The Ground State of the Pseudogap in Cuprate Superconductors

We present studies of the electronic structure of La2-xBaxCuO4, a system where the superconductivity is strongly suppressed as static spin and charge orders or "stripes" develop near the doping level of x=1/8. Using angle-resolved photoemission and scanning tunneling microscopy, we detect an energy gap at the Fermi surface with magnitude consistent with d-wave symmetry and with linear density of states, vanishing only at four nodal points, even when superconductivity disappears at x=1/8. Thus, the non-superconducting, "striped" state at x=1/8 is consistent with a phase incoherent d-wave superconductor whose Cooper pairs form spin/charge ordered structures instead of becoming superconducting.Comment: This is the author's version of the wor

A review of feeding intolerance in critically ill children

© 2018, The Author(s). Ensuring optimal nutrition is vital in critically ill children and enteral feeding is the main route of delivery in intensive care. Feeding intolerance is the most commonly cited reason amongst pediatric intensive care unit healthcare professionals for stopping or withholding enteral nutrition, yet the definition for this remains inconsistent, nebulous, and entirely arbitrary. Not only does this pose problems clinically, but research in this field frequently uses feeding intolerance as an endpoint and the heterogeneity in this definition makes the comparison of studies difficult and meta-analysis impossible. We reviewed the use of, and definitions of, the term feed intolerance in pediatric intensive care research papers in the last 20years. Gastric residual volume remains the most common factor used to define feed intolerance, despite the lack of evidence for this. Healthcare professionals would benefit from further education to improve their awareness of the limitations of the markers to define feeding intolerance, and the international PICU community needs to agree a consistent definition of this phenomenon to improve consistency in both practice and research. Conclusion: This paper will provide a narrative review of the definitions of, evidence for, and markers of feeding intolerance in critically ill children.What is Known?:• Feeding intolerance is a commonly cited reason amongst pediatric intensive care unit healthcare professionals for stopping or withholding enteral nutrition.• There is no agreed definition for feeding intolerance in critically ill children.What is New?:• This paper provides an up to date review of the definitions of, evidence for, and markers of feeding intolerance in critically ill children.• Despite no evidence, gastric residual volume continues to drive clinical bedside decisions about enteral feeding and feeding tolerance

Measurement of an Exceptionally Weak Electron-Phonon Coupling on the Surface of the Topological Insulator Bi2_2Se3_3 Using Angle-Resolved Photoemission Spectroscopy

Gapless surface states on topological insulators are protected from elastic scattering on non-magnetic impurities which makes them promising candidates for low-power electronic applications. However, for wide-spread applications, these states should have to remain coherent at ambient temperatures. Here, we studied temperature dependence of the electronic structure and the scattering rates on the surface of a model topological insulator, Bi$_2$Se$_3$, by high resolution angle-resolved photoemission spectroscopy. We found an extremely weak broadening of the topological surface state with temperature and no anomalies in the state's dispersion, indicating exceptionally weak electron-phonon coupling. Our results demonstrate that the topological surface state is protected not only from elastic scattering on impurities, but also from scattering on low-energy phonons, suggesting that topological insulators could serve as a basis for room temperature electronic devices.Comment: published version, 5 pages, 4 figure

Charge order, metallic behavior and superconductivity in La_{2-x}Ba_xCuO_4 with x=1/8

The ab-plane optical properties of a cleaved single crystal of La_{2-x}Ba_xCuO_4 for x=1/8 (T_c ~ 2.4 K) have been measured over a wide frequency and temperature range. The low-frequency conductivity is Drude-like and shows a metallic response with decreasing temperature. However, below ~ 60 K, corresponding to the onset of charge-stripe order, there is a rapid loss of spectral weight below about 40 meV. The gapping of single-particle excitations looks surprisingly similar to that observed in superconducting La_{2-x}Sr_{x}CuO_4, including the presence of a residual Drude peak with reduced weight; the main difference is that the lost spectral weight moves to high, rather than zero, frequency, reflecting the absence of a bulk superconducting condensate.Comment: 4 pages, with 1 table and 3 figure

Gapped Surface States in a Strong-Topological-Semimetal

A three-dimensional strong-topological-insulator or -semimetal hosts topological surface states which are often said to be gapless so long as time-reversal symmetry is preserved. This narrative can be mistaken when surface state degeneracies occur away from time-reversal-invariant momenta. The mirror-invariance of the system then becomes essential in protecting the existence of a surface Fermi surface. Here we show that such a case exists in the strong-topological-semimetal Bi$_4$Se$_3$. Angle-resolved photoemission spectroscopy and \textit{ab initio} calculations reveal partial gapping of surface bands on the Bi$_2$Se$_3$-termination of Bi$_4$Se$_3$(111), where an 85 meV gap along $\bar{\Gamma}\bar{K}$ closes to zero toward the mirror-invariant $\bar{\Gamma}\bar{M}$ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin-helicity.Comment: 5 pages, 3 figure

High-energy kink in high-temperature superconductors

In conventional metals, electron-phonon coupling, or the phonon-mediated interaction between electrons, has long been known to be the pairing interaction responsible for the superconductivity. The strength of this interaction essentially determines the superconducting transition temperature TC. One manifestation of electron-phonon coupling is a mass renormalization of the electronic dispersion at the energy scale associated with the phonons. This renormalization is directly observable in photoemission experiments. In contrast, there remains little consensus on the pairing mechanism in cuprate high temperature superconductors. The recent observation of similar renormalization effects in cuprates has raised the hope that the mechanism of high temperature superconductivity may finally be resolved. The focus has been on the low energy renormalization and associated "kink" in the dispersion at around 50 meV. However at that energy scale, there are multiple candidates including phonon branches, structure in the spin-fluctuation spectrum, and the superconducting gap itself, making the unique identification of the excitation responsible for the kink difficult. Here we show that the low-energy renormalization at ~50 meV is only a small component of the total renormalization, the majority of which occurs at an order of magnitude higher energy (~350 meV). This high energy kink poses a new challenge for the physics of the cuprates. Its role in superconductivity and relation to the low-energy kink remains to be determined.Comment: 13 pages, 4 figure