Fluffy simulations

By Hamish Johnston in Portland, Oregon

Have you ever wondered why clouds are fluffy?

Well, it’s not an easy question – according to Yong Wang at UCLA. Wang was here at the APS March Meeting to talk about his simulations of cumulus clouds, the fluffy ones that tend to appear after about noon on a sunny day and don’t tend to spoil the rest of the day.

Wang says that these clouds are droplets supported by thermal convection, and their shapes arise because this is a “complex non-linear system” that is driven by thermal plumes.

The simulation begins with a homogeneous layer of water droplets into which small thermal plumes rise. After a while, the jostled droplets look a lot like fluffy cumulus clouds (see above).

Wang didn’t seem to think that there were any practical applications for his work – but I would have thought this could help climate physicists understand why certain clouds form.

You can read more about Wang’s simulations here.

Record breaking accelerator

By Hamish Johnston in Portland, Oregon

Here’s a question for you, what is the most powerful accelerator in the world?

No, it’s not the LHC – that holds the record for energy – the answer is the Spallation Neutron Source (SNS) at the Oak National Lab in Tennessee.

In September 2009 the facility delivered a pulsed beam of 1 GeV protons at a power of 1 MW.

The pulses are fired at a target of liquid mercury, creating copious amounts of neutrons, which can then be slowed down and used for studying solids and liquids.

This afternoon I saw a nice talk by Stuart Henderson of Oak Ridge about recent progress at the SNS. Since experiments began in 2006, the number of instruments attached to the neutron beamline has grown to 12 and he expects that 16 instruments will be running by 2012.

And of course, Oak Ridge hope to upgrade the facility between 2012–2017 – boosting the energy to 1.3 GeV and the power to 3 MW.

Rising plumes

By Hamish Johnston in Portland, Oregon

Any guesses as to what you are looking at?

The red shape is a “person” sitting in a small room. The temperature at the surface of the person is 25 degrees – the temperature of your clothes, apparently – and the temperature of the room is 20 degrees.

The image is from a huge simulation of how air circulates in a room with floor and ceiling vents that was done by John McLaughlin and colleagues at Clarkson University.

The yellow plumes are warm air rising from the sitting person – and McLaughlin looked at how tiny particles comparable to viruses or pollen behaved in the room. He found that the plumes tend to concentrate the particles over the person’s head – and then they fall down onto the poor person!

This could be bad news in a hospital, for example, where there could be lots of nasty bugs floating around.

So if your head is getting dusty, perhaps it’s because you are sitting perfectly still in a small room.

Packed house for Andre Geim

By Hamish Johnston in Portland, Oregon

I had to push my way through the crowd here at the APS March Meeting just to stand at the back of Andre Geim’s talk called “Graphene update”. It seems that there is a still a lot of interest in the wonder material – sheets of carbon just one atom thick – that promises to revolutionize electronics.

The University of Manchester-based graphene guru spoke about a half-dozen or so open questions in the field.

Is graphene ferromagnetic at room temperature, as some have claimed?

“No,” says Geim, who explained how he and his colleagues found no evidence for ferromagnetism down to a chilly 2K.

And what about the vexing question of how to create a bandgap in gapless graphene so that it can be used to create conventional semiconductor devices?

Straining graphene by up to 10% hasn’t worked – and recent calculations suggest that you would have to have to strain the stuff by as much as 25% before a gap appears.

One way forward, according to Geim, is to somehow apply just the right amount of “non-uniform” strain to the material. While this appears to work in theory, it requires the strain to vary on length scales of about a micron – which physicists can’t do today.

A gap could also be introduced by altering the chemistry of graphene. Geim and colleagues have already hydrogenated graphene to create graphane – which has a gap. However, Geim says the material is “unstable” and not suitable for making semiconductor devices.

So if hydrogen doesn’t work, why not try fluorine to create fluorographene? That’s what Geim and colleagues have done – and although the result was a semiconductor with a great deal of disorder, he said that fluorographene could be the way forward to a gap.

Perhaps the most intriguing topic touched on by Geim is the fabrication of “quantum capacitors”, which comprise one graphene plate and one metal plate. In such a device the capacitance is a function of the applied voltage, dipping to zero at zero voltage. And the capacitance oscillates if a magnetic field is applied. I’m not sure what you could do with such a device – but it’s yet another example of the wonders of graphene.

By Hamish Johnston in Portland, Oregon

I know it’s a cliché, but the quantum world gets weirder the more you learn about how it works.

Yesterday I went to a talk by Graeme Smith of IBM Research, whose talk was entitled “Surprises in the theory of quantum communications”.

The surprise that Smith focused on is that two transmission channels – both of which are too noisy or lossy to transmit quantum information individually – can somehow join forces to create a very good channel for transmitting quantum information.

A classical transmission channel fails if you put a signal in one end and get nothing (or just noise) out the other end. By contrast, a quantum channel can fail if you input quantum information but its quantum nature is lost when it gets to the other end – information is transferred, but not quantum information.

But according to Smith, it’s possible that each channel is capable of transmitting a certain subset of the quantum information – but not all of it. The trick is to have two or more channels combine their quantum strengths to overcome their weaknesses.

“The weakness of one is made up for by the strength of the other,” explained Smith.

While it sounds like a great way to build a robust transmission channel from a bunch of bad connections, Smith said that it is not currently clear how to decide which bad channels can be grouped together to create a good channel.

It’s a whopper

By Hamish Johnston in Portland, Oregon

Swinging above the heads of thousands of physicists as they rush to the next session is a very large Foucault pendulum. Indeed, Wikipedia suggests that it is the world’s largest.

It’s day two here at the APS March Meeting and I’m off to hear about how electric and magnetic fields can be synthesized for ultracold neutral atoms.

Steal a speaker’s data at your peril

By Hamish Johnston in Portland Oregon

I couldn’t resist taking a cheeky snap of this sign in a corridor of the convention centre.

Is this a response to the infamous “physics paparazzi”, who take photos of other people’s data during talks and then go off and write a paper that beats the original researchers into publication?

You may recall the scandal surrounding PAMELA data a few years ago when that very thing happened.

When I asked in the press room I was assured that reputable members of the media such as myself were free to take photos – and a press officer is looking into whether there is a ban on delegates taking photos.

As I only saw one sign in the giant convention centre, a more plausible explanation is that the sign was left over from last week’s event.

Zhang (left) next to Molenkamp

By Hamish Johnston in Portland, Oregon

There was a slight panic here in the press room over lunch when we all realized that we will soon be writing about topological insulators. We weren’t exactly sure what they were – but it’s becoming clear that topological insulators are the hot topic here at the APS March Meeting.

Fortunately we received a good introduction by some of the leading lights in the field, including Shou-Cheng Zhang of Stanford University. Zhang described topological insulators as “a new state of matter that has been predicted and discovered”. The prediction – by Zhang, I believe – occurred in 2006 and the first material was made a year later by Laurens Molenkamp at the University of Würzburg, who was also at the press conference.

Topological insulators are actually pretty good conductors (more on that later) and could lead to smaller integrated circuits that run faster and cooler. There is even the suggestion that axions and Majorana fermions could be lurking in these materials.

A simple description of a topological insulator is a material that is an insulator in the bulk, but a very good conductor on the surface.

Why? Well, it seems to have something to do with the quantum spin Hall effect – the accumulation of electrons with opposite spins on opposite sides of a conductor.

Let’s say a spin-up electron is flowing along the surface and scatters off an impurity.
The scattering process involves orbital angular momentum – and thanks to spin-orbit coupling, the spin of the electron is also rotated during the scattering.

Here’s the tricky bit that I didn’t quite understand. If the electron is scattered backwards the spin rotation introduces a phase shift of –1. If you think of this scattering as wave diffraction, destructive interference means that the electron can’t propagate in the opposite direction.

No backscattering means that the resistance of the material is very low, which is very useful if you are trying to make very tiny electronic circuits.

Sounds reasonable, but there are a few things I don’t understand. For one thing, this explanation seems to hinge on the electron only being able to scatter forwards or backwards – but not off to the side.

I’d better start reading-up on topological insulators.