Light-induced Asymmetric Pseudogap below T$_\text{c}$ in cuprates

Abstract

To this day, high-temperature cuprate superconductors remain an unparalleled platform for studying the competition and coexistence of emergent, static and dynamic, quantum phases of matter exhibiting high transition temperature non-s-wave superconductivity, non-Fermi liquid transport and a still enigmatic pseudogap regime. However, how superconductivity emerges alongside and competes with the pseudogap regime remains an open question. Here, we present a high-resolution, time- and angle-resolved photoemission study of the near-antinodal region of optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. For a sufficiently high excitation fluence, we disrupt superconductivity and drive a transient change from a symmetric superconducting-like to an asymmetric pseudogap-like density of states, for electronic temperatures well below the equilibrium superconducting critical temperature. Conversely, when the superconductivity is fully restored, the pseudogap is suppressed, as signaled by a fully particle-hole symmetric density of states. A unique aspect of our experiments is that the pseudogap coexists with superconducting features at intermediate times or at intermediate fluence. Our findings challenge the paradigm that superconductivity emerges by establishing phase coherence in the pseudogap. Instead, our experimental results, supported by phenomenological theory, demonstrate that the two states compete, and that the low-temperature ground state of the cuprates originates from a competition between superconducting and pseudogap states.

Figures (15)

• Figure 1: Emergence of the light-induced pseudogap state.a Pictorial sketch of the equilibrium superconducting (left) and pseudogap (right) states in cuprates. These two states correspond, respectively, to the particle-hole symmetric (red) and asymmetric (blue) density of states (DOS) displayed in b. Light excitation breaks pairs while only slightly increasing the electronic temperature, thereby prompting the emergence of a light-induced pseudogap with a particle-hole asymmetric DOS (panel a, middle, and thick violet curve in b).
• Figure 2: Fermi surface mapping and tracking of the transient electronic temperature.a Fermi surface mapping of Bi2212-OP91 ([-15;15] meV integration window). The red circle shows the maximum momentum reachable with a 6 eV photon probe, corresponding to the edges of the photoemission cone. The green dashed lines indicate the three momentum cuts examined in this work. b ARPES map along the nodal direction. c Transient evolution of the electronic temperature T$_e(\tau)$, extracted by fitting the momentum-integrated nodal energy distribution curves (EDCs, red arrow in panel c) via Fermi-Dirac fit, for two excitation regimes: low fluence (LF, blue) and high fluence (HF, red).
• Figure 3: Transient evolution of the off-nodal and near-antinodal superconducting gap.a and d$\tau <$0 (left) and differential (right, obtained by subtracting the $\tau <$0 map from its counterpart at $\tau =$1 ps) ARPES maps along cuts 1 and 2 defined in Fig. \ref{['Fig1']}a. The red arrow indicates the integration region for the $\text{TDOS}(\omega)$ extraction. b and e transient evolution of the TDOS for cuts 1 and 2, respectively, in the LF case at various pump-probe delays. The EDCs are plotted up to the experimental limit dictated by $4.5\times$$k_\text{B}$T$_e(\tau)$. The superimposed Dynes fit (dashed lines) captures well the evolution of the DOS from the equilibrium SC DOS through a transient intermediate state back to the SC state. Under low fluence conditions (panels b, e) the DOS remains particle-hole symmetric and well fit by the standard Dynes form (dashed lines) indicating a slightly disrupted superconducting state. c and f transient evolution of the TDOS for cuts 1 and 2, respectively, in the HF case at various pump-probe delays. In this case, we report a transient pseudogap-like asymmetric TDOS spectrum, as highlighted by 0.6 ps and 1 ps pump-probe delays. For these delays, the TDOS is fundamentally different from the SC DOS, exhibiting a marked particle-hole asymmetry that is inconsistent with the Dynes fit, as emphasized by the red shadows between the experimental TDOS and the Dynes fit. Additional pump-probe delays are shown in Fig. S2.
• Figure 4: Non-thermal nature of the light-induced pseudogap.a and b Comparison of TDOS curves along cut 1 taken at different pump-probe delays in the LF (0.5 ps and 3 ps) and HF (1.1 ps and 4 ps) regime, but with similar $\text{T}_e$, namely $\sim$78 K in c and $\sim$63 K in d. The spectral weight symmetry is restored at lower $\text{T}_e$, as shown in b. c and d Momentum-integrated nodal EDCs, extracted from Fig. \ref{['Fig1']}b in the LF and HF cases, for the two $\text{T}_e$ discussed in a and b. In both cases, a single Fermi-Dirac fit successfully captures both curves at different pump-probe delays in the LF and HF regimes. e Transient evolution of the linear slope ratio of the TDOS at cut 1 as defined in the main text, extracted in the range [-17;-7] meV and [7;17] meV below and above $E_\text{F}$, respectively. In the LF regime (blue), the ratio is approximately 1, evidence of particle-hole symmetry. Instead, in the HF regime (red), we report a reduction in the slope ratio, with maximal variation at around 1 ps pump-probe delay, highlighting the transient spectral weight asymmetry.
• Figure 5: Competition and coexistence between superconductivity and pseudogap.a Experimental (top) and theoretical (bottom) TDOS in the pseudogap state. Experimental TDOS curves along the three momentum cuts of Fig. \ref{['Fig1']}a have been acquired at 130 K, well above T$_\text{c}$. Theoretical parameters have been adjusted to generate a theoretical pseudogap curve that closely resembles the experimental data (details in the Methods and Supplementary Information). b comparison of three experimental TDOS curves along cut 1: Equilibrium pseudogap phase at 130 K (orange), light-induced pseudogap state at 1 ps in the HF regime (green), and restored superconducting particle-hole symmetric state at 3.5 ps in the HF regime (blue). c Simulated curves using the phenomenological self-energy discussed in the Supplementary Information. When only pseudogap (orange) or SC (blue) correlations are considered, the light-induced pseudogap is not reproduced. Instead, the coexistence of SC and the pseudogap (green) provides a better fit to the experimental evidence in panel b. When long-range SC correlations are restored, the particle-hole symmetric SC contribution alone (blue) reproduces the data.