An interview with nature

Can light shine through walls?

It took 10,000 physicists and a € 6 bn collider to discover the Higgs boson at CERN, but there are easier ways to search for some elementary particles. One is to shine X-rays through a magnetic field and onto a solid wall that has a photon detector on the other side. Classically, no light gets through. But quantum theory allows a virtual photon produced by the magnetic field to combine with a real photon in the beam and form an “axion”, a weakly interacting boson invented to solve problems in particle physics and cosmology. The axion would travel through the wall unimpeded and be converted back into a real photon by interacting with a second virtual photon on the other side.

In 2010, researchers from the ESRF and the Laboratoire National des Champs Magnétiques Intenses in Toulouse searched for such photon regeneration using 50–90 keV X-rays at the ESRF’s ID06 beamline. Two superconducting magnets produced a 3 T field on either side of a 50 mm-think lead shutter, with a high efficiency germanium detector located on the far side (Phys. Rev. Lett. 105 250405). 

Alas, no signal was observed. But with the ESRF’s photons being so energetic the work places a new lower limit on the axion mass: 17 MeV/c². The experiment was a first for synchrotrons, says team member Carsten Detlefs of the ESRF. “With X-ray experiments you can exclude higher axion masses because the axion mass is much smaller than the photon energy, which in laser or microwave experiments is pretty small.”

Why does an electron gas form at interfaces?

Interfaces between transition metal oxides can exhibit electronic properties that are absent in the individual layers, a prominent example being the formation of a conducting electron gas between the insulating compounds LaAlO₃ and SrTiO₃. In an attempt to explain this unusual behaviour, X-ray absorption spectroscopy carried out at the ESRF’s ID08 beamline in 2009 allowed researchers to probe the electronic properties and orbital structure of the interface (Phys. Rev. Lett. 102 166804). The results showed that a structural distortion appears at the interface and in particular that the formation of a conducting 2D electron gas is related to an orbital reconstruction during which the degeneracy of titanium’s 3d electronic states is removed.

Is the speed of light the same to all observers?

That the speed of light is a fundamental constant is the core of Einstein’s special theory of relativity, ensuring that space-time is Lorentz invariant and therefore looks the same no matter which direction you travel in. But this fundamental symmetry might not hold at the most extreme subatomic scales, where uncertainty reigns and nature is thought to be governed by the laws of quantum gravity.

An experiment called GRAAL – GRenoble Anneau Accelerateur Laser – installed at the ESRF several years ago allowed physicists to search for cracks in Lorentz symmetry by studying the Compton backscattering of laser photons off electrons in the storage ring. Electrons and photons collide such that the photon is bounced back and gains energy while the electron loses energy via Compton scattering. Since the energy loss of the electron depends sensitively on the speed of light, any directional variations in the speed of light would show up in the electrons’ energy loss. The team monitored the electron energy loss continually for 24 hours to see if the it changes as the Earth rotates, and the results were published in 2010 (Phys. Rev. Lett. 104 241601). No variations were spotted, placing new limits on anisotropies in the propagation of light at the 10⁻¹⁴ level, as expressed in terms of coefficients in a Lorentz-violating extension of quantum electrodynamics. That’s 10 times better than previous experiments and “represents a billion-fold improvement over the original test performed by Michelson and Morley in 1887”, says co-author Ralf Lehnert of the National University of Mexico.

What is ball lightning?

Ball lightning – a slow-moving orb of light seen hovering above the ground during thunderstorms – has puzzled scientists for a century. It is thought to be a plasma formed when lightning hits the ground and creates a hot-spot, ejecting a plume of silicon, silicon-oxide and silicon-carbide nanoparticles that sustains an optical glow.

To mimic these reported atmospheric events, a team from the University of Rennes, the ESRF and Tel Aviv University used a fireball generator that channels microwaves through a rod into a solid substrate made from glass or some other ceramic (Phys. Rev. Lett. 100 065001). Small-angle X-ray scattering at the ESRF’s ID02 beamline allowed the team to probe the structure of the fireball as it evolved, revealing it to be a dusty plasma consisting of charged nanoparticles with a mean size of 50 nm.

According to other physicists, however, the mystery of ball lightning is a matter of the mind. Recently, Josef Peer and Alexander Kendl of the University of Innsbruck claimed that the phenomenon is an illusion caused by the large magnetic fields created by ordinary lightning (Physics Letters A 374 2932). “We conclude that a plausible interpretation of a large class of reports of luminous phenomena during thunderstorms is the result of electromagnetic pulse induced transcranial magnetic stimulation of phosphenes in the human brain.”

What happens when a fluid goes supercritical?

As a substance goes beyond its critical point at high temperatures and pressures, it enters a single “supercritical fluid” phase in which there is no way to distinguish the gas and liquid states. At least, that’s what the textbooks say. In 2010, using inelastic X-ray scattering and molecular dynamics simulations at the ESRF’s ID28 beamine, a team lead by researchers from the universities of Rome and Florence identified two distinct dynamical regimes (liquid-like and gas-like) in dense, hot supercritical fluid argon, revealing for the first time a crossover between liquid-like and gas-like behaviour (Nature Physics 6 503).

A sharp decrease in the dispersion of sound waves observed at pressures of 0.4 GPa, due to the disappearance of the structural relaxation process, marked the transition from a collective liquid-like to a single particle gas-like behaviour, claims the team. This provides a connection between dynamics and thermodynamics and contradicts the widespread belief of a homogeneous supercritical fluid phase. The crossover corresponds to the extrapolation of the so-called Widom line, which constitutes the locus of maxima of the isobaric specific heat capacity in the supercritical fluid phase. “This offers the first fundamental insight into the correspondence between subcritical and supercritical fluid behaviour,” the team concludes. “These findings open up new territory for which there is at present no theoretical framework.”

What is the smallest atomic displacement?

Multiferroic materials have the unusual property that they can be both magnetically and electrically ordered. In an attempt last year to understand what drives this behaviour, a team led by the ESRF’s Helen Walker showed that the electric polarisation in multiferroics is caused by the relative displacement between charges of different signs. Magnetic polarisation, by contrast, is driven by the alignment of the individual magnetic moments of the atoms in a magnet.

The team’s insight came from a new technique that exploits the interference between two competing processes: charge and magnetic scattering of a polarised X-ray beam. With it they were able to measure displacements of specific atoms in a single crystal of TbMnO₃, which exhibits multiferroic behaviour at temperatures below 30 K, as small as 20 femtometres (about 1/100,000th of the distance between the atoms in the material). Because the displacement involves a high number of electrons, even small displacements can cause significant electrical polarization (Science 333 1273). “I think that everyone involved was surprised, if not staggered, by the result that we can now image the position of atoms with such accuracy,” said team member Des McMorrow of the London Centre for Nanotechnology.

What do quantum effects look like at macroscopic scales?

The Lamb shift, observed by US physicist Willis Lamb in the late 1940s, is a small correction to the ²S_½ and ²P_½ energy levels in the hydrogen atom caused by the emission and reabsorption of ‘virtual’ photons from the vacuum. This causes a tiny shift in the frequency of a spontaneously emitted photon that agrees with the predictions of quantum electrodynamics to one part per million. But when an atom is one of an ensemble, the emitted photon may also be absorbed by other identical atoms – with dramatic effect. The “collective” Lamb shift is predicted to cause a strong acceleration of the collective emission called superradiance, but the effect is extremely difficult to observe in the optical regime.

In 2010, an experiment at the ESRF’s ID22N beamline led by physicists from DESY demonstrated a simple way to observe superradiance and the collective Lamb shift in the X-ray regime (Science 328 1248). The team embedded ultrathin layers of ⁵⁷Fe (a two-level Mössbauer isotope) between two mirrors spaced a few nanometers apart and resonantly excited them with X-ray pulses. Multiple reflections of the radiation within the cavity cause the ensemble of atoms to appear optically thick, and once it was excited into a purely superradiant state, the researchers found that the ensemble decayed almost two orders of magnitude faster than a single atom – in excellent agreement with calculations.

What drives high-temperature superconductors?

Superconductivity, a state of zero electrical resistance exhibited by many elements at very low temperatures, was observed in 1911 in the metal mercury. Half a century later, physicists discovered the microscopic mechanism that drives this exotic and highly useful behaviour: correlations between pairs of electrons at low temperatures, as described by the Bardeen–Cooper–Schrieffer (BCS) theory. But in 1986, astonished physicists discovered superconductivity at much higher temperatures in more complex copper oxide or “cuprate” systems, which has puzzled researchers ever since.

Recently, an international team used resonant soft-X-ray scattering at the ESRF’s ID08 beamline to help identify 2D charge fluctuations in the copper-oxide planes of ceramic yttrium and neodymium barium cuprates (Y, Nd)Ba₂Cu₃3O_6+x. Understanding charge, spin and orbital correlations in the normal state from which superconductivity emerges is key to finding a successful theory of high-temperature superconductivity in the cuprates, which could help researchers exploit this property for more efficient engineering materials, and it is suspected that ordering phenomena are a key factor. A long-standing debate concerns whether “stripe ordering”, a type of antiferromagnetism, is a generic feature of the cuprates and thus whether stripe fluctuations are essential for superconductivity. The ESRF result, taken together with data from the Swiss, Canadian and Bessy light sources, indicates that the correlation length of the charge fluctuations increases as the temperature is lowered and then reverses at the transition temperature, indicating an incipient charge-density wave instability that competes with superconductivity (Science 337 821). 

“We did not expect the charge density waves in the superconducting cuprates because they destroy the superconductivity,” says team member Bernhard Keimer of the Max-Planck-Institute for Solid State Research in Stuttgart. “Superconductivity only just prevailed in this competition.”

Matthew Chalmers

This article originally appeared in ESRFnews, December 2012. 

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Top image: What drives high-temperature superconductors? Charge density waves distort the crystal lattice (blue) and compete with superconductivity.