Archive for the ‘Cold ‘n’ cool’ Category

The iron arsenide superconductivity challenge

Friday, April 25th, 2008

Iron arsenides

Until a few weeks ago, all so-called high temperature superconductors were layered copper oxides of the type discovered by Karl Muller and Georg Bednorz back in 1986. These are so-called because they become superconducting at temperatures above 30K, the theoretical limit predicted by the BCS theory (after Bardeen, Cooper and Schrieffer) of superconductivity that ruled supreme until then. The discovery won Muller and Bednorz the Nobel Prize in 1987.

But in February, Hideo Hosono at the Tokyo Institute of Technology announced that he had discovered a second family of high temperature superconductors based on iron arsenide. Initially these became superconducting at 26K. But within weeks, other groups had upped the temperature to 40K and then to 54K by make some subtle changes to the material’s composition.

Today two groups publish papers on the arXiv claiming versions of this material that superconduct at 41K and 54K. While another group claims to have made low cost wires out of iron arsenides (a considerable feat given that the annealing temperature of the new superconductors is some 1200C ).

These temperatures are s still nowhere near the record of 138K for copper oxides nor the 200K being claimed for aluminum nanoclusters. But its not bad for just 6 weeks work. Who knows what temperatures they might be possible with this material.

And therein lies the challenge. The question now is whether iron arsenides superconduct in the same way copper oxides do. If not, the theorists are going to have one helluva headache working this one out.

Refs: Superconductivity at 41.0 K in the F-doped LaFeAsO1-xFx Superconductivity at 53.5 K in GdFeAsO1-δ Superconductivity of powder-in-tube LaO0.9F0.1FeAs wires

Nanoclusters break superconductivity record

Friday, April 11th, 2008

Al nanoclusters

Wow! Every now and again a paper on the arxiv leaps out at you and today there’s work from Indiana University in Bloomington that has got my eyeballs on stalks.

Get this: a team led by Martin Jarrold is claiming to have found evidence of superconductivity in aluminium nanoclusters at 200 K .

Yep, 200 K. The current world record for high temperature superconductivity is 138 K for a cuprate perovskite so that’s a massive jump.

The background to this is that two years ago Yuri Ovchinnikov at the Landau Institute for Theoretical Physics in Moscow and Vladimir Kresin at the Lawrence Berkeley Laboratory in California predicted that metal nanoclusters with exactly the right number of delocalised electrons (a few hundred or so) could become strong superconductors.

Now Jarrold and his buddies (Kresin and Ovchinnikov among them) have found the first evidence that this prediction is correct in individual aluminium nanoclusters containing 45 or 47 atoms . And they found it at 200 K.

A few caveats. Before a claim of superconductivity can be made, physicists require three unambiguous and repeatable lines of evidence. The first is obviously zero electrical resistance. The second is the Meisner effect in which the superconductor reflects an external magnetic field. And finally there must be evidence of a superconducting phase transition, such as a jump in the material’s heat capacity when superconductivity occurs.

What Jarrold’s team have measured is the last effect–a massive change in an individual nanocluster’s heat capacity at 200 K. That’s an important pillar of evidence which is consistent with superconductivity but it is not yet a slam dunk.

Jarrold and his team are simply time-stamping their efforts by publishing on the arxiv and you can bet your bottom dollar that they’re looking for other evidence right now.

Even with that proviso, this looks to be an important breakthrough which should be straightforward for other groups to replicate. The group’s work is not yet peer-reviewed. That’ll be an important step too.

Jarrold will be only too mindful that the field of high temperature superconductivity is littered with the corpses of physicists who have made premature claims.

But for the moment, sit back and admire. 200K…wow! That’s room temperature in Siberia at certain times of the year.

Ref: Evidence for High Tc Superconducting Transitions in Isolated Al45 and Al47 Nanoclusters

Earlier ref: Shell Structure and Strengthening of Superconducting Pair Correlation in Nanoclusters

First observation of Hawking radiation?

Thursday, March 6th, 2008

Hawking radiation

In 1974, Stephen Hawking predicted that black holes would emit radiation.

So-called Hawking radiation is produced when pairs of virtual particles pop into existence near the event horizon of a black hole (as they do all over the universe). Usually these pairs simply annihilate each other and disappear. But Hawking predicted that in some cases, one of the pair would sucked into hole while the other escaped. When that happened, the black hole would appear to emit radiation.

Nobody has actually observed Hawking radiation because it is too weak to see with our current gear. But perhaps scientists have been looking in the wrong place.

Iacopo Carusotto from the Universita di Trento in Italy and colleagues say they have spotted Hawking radiation in their lab on Earth.

Here’s what they did: the team created a mathematical model of an experiment with a Bose Einstein Condensate. The condensate flows along a waveguide with a particular speed, v. This sets up a kind of sonic horizon: any sound wave with speed less than v travelling back along the condensate can never cross this horizon.

Seems simple enough. But because BECs are no ordinary objects, it turns out that the physics of this situation is exactly analagous to what goes on at the event horizon of a black hole. So Hawking radiation could form at the horizon.

And sure enough, in their simulation, Carusotto and co observed the emission of a particular kind of sound wave called Bogoliubov phonons from the horizon, just as Hawking predicted.

Given that this is a numerical simualiton, the team’s claim that: “our observations can be considered as a first independent proof of the existence of Hawking radiation,” might be a little over-optimistic. But we get the idea.

Anybody got a BEC machine that could do this for real?

Ref: Numerical Observation of Hawking Radiation from Acoustic Black Holes in Atomic BECs

Holographic quantum computing

Wednesday, March 5th, 2008

Holographic quantum computing

After a decade or so in the lab, holographic data storage is about to burst into the hardware market big time.

Its USP is that holographic data is stored globally rather than at specific sites in the storage medium.

It is written using a pair of lasers to create an interference pattern that is recorded in the storage medium. It can then viewed by illuminating that area with a laser to recreate the pattern. Crucially, you can add and view more data by changing the angle at which you address the medium and this gives huge storage potential.

Now Karl Tordrup and colleagues at the University of Aarhus in Denmark have used the idea as inspiration for the design of a quantum computer. Their machine consists of an array of molecules that can each store a qubit. But instead of addressing them individually, Tordrup imagines storing quantum data in them as a group, by zapping them with the right kind of laser-created interference pattern. This is essentially quantum data storage, the holographic way.

What makes the idea interesting is that the group reckons that information can be processed by transferrring it to a nearby superconducting box in which the required operations can be performed. The processed data is then sent back again.

The big advantage of this idea is that, while stored in holographic form, the quantum data is incredibly robust. While any single molecule errors affect all qubits, they do so only very weakly. It also means that the molecules need only be addressed as a group, not as individuals which does away with a significant challenge that other designs of computer face

But there are problems too. The qubits will have to be protected from decoherence while in the superconducting box and travelling to and from it. And although the molecules do not need to be addressed individually, they do need to be held almost perfectly still. Those are toughies.

All they need to do now is build the thing.

Ref: Holographic Quantum Computing

The shower temperature problem

Friday, January 18th, 2008

Water temperature

Here’s an interesting problem. Imagine a large hotel in which many people are taking a shower at the same time. There isn’t enough hot water to give everyone the shower temperature they’d like and a change in temperature in one shower effects everyone else’s.

What strategy should individuals use to achieve the same temperature for everyone without large temperature changes?

That’s the question posed by Christina Matzke, an economist at the University of Bonn in Germany and a pal from Fribourg, in a curious paper on the arXiv today. They first show that there is a solution to this problem in which everyone can achieve the same shower temperature (although it may not be as hot as they’d like).

But achieving this aint’ easy. Much depends on the resolution of the taps: whether you can change the amount of hot water you getting by a small enough amount to fine tune the temperature. If you can’t, then the temperature will jump around like a cat on hot tin roof.

There are two strategies. If everyone starts with same tap settings, there is a risk of large temperature deviations as peopel change the settings in the same way. If everyone starts with different tap settings, the deviations are less but individuals are also less likely to get close to the optimal temperature.

Matzke then goes on to show that one formulation of the problem is NP-complete meaning that there ain’t a quick way of finding the solution other than testing all the combinations of tap settings.

So there ain’t a magic answer to this one. Looks like we’re destined to be unhappy showerers.

Ref: Taking a shower in Youth Hostels: risks and delights of heterogeneity

Tune into the snowflake channel

Thursday, December 20th, 2007


Snowflakes can emit radio signals as they form and a better understanding of this process could provide a new way to monitor and study snow formation in the atmosphere. That’s the ice-cool conclusion of a group o’ physicists from France and Israel who have begun to tease apart some of the more subtle processes at work when snowflakes freeze.

Here’s what’s going on. In normal circumstances, the ions in water provide a convenient route through which water molecules can divest themselves of energy if they need to. But in de-ionised water, the molecules are perfectly insulated dipoles. So when they freeze into a crystal and become oriented in a specific non-random way, the only way their latent heat can be emitted is as radio signals.

It’s actually possible to pick up these low frequency signals of about 1000 Hz.

A new theoretical model built by Mark Perelman and pals from the Hebrew University in Jerusalem puts all this onto a sound footing for the first time and in a way that might mean that future weather forecasts will be based on data from radio transmitters tuning into the sound that snowflakes make as they form.

Cool huh?

Ref: Freezing of Water and Crystals Formation: Double Electric Layer, Radio Emission, Dendrites, Snowflakes

Magnetic cloaking

Wednesday, October 17th, 2007

The world has gone crazy over metamaterials cos they can be used to build invisibility cloaks, as ya’ll saw just the other week. There’s usually some drawback the media coverage never tells ya which means that we ain’t gonna see no Harry Potter-type invisibility cloaks any time soon. But that hasn’t stopped the living God of metamaterials, John “Now-You-See-Him” Pendry at Imperial College in London UK, from dreamin up ever more ingenious designs.

This week, he and a few pals have gone and built a metamaterial that works for light with zero frequency. That’s egghead-talk for a magnetic field. The material is a thin layer of lead squares pasted onto a cover slip and cooled to superconducting temperatures. This actually prevents a magnetic field from passing through it.

Why would anybody need a metamaterial that screens magnetic fields? There are all kindsa researchers playin around with tiny magnetic fields or with experiments in which they don’t want no magnetism at all. Their work is swamped by larger fields nearby. So a magnetic field cloak might be a useful device to have around.

And unlike other cloaking ideas, the technology is up and running now.

Ref: A DC Magnetic Metamaterial

First laser built from an artificial atom

Wednesday, October 10th, 2007

Strike a light, artificial atoms are excitin critters to be playin around with right now. Get this: some nanobods at the NEC Nano Electronics Research Laboratories in Tskuba, Japan, have gone and built a laser out of one. Yep, a single artificial atom that produces laser light.

Here’s what’s goin on. In real atoms, electrons are confined by the positive charge of the atomic nucleus. But electrons can be trapped in other ways: in this case between a pair of electrodes in a superconducting Josephson junction.

Now electrons in real atoms form into patterns called shells that determine the chemical properties of the atom as well as the way the atom lases. The same is true of artificial atoms. In recent years, them physics bods have discovered a whole periodic table of artificial elements whose properties depend on the number of electrons trapped . They’ve even created artificial molecules by bonding artificial atoms together.

So it shouldn’t be a surprise to nobody that Oleg “Ofoot” Astafiev and friends in Japan have built an artificial atom that lases. Oleg Ofoot’s atom lases in the microwave region of the spectrum which could make it useful for all kinds of communications applications. The authors say, tantalisingly, that “artificial-atom masers can be used as on-chip microwave sources and microwave amplifiers”. We’re gonna see alot more o’ this kinda thing, believe me.

Ref: : Single Artificial-atom Lasing

The ball at the end of the solar system

Monday, October 1st, 2007

I know ya’ll think of Pluto as a barren, godforsaken excuse of a planet that ought to be reclassified as a lump of sawdust n’ spit. But that could change when the New Horizons spacecraft arrives at the solar system’s most distant minor second class could-do-better planet (or whatever Pluto is these days) sometime in 2015.

This week the arXiv hosts a series of papers by Alan “Right Hand” Stearn and the New Horizons team outlining the mission, its history and its scientific goals. The papers also include a detailed run down of the various cameras and sensors the spacecraft will use for its sniff ‘n’ lick study of Pluto and its three satellites.

And ah don’t mind tellin yer that Pluto looks a mite more interestin’ than ya’ll would have believed. Turns out Pluto has a variable atmosphere of nitrogen, methane and carbon monoxide that sublimes and condenses with the seasons, as if Pluto were breathin’. These gases interact with sunlight creatin’ a rich photochemistry that has colored the planet red.

Pluto’s largest satellite Charon is a different kettle o fish. Charon has no atmosphere to speak of and bears a striking resemblance to a giant ball of ice, which is probably what it is. Charon is so big that the centre of mass of the two body system lies outside Pluto. So they both circle about a central point. The two other satellites, Nix and Hydra, are so new (discovered in 2005) that nobody knows nothin about em yet.

After almost a decade en route, New Horizons will have only 200 days to take a good look at the Plutonian system. Should be excitin’, if yer can stand the wait.

And if all goes well with the Pluto encounter , New Horizons will continue its journey into the Kuiper Belt to look at one or two of them icy lumps out there (if NASA coughs up the readies, that is).

In the meantime, New Horizons has just finished a fly by of Jupiter which it used for a gravitational slingshot. Expect to see the data from this in the next few months.

Just one thing though fellas. New Horizons? What kind of a half-baked name is that for a spacecraft? Surely Right Hand Stearn can come up with something better than that. And with nothing else on his plate till 2015, he ain’t short of time.

References The New Horizons Pluto Kuiper belt Mission: An Overview with Historical Context New Horizons: Anticipated Scientific Investigations at the Pluto System. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on the New Horizons Mission The New Horizons Spacecraft

Quantum metamaterials: the next generation of superweird stuff

Thursday, September 13th, 2007

Ya’ll heard about metamaterials–that stuff they made invisibility cloaks outta at Duke University last year. It’s mighty strange stuff and it’s about to get a lot weirder in a quantum kinda way.

Metamaterials get their properties from their structure rather than their composition. So chuck a few capacitors, inductors and wires into an eggbox and you gotta material that gives electromagnetic waves some squeezin and a-bendin like nothin’ else on Earth. Metamaterials can bend light backwards, introduce a reverse Doppler shift and turn Cherenkov radiation on its head. But until now they’ve always bin made o’ proper classical bricks n’ mortar.

Now Alexander “Five Fingers” Rakhmanov, currently vacationin’ at the Institute of Physical and Chemical Research (RIKEN) in Japan, says it’s possible to build metamaterials outta quantum building blocks. He and a few pals have come up with a design in which two superconducting rails are connected at regular intervals by tiny superconducting junctions which store a current. These currents (and their direction) can be thought of as bits of quantum information or qubits.

So what ya got here is a line of qubits that can be placed in a quantum superposition.

Now here’s the interestin’ bit. Send an electromagnetic wave down this line and it interacts with superposition of qubits in some mighty fascinatin’ ways. Five Fingers Rakmanov says that by tweaking the qubits, the device can become a photonic crystal with a transparency bandgap that “breathes”, that is changes in size periodically. Or it can behave like an Archimedean Screw which modulates the incident wave and pumps the regions of maximum amplitude along the qubit line at any chosen speed.

That looks like one excitin’ piece o’ kit. Now it’s up to one o’ you guys to rush out ‘n’ build it.

Ref: Quantum Metamaterials: Electromagnetic Waves in a Josephson Qubit Line