Stem canker diseases of eucalypts in Tasmania

In order to evaluate the range of stem canker fungi in natural eucalypt forest and plantations in Tasmania, a systematic survey was conducted. A total of 210 samples representing 30 fungal species were collected. The three species most frequently encountered were Endothia gyrosa, Cytospora eucalypticola and Valsa ceratosperma. Ten of the fungal species detailed in this survey were newly published and five were reported for the first time in Australia. Pathogenicity studies were conducted with 11 fungal species collected from the survey. Three species (E. gyrosa, Phoma sp. and Seiridium eucalypti) could cause significant cankers on both E. nitens and E. globulus. Influences by host species, provenance, age, vigour and bark type on canker development are discussed. The incidence of canker is higher in rough-barked E. nitens compared to smooth-barked trees. Longitudinal cracking in rough bark provides natural infection courts. However, once infected artificially, smooth-barked E. nitens is more susceptible than rough-barked. This susceptibility is attributed to the anatomical structure of smooth bark facilitating post-infection penetration. A high incidence of severe E. gyrosa cankers was observed in 1993 at Tewkesbury (northwestem Tasmania) within a vigorously growing plantation of mixed (smooth or rough barked) provenances of 16 yr old E. nitens. This observation initiated an in-depth investigation of E. gyrosa. Stem inoculations with isolates of E. gyrosa originating from different locations across Australia showed that all can infect E. nitens and E. globulus. However, isolates from Tasmania, Victoria and Western Australia were generally more aggressive than those from the Australian Capital Territory and New South Wales. Endothia gyrosa isolates from Australia and overseas were compared. Four main types of colony morphology were recognised among 133 isolates based on the colour and density of the vegetative mycelium. Vegetative incompatibility was detected using a pH amended medium. Sixteen isolates from different origins in Australia, South Africa, North America and Europe were grouped into 9 vegetative compatibility groups with this method. There was correspondence between the grouping of these sixteen isolates as determined by colony morphology and vegetative compatibility and those revealed by DNA polymorphisms in RFLP and RAPD analyses. Overseas and Australian isolates appear closely related. Within Australia isolates from as geographically distant locations as Western Australia and Victoria were grouped together. The significance of observed levels of intraspecific variation in E. gyrosa is discussed. The potential threat of canker fungi, especially E. gyrosa, to the plantation forestry is reviewed

Topological invariants for interacting topological insulators: II. Breakdown of the Green's function formalism

Topological phase transitions in free fermion systems can be characterized by closing of single-particle gap and change in topological invariants. However, in the presence of electronic interactions, topological phase transitions are more complicated. In paper I of this series (arXiv:1510.07816), we have developed an efficient scheme to evaluate the topological invariants based on Green's function formalism. Here, in paper II, we demonstrate four interaction-drive topological phase transitions (TPTs) in two-dimensional (2D) interacting topological insulators (TIs) via large-scale quantum Monte Carlo (QMC) simulations, based on the scheme of evaluating topological invariants presented in paper I. Across these transitions, the defining symmetries of the TIs have been neither explicitly nor spontaneously broken. In the first two models, the topological invariants calculated from Green's function formalism succeed in characterizing interaction-driven TPTs. However, in the second two models, we find single-particle gap does not close and the topological invariants constructed from single-particle Green's function acquire no change across the TPTs. Unexpected breakdown of the Green's function formalism in constructing topological invariants is thus discovered. We thence classify the TPTs in interacting TIs into two categories: those have noninteracting correspondence can be characterized successfully by the topological invariants constructed from Green's functions, while for the others that do not have noninteracting correspondence, the Green's function formalism experiences a breakdown but more interesting and exciting phenomena, such as emergent collective critical modes at the transition, arise. Discussion on the success and breakdown of topological invariants constructed from the Green's function formalism in the context of symmetry protected topological (SPT) states is presented.Comment: 15 pages, 13 figure

Diagnosis of interaction-driven topological phase via exact diagonalization

We propose a general scheme for diagnosing interaction-driven topological phases in the weak interaction regime using exact diagonalization (ED). The scheme comprises the analysis of eigenvalues of the point-group operators for the many-body eigenstates and the correlation functions for physical observables to extract the symmetries of the order parameters and the topological numbers of the underlying ground states at the thermodynamic limit from a relatively small size system afforded by ED. As a concrete example, we investigate the interaction effects on the half-filled spinless fermions on the checkerboard lattice with a quadratic band crossing point. Numerical results support the existence of a spontaneous quantum anomalous Hall phase purely driven by a nearest-neighbor weak repulsive interaction, separated from a nematic Mott insulator phase at strong repulsive interaction by a first-order phase transition.Comment: 5 pages, 3 figures, 1 table in the main text; 4 pages, 7 figures, 1 table in the supplemental materia

Quantum critical point of Dirac fermion mass generation without spontaneous symmetry breaking

We study a lattice model of interacting Dirac fermions in $(2+1)$ dimension space-time with an SU(4) symmetry. While increasing interaction strength, this model undergoes a {\it continuous} quantum phase transition from the weakly interacting Dirac semimetal to a fully gapped and nondegenerate phase without condensing any Dirac fermion bilinear mass operator. This unusual mechanism for mass generation is consistent with recent studies of interacting topological insulators/superconductors, and also consistent with recent progresses in lattice QCD community.Comment: 5 pages, 5 figures in main text; 10 pages, 6 figures in supplemental materia

Searching for spectral oscillations due to photon-axionlike particle conversion using the Fermi-LAT observations of bright supernova remnants

Axionlike-particles (ALPs) are one promising type of dark matter candidate particle that may generate detectable effects on $\gamma$-ray spectra other than the canonical weakly interacting massive particles. In this work we search for such oscillation effects in the spectra of supernova remnants caused by the photon-ALP conversion, using the Fermi Large Area Telescope data. Three bright supernova remnants, IC443, W44, and W51C, are analyzed. The inclusion of photon-ALP oscillations yields an improved fit to the $\gamma$-ray spectrum of IC443, which gives a statistical significance of $4.2\sigma$ in favor of such spectral oscillation. However, the best-fit parameters of ALPs ($m_{a}=6.6\,{\rm neV}$, $g_{a\gamma}=13.4 \times 10^{-11}\,{\rm GeV}^{-1}$) are in tension with the upper bound ($g_{a\gamma}< 6.6 \times 10^{-11}\,{\rm GeV}^{-1}$) set by the CAST experiment. It is difficult to explain the results using the systematic uncertainties of the flux measurements. We speculate that the "irregularity" displayed in the spectrum of IC443 may be due to the superposition of the emission from different parts of the remnant

Bona fide interaction-driven topological phase transition in correlated SPT states

It is expected that the interplay between non-trivial band topology and strong electron correlation will lead to very rich physics. Thus a controlled study of the competition between topology and correlation is of great interest. Here, employing large-scale quantum Monte Carlo (QMC) simulations, we provide a concrete example of the Kane-Mele-Hubbard (KMH) model on an AA stacking bilayer honeycomb lattice with inter-layer antiferromagnetic interaction. Our simulation identified several different phases: a quantum spin-Hall insulator (QSH), a $xy$-plane antiferromagnetic Mott insulator ($xy$-AFM) and an inter-layer dimer-singlet insulator (dimer-singlet). Most importantly, a bona fide topological phase transition between the QSH and the dimer-singlet insulators, purely driven by the inter-layer antiferromagnetic interaction is found. At the transition, the spin and charge gap of the system close while the single-particle excitations remain gapped, which means that this transition has no mean field analogue and it can be viewed as a transition between bosonic SPT states. At one special point, this transition is described by a $(2+1)d$ $O(4)$ nonlinear sigma model (NLSM) with {\it exact} $SO(4)$ symmetry, and a topological term at {\it exactly} $\Theta = \pi$. Relevance of this work towards more general interacting SPT states is discussed.Comment: 14 pages, 12 figures, 1 table, revised versio

Quantum Monte Carlo study of strange correlator in interacting topological insulators

Distinguishing the nontrivial symmetry-protected topological (SPT) phase from the trivial insulator phase in the presence of electron-electron interaction is an urgent question to the study of topological insulators, due to the fact that most of the topological indices defined for free electron systems are very likely unsuitable for interacting cases. In this work, we demonstrate that the strange correlator is a sensitive diagnosis to detect SPT states in interacting systems. Employing large-scale quantum Monte Carlo (QMC) simulations, we investigate the interaction-driven quantum phase transition in the Kane-Mele-Hubbard model. The transition from the quantum spin Hall insulator at weak interaction to an antiferromagnetic Mott insulator at strong interaction can be readily detected by the momentum space behavior of the strange correlator in single-particle, spin, and pairing sectors. The interaction effects on the symmetry-protected edge states in various sectors, i.e., the helical Luttinger liquid behavior, are well captured in the QMC measurements of strange correlators. Moreover, we demonstrate that the strange correlator is technically easier to implement in QMC and more robust in performance than other proposed numerical diagnoses for interacting topological states, as only static correlations are needed. The attempt in this work paves the way for using the strange correlator to study interaction-driven topological phase transitions in fermionic as well as bosonic systems.Comment: 10 pages, 9 figures, 1 appendi

Visualizing a Bosonic Symmetry Protected Topological Phase in an Interacting Fermion Model

Symmetry protected topological (SPT) phases in free fermion and interacting bosonic systems have been classified, but the physical phenomena of interacting fermionic SPT phases have not been fully explored. Here, employing large-scale quantum Monte Carlo simulation, we investigate the edge physics of a bilayer Kane-Mele-Hubbard model with zigzag ribbon geometry. Our unbiased numerical results show that the fermion edge modes are gapped out by interaction, while the bosonic edge modes remain gapless at the $(1+1)d$ boundary, before the bulk quantum phase transition to a topologically trivial phase. Therefore, finite fermion gaps both in the bulk and on the edge, together with the robust gapless bosonic edge modes, prove that our system becomes an emergent bosonic SPT phase at low energy, which is, for the first time, directly observed in an interacting fermion lattice model.Comment: 5 pages, 5 figures in the main text; 5 pages, 8 figures in the supplemental materia

Searching for the possible signal of the photon-axionlike particle oscillation in the combined GeV and TeV spectra of supernova remnants

The conversion between photons and axionlike particles (ALPs) in the Milky Way magnetic field could result in the detectable oscillation phenomena in $\gamma$-ray spectra of Galactic sources. In this work, the GeV (Fermi-LAT) and TeV (MAGIC/VERITAS/H.E.S.S.) data of three bright supernova remnants (SNRs, ie. IC443, W51C and W49B) have been adopted together to search such the oscillation effect. Different from our previous analysis of the sole Fermi-LAT data of IC443, we do not find any reliable signal for the photon-ALP oscillation in the joint broadband spectrum of each SNR. The reason for the inconsistence is that in this work we use the latest revision (P8R3) of Fermi-LAT data, updated diffuse emission templates and the new version of the source catalog (4FGL), which lead to some modification of the GeV spectrum of IC443. Then we set constraints on ALP parameters based on the combined analysis of all the three sources. Though these constraints are somewhat weaker than limits from the CAST experiment and globular clusters, they are supportive of and complementary to these other results.Comment: 11 pages, 8 figures, Accepted into Physical Review

Search for gamma-ray emission from the nearby dwarf spheroidal galaxies with 9 years of Fermi-LAT data

In this work, we search for $\gamma$-ray emission from the directions of some nearby Milky Way dwarf spheroidal galaxies (dSphs) and candidates with the publicly-available Pass 8 data of Fermi-LAT. Our sample includes 12 sources with the distances $<50$ kpc. Very weak $\gamma$-ray excesses ($\sim 2\sigma$) are found in some dSphs/candidates, consistent with that reported in the previous literature. Intriguingly, the peak test statistic (TS) value of the weak emission from the direction of Reticulum II rises continually. If interpreted as dark matter (DM) annihilation, the peak TS value is 13.5 for the annihilation channel of $\chi\chi \rightarrow \tau^{+}\tau^{-}$ and the DM mass of $m_\chi \sim 16$ GeV. The combination of all these nearby sources yields a more significant (with local significance $> 4\sigma$) $\gamma$-ray signal.Comment: 7 pages, 4 figures, accepted for publication in PR