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In recent years the possibility of electron-like Fermi-surface pockets in the high-Tc cuprates has become an area of intense experimental and theoretical interest. What is the origin of these pockets? Are they connected to the mysterious pseudogap, a depletion in the density of states that dominates thermodynamic and transport properties over a wide range of temperature and doping? Until now, experimental support for these pockets has been confined to samples which, in addition to pseudogap effects, exhibit a separate spin/charge stripe correlation, making their connection to the pseudogap unclear.

In the present paper we calculate the thermopower of high-Tc cuprates from the resonating-valence-bond spin-liquid model developed by Yang, Rice and Zhang, achieving an excellent match with experimental data. A key result of this work is the identification of features in the observed thermopower corresponding to electron pockets in the Fermi-surface appearing with the opening of the pseudogap. These results link the pseudogap with Fermi-surface reconstruction and will be of considerable interest to researchers using photoemission, quantum oscillations and other techniques presently engaged in efforts to detect these pockets directly.

New research shows that the speed of light may not be fixed after all, but rather fluctuates, while its value could be set by the total number of elementary particles in nature. Two EPJ D neighbour papers challenge established wisdom about the nature of vacuum. Vacuum is one of the most intriguing concepts in physics. When observed at the quantum level, vacuum is not empty. It is filled with continuously appearing and disappearing particle pairs such as electron-positron or quark-antiquark pairs.

In the study of article 1, the authors established, for the first time, a detailed quantum mechanism that would explain the magnetisation and polarisation of the vacuum, referred to as vacuum permeability and permittivity, and the finite speed of light. This finding is relevant because it suggests the existence of a limited number of ephemeral particles per unit volume in a vacuum. As a result, there is a theoretical possibility that the speed of light is not fixed, as conventional physics has assumed. Instead, this speed would be dependent on variations in the vacuum properties of space or time.

In the work of article 2, the other authors found that a specific property of vacuum called the impedance, which is crucial in determining the speed of light, depends only on the sum of the square of the electric charges of particles’ pair but not on their masses.  If their idea is correct, the value of the speed of light combined with the value of vacuum impedance gives an indication of the total number of charged elementary particles existing in nature.

The first model demonstrating that it is possible for two particles to cross an energy barrier together, where a single particle could not, is shown here.

A new kind of so-called Klein tunnelling - representing the quantum equivalent of crossing an energy wall - is presented in a model of two interacting particles. The authors relied on an analytical and numerical study of a landmark model of interacting particles, called the Hubbard model. They predict a new type of Klein tunnelling for a couple of interacting particles confronted by an energy barrier. Even though the barrier is impenetrable for single particles, it becomes transparent when the two particles cross the energy barrier together. They expect these predictions to be confirmed experimentally in ultra-cold atoms trapped in optical lattices. If this is the case, similar quantum simulation could be a tool for emulating multiple-particle systems.