Persistent short-range charge correlations revealed by ultrafast melting of electronic order in YBa$_2$Cu$_3$O$_{6+x}$

Abstract

Charge density waves (CDW) are ubiquitous in the complex phase diagram of cuprate superconductors and exhibit both short- and long-range correlations. Using time-resolved resonant X-ray scattering, we investigate the photo-induced dynamics of CDW in YBa$_2$Cu$_3$O$_{6.67}$. We discover an excitation threshold ($Φ$$_\mathrm{C}$ $\approx$ 65 $μ$J/cm$^2$) above which long-range CDW disappear, revealing a persistent CDW peak with short-range correlation length. Ultrafast photo-excitation promptly uncovers this residual short-range CDW correlations, appearing within $\approx$ 0.2 ps. Long-range CDW coherence recovers within $\approx$ 0.6 ps, while the peak intensity remains partially suppressed. We rationalize the dichotomic behavior in the fluence and temporal dependencies as the signature of two coexisting CDW peaks, arising from short- and long-range correlations, which we disentangle through their distinct response to photo-excitation. We provide evidence that the collapse of long-range correlations is driven by an electronic process, while short-range correlations are characterized by distinct timescales and stiffness against photo-excitation. This approach establishes ultrafast X-ray scattering as an effective tool for disentangling coexisting density waves and correlations in quantum materials.

Figures (5)

• Figure 1: (a) Schematic diagram of the experiment. The 1.55 eV laser pump is utilized to perturb, while soft X-ray (931.5 eV) pulses probe the CDW of underdoped YBCO. (b) CDW peak profiles with small and large detector as a function of momentum before photo-excitation. (c) CDW peak dynamics as a function of delay time between pump and probe. The black vertical dotted lines in (b) and (c) mark the peak momentum and delays where the photoinduced signal is maximal, as considered in Figs. 2 and 3.
• Figure 2: (a)-(c) CDW peak dynamics at three different fluence, 25, 51, and 76 $\mu$J/cm$^{2}$. The solid lines are the results of the fit of the data. (d) Fluence dependence of the normalized CDW peak decrease. Red circles represent data collected at a fixed time delay of 0.3 ps with varying pump fluence. Results are consistent with the peak values extracted from the time-domain scans in panels (a)-(c) (green triangles). Black squares represent similar data from Ref. Wandel2022. The gray curve shows a saturation function fit to the data.
• Figure 3: Fluence-dependent evolution of CDW characteristics. (a-e) CDW momentum profiles at $\tau$$\approx$ 0.3 ps for different pump fluences. All data are normalized to the unpumped CDW peak amplitude, following the subtraction of the fitted polynominal background as shown in Fig.1b. (f) Correlation length along the a-axis as a function of fluence. (g) Fluence dependence of the CDW peak amplitude. The thick dark gray line is the same saturation function used in Fig.2(d). The red and blue shaded area indicate that the dominance of the narrow peak (long-range) and the broad peak (short-range). (h) Evolution of peak momentum vector as a function of fluence. Error bars represent one standard deviation derived from the fits in panels (a-e).
• Figure 4: Temporal evolution of CDW dynamics following photo-excitation above the critical fluence threshold. (a) Momentum-resolved CDW scattering profiles at selected time delays relative to pump pulse arrival (t = 0). Color scheme: light pink (before time zero); red, pink, and purple curves represent profiles at 200 fs, 400 fs, and 600 fs post-excitation, respectively. (b) Time-resolved CDW peak intensity normalized to pre-excitation values. Black circles represent experimental data; thick gray line shows bi-exponential fit to the full temporal response. Thick blue line indicates the slow decay component isolated after subtracting the fast decay component from the fit. Black squares denote peak intensities extracted from momentum-resolved profiles in panel (a), demonstrating excellent agreement with time-domain trace at fixed $Q=Q_{CDW}$. (c) Integrated peak area, (d) correlation length, and (e) peak momentum position as functions of pump-probe delay. Thick colored lines represent temporal evolution predicted by fast (red, gray) and slow (blue) decay components derived from the bi-exponential analysis in panel (b), illustrating the distinct recovery dynamics of amplitude versus spatial coherence. Error bars represent one standard deviation derived from the fits in panel (a).
• Figure 5: (a) Schematic evolution of long- (solid red line) and short-range (solid blue line) CDW contributions to the scattering peak using the two-peak model discussed in the main text and in the SI Supplementary. The effect of photo-excitation is primarily to decrease the amplitude of the long-range order (red dashed line). (b) Differential change of the CDW scattering profile measured with enhanced momentum resolution. Red circles denote the experimental data, black line is the best fit with a differential model where the CDW amplitude is decreased following photo-excitation. The dashed blue line shows the case of a differential model where the CDW peak broadens instead. Full data and model in Ref.Supplementary.