Nonlinear Optical Control of Josephson Coupling in Cuprates

Abstract

In High-TC cuprates superconducting Cu-O planes alternate with insulating layers along the crystallographic c-axis, making the materials equivalent to Josephson junctions connected in series. The most intriguing consequence is that the out-of-plane superconducting transport occurs via Cooper pairs tunneling across the insulating layers and can be predicted by the Josephson tunneling equations. Nonlinear interaction between light fields and the superconducting carriers serves as a powerful dynamical probe of cuprates, while offering opportunities for controlling them in an analogous fashion to other stimuli such as pressure and magnetic fields. The main goal of this thesis work is to use intense transient light fields to control the interlayer superconducting transport on ultrafast time scales. This was achieved by tuning the wavelength of such light pulses to completely different ranges, in order to either directly excite Josephson Plasma Waves in the nonlinear regime, or efficiently melt the competing charge and spin order phase, which in certain cuprates quenches the Josephson tunneling at equilibrium. In a first study, I have utilized strong field terahertz transients with frequencies tuned to the Josephson plasma resonance (JPR) to coherently control the c-axis superconducting transport. The Josephson relations have a cubic nonlinearity which is exploited to achieve two related, albeit slightly different, phenomena. Depending on the driving pulse, solitonic breathers were excited with narrow-band multi-cycle pulses in La1.84Sr0.16CuO4 while broad-band half-cycle pulses were employed to achieve a parametric amplification of Josephson Plasma Waves in La1.905Ba0.095CuO4. These experiments are supported by extensive modeling, showing exceptional agreement. A comprehensive study illustrates the strong enhancement of the nonlinear effects near the JPR frequency. Then, I turned to investigate the competition between superconductivity and charge- and spin-order (the so called stripe phase) in La1.885Ba0.115CuO4. I have demonstrated selective melting of the stripe phase through the irradiation with high photon energy pulses, which results in a transient enhancement of the c-axis superfluid density. The dependence of the effect on the wavelength of the pump pulse suggests a dominant energy scale which is at play with superconductivity, supporting the competing nature between the stripe and the superconducting order.

Im Aufbau von Hochtemperatur-Kuprat-Supraleitern wechseln sich entlang der kristallographischen c-Achse Cu-O Ebenen und isolierende Schichten ab, was diese Materialklasse zu in Serie geschalteten Josephson-Kontakten gleichsetzt. Die faszinierendste Konsequenz dieser Anordnung ist, dass der supraleitende Transport senkrecht zu den Materialebenen über das Tunneln von Cooper-Paaren durch die isolierenden Schichten geschieht und durch die Josephson-Gleichungen vorhergesagt werden kann. Nichtlineare Wechselwirkungen zwischen Lichtfeldern und dem supraleitenden Zustand dienen als hilfreiche dynamische Sonden zur Untersuchung solcher Materialien. Gleichzeitig eröffnen sie Kontrollmöglichkeiten analog zu denen anderer Stimuli wie Druck und Magnetfeldstärke, allerdings für ultraschnelle Zeitskalen. In dieser Arbeit nutze ich ultraschnelle Terahertzspektroskopie zur Untersuchung der supraleitenden Eigenschaften zweier Klassen einlagiger La-214 Kupratproben mit und ohne Ausbildung konkurrierender Ladungsstreifen-Ordnung. Die nichtlineare Wechselwirkung der Proben mit Lichtfeldern wird zudem zur gezielten Steuerung der Materialeigenschaften eingesetzt.