Supersolids in confined fermions on one-dimensional optical lattices

Using quantum Monte Carlo simulations, we show that density-density and pairing correlation functions of the one-dimensional attractive fermionic Hubbard model in a harmonic confinement potential are characterized by the anomalous dimension $K_\rho$ of a corresponding periodic system, and hence display quantum critical behavior. The corresponding fluctuations render the SU(2) symmetry breaking by the confining potential irrelevant, leading to structure form factors for both correlation functions that scale with the same exponent upon increasing the system size, thus giving rise to a (quasi)supersolid.Comment: 4 pages, 5 figures, published versio

Proteolytic cleavage of the cell surface protein p160 is required for detachment of the fertilization envelope in the sea urchin

AbstractSea urchin eggs secrete a serine protease activity, CGSP1, at fertilization that is essential for the block to polyspermy. Several targets of this proteolytic activity on the plasma membrane were identified here using a cell surface biotinylation approach. Amino acid microsequencing of one of these proteins led to the identification of a 4.75-kb cDNA clone from a Strongylocentrotus purpuratus ovary cDNA library that encodes a 160-kDa protein called p160. This protein contains five CUB domains and a putative transmembrane domain suggesting that p160 is an integral membrane protein with protein–protein interaction motifs facing the extracellular matrix of the egg. Whole-mount immunolocalization studies demonstrate that p160 is on the surface of the egg, enriched at the tips of microvilli. The protein is removed at fertilization in a protease-dependent manner, and functional assays suggest that p160 serves to link the plasma membrane to the vitelline layer until fertilization. Thus, p160 is a key candidate for a vitelline-layer linker protein, the selective proteolysis of which functions in the block to polyspermy in the sea urchin egg

Penrose Quantum Antiferromagnet

The Penrose tiling is a perfectly ordered two dimensional structure with fivefold symmetry and scale invariance under site decimation. Quantum spin models on such a system can be expected to differ significantly from more conventional structures as a result of its special symmetries. In one dimension, for example, aperiodicity can result in distinctive quantum entanglement properties. In this work, we study ground state properties of the spin-1/2 Heisenberg antiferromagnet on the Penrose tiling, a model that could also be pertinent for certain three dimensional antiferromagnetic quasicrystals. We show, using spin wave theory and quantum Monte Carlo simulation, that the local staggered magnetizations strongly depend on the local coordination number z and are minimized on some sites of five-fold symmetry. We present a simple explanation for this behavior in terms of Heisenberg stars. Finally we show how best to represent this complex inhomogeneous ground state, using the "perpendicular space" representation of the tiling.Comment: 4 pages, 5 figure

Quantum Antiferromagnetism in Quasicrystals

The antiferromagnetic Heisenberg model is studied on a two-dimensional bipartite quasiperiodic lattice. The distribution of local staggered magnetic moments is determined on finite square approximants with up to 1393 sites, using the Stochastic Series Expansion Quantum Monte Carlo method. A non-trivial inhomogeneous ground state is found. For a given local coordination number, the values of the magnetic moments are spread out, reflecting the fact that no two sites in a quasicrystal are identical. A hierarchical structure in the values of the moments is observed which arises from the self-similarity of the quasiperiodic lattice. Furthermore, the computed spin structure factor shows antiferromagnetic modulations that can be measured in neutron scattering and nuclear magnetic resonance experiments. This generic model is a first step towards understanding magnetic quasicrystals such as the recently discovered Zn-Mg-Ho icosahedral structure.Comment: RevTex, 4 pages with 5 figure

Quantum phase transitions in the Kane-Mele-Hubbard model

We study the two-dimensional Kane-Mele-Hubbard model at half filling by means of quantum Monte Carlo simulations. We present a refined phase boundary for the quantum spin liquid. The topological insulator at finite Hubbard interaction strength is adiabatically connected to the groundstate of the Kane-Mele model. In the presence of spin-orbit coupling, magnetic order at large Hubbard U is restricted to the transverse direction. The transition from the topological band insulator to the antiferromagnetic Mott insulator is in the universality class of the three-dimensional XY model. The numerical data suggest that the spin liquid to topological insulator and spin liquid to Mott insulator transitions are both continuous.Comment: 13 pages, 10 figures; final version; new Figs. 4(b) and 8(b

Comment on "Novel Superfluidity in a Trapped Gas of Fermi Atoms with Repulsive Interaction Loaded on an Optical Lattice"

In a recent letter Machida et al. [Phys. Rev. Lett. 93, 200402 (2004)] concluded that in a trapped gas of fermions with repulsive interactions a superfluid phase appears around the Mott-insulator at the center of the trap. They base their conclusion on a negative binding energy, and a large weight for a singlet formed by particles located at opposite sides of the Mott-insulator. We show here that the observed effects are not related to superfluidity.Comment: Revtex file, 1 page, 1 figure, published versio

Universal scaling at field-induced magnetic phase transitions

We study field-induced magnetic order in cubic lattices of dimers with antiferromagnetic Heisenberg interactions. The thermal critical exponents at the quantum phase transition from a spin liquid to a magnetically ordered phase are determined from Stochastic Series Expansion Quantum Monte Carlo simulations. These exponents are independent of the interdimer coupling ratios, and converge to the value obtained by considering the transition as a Bose-Einstein condensation of magnons, alpha_(BEC) = 1.5. The scaling results are of direct relevance to the spin-dimer systems TlCuCl_3 and KCuCl_3, and explain the broad range of exponents reported for field-induced ordering transitions.Comment: 4 pages, 4 eps-figure

Functional imaging of mucociliary phenomena: High-speed digital reflection contrast microscopy

We present a technique for the investigation of mucociliary phenomena on trachea explants under conditions resembling those in the respiratory tract. Using an enhanced reflection contrast, we detect simultaneously the wave-like modulation of the mucus surface by the underlying ciliary activity and the transport of particles embedded in the mucus layer. Digital recordings taken at a speed of 500 frames per second are analyzed by a set of refined data processing algorithms. The simultaneously extracted data include not only ciliary beat frequency and its surface distribution, but also space-time structure of the mucociliary wave field, wave velocity and mucus transport velocity. Furthermore, we propose the analysis of the space and time evolution of the phase of the mucociliary oscillations to be the most direct way to visualize the coordination of the cilia. In particular, this analysis indicates that the synchronization is restricted to patches with varying directions of wave propagation, but the transport direction is strongly correlated with the mean direction of waves. The capabilities of the technique and of the data-processing algorithms are documented by characteristic data obtained from mammalian and avine trachea

Wang-Landau sampling for quantum systems: algorithms to overcome tunneling problems and calculate the free energy

We present a generalization of the classical Wang-Landau algorithm [Phys. Rev. Lett. 86, 2050 (2001)] to quantum systems. The algorithm proceeds by stochastically evaluating the coefficients of a high temperature series expansion or a finite temperature perturbation expansion to arbitrary order. Similar to their classical counterpart, the algorithms are efficient at thermal and quantum phase transitions, greatly reducing the tunneling problem at first order phase transitions, and allow the direct calculation of the free energy and entropy.Comment: Added a plot showing the efficiency at first order phase transition