Recent developments in laser-excited photoemission electron microscopy (laser-PEEM) advance the visualization of electronic nematicity and nematic domain structures in iron-based superconductors. In FeSe and BaFe2(As0.87P0.13)2 superconductors, it has been reported that the thickness of the electronic nematic domain walls is unexpectedly long, leading to the formation of mesoscopic nematicity wave [T. Shimojima et al., Science 373, 1122 (2021)]. This finding demonstrates that the nematic coherence length xi nem can be decoupled from the lattice domain wall. Here, we report that the electronic domain wall thickness shows a distinct variation in related materials: it is similarly long in FeSe0.9S0.1 whereas it is much shorter in undoped BaFe2As2. We find a correlation between the thick domain walls and the nonFermi liquid properties of normal-state resistivity above the nematic transition temperature. This suggests that the nematic coherence length can be enhanced by underlying spin-orbital fluctuations responsible for the anomalous transport properties.

Even before its role in electroweak symmetry breaking, the Anderson-Higgs mechanism was introduced to explain the Meissner effect in superconductors. Spontaneous symmetry-breaking yields massless phase modes representing the low-energy excitations of the Mexican-Hat potential. Only in superconductors the phase mode is shifted towards higher energies owing to the gauge field of the charged condensate. This results in a low-energy excitation spectrum governed by the Higgs mode. Consequently, the Meissner effect signifies a macroscopic quantum condensate in which a photon acquires mass, representing a one-to-one analogy to high-energy physics. We report on the direct observation of the Higgs particle in the high-temperature superconductor Bi-2212 by developing an innovative technique to study its symmetries and energies after a "soft quench" of the Mexican-Hat potential. Population inversion of the metastable Higgs particle induced by an initial laser pulse allows identifying the polarization-dependent Higgs modes as an additional anti-Stokes Raman-scattering signal. Within Ginzburg-Landau theory, the Higgs-mode energy is connected to the Cooper-pair coherence length. Within a BCS weak-coupling model we develop a quantitative and coherent description of single-particle and two-particle channels. This opens the avenue for Higgs Spectroscopy in quantum condensates and provides a unique pathway to control and explore Higgs physics.

A major unsolved puzzle in cuprate superconductivity is that, despite accumulated evidence of more conventional normal state properties over the last 30 years, the superconducting $T_c$ of the overdoped cuprates seems to be still controlled by phase coherence rather than the Cooper pair formation. So far, a microscopic understanding of this unexpected behavior is lacking. Here we report angle-resolved photoemission, magnetic and thermodynamic evidence that Cooper pairs form at temperatures more than 30% above $T_c$ in overdoped metallic Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212). More importantly, our data lead to a microscopic understanding where the phase fluctuation is enhanced by the flat dispersion near the Brillouin zone boundary. This proposal is tested by a sign-problem free quantum Monte Carlo simulation. Such a microscopic mechanism is likely to find applications in other flat band superconductors, such as twisted bilayer-bilayer graphene and NdNiO$_2$

The nature of the pseudogap and its relationship with superconductivity are one of the central issues of cuprate superconductors. Recently, a possible scenario has been proposed that the pseudogap state is a distinct phase characterized by spontaneous rotational symmetry breaking called "nematicity" based on transport and magnetic susceptibility measurements, where the symmetry breaking was observed below the pseudogap temperature T^*. Here, we report a temperature-dependent ARPES study of nematicity in slightly overdoped Bi_1.7Pb_0.5Sr_1.9CaCu_2O_8+δ triggered by a uniaxial strain applied along one of the Cu-O bond directions. While the nematicity was enhanced in the pseudogap state as in the previous studies, it was suppressed in the superconducting state. These results indicate that the pseudogap state is characterized by spontaneous rotational symmetry breaking and that the nematicity may compete with superconductivity. These new experimental insights may provide clues for the nature of the pseudogap and its relation to the superconductivity.

Electronic nematicity, a correlated state that spontaneously breaks rotational symmetry, is observed in several layered quantum materials. In contrast to their liquid-crystal counterparts, the nematic director cannot usually point in an arbitrary direction (XY nematics), but is locked by the crystal to discrete directions (Ising nematics),resulting in strongly anisotropic fluctuations above the transition. Here, we report on the observation of nearly isotropic XY-nematic fluctuations, via elastoresistance measurements, in hole-doped Ba_1-xRb_xFe_2As_2 iron-based superconductors. While for x=0 the nematic director points along the in-plane diagonals of the tetragonal lattice, for x=1 it points along the horizontal and vertical axes. Remarkably, for intermediate doping, the susceptibilities of these two symmetry-irreducible nematic channels display comparable Curie-Weiss behavior, thus revealing a nearly XY-nematic state. This opens a new route to assess this elusive electronic quantum liquid-crystalline state, which is a candidate to host unique phenomena not present in the Ising-nematic case.

We investigated transient optical responses in an optimally-doped high-Tc superconductor La2-xSrxCuO4 (x=0.15) by using 800-nm optical pump and terahertz probe spectroscopy. With increasing the photoexcitation intensities, the Josephson plasma resonance shows a gradual redshift, indicating the suppression of superconductivity by the photoexcitation. With further increasing the photoexcitation intensities, a new longitudinal mode in the loss function spectrum appears and grows from the high energy side, accompanied by a new transverse mode as manifested in the conductivity spectrum. The observed spectra are described by the multilayer model with alternating interlayer Josephson couplings. The new longitudinal and transverse modes sustain much longer than several hundred picoseconds after the photoexcitation, indicating that the new metastable phase with possessing alternating interlayer Josephson couplings is induced by the strong photoexcitation.

In complex materials various interactions have important roles in determining electronic properties. Angle-resolved photoelectron spectroscopy (ARPES) is used to study these processes by resolving the complex single-particle self-energy and quantifying how quantum interactions modify bare electronic states. Here we employ a combined theoretical and experimental treatment of femtosecond time-resolved ARPES and show how population dynamics measured using time-resolved ARPES can be used to separate electron–boson interactions from electron–electron interactions. We demonstrate a quantitative analysis of a well-defined electron–boson interaction in the unoccupied spectrum of the cuprate Bi2Sr2CaCu2O8-x characterized by an excited population decay time that maps directly to a discrete component of the equilibrium self-energy not readily isolated by static ARPES experiments [1].

High transition temperature superconductors in cuprates exhibit the charge-density-wave fluctuations and the ferromagnetic time-reversal-symmetry-breaking fluctuation in the polar Kerr rotation experiments. We demonstrate that they share the same root of origin, and the underlying mechanism also leads to the pseudogap formation. The pseudogap formation, the charge-density-wave fluctuation, and the time-reversal-symmetry-breaking fluctuation are the consequent phenomena of the correlation. They are the basic notions in strongly correlated electron systems.