Correlating quasiparticle excitations with quantum femtosecond magnetism in photoexcited nonequilibrium states of insulating antiferromagnetic manganites

We describe a mechanism for insulator-to-metal transition triggered by spin canting following femtosecond laser excitation of insulating antiferromagnetic (AFM) states of colossal magnetoresistive (CMR) manganites. We show that photoexcitation of composite fermion quasiparticles dressed by spin fluctuations results in the population of a broad metallic conduction band due to canting of the AFM background spins via strong electron-spin local correlation. By inducing spin canting, photoexcitation can increase the quasiparticle energy dispersion and quench the charge excitation energy gap. This increases the critical Jahn-Teller (JT) lattice displacement required to maintain an insulating state. We present femtosecond-resolved pump-probe measurements showing biexponential relaxation of the differential reflectivity below the AFM transition temperature. We observe a nonlinear dependence of the ratio of the femtosecond and picosecond relaxation component amplitudes at the same pump fluence threshold where we observe femtosecond magnetization photoexcitation. We attribute this correlation between nonlinear femtosecond spin and charge dynamics to spin/charge/lattice coupling and population inversion between the polaronic majority carriers and metallic quasielectron minority carriers as the lattice displacement becomes smaller than the critical value required to maintain an insulating state following laser-induced spin canting.

Figure 1

(a) Illustration of the intersite electronic excitations in the CE-AFM/CO/OO three-dimensional unit cell considered here, which consists of 16 sites in two stacked AFM planes and two AFM-coupled FM zigzag chains with alternating JT-distorted (bridge) and undistorted (corner) sites [9]. (b) Hopping of a composite fermion quasiparticle between two AFM sites induces quantum spin canting by forming a local state with total spin $M=S-1/2$ and $J=S+1/2$. Such intersite charge-spin coupling is described by the Hubbard oparator amplitudes $\langle {\widehat{e}}_{{\alpha }^{\prime }{\sigma }^{\prime }}^{†}\left(i^{\prime }{M}^{\prime }\right){\widehat{e}}_{\alpha \sigma }\left(iM\right)\rangle$ as discussed in the text. (c) Calculated time-dependence of the total $z$-component $S_z\left(t\right)$ of the two AFM core spins discussed in the text, driven by the coupling of a 100-fs optical field pulse, for population lifetime $T_1=1$ ps and different Rabi energies $d_0$.

Figure 2

Time-dependent changes in the populations of the local configurations $|i\alpha M\rangle$ (top) and $|im\rangle$ (bottom) of the two AFM sites $i=1$ and 2 discussed in the text for population lifetime $T_1=1$ ps and different dephasing times $T_2$: [(a) and (b)] $T_2=50$, [(c) and (d)] 15, and [(e) and (f)] 8 fs. The spin quantization axis was taken parallel to the equilibrium spin direction in site 1.

Figure 3

Calculated energy dispersions of composite-fermion quasiparticles and comparison to bare electrons for intermediate [(a) and (b)] and large [(c) and (d)] values of JT energy barrier $E_{\mathrm{JT}}$. [(a) and (c)] Quantum spins ($S=3/2$) and [(b) and (d)] classical spins ($S\to \infty$).

Figure 4

Calculated probability of flipped spins for quasiparticles below (left) and above (right) the energy gap. $\sigma =1$ ($-1$) means quasiparticle total spin parallel (antiparallel) to the equilibrium spin direction determined by ${\theta }_i=0$ or $\pi$. The pronounced difference in quantum spin canting between a quasielectron and a quasihole correlates with the difference in their energy dispersions shown in Fig. 3.

Figure 5

Calculated energy levels as function of lattice displacement, $E_{\mathrm{JT}}={\varepsilon }_{\mathrm{JT}}Q$, for different spin canting angles $\theta$. The latter describes FM correlation between the FM chains and planes with respect to the collinear AFM state $\theta =0$. (Top) Bare electrons (adiabatically decoupled classical spins, $S\to \infty$). (Bottom) Composite fermion quasiparticles (strongly coupled quantum spins, $S=3/2$). ${\varepsilon }_{\mathrm{JT}}=1.6t_0$.

Figure 6

Effect of FM correlation on the lattice dependence of the energy gap of Fig. 5: (a) bare electrons, increasing $\theta$, (b) composite fermion quasiparticles, increasing $\theta$, and (c) composite fermion quasiparticles, increasing $\mathrm{\Delta }{J}_z$.

Figure 7

(a) Femtosecond-resolved $\mathrm{\Delta }\mathrm{R}$/R for 1.55 eV pump/probe excitation, plotted on a logarithmic scale. Dashed lines highlight two distinct components of biexponential decay. [(b) and (c)] Comparison of normalized $\mathrm{\Delta }\mathrm{R}$/R for high and low pump fluences at temperatures (b) 30 K (below the AFM transition) and (c) 300 K (above the CO/OO and AFM transitions).

Figure 8

Detailed temperature dependence of the fs-resolved $\mathrm{\Delta }\mathrm{R}$/R, shown on a logarithmic scale together with the pulse autocorrelation (shade) for (a) 300, 220, 180, and 160 K above the AFM transition temperature; (b) 140, 100, 60, and 30 K below the AFM transition temperature. The distinct ${\tau }^{\mathrm{fs}}$ and ${\tau }^{\mathrm{ps}}$ relaxation components are marked for the 30-K trace (blue lines).

Figure 9

(a)–(c) Ultrafast $\mathrm{\Delta }R/R$ dynamics for degenerate 1.55 eV pump/probe photoexcitation. (a) 2D dependence on pump fluence and time delay at 30 K; (b) peak amplitude as function of pump fluence; (c) temporal trace at pump fluence of 3.8 mJ ${\mathrm{cm}}^{-2}$ marked in (a). (d)–(f) Same $\mathrm{\Delta }R/R$ plot as above, but for nondegenerate photoexcitation with 1.55 eV pump but 3.1 eV probe.

Figure 10

Dependence of spin and charge dynamics on photoexcitation intensity. (a) Photoinduced fs magnetization $\mathrm{\Delta }M$ extracted from magneto-optical ellipticity signal $\mathrm{\Delta }{\eta }_k$ at 200 fs (green rectangle). (Inset) Comparison of $\mathrm{\Delta }{\eta }_k$ and $\mathrm{\Delta }{\theta }_k$ (Kerr rotation) dynamics for 5.6 (red) and 0.8 mJ ${\mathrm{cm}}^{-2}$ (black). Both signals show the same “sudden” fs jump above an intensity threshold, which we thus attribute to photoinduced magnetization during 100-fs time scales (arrow). All error bars are within the markers. (b) Amplitudes of fast component $A_{\mathrm{fs}}$ (black dots), slow component $A_{\mathrm{ps}}$ (red dots), and $A^{\mathrm{sum}}$= $A_{\mathrm{fs}}+A_{\mathrm{ps}}$ (inset) of our biexponential fit of $\mathrm{\Delta }R\left(t\right)/R$. (c) Fraction $F=A^{\mathrm{fs}}/A^{\mathrm{sum}}$ (blue rhombus) and the two distinct relaxation times (inset) as function of pump intensity.