Light turns insulator into a superconductor

The team from Oxford, Germany and Japan are said to have observed conclusive signatures of superconductivity after hitting a non-superconductor with a strong burst of laser light. (D. Fausti et al., "Light-Induced Superconductivity in a Stripe-Ordered Cuprate", Science v.331, p.189 (2011)).

Abstract: One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional “striped” spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La1.675Eu0.2Sr0.125CuO4, into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

‘We have used light to turn a normal insulator into a superconductor,’ said Prof Andrea Cavalleri of the Department of Physics at Oxford University and the Max Planck Department for Structural Dynamics, Hamburg. ‘That’s already exciting in terms of what it tells us about this class of materials. But the question now is can we take a material to a much higher temperature and make it a superconductor?’

The material the researchers used is closely related to high-temperature copper oxide superconductors, but the arrangement of electrons and atoms normally act to frustrate any electronic current.

In the journal Science, they describe how a strong infrared laser pulse was used to perturb the positions of some of the atoms in the material. The compound, held at a temperature just 20 degrees above absolute zero, almost instantaneously became a superconductor for a fraction of a second, before relaxing back to its normal state.

Superconductivity describes the phenomenon where an electric current is able to travel through a material without any resistance.

High-temperature superconductors can be found among a class of materials made up of layers of copper oxide, and typically superconduct up to a temperature of around –170°C. They are complex materials where the right interplay of the atoms and electrons is thought to ‘line up’ the electrons in a state where they collectively move through the material with no resistance.

‘We have shown that the non-superconducting state and the superconducting one are not that different in these materials, in that it takes only a millionth of a millionth of a second to make the electrons ‘synch up’ and superconduct,’ said Professor Cavalleri. ‘This must mean that they were essentially already synched in the non-superconductor, but something was preventing them from sliding around with zero resistance. The precisely tuned laser light removes the frustration, unlocking the superconductivity.’

The advance immediately offers a new way to probe with great control how superconductivity arises in this class of materials.

The researchers are hopeful it could also offer a new route to obtaining superconductivity at higher temperatures. If superconductors that work at room temperature could be achieved, it would open up many more technological applications.

‘There is a school of thought that it should be possible to achieve superconductivity at much higher temperatures, but that some competing type of order in the material gets in the way,’ said Prof Cavalleri. ‘We should be able to explore this idea and see if we can disrupt the competing order to reveal superconductivity at higher temperatures. It’s certainly worth trying.’

NRL begins field tests of laser acoustic propagation

An NRL research team led by physicist, Dr. Ted Jones, Plasma Physics Division, performed the first successful long distance acoustic propagation and shock generation demonstration of their novel underwater photo-ionization laser acoustic source. These tests, performed at the Lake Glendora Test Facility of Naval Surface Warfare Center-Crane, expanded on their earlier laboratory research on pulsed laser propagation through the atmosphere.

Using a pulsed Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) 532 nanometer wavelength laser housed in a floating platform, pulses were directed by steering mirrors down through a focusing lens and into the water surface. Each laser pulse produced an acoustic pulse with a sound pressure level of approximately 190 decibels (dBs), which was detected and measured by boat-mounted hydrophones at distances up to 140 meters, roughly the length and a half of a football field. Prior laboratory acoustic propagation distances were limited to about three meters.

"The goal of this laser acoustic source development is to enable efficient remote acoustic generation from compact airborne and ship-borne lasers, without the need for any source hardware in the water," said Jones. "This new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications."

The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. A properly tailored laser pulse has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, a highly useful and efficient tool for undersea communications from aircraft.

Few femtosecond, few kiloampere electron bunch produced by a laser–plasma accelerator

Particle accelerators driven by the interaction of ultraintense and ultrashort laser pulses with a plasma can generate accelerating electric fields of several hundred gigavolts per metre and deliver high-quality electron beams with low energy spread, low emittance and up to 1 GeV peak energy. Moreover, it is expected they may soon be able to produce bursts of electrons shorter than those produced by conventional particle accelerators, down to femtosecond durations and less. Here we present wide-band spectral measurements of coherent transition radiation which we use for temporal characterization. Our analysis shows that the electron beam, produced using controlled optical injection, contains a temporal feature that can be identified as a 15 pC, 1.4–1.8 fs electron bunch (root mean square) leading to a peak current of 3–4 kA depending on the bunch shape. We anticipate that these results will have a strong impact on emerging applications such as short-pulse and short-wavelength radiation sources, and will benefit the realization of laboratory-scale free-electron lasers.

An ultrashort and ultraintense laser pulse (red) is focused on a gas jet in which a plasma wave is excited. Electrons are injected into the plasma wave during the collision with the injection pulse (green), which arrives at a relative angle.

(via Nature Physics doi:10.1038/nphys1872)