Laser-induced transient magnons in Sr3Ir2O7 throughout the Brillouin zone

Abstract

Although ultrafast manipulation of magnetism holds great promise for new physical phenomena and applications, targeting specific states is held back by our limited understanding of how magnetic correlations evolve on ultrafast timescales. Using ultrafast resonant inelastic X-ray scattering we demonstrate that femtosecond laser pulses can excite transient magnons at large wavevectors in gapped antiferromagnets and that they persist for several picoseconds, which is opposite to what is observed in nearly gapless magnets. Our work suggests that materials with isotropic magnetic interactions are preferred to achieve rapid manipulation of magnetism.

Keywords: iridates; time-resolved resonant X-ray scattering; transient magnetic excitations.

Fig. 1.

Demagnetization pathways in antiferromagnets. (A) Multiple spin configurations can give rise to the same macroscopic magnetization. These are indistinguishable in order parameter measurements, as commonly probed in ultrafast studies. Three such cases are sketched, corresponding to a reduced magnetic moment, a collective spin rotation, and a disordered state. The first two cases are uniform perturbations to the entire spin network, whereas the latter is due to short-range disorder of individual spins. (B) Relative magnetic (−3.5, 1.5, 18) Bragg peak intensity in ${\mathrm{S}\mathrm{r}}_3{\mathrm{I}\mathrm{r}}_2{\mathrm{O}}_7$ as a function of time delay (notation in reciprocal lattice units [r.l.u]). The data are plotted up to 7 ps after the arrival of the optical pump at $t$ = 0. The error bars follow Poissonian statistics.

Fig. 2.

Static and transient electronic short-range correlations of ${\mathrm{S}\mathrm{r}}_3{\mathrm{I}\mathrm{r}}_2{\mathrm{O}}_7$. Tr-RIXS spectra at the minimum (A) and maximum (B) of the dispersion. The spectra show an elastic line, a magnon, a magnon continuum, and an orbital excitation at $\sim$680 meV, before the arrival of the laser pump (blue) and for different time delays (orange). Error bars are determined via Poissonian statistics. The dotted lines are contributions of best fits to the data (solid lines), consisting of four Gaussians and a constant background. Their difference spectra are shown in C and D.

Fig. 3.

Ultrafast evolution of electronic correlations in ${\mathrm{S}\mathrm{r}}_3{\mathrm{I}\mathrm{r}}_2{\mathrm{O}}_7$. Time dependence of the relative (A) magnon and (B) orbital amplitude, $A$, and full width at half maximum (FWHM), $\mathrm{\Gamma }$, respectively. The magnon and orbital amplitudes are suppressed in the transient state and their width is enlarged. Note the difference in scale. Error bars are derived from the least-squares fitting algorithm.

Fig. 4.

Spin-bottleneck mechanism. (A) Heisenberg-like ${\mathrm{S}\mathrm{r}}_2{\mathrm{I}\mathrm{r}\mathrm{O}}_4$ features a near-zero-energy Goldstone mode establishing a well-defined channel (indicated schematically by the blue arrows) over which transient correlations decay into lower-energy multiparticle excitations. This leads to strong differences in the ultrafast magnetic response at ($\pi ,\pi$) and $\left(\pi ,0\right)$. (B) Such a decay channel is absent in gapped antiferromagnet ${\mathrm{S}\mathrm{r}}_3{\mathrm{I}\mathrm{r}}_2{\mathrm{O}}_7$, leading to transient magnons that are trapped in the entire Brillouin zone.