Multiple shock reverberation compression experiments are designed and performed to determine the equation of state of neon ranging from the initial dense gas up to the warm dense regime where the pressure is from about 40 MPa to 120 GPa and the temperature is from about 297 K up to above 20 000 K. The wide region experimental data are used to evaluate the available theoretical models. It is found that, for neon below $1.1{\mathrm{g}/\mathrm{cm}}^3$, within the framework of density functional theory molecular dynamics, a van der Waals correction is meaningful. Under high pressure and temperature, results from the self-consistent fluid variational theory model are sensitive to the potential parameter and could give successful predictions in the whole experimental regime if a set of proper parameters is employed. The new observations on neon under megabar ($1\mathrm{Mbar}={10}^{11}\mathrm{Pa}$) pressure and eV temperature ($1\mathrm{eV}\approx {10}^4\mathrm{K}$) enrich the understanding on properties of warm dense matter and have potential applications in revealing the formation and evolution of gaseous giants or mega-Earths.

We examine the topological order in the resonating singlet valence plaquette (RSVP) phase of the hard-core quantum plaquette model (QPM) on the face centered cubic (FCC) lattice. To do this, we construct a Rohksar-Kivelson type Hamiltonian of local plaquette resonances. This model is shown to exhibit a ${\mathbb{Z}}_3$ topological order, which we show by identifying a ${\mathbb{Z}}_3$ topological constant (which leads to a $3^3$-fold topological ground state degeneracy on the 3-torus) and topological pointlike charge and looplike magnetic excitations which obey ${\mathbb{Z}}_3$ statistics. We also consider an exactly solvable generalization of this model, which makes the geometrical origin of the ${\mathbb{Z}}_3$ order explicitly clear. For other models and lattices, such generalizations produce a wide variety of topological phases, some of which are novel fracton phases.

We describe a method to probe the quantum phase transition between the short-range topological phase and the long-range topological phase in the superconducting Kitaev chain with long-range pairing, both exhibiting subgap modes localized at the edges. The method relies on the effects of the finite mass of the subgap edge modes in the long-range regime (which survives in the thermodynamic limit) on the single-particle scattering coefficients through the chain connected to two normal leads. Specifically, we show that, when the leads are biased at a voltage $V$ with respect to the superconducting chain, the Fano factor is either zero (in the short-range correlated phase) or $2e$ (in the long-range correlated phase). As a result, we find that the Fano factor works as a directly measurable quantity to probe the quantum phase transition between the two phases. In addition, we note a remarkable “critical fractionalization effect” in the Fano factor, which is exactly equal to $e$ along the quantum critical line. Finally, we note that a dual implementation of our proposed device makes it suitable as a generator of large-distance entangled two-particle states.

We investigate, in the framework of the macroscopic Dirac model, the spectrum, charge density, and conductivity of metallic armchair graphene nanoribbons in the presence of different mass terms. We reveal the conditions and symmetries governing the existence of edge modes in the system. Depending on the mass terms present, they are exponentially localized gapless or gapped modes. The latter situation is realized, in particular, for a full Kekulé distortion. For this case, we calculate the mean charge and conductivity of the ribbon, and derive the traces of the presence of edge modes suitable for experimental verification.

To address ultimate precision in density functional theory calculations we employ the full-potential linearized augmented plane-wave + local-orbital (LAPW + lo) method and justify its usage as a benchmark method. LAPW + lo and two completely unrelated numerical approaches, the multiresolution analysis (MRA) and the linear combination of atomic orbitals, yield total energies of atoms with mean deviations of 0.9 and $0.2\mu \mathrm{Ha}$, respectively. Spectacular agreement with the MRA is reached also for total and atomization energies of the G2-1 set consisting of 55 molecules. With the example of $\alpha$ iron we demonstrate the capability of LAPW + lo to reach $\mu \mathrm{Ha}/\mathrm{atom}$ precision also for periodic systems, which allows also for the distinction between the numerical precision and the accuracy of a given functional.

We report an optically induced reduction of the $f\text{−}d$ hybridization in the prototypical heavy-fermion compound ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$. We use femtosecond time- and angle-resolved photoemission spectroscopy to monitor changes of spectral weight and binding energies of the Yb $4f$ and Rh $4d$ states before the lattice temperature increases after pumping. Overall, the $f\text{−}d$ hybridization decreases smoothly with increasing electronic temperature up to $\sim 250\mathrm{K}$ but changes slope at $\sim 100\mathrm{K}$. This temperature scale coincides with the onset of coherent Kondo scattering and with thermally populating the first excited crystal electrical field level. Extending previous photoemission studies, we observe a persistent $f\text{−}d$ hybridization up to at least $\sim 250\mathrm{K}$, which is far larger than the coherence temperature defined by transport but in agreement with the temperature dependence of the noninteger Yb valence. Our data underlines the distinction of probes accessing spin and charge degrees of freedom in strongly correlated systems.

We use first-principles calculations to predict the occurrence of half-metallicity and anionogenic ferromagnetism at the heterointerface between two 2p insulators, taking the ${\mathrm{KO}}_2/{\mathrm{BaO}}_2$ (001) interface as an example. Whereas a sharp heterointerface is semiconducting, a heterointerface with a moderate concentration of swapped K and Ba atoms is half-metallic and ferromagnetic at ambient pressure due to the double exchange mechanism. The K-Ba swap renders the interfacial K-O and Ba-O atomic layers electron-doped and hole-doped, respectively. Our findings pave the way to realize metallicity and ferromagnetism at the interface between two $2p$ insulators, and such systems can constitute a new family of heterostructures with novel properties, expanding studies on heterointerfaces from $3d$ insulators to $2p$ insulators.

We model the newly predicted high-$T_c$ superconducting candidates constructed by corner-shared trigonal bipyramidal complexes with an effective three-orbital tight-binding Hamiltonian and investigate the pairing symmetry of their superconducting states driven by electron-electron interactions. Our combined weak- and strong-coupling-based calculations consistently identify the chiral $d+id$ superconductivity as the leading pairing symmetry in a wide doping range with realistic interaction parameters. This pairing state has a nontrivial topological Chern number and can host gapless chiral edge modes, and the vortex cores under magnetic field can carry Majorana zero modes.

We calculate linear and nonlinear optical susceptibilities arising from the excitonic states of monolayer ${\mathrm{MoS}}_2$ for in-plane light polarizations, using second-quantized bound and unbound exciton operators. Optical selection rules are critical for obtaining the susceptibilities. We derive the valley-chirality rule for the second-order harmonic generation in monolayer ${\mathrm{MoS}}_2$ and find that the third-order harmonic process is efficient only for linearly polarized input light while the third-order two-photon process (optical Kerr effect) is efficient for circularly polarized light using a higher order exciton state. The absence of linear absorption due to the band gap and the unusually strong two-photon third-order nonlinearity make the monolayer ${\mathrm{MoS}}_2$ excitonic structure a promising resource for coherent nonlinear photonics.

We show that a pair of overlapping Majorana bound states (MBSs) forming a partially separated Andreev bound state (ps-ABS) represents a generic low-energy feature in spin-orbit-coupled semiconductor-superconductor (SM-SC) hybrid nanowire in the presence of a Zeeman field. The ps-ABS interpolates continuously between the “garden variety” ABS, which consists of two MBSs sitting on top of each other, and the topologically protected Majorana zero modes (MZMs), which are separated by a distance given by the length of the wire. The really problematic ps-ABSs consist of component MBSs separated by a distance of the order of the characteristic Majorana decay length $\xi$, and have nearly zero energy in a significant range of control parameters, such as the Zeeman field and chemical potential, within the topologically trivial phase. Despite being topologically trivial, such ps-ABSs can generate signatures identical to MZMs in local charge tunneling experiments. In particular, the height of the zero-bias conductance peak (ZBCP) generated by ps-ABSs has the quantized value $2e^2/h$, and it can remain unchanged in an extended range of experimental parameters, such as Zeeman field and the tunnel barrier height. We illustrate the formation of such low-energy robust ps-ABSs in two experimentally relevant situations: a hybrid SM-SC system consisting of a proximitized nanowire coupled to a quantum dot and the SM-SC system in the presence of a spatially varying inhomogeneous potential. We then show that, unlike local measurements, a two-terminal experiment involving charge tunneling at both ends of the wire is capable of distinguishing between the generic ps-ABSs and the non-Abelian MZMs. While the MZMs localized at the opposite ends of the wire generate correlated differential conduction spectra, including correlations in energy splittings and critical Zeeman fields associated with the emergence of the ZBCPs, such correlations are absent if the ZBCPs are due to ps-ABSs emerging in the topologically trivial phase. Measuring such correlations is the clearest and most straightforward test of topological MZMs in SM-SC heterostructures that can be done in a currently accessible experimental setup.

We propose a minimal effective two-dimensional Hamiltonian for HgTe/CdHgTe quantum wells (QWs) describing the side maxima of the first valence subband. By using the Hamiltonian, we explore the picture of helical edge states in tensile and compressively strained HgTe QWs. We show that both dispersion and probability density of the edge states can differ significantly from those predicted by the Bernevig-Hughes-Zhang (BHZ) model. Our results pave the way towards further theoretical investigations of HgTe-based quantum spin Hall insulators with direct and indirect band gaps beyond the BHZ model.

Ground-state epitaxial phase diagrams are calculated by density functional theory (DFT) for ${\mathrm{SrTiO}}_3, {\mathrm{CaTiO}}_3$, and ${\mathrm{SrHfO}}_3$ perovskite-based compounds, accounting for the effects of antiferrodistortive and $A$-site displacement modes. Biaxial strain states corresponding to epitaxial growth of (001)-oriented films are considered, with misfit strains ranging between $-4\%$ and 4%. Ground-state structures are determined using a computational procedure in which input structures for DFT optimizations are identified as local minima in expansions of the total energy with respect to strain and soft-mode degrees of freedom. Comparison to results of previous DFT studies demonstrates the effectiveness of the computational approach in predicting ground-state phases. The calculated results show that antiferrodistortive octahedral rotations and associated $A$-site displacement modes act to suppress polarization and reduce the epitaxial strain energy. A projection of calculated atomic displacements in the ground-state epitaxial structures onto soft-mode eigenvectors shows that three ferroelectric and six antiferrodistortive displacement modes are dominant at all misfit strains considered, with the relative contributions from each varying systematically with the strain. Additional $A$-site displacement modes contribute to the atomic displacements in ${\mathrm{CaTiO}}_3$ and ${\mathrm{SrHfO}}_3$, which serve to optimize the coordination of the undersized $A$-site cation.

In an electron backscatter diffraction pattern (EBSP), the angular distribution of backscattered electrons (BSEs) depends on their energy. Monte Carlo modeling of their depth and energy distributions suggests that the highest energy BSEs are more likely to hit the bottom of the detector than the top. In this paper, we examine experimental EBSPs to validate the modeled angular BSE distribution. To that end, the Kikuchi bandlet method is employed to measure the width of Kikuchi bands in both modeled and measured EBSPs. The results show that in an EBSP obtained with a 15 keV primary probe, the width of a Kikuchi band varies by about $0.4^{\circ }$ from the bottom of the EBSD detector to its top. The same is true for a simulated pattern that is composed of BSEs with 5 keV to 15 keV energies, which validates the Monte Carlo simulations.

The stability of GeTe in rhombohedral ($R3m$), face centred cubic ($Fm3m$), and simple cubic ($Pm3m$) phases has been studied using density functional perturbation theory. The rhombohedral phase of GeTe is dynamically stable at 0 GPa, while $Fm3m$ and $Pm3m$ phases are stable at 3.1 and 33 GPa, respectively. The pressure-dependent phonon modes are observed in $Fm3m$ and $Pm3m$ phases at Γ and $M$ points, respectively. The electronic and the thermoelectric properties have been investigated for the stable phases of GeTe. The electronic band gap for rhombohedral and $Fm3m$ phases of GeTe has been observed as 0.66 and 0.17 eV, respectively, while the $Pm3m$ phase shows metallic behavior. We have used the Boltzmann transport equation under a rigid band approximation and constant relaxation time approximation as implemented in boltztrap code for the calculation of thermoelectric properties of GeTe. The metallic behavior of $Pm3m$ phase gives a very low value of Seebeck coefficient compared to the other two phases as a function of temperature and the chemical potential μ. It is observed that the rhombohedral phase of GeTe exhibits higher thermoelectric performance. Due to the metallic nature of $Pm3m$ phase, negligible thermoelectric performance is observed compared to $R3m$ and $Fm3m$-GeTe. The calculated lattice thermal conductivities are low for $Fm3m$-GeTe and high for $R3m$-GeTe. At the relatively higher temperature of 1350 K, the figure of merit $\mathit{ZT}$ is found to be 0.7 for rhombohedral GeTe. The elastic constants satisfy the Born stability criteria for all three phases. The rhombohedral and $Fm3m$ phases exhibits brittleness and the $Pm3m$ phase shows ductile nature.

Using classical Monte Carlo simulations, we study a simple statistical mechanical model of relevance to the emergence of polarization from local displacements on the square and cubic lattices. Our model contains two key ingredients: a Kitaev-like orientation-dependent interaction between nearest neighbors and a steric term that acts between next-nearest neighbors. Taken by themselves, each of these two ingredients is incapable of driving long-range symmetry breaking, despite the presence of a broad feature in the corresponding heat-capacity functions. Instead, each component results in a “hidden” transition on cooling to a manifold of degenerate states; the two manifolds are different in the sense that they reflect distinct types of local order. Remarkably, their intersection, i.e., the ground state when both interaction terms are included in the Hamiltonian, supports a spontaneous polarization. In this way, our study demonstrates how local-order mechanisms might be combined to break global inversion symmetry in a manner conceptually similar to that operating in the “hybrid” improper ferroelectrics. We discuss the relevance of our analysis to the emergence of spontaneous polarization in well-studied ferroelectrics such as ${\mathrm{BaTiO}}_3$ and ${\mathrm{KNbO}}_3$.

Recently, Cichorek et al. have analyzed the magnetic-field-independent $-\left|A\right|T^{1/2}$ term in the resistivity of disordered ${\mathrm{ZrAs}}_{1.58}{\mathrm{Se}}_{0.39}$ within the Altshuler-Arononv model of electron-electron interaction in the presence of disorder [Phys. Rev. Lett. 117, 106601 (2016)]. Making the assumption that diffusion of conduction electrons in this compound is isotropic, they could not get quantitative agreement between the considered model and the experimental results. This led the authors to conclude that the singular $T^{1/2}$ contribution to the resistivity of ${\mathrm{ZrAs}}_{1.58}{\mathrm{Se}}_{0.39}$ could not be caused by electron-electron interaction but could only be explained by a two-channel Kondo effect. Here, we perform a detailed analysis of the $-\left|A\right|T^{1/2}$ correction to the resistivity of disordered ${\mathrm{ZrAs}}_{1.58}{\mathrm{Se}}_{0.39}$ using analogous Altshuler-Aronov relation but taking into account that the diffusion coefficient in the tetragonal ${\mathrm{ZrAs}}_{1.58}{\mathrm{Se}}_{0.39}$ is anisotropic. For the considered resistivity anisotropy, we found that the calculated values of the $A$ coefficient are in very good agreement with those derived from the resistivity measurements. Moreover, analysis indicates that the values of the screening parameter ${\lambda }^{(j=1)}$ are close to zero, which satisfies the condition that the $A$ coefficient does not depend on magnetic field. This shows that the magnetic-field-independent $-\left|A\right|T^{1/2}$ correction to the resistivity of disordered ${\mathrm{ZrAs}}_{1.58}{\mathrm{Se}}_{0.39}$ can be explained within the electron-electron interaction scenario in disordered metallic systems.

We report the discovery of attractive magnetic skyrmions and their clusters in noncentrosymmetric ferromagnets. These three-dimensional solitons have been predicted to exist in the cone phase of chiral ferromagnets [J. Phys: Condens. Matter 28, 35LT01 (2016)] and are fundamentally different from the more common repulsive axisymmetric skyrmions that occur in the magnetically saturated state. We present real-space images of these skyrmion clusters in thin ($\sim 70$ nm) single-crystal samples of ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ taken using transmission electron microscopy and develop a phenomenological theory describing this type of skyrmion.

We report the structure, antiferromagnetism, and superconductivity in $\mathrm{C}{\mathrm{u}}_{1-x}\mathrm{L}{\mathrm{i}}_x\mathrm{FeAs}$ ($0\le x\le 1.0$) samples. A direct evolution from antiferromagnetism to superconductivity is observed as increasing doping level of Li. A phase diagram is constructed to show this evolution, which features no coexistence region between superconductivity and antiferromagnetism. This behavior shows that antiferromagnetic CuFeAs can be regarded as a parent compound to the observed superconductivity by equivalent doping, which is different from the cases with other FeAs-based superconductors. Structural analyses and first-principles calculations indicate that the anion height of $\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ tetrahedral layer plays a crucial role on the physical properties. Moreover, the simple Fermi surface nesting picture adopted to explain the evolution from spin-density wave to superconductor in other FeAs-based superconductors might be not applicable to $\mathrm{C}{\mathrm{u}}_{1-x}\mathrm{L}{\mathrm{i}}_x\mathrm{FeAs}$.