Nematicity and competing orders in superconducting magic-angle graphene

Strongly interacting electrons in solid-state systems often display multiple broken symmetries in the ground state. The interplay between different order parameters can give rise to a rich phase diagram. We report on the identification of intertwined phases with broken rotational symmetry in magic-angle twisted bilayer graphene (TBG). Using transverse resistance measurements, we find a strongly anisotropic phase located in a “wedge” above the underdoped region of the superconducting dome. Upon its crossing with the superconducting dome, a reduction of the critical temperature is observed. Furthermore, the superconducting state exhibits an anisotropic response to a direction-dependent in-plane magnetic field, revealing nematic ordering across the entire superconducting dome. These results indicate that nematic fluctuations might play an important role in the low-temperature phases of magic-angle TBG.

Fluctuations and the Higgs Mechanism in Underdoped Cuprates

The physics of the pseudogap phase of high-temperature cuprate superconductors has been an enduring mystery over the past 30 years. The ubiquitous presence of the pseudogap phase in underdoped cuprates suggests that understanding it is key to unraveling the origin of high-temperature superconductivity. We review various theoretical approaches to this problem, emphasizing the concept of emergent symmetries in the underdoped region of those compounds. We differentiate these theories by considering a few fundamental questions related to the rich phenomenology of these materials. Lastly, we discuss a recent idea regarding two kinds of entangled preformed pairs that open a gap at the pseudogap onset temperature, T*, through a specific Higgs mechanism. We review the experimental consequences of this line of thought.

Dynamical charge density fluctuations pervading the phase diagram of a Cu-based high-Tc superconductor

Charge density modulations have been observed in all families of high–critical temperature (Tc) superconducting cuprates. Although they are consistently found in the underdoped region of the phase diagram and at relatively low temperatures, it is still unclear to what extent they influence the unusual properties of these systems. Using resonant x-ray scattering, we carefully determined the temperature dependence of charge density modulations in YBa2Cu3O7–δ and Nd1+xBa2–xCu3O7–δ for several doping levels. We isolated short-range dynamical charge density fluctuations in addition to the previously known quasi-critical charge density waves. They persist up to well above the pseudogap temperature T*, are characterized by energies of a few milli–electron volts, and pervade a large area of the phase diagram.

Doping dependence of unusual electron spectrum in hole-doped cuprate superconductors

Based on the renormalized Hubbard model, the doping dependence of electron spectrum in cuprate superconductors is discussed within the self-consistent mean field theory. It is shown that the renormalization factor [Formula: see text] (then the quasiparticle coherent weight) increases almost linearly with the doping and plays an important role in the unconventional superconductivity for cuprate superconductors. It suppresses the magnitude of the quasiparticle peak in the electron spectrum, especially in underdoped region. By calculation of the energy and doping dependence of the electron spectral function, the main features of the electron spectrum in cuprate superconductors can be described qualitatively. In particular, with the increasing doping concentration, the position of the quasiparticle peak moves to the Fermi energy and the magnitude of the quasiparticle peak increases monotonically. Our results also show that the superconducting order parameter is determined by product of the renormalization factor and the pseudogap.

ANISOTROPIC RESISTIVITY IN Bi2Sr2CaCu2Oy THIN FILMS GROWN ON VICINAL SrTiO3 SUBSTRATES

The in-plane resistivity ρab(T) and out-of-plane resistivity ρc(T) are measured for Bi 2 Sr 2 CaCu 2 O 8 superconducting thin films successfully deposited by in situ magnetron RF-sputtering on vicinal (100) SrTiO 3 substrates. Oriented film with c axis inclined with respect to the basal plane by a tilt angle α ≈ 4° was obtained. It was found that for the underdoped region ρab(T) deviates from the high T-linear behavior, which evidences the opening of the pseudogap while from the out-of-plane resistivity measurements, we found that the temperature corresponding to the minimum between metallic and semiconducting behavior appears to reflect the opening of the pseudogap for transport along c axis. We also found that the opening of the pseudogap seen in ρab(T) does not coincide with the pseudogap along c axis.