Press releases
Please quote Nature Physics as the source of these items.
The October 2005 issue of Nature Physics is available online.
October 2005
What's shape got to do with it?
Whether or not a material will superconduct depends on its size and shape, report Michael Hermele and co-workers in the November issue of Nature Physics.
If you take a superconductor and simply change its shape, surely it would still carry a current without any resistance? In their theoretical paper, the authors considered two thin-film superconductors connected at a point, in a shape reminiscent of a bowtie. Instead of the expected 'supercurrent' flow, they found that superconductivity could only exist at zero temperature. This means even at the lowest attainable temperature, such thin films will have finite resistance.
Their result is surprising, given that for a larger point of connection, the resulting circuit is at the heart of the most sensitive device for measuring magnetic fields (superconducting quantum interference devices, or SQUIDs). Now that devices are shrinking in size, it will be important to bear this work in mind.
Fate of the Josephson effect in thin-film superconductors pp 117 - 121
Michael Hermele, Gil Refael, Matthew P. A. Fisher and Paul M. Goldbart
Published online: 30 October 2005 | doi 10.1038/nphys154
Spintronics take the next logical step
MP3 players and iPods wouldn't exist if not for the application of 'spin valve' read-heads – in which an electron's spin rather than its charge is manipulated – a technique that has revolutionized the performance of computer hard disk drives. In the November issue of Nature Physics, Christian Schönenberger and colleagues describe a carbon nanotube transistor that operates on a similar principle, opening up a promising avenue towards the introduction of spin-based devices into computer chips.
Conventional electronic circuits in computer chips process information by encoding it in the form of electronic charge. But this is not the only way. The spin of an electron can in principle be used to perform all the same operations as charge, which is one of the key aims of the emerging field of spin-electronics, or spintronics. The putative advantages of spin-based circuits over charge-based circuits include lower power consumption and heat generation, higher speed, smaller devices, and most importantly, the potential to do things, such as quantum computation, that conventional electronics can't.
The authors have developed a device that consists of a single carbon nanotube connected to two magnetic electrodes – which control the orientation of the spins of the electrons injected into the device. Below this, they placed a third isolated magnetic electrode – which controls the passage of injected spins through the device. Theoretical proposals for constructing so-called 'spin transistors' have been around for many years, but this is the first time that such a device has been convincingly realized.
Electric field control of spin transport pp 99 - 102
Sangeeta Sahoo, Takis Kontos, Jürg Furer, Christian Hoffmann, Matthias Gräber, Audrey Cottet and Christian Schönenberger
Published online: 23 October 2005 | doi 10.1038/nphys149
Dark spins light up
Want to see a diamond? Forget the jewellery store – try a physics laboratory. In the November issue of Nature Physics, Ryan Epstein and colleagues demonstrate the power of their microscope for imaging individual nitrogen atoms that sit at vacant sites in the diamond structure. Such 'vacancy' centres have a long lifetime within the diamond host and could be used as the basis for a room-temperature quantum computer.
Because of the potential application as a bit of quantum information, the single magnetic spin (pointing up or down) associated with the extra electron of a nitrogen atom has featured in many different experiments. The latest involves a room-temperature microscope that detects light emitted by a nitrogen vacancy centre. Through their precise control of the alignment of the magnetic field, the researchers can also detect local non-luminescing impurities that couple to the nitrogen vacancy centres. The vacancy centres light the way to neighbouring 'dark' spins that normally would not be detected. These dark spins have a longer life-time than that of the vacancy atoms, and could be potentially more useful for applications involving quantum information processing.
Anisotropic interactions of a single spin and dark-spin spectroscopy in diamond pp 94 - 98
R. J. Epstein, F. M. Mendoza, Y. K. Kato and D. D. Awschalom
Published online: 16 October 2005 | doi 10.1038/nphys141
Atoms almost at a standstill
Using laser beams and a trap made from mirrors, Gerhard Rempe and colleagues can catch single atoms and cool them down to the point at which they nearly stand still. As they report in Nature Physics, using this technique they can trap a well-controlled number of atoms for tens of seconds – which is very long compared with the trapping times of fractions of a second that have so far defined the state of the art. This advance could prove important for the further exploration of how atoms and molecules behave in the quantum world.
These experiments constitute the latest progress in what is known as 'cavity quantum electrodynamics', concerning the trapping of particles in tiny cavities between two highly reflecting mirrors. Such settings enable exquisite control over the interactions between light and matter at the level of single atoms and single photons. The long trapping times now realized mean that, for practical purposes, single atoms can be permanently confined to the cavity – an attractive property with regard to exploiting few-atom systems for fundamental studies and for applications.
Vacuum-stimulated cooling of single atoms in three dimensions pp 122 - 125
Stefan Nußmann, Karim Murr, Markus Hijlkema, Bernhard Weber, Axel Kuhn and Gerhard Rempe
Published online: 9 October 2005 | doi 10.1038/nphys120
Castles made of sand pp 50 - 52
The perfect recipe for building a sandcastle is eight parts sand to one part water. In the October issue of Nature Physics, Arshad Kudrolli and co-workers show how water stabilizes the sand. Aside from sandcastles, wet grains capture the physics of a wide variety of problems ranging from wet milling to debris flow.
The team performed a series of stability measurements using transparent rotating drums. Using different grain sizes, drum diameters and liquids, they recorded the angle of the drum before and after an avalanche. Moreover, the authors propose a liquid-bridge model that does not require any friction between the grains. Their model includes both surface and bulk effects of the sandpile — a hybrid approach that settles years of inconsistent results due largely to different system sizes.
When a sandpile fails, clumps rather than individual grains tend to fall. Those clumps have a characteristic size, or coherence length, points out Peter Schiffer in the accompanying News & Views article. Could this be a connection to the collective behaviour found in traditional hard condensed-matter physics?
Maximum angle of stability of a wet granular pile pp 50 - 52
Sarah Nowak, Azadeh Samadani and Arshad Kudrolli
Published online: 29 September 2005 | doi 10.1038/nphys106
Matter-waves split on a chip pp 57 - 62
A simple device that could aid in the development of ultraprecise gyroscopes and other sensing equipment is described by Peter Krüger and colleagues in the October launch issue of Nature Physics. They manipulate something called a Bose-Einstein condensate – an exotic form of matter that emerges when the atoms or molecules in an ultracold cloud of gas condense into a quantum state, in which they collectively behave as a single, unified whole. The authors' device splits the condensate into two halves, without affecting the phase – a property related to the wave-like nature of Bose-Einstein condensates that governs how they behave and especially how they interact with other condensates.
The authors make their device using conventional lithographic techniques developed by the microelectronics industry for making silicon chips. This means it can be easily integrated with other components designed for manipulating the matter-waves of a Bose-Einstein condensate on a so-called atom chip. The device can also control the distance between the two halves of the condensate, meaning that they can either be moved apart so that each is isolated from the other, or brought close enough together for them to interact. By enabling matter-waves to be manipulated in this way without altering their phase, the device significantly improves the prospects for using atom chips in both fundamental studies and practical applications.
Matter-wave interferometry in a double well on an atom chip pp 57 - 62
T. Schumm, S. Hofferberth, L. M. Andersson, S. Wildermuth, S. Groth, I. Bar-Joseph, J. Schmiedmayer and P. Krüger
Published online: 29 September 2005 | doi 10.1038/nphys125
