University of Maryland, College Park

A physicist peels back the layers of excitement about graphene.

Graphene is an atom-thick sheet of carbon in which electrons behave as if they have no mass. Atomic carbon layers have been grown epitaxially — that is, perfectly aligned with atoms in an underlying crystal surface — on metals and semiconductors for decades, so why the fuss lately?

Well, in the past few years much work in this field has revolved around graphene obtained by 'exfoliating' or peeling it from graphite. By mounting exfoliated graphene on insulating silicon dioxide, researchers observed a half-integer quantum Hall effect, an anomalous measurement that stems from the existence of a Landau level — the quantized orbit of electrons in a magnetic field — at exactly zero energy, a signature property of massless electrons.

But exfoliated graphene is dirty, lumpy and tiny (the biggest pieces are still a tenth of a millimetre in diameter). I wondered whether the older technique of epitaxial growth could produce a better material. In May, a group led by Joseph Stroscio showed that it could (D. L. Miller et al. Science 324, 924–927; 2009). Using scanning tunnelling spectroscopy to study epitaxial graphene on the surface of silicon carbide in a magnetic field, they showed that epitaxial graphene is extraordinarily clean and flat, and clearly exhibits the zero-energy Landau level. For the first time in any material, epitaxial graphene allows direct observation with atomic resolution of the behaviour of electrons in quantized Landau levels, opening a new window on the quantum Hall effect.

Researchers using other techniques, such as cyclotron resonance and photoemission, are also reporting an astonishingly clean electronic system in epitaxial graphene. Experiments on conductivity remain a challenge, but epitaxial graphene seems to have a bright future.

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Massachusetts Institute of Technology

A theoretical physicist examines exotic particles lurking in new materials.

Axions are very light, very weakly interacting particles, whose existence was posited more than 30 years ago [1,2] in order to clean up our ‘standard model’ of particle physics [3]. They close an annoying loophole in Kobayashi and Maskawa’s Nobel-prize-winning explanation of why the microscopic laws of physics look so nearly the same when running backwards as forwards in time (time reversal symmetry).

Despite heroic efforts — and several false alarms — axions have not yet been detected, but they have become increasingly important. They have been warmly embraced in unified field theories and in string theory. And when we run the equations through Big-Bang cosmology, we find that axions should contribute much of the dark matter that astronomers have inferred to explain the Universe [5].

Now Shou-Cheng Zhang and his colleagues (X.-L. Qi et al. Phys. Rev. B78, 195424; 2008) inform us that, all along, axions have been lurking unrecognized on surfaces of bismuth-tin alloys and other materials. To be more precise: the equations that arise in axion physics [6,7] are the same as those that describe the electromagnetic behaviour of a recently discovered class of materials known, collectively, as topological insulators [8,9].
The axion field inside topological insulators is an emergent — and subtle — property of collections of electrons that is connected to their spin–orbit coupling.

These ‘quasi-axions’ don’t improve our standard model, but they do have the charming advantage of being accessible, possibly even useful. There are ideas to exploit their behaviour to make anyons [10], potential building blocks for quantum computation.

No short summary can do justice to the wealth of ideas synthesized in this paper. Powerful, beautiful mathematics is at play in reality.

1. Weinberg. S. Phys. Rev. Lett. 40, 223 (1978).
2. Wilczek, F. Phys. Rev. Lett. 40, 279 (1978).
3. Peccei, R. & Quinn, H. Phys. Rev. Lett. 38, 1440 (1977)
4. Svrcek, P. & Witten, E. J. High Energy Phys. 0606 (2006).
5. Hertzberg, M., Tegmark, M. & Wilczek. F. Phys.Rev. D78, 083507 (2008).
6. Huang, M. & Sikivie, P. Phys. Rev. D32, 1560 (1985).
7. Wilczek, F. Phys. Rev. Lett. 58, 1799 (1987).
8. Kane, C. & Mele,E. Phys. Rev. Lett. 95, 226801 (2005).
9. Fu, L., Kane, C. & Mele, E. Phys. Rev. 98, 106803 (2007).
10. Fu, L. & Kane, C. Phys. Rev. Lett. 100, 096407 (2008).

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University of St Andrews, UK

An optical physicist sees beyond fluorescent labels.

Many a molecular biologist likes to watch molecules move around inside living cells, particularly in real time. The job is usually done by tethering a fluorescent tag to interesting biological molecules and following their movements by means of the tag's glow. But fluorescent tags are often bigger than the molecules they label, so frequently perturb their movements. Better to watch intracellular dramas without millstones around the actors' necks. But how?

A twist on 'Raman scattering' may hold the answer. Normally, when a laser is shone at a molecule, the molecule scatters most of the light at the same frequency at which it was emitted by the laser. A tiny amount — Raman scattered light — is scattered at different frequencies. These frequencies indicate the chemical bonds in the molecule, and can thus identify it as a fingerprint identifies a person. If only Raman signals were stronger, they would be suitable for real-time microscopy on a molecular scale.

A second laser provides the twist — and the necessary amplification. Sunney Xie of Harvard University and his colleagues have found that another laser can enhance the contrast of an image, improving the sensitivity over previous studies by four orders of magnitude (C. W. Freudiger et al. Science 322, 1857–1861; 2008). For this to work, the two lasers must coincide on the sample, and the difference in their frequencies must exactly match that of a specific molecular vibration of a certain chemical bond in the sample. The background noise is eliminated and the signal is amplified.

This method is both versatile and powerful; the authors used it to observe the uptake of omega-3 fatty acids by human lung-cancer cells and the changing distribution of two drugs as they were absorbed by mouse skin. I think this could spur the development of tag-free molecular movie machines for all.

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Kastler Brossel Laboratory, CNRS, France.

A quantum-gas specialist learns about crystals from his own science.

Crystals can behave as electrical insulators or conductors. In a few crystals and under the right conditions, electrons flow perfectly. And in a subset of these superconducting crystals, the minimum temperature for perfect conduction is bizarrely warm.

On the whole, physicists have tried to explain this using models with a small number of parameters, such as the probability of an electron jumping between two sites, and the interaction energy between two neighbouring electrons. Extensive laboratory studies measuring every conceivable property of the curious crystals confirm several predictions of these models, but their general solution is still hotly debated.

Recently, a couple of research groups have been casting around for less obvious ways to understand superconducting crystals, and turned to the field that is my bread and butter: quantum gases. They have modelled electrons zooming through these crystals using gases of cold potassium atoms moving around in a space demarcated by laser beams — a kind of egg box made with light.

In December, a group led by Immanuel Bloch detected cold potassium gas switching to a state with exactly one atom per compartment of the egg box. Such an ordered state is considered a key ingredient for superconductivity. Bloch's team was not the first to see the switch, but the group's measurement of the size of the gas revealed a crucial property of this phase: its incompressibility (U. Schneider et al. Science 322, 1520–1525; 2008).

This means that quantum gases are insulators as well as conductors, making the experimental analogy to superconducting crystals more complete — and making them more useful playthings for scientists studying superconducting crystals.

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Rice University, Houston, Texas

A physicist foresees a new era in electronics.

A material's electronic properties depend largely on its density of mobile charge carriers (electrons and holes). The most common way of tuning that density is 'doping'. This involves carefully adding atoms or molecules that donate or take up electrons from the surrounding material. But doping comes with a downside: these added impurities themselves become charged, so they scatter mobile charge carriers and muddy the predictability of the material's electronic properties.

How to avoid doping? Look to Julius Edgar Lilienfield. In 1925, he proposed what is now called the 'field effect', in which the material of interest functions as one electrode of a capacitor. When a voltage is applied to the other electrode, equal and opposite charge densities accumulate on the sample material. The density of charge carriers can be varied as it is in doping, but not to the same extent. Nonetheless, the field effect has an everyday role in transistors — which are the fundamental parts of consumer electronics.

Another of Lilienfield's inventions, the electrolytic capacitor, holds the key to much higher field-effect charge densities, which could have dramatic consequences. Researchers at Tohoku University in Sendai, Japan, recently used a polymer electrolyte to achieve sufficiently large charge densities at a strontium titanate surface to generate superconductivity (K. Ueno et al. Nature Mater. 7, 855–858; 2008). This has been seen before in doped strontium titanate, but the electrolytic capacitor approach avoids the disorder inherent in doping.

By using mobile ions in an electrolyte to attract charges in the sample, this quirky capacitor can build up charge densities approaching those of chemically doped electronic materials such as high-temperature superconductors. This opens up the possibility of transistor-like devices that can work with very low voltages.

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Northwestern University, Evanston, Illinois

A physical chemist is pleased to learn that 'microscale' swimming isn't that hard after all.

Even if small organisms perfectly mimicked gold medallist Michael Phelps's technique, they wouldn't win a microswimming Olympics. The viscosity of water is so high that these little fellows have had to develop some unusual swimming styles. In 1977, E. M. Purcell formally expressed this idea with his famous 'scallop theorem'. He showed that swimming forwards cannot be achieved at the micrometre-scale with 'time-reversible' motions such as the back-and-forth wiggling of a rigid tail. Instead, tiny organisms must use complex, asymmetrical strokes.

But this is not always the case, according to engineers at the Massachusetts Institute of Technology in Cambridge and the University of California, San Diego. In July, they proved that time-reversible tail-wiggling or wing-flapping can be a viable mode of propulsion through a fluid, provided it is done next to a deformable interface such as a soft membrane (R. Trouilloud et al. Phys. Rev. Lett. 101, 048102; 2008). The reversible motions of the swimmer couple in a nonlinear way to the deformations of the interface, producing additional flows and forces that are sufficient for locomotion.

One of the most exciting extensions of this result might be in creating 'nanosubmarines' — a much-criticized dream of nanotechnologists to have devices navigate blood vessels, finding and fixing damaged organs as they go. The idea has so far seemed implausible because such machines would need elaborate nanopropellers — which are prohibitively difficult to build — to sculpt asymmetrical swimming motions. But what about using a simpler propulsion mechanism and relying on the deformations of blood-vessel walls to move nanosubmarines along? Is there a nanoshipyard out there somewhere to put this idea to the test?

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Howard Hughes Medical Institute, Cornell University, Ithaca, New York

A biophysicist marvels at the idea of grabbing microscopic particles with light by tweaking its phase.

Light carries energy and momentum. Have you ever gazed at a comet on a hot summer night? The dust tail seen streaming out from a comet is caused by sunlight bombarding dust particles from its surface and pushing them away from the Sun. The same radiation pressure can be used to 'trap', or hold, microscopic particles. And if an item of interest — for example, a biological molecule — is attached to a particle subject to trapping, it can then be manipulated as the trap is moved.

So how does one generate optical traps? Conventionally, a laser beam is directed through the objective lens of a microscope and focused to a small spot very close to the specimen. The trapping force relies on the gradient of the laser's intensity — the tighter the focus, the greater the intensity change within the focused beam, and the greater the trapping force.

For a long time, this has been the only type of trap available. But not any more! David Grier and his colleagues have created a new type of trap that relies on the gradient of the 'phase' of a laser's light as well as its intensity (Y. Roichman et al. Phys. Rev. Lett. 100, 013602; 2008). Light waves, like ocean waves, have crests and troughs. The phase of a light wave specifies what position within the wave, from crest to trough, the light is in at a given moment. By tweaking the phase of the laser in the trap, the researchers are able not only to hold a particle steady, but also to move it in a line or spin it around in a circle. It is now possible to design optical traps that are more flexible and versatile, and that can generate as much trapping force as before, but with less light.

I would not be surprised if these traps soon become one of the must-have tools in single-molecule biophysics, cell biology and colloidal physics.

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Weizmann Institute of Science, Rehovot, Israel

A physicist applauds evidence for the quantum spin Hall effect

I have been fascinated by the ballistic (collisionless) motion of charge carriers in solids since the start of my career. In practice this motion is often impeded by unavoidable impurities in the solid. But when it works, the charge carriers maintain their quantum properties while dissipating a minimum amount of energy.

Applying a strong magnetic field perpendicular to a two-dimensional conducting layer can accomplish the feat. Then, the quantum Hall effect kicks in, forcing the charges to the edges of the sample where they skip along in so-called 'chiral edge channels'. Backward scattering is virtually eliminated because that would require the charges to find a way to the opposite edge, where charges move in the opposite direction.

Recently, Laurens Molenkamp of the University of Würzburg in Germany and his colleagues took a step towards verifying the quantum spin Hall effect (M. König et al. Science 318, 766–770; 2007). This is where chiral edge channels form spontaneously in semiconductor insulators with peculiar electronic structures — namely, where the valence band is energetically higher than the conduction band because of the strong spin-orbit interaction between electron spins and electron velocities. This means that spin-up electrons are carried only by edge channels moving in one direction and spin-down elections are carried by edge channels moving in the opposite direction.

Molenkamp's team used a thin layer of mercury telluride sandwiched between two layers of mercury cadmium telluride. Because measuring spin current is difficult, they recorded the conductance of this middle layer to verify the ballistic transport that characterizes edge-channel transport. It was quantized, as predicted.

With further verification, the finding could lead to low-power devices based on the transport of spins rather than charges. Thus a quirk in the scientific field I have always loved might find a practical application.

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Weizmann Institute of Science, Rehovot, Israel

A physicist applauds evidence for the quantum spin Hall effect

I have been fascinated by the ballistic (collisionless) motion of charge carriers in solids since the start of my career. In practice this motion is often impeded by unavoidable impurities in the solid. But when it works, the charge carriers maintain their quantum properties while dissipating a minimum amount of energy.

Applying a strong magnetic field perpendicular to a two-dimensional conducting layer can accomplish the feat. Then, the quantum Hall effect kicks in, forcing the charges to the edges of the sample where they skip along in so-called 'chiral edge channels'. Backward scattering is virtually eliminated because that would require the charges to find a way to the opposite edge, where charges move in the opposite direction.

Recently, Laurens Molenkamp of the University of Würzburg in Germany and his colleagues took a step towards verifying the quantum spin Hall effect (M. König et al. Science 318, 766–770; 2007). This is where chiral edge channels form spontaneously in semiconductor insulators with peculiar electronic structures — namely, where the valence band is energetically higher than the conduction band because of the strong spin-orbit interaction between electron spins and electron velocities. This means that spin-up electrons are carried only by edge channels moving in one direction and spin-down elections are carried by edge channels moving in the opposite direction.

Molenkamp's team used a thin layer of mercury telluride sandwiched between two layers of mercury cadmium telluride. Because measuring spin current is difficult, they recorded the conductance of this middle layer to verify the ballistic transport that characterizes edge-channel transport. It was quantized, as predicted.

With further verification, the finding could lead to low-power devices based on the transport of spins rather than charges. Thus a quirk in the scientific field I have always loved might find a practical application.

Posted by Anna Petherick at 04:06 PM | Permalink | Comments (0) | TrackBacks (0)

University of Manchester, UK

Imploding atoms have softened this experimentalist's teasing views on theoretical physics.

As an experimentalist, I instinctively dislike theory papers. Too many of them seem to be written for the sole purpose of showing off an integral larger than a competitor's, or to present multiple theories just in case one idea proves right and so is hailed as visionary. I feel even less warmly towards theories that are nigh on impossible to check, such as the supposed precursor to a theory of everything, string theory.
But speaking seriously, even the most obscure predictions can turn out to be spectacularly relevant.
In our lab we have been studying graphene, a material that comprises a single layer of carbon atoms arranged similarly to chicken wire. Because electrons in this material mimic ultra-relativistic particles, it should be possible to observe in their behaviour century-long-predicted phenomena such as the Klein paradox (which concerns how highly energetic electrons tunnel through supposedly impenetrable barriers) and zitterbewegung (jittery movements of relativistic wave-packets).

Several recent theory papers on the physics preprint server arXiv predict another coup for graphene (see A. V. Shytov et al. arXiv:0708.0837; 2007).

According to relativistic quantum theory, atoms containing more than 170 protons cannot exist, because electrons around nuclei with such a large charge would fall into the centre. Nuclear physicists have not come close to creating atoms heavy enough to test this prediction. But the recent theory papers suggest that it should be relatively easy to observe the effect in graphene. This is because electrons in this material interact much more strongly than they do in atoms, so should fall down on charged impurities (standing in for nuclei) rather routinely.
This makes me wonder: could we design condensed-matter systems to test the supposedly non-testable predictions of string theory too?

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Boston University, USA

A physicist highlights a three-in-one deal for nonlinear science

As a student of nonlinear phenomena, I am continually amazed by new examples of deterministic chaos, solitary
waves and fractals.

A recent study (R. H. Goodman and R. Haberman Phys. Rev. Lett. 98, 104103; 2007) gave me the rare pleasure of seeing all three of these fundamental manifestations of nonlinearity woven together.

This paper addresses the collisions of solitary waves — localized nonlinear waves that propagate without changing shape and are found in systems ranging from solids to optical fibres.

In the 1980s, with several colleagues, I studied this problem numerically (see, for example, D. K. Campbell and M. Peyrard Physica D 18, 47–53; 1986). We discovered a surprising 'bounce' phenomenon, in which solitary waves would collide, remain trapped for a number (n) of bounces and then escape to infinity. This behaviour occurred only when the waves had specific relative velocities on colliding; these bounce windows were interspersed with regions in which the waves repelled each other immediately.

We developed a heuristic explanation for this behaviour, consistent with the waves behaving like elastic particles that can be deformed, but fell short of developing a full analytical explanation.
Journal club

Goodman and Haberman have now developed an analytical treatment of this effect and have shown, in their words, "that clusters of (n+1)-bounce windows accumulate at the edges of each n-bounce window, repeated at diminishing scales" in an effective fractal structure. This also means that the outcome of a collision is exquisitely sensitive to the initial velocity, a hallmark of deterministic chaos.

If all the above seems dry, take a look at the wonderful graphic (here) from the article, which represents the number of bounces as a function of the collision parameters. The image is certainly worth more than these few hundred words.

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Princeton University, New Jersey, USA

A chemical engineer is struck by the strange properties of 'patchy' colloids.

A recent paper about the behaviour of colloids makes an intriguing prediction — suggesting that they can adopt an 'empty' liquid state.

I study disordered states of matter, such as liquids and glasses. I find colloids interesting because they make phenomena such as crystal nucleation and the glass transition amenable to direct observation. Nanometre- or micrometre-sized particles suspended in liquids are wonderful model atoms. They arrange themselves in the same way that atoms and simple molecules do into solids, liquids or gases.

But controlling the interactions between colloidal particles provides a window into structural and thermodynamic behaviour beyond that found in atomic systems, as this recent theoretical paper shows (E. Bianchi et al. Phys. Rev. Lett. 97, 168301; 2006).

It maps the phase diagrams of 'patchy' colloids. The particles in such colloids are decorated with sticky spots, which tend to bond them together. As the number of bonded neighbours per particle is reduced towards two, the phase diagrams predict liquid states with a vanishing packing fraction. This means the colloidal particles occupy a tiny fraction of the available space — but they still behave as a liquid that is distinct from the gas-like phase of still lower packing fraction.

The low-temperature behaviour of such 'empty' liquids is especially interesting. The calculations suggest that cooling the colloid can freeze in place the empty configuration to give a glassy state of arbitrarily low density.

These predictions have not been tested experimentally. But chemists have already developed techniques for making patchy particles, so the work of Bianchi et al. could guide experimentalists in their exploration of this fascinating form of matter.

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Institut d'Optique, Palaiseau, France

A physicist hopes that cool techniques could show up quantum effects in 'big' systems.

When I was a postdoc in Bell Labs during the 1980s, many of the ideas stimulating our work in quantum optics came from researchers developing sensors for gravitational waves.

Gravitational waves propagate as distortions in space, and a passing wave is expected to have a subtle influence on the oscillation of a heavy bar, or to change by a fraction the separation of two mirrors.

To minimize the uncertainty in measurements of such effects, researchers developed new concepts for manipulating the quantum fluctuations that affect parameters such as an oscillator's position.

Concepts they invented, such as 'quantum non-demolition measurements' and 'squeezed states', have since been demonstrated (sometimes with my help), but with light beams rather than massive objects.

Detecting quantum effects in 'big' systems has remained an elusive goal, despite experiments moving to smaller masses and higher oscillation frequencies to make the quantum noise larger. The stumbling block has been heat — thermal excitations overwhelm the well-hidden quantum noise.

Here, recent work suggests a way forward. Three papers published last autumn (S. Gigan et al. Nature 444, 67–70; O. Arcizet et al. Nature 444, 71–74; D. Kleckner & D. Bouwmeester Nature 444, 75–78; 2006) each show that the techniques used to measure a micromirror's motion can cool the mirror at the same time, pushing its temperature close to absolute zero.

Such cold micromirrors could well become the first 'heavy-weight' quantum-mechanical objects — and the techniques developed in quantum optics may eventually feed back into the gravitational-wave detectors that got us started.

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University of Arizona, Tucson, USA

An expert on instabilities jumps from optically bound plastic beads to the brain.

It's not often that reading scientific papers turns my mind to the melancholic work of great Russian writers, but a recent one did.

The paper reports observations of 'bistability' in a simple optical system. Bistable systems have two stable output states for the same input. In this case, the researchers had studied the behaviour of two plastic spheres, trapped side-by-side in a pair of counter propagating laser beams (N. K. Metzger et al. Phys. Rev. Lett. 96, 068102; 2006). They found that the beads could adopt two stable arrangements, differing in the beads' separation.

Bistability arises in optical systems that show nonlinear responses to changes in light intensity and include some kind of feedback process. Here, one bead feels the position of the other because each affects the light field around it, creating the necessary feedback.

The researchers modelled how the two stable states come about, combining equations that describe the propagation of the light with others that predict the forces on the beads. I was impressed by how many physical effects are taken into account in the model.

And this is what turned my thoughts away from the physics of my research to the literature of my homeland. It is believed that some Russian authors, including Leo Tolstoy, may have suffered from what is now known as a bipolar disorder, characterized by states of euphoria and depression.

I have wondered before whether bistability in optical systems might serve as a simple model to help understand the mechanisms that underlie bistability in the human brain. Papers such as this one put that challenge in perspective — modelling a system that involves just two beads is already nontrivial.

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Johannes Gutenberg University, Mainz, Germany

A cold-matter physicist is amazed by atoms' ability to divide themselves up equally.

Imagine having a box containing an even number of objects, N. You want to divide them into two boxes, each of which contains exactly N/2 objects. Sounds easy, right?

But let's complicate things a bit. Let's suppose you can't count the objects, nor look at them. Will you still be able to make the split fairly?

A collaboration of researchers from the Massachusetts Institute of Technology and Harvard University, both in Cambridge, recently showed that it's possible to do so for atoms. They divided into two equal halves a Bose–Einstein condensate of 1 million sodium atoms (G.-B. Jo et al. Phys. Rev. Lett., in the press; preprint here).

Bose–Einstein condensates are a novel state of matter that forms at a temperature close to absolute zero. They behave like quantum entities with pronounced wave-like properties. These are properties that I exploit in my own work with condensates, and they also underpin the atom division.

The Cambridge team stored their matter waves in microfabricated magnetic traps, made out of thin wires. The researchers changed the currents in the wires to split slowly the one potential well that was holding the atoms into two.

In a non-interacting gas, this splitting process would probably give a skewed distribution of atoms, and the distribution would be different every time. In this case, the quantum interactions favour a system in which each well contains exactly N/2 atoms.

In fact, the evidence suggests that the splitting is accurate to within 50 atoms. I find that truly remarkable from a fundamental point of view. More practically, this dividing of atoms could also be useful in building novel atom interferometers and atomic clocks.

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