Current at a distance and resonant transparency in Weyl semimetals

Surface Fermi arcs are the most prominent manifestation of the topological nature of Weyl semimetals. In the presence of a static magnetic field oriented perpendicular to the sample surface, their existence leads to unique inter-surface cyclotron orbits. We propose two experiments which directly probe the Fermi arcs: a magnetic field dependent non-local DC voltage and sharp resonances in the transmission of electromagnetic waves at frequencies controlled by the field. We show that these experiments do not rely on quantum mechanical phase coherence, which renders them far more robust and experimentally accessible than quantum effects. We also comment on the applicability of these ideas to Dirac semimetals.Comment: 10 pages, 8 figure

Exploring Topological Phases With Quantum Walks

The quantum walk was originally proposed as a quantum mechanical analogue of the classical random walk, and has since become a powerful tool in quantum information science. In this paper, we show that discrete time quantum walks provide a versatile platform for studying topological phases, which are currently the subject of intense theoretical and experimental investigation. In particular, we demonstrate that recent experimental realizations of quantum walks simulate a non-trivial one dimensional topological phase. With simple modifications, the quantum walk can be engineered to realize all of the topological phases which have been classified in one and two dimensions. We further discuss the existence of robust edge modes at phase boundaries, which provide experimental signatures for the non-trivial topological character of the system

Enhanced Pairing in the "Checkerboard" Hubbard Ladder

We study signatures of superconductivity in a 2--leg "checkerboard" Hubbard ladder model, defined as a one--dimensional (period 2) array of square plaquettes with an intra-plaquette hopping $t$ and inter-plaquette hopping $t'$, using the density matrix renormalization group method. The highest pairing scale (characterized by the spin gap or the pair binding energy, extrapolated to the thermodynamic limit) is found for doping levels close to half filling, $U\approx 6t$ and $t'/t \approx 0.6$. Other forms of modulated hopping parameters, with periods of either 1 or 3 lattice constants, are also found to enhance pairing relative to the uniform two--leg ladder, although to a lesser degree. A calculation of the phase stiffness of the ladder reveals that in the regime with the strongest pairing, the energy scale associated with phase ordering is comparable to the pairing scale.Comment: 9 pages, 9 figures; Journal reference adde

Frustrated quantum Ising spins simulated by spinless bosons in a tilted lattice: from a quantum liquid to antiferromagnetic order

We study spinless bosons in a decorated square lattice with a near-diagonal tilt. The resonant subspace of the tilted Mott insulator is described by an effective Hamiltonian of frustrated quantum Ising spins on a non-bipartite lattice. This generalizes an earlier proposal for the unfrustrated quantum Ising model in one dimension which was realized in a recent experiment on ultracold $^{87}$Rb atoms in an optical lattice. Very close to diagonal tilt, we find a quantum liquid state which is continuously connected to the paramagnet. Frustration can be reduced by increasing the tilt angle away from the diagonal, and the system undergoes a transition to an antiferromagnetically ordered state. Using quantum Monte Carlo simulations and exact diagonalization, we find that for realistic system sizes the antiferromagnetic order appears to be quasi-one-dimensional; however, in the thermodynamic limit the order is two-dimensional.Comment: 27 pages, 14 figure

The anomalous Floquet-Anderson insulator as a non-adiabatic quantized charge pump

Periodically driven quantum systems provide a novel and versatile platform for realizing topological phenomena. Among these are analogs of topological insulators and superconductors, attainable in static systems; however, some of these phenomena are unique to the periodically driven case. Here, we show that disordered, periodically driven systems admit an "anomalous" two dimensional phase, whose quasi-energy spectrum consists of chiral edge modes that coexist with a fully localized bulk - an impossibility for static Hamiltonians. This unique situation serves as the basis for a new topologically-protected non-equilibrium transport phenomenon: quantized non-adiabatic charge pumping. We identify the bulk topological invariant that characterizes the new phase (which we call the "anomalous Floquet Anderson Insulator", or AFAI). We provide explicit models which constitute a proof of principle for the existence of the new phase. Finally, we present evidence that the disorder-driven transition from the AFAI to a trivial, fully localized phase is in the same universality class as the quantum Hall plateau transition

Control of vegetative growth of grape vines (Vitis vinifera) with chloroethylphosphonic acid (Ethephon) and other growth inhibitors

In order to arrest vegetative growth of grape shoots, various growth regulating substances were applicated on five grapeVine cultivars: Alphonse Lavallee, Muscat of Hamburg, Cardinal, Perlette, and Queen of the Vineyards. Ethephon at a concentration of 480 ppm efficiently inhibited the terminal growth of the canes and prevented the opening of lateral buds on the shoots for about 8 to 10 weeks. This treatment somewhat enhanced maturation of the current yield. No negative effect on bud differentiation, bud opening and growth in the following season was found. The use of Ethephon was much more effective than the other regulators tested such as Alar, NC 9634, pp 413, and morphactin 7311. Alar caused a shortening of the canes, not effecting the number of nodes. pp 413 had no activity in our system while both the morphactin and NC 9634 did inhibit growth partially. The Ethephon treatment was far more efficient and uniform than repeated manual topping.Regulierung des vegetativen Wachstums von Reben (Vitis vinifera) durch Etephon und andere HemmstoffeDie hemmende Wirkung verschiedener Wachstumsregulatoren wurde an fünf Rebsorten geprüft: Alphonse Lavallee, Muskat Hamburg, Cardinal, Perlette und Queen of the Vineyards. Etephon hemmte bei einer Konzentration von 480 ppm deutlich das Spitzenwachstum der Triebe und verzögerte den Austrieb der lateralen Knospen um 8 bis 10 Wochen. Diese Behandlung erhöhte den Reifegrad der Traubenernte. Ein negativer Einfluß auf Knospendifferenzierung, Knospenaustrieb und Wachstum in der folgenden Wachstumsperiode wurde nicht gefunden. Etephon war erheblich wirksamer als die anderen verwendeten Substanzen: Alar, NC 9634, PP 413 und Morphaktin 7311. Alar verursachte eine- Verkürzung der Triebe, ohne die Zahl der Nodi zu verändern. pp 413 hatte keine Wirkung, während Morphaktin und NC 9634 das Wachstum teilweise hemmten. Etephon wirkte weitaus nachhaltiger und gleichförmiger als wiederholtes Gipfeln in Handarbeit

Spin-polarized superconductivity: order parameter topology, current dissipation, and multiple-period Josephson effect

We discuss transport properties of fully spin-polarized triplet superconductors, where only electrons of one spin component (along a certain axis) are paired. Due to the structure of the order parameter space, wherein phase and spin rotations are intertwined, a configuration where the superconducting phase winds by $4\pi$ in space is topologically equivalent to a configuration with no phase winding. This opens the possibility of supercurrent relaxation by a smooth deformation of the order parameter, where the order parameter remains non-zero at any point in space throughout the entire process. During the process, a spin texture is formed. We discuss the conditions for such processes to occur and their physical consequences. In particular, we show that when a voltage is applied, they lead to an unusual alternating-current Josephson effect whose period is an integer multiple of the usual Josephson period. These conclusions are substantiated in a simple time-dependent Ginzburg-Landau model for the dynamics of the order parameter. One of the potential applications of our analysis is for moir\'e systems, such as twisted bilayer and double bilayer graphene, where superconductivity is found in the vicinity of ferromagnetism.Comment: 12+7 pages, 6 figure

Individual-subject Functional Localization Increases Univariate Activation but Not Multivariate Pattern Discriminability in the "Multiple-demand" Frontoparietal Network.

The frontoparietal "multiple-demand" (MD) control network plays a key role in goal-directed behavior. Recent developments of multivoxel pattern analysis (MVPA) for fMRI data allow for more fine-grained investigations into the functionality and properties of brain systems. In particular, MVPA in the MD network was used to gain better understanding of control processes such as attentional effects, adaptive coding, and representation of multiple taskrelevant features, but overall low decoding levels have limited its use for this network. A common practice of applying MVPA is by investigating pattern discriminability within a ROI using a template mask, thus ensuring that the same brain areas are studied in all participants. This approach offers high sensitivity but does not take into account differences between individuals in the spatial organization of brain regions. An alternative approach uses independent localizer data for each subject to select the most responsive voxels and define individual ROIs within the boundaries of a group template. Such an approach allows for a refined and targeted localization based on the unique pattern of activity of individual subjects while ensuring that functionally similar brain regions are studied for all subjects. In the current study, we tested whether using individual ROIs leads to changes in decodability of task-related neural representations as well as univariate activity across the MD network compared with when using a group template. We used three localizer tasks to separately define subject-specific ROIs: spatial working memory, verbal working memory, and a Stroop task. We then systematically assessed univariate and multivariate results in a separate rule-based criterion task. All the localizer tasks robustly recruited the MD network and evoked highly reliable activity patterns in individual subjects. Consistent with previous studies, we found a clear benefit of the subject-specific ROIs for univariate results from the criterion task, with increased activity in the individual ROIs based on the localizers' data, compared with the activity observed when using the group template. In contrast, there was no benefit of the subject-specific ROIs for the multivariate results in the form of increased discriminability, as well as no cost of reduced discriminability. Both univariate and multivariate results were similar in the subject-specific ROIs defined by each of the three localizers. Our results provide important empirical evidence for researchers in the field of cognitive control for the use of individual ROIs in the frontoparietal network for both univariate and multivariate analysis of fMRI data and serve as another step toward standardization and increased comparability across studies.This work was funded by a Royal Society Dorothy Hodgkin Research Fellowship (United Kingdom) to Yaara Erez (DH130100). Sneha Shashidhara was supported by a scholarship from the Gates Cambridge Trust, Cambridge, United Kingdom. Floortje Spronkers was supported by an Erasmus+ Traineeship grant and a Stichting A.S.C. Academy grant