Ultrafast Magnetic Processes Observed 'Live' Using an X-Ray Laser

Detail of the structure of cupric oxide (CuO). The copper atoms (green) carry a magnetic moment, behaving like small compass needles. The direction of the magnetic moment is illustrated by a red arrow. A point means that the arrow is pointing out of the surface (we are looking at its sharp end), a cross shows that the arrow is pointing into the surface (we are looking at its tail end). The magnetic structure changes significantly as the temperature increases above 213 Kelvin (around -60°C). One aspect of this change is a difference in the period of the magnetic order. Unlike the ordering at low temperatures, the magnetic structure in the temperature range 213 K to 230 K is incommensurate: its period does not ‘fit’ with the period of the crystal structure of copper and oxygen atoms. To be precise, a full rotation of the direction of the magnetic moment does not require exactly four atomic separations, but a little more or a little less, depending on the direction. (Credit: Image courtesy of Paul Scherrer Institut (PSI))

ScienceDaily (Jan. 23, 2012) — In first-of-their-kind experiments performed at the American X-ray laser LCLS, a collaboration led by researchers from the Paul Scherrer Institute has been able to precisely follow how the magnetic structure of a material changes.

This is another milestone, because such investigations will also be a major focus of research at the planned Swiss X-ray Laser, Swiss FEL, at PSI. The results could contribute to the development of new technologies for magnetic storage media for the future.The study was carried out on cupric oxide (CuO). The change of structure was initiated by a laser pulse, and then, with the help of short X-ray pulses, near-instantaneous images were obtained at different points in time for individual intermediate steps during the process. It appears as if the structure begins to change 400 femtoseconds after the laser pulse strikes (1 femtosecond = 0.000 000 000 000 001 seconds). Apparently, the fundamental magnets within the material need that much time to communicate with each other and then react. In addition to this scientific result, the work proves that it is actually possible with X-ray lasers to follow certain types of extremely rapid magnetic processes.

The researchers have reported on their work in the latest edition of the technical journal Physical Review Letters (PRL).

Materials with particular magnetic properties are the basis of many current technologies, in particular, data storage on hard discs and in other media. For this, the magnetic orientation in the material is most often used: the atoms in the material behave to some extent like tiny rod magnets ("spins"). These mini-magnets can be oriented in different ways and information can be stored through their orientation. For efficient data storage, it is crucial that old data can be rapidly overwritten. This is possible if the magnetic orientation in a material can be altered in a very short time. To develop innovative materials which can store data quickly, it is therefore important to understand exactly how this change occurs as a function of time.

Magnetic orientation in motion

In experiments performed at the X-ray laser LCLS at Stanford, California, a collaboration led by researchers from the Paul Scherrer Institute have been able to study the magnetic orientation in cupric oxide, CuO. This material demonstrates completely different magnetic orientations depending on temperature: Below -60°C, the spins, which function in the copper atoms (Cu) like magnets, point periodically in one direction and then the opposite; between -60°C and -43°C, they are arranged helically, as if they were forming a spiral staircase. Although the spin orientations for the two arrangements have been known for some time, the time required to move from one arrangement to the other has only now been shown by the experiment.

"In our investigation, we began with a 'cold' sample and then heated it with an intense flash of light from an optical laser," explains Steven Johnson, spokesman for the PSI experiment. "Shortly after this, we determined the structure of the sample by illuminating it with an extremely short pulse from an X-ray laser. When we repeated this at different time intervals between the flash of light and the X-ray pulse, we were able to reconstruct the course of the change in the magnetic structure."

Mini-magnets need 400 femtoseconds to agree amongst themselves.

The results show that it takes about 400 femtoseconds before the magnetic structure begins to alter visibly. Then the structure gradually reaches its final state. The more intense the initiating flash of light, the faster the change of state. "The spins of all copper atoms are involved in the magnetic structure. Thus the atoms at opposite ends of the material must be coordinated before the structure can change. This takes 400 femtoseconds," explains Urs Staub, one of the PSI researchers responsible. "For cupric oxide, that is the fundamental limit; it simply cannot happen faster than that. This depends upon how strongly the spins are coupled between neighbouring atoms."

There is a good reason why the researchers were particularly interested in cupric oxide. Along with the screw-like magnetic orientation that occurs between -60°C and -43°C, the material is also 'multiferroic', a material where electrical and magnetic processes mutually influence one another. These materials have many different potential areas of application where magnetism and electronics interact.

Story Source:

The above story is reprinted from materials provided byPaul Scherrer Institut (PSI), via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

---

Journal Reference:

1. S. Johnson, R. de Souza, U. Staub, P. Beaud, E. Möhr-Vorobeva, G. Ingold, A. Caviezel, V. Scagnoli, W. Schlotter, J. Turner, O. Krupin, W.-S. Lee, Y.-D. Chuang, L. Patthey, R. Moore, D. Lu, M. Yi, P. Kirchmann, M. Trigo, P. Denes, D. Doering, Z. Hussain, Z.-X. Shen, D. Prabhakaran, A. Boothroyd. Femtosecond Dynamics of the Collinear-to-Spiral Antiferromagnetic Phase Transition in CuO.Physical Review Letters, 2012; 108 (3) DOI:10.1103/PhysRevLett.108.037203

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

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 observe 'campfire effect' in blinking nanorod semiconductors

ScienceDaily (June 24, 2011) — When semiconductor nanorods are exposed to light, they blink in a seemingly random pattern. By clustering nanorods together, physicists at the University of Pennsylvania have shown that their combined "on" time is increased dramatically providing new insight into this mysterious blinking behavior.

The research was conducted by associate professor Marija Drndic's group, including graduate student Siying Wang and postdoctorial fellows Claudia Querner and Tali Dadosh, all of the Department of Physics and Astronomy in Penn's School of Arts and Sciences. They collaborated with Catherine Crouch of Swarthmore College and Dmitry Novikov of New York University's School of Medicine.

Their research was published in the journal Nature Communications.

When provided with energy, whether in the form of light, electricity or certain chemicals, many semiconductors emit light. This principle is at work in light-emitting diodes, or LEDs, which are found in any number of consumer electronics.

At the macro scale, this electroluminescence is consistent; LED light bulbs, for example, can shine for years with a fraction of the energy used by even compact-fluorescent bulbs. But when semiconductors are shrunk down to nanometer size, instead of shining steadily, they turn "on" and "off" in an unpredictable fashion, switching between emitting light and being dark for variable lengths of time. For the decade since this was observed, many research groups around the world have sought to uncover the mechanism of this phenomenon, which is still not completely understood.

"Blinking has been studied in many different nanoscale materials for over a decade, as it is surprising and intriguing, but it's the statistics of the blinking that are so unusual," Drndic said. "These nanorods can be 'on' and 'off' for all scales of time, from a microsecond to hours. That's why we worked with Dmitry Novikov, who studies stochastic phenomena in physical and biological systems. These unusual Levi statistics arise when many factors compete with each other at different time scales, resulting in a rather complex behavior, with examples ranging from earthquakes to biological processes to stock market fluctuations."

Drndic and her research team, through a combination of imaging techniques, have shown that clustering these nanorod semiconductors greatly increases their total "on" time in a kind of "campfire effect." Adding a rod to the cluster has a multiplying effect on the "on" period of the group.

"If you put nanorods together, if each one blinks in rare short bursts, you would think the maximum 'on' time for the group will not be much bigger than that for one nanorod, since their bursts mostly don't overlap," Novikov said. "What we see are greatly prolonged 'on' bursts when nanorods are very close together, as if they help each other to keep shining, or 'burning.'"

Drndic's group demonstrated this by depositing cadmium selenide nanorods onto a substrate, shining a blue laser on them, then taking video under an optical microscope to observe the red light the nanorods then emitted. While that technique provided data on how long each cluster was "on," the team needed to use transmission electron microscopy, or TEM, to distinguish each individual, 5-nanometer rod and measure the size of each cluster.

A set of gold gridlines allowed the researchers to label and locate individual nanorod clusters. Wang then accurately overlaid about a thousand stitched-together TEM images with the luminescence data that she took with the optical microscope. The researchers observed the "campfire effect" in clusters as small as two and as large as 110, when the cluster effectively took on macroscale properties and stopped blinking entirely.

While the exact mechanism that causes this prolonged luminescence can't yet be pinpointed, Drndic's team's findings support the idea that interactions between electrons in the cluster are at the root of the effect.

"By moving from one end of a nanorod to the other, or otherwise changing position, we hypothesize that electrons in one rod can influence those in neighboring rods in ways that enhance the other rods' ability to give off light," Crouch said. "We hope our findings will give insight into these nanoscale interactions, as well as helping guide future work to understand blinking in single nanoparticles."

As nanorods can be an order of magnitude smaller than a cell, but can emit a signal that can be relatively easily seen under a microscope, they have been long considered as potential biomarkers. Their inconsistent pattern of illumination, however, has limited their usefulness.

"Biologists use semiconductor nanocrystals as fluorescent labels. One significant disadvantage is that they blink," Drndic said. "If the emission time could be extended to many minutes it makes them much more usable. With further development of the synthesis, perhaps clusters could be designed as improved labels."

Future research will use more ordered nanorod assemblies and controlled inter-particle separations to further study the details of particle interactions.

This research was supported by the National Science Foundation.

Story Source:

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

Journal Reference:

Siying Wang, Claudia Querner, Tali Dadosh, Catherine H. Crouch, Dmitry S. Novikov, Marija Drndic. Collective fluorescence enhancement in nanoparticle clusters. Nature Communications, 2011; 2: 364 DOI: 10.1038/ncomms1357

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

New light shed on the private lives of electrons: Lasers allow scientists to observe how electrons become entangled

ScienceDaily (July 3, 2011) — A Princeton researcher and his international collaborators have used lasers to peek into the complex relationship between a single electron and its environment, a breakthrough that could aid the development of quantum computers.

The technique reveals how an isolated electron and its surroundings develop a relationship known as a Kondo state -- a state of matter that is of great interest to physicists and engineers. The results not only yield insights into a long-standing quandary in theoretical physics, but also may help scientists understand how to store information at the smallest possible scales, which would open vast new realms of computing power.

"What we've done is illuminate the private life of a single electron," said Hakan Tureci, an assistant professor of electrical engineering at Princeton and a lead researcher on the project. "It's taken nearly a century to isolate, control and probe a single electron in this way -- an extraordinary feat enabled by quantum theory, cryogenics and nanotechnology."

The research was conducted by an international team of scientists from the United States, Germany and Switzerland. The researchers on the project included Tureci, Atac Imamoglu, a professor at Swiss Federal Institute of Technology Zurich in Switzerland, Jan von Delft, a professor at LMU Munich, and Leonid Glazman, a professor at Yale University.

The key theoretical results and a proposal for testing the ideas experimentally were published March 11 in the journal Physical Review Letters.

These theoretical projections were recently confirmed in experiments led by Imamoglu, which were published June 29 in the journal Nature.

The research brings fresh insight to the study of the Kondo problem, a phenomenon first observed in the 1930s, when researchers were surprised to find that resistance to electricity flowing through certain metals increases at very low temperatures. Normally, resistance through metals decreases as temperature is lowered, but that was not the case with these metals.

The phenomenon was explained 30 years later by Japanese scientist, Jun Kondo, as resulting from the presence of cobalt or other magnetic impurities in the metals.

Scientists have further realized that the Kondo effect results from a relationship between electrons known as "entanglement" in which the quantum state of one electron is tied to those of neighboring electrons, even if the particles are later separated by considerable distances. In the case of Kondo effect, a trapped electron is entangled in a complex manner with a cloud of surrounding electrons.

Researchers have been intrigued by the Kondo effect in part because understanding how a trapped electron becomes entangled with its environment could help overcome barriers to quantum computing, which could lead to far more powerful computers than currently exist.

Previous observation methods allowed scientists to make measurements of the Kondo state, but could not provide information on how electrons developed such a relationship with their surroundings.

To better understand how an electron gradually becomes entangled in this manner with its environment, Tureci and his collaborators investigated the idea of using a laser to probe electrons evolving into the Kondo state. They first developed a theory about how laser light scattered off electrons could carry information about this process.

Depending on the state of the electron, they surmised, it should absorb different colors of laser light to varying degrees. The light reflected back would carry a signature of the entangled quantum state, offering a window into the relationship between the trapped electron and its environment.

To isolate the electrons, they proposed using nanostructured devices, small machines built one atom at a time that trap the electrons in small wells. The particles are only provided limited isolation in the wells and so eventually become entangled with a cloud of surrounding electrons in the device.

Tureci's collaborators in Switzerland tested the idea by projecting a laser beam on the device and measuring the light that was transmitted.

The light signature matched theoretical predictions. The researchers also found that they could use the light signatures to confirm when they turned the Kondo state off using a magnetic field.

"By doing this experiment," Tureci said, "we showed that you can extract this information that was previously unavailable in earlier experiments on the Kondo effect."

He said the finding could provide insight into quantum computing because entanglement, depending on its nature, could allow new ways of storing and processing information or could threaten to destabilize the computing process.

Whereas current computers use transistors to store "bits" of information as ones or zeros, scientists believe quantum computers might one day use trapped electrons that are entangled with one another as "qubits," the basic information units of quantum computing, which can have the odd quality of representing a blend of "one" and "zero" simultaneously.

A series of qubits could thus store exponentially more information than the 0 and 1 combination of classical bits.

While quantum computers could theoretically be far smaller and faster than transistor-based machines, using electrons or other sub-atomic particles as storage devices is no trivial feat.

The undesirable entangled relationship between electrons and their environment, such as that seen in the Kondo effect, can destabilize the desired relationship between trapped electrons that form the qubits and gradually destroy the information they store.

"Our technique offers a window into the Kondo state, allowing us a chance to study electrons that are highly entangled with their environments and understand how they got that way," Tureci said.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Princeton University, Engineering School, via EurekAlert!, a service of AAAS.

Journal Reference:

C. Latta, F. Haupt, M. Hanl, A. Weichselbaum, M. Claassen, W. Wuester, P. Fallahi, S. Faelt, L. Glazman, J. von Delft, H. E. Türeci, A. Imamoglu. Quantum quench of Kondo correlations in optical absorption. Nature, 2011; 474 (7353): 627 DOI: 10.1038/nature10204

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