Historical research guide · 0956 · 2014
Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5
Roman Mankowsky · Alaska Subedi · M. Först · S. O. Mariager · Matthieu Chollet · H. T. Lemke · J. S. Robinson · James M. Glownia · M. P. Minitti · A. Frano · M. Fechner · Nicola A. Spaldin · T. Loew · B. Keimer · Antoine Georges · Andrea Cavalleri
In one minute
Roman Mankowsky, Alaska Subedi, Andrea Cavalleri and their colleagues answer a question left open by a startling earlier result: hit the copper-oxide superconductor YBCO with a pulse of mid-infrared light while it is well above the temperature where it normally superconducts, and for a few picoseconds it carries current between its layers as though it were superconducting. What is the crystal actually doing in those picoseconds? The team drove one specific atomic vibration — a 15-micrometre, 83-millielectronvolt mode, hit with a field of 3 megavolts per centimetre — and photographed the lattice with 50-femtosecond X-ray flashes from the LCLS free-electron laser, fitting four Bragg reflections at once. The distortion they find is not a simple squeeze. Inside each copper-oxide bilayer the planes move apart while neighbouring bilayers move together, by about 0.63 per cent, and the in-plane oxygen-copper-oxygen bonds buckle a further third of a degree. Theory says that rearrangement pushes the electronic structure toward the character that favours superconductivity.
Connection
Chapter 11 is about superconductors as the place where fields and matter couple hardest, and this is the paper that showed the coupling can be steered: light reshaping a crystal into a state that conducts like a superconductor roughly fifty kelvin above its own transition, with the atomic positions measured rather than assumed. Chapter 2 cares because the lever is not cooling and not heat but a resonantly driven vibration — an engineered non-equilibrium, read out atom by atom.
From the source
Nonlinear phononics gives the mechanism. Resonantly driving an infrared-active vibration exerts a one-way force along a Raman-active coordinate, shifting the minimum of the crystal’s potential energy for as long as the driven mode keeps oscillating coherently, typically a few picoseconds. In a centrosymmetric crystal such as this one the coupling term survives only for modes that preserve the symmetry of the unit cell.Opening section, the split of the Hamiltonian into linear and nonlinear terms and the coupled equations of motion; Fig. 1a and Fig. 1b
Density functional theory in the local density approximation narrows 72 optical phonon modes to the 11 that preserve the unit cell, and then to four — Ag modes 15, 21, 29 and 74 — that couple strongly to the driven B1u motion. All four move the apical oxygen toward the copper-oxide plane and increase the copper-oxygen buckling; three more couple weakly through a breathing motion of the in-plane oxygens, and four do not couple at all.Fig. 3a to Fig. 3c; Extended Data Figure 3 and Extended Data Table 1
The amplitude was measured, not assumed. Fifty-femtosecond, 6.7-kiloelectronvolt X-ray pulses from the LCLS free-electron laser, synchronised to the mid-infrared pump, recorded four Bragg peaks at 100 kelvin, well above the equilibrium transition temperature of 52 kelvin. All four diffraction curves were fitted simultaneously with only two free parameters, and the peaks relaxed on the same one- and seven-picosecond timescales as the terahertz optical changes reported earlier.The time-resolved X-ray diffraction paragraph; Fig. 4
The transient structure is a staggered motion, not a uniform compression: the copper atoms are driven apart from one another within each bilayer and toward one another between bilayers, by about 0.63 per cent, which is the qualitative shape of the reported decrease in intra-bilayer tunnelling and increase in inter-bilayer tunnelling. On top of it sits an anisotropic increase of 0.32 degrees in the in-plane oxygen-copper-oxygen buckling, different along the two in-plane axes.Fig. 5a for the staggered distances; Extended Data Table 2 for the buckling
The obvious explanation is ruled out by the data. The measured shortening of the apical oxygen to copper distance is far smaller than, and opposite in sign to, the static difference between lanthanum- and mercury-based cuprates whose transition temperatures differ — so the transient enhancement of superconducting transport cannot be read as that equilibrium trend acting quickly.Transient lattice structure section, first element; Fig. 5
What to watch: density functional theory in the distorted structure lowers the oxygen-deficient chain bands by a few tens of millielectronvolts, which strongly reduces their hybridisation with the plane copper orbital and leaves a Fermi surface of stronger copper character and higher hole doping — a change the authors judge likely to favour superconductivity, and which may also destabilise the competing charge-density-wave order. They name the next steps themselves: a full many-body treatment, a fuller experimental characterisation of the transient electronic structure, and the goal of engineering these induced behaviours at equilibrium.The density-functional electronic-structure paragraph; closing paragraph on nonlinear phononics as a tool for dynamical materials discovery
The way in
https://doi.org/10.1038/nature13875How to cite it
Roman Mankowsky, Alaska Subedi, M. Först, S. O. Mariager, Matthieu Chollet, H. T. Lemke, J. S. Robinson, James M. Glownia, M. P. Minitti, A. Frano, M. Fechner, Nicola A. Spaldin, T. Loew, B. Keimer, Antoine Georges, Andrea Cavalleri (2014) Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5 doi:10.1038/nature13875