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
Abstract and summary · read the original at the source
In one page
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.
Why it matters hereChapter 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.
What it claims
01Nonlinear 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
Published and peer-reviewed02Density 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
Published and peer-reviewed03The 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
Published and peer-reviewed04The 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
Published and peer-reviewed05The 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
Published and peer-reviewed06What 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
What to watch
Read it · abstract
Abstract
THz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structure. In complex oxides, this method has been used to melt electronic orders, drive insulator to metal transitions or induce superconductivity. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature in YBa2Cu3O6+x. By combining femtosecond X-ray diffraction and ab initio density functional theory calculations, we determine here the crystal structure of this exotic non-equilibrium state. We find that nonlinear lattice excitation in normal-state YBa2Cu3O6+x at 100 K causes a staggered dilation/contraction of the Cu-O2 intra/inter-bilayer distances, accompanied by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional theory calculations indicate that these motions cause dramatic changes in the electronic structure. Amongst these, the enhancement in the dx²−y² character of the in-plane electronic structure is likely to favor superconductivity.
Roman Mankowsky, Alaska Subedi, Andrea Cavalleri and colleagues at the Max Planck Institute for the Structure and Dynamics of Matter, École Polytechnique, the Paul Scherrer Institut, SLAC, the Max Planck Institute for Solid State Research, ETH Zürich, the Collège de France, the University of Geneva, Oxford and the University of Hamburg. Nature 516, pages 71 to 73, 2014. Accepted manuscript arXiv:1405.2266.
(Abstract only, from the authors’ accepted manuscript — see the rights note for why no further text is reproduced here.)
On this site: the cavity-enhanced superconducting response in an underdoped cuprate is at /library/stm-94b2666369; the cavity-enhanced transition in two-dimensional niobium diselenide at /library/stm-812175a230; and the 2026 evidence that the vacuum field alone can raise a transition temperature at /library/stm-b7a1a66f71. The result this paper was written to explain is Fausti and colleagues, Light-Induced Superconductivity in a Stripe-Ordered Cuprate, Science 331, 189 (2011).
The way in
https://doi.org/10.1038/nature13875The published article is closed at the publisher under its text-and-data-mining licence, which is not a Creative Commons licence. The accepted manuscript is on arXiv as arXiv:1405.2266 under the arXiv non-exclusive distribution licence — also not Creative Commons — and no Creative Commons statement appears in either copy. So only the authors’ own abstract is reproduced here, taken from the accepted manuscript, and the summary and every locator below were written from that complete manuscript. AUTHOR NOTE: the paper prints given names as initials; a given name is expanded here only where an independent authority record confirms it. Affiliations on the manuscript: the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg; the Centre de Physique Théorique at École Polytechnique and CNRS; the Swiss Light Source at the Paul Scherrer Institut; the Linac Coherent Light Source at SLAC; the Max Planck Institute for Solid State Research in Stuttgart; ETH Zürich; the Collège de France; the University of Geneva; the Clarendon Laboratory at Oxford; and the Center for Free-Electron Laser Science and the University of Hamburg. Mankowsky and Subedi contributed equally.
How 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
Where it sits in the curriculum