Signatures of Floquet Engineering in the proximal Kitaev Quantum Spin Liquid H$_3$LiIr$_2$O$_6$ by tr-RIXS

TL;DR

This work investigates whether circularly polarized Floquet driving can dynamically tune magnetic exchanges in the Kitaev magnet candidate H$_3$LiIr$_2$O$_6$ and whether such tuning can transiently realize a Kitaev quantum spin liquid state. By combining a Floquet-Kitaev spin Hamiltonian analysis with time-resolved RIXS measurements at the Ir $L_3$ edge, the authors observe a transient modulation of the magnetic continuum during laser illumination, consistent with non-equilibrium exchange renormalization. However, depth-probe mismatch and the intrinsic complexity of Kitaev magnets limit a definitive identification of the driven ground state or stabilization of the KQSL, highlighting key experimental and theoretical challenges. The study establishes Floquet engineering as a promising route to control Kitaev magnets while outlining concrete strategies—material choices, sample geometries, and theory development—needed to robustly access transient Kitaev quantum spin liquids with RIXS.

Abstract

We present the first circularly polarized Floquet engineering time-resolved Resonant Inelastic X-ray Scattering (tr-RIXS) experiment in H$_3$LiIr$_2$O$_6$, an iridium-based Kitaev system. Our calculations and experimental results are consistent with the modification of the low energy magnetic excitations in H$_3$LiIr$_2$O$_6$ only during illumination by the laser pulse, consistent with the Floquet engineering of the exchange interactions. However, the penetration length mismatch between the X-ray probe and laser pump and the intrinsic complexity of Kitaev magnets prevented us from unequivocally extracting towards which ground H$_3$LiIr$_2$O$_6$ was driven. We outline possible solutions to these challenges for Floquet stabilization and observation of the Kitaev Quantum Spin Liquid limit by RIXS.

Figures (5)

• Figure 1: Tr-Floquet-RIXS in H$_3$LiIr$_2$O$_6$. a) Crystal structure of H$_3$LiIr$_2$O$_6$, illustrating the layered arrangement of the IrO$_{6}$ octahedra (gray) and interlayer $H$ ions. b) Schematic of the electronic transitions involved in the RIXS process at the Ir $L_{3}$ edge. Left: A resonant x-ray excites a $2p_{3/2}$ core electron into an unoccupied $5d$ state above the Fermi level, creating an electron-hole pair. Right: An electron below the Fermi level decays, annihilating the core-hole, emitting a photon and returning to the original state. c) Schematic of the tr-Floquet-RIXS approach. The circularly polarized optical pump will induce non-equilibrium dynamics, and then, after a delay ($\Delta t$), an x-ray probe is used to measure the system's response via RIXS. These inter-orbital transitions provide insight into excited-state dynamics and interactions within the IrO$_{6}$ lattice.
• Figure 2: Characterization of the tr-RIXS experimental setup at the PAL-XFEL. a) X-ray absorption spectrum of IrO$_{2}$ powder sample measured in total fluorescence yield at room temperature. The vertical dashed line indicates the main Ir $L_3$ resonance at $E_{i} = 11214$ eV. b) Elastic scattering from a Scotch tape (circular markers) and fit to a Pseudo-Voigt profile (red dashed line) with a FWHM $= 95.7$ meV. c) Room temperature RIXS spectrum of Na$_2$IrO$_3$ measured at $\Gamma$ for 15 min. d) Transient light-induced modulation of the Bi (111) Bragg peak intensity after arrival of a 1900 nm laser pulse with $F = 97$ mJ cm$^{-2}$.
• Figure 3: Floquet Control of Exchange Parameters in H3LiIr2O6. (a) Light-induced modification of Heisenberg ($J (A,\omega)$) and Kitaev ($K(A,\omega)$) exchange couplings, with respect to their values in equilibrium ($J_{\rm eq}, K_{\rm eq}$), as a function of the Floquet parameter $F$ and pump frequency. (b) Normalized Floquet Kitaev exchange parameter $K_{eff} = K / [|J| + |K| + |\Gamma|]$. (c) Mean and standard deviation of the Floquet Heisenberg and Kitaev exchange parameters for $\Omega = 0.65 {\rm eV}$, averaged over variations of the electronic parameters (inter-orbital hopping and Coulomb interactions).
• Figure 4: Floquet engineering of the magnetic excitations in H$_3$LiIr$_2$O$_6$. a)-c) RIXS spectrum at resonance and $T = 100$ K with (on, red circular markers) and without (off, blue square markers) a $1900$ nm laser pulse at various time delays: a), $\Delta t = -1$ ps, the average of 5 individual scans of 12 min exposure for each condition b), $\Delta t = 0$ ps, the average of 10 individual scans, and c), $\Delta t = 5$ ps, the average of 4 individual scans. The vertical dashed line signal encodes the energy of the laser pump. $\alpha$ is the angle of incidence of the x-ray. d)-e) Relative change of the RIXS spectrum due to the laser pulse at d), $\Delta t = -1$ ps, e), $\Delta t = 0$ ps, and, f), $\Delta t = 5$ ps scans. The gray bar illustrates the mean standard deviation. Red and blue shading indicates increased or decreased intensity due to the laser excitation.
• Figure 5: Effects of the penetration depth mismatch. Evolution of a) the Electric field, $E,$, b) the Floquet parameter,$F$ and c) Floquet-induced modification of the exchange interactions, $\Delta J = J(A,\omega) - J_{eq}; \Delta K = K(A,\omega) - K_{eq}$ as a function of the sample depth.