Ultrafast quasiparticle dynamics in correlated semimetal Ca${}_3$Ru${}_2$O${}_7$

Abstract

The correlated polar semimetal Ca${}_3$Ru${}_2$O${}_7$ exhibits a rich phase diagram including two magnetic transitions ($T_N$=56 K and $T_C$=48 K) with the appearance of an insulating-like pseudogap (at $T_C$). In addition, there is a crossover back to metallic behavior at $T^{\ast }$=30 K, the origin of which is still under debate. We utilized ultrafast optical pump optical probe spectroscopy to investigate quasiparticle dynamics as a function of temperature in this enigmatic quantum material. We identify two dynamical processes, both of which are influenced by the onset of the pseudogap. This includes electron-phonon relaxation and, below $T_C$, the onset of a phonon bottleneck hindering the relaxation of quasiparticles across the pseudogap. We introduce a gap-modified two-temperature model to describe the temperature dependence of electron-phonon thermalization, and use the Rothwarf-Taylor to model the phonon bottleneck. In conjunction with density functional theory, our experimental results synergistically reveal the origin of the $T$-dependent pseudogap. Further, our data and analysis indicate that $T^{\ast }$ emerges as a natural consequence of $T$-dependent gapping out of carriers, and does not correspond to a separate electronic transition. Our results highlight the value of low fluence ultrafast optics as a sensitive probe of low energy electronic structure, thermodynamic parameters, and transport properties of Ruddlesden-Popper ruthenates.

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• Thermodynamic and electron-transport properties of Ca3Ru2O7 from first-principles phonon calculations and Boltzmann transport theory
• Quasiparticle Relaxation Dynamics in URu${}_{2-x}$Fe${}_x$Si${}_2$ Single Crystals
• Nonlinear Transport Signatures of Hidden Symmetry Breaking in a Weyl Altermagnet