Kavli Affiliate: Leon Balents
| First 5 Authors: Gu-Feng Zhang, Sheikh Rubaiat Ul Haque Kelson J. Kaj, Xiang Chen, Urban F. P. Seifert, Jingdi Zhang
| Summary:
Antiferromagnets exhibit rapid spin dynamics in a net zero magnetic
background which enables novel spintronic applications and interrogation of
many-body quantum phenomena. The layered antiferromagnet Sr$_2$IrO$_4$ hosts an
exotic spin one-half Mott insulating state with an electronic gap arising from
on-site Coulomb repulsion and strong spin-orbit coupling. This makes
Sr$_2$IrO$_4$ an interesting candidate to interrogate dynamical attributes of
the magnetic order using ultrafast laser pulses. We investigate the
magnetization dynamics of Sr$_2$IrO$_4$ following circularly-polarized
photoexcitation with below-gap mid-infrared (mid-IR — 9 $mu m$) and above-gap
near-infrared (near-IR — 1.3 $mu m$) pulses. In both cases, we observe
excitation of a zone-center coherent magnon mode featuring a 0.5 THz
oscillation in the pump-induced Kerr-rotation signal. However, only below-gap
excitation exhibits a helicity dependent response and linear (quadratic)
scaling of the coherent magnon amplitude with excitation fluence (electric
field). Moreover, below-gap excitation has a magnon generation efficiency that
is at least two orders of magnitude greater in comparison to above-gap
excitation. Our analysis indicates that the helicity dependence and enhanced
generation efficiency arises from a unique one-photon two-magnon coupling
mechanism for magnon generation. Thus, preferential spin-photon coupling
without photoexcitation of electrons permits extremely efficient magnon
generation. Our results reveal new possibilities for ultrafast control of
antiferromagnets.
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