Superconductor performance improved with nanowires

Superconducting materials have many useful applications, from magnetic resonance imaging (MRI) in medicine, to maglev trains, to particle accelerators. These materials are called superconductors because of their ability to carry electric current with minimal resistance. However, the conducting efficiency of these materials can be compromised when electrons get caught in swirling vortices. Researchers in the laboratory of Nina Markovic, a physicist affiliated with Johns Hopkins Institute for NanoBioTechnology, have discovered a way to use nanowires to prevent these disruptive electron tornados. Read more from Johns Hopkins News and Information below.

Superconductor materials are prized for their ability to carry an electric current without resistance, but this valuable trait can be crippled or lost when electrons swirl into tiny tornado-like formations called vortices. These disruptive mini-twisters often form in the presence of magnetic fields, such as those produced by electric motors.

To keep supercurrents flowing at top speed, Johns Hopkins scientists have figured out how to constrain troublesome vortices by trapping them within extremely short, ultra-thin nanowires. Their discovery was reported Feb. 18 in the journal Physical Review Letters.


This illustration depicts a short row of vortices held in place between the edges of a nanowire developed by Johns Hopkins scientists. (Graphic by Nina Markovic and Tyler Morgan-Wall)

“We have found a way to control individual vortices to improve the performance of superconducting wires,” said Nina Markovic, an associate professor in the Department of Physics and Astronomy in the university’s Krieger School of Arts and Sciences. Markovic is an affiliated faculty member of Johns Hopkins Institute for NanoBioTechnology.

Many materials can become superconducting when cooled to a temperature of nearly 460 below zero F, which is achieved by using liquid helium.

The new method of maintaining resistance-free current within these superconductors is important because these materials play a key role in devices such MRI medical scanners, particle accelerators, photon detectors and the radio frequency filters used in cell phone systems. In addition, superconductors are expected to become critical components in future quantum computers, which will be able to do more complex calculations than current machines.

Wider use of superconductors may hinge on stopping the nanoscopic mischief that electron vortices cause when they skitter from side to side across a conducting material, spoiling the zero-resistance current. The Johns Hopkins scientists say their nanowires keep this from happening.

Markovic, who supervised the development of these wires, said other researchers have tried to keep vortices from disrupting a supercurrent by “pinning” the twisters to impurities in the conducting material, which renders them unable to move.

“Edges can also pin the vortices, but it is more difficult to pin the vortices in the bulk middle area of the material, farther away from the edges,” she said. “To overcome this problem, we made a superconducting sample that consists mostly of edges: a very narrow aluminum nanowire.”

These nanowires, Markovic said, are flat strips about one-billionth as wide as a human hair and about 50 to 100 times longer than their width. Each nanowire forms a one-way highway that allows pairs of electrons to zip ahead at a supercurrent pace.

Vortices can form when a magnetic field is applied, but because of the material’s ultra-thin design, “only one short vortex row can fit within the nanowires,” Markovic said. “Because there is an edge on each side of them, the vortices are trapped in place and the supercurrent can just slip around them, maintaining the resistance-free speed.”

The ability to control the exact number of vortices in the nanowire may produce additional benefits, physics experts say. Future computers or other devices may someday use vortices instead of electrical charges to transmit information, they say.

The lead author of the Physical Review Letters article was Tyler Morgan-Wall, a doctoral student in Markovic’s lab. Along with Markovic, the co-authors were Benjamin Leith, who was an undergraduate at Johns Hopkins when the research took place; Nikolaus Hartman, a graduate student; and Atikur Rahman, who was a postdoctoral fellow in Markovic’s lab.

This research was support by National Science Foundation grants DMR-1106167 and PHYS-1066293. The Physical Review Letters journal article may be viewed at:

Nina Markovic’s Lab Page:


For all press inquiries regarding INBT, its faculty and programs, contact Mary Spiro, or 410-516-4802.


Nanowires Deliver Biochemical Payloads to One Cell Among Many

Imagine being able to drop a toothpick on the head of one particular person standing among 100,000 people in a sports stadium. It sounds impossible, yet this degree of precision at the cellular level has been demonstrated by researchers affiliated with The Johns Hopkins University Institute for NanoBioTechnology. Their study was published online in June in Nature Nanotechnology.

Arrow points to nanowire placed on cell surface. (Image: Levchenko/Chien labs)

The team used precise electrical fields as “tweezers” to guide and place gold nanowires, each about one-two hundredth the size of a cell, on predetermined spots, each on a single cell. Molecules coating the surfaces of the nanowires then triggered a biochemical cascade of actions only in the cell where the wire touched, without affecting other cells nearby. The researchers say this technique could lead to better ways of studying individual cells or even cell parts, and eventually could produce novel methods of delivering medication.

Indeed, the techniques not relying on this new nanowire-based technology either are not very precise, leading to stimulation of multiple cells, or require complex biochemical alterations of the cells. With the new technique the researchers can, for instance, target cells that have cancer properties (higher cell division rate or abnormal morphology), while sparing their healthy neighbors.

“One of the biggest challenges in cell biology is the ability to manipulate the cell environment in as precise a way as possible,” said principal investigator Andre Levchenko, an associate professor of biomedical engineering in Johns Hopkins’ Whiting School of Engineering. In previous studies, Levchenko has used lab-on-a-chip or microfluidic devices to manipulate cell behavior. But, he said, lab-on-a-chip methods are not as precise as researchers would like them to be. “In microfluidic chips, if you alter the cell environment, it affects all the cells at the same time,” he said.

Such is not the case with the gold nanowires, which are metallic cylinders a few hundred nanometers or smaller in diameter. Just as the unsuspecting sports spectator would feel only a light touch from a toothpick being dropped on the head, the cell reacts only to the molecules released from the nanowire in one very precise place where the wire touches the cell’s surface.

With contributions from Chia-Ling Chien, a professor of physics and astronomy in the Krieger School of Arts and Sciences, and Robert Cammarata, a professor of materials science and engineering in the Whiting School, the team developed nanowires coated with a molecule called tumor necrosis factor-alpha (TNF?), a substance released by pathogen-gobbling macrophages, commonly called white blood cells. Under certain cellular conditions, the presence of TNF? triggers cells to switch on genes that help fight infection, but TNF? also is capable of blocking tumor growth and halting viral replication.

Exposure to too much TNF?, however, causes an organism to go into a potentially lethal state called septic shock, Levchenko said. Fortunately, TNF? stays put once it is released from the wire to the cell surface, and because the effect of TNF? is localized, the tiny bit delivered by the wire is enough to trigger the desired cellular response. Much the same thing happens when TNF? is excreted by a white blood cell.

Additionally, the coating of TNF? gives the nanowire a negative charge, making the wire easier to maneuver via the two perpendicular electrical fields of the “tweezer” device, a technique developed by Donglei Fan as part of her Johns Hopkins doctoral research in materials science and engineering. “The electric tweezers were initially developed to assemble, transport and rotate nanowires in solution,” Cammarata said. “Donglei then showed how to use the tweezers to produce patterned nanowire arrays as well as construct nanomotors and nano-oscillators. This new work with Dr. Levchenko’s group demonstrates just how extremely versatile a technique it is.”

To test the system, the team cultured cervical cancer cells in a dish. Then, using electrical fields perpendicular to one another, they were able to zap the nanowires into a pre-set spot and plop them down in a precise location. “In this way, we can predetermine the path that the wires will travel and deliver a molecular payload to a single cell among many, and even to a specific part of the cell,” Levchenko said.

During the course of this study, the team also established that the desired effect generated by the nanowire-delivered TNF? was similar to that experienced by a cell in a living organism.

The team members envision many possibilities for this method of subcellular molecule delivery. “For example, there are many other ways to trigger the release of the molecule from the wires: photo release, chemical release, temperature release. Furthermore, one could attach many molecules to the nanowires at the same time,” Levchenko said. He added that the nanowires can be made much smaller, but said that for this study the wires were made large enough to see with optical microscopy.

Ultimately, Levchenko sees the nanowires becoming a useful tool for basic research. “With these wires, we are trying to mimic the way that cells talk to each other,” he said. “They could be a wonderful tool that could be used in fundamental or applied research.” Drug delivery applications could be much further off. However, Levchenko said, “If the wires retain their negative charge, electrical fields could be used to manipulate and maneuver their position in the living tissue.”

The lead authors for this Nature Nanotechnology article were Fan, a former postdoctoral fellow in the departments of materials science and engineering and in physics and astronomy; and Zhizhong Yin, a former postdoctoral fellow in the Department of Biomedical Engineering. The co-authors included Raymond Cheong, a doctoral student in the Department of Biomedical Engineering; and Frank Q. Zhu, a former doctoral student in the Department of Physics and Astronomy.

Regarding the faculty members’ participation, Chien led the group that developed the electric tweezers technique and collaborated with Levchenko on its biological applications.

The research was funded by the National Science Foundation and the National Institutes of Health.

Johns Hopkins Institute for NanoBioTechnology