Positron annihilation on single crystals of YBa2Cu3O7-x

Multiple Electronic Components and Lifshitz Transitions by Oxygen Wires Formation in Layered Cuprates and Nickelates

There is growing compelling experimental evidence that a quantum complex matter scenario made of multiple electronic components and competing quantum phases is needed to grab the key physics of high critical temperature ( T c ) superconductivity in layered cuprates. While it is known that defect self-organization controls T c , the mechanism remains an open issue. Here we focus on the theoretical prediction of the multiband electronic structure and the formation of broken Fermi surfaces generated by the self-organization of oxygen interstitials O i atomic wires in the spacer layers in HgBa 2 CuO 4 ± δ , La 2 CuO 4 ± δ and La 2 NiO 4 ± δ , by means of self-consistent Linear Muffin-Tin Orbital (LMTO) calculations. The electronic structure of a first phase of ordered O i atomic wires and of a second glassy phase made of disordered O i impurities have been studied through supercell calculations. We show the common features of the influence of O i wires in the electronic structure in three types of materials. The ordering of O i into wires leads to a separation of the electronic states between the O i ensemble and the rest of the bulk. The wire formation first produces quantum confined localized states near the wire, which coexist with, Second, delocalized states in the Fermi surface (FS) of doped cuprates. A new scenario emerges for high T c superconductivity, where Kitaev wires with Majorana bound states are proximity-coupled to a 2D d-wave superconductor.

Proposed parameter-free model for interpreting the measured positron annihilation spectra of materials using a generalized gradient approximation

Positron annihilation spectroscopy is often used to analyze the local electronic structure of materials of technological interest. Reliable theoretical tools are crucial to interpret the measured spectra. Here, we propose a parameter-free gradient correction scheme for a local-density approximation obtained from high-quality quantum Monte Carlo data. The results of our calculations compare favorably with positron affinity and lifetime measurements, opening new avenues for highly precise and advanced positron characterization of materials.

Compton Spectroscopy and Chemical Bonding

In this article we show with the help of two examples how Compton spectroscopy may be used to study the effect of chemical bonding in materials as diverse as a molecular crystal and a high temperature superconductor. Compton spectroscopy has a long history as an investigative method in condensed matter physics and in fact the realisation that the Compton profile is sensitive to the effects of chemical bonding dates back at least fifty years. In the seventies, through the efforts of Weyrich [1] and others [2,3], practical applications of this realisation were first achieved. We argue that such studies are more and more relevant thanks to the availability of synchrotron radiation and efficient computational tools.