Anomalous relaxation kinetics and charge density wave correlations in underdoped BaPb1-xBixO3

Superconductivity often emerges in proximity of other symmetry-breaking ground states, such as antiferromagnetism or charge-density-wave (CDW) order. However, the subtle inter-relation of these phases remains poorly understood, and in some cases even the existence of short-range correlations for superconducting compositions is uncertain. In such circumstances, ultrafast experiments can provide new insights, by tracking the relaxation kinetics following excitation at frequencies related to the broken symmetry state. Here, we investigate the transient terahertz conductivity of BaPb1-xBixO3 - a material for which superconductivity is adjacent to a competing CDW phase - after optical excitation tuned to the CDW absorption band. In insulating BaBiO3 we observed an increase in conductivity and a subsequent relaxation, which are consistent with quasiparticles injection across a rigid semiconducting gap. In the doped compound BaPb0.72Bi0.28O3 (superconducting below Tc=7K), a similar response was also found immediately above Tc. This observation evidences the presence of a robust gap up to T=40 K, which is presumably associated with short-range CDW correlations. A qualitatively different behaviour was observed in the same material fo T>40 K. Here, the photo-conductivity was dominated by an enhancement in carrier mobility at constant density, suggestive of melting of the CDW correlations rather than excitation across an optical gap. The relaxation displayed a temperature dependent, Arrhenius-like kinetics, suggestive of the crossing of a free-energy barrier between two phases. These results support the existence of short-range CDW correlations above Tc in underdoped BaPb1-xBixO3, and provide new information on the dynamical interplay between superconductivity and charge order.Comment: 19 pages, 4 figure

Wavelength dependent optical enhancement of superconducting interlayer coupling in La1.885Ba0.115CuO4

We analyze the pump wavelength dependence for the photo-induced enhancement of interlayer coupling in La1.885Ba0.115CuO4, which is promoted by optical melting of the stripe order. In the equilibrium superconducting state (T < Tc = 13 K), in which stripes and superconductivity coexist, time-domain THz spectroscopy reveals a photo-induced blue-shift of the Josephson Plasma Resonance after excitation with optical pulses polarized perpendicular to the CuO2 planes. In the striped, non-superconducting state (Tc < T < T_SO = 40 K) a transient plasma resonance similar to that seen below Tc appears from a featureless equilibrium reflectivity. Most strikingly, both these effects become stronger upon tuning of the pump wavelength from the mid-infrared to the visible, underscoring an unconventional competition between stripe order and superconductivity, which occurs on energy scales far above the ordering temperature.Comment: 18 pages, 6 figure

Optically induced superconductivity in striped La2-xBaxCuO4 by polarization-selective excitation in the near infrared

We show that superconducting interlayer coupling, which coexists with and is depressed by stripe order in La1.885Ba0.115CuO4, can be enhanced by excitation with near-infrared laser pulses. For temperatures lower than Tc = 13 K, we observe a blue-shift of the equilibrium Josephson plasma resonance, detected by terahertz-frequency reflectivity measurements. Key to this measurement is the ability to probe the optical properties at frequencies as low as 150 GHz, detecting the weak interlayer coupling strengths. For T > Tc a similar plasma resonance, absent at equilibrium, is induced up to the spin-ordering temperature TSO = 40 K. These effects are reminiscent but qualitatively different from the light-induced superconductivity observed by resonant phonon excitation in La1.675Eu0.2Sr0.125CuO6.5. Importantly, enhancement of the below-Tc interlayer coupling and its appearance above Tc are preferentially achieved when the near-infrared pump light is polarized perpendicular to the superconducting planes, likely due to more effective melting of stripe order and the less effective excitation of quasiparticles from the Cooper pair condensate when compared to in-plane excitation.Comment: 19 pages, 5 figure

Nonlinear Optical Control of Josephson Coupling in Cuprates

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

Nonlinear Optical Control of Josephson Coupling in Cuprates

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

Anomalous relaxation kinetics and charge density wave correlations in underdoped BaPb_1-xBixO₃

Superconductivity often emerges in proximity of other symmetry-breaking ground states, such as antiferromagnetism or charge-density-wave (CDW) order. However, the subtle interrelation of these phases remains poorly understood, and in some cases even the existence of short-range correlations for superconducting compositions is uncertain. In such circumstances, ultrafast experiments can provide new insights by tracking the relaxation kinetics following excitation at frequencies related to the broken-symmetry state. Here, we investigate the transient terahertz conductivity of BaPb1−xBixO3––a material for which superconductivity is “adjacent” to a competing CDW phase––after optical excitation tuned to the CDW absorption band. In insulating BaBiO3 we observed an increase in conductivity and a subsequent relaxation, which are consistent with quasiparticles injection across a rigid semiconducting gap. In the doped compound BaPb0.72Bi0.28O3 (superconducting below TC = 7 K), a similar response was also found immediately above TC. This observation evidences the presence of a robust gap up to T ≃ 40 K, which is presumably associated with short-range CDW correlations. A qualitatively different behavior was observed in the same material for T≳ 40 K. Here, the photoconductivity was dominated by an enhancement in carrier mobility at constant density, suggestive of melting of the CDW correlations rather than excitation across an optical gap. The relaxation displayed a temperature-dependent, Arrhenius-like kinetics, suggestive of the crossing of a free-energy barrier between two phases. These results support the existence of short-range CDW correlations above TC in underdoped BaPb1−xBixO3, and provide information on the dynamical interplay between superconductivity and charge order

Terahertz Josephson plasma solitons in high-Tc superconductors

In cuprate high-Tc superconductors, superconducting layers are separated by insulating layers. Cooper pairs can go through the insulating layer via coherent quantum tunnelling. Such structure can be considered as stacks of Josephson junctions, and its spatial and temporal characteristics are described by a nonlinear equation known as the sine-Gordon equation. With a weak electromagnetic driving field, the layered structure shows a typical plasma response which allows the propagation of linear electromagnetic waves (known as Josephson plasma waves) with frequencies only above the Josephson plasma resonance (in the THz region). When the driving field increases, the tunnelling supercurrent approaches its critical value, and the electrodynamics becomes highly nonlinear

Strong-field polarization-state control of higher harmonics generated in crystalline solids

We demonstrate that the polarization states of higher harmonics emitted from crystalline solids (here silicon, quartz) are determined by both crystal symmetry and nonperturbative dynamics, opening the door to strong-field control of the harmonics’ polarization states