Optically induced superconductivity in striped ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ by polarization-selective excitation in the near infrared

We show that superconducting interlayer coupling, which coexists with and is depressed by stripe order in ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}{\mathrm{CuO}}_4$, can be enhanced by excitation with near-infrared laser pulses. For temperatures lower than $T_c=13$ 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 $T>T_c$ a similar plasma resonance, absent at equilibrium, is induced up to the spin-ordering temperature $T_{\mathrm{SO}}\simeq 40$ K. These effects are reminiscent but qualitatively different from the light-induced superconductivity observed by resonant phonon excitation in ${\mathrm{La}}_{1.675}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_{6.5}$. Importantly, enhancement of the below-$T_c$ interlayer coupling and its appearance above $T_c$ 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.

Figure 1

(a) Temperature-doping phase diagram of LBCO, as determined in Ref. [7]. $T_c,T_{\mathrm{CO}},T_{\mathrm{SO}}$, and $T_{\mathrm{LT}}$ indicate the superconducting, charge-order, spin-order, and structural transition temperatures, respectively. Colored circles indicate the different dopings and temperatures for which data are reported here. (b) Periodic stacking of ${\mathrm{CuO}}_2$ planes in the stripe phase. The stripe orientation rotates by 90° between layers. (c) Equilibrium $c$-axis optical properties of LBCO. Left panel: THz reflectivity of the three samples at $T$ = 5 K. The region investigated in this experiment is shaded in gray. Right panel and inset: broadband $c$-axis reflectivity and optical conductivity of LBCO from Ref. [27]. Red arrows indicate the pump photon energy.

Figure 2

THz reflectivity of LBCO displayed at different doping values and temperatures at equilibrium (gray) and 1.5 ps after excitation (colored). Data in panels (a) and (b) have been taken with a pump fluence of ∼2 ${\mathrm{mJ}/\mathrm{cm}}^2$ (a saturation in the fluence dependence of the pump-induced changes was found above ∼1 ${\mathrm{mJ}/\mathrm{cm}}^2)$. Those in panels (c) were taken instead with ∼3 ${\mathrm{mJ}/\mathrm{cm}}^2$. All spectra are shown in the region below 1600 GHz to highlight the pump-induced changes. The low-frequency cutoff is due to sample-size effects. Transient spectra cover a narrower range, due to the higher noise of the pump-probe measurement.

Figure 3

Energy loss function $\left[-\mathrm{Im}(1/\tilde{\varepsilon })\right]$ of ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}{\mathrm{CuO}}_4$ as a function of temperature and pump-probe delay. The lower panels show its evolution throughout the light-induced dynamics. The upper panels show selected line cuts at negative (gray), +1.5 ps (red), and +5 ps (blue) time delay. All data have been taken using a pump fluence of 2 ${\mathrm{mJ}/\mathrm{cm}}^2$.

Figure 4

[Panels (a.1), (a.2), (b.1), (b.2)] Complex optical conductivity of ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}{\mathrm{CuO}}_4$ at 5 and 30 K, shown at different pump-probe delays, for a fluence of 2 ${\mathrm{mJ}/\mathrm{cm}}^2$. In panels (b.1) and (b.2) examples of fits with a Drude model (black dots) and with a perfect-conductor model (red dots) are displayed. [Panels (a.3), (a.4), (b.3), (b.4)] Parameters extracted from the Drude fits as a function of pump-probe delay. The gray shaded region indicates the insulating regime (no Drude fit possible). The red shaded area refers to the highly coherent state (see main text).

Figure 5

Complex optical conductivity of ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}{\mathrm{CuO}}_4$ at 30 K, 1.5 ps after optical excitation with light polarized perpendicular (red) or parallel (black) to the ${\mathrm{CuO}}_2$ planes. Data are taken with the same pump fluence of 2 ${\mathrm{mJ}/\mathrm{cm}}^2$.