The physics of low-temperature superconductivity is fairly well understood, but the ultimate goal of achieving the phenomenon at much higher temperatures remains tantalizingly elusive. The most promising high-temperature superconductor candidates are generally considered to be cuprates with perovskite structures, but it is unclear what mechanisms allow these materials to become superconducting — and how the superconducting temperatures (Tc) can be increased.
By examining the stripe phase-ordering in La1.875Ba0.125CuO4 (LBCO) under high pressure at the U.S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory, a team of researchers from Argonne, Washington University in St. Louis, and Brookhaven National Laboratory probed those questions, specifically, the relationship between stripe ordering and superconductivity. Their work reveals the interplay between stripes, lattice structure, and the superconductivity of LBCO in unprecedented detail and is an important step in understanding high-Tc superconductivity and eventually achieving practical room-temperature superconductors.
The team investigated what happens when the ground state of LBCO1/8 is manipulated with applied pressure, which is known to enhance 3-D superconductivity. Previous work by the Brookhaven co-authors had shown that the charge order component of stripe order was able to survive the disappearance of long-range CuO6 octahedral low-temperature tetragonal (LTT) tilting with the shift to a high-temperature tetragonal (HTT) phase under pressure, and that this charge order coexisted with a marginally enhanced superconducting phase at 2 GPa. Limitations in pressure range prevented suppression of the charge-order phase.
The current experiments extended the pressure range in the HTT phase, where diffraction measurements utilizing high-energy x-ray beams at the X-ray Science Division 4-ID-D beamline of the APS revealed that static charge order completely disappears at 3.6 GPa. With the charge ordering no longer pinned to the lattice structure, superconductivity should again be unhindered. But the experiment team found otherwise, with superconductivity remaining suppressed well below optimal Tc values.
To address this conundrum, high-pressure polarized x-ray absorption fine structure (XAFS) measurements were carried out, also at beamline 4-ID-D, in which the electric field of synchrotron x-rays was oriented along selected crystalline axes. The technique, which allows probing atomic correlations at very short time and length scales, revealed that short-range LTT tilting remains present in the high-pressure HTT phase. The local LTT tilts, which pin charge order, appear to be a manifestation of the presence of dynamic charge correlations sufficiently strong to limit 3-D superconductivity
The extreme sensitivity and fine resolution of the polarized XAFS technique made it possible to uncover these persistent short-range LTT tilts, invisible to longer-range diffraction probes.
The next step involves examining different dopings, pressure ranges, and phases to firmly establish the correlation of stripe order and high-temperature superconductivity. The interplay between stripe order and superconductivity remains a matter of debate. The present results provide indirect evidence that at high pressure, the two may be present simultaneously in a dynamic phase with intertwined order parameters.
Reference: G. Fabbris, M. Hücker, G.D. Gu, J.M. Tranquada, and D. Haskel*, “Local structure, stripe pinning, and superconductivity in La1.875Ba0.125CuO4 at high pressure,” Phys. Rev. B 88, 060507® (2013). DOI: 10.1103/PhysRevB.88.060507
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