H.-B. Yang

Emergence of preformed Cooper pairs from the doped Mott insulating state in Bi 2 Sr 2 CaCu 2 O 8+ δ

Superconductors are characterized by an energy gap that represents the energy needed to break the pairs of electrons (Cooper pairs) apart. At temperatures considerably above those associated with superconductivity, the high-transition-temperature copper oxides have an additional ‘pseudogap’. It has been unclear whether this represents preformed pairs of electrons that have not achieved the coherence necessary for superconductivity, or whether it reflects some alternative ground state that competes with superconductivity. Paired electrons should display particle–hole symmetry with respect to the Fermi level (the energy of the highest occupied level in the electronic system), but competing states need not show such symmetry. Here we report a photoemission study of the underdoped copper oxide Bi2Sr2CaCu2O8+δ that shows the opening of a symmetric gap only in the anti-nodal region, contrary to the expectation that pairing would take place in the nodal region. It is therefore evident that the pseudogap does reflect the formation of preformed pairs of electrons and that the pairing occurs only in well-defined directions of the underlying lattice.

Coupling of spin and orbital excitations in the iron-based superconductor FeSe0.5Te0.5

We present a combined analysis of neutron scattering and photoemission measurements on superconducting FeSe{sub 0.5}Te{sub 0.5}. The low-energy magnetic excitations disperse only in the direction transverse to the characteristic wave vector (1/2,0,0) whereas the electronic Fermi surface near (1/2,0,0) appears to consist of four incommensurate pockets. While the spin resonance occurs at an incommensurate wave vector compatible with nesting, neither spin-wave nor Fermi-surface-nesting models can describe the magnetic dispersion. We propose that a coupling of spin and orbital correlations is key to explaining this behavior. If correct, it follows that these nematic fluctuations are involved in the resonance and could be relevant to the pairing mechanism.

Nearly Perfect Fluidity in a High Temperature Superconductor

Perfect fluids are characterized as having the smallest ratio of shear viscosity to entropy density, η/s, consistent with quantum uncertainty and causality. So far, nearly perfect fluids have only been observed in the quark-gluon plasma and in unitary atomic Fermi gases, exotic systems that are amongst the hottest and coldest objects in the known universe, respectively. We use angle resolved photoemission spectroscopy to measure the temperature dependence of an electronic analog of η/s in an optimally doped cuprate high-temperature superconductor, finding it too is a nearly perfect fluid around, and above, its superconducting transition temperature Tc.

Fine details of the nodal electronic excitations in Bi2Sr2CaCu2O8+δ

Very high energy resolution photoemission experiments on high quality samples of optimally doped Bi

Photoemission Studies of the Pseudogap Regime in the High Tc Cuprate Phase Diagram

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Fine Details of the Nodal Electronic Excitations in Bi

Sr

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Sr

CaCu

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CaCu

O