Transient Superconductivity in optically modulated YBa2Cu3O6+x

 

Stefan Kaiser

 

Max‐Planck Institut für Struktur und Dynamik der Materie,

Center for Free Electron Laser Science, Universität Hamburg, Germany

 

Photo‐stimulation with femtosecond mid‐infrared pulses allows us to induce a

non‐equilibrium superconducting state in YBa2Cu3O6+x at temperatures as high

as 300 K. Its transient response is probed via THz time‐domain spectroscopy.

The parent, undoped compounds of high‐Tc cuprates are antiferromagnetic Mott

insulators, whose physical properties are strongly modified upon doping the Cu‐

O planes with a few percent charge carriers. One region of their phase diagram

which has attracted much attention is the so‐called pseudogap phase [1,2], found

below a characteristic temperature T* in moderately‐doped materials. The origin

of this gapped, but non‐superconducting state has been attributed to a variety of

microscopic phenomena, ranging from a precursor superconducting state with

pre‐formed Cooper pairs without phase coherence, to a scenario where

electronic correlations compete with superconductivity.

So far, only optical excitation in the visible or near infrared has been performed

on high‐Tc cuprates [3‐5]. Such experiments have investigated the relaxation of

hot incoherent quasiparticles back into the Cooper‐pair condensate. Therefore,

superconductivity could not be triggered directly by the light field.

By making use of mid‐infrared pulses, phase transitions have been photoinduced

in strongly‐correlated electron systems through resonant excitation of lattice

vibrational modes [6]. In particular, we have shown that light can induce

superconductivity in La1.675Eu0.2Sr0.125CuO4 at temperatures as high as 20 K, by

melting the competing spin‐ and charge‐order in the so‐called stripe phase [7].

Here, we set out to establish phase coherence in YBa2Cu3O6+x for temperatures

far above Tc. Therefore high intensity optical pulses at THz frequencies are used

to dynamically modulate the crystallographic structure. This creates a high

mobility state for base temperatures far above Tc, as evidenced by the

appearance of a plasma edge in the c‐axis reflectivity and a divergent imaginary

optical conductivity at low frequencies. We identify this state as an

inhomogeneous transient superconductor, which for low doping is observed

even as high as room temperature [8]. Using ultra‐broadband THz pulses we are

able to investigate the redistribution of interlayer coherence within the bi‐layer

structure of YBCO [9]. Ultrafast x‐ray experiments at LCLS allow us observing the

nonlinear phonon dynamics after the lattice excitation and to reconstruct the

crystal structure in the transient superconducting state [10].

 

[1] T. Timusk and B. Statt, "The pseudogap in high‐Tc SC: an experimental survey," Rep. Prog. Phys. 62, 61 (1999).

[2] D. N. Basov and T. Timusk, "Electrodynamics of high‐Tc superconductors," Rev. Mod. Phys. 77, 721 (2005).

[3] J. Demsar, B. Podobnik, V. V. Kabanov, et al., "Superconducting Gap Δc, the Pseudogap Δp, and pair fluctuations above

Tc in overdoped Y1‐xCaxBa2Cu3O7‐δ from Femtosecond Time‐Domain Spectroscopy," Phys. Rev. Lett. 82, 4198 (1999).

[4] R. A. Kaindl, , et al. "Ultrafast Mid‐Infrared Response of YBa2Cu3O7‐δ," Science 287, 470 (2000).

[5] R. D. Averitt, G. Rodriguez, A. I. Lobad, et al., "Nonequilibrium superconductivity and quasiparticle dynamics in

YBa2Cu3O7‐δ," Phys. Rev. B 63, 140502 (2001).

[6] M. Rini, R. I. Tobey, N. Dean, et al. "Control of the electronic phase of a manganite by mode‐selective vibrational

excitation," Nature 449, 72‐74 (2007).

[7] D. Fausti, R. I. Tobey, N. Dean. S. Kaiser, A. Dienst, M. C. Hoffmann, S. Pyon, T. Takayama, H. Takagi, and A. Cavalleri,

"Light‐Induced Superconductivity in a Stripe‐Ordered Cuprate," Science 331, 189 (2011).

[8] S. Kaiser, D. Nicoletti, C. R. Hunt, W. Hu, I. Gierz, H. Y. Liu, M. Le Tacon, T. Loew, D. Haug, B. Keimer, A. Cavalleri,

“Light‐induced inhomogeneous superconductivity far above Tc in YBa2Cu3O6+x” arXiv:1205.4661 (2013)

[9] W. Hu, I. Gierz, D. Nicoletti, S. Kaiser, C. R. Hunt, et al. “Enhancement of superconductivity by redistribution of

interlayer coupling in optically stimulated YBa2Cu3O6.5”, arXiv1308.3204 (2013).

[10] R. Mankowsky, M. Forst et al. (unpublished) .                        

Host: Prof. E. Andrei