Non-Hermitian topology in rock–paper–scissors games

Non-Hermitian topology is a recent hot topic in condensed matters. In this paper, we propose a novel platform drawing interdisciplinary attention: rock–paper–scissors (RPS) cycles described by the evolutionary game theory. Specifically, we demonstrate the emergence of an exceptional point and a skin effect by analyzing topological properties of their payoff matrix. Furthermore, we discover striking dynamical properties in an RPS chain: the directive propagation of the population density in the bulk and the enhancement of the population density only around the right edge. Our results open new avenues of the non-Hermitian topology and the evolutionary game theory.

Inhomogeneous charge distribution of simple substances

Using density function theory calculation method, we investigated the charge distribution in nano- or low-dimension materials and some high pressure three-dimensional solids in many simple substances. It is found that if an atom in simple substances has different atomic environment from the other(s) nearby, they always have spontaneous charge inhomogeneity within local area. Charge inhomogeneity introduced by inequivalent atomic positions is a general phenomenon in simple substances, especially in their nano-materials. Such self-ionization significantly changes elemental matter’s physical and chemical properties. These results can advance the understanding on condensed matters, especially the internal structures and physical or chemical properties of elemental nano-materials, and could be a starting point for experimental investigation of self-ionization in them. The charge inhomogeneity of simple substances means their nonzero oxidation state or nonzero valence.

Creating polar antivortex in PbTiO3/SrTiO3 superlattice

Nontrivial topological structures offer a rich playground in condensed matters and promise alternative device configurations for post-Moore electronics. While recently a number of polar topologies have been discovered in confined ferroelectric PbTiO3 within artificially engineered PbTiO3/SrTiO3 superlattices, little attention was paid to possible topological polar structures in SrTiO3. Here we successfully create previously unrealized polar antivortices within the SrTiO3 of PbTiO3/SrTiO3 superlattices, accomplished by carefully engineering their thicknesses guided by phase-field simulation. Field- and thermal-induced Kosterlitz–Thouless-like topological phase transitions have also been demonstrated, and it was discovered that the driving force for antivortex formation is electrostatic instead of elastic. This work completes an important missing link in polar topologies, expands the reaches of topological structures, and offers insight into searching and manipulating polar textures.

Mixed-cation driven magnetic interaction of interstitial electrons for ferrimagnetic two-dimensional electride

Magnetism of pure electrons is fundamental for understanding diverse magnetic phenomena in condensed matters but has not been fully investigated in experiments due to the lack of a tractable model system. Such an exotic material necessitates an exclusive magnetic interaction of electrons being devoid of orbital and lattice degrees of freedom. Here, we report the two-dimensional mixed-cation [YGdC]2+∙2e− electride, showing ferrimagnetic nature from the direct exchange interaction of magnetic interstitial electrons in interlayer space. We identify that magnetic interstitial electrons are periodically localized in octahedral and tetrahedral cavities between 2D cationic Y2−xGdx arrays. The mixed configuration of non-magnetic and magnetic cations in cavities induces divergent spin states and interactions of magnetic interstitial electrons, in which their direct exchange interaction overwhelms the interactions with magnetic cations, triggering the ferrimagnetic spin-alignment. This discovery facilitates further exploration of magnetic electrides and nurtures the study of two-dimensional magnetism of layered crystals and electron phases.

Unconventional level attraction in cavity axion polariton of antiferromagnetic topological insulator

Strong coupling between cavity photons and various excitations in condensed matters boosts the field of light-matter interaction and generates several exciting sub-fields, such as cavity optomechanics and cavity magnon polariton. Axion quasiparticles, emerging in topological insulators, were predicted to strongly couple with the light and generate the so-called axion polariton. Here, we demonstrate that there arises a gapless level attraction in cavity axion polariton of antiferromagnetic topological insulators, which originates from a nonlinear interaction between axion and the odd-order resonance of cavity. Such a novel level attraction is essentially different from conventional level attractions with the mechanism of either a linear coupling or a dissipation-mediated interaction, and also different from the level repulsion induced by the strong coupling in common polaritons. Our results reveal a new mechanism of level attractions, and open up new roads for exploring the axion polariton with cavity technologies. They have potential applications for quantum information and dark matter research.

A direct measurement method of quantum relaxation time

Quantum relaxation time of electrons in condensed matters is the most important physical property while its direct measurement has been elusive for a century. Here, we report a breakthrough that allows us to directly determine quantum relaxation time at zero and non-zero frequencies using optical measurement. Through dielectric loss function, we connect bound electron effect to the physical parameters of plasma resonance and find an extra term of quantum relaxation time due to inelastic scattering between bound electrons and conduction electrons at non-zero frequencies. We demonstrate here that the frequency dependent inelastic polarization effect of bound electrons is the dominating contribution on quantum relaxation time of conduction electrons at optical frequencies and elastic polarization effect of bound electrons also dramatically changes the plasma resonance frequency through effective screening to charge carriers.

Highly tunable properties in pressure-treated two-dimensional Dion–Jacobson perovskites

The application of pressure can achieve novel structures and exotic phenomena in condensed matters. However, such pressure-induced transformations are generally reversible and useless for engineering materials for ambient-environment applications. Here, we report comprehensive high-pressure investigations on a series of Dion–Jacobson (D-J) perovskites A′An−1PbnI3n+1[A′ = 3-(aminomethyl) piperidinium (3AMP), A = methylammonium (MA),n= 1, 2, 4]. Our study demonstrates their irreversible behavior, which suggests pressure/strain engineering could viably improve light-absorber material not only in situ but also ex situ, thus potentially fostering the development of optoelectronic and electroluminescent materials. We discovered that the photoluminescence (PL) intensities are remarkably enhanced by one order of magnitude at mild pressures. Also, higher pressure significantly changes the lattices, boundary conditions of electronic wave functions, and possibly leads to semiconductor–metal transitions. For (3AMP)(MA)3Pb4I13, permanent recrystallization from 2D to three-dimensional (3D) structure occurs upon decompression, with dramatic changes in optical properties.