Scientists at Brookhaven National Laboratory have uncovered a key factor behind the emergence of superconductivity, the ability to conduct electricity with 100 percent efficiency.
Precisely timed pairs of laser pulses at the SLAC National Accelerator Laboratory’s Linac Coherent Light Source (LCLS) triggered superconductivity in the copper-oxide material under investigation. Researchers took x-ray snapshots of its atomic and electronic structure as superconductivity emerged.
The scientists found that so-called charge stripes of increased electrical charge vanished as superconductivity appeared. The results help rule out the theory that shifts in the material’s atomic lattice hinder the onset of superconductivity.
Light Induced Superconductivity
The new understanding may help scientists develop new techniques to eliminate charge stripes and open the door to room-temperature superconductivity, frequently looked upon as the holy grail of condensed matter physics. The demonstrated ability to rapidly switch between the insulating and superconducting states could also prove useful in advanced electronics and computation.
“The very short timescales and the need for high spatial resolution made this experiment extraordinarily challenging,” said co-author Michael Först. “Now, using femtosecond x-ray pulses, we found a way to capture the quadrillionths-of-a-second dynamics of the charges and the crystal lattice. We’ve broken new ground in understanding light-induced superconductivity.”
The material used in this study was a layered compound made of barium, lanthanum, copper, and oxygen. Each copper oxide layer contained the critical charge stripes.
Strings in a pile of Tennis Racquets
“Imagine these stripes as ripples frozen in the sand,” said John Hill, coauthor on the study. “Each layer has all the ripples going in one direction, but in t
he neighboring layers they run crosswise. From above, this looks like strings in a pile of tennis racquets. We believe that this pattern prevents each layer from talking to the next, thus frustrating superconductivity.”
Mid-infrared laser pulses were used by the scientists used “melt” those frozen ripples, exciting the material and push it into the superconducting phase. Such pulses had previously been shown to induce superconductivity in a related compound at a frigid 10 Kelvin (minus 442 degrees Fahrenheit).
“The charge stripes disappeared immediately,” Hill said. “But specific distortions in the crystal lattice, which had been thought to stabilize these stripes, lingered much longer. This shows that only the charge stripes inhibit superconductivity.”
Catching Charge Stripes in the Act
SLAC’s LCLS x-ray laser works like a camera with a shutter speed faster than 100 femtoseconds, or quadrillionths of a second, and gives atomic-scale image resolution. LCLS uses a section of SLAC’s 2-mile-long linear accelerator to generate the electrons that give off x-ray light.
“This represents a very important result in the field of superconductivity using LCLS,” said Linac Coherent Light Source staff scientist Josh Turner. “It demonstrates how we can unravel different types of complex mechanisms in superconductivity that have, up until now, been inseparable.To make this measurement, we had to push the limits of our current capabilities. We had to measure a very weak, barely detectable signal with state-of-the-art detectors, and we had to tune the number of x-rays in each laser pulse to see the signal from the stripes without destroying the sample.”
For measuring the changes in high spatial resolution, the team used a technique called resonant soft x-ray diffraction. The LCLS x-rays strike and scatter off the crystal into the detector, carrying time-stamped signatures of the material’s charge and lattice structure that the physicists then used to reconstruct the rise and fall of superconducting conditions.
Superior Superconductor Materials
The x-ray scattering measurements showed that the lattice distortion persists for more than 10 picoseconds (trillionths of a second), long after the charge stripes melted and superconductivity appeared, which happened in less than 400 femtoseconds. Slight as it may sound, those extra trillionths of a second make a huge difference.
“The findings suggest that the relatively weak and long-lasting lattice shifts do not play an essential role in the presence of superconductivity,” Hill said. “We can now narrow our focus on the stripes to further pin down the underlying mechanism and potentially engineer superior materials.”
Melting of Charge Stripes in Vibrationally Driven La1.875Ba0.125CuO4: Assessing the Respective Roles of Electronic and Lattice Order in Frustrated Superconductors
M. Först, R. I. Tobey, H. Bromberger, S. B. Wilkins, V. Khanna, A. D. Caviglia, Y.-D. Chuang, W. S. Lee, W. F. Schlotter, J. J. Turner, M. P. Minitti, O. Krupin, Z. J. Xu, J. S. Wen, G. D. Gu, S. S. Dhesi, A. Cavalleri, and J. P. Hill
Phys. Rev. Lett. 112, 157002 – Published 16 April 2014