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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