The non-Abelian topological order has attracted a lot of attention for its fundamental importance and exciting prospect of topological quantum computation. However, explicit demonstration or identification of the non-Abelian states and the associated statistics in a microscopic model is very challenging. Here, based on a density-matrix renormalization-group calculation, we provide a complete characterization of the universal properties of the bosonic Moore-Read state on a Haldane honeycomb lattice model at filling number $\nu =1$ for larger systems, including both the edge spectrum and the bulk anyonic quasiparticle (QP) statistics. We first demonstrate that there are three degenerating ground states for each of which there is a definite anyonic flux threading through the cylinder. We identify the nontrivial countings for the entanglement spectrum in accordance with the corresponding conformal field theory. Through simulating a flux-inserting experiment, it is found that two of the Abelian ground states can be adiabatically connected, whereas the ground state in the Ising anyon sector evolves back to itself, which reveals the fusion rules between different QPs in real space. Furthermore, we calculate the modular matrices $\mathcal{S}$ and $\mathcal{U}$, which contain all the information for the anyonic QPs, such as quantum dimensions, fusion rule, and topological spins.

We present a means of controlling phonons via optical tuning. Taking as a model an array of photoresponsive materials (photoswitches) embedded in a matrix, we numerically analyze the vibrational response of an array of bistable harmonic oscillators with stochastic spring constants. Changing the intensity of light incident on the lattice directly controls the composition of the lattice and therefore the speed of sound. Furthermore, modulation of the phonon band structure at high frequencies results in a strong confinement of phonons. The applications of this regime for phonon waveguides, vibrational energy storage, and phononic transistors is examined.

Frustrated transition-metal compounds in which spin-orbit coupling (SOC) and electron correlation work together have attracted much attention recently. In the case of $5d$ transition metals, where SOC is large, $j_{\text{eff}}=1/2$ bands near the Fermi level are thought to encompass the essential physics of the material, potentially leading to a concrete realization of exotic magnetic phases such as the Kitaev spin liquid. Here, we derive a spin model on a triangular lattice based on $j_{\text{eff}}=1/2$ pseudospins that interact via antiferromagnetic Heisenberg ($J$) and Kitaev ($K$) exchanges, and crucially, an anisotropic $\left(\mathrm{\Gamma }\right)$ exchange. Our classical analysis of the spin model reveals that, in addition to small regions of ${120}^{\circ }, {\mathbb{Z}}_2$/dual-${\mathbb{Z}}_2$ vortex crystal and nematic phases, the stripy and ferromagnetic phases dominate the $J-K-\mathrm{\Gamma }$ phase diagram. We apply our model to the $5d$ transition-metal compound, ${\mathrm{Ba}}_3{\mathrm{IrTi}}_2{\mathrm{O}}_9$, in which the ${\mathrm{Ir}}^{4+}$ ions form layered two-dimensional triangular lattices. We compute the band structure and nearest-neighbor hopping parameters using ab initio calculations. By combining our ab initio and classical analyses, we predict that ${\mathrm{Ba}}_3{\mathrm{IrTi}}_2{\mathrm{O}}_9$ has a stripy ordered magnetic ground state.

Selenium impurities in silicon are deep double donors and their optical and electronic properties have been recently investigated due to their application for infrared detection. However, a singly ionized selenium donor $\left({\mathrm{Se}}^{+}\right)$ possesses an electron spin which makes it a potential candidate as a silicon-based spin qubit, with significant potential advantages compared to the more commonly studied group V donors. Here we study the electron spin relaxation $\left(T_1\right)$ and coherence $\left(T_2\right)$ times of ${\mathrm{Se}}^{+}$ in isotopically purified 28-silicon, and find them to be up to two orders of magnitude longer than shallow group V donors at temperatures above $\sim 15\mathrm{K}$. We further study the dynamics of donor-acceptor recombination between selenium and boron, demonstrating that it is possible to control the donor charge state through optical excitation of neutral ${\mathrm{Se}}^0$.

We theoretically investigate the Landau levels (LLs) and magneto-optical conductivity (MOC) of black phosphorus thin films under a perpendicular magnetic field based on an effective $\mathit{k}·\mathit{p}$ Hamiltonian and linear-response theory. We obtain the analytical expression for LLs, which agrees well with the numerical calculations, and find that the LLs sublinearly depend on the magnetic field and LL index. By using the Kubo formula, we evaluate the longitudinal and Hall optical conductivities as functions of the photon energy and the magnetic field. The analytical optical transition matrix elements reveal unusual selection rules for the interband (intraband) optical transitions between the LLs $\mathrm{\Delta }n=0,\pm 2$ $\left(\pm 1,\pm 3\right)$. The MOC shows strongly anisotropic behaviors of the band structure. For the interband transition, the MOC for linearly polarized light along the armchair direction is three orders of magnitude larger than that along the zigzag direction. Interestingly, we find a beating pattern in the interband MOC due to the interference among the three kinds of optical transitions. For the intraband transition, the MOC can be used to determine the band parameters such as the effective masses and the interband coupling at zero magnetic field. Our results about the MOC can also be applied to the monolayer black phosphorus.

The recent discovery of tunable Dzyaloshinskii-Moriya interactions in layered magnetic materials with perpendicular magnetic anisotropy makes them promising candidates for stabilization and manipulation of skyrmions at elevated temperatures. In this article, we use Monte Carlo simulations to investigate the robustness of skyrmions in these materials against thermal fluctuations and finite-size effects. We find that in confined geometries and at finite temperatures skyrmions are present in a large part of the phase diagram. Moreover, we find that the confined geometry favors the skyrmion over the spiral phase when compared to infinitely large systems. Upon tuning the magnetic field through the skyrmion phase, the system undergoes a cascade of transitions in the magnetic structure through states of different number of skyrmions, elongated and half-skyrmions, and spiral states. We consider how quantum and thermal fluctuations lift the degeneracies that occur at these transitions, and find that states with more skyrmions are typically favored by fluctuations over states with less skyrmions. Finally, we comment on electrical detection of the various phases through the topological and anomalous Hall effects.

We explore by electronic Raman scattering the superconducting state of the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi-2212) crystal by performing a fine-tuned doping study. We find three distinct energy scales in $A_{1g}, B_{1g}$, and $B_{2g}$ symmetries which show three distinct doping dependencies. Above $p=0.22$, the three energies merge; below $p=0.12$, the $A_{1g}$ scale is no longer detectable, while the $B_{1g}$ and $B_{2g}$ scales become constant in energy. In between, the $A_{1g}$ and $B_{1g}$ scales increase monotonically with underdoping, while the $B_{2g}$ one exhibits a maximum at $p=0.16$. The three superconducting energy scales appear to be a universal feature of hole-doped cuprates. We propose that the nontrivial doping dependencies of the three scales originate from the Fermi-surface changes and reveal competing orders inside the superconducting dome.

A local interaction between photons can be engineered by coupling a nonlinear system to a transmission line. The required transmission line can be conveniently formed from a chain of Josephson junctions. The nonlinearity is generated by side-coupling this chain to a Cooper pair box. We propose to probe the resulting photon-photon interactions via their effect on the current-voltage characteristic of a voltage-biased Josephson junction connected to the transmission line. Considering the Cooper pair box to be in the weakly anharmonic regime, we find that the dc current through the probe junction yields features around the voltages $2eV=n\hslash {\omega }_s$, where ${\omega }_s$ is the plasma frequency of the superconducting circuit. The features at $n\ge 2$ are a direct signature of the photon-photon interaction in the system.

A significant accumulation of matter in solid ${}^4\mathrm{He}$ observed during a superflow event, dubbed the giant isochoric compressibility (or the syringe effect), is discussed within the model of dislocations with superfluid core. It is shown that solid ${}^4\mathrm{He}$ in contact with a superfluid reservoir can develop a bistability with respect to the syringe fraction, with the threshold for the bias by chemical potential determined by a typical free length of dislocations with superfluid core. The main implications of this effect are hysteresis and strongly nonlinear dynamical behavior leading to growth, proliferation, and possibly exiting from a crystal of superclimbing dislocations. Three major channels for such dynamics are identified: (i) injection and inflation of the prismatic loops from the boundary, (ii) Bardeen-Herring generation of the loops in the bulk, and (iii) helical instability of the screw dislocations. It is argued that the syringe instability may have already been observed in the experiments on the superflow through solid Helium-4. Several testable predictions for the time and the bias dependencies of the dynamics are suggested.

We investigate by resonant inelastic x-ray scattering the magnetic excitations in thin films of tetragonal CuO. We identify a spin wave excitation, dispersing on two cupratelike antiferromagnetic sublattices. Its energy at the boundary of the Brillouin zone (220 meV), is significantly lower than typical values $(E\sim 300$ meV) found in two-dimensional cuprates. A spin wave expansion starting from an extended Hubbard model suggests two possible scenarios for this energy lowering.

We describe the results of first-principles calculations of the properties of oxygen vacancies in ${\mathrm{LaNiO}}_3$. We consider isolated oxygen vacancies, pairs of vacancies, and vacancies at finite concentrations that form oxygen-deficient phases of ${\mathrm{LaNiO}}_3$. The key electronic structure question we address is whether and to what extent an oxygen vacancy acts as an electron donor to the Fermi level (mobile and conducting electronic states). More generally, we describe how one can quantify, based on electronic structure calculations, the extent to which a localized point defect in a metallic system donates electrons to the Fermi level compared to trapping electrons in localized defect states. For ${\mathrm{LaNiO}}_3$, we find that an oxygen vacancy does not create mobile carriers but instead makes the two Ni sites adjacent to it turn into ${\mathrm{Ni}}^{2+}$ cations. Energetically, we compute the formation energy and diffusion barrier for oxygen vacancies. Structurally, we show that pairs of vacancies prefer to form on opposite sides of a Ni cation, aligning along a pseudocubic axis. For finite concentrations of vacancies, we compute the dependence of the ${\mathrm{LaNiO}}_3$ lattice parameters on the vacancy concentration to provide reliable data for experimental determination of the oxygen content in ${\mathrm{LaNiO}}_3$ and ${\mathrm{LaNiO}}_3$ thin films.

We investigated the coherent phonon generation mechanism by irradiation of an ultrashort pulse with a simple two-level model. Our derived formulation shows that both impulsive stimulated Raman scattering (ISRS) and impulsive absorption (IA) simultaneously occur, and phonon wave packets are generated in the electronic ground and excited states by ISRS and IA, respectively. We identify the dominant process from the amplitude of the phonon oscillation. For short pulse widths, ISRS is very small and becomes larger as the pulse width increases. We also show that the initial phase is dependent on the pulse width and the detuning.

We investigate spin-orbit torques of metallic CuAu-I-type antiferromagnets using spin-torque ferromagnetic resonance tuned by a dc-bias current. The observed spin torques predominantly arise from diffusive transport of spin current generated by the spin Hall effect. We find a growth-orientation dependence of the spin torques by studying epitaxial samples, which may be correlated to the anisotropy of the spin Hall effect. The observed anisotropy is consistent with first-principles calculations on the intrinsic spin Hall effect. Our work demonstrates large tunable spin-orbit effects in magnetically ordered materials.

Frustration in quantum many-body systems is quantified by the degree of incompatibility between the local and global orders associated, respectively, with the ground states of the local interaction terms and the global ground state of the total many-body Hamiltonian. This universal measure is bounded from below by the ground-state bipartite block entanglement. For many-body Hamiltonians that are sums of two-body interaction terms, a further inequality relates quantum frustration to the pairwise entanglement between the constituents of the local interaction terms. This additional bound is a consequence of the limits imposed by monogamy on entanglement shareability. We investigate the behavior of local pair frustration in quantum spin models with competing interactions on different length scales and show that valence bond solids associated with exact ground state dimerization correspond to a transition from generic frustration, i.e., geometric, common to classical and quantum systems alike, to genuine quantum frustration, i.e., solely due to the noncommutativity of the different local interaction terms. We discuss how such frustration transitions separating genuinely quantum orders from classical-like ones are detected by observable quantities such as the static structure factor and the interferometric visibility.

Temperature and fluence dependence of the 1.55-eV optical transient reflectivity in ${\mathrm{BaFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ was measured and analyzed in the low and high excitation density limit. The effective magnitude of the superconducting gap of $\sim 5$ meV obtained from the low-fluence-data bottleneck model fit is consistent with the angle-resolved photoemission spectroscopy results for the $\gamma$- and $\beta -\mathrm{hole}$ Fermi surfaces. The superconducting state nonthermal optical destruction energy was determined from the fluence dependent data. The planar optical destruction energy density scales well with $T_{\mathrm{c}}^2$ and is found to be similar in a number of different layered superconductors.

We construct a generalized quantum dimer model on two-dimensional nonbipartite lattices, including the triangular lattice, the star lattice, and the kagome lattice. At the Rokhsar-Kivelson (RK) point, we obtain its exact ground states that are shown to be a fully gapped quantum spin liquid with the double-semion topological order. The ground-state wave function of such a model at the RK point is a superposition of dimer configurations with a nonlocal sign structure determined by counting the number of loops in the transition graph. We explicitly demonstrate the double-semion topological order in the ground states by showing the semionic statistics of monomer excitations. We also discuss possible implications of such double-semion resonating valence bond states to candidate quantum spin-liquid systems discovered experimentally and numerically in the past few years.

If a positron impacts onto a surface, it may lead to the emission of a positron-electron pair. We have commissioned a laboratory-based positron source and performed a systematic study on a variety of solid surfaces. In a symmetric emission geometry we can explore the fact that positrons and electrons are distinguishable particles. Following fundamental symmetry arguments we have to expect that the available energy is shared unequally among positrons and electrons. Experimentally we observe such a behavior for all materials studied. We find a universal feature for all materials in the sense that, on average, the positron carries a larger fraction of the available energy. A scattering model accounts qualitatively for the observed energy sharing in positron-electron pair emission. A comparison of the intensity levels from the different materials reveals a monotonic relation between the singles and pair coincidence count rates.

We present a dynamical study on the nonlinear conduction behavior in the commensurate charge-density-wave phase of the quasi-one-dimensional conductor ${\mathrm{DCNQI}}_2\mathrm{Cu}$ below 75 K. We can accurately simulate magnitude and time dependence of the measured conductivity in response to large voltage pulses by accounting for the energy exchange between the phononic and electronic subsystems by means of an electrothermal model. Our simulations reveal a distinct nonequilibrium population of optical phonon states with an average energy of $\overline{E_{\text{ph}}}=19$ meV, being half the activation energy of about $\mathrm{\Delta }{E}_a=39$ meV observed in dc resistivity measurements. By inelastic scattering, this hot optical phonon bath generates additional charge-carrying excitations, thus providing a multiplication effect while energy transferred to the acoustic phonons is dissipated out of the system via heat conduction. Therefore, in high electric fields a preferred interaction of charge-carrying excitations with optical phonons compared to acoustic phonon modes is considered to be responsible for the nonlinear conduction effects observed in ${\mathrm{DCNQI}}_2\mathrm{Cu}$.