Light-induced superconducting-like properties in high-Tc cuprates
Charge and spin ordered states are ubiquitous in the phase diagrams of high-Tc superconducting cuprates. A prototypical case is that of the so-called “stripes”. These consist of one-dimensional chains of doped holes separating regions of oppositely phased antiferromagnetism.
Recent experiments suggest that, while the individual Cu-O striped planes may be in a highly coherent state, in which superconductivity is spatially modulated, the stripes completely frustrate the interlayer tunnelling, thus inhibiting a 3D superconducting state.
In our group, using photo-excitation with femtosecond pulses in the mid infrared, we have discovered that stripe-ordered, non-superconducting La1.675Eu0.2Sr0.125CuO4 can be transiently switched into a state which exhibits the optical response of a 3D superconductor . The emergence of coherent interlayer transport at temperatures as high as 20 K was demonstrated via time-domain THz spectroscopy, by detecting a Josephson Plasma Resonance in the transient optical reflectivity [1,2]. In addition, femtosecond resonant soft X-ray diffraction, performed under the same mid-infrared excitation at the Stanford Linac Coherent Light Source LCLS , allowed us to identify the optical melting of charge order as the driving mechanism for light-induced superconductivity in striped cuprates.
More recently, photo-excitation at shorter wavelengths has turned out to be an alternative means to remove stripes and enhance superconductivity transiently. In La2-xBaxCuO4, we have discovered a response with induced Josephson coupling above Tc upon optical driving with near-infrared light pulses [4,5]. This was systematically investigated for different pump photon energies  and in presence of external dc magnetic fields as high as 7 Tesla , allowing us to gain key hints on the interplay between stripes and superconductivity out of equilibrium.
The most striking results of our research on high-Tc cuprates were obtained on the bi-layer compound YBa2Cu3O6+x, which has a maximum superconducting critical temperature at equilibrium, Tc, of about 90 K. Upon photo-stimulation with mid-infrared pulses, which triggered large-amplitude oscillations of the apical oxygen atomic positions, we induced a highly unconventional optical response, compatible with transiently enhanced inter-bilayer Josephson tunnelling [8,9,10]. Strikingly, this effect could be observed all the way up to room temperature, with light-induced superconducting properties emerging from a non-superconducting ground state throughout an extended region of the YBa2Cu3O6+x phase diagram [9,10].
While optical melting of charge order seemed not to play a fundamental role in the case of YBa2Cu3O6+x , femtosecond X-ray crystallography has revealed the emerge of a new, displaced crystal structure , which originates out of equilibrium as a result of a nonlinear coupling between different phonon modes . This atomic rearrangement might be favouring superconductivity at higher temperature, although alternative scenarios which involve a direct modification of the electronic bands within the Cu-O planes shall also be considered to explain the observed dynamics.
Additional studies, that make also use of newly available pump devices , are being carried out, aiming at pinning down the mechanism behind optically-driven superconductivity in YBa2Cu3O6+x, with the perspective of optimising this phenomenon even further .
Related Methods & Experiments
|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)|
|Two distinct kinetic regimes for the relaxation of light-induced superconductivity in La1.675Eu0.2Sr0.125CuO4|
|C. R. Hunt, D. Nicoletti, S. Kaiser, T. Takayama, H. Takagi, and A. Cavalleri|
|Physical Review B 91, 020505(R) (2015)|
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|Physical Review B, 90, 100503(R), (2014)|
|Restoring interlayer Josephson coupling in La1.885Ba0.115CuO4 by charge transfer melting of stripe order|
|V. Khanna, R. Mankowsky, M. Petrich, H. Bromberger, S. A. Cavill, E. Möhr-Vorobeva, D. Nicoletti, Y. Laplace, G. D. Gu, J. P. Hill, M. Först, A. Cavalleri and S. S. Dhesi|
|Physical Review B 93, 224522 (2016)|
|Wavelength-dependent optical enhancement of superconducting interlayer coupling in La1.885Ba0.115CuO4|
|E. Casandruc, D. Nicoletti, S. Rajasekaran, Y. Laplace, V. Khanna, G. D. Gu, J. P. Hill, and A. Cavalleri|
|Physical Review B, 91, 17 (2015)|
Magnetic-Field Tuning of Light-Induced Superconductivity in Striped La2−xBaxCuO4
Physical Review Letters,121, 267003 (2018)
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|Nature Materials, 13, 705–711 (2014)|
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|Phys. Rev. B 89, 184516 (2014)|
|Dynamical decoherence of the light induced inter layer coupling in YBa2Cu3O6+δ|
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|Physical Review B, 94, 224303 (2016)|
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|Physical Review B, 90, 90, 184514 (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|
|Coherent modulation of the YBa2Cu3O6+x atomic structure by displacive stimulated ionic Raman scattering|
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|Generation of narrowband, high-intensity, carrier-envelope phase stable pulses tunable between 4 and 18 THz|
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