Search

Searching for Kitaev quantum spin liquids (QSLs) is a fascinating and challenging problem. Much effort has been devoted to honeycomb lattice candidates with strong spin-orbit coupling in $5d$-electron iridates and $4d$-electron ${\mathrm{RuCl}}_{3}$. Recently, theoretical studies suggested that $3{d}^{7}$ Co-based honeycomb materials with a high spin state $S=3/2$ and effective orbital angular momentum $L=1$ could also be promising candidates of a Kitaev QSL. One of the candidates, ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$, was revisited recently. The long-range magnetic order in ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$ can be suppressed by a very weak in-plane magnetic field, suggesting its proximity to a Kitaev QSL. Here, we perform time-domain terahertz spectroscopy measurements to study the magnetic excitations on ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$. We observe different magnon excitations upon increasing external magnetic field. In particular, the system is easily driven to a field-polarized paramagnetic phase, after the long-range magnetic order is suppressed by a weak field ${H}_{c2}$. The spectra beyond ${H}_{c2}$ are dominated by single-magnon and two-magnon excitations. Therefore, in the measured low-temperature region, the field-induced state is not a Kitaev QSL. We also compared the excitation spectra of ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$ with that of the widely studied $4d$-electron Kitaev candidate ${\mathrm{RuCl}}_{3}$ and addressed their similarities in magnetic excitations.

Here, the authors present a detailed investigation of the electrodynamic response of the ternary chalcogenide Bi${}_{2}$Rh${}_{3}$Se${}_{2}$ (of parkerite type), which is a putative charge density wave material. The study implements a combination of broadband optical conductivity and ultrafast pump-probe measurements of photoinduced changes in the reflectivity. Their measurements provide compelling evidence that the phase transition at 240 K in Bi${}_{2}$Rh${}_{3}$Se${}_{2}$ is not a purely structural phase transition, rather, it is linked to the formation of charge density wave order.

Utilizing femtosecond time-resolved pump probe spectroscopy, we investigated the ultrafast quasiparticle dynamics of the ferromagnetic Kondo lattice compound ${\mathrm{CeAgSb}}_{2}$. The photoinduced reflectivity demonstrates a positive relaxation process connected with simple metal physics at room temperature, while an additional negative decay channel associated with Kondo screening effect can be recognized immediately with temperature decreasing. The temperature-dependent amplitude of the latter exhibits a kink at ${T}^{*}\ensuremath{\approx}$ 45 K, which is close to the energy scale of the first excited crystal field splitting Ce $4f$ level. We propose that an orbital crossover is responsible for this intriguing feature. The dominating $4f$ orbitals that participated in the Kodno $c\ensuremath{-}f$ hybridization varies from the ground state below ${T}^{*}$ to multiple crystal field splitting levels. In the meantime, the hybridization gap changes from 12.6 to 220.4 K correspondingly, which is also in agreement with the fluence-dependent measurements.

We report an intense near-infrared pump, $c$-axis terahertz probe measurement on an electron-doped cuprate superconductor ${\mathrm{Pr}}_{0.88}{\mathrm{LaCe}}_{0.12}{\mathrm{CuO}}_{4}$ with ${T}_{c}=22$ K. The major effect of the intense pump is to induce a splitting of Josephson plasma edge below ${T}_{c}$, similar to the observation on hole-doped cuprate ${\mathrm{La}}_{1.905}{\mathrm{Ba}}_{0.095}{\mathrm{CuO}}_{4}$ in a superconducting state. The photoexcitation induced spectral change does not exhibit significant decay up to the longest measured time delay 210 ps. The result suggests that the intense near-infrared pump drives the system from a superconducting state with a uniform Josephson coupling to a metastable superconducting phase with modulated Josephson coupling strengths below ${T}_{c}$.

We use time-domain terahertz spectroscopy to measure the low energy conductivity and magnons in RuCl$_3$ under external magnetic field. At zero field, an oscillation with a frequency of 0.62 THz is clearly observed in time-domain spectrum below T$_N$, which is identified as a magnon excitation in the magnetic order state. The magnon excitation is not affected by the external magnetic field $\textbf{H}_{DC}$ when it is applied along the c-axis, but is clearly suppressed when $\textbf{H}_{DC}$ is applied within ab plane. More interestingly, when the magnetic component of THz wave $\textbf{h}(t)$ is perpendicular to the applied in-plane magnetic field, we observe another coherent oscillation at slightly higher energy scale at the field above 2 T, which is eventually suppressed for $H_{DC}>$5 T. The measurement seems to indicate that the in-plane magnetic field can lift the degeneracy of two branches of low energy magnons at $\Gamma$ point. The low energy optical conductivity calculated from the measured transmission spectrum is dominated by a broad continuum contribution, which is not affected by changing either temperature or external magnetic field. The continuum is likely to be related to the fractional spin excitation due to dominated Kitaev interaction in the material., Comment: 6 pages

We report near and midinfrared pump, $c$-axis terahertz probe measurements on a superconducting single crystal ${\mathrm{La}}_{1.905}{\mathrm{Ba}}_{0.095}{\mathrm{CuO}}_{4}$ with ${T}_{c}=32$ K. The measurements reveal that the pump-induced change occurs predominantly at the Josephson plasma edge position below ${T}_{c}$. Upon excitation by strong near-infrared pulses, the superconducting state is severely disturbed and incoherent quasiparticle excitations develop in the frequency regime above the static plasma edge. However, within a very short time delay $(\ensuremath{\sim}1.5$ ps) we observe the reappearance of a very sharp Josephson plasma edge at a frequency lower than the static Josephson plasma edge and the emergence of a new light-induced edge at a higher energy. The results imply that the light can induce new Josephson plasmon modes with different coupling strengths. A similar but weaker effect is observed for the midinfrared pump. No pump-induced effect is detected above ${T}_{c}$.