Archive for the ‘The ant playground’ Category

The puzzling wrinkles in graphene

Tuesday, July 22nd, 2008

Graphene

Last year, Jannick Meyer at the Max Plank Institute for Solid State Research in Stuttgart and pals discovered that single sheets of graphene are gently rippled, like the rolling hills of New England. That’s a puzzle because graphene behaves like a perfect 2D crystal. So how do these ripples form  and what role do they play in the crystal properties of the material?

One possibility is that thermal fluctuations cause the wrinkles, in other words, that graphene buckles as it heats up. But today, Rebecca Thompson-Flagg and buddies at the University of Texas at Austin present another idea.

They say that heat is an unlikely to be the cause because the stiffness of graphene ought to ensure that ripples of the observed size vibrate at a frequency of 10^11 Hz. However, Meyer’s observations only make sense if the ripples are static.

Instead, Thompson-Flagg suggests that the ripples are formed by the adsorption of OH molecules at random sites throughout the crystal. In other words, the graphene wrinkles when it’s damp.

They’ve simulated the shape of a damp graphene and the ripples exactly match those seen by Meyer.

That’s an interesting result but not quite a slam dunk. For that, we’ll need to see what graphene looks like and how it behaves when no OH is present.

Ref: arxiv.org/abs/0807.2938: Rippling of Graphene

First X-ray diffraction image of a single virus

Friday, June 20th, 2008

Virus x-ray

X-ray crystallography has been a workhorse technique for chemists since the 1940s and 50s. For many years, it was the only way to determine the 3D structure of complex biological molecules such haemoglobin, DNA and insulin. Many a Nobel prize has been won poring over diffraction images with a magnifying glass.

But x-ray crystallography has a severe limitation: it only works with molecules that form into crystals and that turns out to be a tiny fraction of the proteins that make up living things.

So for many years scientists have searched in vain for a technique that can image single molecules in 3D with the resolution, utility and cost-effectiveness of x-ray diffraction.

That search might now be over. Today, John Miao at the University of California, Los Angeles, makes the claim that he and his team have taken the first picture of a single unstained virus using a technique called x-ray diffraction microscopy. Until now this kind of imaging has only been done with micrometre-sized objects.

Miao’s improvement comes from taking a diffraction pattern of the virus and then subtracting the diffraction pattern of its surroundings. The resolution of his images is a mere 22 nanometres, that’s an improvement of three orders of magnitude.

If confirmed, that’s an extraordinary breakthrough. With brighter x-ray sources, the team says higher resolution images will be possible and that it’s just a matter of time before they start teasing apart the 3D structures of the many proteins that have eluded biologists to date.

But best of all, x-ray diffraction gear is so cheap that this kind of technique should be within reach of almost any university lab in the world.

Ref: arxiv.org/abs/0806.2875: Quantitative Imaging of Single, Unstained Viruses with Coherent X-rays

The trouble with optical invisibility cloaks

Monday, May 19th, 2008

Nanoring

You could be forgiven for thinking that invisibility cloaks are a few R&D dollars away from hitting the high streets. Not so.

While it’s true that a number of high profile cloaks have been built, the best of these work only in the radio and microwave regions of the spectrum and then only in at a single frequency and in two dimensions . So unless you are a flatlander viewing the world through microwave eyes, these cloaks are not much use.

The one claim for an optical invisibility cloak works only inside a strange, exotic material made from gold nanorods and even then over a distance of a only few nanometres.

What an invisibility cloak has to do is steer light around an internal cavity in way that makes it appear to have passed straight through. That’s possible, in theory, if you can design a material in which its permeability and permittivity (the way it interacts with an electromagnetic wave) can be tailored throughout its structure.

That can be done relatively easily at microwave frequencies. The materials in question are extraordinary honeycombs of repeating patterns of split ring resonators and wires. The pattern has to be about the same size as the wavelength of the microwaves– a few centimetres or so.

So why not just make everything smaller to match the wavelength of visible light? The first reason is that we’re talking about a material with a feature size measured in nanometres and that is just beyond what’s possible today. The second is that optical frequencies tend to match the resonant frequency of electrons within these materials. What that means in plain English is that the materials absorb light rather than transmit it (which is why the one demonstration so far has worked only over a distance of a few nanometres before the light was absorbed).

So what to do? One idea is to make the cloaks out of lasing materials which constantly replace the light as it is absorbed. But a better one is to create a material that doesn’t absorb light in the first place. There’s no way to get rid of the electronic resonance that is responsible for absorbing light so the trick is to design a structure in which the resonance can be made to cancel itself out or help to propel the light through the material.

So physicists are desperately examining the properties of various nanostructures to see whether they might have the properties that fit the bill.

Today, Andrea Alu and Nadar Enghet at the University of Pennsylvania in Philadelphia, publish an analysis of just such a metamaterial made of nanoparticles arranged in a ring, as shown above. They say that this material gives “cleaner magnetic dipole response”.

Unfortunately, they are less clear over whether they’ve hit the jackpot with regards optical invisibility. In fact they can’t be sure whether this will have the required properties at optical frequencies or not.

The trouble is that it’s not possible to know the bulk properties of a material made from a particular nanostructure without some heavyweight calculating. And choosing which structure to investigate is little more than guesswork at the moment. As Alu and Enghet show with this work.

Back to the drawingboard, I’d say. Looks as if they’ll need to kiss a few more frogs before they find their prince.

Ref: arxiv.org/abs/0805.2329: Dynamical Theory of Artificial Optical Magnetism Produced by Rings of Plasmonic Nanoparticles

The vibration harvest

Wednesday, February 27th, 2008

Vibration harvester

All them turbines, drills and shakers in our modern factories make one almighty din.

We’re talking about a substantial amount of a-jumpin and a-jiggling which generally goes to waste. Couldn’t there be a way of harvesting this energy so that it can be re-used?

Turns out Tom Sterken and pals at IMEC, an independent nanostuff research lab in Belgium, have thought of a way to do it and have built a device that can do it.

It’s a MEMs gadget that consists of a tiny mass on a spring connected to a capacitor. As the mass bounces around, it generates a voltage which is stored by the capacitor.

When attached to an (unspecified) piece of industrial equipment, Sterken says his device generates 90 nanowatts of power. That doesn’t seem much: it’s not going to recharge your mobile phone or ipod.

But it might power the sensors needed to monitor this and other devices. And these harvesters can only get better.

Ref: arxiv.org/abs/0802.3060: Characterisation of an Electronic Vibration Harvester

Quantum computers and the death of chemistry

Monday, February 11th, 2008

Quantum chemistry

When it comes to chemistry, computer simulations suck. The best they can do is simulate the electron dynamics of a helium atom, which is almost as simple as it gets. Never mind the rest of the periodic table and how the elements interact with each other.

But that’s gonna change when we get quantum computers, says Ivan Kassal, a chemist at Harvard University. He and a few buddies have worked out that a relatively simple quantum computer could generate an “exact, polynomial-time simulation of chemical reactions”.

It might not be long before we see the fruits of this kinda computation. Kassal calculates that a quantum computer with only 100 qubits could simulate the entire quantum dynamics of a lithium atom, a feat that is beyond all the conventional computer power on the planet right now.That’s impressive and destined to make chemistry even more unpopular than it already is. Who’ll want to mess around with test tubes and bunsen burners when a quantum computer can work out the answer while ya put ya feet up?

So how long before the quantum death of chemistry? That’s hard to say. Physicists are already messing around with quantum comuters that work with 10 quibits, not nearly enough to do anything interesting with yet. So we’ll need an order of magnitude improvement.

Sticking my neck out, I’d say we’ll have 100 quibit quantum computers within 7 years, based on nonlinear optical technology (gulp).

Ref: arxiv.org/abs/0801.2986: Quantum Algorithms for the Simulation of Chemical Dynamics

Graphite valley

Thursday, January 3rd, 2008

Graphite

Them chemists have been bewitched by carbon in recent years. Ya can’t move in chemistry departments without being abused ‘n’ bombarded with nanotubes, buckyballs and all mannner of carbononsense. But in all their hurry to blow their own carbon nanotrumpets, it loooks as if they missed a wonder material staring them in the face.

Now physicists are sayin’ that graphene is the material of their dreams and that it could revolitionize everything from fundamental physics to computing. Graphene (a single layer of graphite) has been doing remarkable things for a few years. Physicists have demonstrated the quantum hall effect in graphene and at room temperature for chrissakes! It can switch from a metal-like conductor  to an insulator in the presence of a magnetic field and it is home to a new breed of amazing phantom stuff called 2D Dirac particles that physicists are drooling to get hold of.

So ya can imagine how it could be put to good use to make a new breed of nanodevice. In fact, many commentators expect graphene to play an important role in the new science of spintronics.

Trouble is that graphene ain’t the easiest material to handle. It’s edges are highly reactive and bond to anything within reach and it tends to curl up like paper in fire.

But now Pablo Esquinazi at the University of Leipzig in Germany and a his buddy Yakov say that anything graphene can do, graphite can do better. What they mean is graphite only a few layers thick but this stuff is much easier to make and handle than single layer graphene.

And their conclusion?

“Based on experimental observations, we anticipate that thin graphite samples and not single layers will be the most promising candidates for graphene-based electronics.”

Looks like the area round Palo Alto is gonna need a name change.

Ref: arxiv.org/abs/0712.4020: Graphene Physics in Graphite

How atom lasers are coming of age

Thursday, December 6th, 2007

Atom laser

Atom lasers are gonna be mighty useful for testing quantum mechanics to its limits. But ain’t nobody built one that can operate continuously, which is what yerl need for these kindsa experiments. We’ve had pulsed atom lasers for about a decade and it looks as if a continuous version may be close.

Atom lasers work by cooling a pile ‘o’ atoms so that they occupy the same quantum state, watcha call a Bose Einstein Condensate (BEC) . Then ya allow the condensate to ‘drip’ so that it releases a constant stream of coherent atoms in the same state. But a BEC contains a limited number of atoms so as it drips, the reservoir of atoms has to be replenished. The trouble is that BECs are incredibly fragile, sneeze and they disappear in puff o’ smoke. So physicists have been a-puzzlin’ and a-wondrin’ over how to do the replenishin’.

Now Nick “Dun” Robins at the Australian National University in Canberra says he and a few mates have worked out how to do it by pumping atoms into the BEC from a physically separate cloud nearby. That allows their laser to operate continuously, at least in theory. In practice, they can’t tell whether the laser is actually working cos it produces too few atoms to count.

That’s not a huge barrier to overcome and it should be straightforward to hit the jackpot soon. But that raises the questions: why release these results now? Obviously Robins wants to get them out there before his team is pipped to the post by somebody else.

So sometime soon we can expect to hear somebody (but not necessarily Robins) announce they built of the first continuous pumped atom laser.

Ref: arxiv.org/abs/0711.4418: A pumped atom laser

Nanowire magnets

Tuesday, November 20th, 2007

Co-Ni nanowires

Take a handful of Cobalt-Nickel (Co80-Ni20) alloy nanowires and drop them into a mixture of toluene and the synthetic polymer PMMA. Zap the mixture with a decent magnetic field, sit back and wait.

The field causes the nanowires to align and as the toluene evaporates, the PMMA traps them in place as it solidifies.

The result? A reasonably strong permanent magnet that remains magnetised at temperatures up to 250 degrees C.

Not bad eh?

Frederic “Teap” Ott at the Laboratoire Léon Brillouin in France and colleagues say the new material rivals both SmCo and AlNiCo magnets in various aspects. In particular, they reckon the new magnet is twice as good as existing materials for magnetic media recording.

Ref: arxiv.org/abs/0711.1978: Magnetic Nanowires as Permanent Magnet Materials.

The heterohydrogen question

Monday, November 19th, 2007

Kinetic energy operator

Can hydrogen and and antihydrogen bind to form a stable molecule?

That’s the question that a growing number of particlebods have been scratchin’ their eggs over. And it ain’t merely hypothetical, neither.

In the last few years, engineers at CERN in Switzerland and Fermilab near Chicago have been a-tinkerin’ and a-toyin’ with their particle traps and dramatically improved the numbers of antiprotons they can round up. All of a sudden, physicists have the building (and anti-building) blocks with which to test the question.

Of course, they ain’t got round to it yet which has left the field wide open to theorists. This week, it’s the turn of Mohamed Assad Abdel-Raouf at the United Arab Emirates University in Al Ain City. He’s run the first computations to calculate the bindin’ energies of so-called heterohydrogens (presumably ordinary homohydrogen is illegal in most middle eastern states).

He reckons various molecules of hydrogen and antihydrogen are possible. In particular, them eggs at CERN and Fermilab should keep ‘em peeled for antihydrogen-hydrogen, antihydrogen-deuterium and antihydrogen-tritium, says the man.

Go to it fellas.

Ref: arxiv.org/abs/0711.2283: Possible Coexistence of Antihydrogen with Hydrogen, Deuterium and Tritium Atoms