Scientists observe how superconducting nanowires lose resistance-free state

ScienceDaily (Sep. 22, 2011) — Even with today's invisibility cloaks, people can't walk through walls. But, when paired together, millions of electrons can. The electrons perform this trick, called macroscopic quantum tunneling, when they pair up and move into a region of space that is normally off-limits under the laws of classical mechanics. The problem is that as millions of electrons collectively move through a superconducting nanowire, they use energy and give off heat.

The heat can build, transforming sections of the wire into a non-superconducting state. The process, called a phase slip, adds resistance to an electrical system and has implications for designing new nano-scale superconductors.

Now, scientists have observed individual phase slips in aluminum nanowires and characterized the nature and temperature at which they occur. This information could help scientists remove phase slips from nano-scale systems, which could lead to more reliable nanowires and more efficient nano-electronics, said Duke physicist Albert Chang.

The results appeared online Sept. 21 in Physical Review Letters.

The macroscopic quantum tunneling effect was first observed in a system called a Josephson junction. This device has a thin insulating layer connecting two superconductors, which are several nanometers wide and have a three-dimensional shape.

To study the tunneling and phase slips in a simpler system, however, Chang and his colleagues used individual, one-dimensional nanowires made of aluminum. The new observations are "arguably the first convincing demonstration of tunneling of millions of electrons in one-dimensional superconducting nanowires," said Chang, who led the study.

In the experiment, the wires ranged in length from 1.5 to 10 micrometers, with widths from five to 10 nanometers. Chang cooled the wires to a temperature close to absolute zero, roughly 1 degree Kelvin or -458 degrees Fahrenheit.

At this temperature, a metal's crystal lattice vibrates in a way that allows electrons to overcome their negative repulsion of one other. The electrons make pairs and electric current flows essentially resistance-free, forming a superconductor.

The electron pairs move together in a path in a quantum-mechanical space, which resembles the curled cord of an old phone. On their way around the path, all of the electrons have to scale a barrier or a wall. Moving past this wall collectively keeps the electrons paired and the superconducting current stable.

But, the collective effort takes energy and gives off heat. With successive scaling attempts, the heat builds, causing a section of the wire to experience a phase slip from a superconducting to a non-superconducting state.

To pinpoint precisely how phase slips happen, Chang varied the temperatures and amount of current run through the aluminum nanowires.

The experiments show that at higher temperatures, roughly 1.5 degrees Kelvin and close to the critical temperature where the wires naturally become non-superconducting, the electrons have enough energy to move over the wall that keeps the electrons paired and the superconducting current stable.

In contrast, the electrons in the nanowires cooled to less than 1 degree Kelvin do not have the energy to scale the wall. Instead, the electrons tunnel, or go through the wall together, all at once, said Duke physicist Gleb Finkelstein, one of Chang's collaborators.

The experiments also show that at the relatively higher temperatures, individual jumps over the wall don't create enough heat to cause a breakdown in superconductivity. But multiple jumps do.

At the lowest temperatures, however, the paired electrons only need to experience one successful attempt at the wall, either over or through it, to create enough heat to slip in phase and break the superconducting state.

Studying the electrons' behavior at specific temperatures provides scientists with information to build ultra-thin superconducting wires that might not have phase slips. Chang said the improved wires could soon play a role in ultra-miniaturized electrical components for ultra-miniaturized electronics, such as the quantum bit, used in a quantum computer.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Duke University.

Journal Reference:

Peng Li, Phillip Wu, Yuriy Bomze, Ivan Borzenets, Gleb Finkelstein, A. Chang. Switching Currents Limited by Single Phase Slips in One-Dimensional Superconducting Al Nanowires. Physical Review Letters, 2011; 107 (13) DOI: 10.1103/PhysRevLett.107.137004

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Scientists observe how superconducting nanowires lose resistance-free state

ScienceDaily (Sep. 22, 2011) — Even with today's invisibility cloaks, people can't walk through walls. But, when paired together, millions of electrons can. The electrons perform this trick, called macroscopic quantum tunneling, when they pair up and move into a region of space that is normally off-limits under the laws of classical mechanics. The problem is that as millions of electrons collectively move through a superconducting nanowire, they use energy and give off heat.

The heat can build, transforming sections of the wire into a non-superconducting state. The process, called a phase slip, adds resistance to an electrical system and has implications for designing new nano-scale superconductors.

Now, scientists have observed individual phase slips in aluminum nanowires and characterized the nature and temperature at which they occur. This information could help scientists remove phase slips from nano-scale systems, which could lead to more reliable nanowires and more efficient nano-electronics, said Duke physicist Albert Chang.

The results appeared online Sept. 21 in Physical Review Letters.

The macroscopic quantum tunneling effect was first observed in a system called a Josephson junction. This device has a thin insulating layer connecting two superconductors, which are several nanometers wide and have a three-dimensional shape.

To study the tunneling and phase slips in a simpler system, however, Chang and his colleagues used individual, one-dimensional nanowires made of aluminum. The new observations are "arguably the first convincing demonstration of tunneling of millions of electrons in one-dimensional superconducting nanowires," said Chang, who led the study.

In the experiment, the wires ranged in length from 1.5 to 10 micrometers, with widths from five to 10 nanometers. Chang cooled the wires to a temperature close to absolute zero, roughly 1 degree Kelvin or -458 degrees Fahrenheit.

At this temperature, a metal's crystal lattice vibrates in a way that allows electrons to overcome their negative repulsion of one other. The electrons make pairs and electric current flows essentially resistance-free, forming a superconductor.

The electron pairs move together in a path in a quantum-mechanical space, which resembles the curled cord of an old phone. On their way around the path, all of the electrons have to scale a barrier or a wall. Moving past this wall collectively keeps the electrons paired and the superconducting current stable.

But, the collective effort takes energy and gives off heat. With successive scaling attempts, the heat builds, causing a section of the wire to experience a phase slip from a superconducting to a non-superconducting state.

To pinpoint precisely how phase slips happen, Chang varied the temperatures and amount of current run through the aluminum nanowires.

The experiments show that at higher temperatures, roughly 1.5 degrees Kelvin and close to the critical temperature where the wires naturally become non-superconducting, the electrons have enough energy to move over the wall that keeps the electrons paired and the superconducting current stable.

In contrast, the electrons in the nanowires cooled to less than 1 degree Kelvin do not have the energy to scale the wall. Instead, the electrons tunnel, or go through the wall together, all at once, said Duke physicist Gleb Finkelstein, one of Chang's collaborators.

The experiments also show that at the relatively higher temperatures, individual jumps over the wall don't create enough heat to cause a breakdown in superconductivity. But multiple jumps do.

At the lowest temperatures, however, the paired electrons only need to experience one successful attempt at the wall, either over or through it, to create enough heat to slip in phase and break the superconducting state.

Studying the electrons' behavior at specific temperatures provides scientists with information to build ultra-thin superconducting wires that might not have phase slips. Chang said the improved wires could soon play a role in ultra-miniaturized electrical components for ultra-miniaturized electronics, such as the quantum bit, used in a quantum computer.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Duke University.

Journal Reference:

Peng Li, Phillip Wu, Yuriy Bomze, Ivan Borzenets, Gleb Finkelstein, A. Chang. Switching Currents Limited by Single Phase Slips in One-Dimensional Superconducting Al Nanowires. Physical Review Letters, 2011; 107 (13) DOI: 10.1103/PhysRevLett.107.137004

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Physicists apply Einstein's general theory of relativity to superconducting circuits

ScienceDaily (June 11, 2011) — In recent years, UC Santa Barbara scientists showed that they could reproduce a basic superconductor using Einstein's general theory of relativity. Now, using the same theory, they have demonstrated that the Josephson junction could be reproduced. The results are explained in a recent issue of the journal Physical Review Letters.

The Josephson junction, a device that was first discovered by Brian David Josephson in the early 1960's, is a main ingredient in applications of superconductivity.

Gary Horowitz, professor of physics at UCSB, said that Einstein's general theory of relativity -- which was developed as a theory of gravity and is extremely successful in explaining a wide variety of gravitational phenomena -- is now being used to explain several aspects of non-gravitational physics.

"The basic phenomenon with Josephson junctions is that you can take two superconductors, separate them by a little gap, and still find current going across it, in a specific way," said Horowitz. "And that has found many applications. So the Josephson junction is something we've reproduced using general relativity."

Horowitz said that he and his co-authors used tools from string theory to develop the gravity model of a superconductor. He explained that it was surprising to be able to link Einstein's general theory of relativity to a totally different area of physics. He said he hoped that the new tools would one day be able to shed light on new types of superconductors.

"Most materials, if you cool them down sufficiently, will actually conduct electricity without any resistance," said Horowitz. "These are superconductors. There is a standard theory of superconductivity, discovered about 50 years ago, that has worked well for most of the so-called conventional superconductors."

A new class of materials was discovered 25 years ago. These are superconductors that have zero resistance at somewhat higher temperatures. Physicists are still working on understanding the mechanism.

This new class of materials involves copper-oxygen planes. Another new class of superconductors, based on iron instead of copper, was discovered a couple of years ago. These materials, called iron nictides, also have the property of superconducting at a higher temperature.

"There is a lot of activity and interest in understanding these materials," said Horowitz. "Ultimately, the goal is to have a room-temperature superconductor, which, you can imagine, would have lots of interesting applications."

Horowitz and his research team found what could be called a gravitational model, or a gravitational dual -- a dual description of a superconductor using gravity, black holes, and all of the traditional ingredients of general relativity. "This came as quite a surprise because this is a totally different area of physics, which is now being connected to this condensed matter area," said Horowitz.

The co-authors of the paper are postdoctoral fellow Jorge E. Santos and graduate student Benson Way.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Santa Barbara.

Journal Reference:

Gary Horowitz, Jorge Santos, Benson Way. Holographic Josephson Junctions. Physical Review Letters, 2011; 106 (22) DOI: 10.1103/PhysRevLett.106.221601

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Physicists apply Einstein's general theory of relativity to superconducting circuits

ScienceDaily (June 11, 2011) — In recent years, UC Santa Barbara scientists showed that they could reproduce a basic superconductor using Einstein's general theory of relativity. Now, using the same theory, they have demonstrated that the Josephson junction could be reproduced. The results are explained in a recent issue of the journal Physical Review Letters.

The Josephson junction, a device that was first discovered by Brian David Josephson in the early 1960's, is a main ingredient in applications of superconductivity.

Gary Horowitz, professor of physics at UCSB, said that Einstein's general theory of relativity -- which was developed as a theory of gravity and is extremely successful in explaining a wide variety of gravitational phenomena -- is now being used to explain several aspects of non-gravitational physics.

"The basic phenomenon with Josephson junctions is that you can take two superconductors, separate them by a little gap, and still find current going across it, in a specific way," said Horowitz. "And that has found many applications. So the Josephson junction is something we've reproduced using general relativity."

Horowitz said that he and his co-authors used tools from string theory to develop the gravity model of a superconductor. He explained that it was surprising to be able to link Einstein's general theory of relativity to a totally different area of physics. He said he hoped that the new tools would one day be able to shed light on new types of superconductors.

"Most materials, if you cool them down sufficiently, will actually conduct electricity without any resistance," said Horowitz. "These are superconductors. There is a standard theory of superconductivity, discovered about 50 years ago, that has worked well for most of the so-called conventional superconductors."

A new class of materials was discovered 25 years ago. These are superconductors that have zero resistance at somewhat higher temperatures. Physicists are still working on understanding the mechanism.

This new class of materials involves copper-oxygen planes. Another new class of superconductors, based on iron instead of copper, was discovered a couple of years ago. These materials, called iron nictides, also have the property of superconducting at a higher temperature.

"There is a lot of activity and interest in understanding these materials," said Horowitz. "Ultimately, the goal is to have a room-temperature superconductor, which, you can imagine, would have lots of interesting applications."

Horowitz and his research team found what could be called a gravitational model, or a gravitational dual -- a dual description of a superconductor using gravity, black holes, and all of the traditional ingredients of general relativity. "This came as quite a surprise because this is a totally different area of physics, which is now being connected to this condensed matter area," said Horowitz.

The co-authors of the paper are postdoctoral fellow Jorge E. Santos and graduate student Benson Way.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Santa Barbara.

Journal Reference:

Gary Horowitz, Jorge Santos, Benson Way. Holographic Josephson Junctions. Physical Review Letters, 2011; 106 (22) DOI: 10.1103/PhysRevLett.106.221601

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here