Category Archives: APS March Meeting 2010

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.

Mount Hood in the distance

By Hamish Johnston in Portland, Oregon

I arrived here in Portland on Friday evening and spent the weekend sightseeing with my brother.

Spring also arrived here in the Pacific Northwest, with the daffodils up and the cherry trees in full bloom – the sort of March we’ve come to expect in England, but not this year.

City of bridges

While it’s warm, it’s not warm enough to melt the snow on Mount Hood, which I can see from my hotel window – at least I think that is Mount Hood.

This morning I took the tram over one of Portland’s many bridges to the twin crystal towers of the convention centre for day one of the American Physical Society March Meeting – a solid week of solid-state physics.

I have sat in on two press conferences so far, one on topological insulators and the other on the physics of catastrophes. More on both later…it’s time for lunch in the pressroom.