on the microwave response of the superconducting nanowires to reveal their CPRs. First, we demonstrate a simple nanowire fabrication technique, based on commercially available adhesive tapes, which allows making thin superconducting wire from different metals. We compare the resistance vs temperature curves of Mo(76)Ge(24) and Al nanowires to the classical and quantum models of phase slips. In order to describe the experimentally observed microwave responses of these nanowires, we use the McCumber-Stewart model [McCumber J. Appl. Phys. 1968, 39, 3113; Stewart Appl. Phys. Lett. 1968, 12, 277], which is generalized to include either classical or quantum CPR.
The topic of superconducting nanowires has recently been an interesting field of research which has included the study of the superconductor to insulator transition (SIT), the observation of macroscopic quantum behavior such as quantum phase slips (QPS), and the potential use of nanowires as qubits. Superconducting coplanar microwave waveguide resonators have also become a popular way of studying superconducting junctions and qubits, as they provide an extremely low noise environment. For example, superconducting two-dimensional Fabry-Perot resonators have been used by other groups to make non-demolition measurements of a qubit. The motivation of this thesis will be the merging of the fields of superconducting nanowires and the technique of using superconducting microwave resonators to study junctions by incorporating a nanowire into the resonator itself at a current anti-node. By doing this, the nonlinear effects of the nanowire can be studied which may find application in single photon detectors, mixers, and the readout of qubits. We also employ the technique of molecular templating to fabricate some of the thinnest superconducting nanowires ever studied (down to ~ 5 nm in diameter in some cases). In this thesis, we extend the understanding of the nonlinear properties of a nanowire- resonator system and investigate a new type of nonlinearity that involves a pulsing regime between the superconducting and normal phases of the nanowire. We develop a model, which describes the results quantitatively and by modeling the system, we are able to extract information regarding the relaxation time of the nanowire back into the superconducting state. We also study double nanowire-resonator systems where two closely spaced parallel nanowires interrupt the resonator center conductor and form a loop where vortex tunneling processes can occur. Using a double nanowire-resonator we are able to observe the Little-Parks effect at low temperatures (where the resistance of the wires is immeasurably low) and are able to confirm the multivalued nature of the current phase relationship (CPR) in nanowires. Additionally, we observe an anomalous transmission regime where the periodic pulsing is replaced by stochastic amplitude fluctuations. In addition to microwave measurements on nanowires, we also study the normal state in resistively shunted nanowires with dc measurements where the inclusion of a shunt resistor is observed to change the nature of the normal state from the Joule heated state to a state that preserves phase coherence. Finally, the statistics on switching current events in graphene proximity junctions are analyzed and compared to the well known results for Josephson junctions. Only thermal activation (and no macroscopic quantum tunneling) is observed in graphene proximity superconducting junctions down to temperatures of ~ 300 mK.
Macroscopic quantum tunneling is a process by which a macroscopic object, rather than a single electron, tunnels between two macroscopically distinct quantum states. In this dissertation, we will present the study of macroscopic quantum tunneling of phase-slips in superconducting devices, including one-dimensional nanowires, quasi-two-dimensional superconducting strips and doubly connected superconducting devices composed of two nanowires connected in parallel. We observe macroscopic quantum tunneling in these devices by measuring switching current distributions. It is known that the standard deviation of switching current distributions can be measured on a single nanowire to reveal temperatures at which macroscopic quantum tunneling is responsible for phase-slips. Therefore we begin by studying higher moments of the switching current distributions, namely the skewness S (a measure of the asymmetry of a distribution) and kurtosis K (a measure of its peakedness). We find that the skewness and kurtosis of the switching current distributions obtained via the Kurkijarvi process on devices composed of single nanowires do not depend on whether the switching events are initiated by quantum or thermal phase-slips. Skewness and kurtosis deviate considerably from the values associated with a Gaussian distribution (S=0 and K=3). If, in an experiment these higher moments approach Gaussian values they indicate the existence of noise. Next we study macroscopic quantum tunneling of phase-slips in quasi-two-dimensional superconducting strips, which are commercially available devices used as single-photon detectors. These devices are composed of (250 um) long, (120 nm) wide, meandering superconducting strips. Each time a photon with sufficient energy strikes the strip a voltage pulse is produced and counted. However, the accuracy of these detectors is limited by a rate of dark counts (false events). We find that at sufficiently low temperatures, the macroscopic quantum tunneling of a vortex through an edge barrier on the wire (which results in a phase-slip as the vortex crosses the wide wire) contributes a base-level dark count rate in these detectors which must be considered during operation at low enough temperatures. After studying tunneling in single one-dimensional nanowires and wide, quasi-two-dimensional strips, we move our focus to doubly connected superconducting samples composed of two nanowires connected in parallel. We characterize and model these devices, and show their applicability as nanometer-scale superconducting memory cells. We develop precise algorithms allowing us to write and read the information onto such memory cells. We also observe signatures of macroscopic quantum tunneling in these doubly connected devices by observing a saturation in the standard deviation of switching current distributions at low temperatures. We then discuss how macroscopic quantum tunneling of the memory state can lead to a dissipationless operation of the superconducting memory cell. In each of these devices, whether single one-dimensional nanowires, wide quasi-two-dimensional superconducting strips, or two nanowires in parallel, we find, quite surprisingly, that the same equations can be used to model macroscopic quantum tunneling at the high currents we apply. Therefore we argue that the physics of macroscopic quantum tunneling in wide quasi-two-dimensional strips and doubly connected nanowire devices can be well understood in the context of tunneling in a single, one-dimensional nanowire.
In systems with reduced dimensions quantum fluctuations have a strong influence on the electronic conduction, even at very low temperature. In superconductors this is especially interesting, since the coherent state of the superconducting electrons is strongly interacting with these fluctuations and therefore is a sensitive tool to study them. In this paper, we report on comprehensive measurements of superconducting nanowires in the quantum phase slip regime. Using an intrinsic electromigration process, we have developed a method to lower the resistance of lithographically fabricated highly resistive nanowires in situ and in small consecutive steps. At low temperature we observe critical (Coulomb) blockade voltages and superconducting critical currents, depending on the nanowire's normal-state resistance, in good agreement with theoretical models. Between these two regimes, we find a continuous transition displaying a nonlinear metallic-like behavior. The reported intrinsic electromigration technique is not limited to low temperatures as we find a similar change in resistance that spans over three orders of magnitude also at room temperature. Aside from superconducting quantum circuits, such a technique to reduce the resistance may also have applications in modern electronic circuits.
Superconducting nanowires have been used to detect single photons for more than 20 years. The basic principle is that photons can perturb superconducting Cooper pairs and thereby create a resistive hotspot, which produces a current-crowding effect. This current-crowding effect reduces the superconducting energy gap near the hotspot, which results in the nanowire changing from a superconducting to a normal state near the hotspot. The resulting high impedance causes the current to be squeezed into a high frequency coaxial wire connected in parallel with it, which finally produces a measurable voltage pulse. Thanks to the maturity of current micro- and nano- fabrication techniques, high quality superconducting thin films and nano-wires have been realized in a variety of materials such as NbN, WSi, MoSi and others. These techniques include magnetron sputtering, physical vapor deposition, electron beam lithography, optical lithography, reactive ion etching, wafer cutting, etc. Measurements of superconducting single-photon detectors require cryogenic refrigeration systems, ranging from the initial liquid helium cold tank, to modern dry dilution refrigerators. On one hand, a superconducting device needs to be cooled down below its critical temperature to work. On the other hand, photon-detection events generate hot spots, which release heat to the device substrate, which the refrigerator needs to efficiently take away. Research on superconducting nanowire single-photon detectors (SNSPDs) has made significant advancements over the past two decades, including high detection efficiency, low dark counts, and broad wavelength range capabilities. Beyond improvements in traditional NbN, WSi and MoSi superconducting nanowires, there have been numerous efforts to develop high-temperature superconducting nanowires, such as MgB2 and Bi2Sr2CaCu2O8 (BSCCO), due to the operational limitations imposed by low-temperature superconducting materials. Typically, single-photon detectors require operation temperatures below the critical temperature. High-temperature superconducting nanowire single-photon detectors have currently reported to exhibit the highest operating temperature around 20 K in BSCCO, which is much lower than its critical temperature of 80 K, showing that large-gap superconductors have a comparably low sensitivity to low-energy photons. In recent years since 2018, several groups have reported that micrometer-wide superconducting wires can also be used for single-photon detection, unlike the conventional nanometer-wide superconducting wires. In general, as long as the bias current is uniformly distributed in superconducting wires and approaches the critical Cooper pair-breaking current, the hot spots generated by photon absorption can induce vortices and anti-vortices, and their motion under the action of the Lorentz force generates resistance and voltage, which ultimately produces a normal state region. An increase of the wire width to the micron scale offers more freedom to design new detectors based on current nano- and microfabrication techniques. In this doctoral dissertation, I will first introduce the basic Ginzburg-Landau theory of superconductivity and the measurement theory of superconducting circuits related to single-photon detection. I will then provide a detailed description of the equipment construction and sample fabrication techniques used in the experiment. Finally, I will present my two main studies: in one project I examined the photon-detection performance of superconducting micro-wires with a multi-pinhole structure, the correlation between dark counts and magnetic vortices, as well as the properties of critical current in magnetic fields. In a subsequent study I conducted numerical simulations using the time dependent Ginzburg-Landau equation, to analyze the dynamics of vortices in superconducting micro-strips with regular large-area perforations by pinholes as a function of magnetic field and bias current. The thus obtained predictions were then successfully confirmed in a detailed experimental study on a series of microwires.