Role of thermal friction in relaxation of turbulent Bose-Einstein condensates

In recent experiments, the relaxation dynamics of highly oblate, turbulent Bose-Einstein condensates (BECs) was investigated by measuring the vortex decay rates in various sample conditions [Phys. Rev. A $\bf 90$, 063627 (2014)] and, separately, the thermal friction coefficient $\alpha$ for vortex motion was measured from the long-time evolution of a corotating vortex pair in a BEC [Phys. Rev. A $\bf 92$, 051601(R) (2015)]. We present a comparative analysis of the experimental results, and find that the vortex decay rate $\Gamma$ is almost linearly proportional to $\alpha$. We perform numerical simulations of the time evolution of a turbulent BEC using a point-vortex model equipped with longitudinal friction and vortex-antivortex pair annihilation, and observe that the linear dependence of $\Gamma$ on $\alpha$ is quantitatively accounted for in the dissipative point-vortex model. The numerical simulations reveal that thermal friction in the experiment was too strong to allow for the emergence of a vortex-clustered state out of decaying turbulence.Comment: 7 pages, 5 figure

Photoreductive Dissolution of Iron Oxides Trapped in Ice and Its Environmental Implications

The availability of iron has been thought to be a main limiting factor for the productivity of phytoplankton and related with the uptake of atmospheric CO_2 and algal blooms in fresh and sea waters. In this work, the formation of bioavailable iron (Fe(II)_(aq)) from the dissolution of iron oxide particles was investigated in the ice phase under both UV and visible light irradiation. The photoreductive dissolution of iron oxides proceeded slowly in aqueous solution (pH 3.5) but was significantly accelerated in polycrystalline ice, subsequently releasing more bioavailable ferrous iron upon thawing. The enhanced photogeneration of Fe(II)_(aq) in ice was confirmed regardless of the type of iron oxides [hematite, maghemite (γ-Fe_2O_3), goethite (α-FeOOH)] and the kind of electron donors. The ice-enhanced dissolution of iron oxides was also observed under visible light irradiation, although the dissolution rate was much slower compared with the case of UV radiation. The iron oxide particles and organic electron donors (if any) in ice are concentrated and aggregated in the liquid-like grain boundary region (freeze concentration effect) where protons are also highly concentrated (lower pH). The enhanced photodissolution of iron oxides should occur in this confined boundary region. We hypothesized that electron hopping through the interconnected grain boundaries of iron oxide particles facilitates the separation of photoinduced charge pairs. The outdoor experiments carried out under ambient solar radiation of Ny-Ålesund (Svalbard, 78°55′N) also showed that the generation of dissolved Fe(II)_(aq) via photoreductive dissolution is enhanced when iron oxides are trapped in ice. Our results imply that the ice(snow)-covered surfaces and ice-cloud particles containing iron-rich mineral dusts in the polar and cold environments provide a source of bioavailable iron when they thaw

Evolution of gall inducing Eulophidae (Hymenoptera: Chalcidoidea) on Myrtaceae in Australia

The present thesis consists of two main parts: 1) descriptive works including new species descriptions and a gall community study and 2) phylogenetic studies. The major aim is to determine the evolutionary pathway of gall induction biology among Australian parasitic Tetrastichinae (Hymenoptera: Eulophidae). The descriptive works include three new gall inducing tetrastichines; Quadrastichus erythrinae Kim from Erythrina (Fabaceae) as the first gall-former in the genus, Moona spermophaga Kim and La Salle from seeds of Corymbia (Myrtaceae) and Leprosa milga Kim and La Salle from seeds of Eucalyptus (Myrtaceae). Also, a new gall inducing tribe Boucekelimini has been described from Melaleuca (Myrtaceae), and this new tribe contains two new genera Boucekelimus Kim and La Salle and Tatiana Kim and La Salle. A gall community study including species composition, abundances and seasonal occurrence was conducted. This gall community on Eucalyptus appears very complex in its species composition and interactions among gall-formers, parasitoids and inquilines. The wasp community consists of twelve species of five families in two hymenopteran superfamilies. Two unidentified Ophelimus speices (Eulophidae: Ophelimini) are dominant species among wasps emerged from the galls. The first Ophelimus species was found to be a primary gall-former. The second Ophelimus seems to be a parasitoid or an inquiline of the first Ophelimus. All other associates seem to be parasitoids or inquilines. Morphological and molecular data were used to infer the evolution of gall induction biology on Eucalyptus in Australian Tetrastichinae. Also the combined analysis with both morphological and molecular data was conducted. Each morphological, molecular and the combined analysis yielded contradicting results. 47 characters from 24 tetrastichine species and two outgroup species were used for the morphological analysis. Cladograms were constructed, and the results were compared with Graham's suggestion (1987) about relationships among the tetrastichine genus groups. His suggestion was largely contradicted by the present analyses. The analyses suggested that both the Aprostocetus-complex and the Tetrastichus s. str. are nonmonophyletic. However, some group clustering appeared to fit relatively well with Graham's suggestion: Aprostocetus + Neotrichoporoides, Crataepus + Pronotalia, and the separation of the Australian gall inducing group from the European fauna. The barcoding region (619 bps fragment) of Cytochrome Oxidase subunit I (COI) on the mitochondrial gene was sequenced from 25 tetrastichines as ingroup and one outgroup species. The Barcode gene failed to resolve phylogeny at genus level but is very useful for identification of species in a genus or cryptic species. Molecular analyses found that the Leptocybe species group consists of seven unique sequences. Two species in this species group drew my attention: Leptocybe invasa Fisher and La Salle, which is a devastating invasive pest in Israel, and Leptocybe sp. 9, which was most recently found in Australia. They were thought to be the same species due to the same biology and gall type. However, the molecular analyses suggest that Leptocybe sp. 9 is not L. invasa but a very close species. The evolution of gall induction on Eucalyptus among Australian Tetrastichinae was estimated from the morphological and molecular data. Overall, the morphological analysis suggests two independent origins of gall induction on Myrtaceae in Tetrastichinae while the combined analysis only one origin. Also, the results of the analyses suggest that gall inducing lineages may have evolved from a parasitic progenitor and the ancestral stock of the Australian gall-inducers may have first induced galls on seeds of Eucalyptus. Leaf galling and leaf & twig galling genera except Epichrysocharis may have evolved deep within the seed-galling lineage

Obstructions for Matroids of Path-Width at most k and Graphs of Linear Rank-Width at most k

International audienceEvery minor-closed class of matroids of bounded branch-width can be characterized by a minimal list of excluded minors, but unlike graphs, this list could be infinite in general. However, for each fixed finite field $\mathbb F$, the list contains only finitely many $\mathbb F$-representable matroids, due to the well-quasi-ordering of $\mathbb F$-representable matroids of bounded branch-width under taking matroid minors [J. F. Geelen, A. M. H. Gerards, and G. Whittle (2002)]. But this proof is non-constructive and does not provide any algorithm for computing these $\mathbb F$-representable excluded minors in general. We consider the class of matroids of path-width at most $k$ for fixed $k$. We prove that for a finite field $\mathbb F$, every $\mathbb F$-representable excluded minor for the class of matroids of path-width at most~$k$ has at most $2^{|\mathbb{F}|^{O(k^2)}}$ elements. We can therefore compute, for any integer $k$ and a fixed finite field $\mathbb F$, the set of $\mathbb F$-representable excluded minors for the class of matroids of path-width $k$, and this gives as a corollary a polynomial-time algorithm for checking whether the path-width of an $\mathbb F$-represented matroid is at most $k$. We also prove that every excluded pivot-minor for the class of graphs having linear rank-width at most $k$ has at most $2^{2^{O(k^2)}}$ vertices, which also results in a similar algorithmic consequence for linear rank-width of graphs

Vortex detection in atomic Bose-Einstein condensates using neural networks trained on synthetic images

Quantum vortices in atomic Bose-Einstein condensates (BECs) are topological defects characterized by quantized circulation of particles around them. In experimental studies, vortices are commonly detected by time-of-flight imaging, where their density-depleted cores are enlarged. In this work, we describe a machine learning-based method for detecting vortices in experimental BEC images, particularly focusing on turbulent condensates containing irregularly distributed vortices. Our approach employs a convolutional neural network (CNN) trained solely on synthetic simulated images, eliminating the need for manual labeling of the vortex positions as ground truth. We find that the CNN achieves accurate vortex detection in real experimental images, thereby facilitating analysis of large experimental datasets without being constrained by specific experimental conditions. This novel approach represents a significant advancement in studying quantum vortex dynamics and streamlines the analysis process in the investigation of turbulent BECs.Comment: 10 pages, 6 figure

Refining Generative Process with Discriminator Guidance in Score-based Diffusion Models

The proposed method, Discriminator Guidance, aims to improve sample generation of pre-trained diffusion models. The approach introduces a discriminator that gives explicit supervision to a denoising sample path whether it is realistic or not. Unlike GANs, our approach does not require joint training of score and discriminator networks. Instead, we train the discriminator after score training, making discriminator training stable and fast to converge. In sample generation, we add an auxiliary term to the pre-trained score to deceive the discriminator. This term corrects the model score to the data score at the optimal discriminator, which implies that the discriminator helps better score estimation in a complementary way. Using our algorithm, we achive state-of-the-art results on ImageNet 256x256 with FID 1.83 and recall 0.64, similar to the validation data's FID (1.68) and recall (0.66). We release the code at https://github.com/alsdudrla10/DG.Comment: International Conference on Machine Learning (ICML23

Unavoidable induced subgraphs in large graphs with no homogeneous sets

A homogeneous set of an $n$-vertex graph is a set $X$ of vertices ($2\le |X|\le n-1$) such that every vertex not in $X$ is either complete or anticomplete to $X$. A graph is called prime if it has no homogeneous set. A chain of length $t$ is a sequence of $t+1$ vertices such that for every vertex in the sequence except the first one, its immediate predecessor is its unique neighbor or its unique non-neighbor among all of its predecessors. We prove that for all $n$, there exists $N$ such that every prime graph with at least $N$ vertices contains one of the following graphs or their complements as an induced subgraph: (1) the graph obtained from $K_{1,n}$ by subdividing every edge once, (2) the line graph of $K_{2,n}$, (3) the line graph of the graph in (1), (4) the half-graph of height $n$, (5) a prime graph induced by a chain of length $n$, (6) two particular graphs obtained from the half-graph of height $n$ by making one side a clique and adding one vertex.Comment: 13 pages, 3 figure