Archive for the ‘Mountain climbin'’ Category

Matter wave lithography could carve single nanometre features

Monday, November 24th, 2008

matter-wave-lithography.jpg

Atomic matter waves have been generating a bit of interest of late. The thinking is that atom waves can be manipulated in much the same way as light waves and so could be used to directly print atoms onto microchips to create nanoscale features.

The question is: how small can these features be made and how accurately can they be put in place? An obvious answer is that these features can’t be smaller than the size of the atoms involved and placed no more accurately than the size of the wavelength of these atom ie diffraction limited. And until a few years ago, nobody would have argued with that.

Since then, numerous techniques have been been developed for manipulating light beyond the diffraction limit so it’s reasonable to assume that a similar thing can be done with matter waves.

And so it has come to pass. Jordi Mompart at the Universitat Autonoma de Barcelona in Spain and a few amigos have shown that it is possible to achieve single nanometre resolution with matter waves of rubidium. At least in theory.
All we need now is somebody to step up to the plate and do it for real. Judging by the speed of progress in this field right now, that shouldn’t take long.

And beyond that: the first microchips patterned using matter waves rather than light?

Ref: http://arxiv.org/abs/0811.3409: Coherent Patterning of Matter Waves with Subwavelength Localization

The fine line between the visible and invisible

Monday, September 22nd, 2008

line-cloak.jpg

The man who built the world’s first invisibility cloak is back and this time he’s got an even better idea.

His first design was a triumph for headline writers and Harry Potter fans alike, although most glossed over the fact that this first cloak worked only for microwave-sensitive eyes and even then only at a single specific frequency. Oh, and only in two dimensions. And it wasn’t really a cloak at all, more of an invisibility canister.

Nevertheless, nothing should be taken away from the technical achievements of David Smith and colleagues at Duke University in North Carolina. They’ve done a job almost as spectacular as their PR team.

The new idea gets around one of the most pressing problems associated with invisibility cloaks which is that they are impossible to construct well. Invisibility cloaks work by distorting the permeability and permittivity of the cloaking material in way that forces light to bend around an internal cavity. This makes the cavity invisible to an observer.

But the technique requires the permeability and permittivity to take infinite values at certain points, particularly on the boundary between the cavity and and the cloaking material . And this just isn’t possible.
Various ideas have been proposed to get around this problem but all have their own weaknesses.

So Smith and colleagues say they might as well accept that an invisibility cloak cannot be perfect and use it to their advantage. Instead of attempting to hide the internal cavity completely or crushing it to a point as others have done, the new idea is to make it appear as a single line. Such an invisibility cloak wouldn’t hide an object entirely but instead make it look like a thin line, like a defect in the structure of the cloak.

That’s clever because it dramatically relaxes the constraints placed on the types of metamaterials you can use for cloaking. Smith and his buddies say that such a cloak would be “very easy to realize” using known techniques.

Smith is known for publishing theoretical predictions just ahead of the practical realisation.

So if his past form is anything to go by, we can expect to see a working invisibility cloak that employs this technique in the coming weeks or months.

Ref: arxiv.org/abs/0809.2317/: Invisible Cloak With Easily-Realizable Metamaterials

Predicting mine collapse

Tuesday, September 2nd, 2008

mine-collapse.jpg

Northern France is riddled with limestone mines that occasionally collapse creating a ring-shaped crater on the surface that can cause serious damage to nearby buildings.

Is there any way to predict these failures and thereby attempt to prevent them?

If there is, Siavash Ghabezloo and Ahmad Pouya from the Laboratoire Centrale des Ponts et Chaussées in Paris, seem determined to find it and publish their initial efforts today.

Their approach is to assume that mine failure is the result of weathering in  limestone rock, in other words the reaction of carbon dioxide, water with calcium carbonate. This dissolves the rock, severely weakening it until it collapses.

Ghabezloo and Pouya introduce a set of equations that govern the different hydro/chemo/ mechanical aspects of this weathering phenomenon. They then attempt to model the process numerically.

That’s all very well but does it get us any closer to predicting collapse? Unfortunately not. Validating their model will be difficult to say the least and applying it to a real mine almost impossible.

And even if it could predict how fast such weathering might occur, how would it predict where it was happening?

Nice try but no cigar.

Ref: arxiv.org/abs/0808.4081: Numerical Modelling of the Effect of Weathering on the Progressive Failure of Underground Limestone Mines

Do nuclear decay rates depend on our distance from the sun?

Friday, August 29th, 2008

radioactive-decay.jpg

Here’s an interesting conundrum involving nuclear decay rates.

We think that the decay rates of elements are constant regardless of the ambient conditions (except in a few special cases where beta decay can be influenced by powerful electric fields).

So that makes it hard to explain the curious periodic variations in the decay rates of silicon-32 and radium-226 observed by groups at the Brookhaven National Labs in the US and at the Physikalisch-Technische Bundesandstalt in Germany in the 1980s.

Today, the story gets even more puzzling. Jere Jenkins and pals at Purdue University in Indiana have re-analysed the raw data from these experiments and say that the modulations are synchronised with each other and with Earth’s distance from the sun. (Both groups, in acts of selfless dedication,  measured the decay rates of silicon-32 and radium-226 over a period of many years.)

In other words, there appears to be an annual variation in the decay rates of these elements.

Jenkins and co put forward two theories to explain why this might be happening.

First,  they say a theory developed by John Barrow at the University of Cambridge in the UK and Douglas Shaw at the University of London, suggests that the sun produces a field that changes the value of the fine structure constant on Earth as its distance from the sun varies during each orbit. Such an effect would certainly cause the kind of an annual variation in decay rates that Jenkins and co highlight.

Another idea is that the effect is caused by some kind of interaction with the neutrino flux from the sun’s interior, which could be tested by carrying out the measurements close to a nuclear reactor (which would generate its own powerful neutrino flux).

It turns out, that the notion of that nuclear decay rates are constant has been under attack for some time. In 2006, Jenkins says the decay rate of manganese-54 in their lab decreased dramtically during a solar flare on 13 December.

And numerous groups disagree over the decay rate for elements such as titanium-44, silicon-32 and cesium-137. Perhaps they took their data at different times of the year.

Keep em peeled beause we could hear more about this. Interesting stuff.

Ref: arxiv.org/abs/0808.3283: Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance

The ultimate black hole size limit

Tuesday, August 26th, 2008

ultramassive-black-holes.jpg

We have a pretty good idea that a supermassive black hole is sitting at the center of our galaxy. By supermassive, astronomers mean about 6 millions times as massive as our sun.

That’s pretty big by any standards but how big can black holes get Is there any limit to how big these monsters can become?

According to Priyamvada Natarajan at Yale University and a pal, the answer is yes.  Black holes, they say, cannot be bigger than 10^10 times the mass of the sun. (Or at least, are very unlikely to be bigger than that).

They arrive at this figure by calculating  the rate at which a black hole can swallow stuff and how much it could have gorged on since the universe was born, which seem like reasonable limits.

They call these beasts ultramassive black holes and reckon that there should be around 7×10^−7 of them per cubic megaparsec in the nearby universe. That’s not many. Our bast chance of finding one should be to look in the bright, central cluster galaxies in the local universe.

Better start scanning.

Re: arxiv.org/abs/0808.2813: Is there an upper limit to black hole masses?

Graphene quantum computers could be built with today’s technology

Thursday, August 14th, 2008

graphene-nanoribbon

Is there anything graphene cannot do?

The great graphene gold rush continues today with the news that graphene nanoribbon could be the key ingredient of the next generation of quantum computers.

The trick with quantum computing is to use qubit-carrying particles that are easy to manipulate so that their quibits can be written and read, that interact with each other so that the qubits can be processed in logic gates but are robust in the sense that thay do not easily interact with the environment so that data isn’t needlessly lost.

Photons are the current darlings of the quantum computing crowd because they do not interact easily with the environment and can be relatively easily manipulated themselves (although getting photons to interact with each other is hard).

But electron spins are also a good prospect because they can be easily controlled and interact readily with each other. Their downside is that it is hard to insulate them from stray magnetic and electric fields in the environment, so storing them is hard.

Now it looks as if graphene nanoribbon may come to the rescue. Guo-Ping Guo and pals from the University of Science and Technology of China in Hefei say that z-shaped graphene ribbons can easily store electrons in the corners of their Zs, where they can be read and written to. And by placing two Zs close to each other on a graphene strip, the electrons can also be made to interact with each other.

Materials scientists have recently worked out how to make Z-shaped graphene reliably in the lab so all the ingredients are in place for a test device to be knocked up shortly.

As Guo-Ping Guo and buddies put it: “Due to recent achievement in production of graphene nanoribbon, this proposal may be implementable within the present techniques.”

Ref: arxiv.org/abs/0808.1618: Quantum computation with graphene nanoribbon

How to bury an ion (and find it again later)

Tuesday, June 17th, 2008

Buried ions

The future of computing depends on our ability to bury single ions within the crystal structure of silicon and diamond in a way that allows us to find them again, quickly and repeatably.

The burying part of all this isn’t difficult: simply aim a beam of ions at a substrate and you can be pretty sure one or two of them will end up buried inside the crystal structure. The trick is to know where they’re buried so you can find them again later.

That’s a trick that Thomas Schenkel and pals at the Lawrence Berkeley National Laboratory overlooking San Francisco, seem to have perfected for the first time. Their technique is to fire the ion beam through a hole in the tip of a scanning force microscope. The hole defines the position of the burial while the scanning force microscope can easily find the location again later.

Schenkel has used the technique to bury nitrogen atoms in diamond and antimony in silicon, which is impressive. (The team even says it has discovered that antimony doesn’t migrate when the substrate is later heated, which is an important result in itself. Ions that move after you bury them are difficult to find later!)

But the bigger picture is that ion burial is one of the enabling technologies that will make quantum computing possible. So it’s just possible that Schenkel and co have created the first facility in which quantum computers will be forged.

Ref: arxiv.org/abs/0806.2167: Single-atom Doping for Quantum Device Development in Diamond and Silicon

Western Europe warming much faster than expected

Friday, June 6th, 2008

European warming

There’s little doubt these days over whether the planet is heating up. Temperature measurements clearly show the trend and in recent years, computer models of the Earth’s climate have been able to reproduce these increases pretty accurately when carbon dioxide is injected into their virtual atmospheres.

Where climate models fall down, however, is in predicting how the climate will change on a regional scale. The Netherlands, for example, is represented by a single grid square in global climate models. So that makes it hard to work out how global patterns may influence the climate in the Netherlands.

Today, a group of meterologists from the Royal Netherlands Meterological Institute (KNMI), the Dutch weather forecasting outfit, examine warming trends in western Europe and say the current models of regional climate change have vastly underestimated the rate of change. Yep, that’s underestimated.

What the team has done is identify many of the reasons why regional models fall down. They say the models fail to account for stronger wind circulation patterns in winter and spring, misrepresent the North Atlantic Current that brings warm water from the  equator  to western Europe and underestimate the amount of sunshine in some places. There are also important differnces between observed and modelled effects of aerosols and snow at various places and times too.

That’s important to know because it should be possible to fix the regional climate models  to take account of these effects. At least, in theory.In the meantime, Western Europe is warming much faster than regional climate models have suggested. The message from KNMI is that you live in the Netherlands (or anywhere else in Western Europe), stock up on ice cubes and suncream.

Ref:  arxiv.org/abs/0806.0715: Western Europe is Warming Much Faster Than Expected

They came from Mercury…

Thursday, February 21st, 2008

Mercurian meteorite

Astrobods have found several dozen meteorites from Mars and the Moon that have made their way to Earth over the years. These rocks were launched during major impacts there.

It’s unlikely that we’ll ever see a meteorite from Venus arrive on Earth: the Venusian atmosphere is just too thick. But what of Mercury?

According to Brett Gladman from the University of British Columbia in Vancouver, Mercurian meteorites may be an order of magnitude more common on Earth than we thought.

He and a pal say Mercury is the only planet where impact speeds are routinely 5 to 20 times greater than the escape speed.

So they calculated the percentage of rock likely to escape from Mercury towards Earth after an impact. It turns out that up to 5% of rocks leaving Mercury with speeds greater than 9 km per second could reach Earth within 30 million years.

So Mercurian meteorites probably arrive here at about half the rate of rocks from Mars.

That means that somewhere on Earth, in somebody’s meteorite collection there lies a little bit of Mercury. Get looking!

Ref: arxiv.org/abs/0801.4038: Mercurian Impact Ejecta: Meteorites and Mantle

Extreme ice and the blues

Monday, January 28th, 2008

Ice

There are 15 different types of ice known to science and I’m not talkin’ Baskin Robbins here. These are materials with different structures that form when water freezes at various temperatures and pressures. Types XIII and XIV were only discovered in 2006

Most ice we come across naturally is type I, which forms at ambient pressures and we understand many (but not all) of its properties pretty well. But other phases of ice, which although they form at higher pressures, can be stable at ambient pressures.

So what of the properties of these ices? Renjun Xua and colleagues from the National Laboratory of Superhard Materials at Jilin University in China, have calcuated the optical properties of ices X, XI and (the still theoretical) XV.

They say that the optical properties of these materials are significantly blue shifted and that the range of frequncies at which they absorb and reflect light become broader.

The group hints that these qualities should be taken into account by climatologists trying to understand the effects of ice on our climate.

This is a disingenuous attempt to jump on the climatology bandwagon. Although ice covers 5 per cent of the surface of Earth and has covered much more during the various ice ages in the past, there is no indication that this ice is anything but phase I.

Why bother making this link when a much better one would be to the study of other planets and moons where ice forms in much more extreme conditions?

Ref: arxiv.org/abs/0801.0187: Ab Initio Investigation of Optical Properties of High-Pressure Phases of Ice