Page 1701 – Physics World

Home » Page 1701

Sunlight puts asteroids in a spin

Large asteroids rotate at a range of different speeds, roughly following a bell-shaped “Gaussian” distribution in which most rotate at a speed close to some average, with only a small proportion rotating much faster or much slower then the norm. However, the distribution for asteroids with a diameter less than 10 km is rather different – their families have a large excess of asteroids rotating at the fast and slow extremes.

Researchers had thought that the odd distributions could be a result of infrared photons from the Sun warming an asteroid’s near surface as they are absorbed. These absorbed photons are re-emitted once the surface turns away from the Sun, making the asteroid recoil a tiny amount each time they depart. Although a symmetrical asteroid would be unaffected by these recoils, an irregularly-shaped asteroid would experience a net torque that increases the speed of its rotation over millions of years – the so-called “YORP” effect (named after its originators Yarkovsky, O’Keefe, Radzievskii and Paddack). However, the torque would be so small that it would only cause a significant rotation in small asteroids.

Now, two different groups have accumulated enough optical and radar observations from ground-based telescopes to prove that the YORP effect can indeed produce the odd distributions of small asteroids. A team led by Stephen Lowry from Queen’s University in Belfast in the UK (Science Express doi: 10.1126/science1139040) has analyzed data collected over four years by Patrick Taylor and colleagues (Science Express doi: 10.1126/1139038) of “2000 PH5”, an asteroid with a diameter just over 100 m. They found that 2000 PH5 is gradually rotating faster – currently, it takes 12 minutes to rotate, but in 500,000 years time the YORP effect will have halved that period.

Meanwhile, Mikko Kaasalainen from the University of Helsinki in Finland and colleagues (Nature doi:10.1038/nature05614) have shown that the rotation of the larger asteroid “1862 Apollo” with a radius of 700 m is also increasing because of the YORP effect – but nearly a hundred times slower.

Although past data have hinted at the YORP effect, this is the first time astronomers have been able to rule out other possible causes of rotation acceleration, such as strong “tidal torques” caused by the Earth’s gravity and collisions. “A collision would cause a one-time change in spin rate, not a continuous change,” Taylor told Physics Web. “The odds of a single collision for 2000 PH5 is about one collision per billion years – a collision per year is simple unreasonable.”

The teams also say that the YORP effect could speed up the rotation of asteroids to such an extent that they overcome their own gravity and break into halves, producing a “binary” asteroid system.

'Buckets of BEC with inter-bucket tunnelling'

What? Not Mott?
Pots not dots, lots per pot
…and hot!

You can only get away with describing your experiment with a poem if you have a Nobel Prize — and JILA’s Eric Cornell has one of them.

The pots are the wells within a two-dimensional optical lattice and they were filled with lots of atoms in the Bose-Einstein condensate state. Atoms can tunnel between wells, so you can also think of this as an array of Josephson junctions (still with me?).

“Buckets of BEC with inter-bucket tunnelling”, is how Cornell described it.

Cornell and his team were looking for a Kosterlitz-Thouless transition in the lattice. This occurs when vortices form in the array above a certain temperature.

The lattice is made up of little triangles that look like this (the “O”s are the wells):

O O
O

Atoms moving clockwise (or counter-clockwise) from well to well around the triangle create a vortex.

And that’s exactly what they saw.

Visible invisibility cloak?

Purdue University’s Vladimir Shalaev is giving the following paper tomorrow :

“Negative-Index Metamaterials in the Visible Range” (W38 1).

Could this be the first invisibility cloak for visible light?

Shalaev has already worked out a way to make metamaterials that respond magnetically to visible light, and has come tantalizingly close to creating negative index materials in the visible range.

I asked Shalaev if he would be unveiling an invisibility cloak on Thursday — but he just grinned, said “come to my talk”, and then he vanished into thin air!

Facts and opinion about graphene

Graphene guru Pablo Jarillo-Herrero of Columbia University set me straight on the miraculous flakes of carbon.

-There were 180 papers published on graphene in the last year, but less than 10% were experimental.

-If it’s five or more atomic layers thick, then it’s just plain old graphite.

-If it’s 1-2 layers thick, the electrons think they are confined to two dimensions and the fun begins.

-Graphene is compatible with silicon fabrication processes and transistors can be made from graphene.

-Graphene has high electon mobility and is a superb heat conductor, which could allow graphene transistors to operate at vey high frequencies.

– A paper presented here at the APS has claimed that graphene grown on SiC has an electron energy gap of ~250 meV, which Jarillo-Herrero says is enough to make room temperature transistors.

-Graphene is not flat and its undulating surface affects its electronic properties

-The undulations could be a way of damping out thermal vibrations and therefore graphene could become flat below a certain temperature

-Graphene provides a laboratory for studying a range of fascinating phenomena including the quantum Hall effect, Berry’s phase and Dirac fermions

Commercial high-Tc applications

In my entry on “Rock star physicists” I said that there are no commercially viable applications of high Tc superconductors. I have just discovered that this could be wrong — at least according to Alexis Malozemoff of American Superconductor Corporation.

In his talk “Transforming the Grid with Superconductivity” (L1 5), Malozemoff said that the company had sold two “synchronous condensers” to the Tennessee Valley Authority. These are electric motor-like devices that act as “shock absorbers” in an electricity grid and help keep voltage levels steady. The devices are wound with high Tc superconductor wire.

The company is also working on giant electric motors to power US Navy warships, and Malozemoff said that the company is still in the running for a contract to supply motors for next-generation destroyers.

Three for medical physics

I just came out of a medical physics press conference that presented three very different ways that physics can be put to use saving lives.

The first presentation was from David Nolte of Purdue University who has created a very simple but effective way of measuring motion inside cancer cells. The technique involves splitting a laser beam, reflecting one beam off a tumour and then recombining the two beams at a detector. The two beams interfere and motion within the cancer cells causes the interference to change.

The result is an image of the tumour covered in speckles that change rapidly as the organelles inside the cancer cells move. A moving cell is a healthy cell, so the technique can be used to study how some anti-cancer drugs slow down the movement within cells, ultimately killing them.

Andre Brown of the University of Pennsylvania described his work on fibrin, which are molecular chains that create web-like structures that aid in the clotting of blood. Blood clots cause heart attacks and strokes so it is very important to understand the mechanical properties of fibrin — particulalry how it stretches.

Brown used a technique developed a few years ago whereby the tip of an atomic force microscope (AFM) is used to pick up one end of a fibrin chain and tug on it. He discovered that fibrin was made of a chain of coiled proteins, with each coil unfolding 23nm when pulled hard enough by the AFM. The next step is to work out how this unfolding affects larger fibrin structures.

Finally, Michael Deem of Rice University explained how he has used statistical physics to develop strategies of multiple vaccination to keep the body’s immune system one step ahead of a rapidly mutating virus.

And a big thanks to the APS press office for a fantastic lunch today!

Standing room only for graphene

The room was packed to the rafters for Tsuneya Ando’s talk on “Theory of quantum transport in graphene and nanotubes” (H28 1), which kicked off the first of five focus sessions on graphene. Although it may still be too early to call, I’d say that graphene will be THE topic of this year’s meeting.

I left the session with my head spinning in Landau levels, but I think I got the general idea — graphene is like a very thin motorway for electrons with nothing in the way to slow them down. There’s a press conference later today on graphene — including the latest development in negative refraction — which should be more my speed.

Day two beckons

It’s day two of the March meeting and after cutting my teeth yesterday on some lighter material it’s time to get stuck into some serious physics. The first thing on the agenda is graphene. There are at least a half a dozen sessions on graphene this year, not bad for a material that’s only been around for a few years. Also on my list are a couple talks on spintronics. For some light relief I’ll be popping into a session on nuclear weapons.

The meeting got off to a slow start yesterday and before noon I was wondering where everyone was. But by the end of the day I could easily believe that I was surrounded by 7000 physicists. There will be a total of 6500 talks at the meeting and just over 100 companies are represented at the exhibition.

For the most part, the Colorado Convention Center is easy to navigate and it takes no longer than about five minutes to get from one meeting room to another.

But I don’t think I would be very pleased if I was an exhibitor. The exhibition hall is a good distance from the meeting rooms and areas where delegates tend to congregate. I mentioned this to the folks on our booth, but so far they seem pleased with the traffic they are getting.

Fermilab data hint at Higgs boson

The great triumph of the Standard Model is that it unites two of the fundamental forces – the weak and electromagnetic force – into a single, symmetric “electroweak” force at high energies. But at low energies, a symmetric electroweak theory would imply that particles have no mass, which is clearly wrong.

This is where the Higgs boson comes in – a particle that can break the electroweak symmetry at low energies. If our current Standard Model is correct, the much-sought Higgs would have a mass somewhere in the 100 GeV to 1 TeV region, which should allow physicists to discover it at the 14 TeV Large Hadron Collider at CERN once it starts up in November.

However, theoretical physicists analyzing data taken by the HyperCP experiment at Fermilab in January last year say the US lab might have got there first – that is, if we are prepared to consider an “extension” to the Standard Model. That experiment, which involved firing a proton beam at a fixed target, appeared to show three “events” in which a sigma-plus particle decays into a proton and a muon-antimuon pair. Although just three events would not normally be regarded as significant, German Valencia from Iowa State University in the US and colleagues — who are not part of the HyperCP experiment — suggest that the events could be interpreted as evidence for a new particle with mass 214.3 MeV, which they have dubbed the “HyperCP particle”.

Because it is relatively light and has a low interaction probability, the HyperCP particle does not fit into the current Standard Model. However, it could be explained using the “next-to-minimal supersymmetric standard model” (NMSSM). This model is one of several “supersymmetric” models that attempt to explain why the fundamental forces have such different strengths by proposing twice or more the number of particles. In the NMSSM there are seven Higgs bosons – Valencia’s team thinks the HyperCP particle may be the lightest of these.

Although much more evidence than the three events at HyperCP will be needed to lure physicists towards the NMSSM, Valencia is excited by the thought of physics beyond the current Standard Model. “The probability of this being a fluctuation is about half a percent, about the same as the probability of getting a string of eight heads in a row when flipping a coin,” he told Physics Web.

This is not the first time physicists have laid claim to a Higgs as part of an alternative supersymmetric theory. Earlier this year, John Conway and Tommaso Dorigo suggested that a 160-GeV “bump” in Fermilab data could have been one of five Higgs bosons in the more favoured “minimal supersymmetric standard model” (MSSM).

Rock star physicists

What do superconductor expert Paul Chu and Jimi Hendrix have in common?

They were both on stage at the “Woodstock” of their respective professions — at least according to the APS, which today celebrated the 20th anniversary of a special session on high Tc superconductivity that was held at the 1987 March Meeting in New York City.

The 1987 session occurred just months after Georg Bednorz and Alexander Muller announced their discovery of a material that was superconducting up to 35K — a much higher temperature than was thought possible. Within a year Chu had found another material that remained superconducting above liquid nitrogen temperature and the race was on to find a room temperature superconductor.

In true APS style, the 1987 special session was marked today by holding another special session called “20th Anniversary of High Tc Superconductivity ‘Woodstock’ Session” (B1), which featured many of the original lineup.

According to Brian Maple, who chaired the 1987 session, about 2200 people attended and it went on until 3.15 am — leading someone to dub it the “Woodstock Session”. Not quite the half million people that attended the four-day Woodstock music festival in 1969, but impressive for the normally staid March meeting.

The 1987 session was well attended by the media, and the speakers gained celebrity status as the world went high-Tc mad. At a press conference held today, Woodstock speaker Paul Grant produced a poster from a trendy Manhattan nightclub that had devoted an entire week to a high Tc theme. Grant said that the nightclub’s bouncers led him through the queue like a rock star. Grant also produced a high Tc t-shirt that he bought from a vendor on a California beach.

Bednorz and Muller went on to win the Nobel Prize and more than 100 high Tc compounds have since been discovered — the current record temperature being about 160K. However, Grant admits that the first real commercially viable applications of high Tc materials are still about a decade away and that “there is no generally accepted theory for high Tc superconductivity”. The latter has led to divisions within the high Tc community about the way forward.

So have the high Tc rock stars suffered the same fate as Spinal Tap?

“It’s not fair to say the theory is in great chaos”, said Woodstock attendee Douglas Scalapino when I asked him about the shortcomings of current approaches. He is confident that physicists understand the nature of the high Tc superconducting state, which he believes is caused by the “d-wave” pairing of electrons. What is not understood, says Scalapino, is why the electrons pair up in the first place and this is going to need better experimental data.

Browse articles by content type