Light-induced superconducting-like properties in high-Tc cuprates
Charge and spin orders in high-Tc cuprates and their interaction (and competition) with the superconducting state are believed to carry important information on the mechanism of high-temperature superconductivity. A prototypical case is that of the so-called “stripes” in the Cu-O planes of single-layer La-based compounds. These consist of one-dimensional chains of doped holes separating regions of oppositely phased antiferromagnetism, and this phase is typically stabilized by a distortion of the crystal lattice below a certain temperature (see Figure 1).
Recent experiments suggest that, while the individual Cu-O striped planes may in fact be in a highly coherent state (in which stripes just modulate superconductivity), on the other hand stripes would highly frustrate the interlayer (c direction) coherent transport and therefore inhibit 3D superconductivity.
Using mid-infrared femtosecond pulses, we demonstrated that the cuprate crystal lattice can be directly manipulated to transform non-superconducting, stripe ordered La1.675Eu0.2Sr0.125CuO4 into a transient 3D superconductor . The emergence of coherent interlayer transport at temperatures as high as 20 K is demonstrated via time-domain THz spectroscopy, by detecting a Josephson Plasma Resonance in the transient c-axis optical reflectivity.
Under similar excitation conditions, we performed femtosecond resonant soft X-ray diffraction at the Stanford Linac Coherent Light Source LCLS on the striped compound La1.88Ba0.12CuO4 (LBCO) . In this experiment, the dynamics of different diffraction peaks in the photoinduced superconducting state was mapped out, revealing a decoupling of charge and lattice order on ultrafast timescales. Such result supports a scenario in which charge modulation alone, and not the structural distortion, may be preventing interlayer superconducting coupling at equilibrium.
An alternative means to remove stripe order, different from mid-infrared lattice excitation, was also explored more recently in LBCO. We demonstrated that also high-energy (1.5 eV) photo-excitation, which is often applied to melt charge density waves in various compounds, can strongly enhance the superconducting coupling in striped compounds, in particular for pump polarization perpendicular to the CuO2 planes .
The most striking results of our research on high-Tc cuprates were achieved on the well known bi-layer compound YBa2Cu3O6+x, which has a maximum critical temperature at equilibrium Tc ≃ 90 K. By using mid-infrared pulses to induce large-amplitude modulations of the lattice (and in particular of the position of the so-called apical oxygen atoms), we could transiently induce highly unconventional electronic properties in this material [4,5A].
Detailed analysis of the broadband optical response after excitation revealed that the inter-bilayer superconducting coherence is transiently enhanced at the expense of intra-bilayer coupling (see Figure 3) . Strikingly, this effect is observed up to room temperature, with light-induced superconducting-like properties emerging from the incoherent ground state in an extended region of the YBa2Cu3O6+x phase diagram (Figure 4) .
Measurements of the transient lattice structure following this vibrational excitation have revealed that the atoms do not only oscillate but are also shifted away from their equilibrium positions (Figure 5). This deformation of the crystal lattice appears and relaxes on the same timescale as the transient superconducting-like state and is likely to favor superconductivity . In addition, the driven state involves melting of charge density wave correlations , which compete with the superconducting state at equilibrium.
Figure 5: Transient lattice structure of and vibrational exitation between two CuO2 double layers of YBa2Cu3O6.5.
Related Methods & Experiment
|Light induced Superconductivity in a Stripe-ordered Cuprate|
|D. Fausti, R.I. Tobey, N. Dean, S. Kaiser, A. Dienst, M.C. Hoffmann, S. Pyon, T. Takayama, H. Takagi and A. Cavalleri|
|Science, 331, 6014 189-191 (2011)|
|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|
|Physical Review Letters, 112, 157002|
|Optically-induced superconductivity in striped La2-xBaxCuO4 by polarization-selective excitation in the near infrared|
|D. Nicoletti , E. Casandruc, Y. Laplace, V. Khanna, C. R. Hunt, S. Kaiser, S. S. Dhesi, G. D. Gu, J. P. Hill, A. Cavalleri|
|Physical Review B, 90, 100503(R), (2014)|
|Optically enhanced coherent transport in YBa2Cu3O6.5 by ultrafast redistribution of interlayer coupling|
|W. Hu, S. Kaiser, D. Nicoletti, C. R. Hunt, I. Gierz, M. C. Hoffmann, M. Le Tacon, T. Loew, B. Keimer & A. Cavalleri|
|Nature Materials, 13, 705–711 (2014)|
|⇒ News & Views by N. Peter Armitage||→ Press releases|
|Optically induced coherent transport far above Tc in underdoped YBa2Cu3O6+δ|
|S. Kaiser, C. R. Hunt, D. Nicoletti, W. Hu, I. Gierz, H. Y. Liu, M. Le Tacon, T. Loew, D. Haug, B. Keimer, and A. Cavalleri|
|Phys. Rev. B 89, 184516 (2014)|
|Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5|
|R. Mankowsky, A. Subedi, M. Först, S.O. Mariager, M. Chollet, H. Lemke, J. Robinson, J. Glownia, M. Minitti, A. Frano, M. Fechner, N. A. Spaldin, T. Loew, B. Keimer, A. Georges, A. Cavalleri|
|Nature 516 , 71–73 (2014)||⇒ MPG Press release ⇒ SLAC Press release|