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Home » Page 1749

Critical breakthrough

Andrew Huxley and colleagues at the CEA laboratory in Grenoble and the Grenoble High Magnetic Field Laboratory (GHMFL) cooled a sample of uranium rhodium germanium (URhGe) until it became a superconductor. In the absence of a magnetic field the superconducting transition temperature, Tc, was about 280 milliKelvin. As expected Tc became smaller as the magnetic field was increased to about 2 Tesla, and the superconducting properties disappeared above this field.

However, when Huxley and co-workers increased the magnetic field to 8 Tesla, the superconducting behaviour returned. Indeed, Tc reached a value of about 400 milliKelvin before the superconductivity disappeared again at about 13 Tesla. The Grenoble team also found that URhGe experiences a phase transition between two different magnetic states at a field of 12 Tesla.

The new work is part of a growing effort to understand the properties of quantum phase transitions and quantum critical points. Ordinary phase transitions – such as the melting of ice to form water, or the transition from ferromagnetism to paramagnetism – are caused by thermal fluctuations. However, the temperature at which these transitions occur can be changed by, for example, applying pressure.

This leads to the possibility of the transition temperature being reduced to close to absolute zero to produce a quantum critical point. Although the thermal fluctuations would disappear in this case, quantum fluctuations would remain and should be capable of inducing quantum phase transitions between different types of magnetic order or between normal and superconducting states. It is also possible that the quantum critical point might influence the room-temperature behaviour of these materials.

Novel forms of magnetic order have been observed at low temperatures and under high fields or pressures before, and superconductivity has been observed near quantum critical points in several materials. However, the latest result comes as a surprise because strong magnetic fields normally destroy the superconducting state.

The breakthrough was made possible, says Huxley, by the availability of high-quality single crystals of URhGe and high magnetic fields at the GHMFL. The D23-CRG instrument at the ILL neutron source in Grenoble was also used to probe the magnetic transition. The next challenge, he says, is to understand more about the superconductivity. For instance, is the same mechanism responsible for both superconducting regions in URhGe?

How the Earth spins

The inner core of the Earth is a dense ball of solid iron and nickel with a radius of about 1280 km that is surrounded by the outer core – a 2210-km thick layer of liquid metal. The next layer, the mantle, is made of molten rock and has a thickness of about 2850 km, while the topmost layer, the crust, is less than 100 km thick. It is thought that the Earth’s magnetic field is produced by the rotation of the outer core and mantle.

Just like electromagnetic waves, the seismic waves produced by earthquakes refract at boundaries between two different media. Therefore by detecting seismic waves that have been created on the other side of the globe, it is possible to extract information about the earth’s interior.

Zhang, Song and co-workers compared the seismic waves from “Earthquake doublets” – pairs of quakes which occurred at the same location but at different times. They studied 18 doublets in which the waves were created by quakes in the South Sandwich Islands in the South Atlantic Ocean and detected at 58 seismic stations in and near Alaska. The largest gap between the two quakes in any doublet was 35 years. Each quake results in a characteristic signal known as a “waveform”.

They found that when the waves did not pass through the inner core, both waveforms in the doublet were the same. However, if the waves had passed through the inner core and were more than four years apart, the waveforms were different. This means that something must have changed along the path followed by the waves during this period.

“This shows unambiguously that the inner core is moving relative to the mantle and crust,” says Song. The next challenge for Song and other geophysicists is to develop a better model of the inner core and to explore its rotation is more detail.

Making ice at room temperature

Kang and co-workers trapped the water in a nanometre wide gap between the gold-plated tip of a scanning tunnelling microscope (STM) and a gold surface. Previously it had been predicted that water would freeze above its normal freezing point if an electric field of 109 volts per metre was applied. However, the Seoul team found that the water froze in a much weaker electric field of just 106 volts per metre.

The team detected the ice through its effect on the vibrating tip of the STM. The tip was made to oscillate with a small amplitude and high-frequency as it was moved slowly toward the surface. The ice resists the motion of the tip much more than liquid water. Ice has been observed at room temperature before, but only under extremely high pressures.

“The discovery may affect our understanding of ice formation phenomena in diverse natural environments, such as within crevices in rocks and at biological and electrochemical interfaces,” Kang told PhysicsWeb. “Interfacial water freezing might also affect nano-sized electrical devices.”

Kang is now thinking about further experiments to determine how the combination of confinement and an electric field leads to freezing at such high temperatures.

Fibres control the speed of light

Over the past decade physicists have used exotic media such as ultracold atomic gases and various crystals to make “slow” or “fast” light. Some groups have managed to stop and store light, while others have demonstrated group velocities greater than the speed of light in vacuum.

Although some of these techniques have worked at room temperature, they have not been suitable for deployment in a fibre-optic network. Now, however, Miguel González-Herráez, Kwang-Yong Song and Luc Thévenaz of the École Polytechnique Fédérale de Lausanne (EPFL) have shown that slow and fast light can be made by exploiting an effect known as stimulated Brillouin scattering (SBS) in an ordinary optical fibre.

“Using this simple and flexible approach we have achieved nearly all the results obtained [previously by others] using atomic transitions”, says Thévenaz. “This experiment can be realized on a tabletop in normal environmental conditions so it could be a platform for the development of a wide range of applications in the real world.”

In addition to being more practical, says González-Herráez, who is based at the University of Alcalá in Spain, the fibre-based approach also has a bandwidth that is 10 times greater than that of the other approaches.

In stimulated Brillouin scattering (SBS) a “pump” laser can be used to amplify a “probe” laser at a slightly longer wavelength, with the difference in the energy of the pump and probe photons being equal to the energy of acoustic waves or vibrations called phonons in the fibre.

However, by adjusting the pump power and the wavelength of the probe beam, Thévenaz and co-workers found that the SBS effect could also increase or decrease the speed of the probe pulses. They managed to reduce the speed of light by a factor of 4.3 and, under different conditions, increase it by a factor of 1.4.

One problem with the current approach is that the strength of the probe pulse varies with its speed. The team also wants to increase the bandwidth even further.

Superlens breaks optical barrier

One of the best known properties of light is that it diffracts, bending or spreading around objects that lie in its path. A familiar example is when a collimated beam of light passes through a small aperture in an opaque barrier. If the aperture is large, the light emerges as a beam with the same radius as that of the aperture. But if the size of the aperture is similar to the wavelength of the incident light, the emerging light flares out from the aperture and forms a diffraction pattern whereby the intensity of the transmitted light has a broad central peak.

Diffraction restricts the resolution of microscopes and other optical devices to the wavelength of light used. To see why, imagine two widely spaced apertures that are illuminated by the same beam of light so that each aperture produces its own diffraction pattern. If the apertures are moved closer together, their diffraction patterns overlap until they eventually merge to form a single peak. The individual apertures can then no longer be resolved by observing the transmitted light. This unwanted effect is known as the diffraction limit.

As is often the case in physics, this simple picture is a little more complicated in practice because light that squeezes through a sub-wavelength aperture emerges in two portions. First there is a “far-field” portion that propagates away from the aperture and can be refocused by conventional lenses. Then there is a “near-field” portion that stays put, remaining localized around the aperture over a region less than a wavelength in size.

The near-field portion contains all of the sub-wavelength spatial details about an object, but it decays quickly as a function of distance from the object. Conventional optical devices are therefore unable to convey these finer details to an image. Instead, such instruments are constrained to recover as much of the far-field light as possible, limiting their resolution to roughly the wavelength of light.

But if a “perfect” lens existed that could recover all of the near-field and all of the far-field light, then an exact image of the object could be formed with perfect resolution. Even if the lens could recover only a fraction of the near-field light it would still produce an image with a resolution that would be well beyond what is currently possible. Two independent teams have now managed to build such a lens – dubbed a “superlens” – from a thin layer of silver. Their lenses, which work with visible light, can be used to image structures with a resolution as high as one-quarter the wavelength of the incident light.

Superlenses in theory and practice

The idea of a lens that can beat the diffraction limit was first proposed by John Pendry of Imperial College, London, five years ago. To the eye, Pendry’s superlens would look like nothing more than a thin planar film of material. However, there would be one crucial difference: the lens would have a negative refractive index. Pendry found that if the value of the refractive index, n, were just right (usually about −1) then the superlens could focus not just the far-field light but also some of the near-field light. This would not be possible if the material had a positive refractive index.

The near-field light can be pictured as a collection of “evanescent” waves that decay exponentially with distance from source. The waves that decay fastest convey the smallest spatial features, which means that the resolution of the image can be improved by detecting as many of the evanescent waves as possible. Pendry’s superlens would do this by reversing the decay of the evanescent waves, causing them to grow exponentially as they pass from one side of the superlens to the other, where they would decay once again and be recovered at the image plane (see figure).

So how do you build a superlens? Unfortunately, nature does not provide us with materials that have a negative refractive index. But nearly 40 years ago the Russian physicist Victor Veselago postulated that such a material could exist in theory, provided that its electric permittivity, ε, and magnetic permeability, μ, are both less than zero. The material’s refractive index, which is defined in this case as n = −√(εμ), would then also be less than zero. However, physicists had to wait until 2000 before an artificially structured composite material with a negative refractive index was developed for the first time. The material was designed so that both ε and μ were less than zero over certain microwave frequencies.

This breakthrough triggered an explosion of interest in the properties of artificial negative-refractive-index “metamaterials”. But although they can easily be designed to operate at lower frequencies, it is difficult to make such materials small enough to operate with visible light. In separate recent experiments, Steve Brueck and colleagues from Columbia University, and Vladimir Shalaev and colleagues from Purdue University managed to make nano-patterned structures that have a negative index at wavelengths of 1.5-2 μm, which is almost in the visible region of the spectrum. Unfortunately, these particular structures absorbed a lot of light and thus are not yet good candidates for a superlens.

While many materials have a negative electrical permittivity at optical wavelengths, it is not easy to ensure that μ < 0. To get round this problem, Pendry noted that a planar slab of material with ε = −1 could act as a partial superlens even if μ itself is positive. The material would then recover enough of the near-fields to beat the resolution of any conventional lens.

The two new experiments – one by Xiang Zhang and co-workers at the University of California at Berkeley (N Fang et al. 2005 Science 308 534) and the other by Richard Blaikie’s group at the University of Canterbury in New Zealand (D Melville et al. 2005 Optics Express 13 2127) – have independently confirmed the phenomenon of superlensing in the context of optical lithography. This new work comes on the heels of a demonstration of superlensing in a guided-wave structure at microwave frequencies by George Eleftheriades and colleagues at the University of Toronto, reported last year (see “Beating the diffraction limit” Physics World May 2004 pp23-24).

Great pictures

Both new experiments involve building a superlens from an evaporated thin silver film, the thickness of which is carefully selected to optimize the refocusing of evanescent waves (30-50 nm). This thickness is a trade-off; make the lens too thick, and losses start reducing the resolution; make the lens too small, however, and the experiment becomes impractical because the object has to placed at least a distance of d/2 from the centre of the lens, where d is the thickness of the film. Silver is a convenient choice for the superlens because it has relatively low losses at optical wavelengths. Moreover, ε = −1 for silver at ultraviolet wavelengths, where optical lithography and imaging can easily be performed.

To demonstrate superlensing, the two teams had to fabricate all of the essential elements of an optical system. These included an object, a dielectric spacer layer to provide an adequate working distance, the superlens itself, a further dielectric spacer layer and finally an image plane. The object in these experiments was not a lump of material, but instead was the light passing through various lines etched onto an opaque tungsten or chromium mask. The problem with imaging an object such as a piece of silicon is that you would have to find a way of illuminating the silicon while rejecting the incident light, which would be tricky. Here, the researchers did not have to worry about this because the mask blocked all of the unwanted light.

The features of the mask were written using either an electron or an ion beam, both of which create lines that are narrower than the 365 nm wavelength used for imaging. To view the image, a layer of photoresist was placed where the image was expected to form. The light from the mask exposes the photoresist, creating a sub-wavelength lithographic pattern that can then be imaged later, for example with an atomic force microscope.

The light that passes through the object mask undergoes diffraction. But if the superlens is inserted between the object mask and the image plane, then the resolution of the image is significantly improved due to the refocusing of evanescent waves. Blaikie and colleagues illustrated this improvement by comparing images of patterned gratings with progressively smaller periods. With the superlens present, gratings with periods as small as 120 nm – one-third that of the illuminating wavelength – could be imaged.

In addition to gratings, Zhang and co-workers also patterned the word “NANO” onto a mask, with each of the lines in the letters having a width of 40 nm. In the absence of the superlens, the pattern formed a fuzzy image at the image plane, with the imaged linewidths being roughly 320 nm. With the superlens in place, however, the image was considerably sharper, with the linewidths less than 90 nm – one-quarter of the incident wavelength.

The price of perfection

These two experiments demonstrate the phenomenon of evanescent-wave refocusing and prove that the superlens is much more than a theoretical curiosity. But superlensing does come at a cost because the object, lens and image are all very close together, spaced over a distance considerably less than one wavelength. In other words, to image the near-field it is necessary to remain in the near-field, otherwise those components of light that contain the sub-wavelength spatial details decay to the point that they cannot be recovered.

Still, the superlens is a conceptually new and intriguing device that has already altered long-held notions of optics. Moreover, superlensing may find practical applications in industries such as optical lithography or optical storage, where, among other things, it will enable manufacturers to fabricate devices on smaller scales.

John Bahcall dies

In 1964 Bahcall and Ray Davis Jr of Brookhaven published back-to-back papers in Physical Review Letters that essentially defined what became known as the solar neutrino problem. In the decades that followed Davis consistently measured less than half the flux of solar neutrinos that had been predicted by Bahcall. Either Bahcall’s theory or Davis’ experiment was wrong, or possibly both, or else neutrinos behaved in unexpected ways.

It turned out that both Bahcall and Davis were right – the electron neutrinos produced by the Sun were oscillating into muon neutrinos that did not show up in Davis’ detector. In 2002 Davis and Masatoshi Koshiba of the University of Tokyo shared the Nobel Prize for their work on solar neutrinos. It was widely expected that Bahcall would share a subsequent prize for the discovery of neutrino oscillations.

Bahcall published almost 500 papers on a variety of topics in astrophysics including quasars, ultra-high-energy cosmic rays and the composition of the Sun. He was also a member of many committees and chaired the panel that produced an influential report that set the direction for astronomy and astrophysics research in the US in the 1990s.

Bahcall was born in Shreveport, Louisiana, in 1934 and attended the University of California at Berkeley, the University of Chicago and Harvard, where he received his PhD in 1961. After periods at Indiana University and Caltech he moved to the Institute for Advanced Study in 1968, where he spent the rest of his career. He was married to the astrophysicist Neta Bahcall and had three children.

Nanotubes make perfect diodes

The feature sizes in conventional microelectronic circuits are getting smaller and smaller and look set to reach the limit imposed by the fundamental properties of silicon in a decade or so. The semiconducting properties of carbon nanotubes – rolled up sheets of graphite just nanometres in diameter – make them a promising alternative to silicon, and nanotubes have already been used to fabricate a variety of electronic components, including diodes and field-effect transistors.

Diodes are fundamental semiconductor devices that form the basic building blocks of many electronic devices, such as transistors and light-emitting diodes (LEDs). A diode is normally made by joining a p-type semiconducting material — which has been doped with impurities to add extra “holes” — to an n-type semiconductor that has an excess of electrons. However, it is almost impossible to do this in a carbon nanotube.

Last year, Ji Ung Lee of GE Global Research in New York solved this problem by using an electric field to create the p and n regions instead. He placed two separate gates underneath a single-walled nanotube so that one gate coupled to one half of the nanotube and the other gated coupled to the other half. By biasing one gate with a negative voltage and the other with a positive voltage, he created a p-n junction that behaved as an almost ideal diode.

Lee made his device using standard optical lithography techniques and placed the carbon nanotube on top of a silicon dioxide substrate, which acted as the gate dielectric. Now, he has transformed this structure into an ideal diode by simply suspending the nanotube across two silicon dioxide slabs (see figure). According to Lee, the nanotube no longer interacts with the surface on which it rests which means that extra states that can lower the device’s performance are not created. It may also be possible to make the diode operate as an LED.

“My results are not only a direct confirmation of the structural purity of single-walled nanotubes but a strong affirmation of their potential as electronic materials,” says Lee. He now plans to further investigate the optical properties of the nanotubes and make a photodetector.

Photonic crystals move into new areas

Photonic crystals are materials in which a periodic variation of the dielectric constant results in a photonic band-gap. This means that photons with wavelengths or energies in this band-gap cannot travel through the crystal. Quasicrystals, on the other hand, are materials that do not have a periodic lattice structure but still display subtle long-range order not possible in regular crystals. Researchers have already made one- and two-dimensional quasicrystals that have photonic band-gaps. Now Paul Steinhardt, Weining Man, Paul Chaikin and Mischa Megens of Princeton University, New York University and Philips Research Laboratories in Eindhoven have performed this feat in three-dimensional quasicrystals for the first time (Nature 436 993).

Deciding that it was too difficult to explore the problem theoretically or computationally, Steinhardt and co-workers decided to make a 3D icosahedral quasicrystal and see if it demonstrated a band-gap. They joined several thousand one-centimetre long plastic rods into a diamond crystal array – a structure known to have a photonic band gap – and shone microwaves at it. They found that light is trapped by the structure in certain planes, and that these planes enclose an almost spherical, 30-sided polyhedron called a triacontahedran (figure 1).

According to Steinhardt, quasicrystals are ideal candidates for making photonic band-gap structures that could be used in a variety of applications in computation and communication. The team is now exploring three different areas of research: the use of optical tweezers to make materials that have band-gaps at optical rather than microwave wavelengths; the use of rods spheres and surfaces – as opposed to just rods – to make the materials; and exploring different types of photonic, electronic and even acoustic applications.

Figure 2
Figure 2

Meanwhile, Evgenya Smirnova and colleagues at the Massachusetts Institute of Technology (MIT) have used photonic crystals to accelerate particles for the first time. This result could help researchers make a table-top particle accelerator that is capable of accelerating particles into the TeV range (Phys. Rev. Lett. 95 074801).

Smirnova and colleagues made a two-dimensional photonic crystal with a periodic lattice of metal rods and then converted it into a waveguide by removing a rod from the centre of the lattice (figure 2). When the waveguide was placed in a test beam at the MIT accelerator laboratory and powered with 2 megawatts of microwaves it increased the energy of a 16.5 MeV electron beam by 1.4 MeV. The electrons were accelerated as a result of interactions with high-frequency microwaves inside the waveguide.

The main advantage of the new approach is that it suppresses unwanted “wake fields” that are created by the electron beam. Wake fields are a problem for conventional accelerators because they lead to beam loss.

“Our photonic band gap waveguide is designed so that is only supports the particle accelerating modes and not any wake fields, thus making highly efficient acceleration possible,” says Smirnova. The MIT team now plans to build a bigger stand-alone photonic crystal accelerator and power it with more microwaves to increase the acceleration.

Number theory

While the number of papers published by a scientist provides a measure of their productivity, it says nothing about the quality of their work. The number of citations received by a scientist is a better indicator of quality, but co-authoring a handful of articles that are cited widely could “inflate” the reputation of a scientist. Hirsch says that his new approach overcomes these problems. A scientist with a h-index of 10, say, will have published 10 papers that have received at least 10 citations each. The best researchers should therefore have the highest h-indexes.

“A high h is a very accurate indicator of scientific achievement,” says Hirsch. “I have looked at the h-index of many physicists in the subfields I am familiar with and have found that there is a very strong correlation between scientists for whom I have a high regard and their high h.”

Hirsch says that it only takes a few seconds to find the h-index for a scientist – providing they don’t have a common name – on the ISI Web of Knowledge database. For example, the physicist with the highest h-index is the string theorist Edward Witten of the Institute for Advanced Study in Princeton, who has a h-index of 110. This means that Witten has published 110 papers with at least 110 citations each.

Other highly ranked physicists include: Marvin Cohen (94), a condensed matter theorist at the University of California at Berkeley; Philip Anderson (91), a condensed matter theorist at Princeton University; Steven Weinberg (88), a particle theorist at the University of Texas at Austin; and Michael Fisher (88), a mathematical physicist at the University of Maryland (88).

Hirsch, who has a h-index of 49, says that a “successful scientist” will have an index of 20 after 20 years; an “outstanding scientist” will have an index of 40 after 20 years; and a “truly unique individual” will have an index of 60 after 20 years. Moreover, he goes on to propose that a researcher should be promoted to associate professor when they achieve a h-index of around 12, and to full professor when they reach a h about of 18.

However, Hirsch recognizes that the average h-index might be different for different subfields of physics: “One should make sure one knows what the typical values in each subfield are if one is comparing individuals from different subfields,” he says.

The ups and downs of doping

Doping generally involves adding impurities or charge carriers – which can be electrons or “holes” – to inert materials. The challenge is to produce the required electronic properties in the material with the dopant atoms, which are randomly distributed, without causing electronic disorder.

Davis and colleagues at Cornell, the University of California at Berkeley, the AIST laboratory in Tsukuba and the University of Tokyo studied crystals of bismuth strontium calcium copper oxide (Bi2Sr2CaCu2O8+x), also known as Bi-2212. This material is normally an insulator but it becomes a superconductor when extra oxygen atoms – which are a source of holes – are added. Superconductors lose their resistance to electric current when they are cooled below a transition temperature, Tc, that varies with the amount of doping. The basic phenomena underpinning superconductivity is the formation of Cooper pairs by the charge carriers.

Physicists have long suspected that dopant atoms lead to electronic disorder in Bi-2212 but there was no experimental evidence. Now Davis and co-workers have used a high-energy scanning tunnelling microscope (STM) to show that this disorder is caused by atomic-scale impurity states, and to show that it is highly likely that these impurity states are actually the dopant atoms. “If so,” says Davis, “the doping process, although necessary to create superconductivity, also damages it near the dopant atom. The way this damage is caused is also completely different to what we expected and to what happens in conventional superconductors.”

The results agree well with calculations performed by Peter Hirschfeld and colleagues at the University of Florida, which showed that the dopant atom distorts the “cage” of atoms around it and therefore changes the local electronic structure. Hirschfeld says that this could lead to an observable change in the local pairing interaction which, in turn, would lead to a change in the superconducting gap – the energy needed to break up the Cooper pairs.

Davis believes that better superconductors could be made by controlling the location of the dopant atoms. Indeed, two of his co-workers – Hiroshi Eisaki and Shin-ichi Uchida – have already increased the Tc of Bi-2212 to almost 100 Kelvin by minimising the disorder in the strontium-oxygen layer. This work could also help in the search to find a theory of high-temperature superconductors.

In conventional superconductors the formation of pairs and the onset of superconductivity are closely related. In the cuprates, on the other hand, they are quite independent. “The results in the Science paper gives us a hint about the mechanism responsible for the onset of superconductivity,” says Uchida. “They indicate the presence of a parameter – in addition to the doping concentration and independent of the pairing mechanism – that is sensitive to the disorder or small changes in the environment around the copper oxide plane on the atomic scale.”

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