Magnetic-Field Tuning of Light-Induced Superconductivity in Striped ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$

Optical excitation of stripe-ordered ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ has been shown to transiently enhance superconducting tunneling between the ${\mathrm{CuO}}_2$ planes. This effect was revealed by a blueshift, or by the appearance of a Josephson plasma resonance in the terahertz-frequency optical properties. Here, we show that this photoinduced state can be strengthened by the application of high external magnetic fields oriented along the $c$ axis. For a 7 T field, we observe up to a tenfold enhancement in the transient interlayer phase correlation length, accompanied by a twofold increase in the relaxation time of the photoinduced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically enhanced interlayer coupling in ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photoinduced state may emerge from activated tunneling between optically excited stripes in adjacent planes.

Figure 1

(a) Temperature-doping phase diagram of ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$, as determined in Ref. [9]. $T_C$, $T_{\mathrm{SO}}$, and $T_{\mathrm{CO}}$ indicate the superconducting, spin-order, and charge-order transition temperatures, respectively. The inset shows the 90° periodic stacking of ${\mathrm{CuO}}_2$ planes in the stripe phase. (b) Cartoon depicting the experimental geometry. We used optical pump and THz probe pulses polarized both along the $c$ axis. The experiment was performed in presence of magnetic fields up to 7 T, oriented along the $c$ direction. (c) Equilibrium $c$-axis optical properties of LBCO. Left panel: THz reflectivity of both samples at $T=5\mathrm{K}$ (no magnetic field), resulting from fits to the experimental data [10]. Right panel and inset: broadband $c$-axis reflectivity and optical conductivity of LBCO ($x=9.5\%$) from Ref. [19]. Red arrows indicate the pump photon energy.

Figure 2

(a),(b) $c$-axis THz reflectivity of ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}{\mathrm{CuO}}_4$ measured at $T=5\mathrm{K}$ (no magnetic field), at equilibrium (gray lines) and at two pump-probe time delays ($\tau =1.4$ and $\tau =2\mathrm{ps}$) after excitation (blue circles). Fits to the transient spectra are shown as blue lines. (e),(f) Same quantities as in (a),(b) measured in presence of a 7 T magnetic field (transient data are shown here in red). (c),(d) Zero-field imaginary conductivity spectra corresponding to the reflectivities shown in (a),(b). (g),(h) In-field imaginary conductivities corresponding to the data of (e),(f).

Figure 3

Dynamical evolution of the interlayer phase correlation length measured in ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}{\mathrm{CuO}}_4$ for different temperatures and applied magnetic fields. The values were extracted from the fits of Fig. 2 [10].

Figure 4

Dynamical evolution of the energy loss function of photostimulated LBCO under various starting conditions (different doping levels, sample temperatures, and applied magnetic field values), as extracted from fits to the data. Panels (a),(b) refer to ${\mathrm{La}}_{1.905}{\mathrm{Ba}}_{0.095}{\mathrm{CuO}}_4$ at $T=5\mathrm{K}$ in absence (a) and in presence (b) of a 7 T magnetic field. Panels (c)–(f) show instead the response of ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}{\mathrm{CuO}}_4$, measured at $T=5\mathrm{K}$ and $H=0$ (c), $T=30\mathrm{K}$ and $H=0$ (d), $T=5\mathrm{K}$ and $H=3.5\mathrm{T}$ (e), $T=5\mathrm{K}$ and $H=7\mathrm{T}$ (f) [10].