Session L54: Superconductivity: Mostly Devices and Applications

Proximity Induced Superconductivity in Ferromagnetic Nanowires: Magnetoresistance Oscillations and Steps in I-V Characteristics

Ferromagnetic order requires electronic spins to be aligned parallel whereas singlet superconducting order requires spins to be aligned antiparallel. This spin incompatibility limits the superconducting proximity effect to $\sim $1 nm in bulk ferromagnets. In ferromagnetic nanowires contacted with superconducting W electrodes however, the proximity effect is seen to extend to $\sim $ 400 nm [Wang et al., Nat. Phys. 6, 389 (2010)]. The mechanism behind the long range proximity in these systems is not certain. We have studied single crystalline Co nanowires contacted with normal electrodes with a single superconducting W strip patterned on the nanowire. The long range proximity effect is found to persist in this geometry. Robust magnetoresistance oscillations were found when an external field parallel to the axis of the wire was applied. In addition, regularly spaced peaks were seen in the dI/dV vs. V characteristics of the sample. The origin of the oscillations and the peaks is not understood.   

Imaging point-like defects in two dimensional superconductors: theory and experiment

Point defects in two-dimensional superconductors produce significant local variations in the superconducting properties. A model [1] of point defects in a weak, two-dimensional superconductor, based on London's equations, shows that defects appear as haloes of decreased diamagnetic susceptibility as seen by the imaging kernel of a scanning SQUID susceptometer. We report theoretical limits on the defect strength and superconducting Pearl length $\Lambda$ required for defects to be visible. We compare these models to our experimental data showing similar haloes in the superconducting state of several types of superconducting films as a function of magnetic field, gate voltage, temperature, and height. Our ability to image these defects offers new possibilities for studying the interplay between materials properties and superconducting phenomena in thin film systems.\\[4pt] [1] V.G. Kogan and J.R. Kirtley, Phys. Rev. B \textbf{83}, 214521 (2011).   

Half-quantum vortices in superconducting networks

We study numerically the magnetic field and temperature dependence of vortex occupation in superconducting wire networks assuming that the underlying material allows for the presence of half-quantum vortices. We focus on thermodynamic stability of half-quantum vortices, which are believed to be present in large size networks even when a stability of a single isolated half-quantum vortex is not expected. The spatial arrangements of half- and full vortices in networks are studied as well.   

DNA templating for \textit{in situ} growth and measurement of 1D superconducting wires

Controllable and reproducible fabrication of sub-10 nm metal wires is of broad scientific and technological interest. One notable example is nanowires of disordered superconductors; such a small width is generally required to place them in the 1D regime where questions remain on the nature of superconductivity and superconducting fluctuations. Molecular templating based on macromolecules, such as carbon nanotubes and DNA strands, offers a relatively straightforward and reliable method of producing nm-wide wires. Here, we report the fabrication of nanowire templates using a unique procedure of DNA stretching.\footnote{J.J. Guan \textit{et al}, Adv. Mater. 19, 1212 (2007).} DNA wires are assembled onto Si/SiO$_{2}$/Si$_{3}$N$_{4}$ substrates across trenches with an overhang, fabricated via e-beam lithography and combination of dry and wet etching. The location and orientation of the DNA wires can be well controlled. The template will be used as the substrate for \textit{in situ} growth and electrical measurement of 1D superconductor nanowires in a custom dilution refrigerator. The experiments are expected to enable a close examination of the emergence and evolution of superconductivity in true 1D limit as the wire cross-section is varied in situ on one and the same sample.   

The role of dissipation in quantum phase slip production in superconducting nanowires

We study dynamics of quantum phase slips (QPS) in ultrathin superconducting wires. At sufficiently low temperatures momentum conservation is known to pose a constraint on QPS creation, which can be lifted due to inhomogeneous external potential or due to production of quasiparticles. We argue that while the former mechanism is weak in nanowires with diameters significantly exceeding the Fermi wavelength (which is the case for most up-to-date QPS experiments), the dissipation caused by quasiparticles is likely to be the major source for the QPS generation. We argue that dissipation can be enhanced in superconductors with broken time reversal symmetry, and therefore such systems are likely to exhibit phase slips at ultralow temperatures. Our studies may clarify recent experiments (Nature Physics 5, 503 (2009)) showing the evidence of QPS at sufficiently high values of bias-current.   

Quantum oscillations in superconducting mesoscopic rings of Sr$_2$RuO$_4$

Sr$_2$RuO$_4$ has attracted considerable amount of attention recently because of its potential application in topological quantum computing as a $p_x\pm ip_y$ superconductor. Cantilever magnetometry measurements carried out on doubly connected Sr$_2$RuO$_4$ samples of a micron size were found to exhibit jumps in magnetization suggesting the presence of both integer and half-integer flux quanta. To search for quantum oscillations with periods corresponding to integer and half-integer flux, we prepared micron size rings of Sr$_2$RuO$_4$ from thin flakes of Sr$_2$RuO$_4$ obtained by mechanical exfoliation and carried out transport measurements at low temperatures. Photolithography technique was applied to make contact probes on the flake and focused ion beam (FIB) was used in the fabrication. Disorder introduced by the FIB process employing high-energy beams of Ga ions was characterized by transmission electron microscopy. We succeeded in the preparation of superconducting rings of roughly 1 $\mu$m in diameter with 200$\sim$400nm line width. Strong oscillations of ring resistance with a conventional period of full flux quantum have been observed. In some samples a different period of oscillations was also found at high field range. The nature of these oscillations will be discussed.   

Superconductivity of a Sn film controlled by an array of Co nanowires

Superconducting properties of a hybrid structure composed of ferromagnetic Co nanowire arrays and a superconducting Sn film have been investigated. Ordered Co nanowires arrays with 60 nm, 150 nm and 200 nm diameter were electroplated into the pores of self organized anodic aluminum oxide (AAO) membranes. Hysteretic dependence of the Sn film superconducting properties on applied magnetic field and critical current enhancement at moderate fields has been observed. This behavior strongly depends on the ratio of the Sn film thickness to the Co nanowire diameter.   

Measurements on Superconducting Nanorings Smaller than the Coherence Length

The Little-Parks experiment on superconducting cylinders is an important demonstration of fluxoid quantization in superconductors. The transition temperature oscillations in magnetic field have a period of $h/2e$ for the micro cylinders in their studies, which was further evidence for Cooper paring at the time {[}W. A. Little, R. D. Parks, \emph{PRL} \textbf{9}, 9 (1964){]}. However recent theoretical works have suggested that in superconducting loops smaller than the coherence length this period changes from $h/2e$ to $h/e$, for details see {[}F. Loder, et al. \emph{PRB} \textbf{78}, 174526 (2008){]} and references therein. The destructive regime has also been observed experimentally in cylinders whose diameter is small compared to the coherence length {[}Y. Liu, et al. \emph{Science} \textbf{294}, 2332 (2001){]}. We present experimental work in an effort to achieve this limit in Al nanorings prepared by electron beam lithography. These measurements achieve a regime hitherto unexplored in nanorings with interesting consequences.   

Flux noise in SQUIDs: calculations of geometrical dependence

Low frequency (1/$f$) magnetic flux noise in SQUIDs is understood to arise from the random reversal of electron spins localized at the surface of the superconducting film. Analytical results$^1$ that assume independent electron spins predict that the spectral density at 1~Hz scales with the washer geometry approximately as $R/W$ in the limit $R/W\gg1$. Here, $R$ is the outer radius and $W$ is the linewidth. We present numerical calculations that reproduce the analytical result in the appropriate limit and extend these results to arbitrary values of $R/W$. In addition, a logarithmic dependence on $W$, evident when $R/W$ is fixed and $W$ is varied, is reproduced and discussed. The contribution of spins at the edge of the film is also computed. We compare the predicted geometrical scaling to our recent measurements of several SQUIDs with varying geometries. Our calculation that is valid for all values of $R/W$ enables us to investigate a possible breakdown of the independent spin model in our experimental data. $^1$R. C. Bialczak \textit{et al}., Phys. Rev. Lett. \textbf{99}, 187006 (2007).   

Geometry and temperature dependence of low frequency flux noise in dc SQUIDs

Measurements of low frequency magnetic flux noise in dc SQUIDs demonstrate a spectral density $S_\Phi(f)=A^2/f^\alpha$ in which the magnitude $A$ scales only weakly with the washer geometry and typically $0.5<\alpha<1$. An analytical model assuming non-interacting spins localized to the surface of the SQUID loop predicts that $A^2\propto R/W$ for $R/W\gg1$. Here, $R$ and $W$ are the outer radius and linewidth, respectively. Our numerical calculations extend the analytical results to arbitrary washer geometries. We compare both models to flux noise measurements on several dc SQUIDs fabricated simultaneously on a single chip using a Nb trilayer process. Multiple SQUIDs were measured within a single cool-down. The SQUID geometries were divided into two categories: fixed $W$=500~nm with $3 [Preview Abstract]

Investigation of 1/f flux noise in superconducting circuits

Low-frequency 1/f flux noise is a dominant source of dephasing in the Josephson phase and flux qubits. Recent work has revealed the presence of a high density of unpaired spins at the surfaces of superconducting thin films; it is now believed that these spins are the source of the noise, although the microscopic noise mechanism is not understood. Here we describe experiments on SQUIDs and Josephson phase qubits designed to shed light on the underlying noise mechanism, and we describe efforts to develop novel materials with reduced levels of noise.   

Critical Exponents and Cluster Analysis on STM Studies of a Cuprate Superconductor

In this presentation we will show results from a new kind of analysis for surface probes allowing us to gather some information about the bulk behavior of the sample that is studied. This presentation will introduce the analysis in as general a way as practical and use a particular example to concretely demonstrate how it can be applied. The particular example is the case of a local nematic order observed in a Scanning Tunneling Microscopy (STM) experiment on a BSCCO sample from which we determine which model best describes the behavior of nematic orientation using critical exponents derived from the properties of clusters and correlations of these orientations. These results would highlight key physical contributions to the nematic's orientation and guide further studies into the underlying physics. We fully expect similar impact on the study of other systems for which this analysis can be performed.   

Extracting the Bosonic Spectra of Pb Using Superconducting-Tip STS and Comparing it with the Cuprates

In high-temperature superconductors the meaning of the common feature labeled ``peak-dip-hump'' is still a point of great debate. In terms of scanning tunneling spectroscopy (STS) this refers to the shape of satellite features that occur outside the coherence peaks in the $dI/dV$ spectra. There are many conflicting interpretations and labeling schemes for this feature in both the hole- and electron-doped cuprates. The path to resolving this confusion is to study a well-understood BCS superconductor to better observe the way that the STM measures bosonic information. Utilizing the ultra-low electronic noise of our home-built low-temperature STM, and utilizing a superconducting tip for increased spectral resolution, we recreate the original McMillan and Rowell S-I-S junction\footnote{W. L. McMillan and J. M. Rowell Phys. Rev. Lett., \textbf{14}, 108-112 (1965)} with the STM equivalent (S-Vacuum-S). This method provides very high energy resolution for both the filled and empty electronic states in both the superconducting and normal state. We compare this data to first-principle Eliashberg calculations and relate this data to ``peak-dip-hump'' in the high T$_c$ case.   

Comparison between Simmons's Equations and Quantum Tunneling Experimental Results in A Thin Film

The theoretical predictions of J.G. Simmons's equations are compared with quantum tunneling experimental results and a discrepancy is found at bias voltage $V_{b}=U$/$e$, where U is the barrier's potential height and e is the electron charge. Specifically, he divided the bias voltage into 2 regions: $V_{b} \quad < \quad U$/$e$ and $V_{b} \quad > \quad U$/$e$, and the I -- V characteristics are different in these two regions. The derived equations show a kink on differential conductance d$I$/d$V$ vs. $V_{b}$ at $V_{b}=U$/$e$, because starts at this bias the thickness of the insulation film decreases with $V_{b}$ in addition to the lowering of the barrier's average height. Therefore, the differential conductance decreases more rapidly in the region $V_{b} \quad > \quad U$/$e$ than in the region $V_{b} \quad < \quad U$/$e$. However, in tunneling experiment in which Pt is used as conductor and solid neon as insulator, we have not observed such kink even the bias was increased to 4 volts. Our speculation is either 1) there should not be a kink on conductance at $V_{b}=U$/$e$ so Simmons's equations need to be modified; 2) the kink should exist but bias voltage is not high enough to observe it in the experiments.   

Size effect on supercurrent screening in large array superconducting anti-dot thin films

We fabricated well-ordered and large array niobium (Nb) anti-dot thin films using a monolayer of polymer/nanosphere hybrid as a template. The hole diameter and center-to-center distance can be tuned independently. By applying a perpendicular magnetic field, we observed pronounced oscillations with field in both magnetization and resistivity due to the criteria of the flux quantization, which is reminiscent of the well-known Little-Parks experiment in a thin-walled superconducting cylinder. By varying the hole diameter, the detailed size effect on supercurrent screening can be explored and will be discussed.