Terahertz light driven coherent excitation of a zone-folded Raman-active phonon mode in the Spin-Ladder System $α'$-NaV$_2$O$_5$

TL;DR

The paper demonstrates THz-driven nonlinear phononics as a mechanism to coherently excite a zone-folded Raman-active phonon in the spin-ladder compound alpha'-NaV2O5, via resonant driving of IR-active phonons by intense THz pulses. By combining pump–probe measurements with a coupled-phonon equation-of-motion model, the authors show that the Raman mode is excited through a quadratic IR-phonon coupling term, requiring spectral overlap with IR-active modes and exhibiting a quadratic dependence on the THz field. The NIR pump fails to generate similar coherent dynamics, highlighting the selectivity and efficiency of THz-driven phononic pathways. The work reveals symmetry- and charge-order–dependent nonlinear couplings in a strongly correlated lattice, and points to ab initio and ultrafast X-ray studies to quantify the couplings and to control spin–lattice dynamics for future ultrafast devices.

Abstract

We investigate the out-of-equilibrium lattice dynamics in the spin-ladder system $α'$-NaV$_2$O$_5$ using intense terahertz (THz) pump and near-infrared (NIR) probe spectroscopy. When quasi-single-cycle THz pulses interact with $α'$-NaV$_2$O$_5$ in its low-temperature, dimerized charge-ordered phase, they induce coherent oscillations in the time domain at the zone-folded Raman-active phonon frequency of 1.85 THz. By combining pump-probe measurements with lattice dynamics modeling based on equation-of-motion approach, we propose that these oscillations arise from a nonlinear coupling between Raman-active and infrared (IR)-active phonon modes, with the latter being resonantly excited by the THz pulses. In contrast, excitation with NIR femtosecond laser pulses does not produce measurable vibrational dynamics, highlighting the unique potential of THz-driven, nonlinear light-matter interactions for the coherent and selective control of structural dynamics in quantum materials.

Figures (9)

• Figure 1: (a) Schematic representation of the spin-ladder system formed by the Vanadium (V) and oxygen atoms (O). Below $T_c=34 K$, the system is in a charge-ordered phase, with in-equivalent $V^{4+}$ and $V^{5+}$. (b) Experimental setup. An intense THz pulse (yellow) with the electric field polarized linearly along the spin-ladder (b-axis) of $\alpha'$-NaV$_2$O$_5$ excites the crystal and induces a polarization change on a femtosecond NIR probe pulse (red). (c-d) THz electric field, measured by electro-optic sampling (EOS), and induced polarization changes of the probe as a function of the time delay $t_{pp}$. (e) Fourier amplitude spectrum of the THz pump waveform (yellow solid line) and real part of the optical conductivity $\sigma_1$ along the $b$-axis of $\alpha'$-NaV$_2$O$_5$ at 4.4 K, taken from room04. (f) Fourier transform of the temporal derivative of $\Delta R/R$ shown in panel (d). Black arrows indicate the frequencies of the Raman-active modes from Ref. lemmens1998.
• Figure 2: (a) Temperature evolution of the THz pump-induced $\Delta R/R$ in $\alpha'$-NaV$_2$O$_5$. No prominent signature of coherent oscillations is observed above $Tc=34 K$. Vertical offsets have been added to the curves for better visualization. (b) Fourier transform of $d(\Delta R/R)/dt_{pp}$ for T= 5 and 60 K. Black dotted curves are Lortentzian fit, to evaluate the amplitude and the center mode frequency $\Omega_R$. (c-d) Normalized amplitude and center frequency $\Omega_R$ as a function of temperature determined by Lorentzian fit.
• Figure 3: (a), Normalized oscillations amplitude (Norm. Osc. Ampl.) vs normalized THz field strength. The amplitude scales quadratically with the pump electric field (the light blue solid line represents a quadratic fit). Error bars indicate the standard deviations of the mean determined from pump-probe scan measurements, (b) THz pump electric field waveform $E^{(+)}$ (dashed line) and polarity inverted one $E^{(-)}$ and corresponding THz induced $\Delta R/R(t_{pp})$ dynamics.
• Figure 4: (a)-(d), THz pump excitation spectra. The dotted lines indicate the resonance frequency of the Raman phonon, $\Omega_R$, and IR-active phonons, $\Omega_{IR,1}$, $\Omega_{IR,2}$, and $\Omega_{IR,3}$. (e)-(f), Corresponding THz-induced $\Delta R/R$ dynamics. $\alpha'$-NaV$_2$O$_5$ is kept at 5 K. The black dashed line in panel (h) is the exponential fit, see main text. (i)-(n), Numerical calculation using Eq. (4) based on nonlinear phononic model.
• Figure 5: Comparison between THz pump and NIR pump induced $\Delta R/R(t_{pp})$ in $\alpha'$-NaV$_2$O$_5$. The sample is kept at 5 K. NIR pump fluence is $\sim$ 3 mJcm$^{-2}$. The dotted black lines are single-exponential fit. Vertical offset has been added for better visualization.