Archive for the ‘Materials Science’ Category

Video: Light creates instant origami

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Too lazy to learn origami? Now you can sit back and let heat do the work, thanks to a new technique developed by Michael Dickey and his team at North Carolina State University that uses a material that can fold up on its own.

The researchers used plastic sheets that shrink when placed under a heat source such as infrared light. Using an ordinary inkjet printer, they printed black ink along the lines to be folded. Since black absorbs more energy than paler colours, the lines first shrink more than the surrounding material and then fold when subjected to light. Increasing the width of the line increases the angle of the fold, making it possible to produce a range of shapes.

It’s not the first self-folding technique. Last year, researchers at Harvard University and Massachusetts Institute of Technology used “shape memory” alloy foil and flexible silicon rubber to create a material that folds into a previously-held shape when heated. However, the new method should be more useful as it can be integrated with existing commercial printing techniques.
Jacob Aron –

Written by physicsgg

November 13, 2011 at 9:04 am

Posted in Materials Science, TECHNOLOGY

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Could graphene be the new silicon?

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A 3D model of graphene's chicken wire structure. Photograph: nobeastsofierce/Alamy

It started with a few experiments with Scotch tape and a pencil. Then graphene, stronger than steel, one atom thick and a super-conductor, was born, a wonder material that could be as revolutionary as silicon, say its Nobel prize-winning creators. Now with £50m from the UK government, they’re out to prove it.
Somehow it seems appropriate that the government might be basing some of its hopes for the economy’s recovery on a substance that is one atom thick. The substance in question – graphene – 200 times as strong as steel, seems to some designed to carry the weight of almost anything – but George Osborne’s Plan A? That would indeed make it a miracle material.

Nevertheless the chancellor made a detour from the Tory conference in Blackpool in September to visit Manchester University, graphene’s spiritual home, and to announce a £50m investment. Graphene is claimed by some as an innovation that will prove as revolutionary as the silicon chip, or even plastics, both of which it may supersede. A poster campaign around Manchester currently reminds you that the industrial revolution was born in the city at the beginning of the 19th century. Two hundred years on the challenge is to keep the “graphene revolution” in the north west, too.

Sitting in his lab at the university, Konstantin Novoselov one half of the 2010 physics Nobel prize-winning team that “discovered” graphene, runs through the superlatives of his material – uniquely strong and flexible and the best conductor of electricity yet found – with a kind of amused pride before explaining its genesis. Graphene wasn’t so much of a eureka moment as a eureka year or two, but since it was first identified the exclamation marks have kept coming. What they began with, however, was some pencil lead and a roll of Scotch tape.

In 2004 Novoselov, a 37-year old from the Ural mountains with a deadpan wit, was a post-doctorate researcher in conductivity in a department run by fellow Russian émigré Andre Geim. “It was always the style in our lab to have side projects going on,” he recalls. “We were working on issues of microscopic electromagnetism during the day, but we had a few after-hours projects on the go mainly for fun.”
At the time, Andre Geim was probably best known for his “frog levitation” experiment. This showed that if you placed small amphibians between two large electromagnets they would defy gravity and swim in the air. The experiment won him an Ig-Nobel prize (awarded for the most enjoyably pointless research of the year; Geim remains the only recipient both of an Ig-Nobel and the real thing).
It was in the same spirit…………….
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November 13, 2011 at 7:59 am


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Electromagnons open up new opportunities to control electric and magnetic properties.

Michel Kenzelmann

Figure 1 Electromagnons can be excited with light in materials such as YMn2O5 , shown here with its crystal structure projected onto a plane. When the ordered moments (black arrows) on the manganese sites (labeled M) fluctuate, some nearest-neighbor moments will be more parallel, others more antiparallel. (The blue arrows indicate the possible direction of the fluctuations.) This can lead to a dynamic modulation of the magnetic interactions and, in the process, a coupling of the magnetic fluctuations to the fluctuating electric dipoles.

This week, the Magnetism and Magnetic Materials Conference takes place in Scottsdale, Arizona. This article takes a look back on the discovery of electromagnons, and how it influenced the search for materials with coupled electric and magnetic properties.

Physical phenomena living in two worlds have long inspired scientists and popular culture alike. A good example is light, which consists of coupled electric and magnetic fields that oscillate in time. Light is the consummate example of electric and magnetic fields “working together.” In solid-state materials, however, it has proven difficult to discover and design materials with useful electromagnetic properties. The bulk of today’s device applications are based on materials where electric properties are controlled by electric fields, or where magnetic properties are controlled by magnetic fields.

Materials in which these properties are coupled would be advantageous because of how electric currents and magnetic fields are generated and measured. While it is easy to generate large electric fluctuations, the same facility does not exist for magnetic fields. From an applications standpoint, being able to store magnetic information with electric fields or currents and to manipulate domains in ferroelectrics—materials with long-range electric order—with magnetic fields would be particularly attractive. Dynamic magnetoelectric effects could enable ultrafast switching behavior in materials allowing the manipulation of electronic properties using short-pulsed lasers instead of ultrafast switching magnetic fields.

Recent developments in pursuit of this kind of control have occurred in multiferroics—materials that exhibit both magnetic and electric long-range order—which occasions us to revisit a paper by Andrei Sushkov of the University of Maryland, College Park and his collaborators. In 2007 they reported they were able to activate magnetic excitations (magnons) in manganese oxides using electric fields. These excitations, called “electromagnons,” were electric-dipole-active, that is, the electric fields stimulate the electric dipoles that affect the magnetic structure [1]. Since these magnetic excitations are hybrid magnetoelectric excitations that can be manipulated electrically and magnetically, they bridge the gap between electricity and magnetism in a solid-state system, and their discovery has launched an entire field of inquiry. While not the first observation of electromagnons, Sushkov et al.’s study made an important step toward identifying the origin for this new type of excitation.

To appreciate why the discovery of electromagnons caused so much excitement, consider that it is not straightforward to couple electric and magnetic degrees in freedom in an insulating material. Ferroelectrics are insulators where the charge fluctuations can be associated with so-called electric-dipole-active lattice vibrations. These excitations can drive phase transitions to ferroelectric, or charge-ordered phases. Magnetic fluctuations in insulators, on the other hand, are associated with localized magnetic moments, which are ordered at sufficiently low temperature and support coherent magnetic waves. Because the origin of the electro- and magneto-active excitations is very different, it is not easy to find coupled electromagnetic or magnetoelectric excitations in insulating materials. To make matters worse, the mechanism that leads to ferroelectricity in conventional ferroelectrics like BaTiO3, which should exhibit electric-dipole-active excitations, impedes the presence of magnetism.

Since the early efforts on classical ferroelectrics, research has rapidly progressed, and several different classes of multiferroics have been discovered [2, 3], with coupled ferroelectric and magnetic degrees of freedom. While ferroelectricity in a classical ferroelectric like BaTiO3 is driven by a hybridization of empty -shell orbitals on the transition-metal site with occupied shells on the oxygen sites, ferroelectricity in these new classes of materials arises from different mechanisms, such as lone-pair ions, topological effects of the chemical lattice, or magnetically driven effects in frustrated materials. Not long after the discovery of these new multiferroic materials, it was shown that such materials could, as suspected, support electromagnetic excitations in solid matter.

In 2006, Pimenov et al. [4] reported that, using terahertz light, they were able to excite spin waves, which they called electromagnons, in . They also showed the electromagnons could be suppressed by applying a magnetic field, directly demonstrating magnetic-field-tuned electric-dipole-active excitations. The understanding of these electromagnons was, however, tentative. For example, it was not clear whether the electromagnon is associated with the transition metal or rare-earth-metal ion magnetism in this material, thus leaving the origin of electromagnons as an open question. In , it is the magnetic interactions between the manganese () ions that gives rise to ferroelectricity. But is also magnetic because of the electrons of the rare-earth-metal ions, so it was not clear whether these excitations arose purely from the transition-metal magnetism.

Published shortly afterwards, Sushkov et al.’s paper answered this question [1]. They observed the same effects in the multiferroic material , which contains no rare-earth-metal magnetism (Fig. 1), putting any doubts that the electromagnons were not associated with transition-metal magnetism to rest. Since these two ground-breaking studies, numerous observations of electromagnons in very different materials have been published. Electromagnons appear most commonly in magnetically induced ferroelectrics, including a number of spin spiral ferroelectrics.

It is now known that electromagnons can be in multiferroics where ferroelectricity does not arise from magnetic order. For example, these excitations occur in multiferroic , where ferroelectricity arises from the lone-pair mechanism and a low-pitch antiferromagnetic spiral does not form except at a much lower temperature [5]. Recently (and somewhat surprisingly), electromagnons were reported in the paraelectric phase of multiferroic materials. For example, electromagnons have been observed in a very different material with a very different electronic and physical structure: a conical-spin magnetically ordered phase of the paraelectric phase of the hexaferrite [6]. This is an exciting discovery, as it suggests that electric-dipole-active magnons can exist in nonmultiferroic materials, and that many magnetically ordered insulators with complex noncollinear magnetic structures may support electromagnon excitations.

Progress in this field is not just a matter of adding to the catalog of multiferroics exhibiting electromagnons. Initially, electromagnetic excitations were mostly observed at relatively low frequencies, where they were expected to drive the condensation of boson excitations associated with the phase transition. However, it has now been shown, contrary to expectation, that in rare-earth and electromagnetic excitations exist also at relatively high frequencies [7, 8]. It has been suggested that at least some of the high-frequency excitations are two-magnon excitations, and these excitations are strongly coupled to some of the phonons in that energy range. The prospect of multiparticle magnon states that couple strongly to the lattice can lead to novel strongly correlated effects in these materials and other unusual interactions.

What sets the time scale of magnetoelectric switching in multiferroics is also an exciting open question. A recent theoretical study [9] predicts that electromagnons can be used to switch ferroelectric polarization in rare-earth manganites at a picosecond time scale using terahertz optical pulses. This would be due to the dynamic magnetoelectric effects that are larger than the spin-orbit interactions, leading to static magnetoelectric effects. Recent studies on suggest, however, that the switching time scale is considerably longer, in the millisecond range [10]. This time scale is much longer than what would be expected even if the dynamic magnetoelectric effects in this material were completely governed by spin-orbit interactions and not by symmetric exchange as in the rare-earth manganites.

Since the publication of Sushkov et al.’s work [1], the study of electromagnons and the multiferroics in which they appear has opened up a new field of research that continues to surprise. The challenge of finding materials that tick all the boxes of desirable features—abundance, operating temperatures, and controllability of magnetic and electrical switching—ensures exciting new discoveries for years to come.


  1. A.B. Sushkov, R. Valdés Aguilar, S. Park, S-W. Cheong, and H. D. Drew, Phys. Rev. Lett. 98, 027202 (2007).
  2. S.-W. Cheong and M. Mostovoy, Nature Mater. 6, 13 (2007).
  3. D. Khomskii, Physics 2, 20 (2009); see also
  4. A. Pimenov, A. A. Mukhin, V. Yu. Ivanov, V. D. Travkin, A. M. Balbashov, and A. Loidl, Nature Phys. 2, 97 (2006).
  5. M. Cazayous, Y. Gallais, A. Sacuto, R. de Sousa, D. Lebeugle and D. Colson, Phys. Rev. Lett. 101, 037601 (2008).
  6. N. Kida, D. Okuyama, S. Ishiwata, Y. Taguchi, R. Shimano, K. Iwasa, T. Arima, and Y. Tokura, Phys. Rev B 80, 220406 (2009).
  7. Y. Takahashi, N. Kida, Y. Yamasaki, J. Fujioka, T. Arima, R. Shimano, S. Miyahara, M. Mochizuki, N. Furukawa, and Y. Tokura, Phys. Rev. Lett. 101, 187201 (2008).
  8. A. M. Shuvaev, F. Mayr, A. Loidl, A. A. Mukhin, and A. Pimenov, Eur. Phys. J. B 80, 351 (2011).
  9. M. Mochizuki and N. Nagaosa, Phys. Rev. Lett. 105, 147202 (2010).
  10. T. Hoffmann, P. Thielen, P. Becker, L. Bohaty, and M. Fiebig, arXiv:1103.2066.
  11. J. H. Kim, M. A. van der Vegte, A. Scaramucci, S. Artyukhin, J.-H. Chung, S. Park, S-W. Cheong, M. Mostovoy, and S.-H. Lee, Phys. Rev. Lett. 107, 097401 (2011).

Written by physicsgg

October 31, 2011 at 9:54 pm

A new kind of superconductivity

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Physicists unveil a theory for a new kind of superconductivity

The superfow of two kinds of superconducting electrons (arrows show their velocities) as calculated on supercomputers. Graphic 2 shows superflow of the other subpopulation of electrons on the surface of a vortex cluster. Graphics courtesy of Egor Babaev.

In this 100th anniversary year of the discovery of superconductivity, physicists at the University of Massachusetts Amherst and Sweden’s Royal Institute of Technology have published a fully self-consistent theory of the new kind of superconducting behavior, Type 1.5, this month in the journal Physical Review B.

In three recent papers, the authors report on their detailed investigations to show that a Type 1.5 superconducting state is indeed possible in a class of materials called multiband superconductors.
For years, most physicists believed that superconductors must be either Type I or Type II. Type 1.5 superconductivity is the subject of intense debate because until now there was no theory to connect the physics with micro-scale properties of real materials, say Egor Babaev of UMass Amherst, currently a fellow at the technology institute in Stockholm, with Mikhail Silaev, a postdoctoral researcher there.
Their new papers now provide a theoretical framework to allow scientists to calculate conditions necessary for the appearance of Type 1.5 superconductivity, which exhibits characteristics of Types I and II previously thought to be antagonistic.
Superconductivity is a state where electric charge flows without resistance. In Type I and Type II, charge flow patterns are dramatically different. Type I, discovered in 1911, has two state-defining properties: Lack of electric resistance and the fact that it does not allow an external magnetic field to pass through it. When a magnetic field is applied to these materials, superconducting electrons produce a strong current on the surface which in turn produces a magnetic field in the opposite direction. Inside this type of superconductor, the external magnetic field and the field created by the surface flow of electrons add up to zero. That is, they cancel each other out.
Type II superconductivity was predicted to exist by a Russian theoretical physicist who said there should be superconducting materials where a complicated flow of superconducting electrons can happen deep in the interior. In Type II material, a magnetic field can gradually penetrate, carried by vortices like tiny electronic tornadoes, Babaev explains. The combined works that theoretically described Type I and II superconductivity won the Nobel Prize in 2003.
Classifying superconductors in this way turned out to be very robust: All superconducting materials discovered in the last half-century can be classified as either, Babaev says. But he believed a state must exist that does not fall into either camp: Type 1.5. By working out the theoretical bases for superconducting materials, he had predicted that in some materials, superconducting electrons could be classed as two competing types or subpopulations, one behaving like electrons in Type I material, the other behaving like electrons in a Type II material.
Babaev also said that Type 1.5 superconductors should form something like a super-regular Swiss cheese, with clusters of tightly packed vortex droplets of two kinds of electron: one type bunched together and a second type flowing on the surface of vortex clusters in a way similar to how electrons flow on the exterior of Type I superconductors. These vortex clusters are separated by “voids,” with no vortices, no currents and no magnetic field.
The major objection raised by skeptics, he recalls, is that fundamentally there is only one kind of electron, so it’s difficult to accept that two types of superconducting electron populations could exist with such dramatically different behaviors.
To answer this, Silaev and Babaev developed their theory to explain how real materials can give raise to Type-1.5 superconductivity, taking into account interactions at microscales. In a parallel effort, their colleagues at UMass Amherst and in Sweden including Johan Carlstrom and Julien Garaud, with Babaev, used supercomputers to perform large-scale numerical calculations modeling the behavior of superconducting electrons to better understand the structure of vortex clusters and what they look like in a Type-1.5 superconductor.
They found that under certain conditions they could describe new, additional forces at work between the Type-1.5 vortices, which can give vortex clusters very complicated structure. As more work is done on superconductivity, the team of physicists in Stockholm and at UMass Amherst say the family of multi-band superconducting materials will grow. They expect that some of the newly discovered materials will belong in Type 1.5.
“With the development of theory that works on the microscopic level, as well as our better understanding of inter-vortex interaction, we can now connect the properties of vortex clusters with the properties of electronic structure of concrete materials. This can be useful in establishing whether materials belong in the Type 1.5 superconductivity domain,” says Babaev.
More information: … /i13/e134515 … 4/i9/e094515 … /i13/e134518

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October 24, 2011 at 12:39 pm

Posted in Materials Science

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Supercapacitor electrodes go for a dip

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Conductive wrapping can improve the performance of supercapacitors.

A new and simple “dipping” technique that can significantly improve the performance of supercapacitors has been developed by researchers at Stanford University in the US. The method, dubbed “conductive wrapping”, could be applied to a range of electrode materials. It might even be used to improve next-generation electrodes made from sulphur, lithium manganese phosphate and silicon for use in lithium-ion batteries.

Supercapacitors – more accurately known as electric double-layer or electrochemical capacitors – can store much more charge than a conventional capacitor. This is thanks to a double layer that forms at the electrolyte–electrode interface of such devices when a voltage is applied.

The conductive-wrapping technique can further increase the capacitance of a supercapacitor by boosting the conductivity of the electrodes – which enhances the device’s ability to store charge. Developed by Zhenan Bao, Yi Cui and colleagues, the process involves dipping a composite electrode made of graphene and manganese oxide into a solution containing either carbon nanotubes (CNTs) or a conductive polymer. The CNTs or polymer coat the electrode and boost its ability to store charge by more than 20% for the CNT coating and 45% for the polymer.

Higher specific capacitance

The specific capacitance obtained by the researchers (about 380 F/g) is comparable to other manganese-oxide-based electrodes, which typically have specific capacitances of between 250–400 F/g. However, the hybrid electrodes also show good “rate capability” – which means that they maintain their high capacitance at high charging and discharging rates. This is in contrast to conventional metal-oxide-based electrodes, which usually have poor rate capability because they have low electronic and ionic conductivity.

As a result, the new electrodes can also be used for more than 3000 charge–discharge cycles while retaining more than 95% of their capacitance. When combined with the fact that the electrodes have a much higher specific capacitance than existing commercial carbon-based supercapacitors (150–250 F/g), the conductive-wrapping technique looks promising.

Large-scale energy-storage applications

“The hybrid electrode system we have developed shows promise for large-scale energy-storage applications,” says team member Guihua Yu. “From the perspective of materials selection, both graphene and MnO2 are attractive electrode materials given that both carbon and manganese are cheap and abundant. From a processing point of view, our coating method is solution-based and easy to scale up.”

The researchers are now busy working on improving the performance of the electrodes in lithium-ion batteries using the method. “Our novel approach could be applied to a wide range of energy-storage electrode materials that have high energy density but that show limited performance because of their insulating nature,” says Yu.

The results are reported in Nano Letters.
Belle Dumé –

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October 22, 2011 at 9:13 am

Posted in Materials Science, Thermodynamics

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Graphene: 2010 Nobel Lectures

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October 10, 2011 at 6:15 pm

Magnetic joystick

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Regulating Brownian Fluctuations with Tunable Microscopic Magnetic Traps
A. Chen, G. Vieira, T. Henighan, M. Howdyshell, J. A. North, A. J. Hauser, F. Y. Yang, M. G. Poirier, C. Jayaprakash, and R. Sooryakumar
Phys. Rev. Lett. 107, 087206 (Published August 18, 2011)

The zigzagging nanowire with magnetic traps at each vortex, which can pin down magnetic beads

Magnetic particles can be guided with external fields through small-scale fluidic environments, bringing with them a biological molecule hitching a ride. A paper appearing in Physical Review Letters presents a two-dimensional magnetic trap that uses this type of magnetic remote control to guide the thermal motion of submicron magnetic beads.
Following a magnetic trap design from their earlier work, Aaron Chen at The Ohio State University in Columbus and his colleagues deposit a 2-micron-wide magnetic wire in the shape of a zigzag on a silicon surface. Chen et al. apply a one-time, large, in-plane magnetic field of 1000 oersted to polarize the legs of the zigzag shape, resulting in a sequence of head-to-head and tail-to-tail magnetic domain walls which meet at the kinks in the wire. Embedding the trap in a solution of magnetic beads, the team coaxes the beads to the large magnetic trapping gradients near the kinks using fairly weak (less than 100 oersted) external magnetic fields. The key control parameter is the strength of the external field perpendicular to the trap.
This setup allows exploration between two types of particle motion: one where the beads are tightly confined near a wire kink and another where the motion, driven by thermal fluctuations, spreads out around the kink. A magnetic trap such as this has the additional benefit that it does not rely on strong fields to move the particles or generate heat, both of which could perturb the environment studied.
Read also: Zigzag nanowire regulates Brownian motion

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August 29, 2011 at 11:18 am

Candles shine new light on diamonds

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By Christine Lavelle
Candle flames contain millions of tiny diamond particles, a university professor has discovered.
Research by Wuzong Zhou, a professor of chemistry at the University of St Andrews in Fife, revealed that around 1.5 million diamond nanoparticles are created in a candle flame every second it is burning.

Discovery could have implications for diamond industry

Dr Zhou used a new sampling technique to remove particles from the centre of the flame, which is believed to have never been done before, and found that it contained all four known forms of carbon.
He said: “This was a surprise because each form is usually created under different conditions.”
Dr Zhou added that the diamond particles are burned away in the process, but the discovery could lead to future research into how diamonds could be created more cheaply, and in a more environmentally friendly way.
He said: “This will change the way we view a candle flame forever.”
The academic said he uncovered the secret after a challenge from a fellow scientist in combustion.
Dr Zhou said: “A colleague at another university said to me: ‘Of course no one knows what a candle flame is actually made of.
“I told him I believed science could explain everything eventually, so I decided to find out.”
The first candle is said to have been invented in China more than 2,000 years ago, and previous research has shown that hydro-carbon molecules at the bottom of the flame are converted into carbon dioxide by the top of the flame.
However, the process in between has remained a mystery until now, with the discovery of the diamond nanoparticles, as well as fullerenic particles and graphitic and amorphous carbon.
Rosey Barnet, artistic director of one of Scotland’s biggest candle manufacturers, Shearer Candles, said the discovery was “exciting”.
She said: “We were thrilled to hear about the discovery that diamond particles exist in a candle flame.
“Although currently there is no way of extracting these particles, it is still an exciting find and one that could change the way people view candles.
“The research at St Andrews University will be of interest to the entire candle making industry.
“We always knew candles added sparkle to a room but now scientific research has provided us with more insight into why.”

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August 18, 2011 at 8:22 am