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Abstract We examine key aspects of the theory of the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein condensation (BEC) crossover, focusing on the temperature dependence of the chemical potential,μ. We identify an accurate method of determining the change ofμin the cuprate high temperature superconductors from angle-resolved-photoemission data (along the ‘nodal’ direction), and show thatμvaries by less than a few percent of the Fermi energy over a range of temperatures from far below to several times above the superconducting transition temperature,T_c. This shows, unambiguously, that not only are these materials always on the BCS side of the crossover (which is a phase transition in thed-wave case), but are nowhere near the point of the crossover (where the chemical potential approaches the band bottom). 

Two-dimensional electron gas (2DEG) states at oxide interfaces between two ferroic materials have been fertile ground to realize controllable multiferroicity. Here, we investigate the 2DEG states at the interface of ferroelectric BaTi⁢O3 and a magnetic layer of iron using angle-resolved photoemission spectroscopy. Orbital-selective charge transfer occurs on the surprisingly robust 2DEG. Based on first-principles calculations, we show how the interfacial hybridization can give rise to the unexpected charge transfer in the magnetic 2DEG. Our study reveals a close interplay on a 2DEG between magnetic and ferroelectric interfaces, which sheds light on future design principles of multiferroic 2DEG states. 

Electron-hole bound pairs, or excitons, are common excitations in semiconductors. They can spontaneously form and condense into a new insulating ground state—the so-called excitonic insulator—when the energy of electron-hole Coulomb attraction exceeds the band gap. In the presence of electron-phonon coupling, a periodic lattice distortion often concomitantly occurs. However, a similar structural transition can also be induced by electron-phonon coupling itself, therefore hindering the clean identification of bulk excitonic insulators (e.g., which instability is the driving force of the phase transition). Using high-resolution synchrotron x-ray diffraction and angle-resolved photoemission spectroscopy, we identify key electron-phonon coupling effects in a leading excitonic insulator candidate Ta 2 NiSe 5 . These include an extensive unidirectional lattice fluctuation and an electronic pseudogap in the normal state, as well as a negative electronic compressibility in the charge-doped broken-symmetry state. In combination with first principles and model calculations, we use the normal state electronic spectra to quantitatively determine the electron-phonon interaction vertex g and interband Coulomb interaction V in the minimal lattice model, the solution to which captures the experimental observations. Moreover, we show how the Coulomb and electron-phonon coupling effects can be unambiguously separated based on the solution to quantified microscopic models. Finally, we discuss how the strong lattice fluctuations enabled by low dimensionality relate to the unique electron-phonon interaction effects beyond the textbook Born-Oppenheimer approximation. 

In heavily hole-doped cuprates, superconductivity does not die by simply dissolving into a uniform metal due to the lack of pairing, but rather survives by shattering into nanoscale superconducting puddles.