Shangfei Wu

Spontaneous symmetry breaking—the phenomenon in which an infinitesimal perturbation can cause the system to break the underlying symmetry—is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher-temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon upon cooling in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. FeGe has a kagome lattice structure with simple A-type antiferromagnetic order below Néel temperature T N≈ 400 K and a charge density wave (CDW) transition at T CDW≈ 110 K, followed by a spin …

We employ polarization-resolved electronic Raman spectroscopy and density functional theory to study the primary and secondary order parameters, as well as their interplay, in the charge density wave (CDW) state of the kagome metal A V 3 Sb 5. Previous x-ray diffraction data at 15 K established that the CDW order in CsV 3 Sb 5 comprises of a 2× 2× 4 structure: one layer of inverse-star-of-David and three consecutive layers of star-of-David pattern. We analyze the lattice distortions based on the 2× 2× 4 structure at 15 K, and find that the U 1 lattice distortion is the primarylike (leading) order parameter while M 1+ and L 2− distortions are secondarylike order parameters for vanadium displacements. This conclusion is confirmed by the calculation of bare susceptibility χ 0′(q) that shows a broad peak at around q z= 0.25 along the hexagonal Brillouin zone face central line (U line). We also identify several phonon …

We use polarization-resolved electronic Raman spectroscopy to study quadrupolar charge dynamics in a nonmagnetic superconductor. We observe two types of long-wavelength symmetry excitations: 1) a low-energy quasi-elastic scattering peak (QEP) and 2) a broad electronic continuum with a maximum at 55 meV. Below the tetragonal-to-orthorhombic structural transition at , a pseudogap suppression with temperature dependence reminiscent of the nematic order parameter develops in the symmetry spectra of the electronic excitation continuum. The QEP exhibits critical enhancement upon cooling toward . The intensity of the QEP grows with increasing sulfur concentration and maximizes near critical concentration , while the pseudogap size decreases with the suppression of . We interpret the development of the pseudogap in the quadrupole scattering channel as a …

High-T c cuprate superconductors host spin, charge, and lattice instabilities. In particular, in the antiferromagnetic glass phase, over a large doping range, lanthanum-based cuprates display a glass-like spin freezing with antiferromagnetic correlations. Previously, sound velocity anomalies in La 2− x Sr x CuO 4 (LSCO) for hole doping p= x≥ 0.145 were reported and interpreted as arising from a coupling of the lattice to the magnetic glass [M. Frachet, I. Vinograd et al., Nat. Phys. 16, 1064 (2020)]. Here we report both sound velocity and attenuation in LSCO p= 0.12, ie, at a doping level for which the spin freezing temperature is the highest. Using high magnetic fields and comparing with nuclear magnetic resonance measurements, we confirm that the anomalies in the low temperature ultrasound properties of LSCO are produced by a coupling between the lattice and the spin glass. Moreover, we show that both sound …

We study the coupling of the fully symmetric vibration mode of arsenic atoms to magnetism in a Ba (Fe 1− x Au x) 2 As 2 system by polarization-resolved Raman spectroscopy and neutron diffraction. In this system, there are two phase transitions: a tetragonal-to-orthorhombic structural phase transition at temperature T S and a magnetic phase transition into collinear spin-density wave (SDW) state at temperature T N (≤ T S). T S and T N almost coincide in the pristine compound, whereas they differ by as much as 8 K for compounds with dilute gold substitution for iron. Raman coupling to the A g (As) phonon is forbidden for the X Y scattering geometry in the tetragonal phase above T S, whereas it becomes allowed in the orthorhombic phase below T S: The emerging mode's intensity indicates the lattice orthorhombicity. We find that upon cooling below T S, first, weak A g (As) phonon mode intensity appears in the X Y …

Kagome-nets, appearing in electronic, photonic and cold-atom systems, host frustrated fermionic and bosonic excitations. However, it is rare to find a system to study their fermion–boson many-body interplay. Here we use state-of-the-art scanning tunneling microscopy/spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet, CoSn. We image the kagome structure with unprecedented atomic resolution and observe the striking bosonic mode interacting with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion exhibits a pronounced renormalization, signaling a giant coupling to bosons. Through the self-energy analysis, first-principles calculation, and a lattice vibration model, we present evidence that this mode arises from the geometrically frustrated phonon flat-band, which is the lattice bosonic analog of …

We study a Fe 1+ y Te 0.6 Se 0.4 multiband superconductor with T c= 14 K by polarization-resolved Raman spectroscopy. Deep in the superconducting state, we detect pair-breaking excitation at 2 Δ= 5.6 meV in the X Y (B 2 g) scattering geometry, consistent with the fundamental gap energy Δ= 3 meV revealed by ARPES on the holelike Fermi pocket with d x z/d y z character. We analyze the superconductivity-induced phonon self-energy effects for the B 1 g (Fe) phonon mode and derive the electron-phonon coupling constant λ Γ≈ 0.026, which is insufficient to explain superconductivity with T c= 14 K.

The nature of the pseudogap phase of hole-doped cuprate superconductors is still not understood fully. Several experiments have suggested that this phase ends at a critical hole doping level p*, but the nature of the ground state for lower doping is still debated. Here, we use local nuclear magnetic resonance and bulk ultrasound measurements to show that, once competing effects from superconductivity are removed by high magnetic fields, the spin-glass phase of La2−xSrxCuO4 survives up to a doping level consistent with p*. In this material, the antiferromagnetic-glass phase extends from the doped Mott insulator at p = 0.02 up to p* ≈ 0.19, which provides a connection between the pseudogap and the physics of the Mott insulator. Furthermore, the coincidence of the pseudogap boundary with a magnetic quantum phase transition in the non-superconducting ground state has implications for the interpretation of …

We study the in-plane electronic anisotropy in the parent compounds of several families of Fe-based superconductors (BaFe 2 As 2, EuFe 2 As 2, NaFeAs, LiFeAs, FeSe, and LaFeAsO) by polarization-resolved Raman scattering. We measure intensity of the fully symmetric c-axis vibration of As atom mode in the X Y scattering geometry and notice that the mode's intensity is significantly enhanced below the magnetostructural transition only for compounds showing magnetic ordering. In particular, we find that the intensity ratio of this As phonon in the X Y vs. X X scattering geometries is proportional to the square of the ordered magnetic moment. We relate this As phonon intensity enhancement below the Néel temperature in iron pnictides to in-plane electronic anisotropy induced by the collinear spin-density wave order.