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Lithium feels the pressure to become a superconductor

At ambient conditions the conduction electrons in lithium the first metal in the periodic table are only weakly perturbed by its atomic cores, which are arranged in a highly symmetric body-centred cubic lattice. For many years it was thought that lithium would remain metallic at high pressures, retaining some form of highly symmetric cubic structure. However, in 1999 Jeffrey Neaton and Neil Ashcroft of Cornell University predicted that lithium would undergo various structural phase transitions as the pressure was increased, leading to a “paired atom” or molecular phase with low symmetry. Moreover, the electronic properties of this phase are expected to be close to those of an insulator.

A year later X-ray diffraction experiments by the current author and co-workers showed that at pressures near 40 GPa (400 000 atmospheres) lithium transforms from a face-centred cubic phase, via an intermediate rhombohedral phase, to a cubic polymorph with 16 atoms per unit cell called cI16 (see figure). This structure can be viewed as a 2 x 2 x 2 super cell of a body-centred cubic structure but with the atoms displaced diagonally.

Calculations of total energy show that this structure is stable with respect to all the known structures for metals to pressures of at least 165 GPa. Moreover, the decreased symmetry caused by the diagonal displacement of the atoms changes the Fermi energy in a way that means lithium is no longer a simple metal. Although this cI16 structure was different from the “paired atom” structure proposed by Neaton and Ashcroft, it provided further evidence that lithium adopted low-symmetry structures at high pressure.

Experiments under pressure

The existence of low-symmetry phases at high pressure immediately poses the question of superconductivity in lithium. The only known superconducting alkali metal is caesium, which becomes superconducting at 12.5 GPa with a critical temperature of about 1.5 K. At this pressure it transforms into a low-symmetry orthorhombic phase with 16 atoms per unit cell. Transitions of the heavy alkali metals (caesium, rubidium and potassium) to low-symmetry structures are driven by electronic transitions in which delocalized s-valence electrons become more localized d-valence electrons. Similar transitions in lithium and sodium are driven by transitions in which s-electrons become p-electrons.

This possibility of pressure-induced superconductivity in lithium was initially studied theoretically. In 2001 Niels Christensen and Dmitri Novikov predicted that lithium would become superconducting at high pressures and that the critical temperature would reach 50-70 K in the region of the face-centred cubic to cI16 transition, and could be as high as 60-80 K in the cI16 phase.

Two experimental groups have now observed superconductivity in lithium at high pressures, albeit with lower critical temperatures than predicted by theory. Both groups used so-called diamond-anvil cells to compress the lithium.

Katsuya Shimizu of Osaka University in Japan and co-workers measured the electrical resistance of lithium as a function of temperature and observed a sudden drop in the resistance when the sample was cooled below 7 K at a pressure of 30 GPa (K Shimizu et al. 2002 Nature 419 597). The researchers attribute this drop in resistance to a transition to a superconducting state. Moreover, this transition temperature increases with pressure to reach a maximum of 20 K at 48 GPa. The Japanese team also demonstrated that the effect is suppressed by a magnetic field and disappears completely for fields above 3 T. This is generally considered as strong experimental evidence for superconductivity.

Shortly afterwards these results were confirmed by a team at the Carnegie Institute of Washington in the US that studied both the electrical resistance and the magnetic susceptibility, which is considered the ultimate evidence for superconductivity (V V Struzhkin et al. 2002 Science 298 1213). The critical temperatures from both methods were in good agreement.

The Carnegie researchers found that the critical temperature increased from 9 K at 23 GPa to about 15 K at 36 GPa, and they attribute this to the face-centred cubic structure. The critical temperature remained constant at about 16 K for pressures between 40 and 70 GPa, which they suggest is due to the cI16 phase.

The results of the Osaka and Carnegie experiments are in good agreement and the small differences between them are probably due to way the lithium was confined in the diamond-anvil cell. Lithium is very difficult to study at high pressure because it reacts with both the diamond anvils and the gasket material surrounding the sample. Isolating the electrodes that are used to measure the resistance from the gasket, which is normally made of metal, makes the experiments even more complicated. While the Osaka group confined the lithium sample in a small pit drilled in one of the diamond anvils with a laser, the Carnegie team used a metallic but nonmagnetic gasket for its susceptibility measurements.

The results also suggest that a sudden drop in the electrical resistance of lithium observed in an experiment at the University of California at Los Angeles in 1986 was due to a superconducting phase transition. Tzer-Hso Lin and Keh-Jim Dunn observed the drop at a temperature of 7 K and pressures between 22 and 32 GPa.

Theory under pressure

Lithium brings the number of elements that superconduct under pressure to 23, which is close to the number of elements that are superconductors at ambient pressure (29). Among the high-pressure superconductors are such unlikely candidates as oxygen and sulphur, and even iron becomes superconducting in a small pressure range. However, the maximum critical temperature observed in lithium (between 16 and 20 K) is among the highest observed for any element, and attention will now turn to the other alkali metals.

Rubidium and potassium have the same low-symmetry structure as caesium, only at higher pressures, while sodium follows a sequence of phases that is similar to lithium, again at higher pressures. The discovery of superconductivity in lithium and its explanation will also have important implications for superconductivity in highly compressed metallic hydrogen. Hydrogen is the only element in the first group of the periodic table that is not a metal at ambient pressure (see Physics World July 1995 pp4347).

The discoveries of low-symmetry phases and superconductivity in lithium are good examples of the intensive interactions between theory and experiment in modern high-pressure physics. Theoretical predictions stimulate experimental studies, which result in the need for more theoretical work, which in turn leads to further experimental measurements. However, the agreement between theory and experiment is not always perfect. The “paired atom” phase has not been observed experimentally and the critical temperatures measured by the Osaka and Carnegie teams are lower than the theoretical predictions by a factor of three or four. The challenge is now on for the theorists to refine their calculations and explain the latest experimental results.

Finding the flaw in falsifiability

As a philosopher of science, I should honour my field’s heroes, especially those recognized by outsiders. I should, in particular, cherish those whose doctrines are understood and valued by scientists themselves. Thus, 2002 should have found me busy celebrating the centenary of the birth of Karl Popper. However, I was unenthusiastic.

Popper, who died in 1994, is hailed on one Web page as “the most important philosopher of science since Francis Bacon” (1561-1626). Cosmologist Frank Tipler has called Popper’s The Logic of Scientific Discovery “the most important book of its century”. Popper’s views strongly shaped the ideas of art historian Ernst Gombrich, and his ideas are cited in a landmark decision by the US Supreme Court, Daubert versus Dow Pharmaceutical, on the legal procedures for the admission of scientific expert testimony in court. Popper is also the only philosopher of science who is a household name among scientists.

Most can state the essentials of his chief teaching the “principle of falsifiability”. This is the idea that science progresses by “conjectures and refutations” and not (as Bacon suggested) through “inductive generalizations”, in which one generalizes on the basis of a sample. Popper, in other words, thought that a theory cannot be proved right, only wrong. A theory becomes scientific by exposing itself to the possibility of being proved incorrect.

This principle seems to solve at a stroke what philosophers of science call the problem of demarcation, or how to draw the line between science and non-science. “The criterion of the scientific status of a theory is its falsifiability, or refutability, or testability,” wrote Popper in Conjectures and Refutations in 1963. Einstein’s relativity theory, for example, is scientific, while astrology,

Marxism, psychoanalysis and new-age systems are not. The problem with such dogmas and ideologies, for Popperians, is not that they cannot be confirmed, but that they find confirmation everywhere. They are unscientific because they do not offer themselves up to potential refutation.

Popper’s principle is beloved by crusaders against junk- and pseudo-science, for it simplifies demarcation. But, however attractive Popper’s falsifiability principle might sound, it is not good philosophy of science.

Describing science neither in practice…

The history of science is replete with examples that show Popper’s principle to be wrong. Consider the scientific community’s response to an experiment that the physicist Dayton Miller conducted in 1925. Miller attempted to repeat Albert Michelson and Edward Morley’s famous experiment that showed that the speed of light is constant, regardless of the velocity of the source a result that lies at the foundation of Einstein’s theory of relativity. Miller, however, found a slight difference in speed. He reported this result to a meeting of the American Physical Society (APS), and interpreted it as a refutation of Einstein’s theory.

But was it? The APS’s members did not think so. Hundreds of other experiments agreed with Michelson and Morley’s work, and relativity was already tightly woven into contemporary science. Evidently, a gut feeling was telling the sceptical professionals that something was amiss with Miller’s results. Miller did find some champions. One was the Soviet academician A K Timiriazev, who was seeking to destroy Einstein’s influence among Soviet colleagues. William Broad and Nicholas Wade, meanwhile, have argued that scientists should have taken Miller’s apparent falsification more seriously. In their 1983 book Betrayers of the Truth: Fraud in Science, they looked at why the APS audience refused to consider Miller’s work as a refutation of Einstein. According to Broad and Wade, who are journalists, the physicists’ refusal was evidence of incompetence and unprofessional conduct. “[T]he audience”, they write, “should instantly have abandoned the theory or at least assigned it to provisional status.”

But to have suspended Einstein’s achievement because of a single contrary experiment would have been irrational and unscientific. It was the ideologues those with antiscientific axes to grind who were insisting that a theory should be tossed out because of a falsification. The principle of falsifiability can thus promote a damaging image of science. For even newspaper accounts of routine scientific work can turn up discrepancies between Popper’s picture and actual practice, implying that scientists are bunglers and frauds.

…nor in theory

Popper, I know, would say that we should pay close attention to claimed results, such as Miller’s, because they might be falsifications. And Popper knew that claimed falsifications are not necessarily real. The principle of falsifiability was not meant to be a description or a “recipe” for science. For him, the falsifiability criterion was not itself falsifiable. It is a methodological principle a philosophical test or model of what science would look like if reconstructed in logical terms.

But here, too, it fails. Theoretical “guesses” and experiments to test them are based on assumptions that we inherit from the entire past history of science. What shows up in the laboratory may not merely confirm or falsify the guesses, but rather call into question the background assumptions from which the guesses arise, forcing us to review and rethink the assumptions

In this extremely important interpretative process, a scientist must judge what is reliable and promising, and what is not. This is why great scientists are often people of strong purpose, whose very obtuseness and reluctance to be distracted by contrary empirical evidence in this interpretative process is a source of their success.

The critical point

Science is not a robotic process of conjecture and refutation. It involves the ability to call into question inherited assumptions that are elements of our background framework, thereby opening up possibilities that could not have been foreseen at the start. What we do in laboratories is both inquiry into nature and self-inquiry. Those efforts put our guesses about nature to the test and force us to reinterpret the assumptions on which these guesses are made.

It is tempting to seek a simple single principle with which we can grasp the essence of science and toss out all else. Unfortunately, science is too complicated for that.

A very strange year

The year has been dominated by the story of scientific misconduct at Bell Labs in the US (see Physics World November pp6-7 and pp17-18). Last month Jan Hendrik Schön and co-authors retracted eight papers reporting high-profile research results on the electronic properties of organic materials, and more papers are sure to be withdrawn. The retractions followed the publication of a report by a panel that Bell Labs had established to investigate the affair. The panel found Schön guilty on 16 out of 24 charges of misconduct and he was promptly fired by the company. All of his co-authors were cleared but serious questions were raised about their responsibilities.

The panel’s report is probably the most fascinating document published in physics this year, and its findings are starting to filter through to the physics community. The American Physical Society, for instance, has just released a set of statements on professional conduct including guidelines on the responsibilities of co-authors and collaborators and policies for handling allegations of research misconduct (www.aps.org). Schön was not the only physicist to be fired for misconduct this year: in June, Victor Ninov was dismissed by the Lawrence Berkeley National Laboratory after a committee found that he had fabricated data concerning the discovery of what would have been the heaviest element ever (element 118).

Compared with these two episodes, the Bogdanov affair is fairly harmless. The University of Bourgogne might want to look more closely at its processes for awarding PhDs and referees might need to be more rigorous on occasion, but there is no evidence that anyone has radically changed the direction of their research to follow the Bogdanovs. Indeed, some good might come out of the affair as more outré theories become subject to reality checks more often.

But there were plenty of physics highlights in 2002 as well the production of large numbers of cold antihydrogen atoms and the first measurements of the polarization of the cosmic microwave background were the results of heroic experimental efforts. Optical physics firmly entered the attosecond regime, quantum cryptography moved ever closer to real-world applications, and research on degenerate quantum gases continued to move forward at a rapid pace with both bosons and fermions. Researchers in Australia managed to beat the second law of thermodynamics for short times at least and physicists in Grenoble observed bound quantum states for neutrons moving in a gravitational potential.

Meanwhile, as X-ray astronomers revelled in the floods of data from Chandra and XMM-Newton and a Nobel prize for Riccardo Giacconi other “new windows” opened on the universe. Gamma-ray astronomers can look forward to data from the Integral satellite and the HESS array in Africa, and there was significant progress in large facilities dedicated to neutrino and gravitational-wave astronomy. Of course, the capabilities of optical and infrared telescopes also continued to improve. The strangest event in astronomy was the decision to name the next-generation space telescope which is due to replace the Hubble telescope in 2010 the James Webb Space Telescope in recognition of a previous NASA administrator.

Finally, female physicists from around the world met in Paris in March for the first international conference on women in physics. Reports from the meeting are now being followed up by action plans and concrete targets. The Institute of Physics, for instance, has set the ambitious target that women should account for 40% of physics undergraduates and new university appointments in the UK by 2015. Strange as such a world would seem compared with today’s male-dominated physics community, it would certainly be an achievement worth celebrating.

Antiprotons probe hyperfine splitting

Widmann and co-workers have previously observed hyperfine splitting within anti-protonic helium. This is caused by the magnetic interaction between the orbital angular momentum of the anti-proton and the spin of the electron. The latest results provide a more accurate measurement of this phenomenon and also confirm the theoretical prediction made by Dimitar Bakalov of the Bulgarian Academy of Sciences in Sofia and Vladimir Korobov of the Joint Institute for Nuclear Research in Dubna that the two levels in each hyperfine “doublet” are themselves split into two further sublevels. This superhyperfine effect arises from the weaker magnetic interaction between the spin of the anti-proton and the other angular momenta. In the two sublevels within each half of the doublet, the electron spins are parallel and the anti-proton spins antiparallel.

The researchers created antiprotonic helium atoms by directing pulses of antiprotons from CERN’s Antiproton Decelerator into helium gas. Initially the two levels in each hyperfine doublet were equally populated. Then by firing a laser pulse into the helium the researchers were able to reduce the population of the lower level relative to the higher level by sending antiprotons in atoms lying in the lower level into atomic states from which they instantly annihilated. They then sent a microwave pulse into the gas to stimulate repopulation of the lower level, and used a second laser to measure the new population of the lower level.

The frequency of the microwave pulse was chosen to match the two theoretically allowed transitions that occur between sublevels with the same anti-proton spin in different halves of the doublet. By recording a repopulation of the lower half of the doublet, the researchers verified that these transitions do indeed occur and, therefore, that anti-protonic helium atoms undergo hyperfine splitting. Their results agreed with theory to a level of six parts in 10⁵. They also confirm the expected superhyperfine splitting with a precision of 1.6%.

Physics teaching comes down to earth

King and Kennett, who are members of the Earth Science Teachers’ Association (ESTA) say that there is a clear need for better Earth Science teaching and that it should be linked to the traditional science disciplines of physics, biology and chemistry. The Earth provides a variety of contexts for teaching because it helps students, particularly girls, relate physics concepts to the ‘real world’ around them.

The Institute of Physics, the Royal Society of Chemistry and the Institute of Biology plan to work with the ESTA in the Joint Earth Science Education Initiative (JESEI). The JESEI material is still being written but the main areas that could benefit from using Earth Science in their teaching include:

Electricity and magnetism – teaching circuits and magnetic fields using the Earth’s magnetic field and remanent magnetization preserved in rocks.
Forces and motion – teaching mass and weight, friction, force and pressure using the Earth’s gravitational field, earthquakes, oceans, the atmosphere and lithosphere.
Light and sound – teaching reflection using albedo effects on temperature.
Waves – teaching reflection and refraction, the electromagnetic spectrum and energy transfer, sound and ultrasound using coastal and seismic waves and the greenhouse effect.
Radioactivity – teaching radiation sources and ‘half-life’ using the dangers of radon, radioactive heat energy in the Earth and half-life simulation.

The review refers to the National Curriculum for Science in England but can easily be adapted for many countries around the world say King and Kennett.

Light powers artificial membrane

Synthetic systems that transport metal ions across membranes made of water-insoluble lipid molecules are well known. The ions are transported by a carrier molecule located in the membrane. The molecule binds a metal ion from an aqueous solution on one side of the membrane and releases it on the other. The driving force behind this transport is the difference in the ion concentration between the two sides of the membrane.

The artificial membrane developed by Gust and Moore uses a very different approach. In their membrane, the transport is driven not by concentration gradients but by light. Their system consists of a bilayer lipid membrane containing “shuttle” molecules that are soluble within the membrane but not in the aqueous solution on either side.

This shuttle molecule – through the addition and removal of electrons – binds calcium ions at the outer surface of the membrane. The molecule then takes the ions across the membrane and releases them at the inner surface of the membrane. The ions, which cannot remain in the lipid environment, then enter the aqueous solution inside the cell.

The transport process is controlled by a special molecule in the membrane known as an “artificial reaction centre”. Based on molecules used in biological photosynthesis, the reaction centre molecule is placed across the membrane and donates and reabsorbs electrons at opposite ends in response to light.

‘Cellular machines’ such as this could be used as ‘nanofactories’ that transport reactants in, and products out, of a biological membrane. There may also be applications for converting solar energy into electrical current.

Magnets double up

‘Exchange-spring magnets’ are promising candidates for advanced permanent magnetic applications, such as recording and storage devices, because they have a large energy product -the figure of merit for a magnet’s strength. A high energy product requires the material to have a large magnetization and a large ‘coercivity’ – the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero.

Exchange spring magnets contain a magnetically hard phase, which has a high coercivity, and a soft phase, with low coercivity. These two phases interact by ‘exchange coupling’. The hard phase provides high anisotropy and the soft phase high magnetization. For the exchange coupling to be effective, however, the hard and soft phases must be controlled at the nanometre scale, which can be difficult.

Now, Hao Zeng and co-workers at the IBM TJ Watson Research Center in New York, with colleagues at Louisiana Tech University and the Georgia Institute of Technology have devised a novel nanoparticle self-assembly method. They use nanoscale iron-platinum and iron oxide (Fe₃O₄) particles as the ‘building blocks’ in the assembly. The components are mixed together and allowed to self-organise.

Optimum exchange coupling, and therefore the maximum energy product, can be obtained by changing the size and composition of the individual building blocks. The energy product of this two-phase material is 20.1 mega gauss oersteds, which is over 50% higher than the value for conventional iron-platinum magnets.

The workers now plan to compress this material to make high-density magnets and to improve the alignment of the axes of the hard phase grains to increase the magnetization value of the composite. They also hope to look at other magnetic materials such as samarium cobalt and neodymium iron boride.

Semiconductor lights up solar cells

A fundamental limit to the efficiency of a solar cell is the band gap of the semiconductor from which it is made. The band gap is the energy difference between the conduction and valence bands in a semiconductor. It is difficult to find a single semiconducting material that can match the broad range of energies found in solar radiation. Light below the band gap of the semiconductor is not absorbed, while light above the band gap is absorbed but the excess is lost as heat.

Higher efficiencies can be achieved by using stacks of different semiconductor materials – the higher gap materials convert the most energetic photons into electrical current, leaving the lower energy photons to pass through to the lower gap materials. The highest efficiency observed to date with these types of systems is about 30%. These systems have a maximum theoretical efficiency of 70% when many different semiconductor layers are stacked on top of each other, but problems related to mismatches between the layers can destroy the optical properties of the device.

Now Wladek Walukiewicz and co-workers have measured the optical properties of pure indium nitride and a wide range of alloys made of indium, gallium and nitrogen. They find that the band gap can vary between 0.7 and 3.4 eV, which covers the entire solar spectrum.

The researchers believe their results to be more reliable than previous results because they used higher quality samples grown with epitaxial techniques. Although these samples were grown on lattice-mismatched substrates, they still have strong optical properties. Indium nitride materials seem to be able to accommodate large lattice mismatches without much effect on their optoelectronic properties.

The team also found that alloys made from indium, aluminium and nitrogen had an even wider range of band gaps – from 0.7 to 6.2 eV. This should allow nitride-based alloys to be used in a range of optoelectronic applications from the near infrared to the far ultraviolet.

Imaging an infant brain

Brain injury is a common cause of disability in prematurely born infants who survive intensive care. Currently, there are no reliable methods for assessing the degree of injury or the effectiveness of procedures that can be safely used on such infants. This has led researchers to investigate methods based on the transmission of visible and near infrared light.

Hebden and co-workers’ new technique – an improvement on an existing technique known as “optical tomography” – allows them to acquire images of the entire three-dimensional volume of the brain. Optical tomography uses a finite set of measurements of transmitted light between pairs of points on the surface of an object to construct either a transverse slice or a 3D image of the whole object.

The method needs to be sensitive to deep tissues which means that the transmitted light must be integrated over periods of several seconds – or longer – to obtain an adequate signal. Although this can prevent effective imaging of some fast phenomena, it is useful for monitoring long-term changes throughout the entire brain over several hours or even days.

The UCL imaging system measures the arrival time of photons from lasers at wavelengths of 780 nm and 815 nm at 32 different points on the baby’s head. The helmet, used as the interface between the infant and the instrumentation, contains a fibre source coupled with a single detector fibre bundle.

Work still needs to be done on the development of the complex head models. “These models must be able to represent all the possible geometries and optical properties of the infant head in order to get accurate images,” Hebden told PhysicsWeb. “We are steadily introducing more complexities into our models as this work progresses.”

The researchers hope that their method will one day become a routine technique for the monitoring of premature infants in intensive care.

Plasma accelerators take shape

The performance of an accelerator can be described in terms of a voltage gradient. Traditional acceleration technologies cannot exceed gradients of 55 megavolts per metre, which is why particles accelerators have to be tens of kilometres long. To reach the energies of 10¹¹ eV or higher that are needed to explore the unification of the fundamental forces, it is therefore necessary to build even longer accelerators – or to find new ways to increase the voltage gradient. Plasmas are promising candidates for use in the next generation of particle accelerators because they can support electric fields of greater than several hundred gigavolts per metre.

Over the past two decades physicists have demonstrated a number of plasma-based approaches in which the electrons are accelerated by waves in the plasma. In the latest experiments Victor Malka of the École Nationale Supérieure des Techniques Avancées in Paris and co-workers at the CEA/DAM laboratory in Bruyeres-le-Chatel, the University of Bordeaux and Imperial College in London have accelerated electrons to energies of 200 MeV – twice as high as the previous best. Malka and co-workers focused a 30 femtosecond pulse from a 30 terawatt laser beam onto a jet of helium gas to generate an ultrashort beam of energetic electrons. They used a “forced laser wake field” approach in which the plasma wave actually “breaks”.

“The main significance of our experiment is the production of an ultra-short, energetic and very collimated electron beam,” Malka told PhysicsWeb. “This new table-top electron source could be used for many applications in the near future.” The researchers now hope to increase the laser power to generate a mono-energetic electron beam at several hundred MeV for use in accelerators. They would also like to repeat their experiment with protons, with energies up to 10 MeV.

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