Ultrafast Electronic Phase Transition in ${\mathrm{La}}_{1/2}{\mathrm{Sr}}_{3/2}{\mathrm{MnO}}_4$ by Coherent Vibrational Excitation: Evidence for Nonthermal Melting of Orbital Order

An ultrafast electronic phase transition, associated with melting of orbital order, is driven in ${\mathrm{La}}_{1/2}{\mathrm{Sr}}_{3/2}{\mathrm{MnO}}_4$ by selectively exciting the Mn-O stretching mode with femtosecond pulses at $16\mu \mathrm{m}$ wavelength. The energy coupled into this vibration is less than 1% of that necessary to induce the transition thermally. Nonthermal melting of this electronic phase originates from coherent lattice displacements comparable to the static Jahn-Teller distortion.

Figure 1

Structure of ${\mathrm{La}}_{1/2}{\mathrm{Sr}}_{3/2}{\mathrm{MnO}}_4$. (a) Three dimensional structure of single-layer ${\mathrm{La}}_{1/2}{\mathrm{Sr}}_{3/2}{\mathrm{MnO}}_4$ consists of planes of oxygen octahedra separated by lanthanum and strontium dopants. A manganese atom occupies the center of the oxygen octahedra. (b) Below $T_{00}=220\mathrm{K}$, lattice-commensurate orbital ordering of Mn $3d$ $e_g$-like breaks the symmetry of the tetragonal lattice and results in optical birefringence. Near-edge optical properties derived from electronic hopping between adjacent Mn $3d$ atomic orbitals via hybridization with intermediate oxygen atoms.

Figure 2

Static and transient reflectivity spectra. Top: The vibrationally induced changes in reflectivity at 90 K, probed at 650 nm, reveal a prompt and long-lived product phase. Middle: The spectral response at 100 ps time delay (data points) closely corresponds to the observed changes in reflectivity when the sample is heated above $T_{00}$ (solid line), but not when the sample temperature is increased by 20 K (dashed line). Bottom: The spectral feature at 1.3 eV (arrow) is associated with orbital ordering. Above $T_{00}$ (290 K), spectral weight shifts to lower energies.

Figure 3

Ultrafast loss of birefringence. Selective excitation of the vibrational resonance at 80 meV results in the prompt destruction of orbital ordering and a loss of birefringence at 650 nm (black line). A cross correlation [gray (red) line] of the mid-IR pump and 650 nm indicates that melting occurs within the pulse width of the pump.

Figure 4

Threshold behavior. As the pump wavelength is tuned to resonance the threshold fluence drops dramatically. The central photon energy is shown and the horizontal bars represent the full width at half maximum of the pump bandwidth measured using linear interferometric autocorrelation. The complex part of the index of refraction is shown as a guide for the eye (solid line). Inset shows threshold behavior for three pump photon energies.