Charge Density Wave Melting in One-Dimensional Wires with Femtosecond Subgap Excitation

Charge density waves (CDWs) are symmetry-broken ground states that commonly occur in low-dimensional metals due to strong electron-electron and/or electron-phonon coupling. The nonequilibrium carrier distribution established via photodoping with femtosecond laser pulses readily quenches these ground states and induces an ultrafast insulator-to-metal phase transition. To date, CDW melting has been mainly investigated in the single-photon regime with pump photon energies bigger than the gap size. The recent development of strong-field midinfrared sources now enables the investigation of CDW dynamics following subgap excitation. Here we excite prototypical one-dimensional indium wires with a CDW gap of $\sim 300\mathrm{meV}$ with midinfrared pulses at $\hslash \omega =190\mathrm{meV}$ with $\mathrm{MV}/\mathrm{cm}$ field strength and probe the transient electronic structure with time- and angle-resolved photoemission spectroscopy. We find that the CDW gap is filled on a timescale short compared to our temporal resolution of 300 fs and that the band structure changes are completed within $\sim 1\mathrm{ps}$. Supported by a minimal theoretical model we attribute our findings to multiphoton absorption across the CDW gap.

Figure 1

Equilibrium phase transition. Measured room temperature Fermi surface together with guides to the eye (black), a sketch of the ($4\times 1$) Brillouin zone (gray), and a red line indicating the direction along which the dispersion in (b) and (c) is measured. Measured band structure at room temperature (b) and at $T=40\mathrm{K}$ (c). (d) Difference between panel (c) and (b). Insets in (b) and (c) show the respective structure of the Indium wires. White lines in (b) are guides to the eye based on Ref. [31] that mark the position of the bands.

Figure 2

Light-induced phase transition. (a) Measured band structure at negative pump-probe delay. (b) Measured photocurrent in the presence of the 300 fs pump pulse at $\hslash \omega =190\mathrm{meV}$ with a peak electric field of $0.9\mathrm{MV}/\mathrm{cm}$. (c) Same as (b) but for a pump-probe delay of 1 ps. (d),(e) The pump-induced changes of the photocurrent obtained by subtracting panel (a) from panels (b) and (c), respectively. Black boxes in (e) indicate the area over which the pump-probe signal in Fig. 3 was integrated.

Figure 3

CDW melting dynamics. Pump-probe signal obtained by integrating the pump-induced changes of the photocurrent over the areas marked by the black boxes in Fig. 2. The gray-shaded region is the pump pulse. (a) In-gap spectral weight as a function of pump-probe delay for different peak field strengths from box (1). (b) Downshift of $m_1$ from box (2) with a rise time of $1.0\pm 0.2\mathrm{ps}$. (c) Upshift of $m_3$ from box (3) with a rise time of $0.9\pm 0.2\mathrm{ps}$ [37]. (d) Long-time dynamics of the spectral weight in boxes 1–3 yielding exponential life times of $6\pm 1\mathrm{ps}$. Thick lines in (b)–(d) are fits to the data [37].

Figure 4

Simulation. (a) Change in total energy $\mathrm{\Delta }E$ of the system as a function of time for different peak field strengths. The dashed line indicates the CDW condensation energy of 32 meV per ($4\times 2$) unit cell [37]. The gray-shaded area is the pump pulse. (b) Log-log plot of absorbed energy vs the square of the peak field. The red line has a slope of 2 as expected for two-photon absorption. White and blue areas distinguish the multiphoton ($\gamma >1$) and the tunneling regime ($\gamma <1$), respectively.