Physicists capture first images of atomic spin

ATHENS, Ohio (April 26, 2010) – Though scientists argue that the emerging technology of spintronics may trump conventional electronics for building the next generation of faster, smaller, more efficient computers and high-tech devices, no one has actually seen the spin—a quantum mechanical property of electrons—in individual atoms until now.
In a study published as an Advance Online Publication in the journal Nature Nanotechnology on Sunday, physicists at Ohio University and the University of Hamburg in Germany present the first images of spin in action.

The researchers used a custom-built microscope with an iron-coated tip to manipulate cobalt atoms on a plate of manganese. Through scanning tunneling microscopy, the team repositioned individual cobalt atoms on a surface that changed the direction of the electrons’ spin. Images captured by the scientists showed that the atoms appeared as a single protrusion if the spin direction was upward, and as double protrusions with equal heights when the spin direction was downward.

The study suggests that scientists can observe and manipulate spin, a finding that may impact future development of nanoscale magnetic storage, quantum computers and spintronic devices.

“Different directions in spin can mean different states for data storage,” said Saw-Wai Hla, an associate professor of physics and astronomy in Ohio University’s Nanoscale and Quantum Phenomena Institute and one of the primary investigators on the study. “The memory devices of current computers involve tens of thousands of atoms. In the future, we may be able to use one atom and change the power of the computer by the thousands.”

Unlike electronic devices, which give off heat, spintronic-based devices are expected to experience less power dissipation.

The experiments were conducted in an ultra-high vacuum at the low temperature of 10 Kelvin, with the use of liquid helium. Researchers will need to observe the phenomenon at room temperature before it can be used in computer hard drives.

But the new study suggests a path to that application, said study lead author Andre Kubetzka of the University of Hamburg. To image spin direction, the team not only used a new technique but also a manganese surface with a spin that, in turn, allowed the scientists to manipulate the spin of the cobalt atoms under study.

“The combination of atom manipulation and spin sensitivity gives a new perspective of constructing atomic-scale structures and investigating their magnetic properties,” Kubetzka said.

The research, which was carried out at the University of Hamburg, was supported by the German Research Foundation and a Partnership for International Collaboration and Education (PIRE) grant from National Science Foundation.

The research is the result of a collaboration among three research teams: a spin-polarized scanning tunneling microscopy group of Professor Roland Wiesendanger led by Kubetzka at the University of Hamburg, Germany; Hla, an expert in atom manipulation at Ohio University; and two theorists, Professor Stefan Heinze and Paolo Ferriani, now at the Christian-Albrechts-Universität Kiel, in Germany.

Obtenido de: http://www.ohio.edu/research/communications/spin.cfm
Tirso Ramírez
CRF

FRICTION DIFFERENCES OFFER NEW MEANS FOR MANIPULATING NANOTUBES

Nanotubes and nanowires are promising building blocks for future integrated nanoelectronic and photonic circuits, nanosensors, interconnects and electro-mechanical nanodevices. But some fundamental issues remain to be resolved—among them, how to position and manipulate the tiny tubes.


Publishing in the journal Nature Materials, researchers from four different institutions report measuring different friction forces when a carbon nanotube slides along its axis compared to when it slides perpendicular to its axis. This friction difference has its origins in soft lateral distortion of the tubes when they slide in the transverse direction.

The findings not only could provide a better understanding of fundamental friction issues, but from a more practical standpoint, offer a new tool for assembling nanotubes into devices and clarify the forces acting on them. Asymmetries in the friction could potentially also be used in sorting nanotubes according to their chirality, a property that is now difficult to measure with other means.

When an atomic force microscope (AFM) tip is scanned transversely across a multi-walled carbon nanotube, the amount of friction measured is twice as much as when the same tube is scanned longitudinally, along the length of the tube. The researchers attribute this difference to what they call “hindered rolling”—additional effort required to overcome the nanotube’s tendency to roll as the AFM tip strokes across it rather than along it.

“Because the energy required to move in one direction is twice as much as required to move in the other direction, this could be an easy way to control the assembly of carbon nanotubes for nanoelectronics, sensors and other applications,” said Elisa Riedo, co-author of the study and an associate professor in the School of Physics at the Georgia Institute of Technology. “To assemble nanotubes on a surface, you need to know how they interact and what force is needed to move them.”

The combined theoretical and experimental study was supported by the U.S. Department of Energy. Other institutions contributing to the project include the International Centre for Theoretical Physics, International School for Advanced Studies and CNR Democritos Laboratory—all in Trieste, Italy—and the University of Hamburg in Germany. The paper was published in advance online on September 13 by the journal Nature Materials.

Carbon nanotubes have exceptional thermal, mechanical and electrical properties that have generated considerable interest since they were first reported in 1991. Though friction has been studied before in nanotubes, this research is the first to provide detailed information about the frictional forces at work in both the longitudinal and transverse directions when the tubes interact with an AFM tip.

Friction is one of the oldest problems in physics and one of the most important to everyday life. It is estimated that the losses in the U.S. economy due to friction total about 6 percent of the gross national product. Friction is even more important to micro-electromechanical systems (MEMS) and nanoscale devices because these smaller systems are more affected by surface forces than large systems.

“As systems become smaller and smaller, it becomes more important to understand how to address friction,” said Riedo. “Surface forces can prevent micro and nano systems from operating at all.”

Experimentally, the researchers scanned an atomic force microscope tip longitudinally along a series of multiwalled carbon nanotubes held stationary on a substrate. They also conducted a series of similar scans in the transverse direction. The researchers applied a consistent force on the AFM tip in both scanning directions, and relied on powerful Van der Waals forces to hold tubes in place on the substrate.

“When you scan a nanotube transversely, you are probing something very different,” said Riedo. “You are also probing additional dissipation modes due to a kind of swaying motion in which energy is also dissipated through movement of the nanotube as it alters its cross section.”

The experiment showed that greater forces were required to move the tip in the transverse direction. Using molecular dynamics simulations, collaborators Erio Tosatti and Xiaohua Zhang at the International Centre for Theoretical Physics, International School for Advanced Studies and CNR Democritos Laboratory analyzed the phenomenon to understand what was happening.

“In principle, there seems to be no reason why the frictional forces required to move the AFM tip would be different in one direction,” Riedo noted. “But the theory confirmed that this ‘hindered rolling’ and soft mode movement of the nanotubes are the sources of the higher friction when the tip moves transversely.”

Because the nanotube-tip system is so simple, it offers an ideal platform for studying basic friction principles, which are important to all moving systems.

“This kind of system gives you the opportunity to explore friction using an ideal experiment so you can really probe the energy dissipation mechanism,” Riedo explained. “The system is so simple that you can distinguish between the dissipation mechanisms, which you can’t usually do well in macro-scale systems.”

Based on the molecular dynamics simulations, Riedo and Tosatti believe that the friction anisotropy will be very different in chiral nanotubes versus non-chiral—left-to-right symmetric—nanotubes.

“Because of the chirality, the tip moves in a screw-like fashion, creating hindered rolling even for longitudinal sliding,” Tosatti said. “Thus, the new measuring technique may suggest a simple way to sort the nanotubes; among the next steps in the research will be to show experimentally that this can be done.”

In addition to the researchers already mentioned, co-authors for this paper include Christian Klinke at the Institute of Physical Chemistry at the University of Hamburg, and Marcel Lucas and Ismael Palaci at Georgia Tech.

“Understanding the basic mechanism of friction in carbon nanotubes will help us in designing devices with them in the future,” Riedo added. “We have shown an anisotropy in the friction coefficient of carbon nanotubes in the transverse and longitudinal directions, which has its origin in the soft lateral distortion of tubes when the tip-tube contact is moving in the transverse direction. Our findings could help in developing better strategies for chirality sorting, large-scale self-assembling of nanotubes on surfaces, and designing nanotube adhesives and nanotube-polymer composite materials.”

Obtenido de: http://www.scitech-news.com/search/label/Nanotechnology
Tirso Ramírez
CRF

PARTICLE CHAMELEON CAUGHT IN THE ACT OF CHANGING

Researchers on the OPERA experiment at the INFN1’s Gran Sasso laboratory in Italy announced the first direct observation of a tau particle in a muon neutrino beam sent through the Earth from CERN2, 730km away. This is a significant result, providing the final missing piece of a puzzle that has been challenging science since the 1960s, and giving tantalizing hints of new physics to come.

The neutrino puzzle began with a pioneering and ultimately Nobel Prize winning experiment conducted by US scientist Ray Davis beginning in the 1960s. He observed far fewer neutrinos arriving at the Earth from the Sun than solar models predicted: either solar models were wrong, or something was happening to the neutrinos on their way. A possible solution to the puzzle was provided in 1969 by the theorists Bruno Pontecorvo and Vladimir Gribov, who first suggested that chameleon-like oscillatory changes between different types of neutrinos could be responsible for the apparent neutrino deficit.

Several experiments since have observed the disappearance of muon-neutrinos, confirming the oscillation hypothesis, but until now no observations of the appearance of a tau-neutrino in a pure muon-neutrino beam have been observed: this is the first time that the neutrino chameleon has been caught in the act of changing from muon-type to tau-type.

Antonio Ereditato, Spokesperson of the OPERA collaboration described the development as: “an important result which rewards the entire OPERA collaboration for its years of commitment and which confirms that we have made sound experimental choices. We are confident that this first event will be followed by others that will fully demonstrate the appearance of neutrino oscillation”.

“The OPERA experiment has reached its first goal: the detection of a tau neutrino obtained from the transformation of a muon neutrino, which occurred during the journey from Geneva to the Gran Sasso Laboratory,” added Lucia Votano, Director Gran Sasso laboratories. “This important result comes after a decade of intense work performed by the Collaboration, with the support of the Laboratory, and it again confirms that LNGS is a leading laboratory in Astroparticle Physics”.

The OPERA result follows seven years of preparation and over three years of beam provided by CERN. During that time, billions of billions of muon-neutrinos have been sent from CERN to Gran Sasso, taking just 2.4 milliseconds to make the trip. The rarity of neutrino oscillation, coupled with the fact that neutrinos interact very weakly with matter makes this kind of experiment extremely subtle to conduct. CERN’s neutrino beam was first switched on in 2006, and since then researchers on the OPERA experiment have been carefully sifting their data for evidence of the appearance of tau particles, the telltale sign that a muon-neutrino has oscillated into a tau-neutrino. Patience of this kind is a virtue in particle physics research, as INFN President Roberto Petronzio explained:

“This success is due to the tenacity and inventiveness of the physicists of the international community, who designed a particle beam especially for this experiment,” said Petronzio. “In this way, the original design of Gran Sasso has been crowned with success. In fact, when constructed, the laboratories were oriented so that they could receive particle beams from CERN”.

At CERN, neutrinos are generated from collisions of an accelerated beam of protons with a target. When protons hit the target, particles called pions and kaons are produced. They quickly decay, giving rise to neutrinos. Unlike charged particles, neutrinos are not sensitive to the electromagnetic fields usually used by physicists to change the trajectories of particle beams. Neutrinos can pass through matter without interacting with it; they keep the same direction of motion they have from their birth. Hence, as soon as they are produced, they maintain a straight path, passing through the Earth's crust. For this reason, it is extremely important that from the very beginning the beam points exactly towards the laboratories at Gran Sasso.

“This is an important step for neutrino physics,” said CERN Director General Rolf Heuer. “My congratulations go to the OPERA experiment and the Gran Sasso Laboratories, as well as the accelerator departments at CERN. We’re all looking forward to unveiling the new physics this result presages.”

While closing a chapter on understanding the nature of neutrinos, the observation of neutrino oscillations is strong evidence for new physics. In the theories that physicists use to explain the behaviour of fundamental particles, which is known as the Standard Model, neutrinos have no mass. For neutrinos to be able to oscillate, however, they must have mass: something must be missing from the Standard Model. Despite its success in describing the particles that make up the visible Universe and their interactions, physicists have long known that there is much the Standard Model does not explain. One possibility is the existence of other, so-far unobserved types of neutrinos that could shed light on Dark Matter, which is believed to make up about a quarter of the Universe’s mass.

Obtenido de: http://www.scitech-news.com/2010/06/particle-chameleon-caught-in-act-of.html
Tirso Ramírez
CRF

All-in-one nanoparticle: A Swiss Army knife for nanomedicine

Nanoparticles are being developed to perform a wide range of medical uses -- imaging tumors, carrying drugs, delivering pulses of heat. Rather than settling for just one of these, researchers at the University of Washington have combined two nanoparticles in one tiny package.

The result is the first structure that creates a multipurpose nanotechnology tool for medical imaging and therapy. The structure is described in a paper published online this week in the journal Nature Nanotechnology.A quantum dot (red) encapsulated in a gold shell, combining two useful nanoparticles in one package. The total structure measures less than 20 nanometers across

"This is the first time that a semiconductor and metal nanoparticles have been combined in a way that preserves the function of each individual component," said lead author Xiaohu Gao, a UW assistant professor of bioengineering.

The current focus is on medical applications, but the researchers said multifunctional nanoparticles could also be used in energy research, for example in solar cells.

Quantum dots are fluorescent balls of semiconductor material just a few nanometers across, a small fraction of the wavelength of visible light (a nanometer is 1-millionth of a millimeter). At this tiny scale, quantum dots' unique optical properties cause them to emit light of different colors depending on their size. The dots are being developed for medical imaging, solar cells and light-emitting diodes.

Glowing gold nanoparticles have been used since ancient times in stained glass; more recently they are being developed for delivering drugs, for treating arthritis and for a type of medical imaging that uses infrared light. Gold also reradiates infrared heat and so could be used in medical therapies to cook nearby cells.

But combine a quantum dot and a gold nanoparticle, and the effects disappear. The electrical fields of the particles interfere with one another and so neither behaves as it would on its own. The two have been successfully combined on a surface, but never in a single particle.

The paper describes a manufacturing technique that uses proteins to surround a quantum dot core with a thin gold shell held at 3 nanometers distance, so the two components' optical and electrical fields do not interfere with one another. The quantum dot likely would be used for fluorescent imaging. The gold sphere could be used for scattering-based imaging, which works better than fluorescence in some situations, as well as for delivering heat therapy.

The manufacturing technique developed by Gao and co-author Yongdong Jin, a UW postdoctoral researcher, is general and could apply to other nanoparticle combinations, they said.

"We picked a tough case," Gao said. "It is widely known that gold or any other metal will quench quantum dot fluorescence, eliminating the quantum dot's purpose."

Gao and Jin avoided this problem by building a thin gold sphere that surrounds but never touches the quantum dot. They carefully controlled the separation between the gold shell and the nanoparticle core by using chains of polymer, polyethylene glycol. The distance between the quantum dot core and charged gold ion is determined by the length of the polymer chain and can be increased with nanometer precision by adding links to the chain. On the outside layer they added short amino acids called polyhistidines, which bind to charged gold atoms.

Gao compares the completed structure to a golden egg, where the quantum dot is the yolk, the gold is the shell, and polymers fill up the space of the egg white.

Using ions allowed the researchers to build a 2- to 3-nanometer gold shell that's thin enough to allow about half of the quantum dot's fluorescence to pass through.

"All the traditional techniques use premade gold nanoparticles instead of gold ions,"Gao said. "Gold nanoparticles are 3 to 5 nanometers in diameter, and with factoring in roughness the thinnest coating you can build is 5-6 nanometers. Gold ions are much, much smaller.”

The total diameter of the combined particle is roughly 15-20 nanometers, small enough to be able to slip into a cell.

Incorporating gold provides a well-established binding site to attach biological molecules that target particular cells, such as tumor cells. Gold could also potentially amplify the quantum dot's fluorescence by five to 10 times, as it has in other cases.

The gold sphere offers one further benefit. Gold is biocompatible, is medically approved and does not biodegrade. A gold shell could thus provide a durable non-toxic container for nanoparticles being used in the body, Gao said.

The research was supported by the National Institutes of Health, the National Science Foundation, the Seattle Foundation and the UW's Department of Bioengineering.

Obtenido de: http://uwnews.org/article.asp?articleID=51016
Tirso Ramírez
CRF

Answer from 'Dusty Shelf' Aids Quest to See Matter as it Was Just After Big Bang

Scientists trying to recreate conditions that existed just a few millionths of a second after the big bang that started the universe have run into a mysterious problem -- some of the reactions they are getting don't mesh with what they thought they were supposed to see.

Now, two University of Washington physicists have dusted off a quantum mechanics technique usually associated with low-energy physics and applied it to results from experiments at Brookhaven National Laboratory on New York's Long Island that produce high-energy collisions between gold nuclei. The result is data much more in line with what theorists expected from the experiments, said John Cramer, a UW physics professor. That means physicists at Brookhaven probably have actually succeeded in creating quark-gluon plasma, a state of matter that has not existed since a microsecond after the big bang that began the universe.

As it turns out, the model the physicists were using was missing some pieces, say Cramer and Gerald Miller, also a UW physics professor, whose findings will be published this month in Physical Review Letters, a journal of the American Physical Society.

"We think we've solved the puzzle by identifying important phenomena that were left out of the model," Cramer said.

Since 2000, scientists have been using the Relativistic Heavy Ion Collider at Brookhaven to collide gold nuclei with each other at nearly the speed of light. They are trying to get subatomic particles called quarks and gluons to separate from the nuclei and form a superheated quark-gluon plasma, 40 billion times hotter than room temperature.

Physicists used a technique called Hanbury Brown-Twiss Interferometry, originally used by astronomers to measure the size of stars, to learn the size and duration of a fireball produced in the collision of two gold nuclei. The technique focuses on momentum differences between pairs of pions, the particles produced in the fireball.

Before the collider experiment began, scientists expected a quark-gluon plasma to fuel a large and long-lasting fireball. Instead, the interferometry data showed a fireball similar in size and duration to those seen at much lower energies. Researchers also expected to see pions pushed out of the plasma gradually, but instead they seem to explode out all at once.

"We expected to bring the nuclear liquid to a boil and produce a steam of quark-gluon plasma," Cramer said. "Instead, the boiler seems to be blowing up in our faces."

While other evidence suggested that the collider experiment had created a quark-gluon plasma, the interferometry data pointed away from that possibility. To solve the puzzle, Cramer and Miller used a phenomenon called chiral symmetry restoration, which predicts that subatomic particles will change in mass and size depending on their environment -- in a hot, dense plasma as opposed to a vacuum, for instance.

By adding that process to the model, they found that pions in the plasma have to expend a large amount of energy to escape, as if they were stuck in a deep hole and had to climb out. That is because chiral symmetry gives pions a low mass when they are inside the plasma but a much higher mass once outside. The scientists also allowed for some pions to disappear completely, to transform into some other type of particle as they emerge from the plasma.

The result reconciles all the evidence from the collider experiments, supporting the possibility that a quark-gluon plasma actually has been created.

"We have taken a quantum mechanics technique, called the nuclear optical model, from an old and dusty shelf and applied it to puzzling new physics results," Miller said. "It's really a scientific detective story."

The work, supported by U.S. Department of Energy grants, adds to the general understanding of what happened in the first microseconds after the big bang, he said, "and what we bring to bear is a better microscope, the microscope of quantum mechanics."

Cramer noted that adding chiral symmetry restoration to the picture achieved results very close to what computer models told scientists to expect, and did so without forcing the experimental data to fit preconceived standards.

"A microsecond after the big bang, there was a state of matter that no one was able to investigate until very recently," he said. "We are still learning, but our understanding is growing."

For more information, contact Cramer at (206) 616-4635, (206) 543-9194 or cramer@phys.washington.edu, or Miller at (206) 543-2995 or miller@phys.washington.edu

Obtenido de: http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=05-X3
Tirso Ramírez
CRF

Dartmouth researchers propose new way to reproduce a black hole

Despite their popularity in the science fiction genre, there is much to be learned about black holes, the mysterious regions in space once thought to be absent of light. In a paper published in the August 20 issue of Physical Review Letters, the flagship journal of the American Physical Society, Dartmouth researchers propose a new way of creating a reproduction black hole in the laboratory on a much-tinier scale than their celestial counterparts.

Miles Blencowe (photo by Joseph Mehling '69)

The new method to create a tiny quantum sized black hole would allow researchers to better understand what physicist Stephen Hawking proposed more than 35 years ago: black holes are not totally void of activity; they emit photons, which is now known as Hawking radiation.

"Hawking famously showed that black holes radiate energy according to a thermal spectrum," said Paul Nation, an author on the paper and a graduate student at Dartmouth. "His calculations relied on assumptions about the physics of ultra-high energies and quantum gravity. Because we can't yet take measurements from real black holes, we need a way to recreate this phenomenon in the lab in order to study it, to validate it."

In this paper, the researchers show that a magnetic field-pulsed microwave transmission line containing an array of superconducting quantum interference devices, or SQUIDs, not only reproduces physics analogous to that of a radiating black hole, but does so in a system where the high energy and quantum mechanical properties are well understood and can be directly controlled in the laboratory. The paper states, "Thus, in principle, this setup enables the exploration of analogue quantum gravitational effects."

"We can also manipulate the strength of the applied magnetic field so that the SQUID array can be used to probe black hole radiation beyond what was considered by Hawking," said Miles Blencowe, another author on the paper and a professor of physics and astronomy at Dartmouth.

This is not the first proposed imitation black hole, says Nation. Other proposed analogue schemes have considered using supersonic fluid flows, ultracold bose-einstein condensates and nonlinear fiber optic cables. However, the predicted Hawking radiation in these schemes is incredibly weak or otherwise masked by commonplace radiation due to unavoidable heating of the device, making the Hawking radiation very difficult to detect. "In addition to being able to study analogue quantum gravity effects, the new, SQUID-based proposal may be a more straightforward method to detect the Hawking radiation," says Blencowe.

In addition to Nation and Blencowe, other authors on the paper include Alexander Rimberg at Dartmouth and Eyal Buks at Technion in Haifa, Israel.

Obtenido de: http://www.dartmouth.edu/~news/releases/2009/08/21a.html
Tirso Ramírez
CRF

Conjuntos de Electrones Moviéndose Como Si Hubiera un Campo Magnético

Un equipo internacional de científicos dirigido por un grupo de la Universidad de Princeton ha descubierto recientemente que, en la superficie de ciertos materiales, los conjuntos de electrones se mueven de maneras que imitan la presencia de un campo magnético donde no hay ninguno presente. El hallazgo representa la constatación de uno de los más exóticos fenómenos cuánticos macroscópicos en la física de la materia condensada.
La investigación podría llevar a importantes avances en la construcción de un nuevo tipo de computadora cuántica que tendría la flexibilidad suficiente como para operar a temperaturas moderadas, en vez de a las bajas temperaturas que son un requisito ineludible para los diseños actuales.

Anteriormente, los científicos podían observar movimientos similares de los electrones sólo bajo fuertes campos magnéticos y bajas temperaturas, en lo que se conoce como Efecto Hall Cuántico, que condujo a la concesión de dos Premios Nobel de Física, en 1985 y 1998.

Sin embargo, unos teóricos de la Universidad de Pensilvania y la Universidad de California en Berkeley propusieron que en los límites de ciertos materiales tridimensionales, el espín de los electrones individuales y la dirección en que se mueven, estarían alineados directamente con los electrones correspondientes sin necesitar de altos campos magnéticos o temperaturas muy bajas. Los investigadores también teorizaron que para que esto sucediera, los electrones necesitaban moverse a velocidades sumamente altas.

Ahora, Zahid Hasan, profesor de física en la Universidad de Princeton, y sus colegas, han conseguido observar los espines sincronizados de muchos electrones moviéndose en un material exótico, un cristal de antimonio enlazado con bismuto.

"Este resultado es bastante asombroso porque estamos viendo a los electrones comportarse de una manera que es muy similar al modo en que lo hacen cuando está presente un campo magnético fuerte, pero no había ninguno en nuestro experimento", explica Hasan, quien dirigió la colaboración internacional con científicos de EE.UU., Suiza y Alemania.

Obtenido de: http://www.amazings.com/ciencia/noticias/250309b.html
Tirso Ramírez
CRF