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Henkel is focused on developing next-generation materials for a variety of applications in semiconductor packaging, industrial, consumer, displays and emerging electronics market sectors. With a broad portfolio of silver, carbon, dielectric and clear conductive inks, Henkel is making today’s medical solutions, in-home conveniences, handheld connectivity, RFID and automotive advances reliable and effective. Watch an interview taken at the IDTechEx Printed Electronics event at this link: www.IDTechEx.com/peusa
Genesis Nanotechnology, Inc. ~ “Great Things from Small Things”
Graphene is a single-atomic carbon sheet with a hexagonal honeycomb network. Electrons in graphene take a special electronic state called Dirac-cone where they behave as if they have no mass. This allows them to flow at very high speed, giving graphene a very high level of electrical conductivity.
This is significant because electrons with no mass flowing with no resistance in graphene could lead to the realization of an ultimately high-speed nano electronic device.
The collaborative team of Tohoku University and the University of Tokyo has developed a method to grow high-quality graphene on a silicon carbide (SiC) crystal by controlling the number of graphene sheets. The team fabricated bilayer graphene with this method and then inserted calcium (Ca) atoms between the two graphene layers like a sandwich.
They measured the electrical conductivity with the micro four-point probe method and found that the electrical resistivity rapidly drops at around 4 K (-269 °C), indicative of an emergence of superconductivity.
The team also found that neither genuine bilayer graphene nor lithium-intercalated bilayer graphene shows superconductivity, indicating that the superconductivity is driven by the electron transfer from Ca atoms to graphene sheets.
The success in fabricating superconducting graphene is expected to greatly impact both the basic and applied researches of graphene.
It is currently not clear what phenomenon takes place when the Dirac electrons with no mass become superconductive with no resistance. But based on the latest study results, further experimental and theoretical investigations would help to unravel the properties of superconducting graphene.
The superconducting transition temperature (Tc) observed in this study on Ca-intercalated bilayer graphene is still low (4 K). This prompts further studies into ways to increase Tc, for example, by replacing Ca with other metals and alloys, or changing the number of graphene sheets.
From the application point of view, the latest results pave the way for the further development of ultrahigh-speed superconducting nano devices such as a quantum computing device, which utilizes superconducting graphene in its integrated circuit.
- Satoru Ichinokura, Katsuaki Sugawara, Akari Takayama, Takashi Takahashi, Shuji Hasegawa. Superconducting Calcium-Intercalated Bilayer Graphene. ACS Nano, 2016; DOI: 10.1021/acsnano.5b07848
16 Feb 2016
University of Utah engineers have discovered a new kind of 2D semiconducting material for electronics that opens the door for much speedier computers and smartphones that also consume a lot less power.
The semiconductor, made of the elements tin and oxygen, or tin monoxide (SnO), is a layer of 2D material only one atom thick, allowing electrical charges to move through it much faster than conventional 3D materials such as silicon. This material could be used in transistors, the lifeblood of all electronic devices such as computer processors and graphics processors in desktop computers and mobile devices. The material was discovered by a team led by University of Utah materials science and engineering associate professor Ashutosh Tiwari.
A paper describing the research was published online Monday, Feb. 15, 2016 in the journal, Advanced Electronic Materials. The paper, which also will be the cover story on the printed version of the journal, was co-authored by University of Utah materials science and engineering doctoral students K. J. Saji and Kun Tian, and Michael Snure of the Wright-Patterson Air Force Research Lab near Dayton, Ohio.
Transistors and other components used in electronic devices are currently made of 3D materials such as silicon and consist of multiple layers on a glass substrate. But the downside to 3D materials is that electrons bounce around inside the layers in all directions.
The benefit of 2D materials, which is an exciting new research field that has opened up only about five years ago, is that the material is made of one layer the thickness of just one or two atoms. Consequently, the electrons “can only move in one layer so it’s much faster,” says Tiwari.
While researchers in this field have recently discovered new types of 2D material such as graphene, molybdenun disulfide and borophene, they have been materials that only allow the movement of N-type, or negative, electrons. In order to create an electronic device, however, you need semiconductor material that allows the movement of both negative electrons and positive charges known as “holes.” The tin monoxide material discovered by Tiwari and his team is the first stable P-type 2D semiconductor material ever in existence.
Now that Tiwari and his team have discovered this new 2D material, it can lead to the manufacturing of transistors that are even smaller and faster than those in use today. A computer processor is comprised of billions of transistors, and the more transistors packed into a single chip, the more powerful the processor can become.
Transistors made with Tiwari’s semiconducting material could lead to computers and smartphones that are more than 100 times faster than regular devices. And because the electrons move through one layer instead of bouncing around in a 3D material, there will be less friction, meaning the processors will not get as hot as normal computer chips. They also will require much less power to run, a boon for mobile electronics that have to run on battery power. Tiwari says this could be especially important for medical devices such as electronic implants that will run longer on a single battery charge.
“The field is very hot right now, and people are very interested in it,” Tiwari says. “So in two or three years we should see at least some prototype device.”
Explore further: Semiconductor miniaturisation with 2D nanolattices
28 Dec 2015
|A group of researchers in Japan and China identified the requirements for the development of new types of extremely low power consumption electric devices by studying Cr-doped (Sb,Bi)2Te3 thin films. This study has been reported in Nature Communications (“Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)2Te3“).|
|At extremely low temperatures, an electric current flows around the edge of the film without energy loss, and under no external magnetic field. This attractive phenomenon is due to the material’s ferromagnetic properties; however, so far, it has been unclear how the material gains this property. For the first time, researchers have revealed the mechanism by which this occurs. “Hopefully, this achievement will lead to the creation of novel materials that operate at room temperature in the future,” said Akio Kimura, a professor at Hiroshima University and a member of the research group.|
|Ferromagnetism mediated by Sb or Te atoms. (Image: Hiroshima University)|
|Their achievement can be traced back to the discovery of the quantum Hall effect in the 1980’s, where an electric current flows along an edge (or interface) without energy loss. However, this requires both a large external magnetic field and an extremely low temperature. This is why practical applications have not been possible. Researchers believed that this problem could be overcome with new materials called topological insulators that have ferromagnetic properties such as those found in Cr-doped (Sb,Bi)2Te3.|
|A topological insulator, predicted in 2005 and first observed in 2007, is neither a metal nor an insulator, and has exotic properties. For example, an electric current is generated only at the surface or the edge of the material, while no electric current is generated inside it. It looks as if only the surface or the edge of the material has metallic properties, while on the inside it is an insulator.|
|At extremely low temperatures, a thin film made of Cr-doped (Sb,Bi)2Te3 shows a peculiar phenomenon. As the film itself is ferromagnetic, an electric current is spontaneously generated without an external magnetic field and electric current flows only around the edge of the film without energy loss. However, it was previously unknown as to why Cr-doped (Sb,Bi)2Te3 had such ferromagnetic properties that allowed it to generate electric current.|
|“That’s why we selected the material as the object of our study,” said Professor Kimura.|
|Because Cr is a magnetic element, a Cr atom is equivalent to an atomic-sized magnet. The N-S orientations of such atomic-sized magnets tend to be aligned in parallel by the interactions between the Cr atoms. When the N-S orientations of Cr atoms in Cr-doped (Sb,Bi)2Te3 are aligned in parallel, the material exhibits ferromagnetism. However, the interatomic distances between the Cr atoms in the material are, in fact, too long to interact sufficiently to make the material ferromagnetic.|
|The group found that the non-magnetic element atoms, such as the Sb and Te atoms, mediate the magnetic interactions between Cr atoms and serve as the glue to fix the N-S orientations of Cr atoms that face one direction. In addition, the group expects that its finding will provide a way to increase the critical temperature for relevant device applications.|
|The experiments for this research were mainly conducted at SPring-8. “We would not have achieved perfect results without the facilities and the staff there. They devoted themselves to detecting the extremely subtle magnetism that the atoms of non-magnetic elements exhibit with extremely high precision. I greatly appreciate their efforts,” Kimura said.|
|Source: Hiroshima University|
Researchers in the Cockrell School of Engineering at The University of Texas at Austin have developed a first-of-its-kind self-healing gel that repairs and connects electronic circuits, creating opportunities to advance the development of flexible electronics, biosensors and batteries as energy storage devices.
Although technology is moving toward lighter, flexible, foldable and rollable electronics, the existing circuits that power them are not built to flex freely and repeatedly self-repair cracks or breaks that can happen from normal wear and tear.
Until now, self-healing materials have relied on application of external stimuli such as light or heat to activate repair. The UT Austin “supergel” material has high conductivity (the degree to which a material conducts electricity) and strong mechanical and electrical self-healing properties.
“In the last decade, the self-healing concept has been popularized by people working on different applications, but this is the first time it has been done without external stimuli,” said mechanical engineering assistant professor Guihua Yu, who developed the gel. “There’s no need for heat or light to fix the crack or break in a circuit or battery, which is often required by previously developed self-healing materials.”
Yu and his team created the self-healing gel by combining two gels: a self-assembling metal-ligand gel that provides self-healing properties and a polymer hydrogel that is a conductor. A paper on the synthesis of their hydrogel appears in the November issue of Nano Letters.
In this latest paper, the researchers describe how they used a disc-shaped liquid crystal molecule to enhance the conductivity, biocompatibility and permeability of their polymer hydrogel. They were able to achieve about 10 times the conductivity of other polymer hydrogels used in bioelectronics and conventional rechargeable batteries. The nanostructures that make up the gel are the smallest structures capable of providing efficient charge and energy transport.
In a separate paper published in Nano Letters in September, Yu introduced the self-healing hybrid gel. The second ingredient of the self-healing hybrid gel is a metal-ligand supramolecular gel. Using terpyridine molecules to create the framework and zinc atoms as a structural glue, the molecules form structures that are able to self-assemble, giving it the ability to automatically heal after a break.
When the supramolecular gel is introduced into the polymer hydrogel, forming the hybrid gel, its mechanical strength and elasticity are enhanced.
To construct the self-healing electronic circuit, Yu believes the self-healing gel would not replace the typical metal conductors that transport electricity, but it could be used as a soft joint, joining other parts of the circuit.
“This gel can be applied at the circuit’s junction points because that’s often where you see the breakage,” he said. “One day, you could glue or paste the gel to these junctions so that the circuits could be more robust and harder to break.”
Yu’s team is also looking into other applications, including medical applications and energy storage, where it holds tremendous potential to be used within batteries to better store electrical charge.
Yu’s research has received funding from the National Science Foundation, the American Chemical Society, the Welch Foundation and 3M.
- Yaqun Wang, Ye Shi, Lijia Pan, Yu Ding, Yu Zhao, Yun Li, Yi Shi, Guihua Yu. Dopant-Enabled Supramolecular Approach for Controlled Synthesis of Nanostructured Conductive Polymer Hydrogels. Nano Letters, 2015; 15 (11): 7736 DOI: 10.1021/acs.nanolett.5b03891
06 Oct 2015
|A single-electron transistor (SET) is an electrical device that takes advantage of a strange quantum phenomenon called tunneling to transport single electrons across a thin insulator. The device serves as an on/off switch on the tiniest scale and could play an important role in quantum computing.|
|A group of researchers in Japan is exploring the behavior of a certain type of SET made from two quantum dots, which are bits of material so small they start to exhibit quantum properties. The group has produced a detailed analysis of the electrical characteristics of the so-called double-quantum-dot SETs, which could help researchers design better devices to manipulate single electrons. They report their findings in the Journal of Applied Physics, from AIP Publishing (“Chemically assembled double-dot single-electron transistor analyzed by the orthodox model considering offset charge”).|
|Left to right: a scanning electrode microscopy shot of the series of double-dot single-electron transistor (bright spots correspond to the cores of the gold nanoparticles); a schematic of the device; an experimental stability diagram. (Image: Majima/Tokyo Institute of Technology)|
|The team began their work by fabricating the electrodes of the SET, which were separated by a nanometer scale gap, with an electroless gold-plating technique. They then synthesized size-controlled gold nanoparticles within the gap.|
|To do this, they “chemically assembled a series of double-dot SETs by anchoring two gold nanoparticles between the nanogap electrodes with alkanedithiol molecules to form a self-assembled monolayer,” explained Yutaka Majima, a professor in the Materials and Structures Laboratory at the Tokyo Institute of Technology.|
|The team tested the electrical properties of the device and found that regions within the quantum dots exhibited zero conductance and a stable electron number — both highly desirable traits for quantum computing. Such regions are called Coulomb diamonds and their properties are “extraordinarily stable and coveted,” Majima said.|
|The same researchers had earlier found Coulomb diamonds in single-quantum-dot SETs.|
|The group — which also includes members from Kyoto University, the University of Tsukuba, and Japan Science and Technology Agency (JST) — was then able to determine, through both theoretical and experimental analysis, many additional important electrical parameters of the SETs. The team then linked these parameters to the geometry of the device.|
|“Thanks to [the Coloumb diamond] stability, we could determine the equivalent circuit parameters with accuracy by analyzing the device’s electrical characteristics,” said Majima. “Precise estimation of the circuit parameters results in the determination of double-dot structures, which can be critical for reproducible single-electron devices.”|
|Majima and colleagues found that the evaluated parameters “corresponded well to the geometrical structures of the device,” which they were able to observe via scanning electron microscopy.|
|In terms of applications, it’s quite possible that the team’s work with double-dot SETs will find future use within quantum electronics to manipulate a single electron and its spin.|
|The researchers’ next goal is “to manipulate and control a single electron and its spin on double-dot single-electron devices by using asymmetric side-gate electrodes to demonstrate spin qubits,” said Majima.|
|Qubits, aka quantum bits, can encode both a “zero” and a “one” at the same time within their relative spin, so they are being pursued for storing and manipulating information in quantum computers.|
|Source: American Institute of Physics|
Researchers have discovered a new stretchable, transparent conductor that can be folded or stretched and released, resulting in a large curvature or a significant strain, at least 10,000 times without showing signs of fatigue. This is a crucial step in creating a new generation of foldable electronics – think a flat-screen television that can be rolled up for easy portability – and implantable medical devices. The work, published Monday in the Proceedings of the National Academy of Sciences (“Fatigue-free, superstretchable, transparent, and biocompatible metal electrodes”), pairs gold nanomesh with a stretchable substrate made with polydimethylsiloxane, or PDMS.
|Fatigue-free flexible transparent electrode for stretchable and bendable electronics. (Image: University of Houston)|
|The substrate is stretched before the gold nanomesh is placed on it – a process known as “prestretching” – and the material showed no sign of fatigue when cyclically stretched to a strain of more than 50 percent.|
|The gold nanomesh also proved conducive to cell growth, indicating it is a good material for implantable medical devices.|
|Fatigue is a common problem for researchers trying to develop a flexible, transparent conductor, making many materials that have good electrical conductivity, flexibility and transparency – all three are needed for foldable electronics – wear out too quickly to be practical, said Zhifeng Ren, a physicist at the University of Houston and principal investigator at the Texas Center for Superconductivity, who was the lead author for the paper.|
|The new material, produced by grain boundary lithography, solves that problem, he said.|
|In addition to Ren, other researchers on the project included Chuan Fei Guo and Ching-Wu “Paul” Chu, both from UH; Zhigang Suo, Qihan Liu and Yecheng Wang, all from Harvard University, and Guohui Wang and Zhengzheng Shi, both from the Houston Methodist Research Institute.|
|In materials science, “fatigue” is used to describe the structural damage to a material caused by repeated movement or pressure, known as “strain cycling.” Bend a material enough times, and it becomes damaged or breaks. That means the materials aren’t durable enough for consumer electronics or biomedical devices.|
|“Metallic materials often exhibit high cycle fatigue, and fatigue has been a deadly disease for metals,” the researchers wrote.|
|“We weaken the constraint of the substrate by making the interface between the Au (gold) nanomesh and PDMS slippery, and expect the Au nanomesh to achieve superstretchability and high fatigue resistance,” they wrote in the paper. “Free of fatigue here means that both the structure and the resistance do not change or have little change after many strain cycles.”|
|As a result, they reported, “the Au nanomesh does not exhibit strain fatigue when it is stretched to 50 percent for 10,000 cycles.”|
|Many applications require a less dramatic stretch – and many materials break with far less stretching – so the combination of a sufficiently large range for stretching and the ability to avoid fatigue over thousands of cycles indicates a material that would remain productive over a long period of time, Ren said.|
|The grain boundary lithography involved a bilayer lift-off metallization process, which included an indium oxide mask layer and a silicon oxide sacrificial layer and offers good control over the dimensions of the mesh structure.|
|The researchers used mouse embryonic fibroblast cells to determine biocompatibility; that, along with the fact that the stretchability of gold nanomesh on a slippery substrate resembles the bioenvironment of tissue or organ surfaces, suggest the nanomesh “might be implanted in the body as a pacemaker electrode, a connection to nerve endings or the central nervous system, a beating heart, and so on,” they wrote.|
|Ren’s lab reported the mechanics of making a new transparent and stretchable electric material, using gold nanomesh, in a paper published in Nature Communications in January 2014.|
|This work expands on that, producing the material in a different way to allow it to remain fatigue-free through thousands of cycles.|
|Source: University of Houston|
Today’s smartphones are well-equipped to satiate our appetites for instant gratification. We stream live video, look up facts on a whim, receive breaking news alerts and stay connected to our friends via social media. But one thing has lagged behind this culture of immediacy: smart phones’ batteries.
Now, it looks like recharging our phones could finally keep pace with the demands of our fast-moving culture. Yesterday an Israeli company called StoreDot unveiled a new smartphone battery that fully recharges in just 30 seconds. In contrast the couple hours it takes for a typical smartphone to fully charge, seems hopelessly outdated.
The battery manages such a speedy charge by utilizing quantum dot technology. Quantum dots are tiny bio-organic nanocrystals made of semiconducting materials. The battery is just a prototype at this point, and it’s still big and clunky — about the size of a laptop charger. However, the company plans to scale down its size and begin mass production of the device in 2016, the Wall Street Journal reports.
Check out this video of the battery recharging in real time:
17 Jul 2015
Rice researchers Rouzbeh Shahsavari and Navid Sakhavand have completed the first theoretical analysis of how 3-D boron nitride might be used as a tunable material to control heat flow in such devices.
Their work appears in Applied Materials and Interfaces.
In its 2-D form, hexagonal boron nitride (h-BN), aka white graphene, looks just like the atom-thick form of carbon known as graphene. One well-studied difference is that h-BN is a natural insulator, where perfect graphene presents no barrier to electricity.
But like graphene, h-BN is a good conductor of heat, which can be quantified in the form of phonons. (Technically, a phonon is one part—a “quasiparticle”—in a collective excitation of atoms.) Using boron nitride to control heat flow seemed worthy of a closer look, Shahsavari said.
“Typically in all electronics, it is highly desired to get heat out of the system as quickly and efficiently as possible,” he said. “One of the drawbacks in electronics, especially when you have layered materials on a substrate, is that heat moves very quickly in one direction, along a conductive plane, but not so good from layer to layer. Multiple stacked graphene layers is a good example of this.”
Heat moves ballistically across flat planes of boron nitride, too, but the Rice simulations showed that 3-D structures of h-BN planes connected by boron nitride nanotubes would be able to move phonons in all directions, whether in-plane or across planes, Shahsavari said.
A 3-D structure of hexagonal boron nitride sheets and boron nitride nanotubes could be a tunable material to control heat in electronics, according to researchers at Rice Univ. Image: The Shahsavari Group
The researchers calculated how phonons would flow across four such structures with nanotubes of various lengths and densities. They found the junctions of pillars and planes acted like yellow traffic lights, not stopping but significantly slowing the flow of phonons from layer to layer, Shahsavari said. Both the length and density of the pillars had an effect on the heat flow: more and/or shorter pillars slowed conduction, while longer pillars presented fewer barriers and thus sped things along.
While researchers have already made graphene/carbon nanotube junctions, Shahsavari believed such junctions for boron nitride materials could be just as promising. “Given the insulating properties of boron nitride, they can enable and complement the creation of 3-D, graphene-based nanoelectronics.
“This type of 3-D thermal-management system can open up opportunities for thermal switches, or thermal rectifiers, where the heat flowing in one direction can be different than the reverse direction,” Shahsavari said. “This can be done by changing the shape of the material, or changing its mass—say one side is heavier than the other—to create a switch. The heat would always prefer to go one way, but in the reverse direction it would be slower.”
Source: Rice Univ.
22 Jun 2015
Physicists at UC San Diego have developed a new way to control the transport of electrical currents through high-temperature superconductors — materials discovered nearly 30 years ago that lose all resistance to electricity at commercially attainable low temperatures.
Their development, detailed in two separate scientific publications, paves the way for the development of sophisticated electronic devices capable of allowing scientists or clinicians to non-invasively measure the tiny magnetic fields in the heart or brain, and improve satellite communications.
‘We believe this new approach will have a significant and far-reaching impact in medicine, physics, materials science and satellite communications,’ said Robert Dynes, a professor of physics and former chancellor of UC San Diego. ‘It will enable the development of a new generation of superconducting electronics covering a wide spectrum, ranging from highly sensitive magnetometers for biomagnetic measurements of the human body to large-scale arrays for wideband satellite communications. In basic science, it is hoped it will contribute to the unravelling of the mysteries of unconventional superconductors and could play a major role in new technologies, such as quantum information science.’
The research team headed by Dynes and Cybart, summarized its achievements in this week’s issue of Applied Physics Letters. Another paper outlining the initial discovery was published online April 27 in the journal Nature Nanotechnology.
The developments breathe new life into the promise of electronics constructed from ceramic materials that become superconducting — that is, lose all resistance to electricity — at temperatures that can be easily achieved in the laboratory with liquid nitrogen, which boils at 77 degrees Kelvin or 77 degrees above absolute zero.
Physicists first discovered high-temperature superconductivity in a copper-oxide materials in 1986, setting off an intense effort to develop new kinds of electronics and other devices with this new material.
‘Scientists and engineers worked with fervor to develop these new exciting materials, but soon discovered that they were much more complicated and difficult to work with than imagined,’ said Dynes. ‘These new materials demanded novel device architectures that proved very difficult to realize.’
The UC San Diego physicists found a way to control electrical transport through these materials by building a device within the superconducting material called a ‘Josephson junction,’ analogous in function to the transistor in semiconductor electronics. It’s composed of two superconducting electrodes separated by about one nanometer or a billionth of a meter.
‘Circuits built from Josephson junctions called Superconducting QUantum Interference Devices (SQUIDS), are used for detectors of extremely small magnetic fields, more than 10 billion times smaller than that of Earth,’ said Dynes. ‘One major drawback to these earlier devices is the low temperatures required for their operation, typically just 4 degrees above absolute zero. This requires intricate and costly cooling systems.’
‘Nearly three decades have passed since the discovery of the first high-temperature superconductor and progress in constructing electronic devices using these materials has been very slow because process control at the sub-10-nanometer scale is required to make high quality Josephson junctions out of these materials,’ he explained.
The UC San Diego physicists teamed up with Carl Zeiss Microscopy in Peabody, Mass., which has a facility capable of generating highly focused beams of helium ions, to experiment with an approach they believed might avoid previous problems.
‘Using the Zeiss Orion’s finely focused helium beam, we irradiated and hence disordered a nanoscale region of the superconductor to create what is called a ‘quantum mechanical tunnel barrier’ and were able to write Josephson circuits directly into a thin film of the oxide superconductor,’ said Shane Cybart, a physicist in Dynes’ laboratory who played a key role in the discoveries. ‘Using this direct-write method we eliminated the lithographic processing and offered the promise of a straightforward pathway to quantum mechanical circuits operating at more practical temperatures.’
‘The key to this method is that these oxide superconductors are very sensitive to the point defects in the crystal lattice caused by the ion beam. Increasing irradiation levels has the effect of increasing resistivity and reducing the superconducting transition temperature,’ said Cybart. ‘At very high irradiation levels the superconductor becomes insulating and no longer conducts or superconducts. This allows us to use the small helium beam to write these tunnel junctions directly into the material.’
The Nature Nanotechnology paper describes the development of the basic Josephson junction, while the Applied Physics Letters paper describes the development of the magnetic field sensor built from two junctions.
The UC San Diego physicists, who filed a patent application to license their discovery, are now collaborating with medical researchers to apply their work to the development of devices that can non-invasively measure the tiny magnetic fields generated within the brain, in order to study brain disorders such as autism and epilepsy in children.
‘In the communications field, we are developing wide bandwidth high data throughput satellite communications,’ said Cybart. ‘In basic science, we are using this technology to study ceramic superconducting materials to help determine the physics governing their operation which could lead to improved materials working at even higher temperatures.’
- E. Y. Cho, M. K. Ma, Chuong Huynh, K. Pratt, D. N. Paulson, V. N. Glyantsev, R. C. Dynes and Shane A. Cybart. YBa2Cu3O7− δ superconducting quantum interference devices with metallic to insulating barriers written with a focused helium ion beam. Applied Physics Letters, 2015 DOI: 10.1063/1.4922640